Anatomy chapter5

Classification by Method of Secretion

Merocrine Apocrine Holocrine

Simple Columnar Epithelium

A simple columnar epithelium is composed of a single layer of cells that are taller than they are wide. The nucleus is oval, oriented lengthwise, and located in the basal region of the cell. This type of epithelium is ideal for both secretory and absorptive functions. Simple columnar epithelium has two forms: One type has no cilia, whereas the apical surface of the other type is covered with cilia.

Selective permeability

All substances that enter or leave the body must pass through an epithelium, and thus epithelial cells act as "gatekeepers." An epithelium typically exhibits a range of permeability; it may be relatively impermeable to some substances, while promoting and assisting the passage of other molecules.

Physical protection

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

Columnar

are slender and taller than they are wide. The cell nucleus is oval and usually oriented lengthwise and in the basal region of the cell.

Mesothelium

is the name given to the simple squamous epithelium that forms the serous membranes of body cavities. Mesothelium gets its name from the embryonic primary germ layer called mesoderm, from which it is derived

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, both damaged and lost epithelial cells generally are replaced quickly because epithelia have a high regeneration capacity. In other words, the cells undergo mitosis frequently. The continual replacement occurs through the mitotic divisions of the deepest epithelial cells (called stem cells), which are adjacent to the basement membrane.

reticular lamina

Cells in the underlying connective tissue secrete the reticular lamina and it contains protein fibers and carbohydrates

Classification by Anatomic Form

Simple glands compound glands

Classification by Cell Shape

Squamous (skwā′mŭs; squamosus = scaly) cells Cuboidal (kū-boy′dăl) cells Columnar transitional.

Attachment to a basement membrane.

The epithelial layer is bound to a thin basement membrane at its basal surface. This is a complex structure produced by the collective efforts of the epithelium and underlying connective tissue.

Polarity.

. An epithelium has an apical (āp′i-kăl) surface (free, or superficial, surface), which is exposed either to the external environment or to some internal body space. The apical surface may have either microvilli or cilia.

intercellular junctions

1-tight junction also known as zonula occludens (attach the cell to the neighbors) 2-adehering junction also known as zonula adhering (includes microfilaments are located deep to the tight junctions ) 3-desmosome also known as macula adherens , they don't encircle the entire cell 4-gap junction small solutes such as ions , glucose ,and amino acids pass through connexons from the cytoplasm of one cell into a neighbor cell

Simple Cuboidal Epithelium

A simple cuboidal epithelium contains uniformly shaped cells, and it is ideal for building small ducts and glands. This epithelium consists of a single layer of cells that are about as tall as they are wide (table 5.2b). A spherical nucleus is located in the center of the cell. function A simple cuboidal epithelium functions primarily to absorb fluids and other materials across its apical surface, and to secrete specific molecules. location 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. It also forms the secretory regions of most glands and smaller ducts of exocrine glands, which secrete materials. Simple cuboidal epithelium covers the surface of the ovary and also lines the follicles of the thyroid gland.

simple epithelium

A simple epithelium is one cell layer thick, and all of the epithelial cells are in direct contact with the basement membrane. A simple epithelium is found in areas where stress is minimal and where filtration, absorption, or secretion is the primary function. Examples of locations include the lining of the air sacs of the lung, the intestines, and blood vessels.

simple squamous epithelium

A simple squamous epithelium is the thinnest possible barrier, because it consists of a single layer of flattened cells (table 5.2a). When viewed "en face" (looking onto the surface), the irregularly shaped cells display a spherical to oval nucleus, and the cells are tightly bound together. Each squamous cell resembles a fried egg, with the nucleus of the cell representing the yolk. This epithelium is extremely delicate and highly specialized to allow rapid movement of molecules across its surface by diffusion, osmosis, or filtration. location it forms the lining of the air sacs (alveoli) of the lung, where this thin epithelium is well suited for the exchange of oxygen and carbon dioxide between the blood and the inhaled air. also is found lining the lumen (inside space) of blood vessel walls, where it allows for rapid exchange of nutrients and waste between the blood and the interstitial fluid surrounding the blood vessels. The simple squamous epithelium that lines the blood vessel and lymph vessel walls is termed endothelium (en-dō-thē′lē-ŭm; endon = within). Mesothelium (mez-ō-thē′-ŭm; mesos = middle) is the name given to the simple squamous epithelium that forms the serous membranes of body cavities. Mesothelium gets its name from the embryonic primary germ layer called mesoderm, from which it is derived (see section 5.6a).

Stratified Columnar Epithelium

A stratified columnar epithelium is relatively rare in the body. It consists of two or more layers of cells, but only the cells at the apical surface are columnar in shape (table 5.4d). This type of epithelium protects and secretes. It is found in the large ducts of salivary glands and in the membranous segment of the male urethra.

Stratified Cuboidal Epithelium

A stratified cuboidal epithelium contains two or more layers of cells, and the superficial cells tend to be cuboidal in shape (table 5.4c). This tissue forms the walls of the ducts of most exocrine glands, such as the ducts of the sweat glands in the skin. Although the function of stratified cuboidal epithelium is mainly protective, it also serves to strengthen the walls of the gland ducts, and some sections of male urethra.

Transitional Epithelium

A transitional epithelium is limited to the urinary tract (urinary bladder, ureters, and part of the urethra). It varies in appearance, depending upon whether it is in a relaxed state or a stretched state (table 5.4e). In a relaxed state, the basal cells appear cuboidal or polyhedral, and the apical cells are large and rounded. When transitional epithelium stretches, it thins and the apical cells flatten and become almost squamous in shape. One distinguishing feature of transitional epithelium is the presence of some binucleated (containing two nuclei) cells. By being able to stretch as the bladder fills, this epithelium ensures that toxic urine does not seep into the underlying tissues and structures of these organs.

Avascularity

All epithelial tissues lack blood vessels. Nutrients for epithelial cells are obtained either directly across the apical surface or by diffusion across the basal surface from the underlying connective tissue.

Apocrine

Apocrine (ap′ō-krin; apo = away from, off) glands produce their secretion in the following way: The apical membrane around a portion of the glandular cell cytoplasm with the secretory product pinches off and becomes the secretion. The glandular cells repair the damage and then continue to produce new secretions in the same manner. Examples include the mammary glands and some sweat glands in the axillary and pubic regions.

Characteristics of Epithelial Tissue

Cellularity Polarity. Attachment to a basement membrane. Avascularity Extensive innervation High regeneration capacity

Ciliated simple columnar epithelium

Ciliated simple columnar epithelium has cilia that project from the apical surfaces of the cells (table 5.2d). Mucus covers these apical surfaces and is moved along by the beating of the cilia. Goblet cells typically are interspersed throughout this epithelium. Ciliated columnar epithelium is present in the bronchioles (smaller air passageways) in the lung. It also lines the luminal (internal) surface of the uterine tubes, where it helps move an oocyte from the ovary to the uterus.

Endocrine glands

Endocrine (en′dō-krin; endon = within, krino = to separate) glands lack ducts and secrete their products, called hormones, directly into the interstitial fluid and blood. Hormones act as chemical messengers to influence cell activities elsewhere in the body.

Extensive innervation

Epithelia are richly innervated to detect changes in the environment at that body or organ region.

Cellularity

Epithelial tissue is composed almost entirely of tightly packed cells. A minimal amount of extracellular matrix separates the cells in an epithelium.

Sensations

Epithelial tissues contain 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. Additionally, several organs contain a specialized epithelium, called a neuroepithelium, that houses specific cells responsible for the senses of sight, taste, smell, hearing, and equilibrium.

Exocrine glands

Exocrine (ek′sō-krin) glands typically originate from an invagination of epithelium that burrows into the deeper connective tissues. These glands usually maintain their connection with the epithelial surface by means of a duct, an epithelium-lined tube through which the gland sections are discharged onto the epithelial surface. Examples of exocrine glands include sweat glands, mammary glands, and salivary glands.Exocrine glands may be unicellular (one-celled) or multicellular.

Glands

Glands are either individual cells or multicellular organs composed predominantly of epithelial tissue. They secrete substances either for use elsewhere in the body or for elimination from the body. Glandular secretions may include mucin, electrolytes, hormones, enzymes, and urea (a nitrogenous waste produced by the body).

Holocrine

Holocrine (hōl′ō-krin; holos = whole) glands are formed from cells that accumulate a product; the entire cell then disintegrates. Thus, a holocrine secretion is a viscous mixture of both cell fragments and the product the cell produced prior to its disintegration. The ruptured, dead cells are continuously replaced by other epithelial cells undergoing mitosis. The oil-producing glands (sebaceous glands) in the skin are examples of holocrine glands.

Merocrine

Merocrine (mer′ō-krin; meros = share) glands package their secretions into secretory vesicles and release the secretions by exocytosis. The glandular cells remain intact and are not damaged in any way by producing the secretion. Examples of merocrine glands include lacrimal (tear) glands; salivary glands; some sweat glands, also known as eccrine glands; the exocrine glands of the pancreas; and the gastric glands of the stomach.

Nonciliated simple columnar epithelium

Nonciliated simple columnar epithelium often contains microvilli and a scattering of unicellular glands called goblet cells (table 5.2c). Individual microvilli cannot be distinguished under the microscope; rather, the microvilli collectively appear as a bright, fuzzy structure known as a brush border. Goblet cells secrete mucin (mū′sin), which is 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.

Functions of Epithelial Tissue

Physical protection Selective permeability Secretions Sensations

Secretions

Some epithelial cells are specialized to produce secretions. Individual gland cells may be scattered among other cell types in an epithelium, or a large group of gland cells may form either an exocrine gland or an endocrine gland that produces specific secretions.

basement membrane

The basement membrane may be observed as a single layer internal to the epithelium using the light microscope—however, in reality it consists of three layers when viewed using an electron microscope: the lamina lucida, the lamina densa, and the reticular lamina.

nonkeratinized stratified squamous epithelium

The cells in nonkeratinized stratified squamous epithelium remain alive all the way to the tissue's apical surface, and they are kept moist with secretions such as saliva or mucus. These cells lack keratin (ker′ă-tin; keras = horn), the tough, protective protein that is abundant in the keratinized form of this epithelium (described next). Because all of the cells are still alive, the flattened nuclei characteristic of squamous cells are visible throughout the tissue (table 5.4a). Nonkeratinized stratified squamous epithelium lines the oral cavity (mouth), part of the pharynx (throat), the esophagus, the vagina, and the anus.

the lamina lucida, the lamina densa

The two lamina closest to the epithelium contain collagen fibers as well as specific proteins and carbohydrates, some of which are secreted by the epithelial cells.

Cuboidal (kū-boy′dăl) cells

are about as tall as they are wide. The cells do not resemble perfect cubes because their edges are somewhat rounded. The cell nucleus is spherical and located within the center of the cell

Squamous (skwā′mŭs; squamosus = scaly) cells

are flat, wide, and somewhat irregular in shape. The cells are arranged like flattened floor tiles, and the nucleus is somewhat flattened

pseudostratified ciliated columnar epithelium

contains cilia on its apical surface The ciliated form houses goblet cells—like goblet cells in simple columnar epithelium, these cells secrete mucin, which hydrates to become the mucus that traps foreign particles and is moved by the beating cilia. This type is found in the larger air passageways of the respiratory system (e.g., the nasal cavity, part of the pharynx [throat], larynx [voice box], trachea, and bronchi

stratified epithelium

contains two or more layers of epithelial cells. Only the cells in the deepest (basal) layer are in direct 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. A stratified 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 the wear and tear (e.g., the skin, internal lining of the pharynx and esophagus). Cells in the basal layer continuously regenerate as the cells in the apical layer are lost due to abrasion or stress.

Epithelial Tissue

covers the body surfaces, lines the body cavities, and forms the majority of glands. An epithelium (pl., epithelia) is composed of one or more layers of closely packed cells, and it contains little to no extracellular matrix between these cells. Additionally, no blood vessels penetrate an epithelium.

Two types of transitional epithelium

found in the urinary tract is a stratified epithelium that gives the impression of transitioning from cuboidal when the epithelium relaxed to squasum when it is stretched .

keratinized stratified squamous epithelium

he superficial layers are composed of cells that are dead. These cells lack nuclei and all organelles when observed under the microscope, and instead are filled with the protein keratin (table 5.4b). New cells produced in the basal region of the epithelium migrate toward the apical surface of the tissue. During their migration, the cells fill with keratin they produce, which makes them very strong, but as a consequence the cells lose their organelles and nuclei, and die. Thus, the strength of keratin has a trade-off. The epidermis (outer layer) of the skin consists of keratinized stratified squamous epithelium.

transitional.

hese cells can readily change their shape, 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 transitional epithelium is in a relaxed state, the cells are polyhedral in shape, whereas when this epithelium is stretched, the surface cells become more flattened.

Stratified Squamous Epithelium

is designed to protect against abrasion and friction. This tissue has multiple cell layers, and only the deepest layer of cells is in direct contact with the basement membrane. The cells in the basal layers have a cuboidal or polyhedral shape, 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 the apical cells. This epithelium is adapted to protect underlying tissues from damage due to activities that cause abrasion and friction. Stem cells in the basal layer continuously divide, to produce a new stem cell and a committed cell that is gradually displaced toward the surface to replace those cells that have been lost. This type of epithelium exists in two forms: nonkeratinized and keratinized.

pseudostratified (sū′dō-strat′i-fīd; pseudo = false, stratum = layer) epithelium

looks layered (stratified) because the cells' nuclei 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. For our purposes, we have classified pseudostratified epithelium as a type of simple epithelium, because all of the cells are attached to the basement membrane.

multicellular exocrine glands

multicellular exocrine glands contain numerous cells that work together to produce a secretion (figure 5.4). The gland consists of acini (as′i-nī; acinus = grape), which are the clusters of cells that produce the secretion, and one or more smaller ducts, which merge to form a larger duct that transports the secretion to the epithelial surface. Multicellular exocrine glands typically are surrounded by a fibrous capsule, and extensions of the capsule called septa partition the gland into lobes.

Pseudostratified Columnar Epithelium

pseudostratified columnar epithelium is so named because upon first glance, it appears to consist of multiple layers of cells. Note that this epithelium is not really stratified, because all of its cells are in direct contact with the basement membrane. It may look stratified because the nuclei are scattered at different distances from the basal surface, but not all of the cells reach the apical surface in this epithelium (table 5.3). The columnar cells within this epithelium always reach the apical surface, and the shorter cells are stem cells that give rise to the columnar cells. Pseudostratified columnar epithelium consists of two forms: pseudostratified ciliated columnar epithelium, and pseudostratified nonciliated columnar epithelium

Classification by Number of Cell Layers

simple epithelium stratified epithelium pseudostratified (sū′dō-strat′i-fīd; pseudo = false, stratum = layer) epithelium

glands may be classified according to the shape of their secretory portions

tubular acinar tubuloacinar gland

Unicellular exocrine glands

typically 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, which is commonly found in both simple columnar epithelium and pseudostratified ciliated columnar epithelium

pseudostratified nonciliated columnar epithelium

which lacks cilia The nonciliated form is rare, lacks goblet cells and cilia, and occurs primarily in part of the male urethra and epididymis.