Chapter 4 exam 1

cuboidal epithelium

cells are cube shape, nucleus is found in the center.

fibrosis

cells are permanently replaced by fibrous connective tissue. Such permanent replacement of normal tissues is called

Connective Tissue Fibers

collagen, elastic, and reticular— are formed from protein subunits secreted by fibroblasts

dense connective tissue

consist mostly of collagen fibers. For this reason, they are also called fibrous, or collagenous, tissues.

Cartilage

is a firm gel containing embedded fibers.

functions of epithelial tissue

physical protection control permeability provide sensation produce specialized secretion

neuroglia

provide physical support for neural tissue, maintain the chemical composition of the neural tissue fluids, supply nutrients to neurons, and defend the tissue from infection.

Stratified Epithelia

provides greater protection because it has several layers of cells above the basement membrane. __________ __________ are usually found in areas exposed to mechanical or chemical stresses. Examples include the surface of the skin and the lining of the mouth.

simple cuboidal epithelium

provides limited protection and occurs where secretion or absorption takes place. These functions are enhanced by larger cells, which have more room for the necessary organelles. located in the pancreas and salivary glands secrete enzymes and buffers and line the ducts that discharge these secretions, also line portions of the kidney tubules involved in producing urine.

simple columnar epithelium

provides some protection and may also occur in areas of absorption or secretion. This type of epithelium lines the stomach, the intestinal tract, and many excretory ducts.

Dendrites

receive information, typically from other neurons.

Three types of cell junctions

tight junctions, gap junctions, desmosomes

Ligaments

resemble tendons but connect one bone to another. ________ often contain elastic fibers as well as collagen fibers and thus can tolerate a modest amount of stretching.

Exocrine

secretions are discharged onto the surface of the epithelium. Examples include enzymes entering the digestive tract, perspiration on the skin, and milk produced by mammary glands.

Endocrine

secretions are released into the surrounding tissue (interstitial) fluid and blood. These secretions, called hormones, act as chemical messengers and regulate or coordinate the activities of other tissues, organs, and organ systems. (We discuss hormones further in Chapter 10.) Endocrine secretions are produced in such organs as the pancreas, and thyroid and pituitary glands

serous secretion

secretions of the perotid salivary gland

mix secretion

secretions of the sub-mandibular salivary gland

mucous secertion

secretions of the sublingual salivary gland

three types of secretion

serous, mucous, mixed

columnar epithelium

the cells are also hexagonal but taller and more slender, resembling rectangles in sectional view. The nuclei are crowded into a narrow band close to the basement membrane. The height of the epithelium is several times the distance between two nuclei.

Squamous epithelium

the cells are thin and flat. The nucleus occupies the thickest portion of each cell. Viewed from the surface, the cells look like fried eggs laid side by side.

dense regular connective tissue

the collagen fibers are parallel to each other, packed tightly, and aligned with the forces applied to the tissue.

Epithelial repair and renewal

An epithelium must continually repair and renew itself. Epithelial cells may survive for just a day or two, because they are lost or destroyed by exposure to disruptive enzymes, toxic chemicals, pathogenic microorganisms, or mechanical abrasion. The only way the epithelium can maintain its structure over time is through the continuous division of unspecialized cells known as stem cells, or germinative cells. These cells are found in the deepest layers of the epithelium, near the basement membrane.

The four types of tissues are

1. Epithelial 2. Connective 3. Muscle 4. Nervous

epithelial tissue

A body tissue that covers the surfaces of the body, inside and out. includes epithelia, and glands

Dysplasia

A change in the normal shape, size, and organization of tissue cells.

Metaplasia

A structural change in cells that alters the character of a tissue.

Pericarditis

An inflammation of the pericardial lining that may lead to the accumulation of pericardial fluid (pericardial effusion).

Peritonitis

An inflammation of the peritoneum after infection or injury.

Pleuritis (pleurisy)

An inflammation of the pleural cavities.

anaplasia

An irreversible change in the size and shape of tissue cells.

Apocrine secretion

Apocrine secretion (AP-ō-krin; apo-, off) involves the loss of both cytoplasm and the secretory product (Figure 4-6b). The outermost portion of the cytoplasm becomes packed with secretory vesicles before it is shed. Milk production in the mammary glands involves both merocrine and apocrine secretions.

Tight junctons

At a _____ _________ , the lipid layers of adjacent plasma membranes are tightly bound together by interlocking membrane proteins (Figure 4-2a,c). Inferior to the _____ _________, a continuous adhesion belt forms a band that encircles cells and binds them to their neighbors. The bands are connected to a network of actin filaments in the cytoskeleton. Tight junctions prevent the passage of water and solutes between cells. These junctions are common between epithelial cells exposed to harsh chemicals or powerful enzymes. For example, _____ _________ between epithelial cells lining the digestive tract keep digestive enzymes, stomach acids, or waste products from damaging underlying tissues.

synovial membrane (si-NŌ-vē-ul)

Bones contact one another at joints, or articulations (ar-tik-ū-LĀ-shuns). Joints that allow free movement are surrounded by a fibrous capsule and contain a joint cavity lined by a ______ _________. Unlike the other three membranes, the ________ __________ consists primarily of areolar tissue and an incomplete layer of epithelial tissue. In freely movable joints, the bony surfaces do not come into direct contact with one another. If they did, impacts and abrasion would damage the opposing surfaces, and smooth movement would become almost impossible. Instead, the ends of the bones are covered with hyaline cartilage and separated by a viscous synovial fluid produced by fibroblasts in the connective tissue of the _________ ________. The synovial fluid helps lubricate the joint and permits smooth movement.

important characteristics of epithelial tissue

Cells that are bound closely together. In other tissue types, the cells are often widely separated by extracellular materials. A free (apical) surface exposed to the environment or to an internal chamber or passageway. Attachment to underlying connective tissue by a basement membrane. The absence of blood vessels. Because of this avascular (ā-VAS-ku-lar; a-, without + vas, vessel) condition, epithelial cells must obtain nutrients across their attached surface from deeper tissues or across their exposed surfaces. Continual replacement or regeneration of epithelial cells that are damaged or lost at the exposed surface

basement membrane

Epithelial cells not only must adhere to one another but also must remain firmly connected to the rest of the body. This function is performed by the ________ ________, which lies between the epithelium and underlying connective tissues (Figure 4-3). There are no cells within the ________ ________, which consists of a network of protein fibers. The epithelial cells adjacent to the ________ ________ are firmly attached to these protein fibers by hemidesmosomes. In addition to providing strength and resisting distortion, the ________ ________ also acts as a barrier. It restricts proteins and other large molecules from moving from the underlying connective tissue into the epithelium.

Holocrine secretion

Holocrine secretion (HOL-ō-krin; holos, entire) does not leave a cell intact. Instead, the entire cell becomes packed with secretory vesicles and then bursts, releasing the secretion, but killing the cell (Figure 4-6c). Sebaceous glands, associated with hair follicles, produce an oily hair coating by holocrine secretion.

Periosteum

Except in joint cavities, where a layer of hyaline cartilage covers bone, all other bone surfaces are surrounded by a __________, a covering made up of fibrous (outer) and cellular (inner) layers

canaliculi

Hairlike canals that connect lacunae to each other and the central canal

merocrine secretion

In merocrine secretion (MER-u-krin; meros, part + krinein, to secrete), the product is released from secretory vesicles by exocytosis. (Figure 4-6a). This is the most common mode of exocrine secretion. One product of merocrine secretion, mucin, mixes with water to form mucus. Mucus is an effective lubricant, a protective barrier, and a sticky trap for foreign particles and microorganisms.

Regeneration

In the second phase following injury, damaged tissues are replaced or repaired to restore normal function. This repair process is called regeneration. During regeneration, fibroblasts produce a dense network of collagen fibers known as scar tissue or fibrous tissue. Over time, scar tissue gradually assumes a more normal appearance. Each organ has a different tissue organization, and this organization affects its ability to regenerate after injury. Epithelia, connective tissues (except cartilage), and smooth muscle tissue usually regenerate well. Other muscle tissues and neural tissue regenerate relatively poorly, if at all.

Epithelia

Layers of cells covering internal or external surfaces

cell junctions

More specialized attachment sites that attach a cell to another cell or to extracellular materials

Desmosomes

Most epithelial cells are subject to mechanical stresses such as stretching, bending, twisting, or compression. For this reason, they must have durable interconnections. At a __________, the plasma membranes of two cells are locked together by CAMs and proteoglycans between the opposite dense areas of each cell. Each dense area is linked to the cytoskeleton by a network of intermediate filaments

pseudostratified columnar epithelium

Portions of the respiratory tract contain ________________ __________ ___________, a columnar epithelium that includes a mixture of cell types. The distances between the cell nuclei and the exposed surface vary, so the epithelium appears layered, or stratified (Figure 4-5b). It is not truly stratified, though, because all the cells contact the basement membrane. Epithelial cells of this tissue typically possess cilia. A pseudostratified ciliated columnar epithelium lines most of the nasal cavity, the trachea (windpipe) and bronchi, and portions of the male reproductive tract.

Adhesions

Restrictive fibrous connections that can result from surgery, infection, or other injuries to serous membranes.

gap junctions

Some epithelial functions require rapid intercellular communication. At a ___ _________, two cells are held together by embedded membrane proteins called connexons (Figure 4-2b). Together, the connexons form a narrow passageway that lets small molecules and ions pass from cell to cell. ___ _________ are most abundant in cardiac muscle and smooth muscle tissue, where they are essential to the coordination of muscle contractions. They also interconnect cells in ciliated epithelia (discussed shortly).

perichondrium

Structures of cartilage are covered and set apart from surrounding tissues by a __________________ made up of an inner cellular layer and an outer fibrous layer.

connective tissue functions

Support and protection. The minerals and fibers produced by connective tissue cells form a strong structural framework for the body, protect delicate organs, and surround and interconnect other tissue types. Transportation of materials. Fluid connective tissues move dissolved materials efficiently from one region of the body to another. Storage of energy reserves. Fats are stored in connective tissue cells called adipose cells until needed. Defense of the body. Specialized connective tissue cells respond to invasions by microorganisms through cell-to-cell interactions and the production of antibodies.

Pleural effusion

The accumulation of fluid within the pleural cavities as a result of chronic infection or inflammation of the pleura.

Matrix

The extracellular fibers and ground substance form the ______ that surrounds the cells. This ___________ ______ accounts for most of the volume of connective tissues. Connective tissue differs in this way from epithelial tissues, which consist almost entirely of cells.

Cell Adhesion Molecule's

The large areas of opposing plasma membranes are interconnected by transmembrane proteins called ________ (CAMs).These proteins bind to each other and extracellular materials by a thin layer of proteoglycans (a protein-polysaccharide mixture).

Necrosis

Tissue destruction that occurs after cells have been injured or killed.

Neural tissue

also known as nervous tissue or nerve tissue, is specialized for propagating (transmitting) electrical impulses from one region of the body to another. Most neural tissue (98 percent) is concentrated in the brain and spinal cord, the control centers for the nervous system.

Glands

are composed of fluid-secreting cells derived from epithelia

Tendons

are cords of dense regular connective tissue that attach skeletal muscles to bones. Their collagen fibers run along the length of the tendon and transfer the pull of the contracting muscle to the bone.

Macrophages

are large phagocytic cells (phagocytes) scattered throughout the matrix. These "big eater" cells engulf, or phagocytize, damaged cells or pathogens that enter the tissue. They also release chemicals that mobilize the immune system, attracting additional ____________ and other cells involved in tissue defense. _______________ that spend long periods of time in connective tissue are known as fixed _____________. When an infection occurs, migrating ______________ (called free ____________) are drawn to the affected area.

Collagen fibers

are long, straight, and unbranched. These strong but flexible fibers are the most common fibers in connective tissue proper.

Reticular fibers

are made up of the same protein subunits as collagen fibers, but arranged differently. The least common of the three fibers, they are thinner than collagen fibers. form a branching, interwoven framework in various organs.

Fibrocytes

are next in abundance and differentiate from fibroblasts. ___________ maintain the connective tissue fibers of connective tissue proper.

Adipocytes

are permanent residents. A typical __________ contains a large lipid droplet that squeezes the nucleus and other organelles to one side of the cell. The number of ______________ varies from one connective tissue to another, from one region of the body to another, and among individuals.

Mast cells

are small, mobile connective tissue cells often found near blood vessels. The cytoplasm of a ____ ____ is packed with granules filled with histamine and heparin. These chemicals are released to begin the body's defensive activities after an injury or infection.

Chondrocytes

are the only cells found within the cartilage matrix. They occupy small pockets known as lacunae. Unlike other connective tissues, cartilage is avascular, so chondrocytes must obtain nutrients and eliminate waste products by diffusion through the matrix. This lack of a blood supply also limits the repair capabilities of cartilage.

Fibroblasts

are the only cells that are always present in connective tissue proper. They are also the most abundant permanent, or fixed, cells in connective tissue proper. ________ produce connective tissue fibers and ground substance.

Axon

carry information to other cells

A serous membrane

consists of a simple epithelium supported by areolar tissue. _________ _________ line the sealed, internal cavities of the trunk, which are not open to the exterior. There are three ________ ________. The pleura (PLOO-ra; pleura, rib) lines the pleural cavities and covers the lungs. The peritoneum (per-i-tō-NĒ-um; peri, around + teinein, to stretch) lines the peritoneal cavity and covers the surfaces of enclosed organs such as the liver and stomach. The pericardium (per-i-KAR-dē-um) lines the pericardial cavity and covers the heart. has parietal and visceral portions that are in close contact at all times. The parietal portion lines the inner surface of the cavity. The visceral portion, or serosa, covers the outer surface of organs projecting into the body cavity. For example, the visceral pericardium covers the heart, and the parietal pericardium lines the inner surfaces of the pericardial sac that surrounds the pericardial cavity. The primary function is to reduce friction between the opposing parietal and visceral surfaces when an organ moves or changes shape. Friction is reduced by a watery, serous fluid formed by fluids diffusing from underlying tissues.

simple epithelia

consists of a single layer of cells covering the basement membrane. ______ __________ are thin. A single layer of cells is fragile and cannot provide much mechanical protection. For this reason, ______ ________ are found only in protected areas inside the body. They line internal compartments and passageways, including the pleural, pericardial, and peritoneal cavities, the heart chambers, and blood vessels. ______ _________ are also characteristic of regions where secretion or absorption occurs. Examples are the lining of the intestines and the gas-exchange surfaces of the lungs. In these places, thinness is an advantage. It reduces the time for materials to cross the epithelial barrier.

loose connective tissue

consists of areolar tissue, adipose tissue, and reticular tissue.

Connective tissue proper

consists of many cell types within a matrix containing extracellular fibers and a syrupy ground substance. Examples are the tissue that underlies the skin, fatty tissue, and tendons and ligaments.

Elastic fibers

contain the protein elastin. _____ ______ are branched and wavy. After stretching, they return to their original length.

Elastic cartilage

contains numerous elastic fibers that make it extremely resilient and flexible. forms the external flap (the auricle, or pinna) of the external ear, the epiglottis, an airway to the middle ear (the auditory tube), and small cartilages in the larynx (voice box).

Skeletal muscle tissue

contains very large, multinucleated cells (Figure 4-14a). A skeletal muscle cell may be 100 micrometers (μm; 1 μm = 0.001 mm = 1/25,000 in.) in diameter and up to 0.3 m (1 ft) long. The individual muscle cells are usually called muscle fibers because they are relatively long and slender. Skeletal muscle fibers are incapable of dividing, but new muscle fibers are produced through the divisions of stem cells in adult skeletal muscle tissue. As a result, at least partial repairs can take place after an injury. In skeletal muscle fibers, actin and myosin filaments are organized into repeating patterns that give the cells a striated, or banded, appearance. The striations, or bands, are easy to see in light micrographs. Skeletal muscle fibers will not usually contract unless stimulated by nerves. Because the nervous system provides voluntary control over its activities, skeletal muscle is described as striated voluntary muscle.

inflammatory response

coordinates the activities of several different tissues. coordinates the activities of several different tissues Many stimuli can produce inflammation. They include impact, abrasion, chemical irritation, infection (invasion and presence of pathogens, such as harmful bacteria or viruses), and extreme temperatures (hot or cold). When any of these stimuli either kill cells, damage fibers, or injure tissues, they trigger the inflammatory response by stimulating mast cells. p. 103 The mast cells release chemicals (histamine and heparin) that cause local blood vessels to dilate (enlarge in diameter) and become more permeable. As a result, increased blood flow to the injured region makes it red and warm to the touch, and the diffusion of blood plasma causes the injured area to swell. The abnormal tissue conditions and chemicals released by the mast cells also stimulate sensory nerve endings that produce sensations of pain. These local circulatory changes increase the delivery of nutrients, oxygen, phagocytic white blood cells, and blood-clotting proteins. They also speed up the removal of waste products and toxins. Over a period of hours to days, this coordinated response generally succeeds in eliminating the inflammatory stimulus.

Ground Substance

fluid or semi-fluid portion of the matrix fills the spaces between cells and surrounds connective tissue fibers. In normal connective tissue proper, it is clear, colorless, and similar in consistency to maple syrup. This dense consistency slows the movement of bacteria and other pathogens, making them easier for phagocytes to catch.

dense irregular connective tissue

forms an interwoven meshwork in no consistent pattern. This tissue strengthens and supports areas subjected to stresses from many directions. gives skin its strength and covers bone and cartilage (except at joints). It also forms a thick fibrous layer, or capsule, which surrounds internal organs such as the liver, kidneys, and spleen and encloses joint cavities.

Fibrocartilage

has little ground substance, and its matrix is dominated by collagen fibers. These fibers are densely interwoven, making this tissue extremely durable and tough. Pads of ______________ lie between the spinal vertebrae, between the pubic bones of the pelvis, around tendons, and around or within a few joints. In these positions the pads resist compression, absorb shocks, and prevent damaging bone-to-bone contact. Cartilages heal poorly, and damaged fibrocartilage in joints such as the knee can interfere with normal movements.

Fluid connective tissues

have a distinctive population of cells suspended in a matrix of watery ground substance containing dissolved proteins. The two fluid connective tissues are blood and lymph.

Supporting connective tissues

have a less diverse cell population than connective tissue proper, and a matrix of dense ground substance and closely packed fibers. The body contains two supporting connective tissues: cartilage and bone. The fibrous matrix of bone is said to be calcified because it contains mineral deposits (primarily calcium salts) that give the bone strength and rigidity.

Adipose tissue

is a loose connective tissue containing large numbers of fat cells, or adipocytes. The difference between loose connective tissue and ________ _________ is one of degree. A loose connective tissue is called adipose tissue when it becomes dominated by fat cells. Adipose tissue is another source of padding and shock absorption for the body. It also serves as insulation that slows heat loss through the skin, and it stores energy. __________ _________ is common under the skin of the flanks (between the last rib and the hips), buttocks, and breasts. It fills the bony sockets behind the eyes and surrounds the kidneys. It is also common beneath the epithelial lining of the pericardial and peritoneal cavities.

Reticular tissue

is a loose connective tissue whose reticular fibers form a complex three-dimensional network. They stabilize the positions of functional cells in lymph nodes and bone marrow and in organs such as the spleen and liver. Fixed macrophages, fibroblasts, and fibrocytes are associated with the reticular fibers. These cells are seldom visible, however, because specialized cells with other functions dominate the organs.

transitional epithelium

is a stratified epithelium that tolerates repeated cycles of stretching and recoiling. It is called ___________ because the appearance of the epithelium changes as stretching takes place. It lines the ureters and urinary bladder, where large changes in volume occur (Figure 4-5c). In an empty urinary bladder, the epithelium is layered, and the outermost cells appear plump and cuboidal. In a full urinary bladder, when the volume of urine has stretched the lining to its limits, the epithelium appears flattened. It looks more like a stratified squamous epithelium.

stratified columnar epithelium

is also relatively rare. It is found along portions of the pharynx, epiglottis, anus, urethra, and a few, large excretory ducts. If more than two layers are present, only the superficial cells are columnar. (It is also not shown with the other stratified epithelia.)

simple squamous epithelium

is found in protected regions where absorption takes place or where a slippery surface reduces friction. Examples are portions of the kidney tubules, the exchange surfaces of the lungs, the linings of the pericardial, pleural, and peritoneal cavities, blood vessels, and the inner surfaces of the heart.

Cardiac muscle tissue

is found in the heart. Like skeletal muscle tissue, cardiac muscle tissue is striated. A typical cardiac muscle cell, or cardiocyte, is much smaller than a skeletal muscle fiber and usually has only a single nucleus (Figure 4-14b). Cardiac muscle cells branch and form extensive connections with one another. These cells are interconnected at intercalated (in-TER-ka-lā-ted) discs, specialized attachment sites containing gap junctions and desmosomes. Cardiac muscle cells thus form a network that efficiently conducts the stimulus and force for contraction from one area of the heart to another. Cardiac muscle tissue has a very limited ability to repair itself. Stem cells are lacking. Some cardiac muscle cells do divide after an injury to the heart, but the repairs are incomplete. Cardiac muscle cells do not rely on nerve activity to start a contraction. Instead, specialized pacemaker cells establish a regular rate of contraction. The nervous system can alter the rate of pacemaker activity, but it does not provide voluntary control over individual cardiac muscle cells. For this reason, cardiac muscle is called striated involuntary muscle.

Smooth muscle tissue

is found in the walls of blood vessels; around hollow organs such as the urinary bladder; and in layers around the respiratory, circulatory, digestive, and reproductive tracts. A smooth muscle cell is small and slender. It tapers to a point at each end and contains a single nucleus (Figure 4-14c). Unlike skeletal and cardiac muscle, smooth muscle cells have no striations. The reason is because their actin and myosin filaments are scattered throughout the cytoplasm. Smooth muscle cells can divide, so smooth muscle tissue can regenerate after injury. Smooth muscle cells may contract on their own, or their contractions may be triggered by neural activity. The nervous system usually does not provide voluntary control over smooth muscle contractions, so smooth muscle is known as nonstriated involuntary muscle.

stratified squamous epithelium

is found where mechanical stresses are severe. Examples include the surface of the skin and the lining of the mouth, tongue, esophagus, and anus

Stratified cuboidal epithelium

is relatively rare. It occurs along the ducts of sweat glands and in the larger ducts of the mammary glands. (It is not shown with the other stratified epithelia.)

Muscle tissue

is specialized for contraction. Muscle cells contract due to interaction between filaments of the proteins myosin and actin. p. 70 These two proteins are found in the cytoskeletons of many cells, but in muscle cells, the filaments are more numerous and arranged so that their interaction produces a contraction of the entire cell. There are three types ______ ______ in the body—skeletal, cardiac, and smooth muscle tissues (Figure 4-14). The contraction process is the same in all of them, but the organization of their actin and myosin filaments differs. In this discussion we will focus on general characteristics rather than specific details

Areolar tissue

is the least specialized connective tissue in adults. It contains all the cells and fibers found in any connective tissue proper, as well as an extensive blood supply. Also forms a layer that separates the skin from deeper structures. In addition to providing padding, its elastic properties allow a considerable amount of independent movement. Pinching the skin of your arm, for example, does not distort the underlying muscle. Conversely, a contracting muscle does not pull against the skin. As the muscle bulges, the ________ _______ The ample blood supply in this tissue carries wandering cells to and from the tissue and provides for the metabolic needs (oxygen and nutrients) of nearby epithelial tissue. type of loose connective tissue

Hyaline cartilage

is the most common type of cartilage. The matrix contains closely packed collagen fibers, making ________ _________ tough but somewhat flexible. This type of cartilage connects the ribs to the sternum (breastbone), supports the conducting passageways of the respiratory tract, and covers opposing bone surfaces within joints.

Connective tissue

is the most diverse tissue of the body. Bone, blood, and fat are familiar __________ ________ with very different functions and characteristics. All __________ _______ have three basic components: (1) specialized cells, (2) extracellular protein fibers, and (3) ground substance.

cutaneous membrane

is the skin that covers the surface of your body It consists of a stratified squamous epithelium and a layer of areolar tissue reinforced by underlying dense irregular connective tissue. In contrast to serous or mucous membranes, the ________ ________ is thick, relatively waterproof, and usually dry.

Mucous membranes (mucosae)

line passageways and chambers that open to the exterior, including those in the digestive, respiratory, reproductive, and urinary tracts (Figure 4-13a). The epithelial surfaces are kept moist at all times, typically by mucous secretions from mucous cells or multicellular glands, or by fluids such as urine or semen. The areolar tissue portion is called the lamina propria (PRŌ-prē-uh). Many contain simple epithelia with absorptive or secretory functions, such as the simple columnar epithelium of the digestive tract. However, other types of epithelia may be involved. For example, a stratified squamous epithelium is part of the mucous membrane of the mouth. Also, the ________ ________ along most of the urinary tract has a transitional epithelium.

glandular epithelium

most or all of the cells actively produce secretions. These secretions are classified according to where they are discharged

Avascular

without blood vessels