רקמות Flashcards
Types of Tissues
- Epithelial tissue
- Connective tissue
- Muscular tissue
- Nervous tissue
Epithelial tissue
covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands. This tissue
allows the body to interact with both its internal and external environments.
Epithelial tissue has three major functions. It serves as (1) a selective barrier that limits or aids the transfer of substances into and out of the body;
(2) a secretory surface that releases products produced
by the cells onto its free surfaces; and
(3) a protective surface that resists the abrasive influences of the environment.
Connective tissue
protects and supports the body and its organs. Various types of connective tissues bind organs together,
store energy reserves as fat, and help provide the body with immunity to disease-causing organisms.
Cell junctions
Cell junctions are contact points between the plasma
membranes of tissue cells. Here we consider the five most important types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions
Tight Junctions
Tight junctions consist of weblike strands of transmembrane proteins that fuse together the outer surfaces of adjacent plasma membranes to seal off passageways between adjacent cells (Figure 4.2a). Cells of epithelial tissue that lines the stomach, intestines, and urinary bladder have many tight junctions. They inhibit the passage of substances between cells and prevent the contents of these organs from leaking into the blood or surrounding tissues.
Adherens Junctions
Adherens junctions (ad-HEˉR-ens) contain plaque (PLAK), a dense layer of proteins on the inside of the plasma membrane that attaches both to membrane proteins and to microfilaments of the cytoskeleton (Figure 4.2b). Transmembrane glycoproteins called cadherins join the cells. Each cadherin inserts into the plaque from the opposite side of the plasma membrane, partially crosses the intercellular space (the space between the cells), and connects to cadherins of an adjacent cell. In epithelial cells, adherens junctions often form extensive zones called adhesion belts because they encircle the cell similar to the way a belt encircles your waist. Adherens junctions help epithelial surfaces resist separation during various contractile activities, as when food moves through the intestines.
Desmosomes
Like adherens junctions, desmosomes (DEZ-moˉ-soˉms; desmo-band) contain plaque and have transmembrane glycoproteins (cadherins) that extend into the intercellular space between adjacent cell membranes and attach cells to one another (Figure 4.2c).
However, unlike adherens junctions, the plaque of desmosomes does not attach to microfilaments. Instead, a desmosome plaque attaches to elements of the cytoskeleton known as intermediate filaments, which consist of the protein keratin. The intermediate
filaments extend from desmosomes on one side of the cell across the cytosol to desmosomes on the opposite side of the cell. This structural arrangement contributes to the stability of the cells and tissue. These spot weld–like junctions are common among the cells that make up the epidermis (the outermost layer of the skin)
and among cardiac muscle cells in the heart. Desmosomes prevent epidermal cells from separating under tension and cardiac muscle cells from pulling apart during contraction.
Hemidesmosomes
Hemidesmosomes (hemi- half) resemble desmosomes, but they do not link adjacent cells. The name arises from the fact that they look like half of a desmosome (Figure 4.2d). However, the transmembrane glycoproteins in hemidesmosomes are integrins rather than cadherins. On the inside of the plasma membrane, integrins attach to intermediate filaments made of the protein keratin. On the outside of the plasma membrane, the integrins attach
to the protein laminin, which is present in the basement membrane (discussed shortly). Thus, hemidesmosomes anchor cells not to each other but to the basement membrane.
Gap Junctions
At gap junctions, membrane proteins called connexins form tiny fluid-filled tunnels called connexons that connect neighboring cells (Figure 4.2e). The plasma membranes of gap junctions are not fused together as in tight junctions but are separated by a very narrow intercellular gap (space). Through the connexons, ions
and small molecules can diffuse from the cytosol of one cell to another, but the passage of large molecules such as vital intracellular proteins is prevented. The transfer of nutrients, and perhaps wastes, takes place through gap junctions in avascular tissues such as the lens and cornea of the eye. Gap junctions allow the
cells in a tissue to communicate with one another. In a developing embryo, some of the chemical and electrical signals that regulate growth and cell differentiation travel via gap junctions. Gap junctions also enable nerve or muscle impulses to spread rapidly
among cells, a process that is crucial for the normal operation of some parts of the nervous system and for the contraction of muscle in the heart, gastrointestinal tract, and uterus.
apical (free) surface
The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of an internal organ, or a tubular duct that receives cell secretions (Figure 4.4). Apical surfaces may contain cilia or microvilli.
lateral surfaces
The lateral surfaces of an epithelial cell, which face the adjacent cells on either side, may contain tight junctions, adherens junctions, desmosomes, and/or gap junctions.
basal surface
The basal surface of an epithelial cell is opposite the apical surface. The basal surfaces of the deepest layer of epithelial cells adhere to extracellular materials such as the basement membrane. Hemidesmosomes in the basal surfaces of the deepest layer of epithelial cells anchor the epithelium to the basement membrane (described next). In discussing epithelia with multiple layers, the term apical layer refers to the most superficial layer of cells, and the basal layer is the deepest layer of cells.
basement membrane
The basement membrane is a thin extracellular layer that commonly consists of two layers, the basal lamina and reticular lamina. The basal lamina (lamina thin layer) is closer to—and secreted by—the epithelial cells. It contains proteins such as laminin and collagen (described shortly), as well as glycoproteins and
proteoglycans (also described shortly). As you have already learned, the laminin molecules in the basal lamina adhere to integrins in hemidesmosomes and thus attach epithelial cells to the basement membrane (see Figure 4.2d). The reticular lamina is closer to the underlying connective tissue and contains proteins
such as collagen produced by connective tissue cells called fibroblasts (see Figure 4.8). In addition to attaching to and supporting the overlying epithelial tissue, basement membranes have other functions. They form a surface along which epithelial cells migrate during growth or wound healing, restrict passage of larger molecules between epithelium and connective tissue, and participate in filtration of blood in the kidneys.
avascular
Epithelial tissue has its own nerve supply but, as mentioned previously, is avascular (a- without; -vascular vessel), relying on the blood vessels of the adjacent connective tissue to bring nutrients and remove wastes. Exchange of substances between an epithelial tissue and connective tissue occurs by diffusion.
Covering and lining epithelium
Covering and lining epithelium forms the outer covering of the skin and some internal organs. It also forms the inner lining of blood vessels, ducts, and body cavities, and the interior of the respiratory,
digestive, urinary, and reproductive systems.
Glandular epithelium
Glandular epithelium makes up the secreting portion of glands such as the thyroid gland, adrenal glands, and sweat glands.
Types of covering and lining epithelial tissue
are classified according to two characteristics: the arrangement of cells into layers and the shapes of the cells
Arrangement of cells in layers
- Simple epithelium
- Pseudostratified epithelium
- Stratified epithelium
Simple epithelium
Simple epithelium is a single layer of cells that functions in diffusion, osmosis, filtration, secretion, or absorption. Secretion is the production and release of substances such as mucus, sweat, or enzymes. Absorption is the intake of fluids or other substances such as digested food from the intestinal tract.
Pseudostratified epithelium
Pseudostratified epithelium (pseudo- false) appears to have multiple layers of cells because the cell nuclei lie at different levels and not all cells reach the apical surface; it is actually a simple epithelium because all its cells rest on the basement membrane. Cells that do extend to the apical surface may contain cilia; others (goblet cells) secrete mucus
Stratified epithelium
Stratified epithelium (stratum layer) consists of two or more layers of cells that protect underlying tissues in locations where there is considerable wear and tear.
Cell shapes epithelium
- Squamous
- Cuboidal
- Columnar
- Transitional
Squamous
Squamous cells (SKWA¯-mus flat) are thin, which allows for the rapid passage of substances through them.
Cuboidal
are as tall as they are wide and are shaped like cubes or hexagons. They may have microvilli at their apical surface and function in either secretion or absorption.
Columnar
Columnar cells are much taller than they are wide, like columns, and protect underlying tissues. Their apical surfaces may have cilia or microvilli, and they often are specialized for secretion and absorption.
