Anatomy25 chapter 4

Simple Columnar Epithelium

A simple columnar epithelium is composed of a single layer of tall, narrow cells. The nucleus is oval, oriented lengthwise, and located within the basal region of the cell. Active movement of molecules oc- curs across this type of epithelium by either absorption or secretion. Simple columnar epithelium has two forms; one type has no cilia, whereas the apical surface of the other type is lined by cilia.

Cardiac Muscle Tissue

Cardiac tissue is confined to the thick middle layer of the heart wall (called the myocardium). Microscopically, cardiac muscle tissue resembles skeletal muscle in that both contain visible striations (table 4.12b), However, several obvious cellular differences distinguish the two types. First, the typical cardiac muscle cell is much shorter than a typical skeletal muscle fiber. Second, a cardiac muscle cell contains only one or two centrally located nuclei. Third, the cardiac muscle cell often bifurcates (branches), thus resembling a Y in shape. Finally, cardiac muscle cells are connected by intercalated discs ( in - ter , ka - la - ted ; intercalates = inserted between ) , which have strong desmosomes and gap junctions between the cells. The intercalated discs promote the rapid transport of an electrical activity through many cardiac muscle cells at once, allowing the entire muscle wall to contract as a unit. When you view cardiac muscle tissue under the microscope the intercalated discs appear as thick, dark lines between the cells Cardiac muscle is responsible for the rhythmic heart contrac- tions that pump blood throughout the blood vessels of the body. Cardiac muscle cells are considered involuntary because they do not cialized cardiac muscle cells in the heart wall initiate the contraction require nervous system activity to initiate a contraction; instead, spe- Thus, cardiac muscle tissue is both striated and involuntary.

Supporting Connective Tissue

Cartilage and bone are types of supporting connective tissue because they form a strong, durable framework that protects and supports the soft body tissues. The extracellular matrix in support ing connective tissue contains many protein fibers and a ground substance that ranges from semisolid to solid. In general, cartilage has a semisolid extracellular matrix, and bone has a solid extracellular matrix.

perichondrium

Chondrocytes produce a chemical that prevents blood vessel formation and growth within the extracellular matrix. Thus, mature cartilage is avascular, meaning without blood vessels. Therefore, the chondrocytes must exchange nutrients and waste products with blood vessels outside of the cartilage by diffusion. Cartilage usu- ally has a covering called the perichondrium ( peri - kondre - tim ; peri = around ) . Two distinct layers form the perichondrium : an outer , fibrous region of dense irregular connective tissue and an inner, cel- lular layer. The fibrous layer provides protection and mechanical support, and secures the perichondrium to the cartilage and to other structures. The cellular layer contains stem cells (chondroblasts) nec- essary for the growth and maintenance of the cartilage.

Adipocytes

Adipocytes ( adi - pō - sit , adip = fat ) are also called fat cells They often appear in small clusters within some types of connective tissue proper. If a larger cluster of these cells dominates an area, the connective tissue is called adipose connective tissue.

4.1a Characteristics of Epithelial Tissue

All epithelia exhibit several common characteristics, some of which are shown in figure 4.1:

Avascularity.

All epithelial tissues lack blood vessels. Epithelial cells obtain nutrients either directly across the apical surface or by diffusion across the basal surface from the underlying connective tissue

Selective permeability.

All substances that enter or leave the body must pass through an epithelium; thus, epithelial cells act as "gatekeepers." Sometimes an epithelium exhibits a range of permeability; that is, it may be relatively impermeable to some substances, at the same time promoting and assisting the passage of other molecules by absorption or secretion

4.6b Tissue Aging

All tissues change as a result of aging. Proper nutrition, good hee normal circulation, and relatively infrequent wounds romote tinued normal tissue functioning past middle . Thereafter 、 support, maintenance, and replacement of cells and extraceit matrix become less efficient. Physical damage, chemical cha and physiologic changes can alter the structure and chemical com position of many tissues. For example, adequate intake of protein is uired to enable the cells to continue synthesizing new proteins, the y's structural and functional building blocks. As individuals age, epithelia become thinner, and connective tissues lose their pliability and resilience. Because the amount of collagen in the body declines e rpair and wound healing lose efficiency. Bones become brittle; oint pains and broken bones are common. Muscle and nervous tissue begin to atrophy. Diet and circulation problems contribute to these tissue declines. Eventually, cumulative losses from relatively minor damage or injury contribute to major health problems

Characteristics of Connective Tissue

Although the types of connective tissue are diverse, all of them share three basic components: cells, protein fibers, and ground substance (figure 4.8). Their diversity is due to varying proportions of these components as well as to differences in the types and amounts of protein fibers.

High regeneration capacity.

Because epithelial cells have an apical surface that is exposed to the environment, they are frequently damaged or lost by abrasion. However, damaged or lost epithelial cells generally are replaced as fast as they are lost because epithelia have a high regeneration capacity. The continual replacement occurs through the mitotic divisions of the deepest epithelial cells.wich are found within the epithelium near its base.

4.4a Classification of Muscle Tissue

The three histologic types of muscle in the body are skeletal muscle, cardiac muscle, and smooth muscle. The contraction mechanism is somewhat similar in all three, but they vary in their appearance, location, physiology, internal organization, and means of control by the nervous system. Specific details about the muscular system are aiscussed in chapter 10.

Plasma cells.

When B-lymphocytes (a type of white blood cell) are activated by exposure to foreign materials, the cells mature into plasma cells. These cells are small "factories that synthesize disease-fighting proteins called antibodies ( ante - bode : anti against bodig - corpus ) , Antibodies attach to foreign material and prevent it from causing further damage. Usually, plasma cells are found in the intestinal walls, spleen and lymph nodes.

Tissues

are groups of similar cells and extracellular products that carry out a common function, such as providing protection or facilitating body movement.

Reticular fibers

are thin- ner than collagen fibers. They contain the same protein subunits that collagen has, but their subunits are combined in a different way and they are coated with a glycoprotein (a protein with some carbohydrate attached to it.

Tight Junctions

called a zonula ( zo'nyi - la ) occludens ("occluding belt"), encircles epithelial cells near their api- cal surface and completely attaches each cell to its neighbors. Plasma membrane proteins among neighboring cells fuse, so the apical surfaces of the cells are tightly connected everywhere around the cell. This seals off the intercellular space and prevents substances from passing betweern the epithelial cells. The tight junction forces almost all materials to move through, rather than between, the epithelial cells in order to cross the epithelium. Thus, epithelia control whatever enters and leaves the body For example, in the small intestine, tight junctions prevent digestive enzymes that degrade molecules from moving between epithelial cells into underlying connective tissue.

Elastic cartilage

cartilage is so named because it contains numerous astic fibers in its matrix (table 4.9c). The higher concentration of elastic fibers in this cartilage causes appear yellow in fresh sections. The chondrocytes of elastic cartilage are almost indist in guishable from those of hyaline cartilage. They are typically closely packed and sorrounded by only a small amount of extracellular matrix.

Simple Cuboidal Epithelium

consists of a single layer of cells that are as tall as they are wide (table 4.2b). A spherical nucleus is located in the center of the cell. A simple cuboidal epithelium functions primarily for absorption and secretion. It forms the walls of kidney tubules, where it participates in the reabsorption of nutrients, ions and water that are filtered out of the blood plasma. It also forms the ducts of exocrine glands, which secrete materials. Simple cuboidal epithelium covers the surface of the ovary and also lines the follicles he thyroid gland.

Loose Connective Tissue Loose connective tissue

contains relatively and protein fibers than dense connective protein fibers in loose connective tissue are loosely arranged rather than tightly packed together. Usually, this tissue occupies the spaces between and around organs. Loose connective tissues support the overlying epithelia and provide cushioning around organs, sup- port and surround blood vessels and nerves, store lipids, and provide a medium for the diffusion of materials. Thus, loose connective tis- sues act as the body's '"packing material." There are three types o loose connective tissue: areolar connective tissue, adipose connective cissue, and reticular connective tissue.

Stratified Cuboidal Epithelium

contains two or more layers of cells, and the apical cells tend to be cuboidal in shape (table 4.3c). This type of epithelium forms the walls of the larger ducts of most exocrine glands, such as the sweat glands in the skin. Although the function of stratified cuboidal epithelium is mainly protective, it also serves to strengthen the wall of gland ducts.

Gap Junctions

is formed across the intercellular gap between neighboring cells. This gap (about 2 nanometers in length) is bridged by structures called connexons ( kǒ_neks'ǒnz ) . Each connexon consists of six transmembrane proteins, arranged in a circular fashion to form a tiny, fluid-filled tunnel or pore. Gap junctions provide a direct for small molecules traveling between neighboring cells. passageway lons, glucose, amino acids, and other small solutes can pass directly from the cytoplasm of one cell into the neighboring cell through these channels. The flow of ions between cells coordinates such cellular activites as the beating of the cilia.Gap junctions are also seen in certain types of muscle tissue, where they help coordinate contraction activities.

Example

of dense irregular connective tissue is the deep portion of the dermis, which lends strength to the skin and permits it to withstand applied forces from any direction. Dense irregular con- nective tissue also forms a supporting layer around cartilags (called e perichondrium) and around bone (called the periosteun;, except et oints. In addition, it forms a thick, fibrous capsule that supports such as the liver, kidneys, and spleen ouses internal organs,such as liver,kidneys, and spleen

Categories of Connective Tissue Proper

onnective tissue proper is divided into two broad categories: loose connective tissue and dense connective tissue (table 4.6). This classi ation is based upon the relative proportions of cells, protein fibers, nd ground substance

Merocrine glands

package their secretions in structures called secretory vesicles and release their secretion by exocytosis. The cells remain intact and are not damaged in any way by producing the secretion. Lacrimal (tear) glands, sali- vary glands, sweat glands, the exocrine glands of the pancreas, and the gastric glands of the stomach are examples of merocrine glands. Some merocrine glands are also called eccrine glands, to denote a type of sweat gland in the skin that is not connected to a hair follicle (see section 5.5c). AP R

Epithelial

tissue covers or lines every body surface and all body cavities: thus it forms both the external and internal lining of many organs, and it constitutes the majority of glands.

dense irregular connective tissue,

individual bundles of collagen fibers extend in all directions in a scattered meshwork. These bundles of collagen fibers appear in clumps throughout the tissue, rather than arranged in parallel as seen in dense regular con- nective tissue (table 4.8b). Dense irregular connective tissue provides support and resistance stress in multiple directions.

epithelium

is composed of one or more layers of closely packed cells between two compartments having different components. There is little to no extracellular matrix between epithelial cells; additionally, no blood vessels penetrate an epithelium.

The resident cells of the connective tissue proper include the following types:

Fibroblasts ( fibro - blast ; fibra fiber blastos = germ ) are large, relatively flat cells with tapered ends. They are the most abundant resident cells in connective tissue proper. They produce the fibers and ground substance components of the extracellular matrix.

Secretion methods

Glands are also classified by their mechanism of discharging secretory product as merocrine glands, holocrine glands, or apocrine glands (figure 4.7).

Other leukocytes.

In addition to the leukocytes just mentioned, leukocytes migrate through the blood vessel walls the connective tissue where they spend most of their time. The majority of these leukocytes are neutrophils, a type of white blood cell that seeks out and phagocytizes bacteria. The rest are lymphocytes, which atack and destroy foreign materials.

Osteons

It is formed from cylindrical structures called osteons, or Haversian Compact bone has an ordered histologic pattern (table 4.10) systens. Osteons run parallel to the shafts of long bones.

Columnar

( kō_lǔm'nǎr ; columna = column ) cells are slender and taller than they are wide . The cells look like a group of hexagonal columns aligned next to each other. Each cell nucleus is oval and usually oriented lengthwise and located in the basal region of the cell.

Fixed macrophages

( makTO fai ; makros large , phago - to eat) are relatively large, irregular-shaped cells with numerous surface folds and projections. They are derived from one type of leukocyte (called a monocyte) and dispersed throughout the extracellular matrix, where they phagocytize damaged cells or pathogens. When the fixed macrophages encounter foreign materials, the cells release chemicals that stimulate the immune system and lure numerous wandering cells involved in body defense to the foreign materials.

Mesothelium

( mez'o - the lē_ǔm ; mesos = middle ) is the simple squamous epithe- lium of the serous membrane (discussed in section 1.4e) that lines the internal walls of the pericardial, pleural, and peritoneal cavities as well as the external surfaces of the organs within those cavities Mesothelium gets its name from the primary germ layer mesoderm from which it is derived.

mucous connective tissue

(Wharton's jelly). The immature protein fibers in mucous connective tissue are more than those within mesenchyme (table 4.4b). Mucous connective tissue is located within the umbilical cord only

Adipose connective tissue

(commonly known as "fat") is a loose connective tissue composed primarily of cells called adipocytes (table 4.7b). Adipocytes (white fat) usually range from 70 um to 120 um in diameter. In life, adipocytes are filled with one lipid droplet. On a histology slide, the lipid has been extracted during preparation, so all that is left is the plasma membrane of the adipocyte, with the nucleus pushed to the side of a round, clear space looking much like a ring. Adipose connective tissue serves as packing aroand structures and provides padding, cushions shocks, and acts as an insulator to slow heat loss through the skin. Adipose connective tissue is substance than does loose connective tissue.Dense connective tissue is sometimes called collagenous tissue because collagen fibers are the dominant fiber type.there are three categories of dense connective tissue:(1)dense regular connective tissue(2)dense irregular connective tissue(3)elastic connective tissue.

Two classes of cells

(table Resident cells are stationary cells permanently contained within the connective tissue. They help support, maintain, and repair the extracellular matrix. Wandering cells continuously move throughout the connective ti ssue and are involved in immune protec- tion and repair of damaged extracellular matrix. The number of wan- ing cells at any given moment varies depending on local conditions.

Sensation

. Epithelial tissues are innervated by nerve endings to detect changes in the external environment at their surface. These sensory nerve endings and those in the underlying connective tissue continuously supply information to the nervous system concerning touch, pressure, temperature, and pain. For example, receptors in the epithelium of the skin respond to pressure by stimulating adjacent sensory nerves. Additionally, several organs contaitn a specialized epithelium, called a neuroepithelium, that houses specifie cells responsible for the senses of taste, smell, vision, hearing, and equilibrium (as described i sections 19.3 to 19.6)

Simple Squamous Epithelium

A simple squamous epithelium cells (table 4.2a). When viewed consists of a single layer of flattened 'en face," the irregularly shaped cells display a spherical to oval nucleus, and they appear tightly bound together. Each squamous cell resembles a fried egg, with the nucleus representing the yolk.this epithelium is extremly delicate and highly specialized to allow rapid movement of molecules and ions across its surface by membrane transport process(see section 2.3c)siple squamous epithelium is found only in protected regions where moist surfaces reduce friction and abrasion found in blood vessels and the lung air sacs called alveoli.

Stratified Squamous Epithelium

A stratified squamous epithelium has multiple cell layers, and only the deepest layer of cells is in direct contact with the basement mem brane. The cells in the basal layers appear cuboidal or polyhedral, whereas the apical cells display a flattened, squamous shape. Thus, stratified squamous epithelium is so named because of its multiple cell layers and the shape of its apical cells. This epithelium is adapted to protect underlying tissues from damage due to activities that are abrasive and cause friction. Stem cells in the basal layer continuously divide to produce a new stem cell and a committed cell that gradually moves toward the surface to replace the cells lost during protective activities. This type of epithelium exists in two forms: keratinized and nonkeratinized

Exocrine Gland Structure

An exocrine gland may be unicellular or multicellular. A unicel lular exocrine gland is an individual exocrine cell located within an epithelium that is predominantly nonsecretory. Unicellular exocrine glands do not contain a duct, and they are located close to the surface of the epithelium in which they reside. The most common type of unicellular exocrine gland is the goblet cell (figure 4.4). It is usually found in both a simple columnar epithelium and pseudostratified cili- ated columnar epithelium. For example, the respiratory tract is lined mainly by pseudostratified ciliated columnar epithelium, which also contains some mucin-secreting goblet cells. Mucus then coats t inner surface of the respiratory passageway to cover and protect its lining and to help warm, humidify, and cleanse the inhaled air before reaches the gas exchange surfaces in the lungs Multicellular exocrine glands are composed of numerous cells that work together to produce a secretion and secrete it onto the surface of an epithelium. The gland often consists of acini (as'i-nT: sing , as - nǔs ; acinus grape ) , sacs that produce the secretion , and one or more smaller ducts, which merge to eventually form a largerFic duct that transports the secretion to the epithelial surface (figure 4.5). Acini are the secretory portions, and ducts are the conducting por- tions of these glands S. Most multicellular exocrine glands are enclosed within a f capsule. Extensions of this capsule, called septa or trabeculae,partition the gland internalky inti conpartments called lobes.

transitional

Another shape that occurs in epithelial cells is called transitional ( tran - si , shǔn - dl ; transitio = to go across ) . These cells can readily change their shape or appearance depending upon how stretched the epithelium becomes. They occur where the epithelium cycles between distended and relaxed states, such as in the lining of the bladder, which fills with urine and is later emptied. When the transitional epithelium is in a relaxed state, the cells are described as polyhedral, which means "many-sided" and reflects the ranges in shape that are possible in this type of epithelium. When transitional epithelium is stretched, the surface cells resemble squamous cells.

4.2b Functions of Connective Tissue

As a group, the many types of connective tissue perform a wide va- riety of functions, including the following:

Glands

As epithelial tissue develops in the embryo, small invaginations from this epithelium into the underlying connective tissue give rise to specialized secretory structures called glands. Glands are either individual cells or multicellular organs composed predomi- nantly of epithelial tissue. They secrete substances either for use elsewhere in the body or for elimination from the body. Glandular secretions include mucin, electolytes, hormones, enzymes, and waste products.

Protein Fibers Connective Tissue Proper

As mentioned previ- ously, the types of protein fibers in connective tissue proper are collagen fibers, elastic fibers, and reticular fibers. Fibroblasts synthesize the components of all three fiber types, and then secrete these protein subunits into the interstitial fluid. The subunits combine or aggregate within the matrix and form the completed fiber.

Attachment to a basement membrane.

At the basal surface of an epithelium, the epithelial layer is bound to a thin basement membrane, a complex molecular structure produced by both the epithelium and the underlying connective tissue.

plasma-(lymph)

Besides the formed elements, blood contains dissolved protein fibers in a watery ground substance. Together, the dissolved protein fibers and the watery ground substance form an extracellular matrix called plasma. Plasma transports nutrients, wastes, and hormones throughout the body. Lymph is derived from blood plasma and is returned to the blood. As it has no formed elements, it is not shown histologically in table 4.11. Blood and lymph are discussed in greater detail in chapter 21 and section 24.2, respecively, APR

Transport

Blood carries nutrients , gases , hormones, wastes, and blood cells between different regions of the body.

support and structural framework .

Bones provide the framework for the adult body and support the soft tissues cartilage supports such body structures as the trachea, bronchi, ears, and nose; connective tissue sheets form capsules to support body organs such as the spleen and kidneys.

Ground Substance

Both the cells and the protein fibers reside within a material called ground substance. This nonliving material is produced by the con- nective tissue cells. It primarily consists of protein and carbohydrate molecules and variable amounts of water. The ground substance may be viscous (as in blood), semisolid (as in cartilage), or solid (as in bone). Together, the ground substance and the protein fibers form an extracellular matrix. Most connective tissues are composed primarily of an extracellular matrix, with relatively small proportions of cells.

lamellae -central canal

Each contains concentric rings of bone called lamellae (lă-mel'e; lamina- plate), which encircle a central canal (Haversian canal). Blood ves- els and nerves travel through the central canals of osteons.

Cells

Each type of connective tissue contains specific types of cells. For example, connective tissue proper contains fibroblasts, fat contains adipocytes,cartilage contains chondricytes, and bone contains osteocytes.

In tissue sections, collagen forms coarse, sometimes wavy bundles. The parallel structure and arrangement of collagen bundles in tendons and ligaments allows them to withstand enormous forces in one direction

Elastic fibers contain the proteir collagen fibers. They stretch easily. branch, rejoin, and appear wavy. The coiled structure of elastin al- lows it to stretch and recoil like a rubber band when the deforming force is withdrawn. Elastic fibers permit the skin, lungs, and arteries to return to their normal shape after being stretched. Fresh elastic fi bers have a yellowish color and are called yellow fibers. In tissue sec- tions, elastic fibers are visible only when stained with special stains, uch as Verhoff stain, which makes elastic fibers appear black.

Innervation

Epithelia are richly innervated to detect changes in the environment at a particular body or organ surface region. Most nervous tissue is in the underlying connective issue.

Storage

Fat is the major energy reserve in the body;: bone is a large reservoir for calcium and phosphorus.

simple epithelium

Epithelia may be classified as either simple or stratified (igure 4.2a), A simple epithelium is one cell layer thick, and all of the epithelial cells are in direct contact with the basement membrane. A simple epi- thelium is found in areas where stress is minimal and where filtration, absorption, or secretion is the primary function. Examples of these locations include the linings of the air sacs in the lungs, intestines. and blood vessels.

Intercellular Junctions

Epithelial cells are strongly bound together by specialized connections in the plasma membranes of their lateral surfaces called intercellular junctions. There are four types of junctions: tight junctions, adhering junctions, desmosomes, and gap junctions (figure 4.1b). Each of these types of junctions has a specialized structure

Cellularity.

Epithelial tissue is composed almost entirely of ceils. The cells of an epithelium are bound closely together by different types of intercellular junctions. A minimal amount of extracellular matrix separates the cells.

Physical protection.

Epithelial tissues protect both exposed and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents

Polarity

Every epithelium has an apical ( api - kan surface environment or to some internal body space. Lateral surfaces (free or top surface), which is exposed either to the external have intercellular junctions (see section 4.lc). Additionally each epithelium has a basal (ba'săl) surface (fixed or bottom surface) where the epithelium is attached to the underlying connective tissue.

Secretion Types

Exocrine glands are classified by the nature of their secretions as serous glands, mucous glands, or mixed glands. Serous ( se'rüs : serum - whey ) glands produce and secrete a non- viscous, watery fluid, such as sweat, milk, tears, or digestive juices. This fluid carries wastes (sweat) to the surface of the skin, nutrients (milk) to a nursing infant, or digestive enzymes from the pancreas to the lumen of the small intestine.

Endocrine and Exocrine Glands

Glands are classified as either endocrine or exocrine, depending upon whether they have a duct connecting the secretory cells to the surface of an epithelium. Endocrine ( endō - krin : endon within , krino = to separate ) glands lack ducts and secrete their products directly into the inter- stitial fluid and blood. The secretions of endocrine glands, called hormones, act as chemical messengers to influence cell activities elsewbere in the body. Endocrine glands are discussed in depth in section 20.1

osteocytes-(canaliculi)

Lacunae between neighboring concentric lamellae house bone cells, called osteocytes (os'te-ö-sTt). Diffusion of nutrients and waste products annot occur through the hard matrix of bone, so osteocytes must communicate with one another, and ultimately with the blood ves els in the central canal, through minute passageways in the matrix called canaliculi ( kan - a - lik'yū - li ; canalis = canal ) . Together , all of the canaliculi form a branching network throughout compact bone for the exchange of materials between the blood vessels and the os teocytes within the lacunae.

Binding of structures

Ligaments bind bone to bone; tendons bind muscle to bone; dense irregular connective tissue binds skin to underlying muscle and bone.

Immune protection

Many connective tissue types contain white blood cells (leukocytes), which protect the body against disease and mount an immune response when the body is exposed to something foreign. Additionally, the extracellular matrix is a viscous material that interferes with the movement and spread of disease-causing organisms.

Classification of Exocrine Glands

Multicellular exocrine glands may be classified according to three criteria: (1) form and structure (morphology), which is considered an anatomic classification; (2) type of secretion; and (3) method of secretion. The latter two are considered physiologic classifications. Form and Structure Exocrine glands are considered either simple or compound based upon the structure and complexity of their ducts Simple glands have a single, unbranched duct: compound glands exhibit branched ducts. Exocrine glands are also classified according to the shape or organization of their secretory portions. If the secretory portion and the duct are of uniform diameter, the gland is called tubular. If the secretory cells form an expanded sac, the gland is called acinar ( asi - nar ) . Finally , a gland with both secretory tubules and secretory acini is called a tubuloacinar gland. Figure 4.6 shows the several types of exocrine glands as classified by morphology

4.5a Characteristics of Neurons

Neurons are specialized to detect stimuli, process information quickly, and rapidly transmit electrical impulses from one region of the body to another. Each neuron has a prominent cell body, or soma, that houses the nucleus and most other organelles. The cell body is the "head" that controls the rest of the cell and produces proteins for the cell. Extending from the cell body are branches called nerve cell dendries relating to a tree), which receive incoming signals from processes. The short, branched processes are dendrites (den'drites: other celis and transmit the information to the cell body. The long nerve celi- ) " , cess extending from a cell body is the axon ( akson : axon of an axon, neurons are usually the longest cells in the body; some are axis), which carries outgoing signals to other cells. Due to the length trated in the brain and spinal cord, the control centers for the nervous longer than a meter. Much of the nervous tissue in the body is concen- system. APIR

Noncilisted simple columnar

Nonciliated simple columnar epithelium often contains microvilli and a scattering of unicellular glands called goblet cells (table 4.2c). Recall that microvilli are tiny, cytoplasmic projections on the apical surface of the cell that increase the surface area for secre- tion and absorption (see table 2.2). You cannot distinguish individual microvilli under the microscope; rather, the microvilli collectively appear as a more darkly stained, fuzzy structure known as a brush border . Goblet cells secrete mucin ( myūsin ; mucus = mucus ) , a glycoprotein that when hydrated (mixed with water) forms mucus Nonciliated simple columnar epithelium lines most of the digestive tract, from the stomach to the anal canal.

Bone connective tissue

Or osseous tissue makes up the mass of most of the body structures teferred to as bones.bone is more solid than cartilage and provides greater support

nonkeratinized stratified squamous epithelium

Remain alive all the way to its apucal surface,and they are kept moist with secretions such as saliva or mucous.these cells lack keratin.because all of the cells are still alive, the flattened nuclei characteristic of squamous cells are visible even in the mois superficial cells. Nonkeratinized stratified squamous epithelium lines the vagina,the oral cavity(mouth),partnof the pharynx(throat),the esophagus,and the anus.

Mucous glands

Secrete mucin, which forms mucus when mixed with water. Mucous glands occur in such places as the roof of the oral cavity and the surface of the tongue.

Skeletal muscle is voluntary

Skeletal muscle is described as striated and voluntary. Under the light microscope, the cells of skeletal muscle exhibit alternat- ing light and dark bands , termed striations ( stri - d' shǔn ; striatus- furrow), that reflect the overlapping pattern of parallel thick and thin contractile protein filaments inside the cell. Additionally, skeletal muscle is considered voluntary because it usually does not contract unless stimulated by the somatic (voluntary) nervous system. Skeletal muscle attaches to the bones of the skeleton and also forms muscles ssociated with the skin, such as the muscles of facial expression nd those forming body sphincters that help control waste removal When skeletal muscles contract and relax, they produce heat for the body, which is why we become warmer when we "work out" at the gym or fitness center

Skeletal Muscle Tissue

Skeletal muscle tissue is composed of cylindrical muscle cells called muscle fibers (table 4.12a). Individual skeletal muscle cells are slender and often long (sometimes the length of the entire muscle). Such long cells need more than one nucleus to control and carry out all cellular functions, so each skeletal muscle fiber is multinucleated me contain hundreds of nuclei. These multiple nuclei form when smaller embryo ic muscle cells fuse early in the development of the skeletal muscle fiber. The nuclei in skeletal muscle fibers are located at the edge of the cell(called the periphery)imediately internal to the plasma membrane.

Smooth Muscle Tissue

Smooth muscle tissue is so named because it lacks the striations seen in the other two types of muscle tissue, so the cells appear smooth (table 4.12c). Smooth muscle tissue is also called visceral muscle tis stom- ach, urinary bladder, and blood vessels. The contraction of smooth muscle helps propel and control the movement of material through these organs. Smooth muscle cells are fusiform (spindle-shaped), which means they are thick in the middle and tapered at their ends. The cells are also relatively short. Each cell has one centrally placed nucieus. Smooth muscle is considered involuntary because we do not have voluntary control over it. For example, you cannot voluntarily food or your blood vessels stop your stomach from digesting your fromn transporting your blood . 2PRI

Secretion.

Some epithelial cells, called exocrine glands,a specialized to produce secretions. Individual gland cells may be scattered among other cell types in an epithelium, or a large group of epithelial secretory cells may form a gland to produce specific secretions (see section 4.1f).

synovial membrane

Some joints of the skeletal system are lined by a synovial si - nō've - dl ; syn together , ovum = egg ) membrane , which is com- posed of a well-vascularized areolar, fibrous, or adipose connective tissue under a superficial highly cellular lining. Some of the cells se- crete a synovial fluid that reduces friction in the joint cavity and dis- tributes nutrients to the cartilage on the joint surfaces of the bone.

Siple squamous epithelia

That line closed internal body cavities and all circulatory structures have special names.

hemidesmosomes,

The basal epithelial tissue exhibit structures called hemidesmosomes, half-desmosomes that anchor them to the underlying basement membrane.

Physical protection.

The bones of the cranium, sternum, and thoracic cage protect delicate organs, such as the brain, heart, and lungs; fat packed around the kidneys and at the posterior side of the eyes within the skull protects these organs.

Ground Substance of Connectire Tissue Proper

The cellular and fibrous components of the connective tissue proper sus- pended within the ground substance, a colorless, featureless, viscous olution. Ground substance usually has a gelatinous, almost rubbery consistency due to the mixture of its component molecules, which ary both in their size and in their proportions of proteins and carbo- hydrates. The different molecules in the ground substance are called glycosaminoglycans, proteoglycans, and structural glycoproteins

Classification of Connective Tissue

The connective tissue types present after birth are classified into three broad categories: connective tissue proper, supporting connective tissue, and fluid connective tissue. Figure 4.9 provides an overview of these tissue types and the subcategories within them, each of which is described in detail next.

Extracellular matrix

The four tissue types vary in terms of the structure and function of their specialized cells, as well as the presence of an extracellular matrix ( matriks ; matrix = womb ) that not only is produced by the cells but also surrounds them. The ex tracellular matrix is composed of varying amounts of water, protein fibers, and dissolved molecules (e.g., glucose, oxygen). Its consistency ranges from fluid to quite solid. Epithelial, muscle, and nervous tissues have relatively little extracellular matrix. In contrast, con nective tissue types contain varying amounts of extracellular matrix that differ in the volume of space occupied, the relative amounts of the extracellular matrix components, and the consistency (fluid to solid) of the extracellular matrix.

cutaneous membrane

The largest body membrane is the cutaneous ( kyi - tane_ǔs ; cutis = skin ) membrane , more commonly called the skin . The cu- taneous membrane is composed of a keratinized stratified squamous epithelium (called the epidermis) and a layer of connective tissue (termed the dermis) upon which the epithelium rests (see sections 5.2 and 5.3). It differs from the other membranes discussed so far in that it is relatively dry. Its many functions include protecting internal organs and preventing water loss.

periosteum

The minerals are deposited onto the collagen fibers resulting in a structure that is strong and durable but not brittle. Al most all bone surfaces (except for the surfaces of the joints of long bones) are covered by a dense irregular connective tissue called the periosteum ( pere oste - dm ; osteon = bone ) , which is similar to the perichondrium of cartilage

Development of Connective Tissue

The primary germ layer mesoderm forms all connective tissues. There are two t mucous connective tissue. In the developing embryo, mesenchyme ( mes eng - kīm ; mesos middle , enkyma - infusion ) is the first type of connective tissue to emerge. It has star-shaped (stellate) or spindle shaped mesenchymal cells dispersed within a gel-like ground sub- stance that contains fine, immature protein fibers (table 4.4a). In fact ortionately more ground substance than mesenchymal cells in this type of embryonic connective tissue. Mesenchyme is the source of all other connective tissues. Adult connective tissues often house numerous mesenchymal (stem) cells that support the repair of the tissue following damage or injury

endothelium

The simple squamous epithelium that lines the lumen of the blood and lymphatic vessels and the heart and its chambers is termed endothelium.

histology.

The study of tissues and their relationships within organs is called histology.

mucous membrane,

The two principal kinds of internal membranes are mucous and serous membranes. A mucous membrane, also called a mucosa ( myū - kōsǎ ) , lines body passageways and compartments that eventu- ally open to the external environment; these include the digestive, respiratory, reproductive, and urinary tracts. Mucous membranes perform absorptive, protective, and/or secretory functions. A mu- cous membrane is composed of an epithelium and underlying con- nective tissue called the lamina propria. Often, it is covered with a thin layer of mucus derived from goblet cells, multicellular mucous glands, or both. The mucus prevents underlying layer of cells from drying out (a process called desiccation), provides lubrication, and traps bacteria and foreign particles to prevent them from invad- ing the body

leukocytes

The wandering cells of connective tissue proper are primarily types of leukocytes their are diffrent types of white blood cells that help aid in our bodies.

Compact bone

There are two forms of bone connective tissue bone and spongy bone. Both types are typically found in all bones by a number of vascular canals. It usually forms the hard outer of the body. Compact bone appears solid but is in fact perforated shell of the bone. Spongy bone (or cancellous bone) is located spongy bone contains spaces, and the bone connective tissue within the interior of a bone. Instead of being completely solid forms a latticework structure that is very strong, yet lightweight This design allows our bones to be both strong and lightweight the same time

Pseudostratified ciliated columnar epithelium pseudostratified nonciliated columnar epithelium

There are two forms of pseudostratified columnar epithe lium: Pseudostratified ciliated columnar epithelium has cilia on its apical surface, whereas pseudostratified nonciliated columnar epithelium lacks cilia. Both types of this epithelium perform protective functions. The ciliated form houses goblet cells, which secrete mucin that forms mucus. This mucus traps foreign particles and is moved along the apical surface by the beating of the cilia. Pseudostratified ciliated columnar epithelium lines much of the larger portions of the respiratory tract, including the nasal cavity part of the pharynx (throat), the larynx (voice box), the trachea, and the bronchi. The cilia in this epithelium help propel dust particles and foreign materials away from the lungs and to the nose and mouth. In ontrast, the nonciliated form of this epithelium has no goblet cells. It a rare epithelium that occurs primarily in part of the male urethra and the epididymis.

Fluid Connective Tissue

There are two types of fluid connective tissue: blood and lymph Blood is a fluid connective tissue composed in part of cells and cell fragments called formed elements. These formed elements are erythrocytes (red blood cells), leukocytes (white blood cells), platelets (table 4.11). The erythrocytes transport oxygen and car- dioxide between the lungs and the body tissues, while some ieu eeytes mount an adaptive immune response and others respond ( ) ign pathogens such as bacteria viruses , fungi , and parasites Platelets are involved in blood clotting.

stroma,

These fibers form a branching, interwoven framework that is tough but flexible. Reticular fibers are especially abundant in the stroma, a structural connective tissue framework in organs such as the lymph nodes, spleen, and liver. The meshlike arrangement of the reticular fibers permits them to physically support organs and resist external forces that may damage the organ's cells and blood vessels

Free macrophages.

These mobile, phagocytic cells are formed from monocytes (a type of white blood cell) that migrate out of the bloodstream. They wander through connective tissue and engulf and destroy any bacteria, foreign particles, or damaged cells and debris they encounter.

Mast cells

These small , mobile cells contain a granule - filled cytoplasm. They are usually found close to blood vessels; they secrete heparin to inhibit blood clotting, and histamine to dilate blood vessels and increase blood flow.

Types of Cartilage

Three major types of cartilage occur in the body: hyaline cartilage, fibrocartilage, and elastic cartilage. The cartilage s exhibit differences in density and dispersal of chondrocytes within the extracellular matrix.

Cells of connective tissue proper

Two classes of celks form the connective tissue proper:resident cells and wandering cells

4.le Types of Epithelium

Using the classification system just described, epithelium car ken down into the primary types shown in figure 4.3. In this section, we describe the characteristics of these types of epithelium and show w each appears under the microscope

4.6a Tissue Change

adapts to environmental chea to a stratified squamous epithelium. Thus, the act of smoking normal pseudostratified ciliated columnar epithelium lining the tra- and its by-products are the environmental stressors that change the changes in the tracheal epithelium. The smoke conditions. For example, smokers typically may occur as an epithelium ture epithelium , a phenomenon called metaplasia ( met , a - pla'ze - d ; Sometimes a mature epithelium changes to a different form of ma metaplasis transformation). Metaplasia experience metaplastic causes metaplastic changes in the epithelium of the airway. Tissues can grow or shrink in two ways: by a change in cell size or by a change in cell number. Increase in the size of existing cells is called hypertrophy; increase in the number of cells in the tissue due to mitosis is called hyperplasia. When growth proceeds out of control, a tumor the condition is termed neoplasia ( neo - pla'ze - d ; neo = new , plasis = that is composed of abnormal tissue develops molding) Shrinkage of tissue by a decrease in either cell number or cell size is called atrophy (at 'ro-fe). Atrophy may result from normal aging (senile atrophy) or from failure to use an organ (disuse atrophy) When people do not perform normal activities, their muscles exhibit disuse atrophy as the cells become smaller.

Desmosomes

also called a macula adherens ("adhering spot"), is like a button or snap between adjacent epithelial cells. Each cell contributes half of the complete desmosome. It is a small region that holds cells together and provides resistance to mechanical stress at a single point, but it does not totally encircle the cell. In contrast to tight junctions, which encircle the cell to secure it to its neighbors everywhere around its periphery the desmosome attaches a cell to its neighbors only at potential stress points. The neighboring cells are separated by a small space that is span ned by a fine web of protein filaments. These filaments anchor into a thickened protein plaque located at the internal surface of the plasma membrane. On the cytoplasmic side of each plaque , intermediate filaments of the cytoskeleton penetrate the plaque to extend through out the cell the support and strength supplied between the cells by the desmosome.

Adhering Junctions

also called a zonula adherens ('adhesion belt), is formed completely around the cell This type of junction occurs when extensive zones of microfilaments extend from the cytoplasm into the plasma membrane, forming a supporting and strengthening belt within the plasma membrane that completely encircles the cell immediately adjacent to all of its neighbors. Typically, adhering junctions are located deep to the tight junctions; the anchoring of the microfilament proteins within this belt provides the only means of junctional support for the apical sur face of the cell. The ultra-strong tight junctions are needed only near the apical surface and not along the entire length of the cell. Once neighboring cells are fused together by the tight junctions near the apical surface, the adhering junctions support the apical surface and provide for a small space between neighboring cells in the direction of the basal surface. Thus, the junction affords a passageway between cells for materials that have already passed through the apical surface of the epithelial cell and can then exit through the membranes on the lateral surface and continue their journey toward the basement membrane

pseudostratified epithelium

appears layered (stratified) because the cells' nu- lei are distributed at different levels between the apical and basal surfaces. Although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface. Because all the cells are attached to the basement membrane, we have classified pseudostratified epithelium as a type of simple epithelium A ciliated pseudostratified epithelium lines the nasal cavity and the respiratory passageways.

Mesenchymal cells

are a type of embryonic stem cell contained within connective tissue. As a result of local injury or connective tissue damage, mesenchymal stem cells divide. One of the cells produced is the replacement mesenchymal cell and the other becomes a contaminated cell that moves into the damaged or injured area and differentiates into the type of connective tissue cell that is needed.

Apocrine glands

are composed of cells that accumulate their secretory products within the apical portion of their cytoplasm. Secretion occurs when the cell's apical portion pinches off, releasing cytoplasmic content. Thereafter the cell repairs itself in order to repeat its secretory activity. Mam- mary glands and ceruminous glands are apocrine glands. AP R

Squamous

are flat , wide , and somewhat irregular in shape. The nucleus looks like a flattened disc.

Holocrine

are formed from cells that accumulate a product and then the entire cell disintegrates. Thus, a holocrine secretion is a viscous mixture of cell fragments and the product the cell synthesized prior to its destruction. The ruptured, dead cells are continuously replaced by other epithelial cells undergo- ing mitosis. The oil-producing glands (sebaceous glands) in the skin are an example of holocrine glands. (So the oily secretion you feel on our skin is actually composed of ruptured, dead cells!)

stratified epithelium

contains two or more layers of epi- thelial cells. Only the cells in the deepest (basal) layer are in contact with the basement membrane. A stratified epithelium resembles a brick wall, where the bricks in contact with the ground represent the basal layer and the bricks at the top of the wall represent the apical (superficial) layer. The multiple cell layers of a stratified epithelium make it strong and capable of resisting stress and protecting underly- ing tissue. This epithelium is found in areas likely to be subjected to abrasive activities or mechanical stresses, as multiple layers of cells are better able to resist this wear and tear (e.g., skin, internal lining of the esophagus, pharynx, or vagina). Cells in the basal layer continuously regenerate as the cells in the apical layer are lost due to abrasion or stress.

Most connective tissue

each other, but are scattered throughout the tissue. This differs mark- Most connective tissue cells are not in direct contact with little to no extracellular matrix surrounding them. edly from epithelial tissue, whose cells crowd closely together with little to no extracellular matrix sorrounding them.

Fibrocartilage

fi'brō - karti - lij ; fibro = fiber ) has numerous coarse, readily visible fibers in its extracellular matrix (table 4.9b) The fibers are arranged as irregular bundles between large chondro- cytes. There is only a sparse amount of ground substance, and often the chondrocytes are arranged in parallel rows. The densely interwo- ven collagen fibers contribute to the extreme durability of this type of c artilage. It has no perichondrium because stress applied at the urface of the fibrocartilage would destroy this tissue layer. Fibrocartilage is found in the intervertebral discs (circular structures between adjacent vertebrae), the pubic symphysis (a pad of cartilage between the anterior parts of the pelvic bones and the menisci (C-shaped cartilage pads) of the knee joint. In these locations, fibrocartilage acts as a shock absorber and resists compression

Connective Tissue Proper

includes those types of connective tissue that exhibit a variable mixture of both connective tissue cell types and extracellular protein fibers suspended within a viscous ground substance. These connective tissue types differ with respect to their numbers and types of cells and relative properties and proportions of their fibers and ground substance.

Protein Fibers

fibers strengthen and support connective tissue. The type and abun- Most connective tissue contains protein fibers throughout. These dance of these fibers indicate to what extent the particular connective tissue is responsible for strength and support. Three types of protein fibers occur in connective tissue: collagen fibers, which are strong and stretch-resistant; elastic fibers, which are flexible and resilient; and reticular fibers, which form an interwoven framework

Mixed glands,

glands, such as the two pairs of salivary glands inferior to the oral cavity, contain both serous and mucous cells produce a mixture of the two types of secretions.

Cartilage

has a firm, gel-like extracellular matrix com- posed of both protein fibers and ground substance. Mature cartilage cells are called chondrocytes (kon'dro-sit; chondros gristle or cartilage.

Elastic connective tissue

has branching elastic fibers and more ibroblasts than loose connective tissue in addition to packed collagen bers (table 4.8c). The elastic fibers provide resilience and the ability o deform and then return to normal shape. Examples of structures omposed of elastic connective tissue are the vocal cords, the suspen- ory ligament of the penis, and some ligaments of the spinal column sheets in the walls Elastic connective tissue also is present as wavy of large and medium arteries.

Ciliated simple columnar epithelium

has cilia that project from the apical surfaces of the cells (table 4.2d). Mucus covers surfaces and is moved along by the beating of the cilia. Goblet typically are interspersed throughout this epithelium. This type or where it helps move an oocyte from the ovary to the uterus A ciliated epithelium lines the luminal (internal) surface of the uterinet simple columnar epithelium is also present in the bronchioles (smaller air tubes) of the lung

Dense regular connective tissue

has collagen fibers that are packed tightly and aligned parallel to an applied force (table 4.8a). This tissue type is found in tendons and ligaments, where stress is applied in a single direction. Dense regular connective tissue has few blood vessels, and thus takes a long time to heal following injury, because a rich blood supply is necessary for good healing.

Areolar connective tissue

is highly variable in appearance and the least specialized connective tissue in the body table 4.7a ) . It has a loosely organized array of collagen and elastic fibers and an abundant distribution of blood vessels. Areolar connec- tive tissue contains all of the cell types of connective tissue proper although the predominant cell is the fibroblast. A viscous ground sub tance occupies the spaces between fibers and accounts for most of the volume of areolar connective tissue. The ground substance cus ons shocks, and the loosely organized fibers ensure that this type of connective tissue can be distorted without damage. Additionally elastic properties of this tissue promote independent movements. For instance, under the dermis of the skin is a superficial laver of areola connective tissue, thus tugging on the skin of the leg, for example does not affect the underlying muscle

Stratified Columnar Epithelium

is relatively rare in the body. It consists of two or more layers of cells, but only the apical surface cells are columnar in shape (table 4.3d). This type of epithelium is found in the membranous segment of the male urethra and in the large ducts of the salivary glands

Pseudostratified Columnar Epithelium

is so named because upon first glance, it appears to consist of multiple layers of cells. How ever, this epithelium is not really stratified, because all of its celis are in direct contact with the basement membrane. Although it may look stratified because the nuclei are scattered at different distances from the basal surface, not all of the cells reach the apical surface (table 4.2e). The columnar cells within this epithelium always reach the apical surface; the shorter cells are stem cells that give rise to the columnar cells.

Nervous tissue

is sometimes tissue. It consists of cells called neurons (nu'ron), or nerve cells, and a larger number of different types of glial cells (or supporting cells) that support, protect, and provide a framework for neurons (table 4.13). This tis- sue will be discussed in detail in section 14.2, but we provide a brief description here.

Connective tissue

is the most diverse, abundant, widely distributed, and microscopically variable of the tissues. Connective tissue is designed to support, protect, and bind organs. As its name implies it is the glue that bunds body structures together like fibrous tendons and ligaments, body fat,the cartilage that connects the ends of ribs to the sternum, the bones of the skeleton, and the blood.

Cuboidal

ka - boy ' dǎl ; kybos = cube , eidos resemblance ) cells re about as tall as they are wide. The cells do not resemble perfect "cubes," because their edges are rounded.

Collagen

kol'lǎ - jen : koila glue gen producing fibers are long, unbranched, "cablelike" fibers composed of the protein collagen. They are strong, flexible, and resistant to stretching Collagen forms about 25 % of the body 's protein , making it the most abundant protein in the body. In fresh tissue, collagen fibers appear white, and thus they are often called white fibers. In tissue sections stained with hematoxylin and eosin to give contrast, they appear pink.

Hyaline

lin " -len ; hvalos = glass ) cartilage is the most type of cartilage and also the weakest. It provides its support through flexibility and resilience. Hyaline cartilage is surrounded by perichondrium. It is named for its clear, glassy appearance under the microscope. The chondrocytes within their lacunae are irregularly scattered throughout the extracellular matrix (table 4.9a). However the collagen within the matrix is not readily seen by light micros- copy. If the hyaline cartilage tissue is stained by hematoxylin and eosin and then examined under the microscrope, the tissue resembles carbonated grape soda, where the lacunae represent the bubbles in the soda. Hyaline cartilage has many functions in addition to its pri- mary one of supporting soft tissue. It forms most of the fetal skel eton and is a model for most future bone growth. The cartilage at the articular ends of long bones allows the bones in a joint to move freelv and easily . Hyaline cartilage is found in many other areas of the body, including the nose, trachea, most of the larynx, costal cartilage (the cartilage attached to the ribs), and the articular ends of long bones.

serous membrane,

serous membrane, also termed a serosa (se-rö'să), is com posed of a simple squamous epithelium called mesothelium and a thin underlying layer of loose connective tissue. The mesothelium is so named because it is derived from mesoderm. Serous membranes produce a thin, watery serous fluid, or transudate (tranz yü-dat; trans = across , sudo = to sweat ) , which is derived from blood plasma Serous membranes are composed of two parts: a parietal layer that lines the body cavity and a visceral layer that covers organs. The parietal and visceral layers are in close contact; a thin layer of serous fluid between them reduces the friction between their opposing sur- faces. Examples of serous membranes include the pericardium, the peritoneum, and the pleura.

lobules

subdivisions of the septa within each lobe form microscopic lobules (lob'yal). The septa contain ducts, blood vessels, and nerves supply- ing the gland. The connective tissue framework of the gland is called the stroma. The stroma supports and organizes the parenchyma ( pa - rengki mi ) , the functional cells of the gland that produce and secrete the gland products. These cells are usually simple cuboidal or columnar epithelial cells.multicellular exocrine glands occur in the mammary glands,pancreas,and salivary glands.

keratinized stratified squamous epithelium,

the apical surface is composed of layers of cells that are dead, these cells lack nuclei and all organelles produced and are filled with keratin , a tough . protective fibrous protein. It is obvious that the superficial cells lack nuclei when they are viewed through microscope. New commited cells produced in the basal region of the epithelium migrate toward the apical surface.During their migration,they fill with keratin.lose their organelles and nuclei,and die.However,the keratin in these dead cells make them very strong.thus,there is a tradeoff with the appearance of keratin,in that the tissue becomes very strong,but the cells must die as a result.The epidermis(outer layer) of the skin consists of keratinized startified squamous epithelium.

-sensation

the attachment and form a selective molecular barrier between the epithelium and the underlying connective tissue. The basement membrane has the following functions: . Providing physical support for the epithelium Anchoring the epithelium to the connective tissue Acting as a barrier to regulate the movement of large molecules between epithelium and the underlying connective tissue

Exocrine glands

typically origi - nate from an invagination of epithelium that burrows into the deeper connective tissues. These glands usually maintain their contact with the epithelial surface by means of a duct, an epithelium-lined tube through which secretions of the gland are discharged onto the epithe- lial surface. This duct may secrete materials onto the surface of the skin (eg., sweat from sweat glands or milk from mammary glands) or onto an epithelial surface lining an internal passageway (e.g., en- zymes from the pancreas into the small intestine or saliva from the salivary glands into the oral cavity)

Transitional Epithelium

varies in appearance, depending upon whether it is in a relaxed or a stretched state (table 4.3e). In a relaxed state, the basal cells appear almost cuboidal, and the apical cells are large and rounded. During stretching, the transitional epithelium thins, and the apical cells continue to flatten, becoming almost squamous. In this distended state, it may be difficult to distinguish a transitional epithelium from a squamous epithelium. However, one distinguishing feature of transitional epithelium is the presence of a handful of binucleated (double-nucleus-containing) cells. This epi- thelium lines the urinary bladder, an organ that changes shape as it fills with urine. It also lines the ureters and the proximal part of the urethra. Transitional epithelium permits stretching and ensures that toxic urine does not seep into the underlying tissues and structures of these organs.

hemopoietic cells

which form reticular connective tissue that responsible producing blood cells (a process called hemopoiesis) Thus, the connective tissue that produces our blood cells is stored within our spongy bone.

lacunae

within the extracellular matrix. The physical properties of cartilage vary with the extracellular matrix contents. Cartilage is stronger and more resilient than any previously discussed connective tissue,and it provides more more flexibility than bone.collagen fibers within the matrix give cartilage its tensile strength it resiliance us atributed ti elastic fibers and variations in the kinds and amounts of ground substance components,including water.cartilage is found in areas of the body that need support and must withstand deformation,such as the tip of the nose or the external part of the ear(auricle).