Answers to Biology Corner Review Guide - Tissues (Chapter 5)

Chapter 10

Epithelial Tissue

By the finish of this section, you will be able to:

Explain the structure and role of epithelial tissue
Distinguish betwixt tight junctions, anchoring junctions, and gap junctions
Distinguish between uncomplicated epithelia and stratified epithelia, besides every bit between squamous, cuboidal, and columnar epithelia
Describe the structure and role of endocrine and exocrine glands and their respective secretions

Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside globe and lining the outside of organs. Epithelium likewise forms much of the glandular tissue of the body. Skin is not the simply surface area of the trunk exposed to the outside. Other areas include the airways, the digestive tract, likewise as the urinary and reproductive systems, all of which are lined past an epithelium. Hollow organs and trunk cavities that practice not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type of epithelium.

Epithelial cells derive from all 3 major embryonic layers. The epithelia lining the pare, parts of the oral cavity and nose, and the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm. The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an endothelium.

All epithelia share some of import structural and functional features. This tissue is highly cellular, with footling or no extracellular material nowadays between cells. Adjoining cells form a specialized intercellular connection between their prison cell membranes chosen a cell junction. The epithelial cells showroom polarity with differences in structure and function between the exposed or apical facing surface of the cell and the basal surface shut to the underlying trunk structures. The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating information technology from underlying connective tissue. The basal lamina attaches to a reticular lamina, which is secreted past the underlying connective tissue, forming a basement membrane that helps agree it all together.

Epithelial tissues are well-nigh completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come past improvidence or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a feature of surface epithelium and allows our airways and digestive tracts to chop-chop supercede damaged cells with new cells.

Generalized Functions of Epithelial Tissue

Epithelial tissues provide the torso'south showtime line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium deed as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical bulwark. All substances that enter the torso must cantankerous an epithelium. Some epithelia often include structural features that allow the selective ship of molecules and ions across their prison cell membranes.

Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their upmost surfaces. The epithelium of the small intestine releases digestive enzymes, for case. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.

The Epithelial Cell

Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins betwixt their basal and apical surfaces. Particular structures found in some epithelial cells are an accommodation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.

Cilia are microscopic extensions of the apical prison cell membrane that are supported past microtubules. They beat in unison and motion fluids as well every bit trapped particles. Ciliated epithelium lines the ventricles of the encephalon where it helps broadcast the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator considering it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket downwardly towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic surroundings of your breadbasket.

Cell to Jail cell Junctions

Cells of epithelia are closely continued and are non separated by intracellular cloth. Three basic types of connections allow varying degrees of interaction betwixt the cells: tight junctions, anchoring junctions, and gap junctions (Effigy 1).

These three illustrations each show the edges of two vertical cell membranes. The cell membranes are viewed partially from the side so that the inside edge of the right cell membrane is visible. The upper left image shows a tight junction. The two cell membranes are bound by transmembrane protein strands. The proteins travel the inside edge of the right cell membrane and cross over to the left cell membrane, cinching the two membranes together. The cell membranes are still somewhat separated in between neighboring strands, creating intercellular spaces. The upper right diagram shows a gap junction. The gap junctions are composed of two interlocking connexins, which are round, hollow tubes that extend through the cell membranes. Two connexins, one from the left cell membrane and the other from the right cell membrane, meet between the two cells, forming a connexon. Even at the site of the connexon, there is a small gap between the cell membranes. On the inside edge of the right cell membrane, the gap junction appears as a depression. Three connexins are embedded into the membranes like buttons on a shirt. The bottom images show the three types of anchoring junctions. The left image shows a desmosome. Here, the inside edge of both the right and left cell membranes have brown, round plaques. Each plaque has tentacle-like intermediate filaments (keratin) that extend into each cell's cytoplasm. The two plaques are connected across the intercellular space by several interlocking transmembrane glycoproteins (cadherin). The connected glycoproteins look similar to a zipped-up zipper between the right and left cell membranes. The right image shows an adheren. These are similar to desmosomes, with two plaques on the inside edge of each cell membrane connected across the intercellular space by glycoproteins. However, the plaques do not contain the tentacle-like intermediate filaments branching into the cytoplasm. Instead, the plaques are ribbed with green actin filaments. The filaments are neatly arranged in parallel, horizontal strands on the surface of the plaque facing the cytoplasm. The bottom image shows a hemidesmosome. Rather than located between two neighboring cells, the hemidesmosome is located between the bottom of a cell and the basement membrane. A hemidesmosome contains a single plaque on the inside edge of the cell membrane. Like the desmosome, intermediate filaments project from the plaque into the cytoplasm. The opposite side of the plaque has purple, knob-shaped integrins extending out to the basal lamina of the basement membrane. — Figure ane: The iii basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.

At 1 stop of the spectrum is the tight junction, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to human activity as selective barriers. An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are 3 types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the jail cell's membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of next cells. These connections are especially important in holding cells together. Hemidesmosomes, which look similar half a desmosome, link cells to the extracellular matrix, for example, the basal lamina. While like in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the contractile poly peptide actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like construction inside the cell. These junctions influence the shape and folding of the epithelial tissue.

In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions betwixt the cytoplasm of next cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.

Classification of Epithelial Tissues

Epithelial tissues are classified according to the shape of the cells and number of the jail cell layers formed (Effigy two). Jail cell shapes can be squamous (flattened and thin), cuboidal (indigestible, as wide as information technology is tall), or columnar (rectangular, taller than it is broad). Similarly, the number of prison cell layers in the tissue tin exist ane—where every cell rests on the basal lamina—which is a simple epithelium, or more than than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina. Pseudostratified (pseudo- = "faux") describes tissue with a single layer of irregularly shaped cells that give the appearance of more than i layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.

This figure is a table showing the appearance of squamous, cuboidal and columnar epithelial tissues. Simple and compound forms are shown for each tissue type. In a simple squamous epithelium, the cells are flattened and single layered. In a simple cuboidal epithelium, the cells are cube shaped and single layered. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. A stratified squamous epithelium contains many layers of flattened cells. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells. — Effigy 2: Elementary epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells.

Simple Epithelium

The shape of the cells in the unmarried prison cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous jail cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the jail cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular organization, and it is made upwards of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the prison cell, is present where rapid passage of chemic compounds is observed. The alveoli of lungs where gases lengthened, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a shine and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.

In unproblematic cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the middle of the cell. These epithelia are active in the secretion and absorptions of molecules. Elementary cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.

In unproblematic columnar epithelium, the nucleus of the tall cavalcade-similar cells tends to exist elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the assimilation and secretion of molecules. Uncomplicated columnar epithelium forms the lining of some sections of the digestive organization and parts of the female reproductive tract. Ciliated columnar epithelium is composed of unproblematic columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory arrangement, where the chirapsia of the cilia helps remove particulate matter.

Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified only instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at dissimilar levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some practice not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.

Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a goblet jail cell is a mucous-secreting unicellular "gland" interspersed between the columnar epithelial cells of mucous membranes (Figure 3).

This illustration shows a diagram of a goblet cell. The goblet cell is shaped roughly like an upside down vase. The enlarged end at the top contains six finger like projections labeled microvilli. Between the microvilli, secretary vesicles containing mucin are moving from the upper half of the cell toward the microvilli. Below the secretory vesicles are several rough endoplasmic reticula and an irregularly shaped Golgi apparatus with secretory vesicles budding off of it. The narrow, lower half of the cell contains the oval-shaped nucleus as well as a few mitochondria and segments of the endoplasmic reticulum. — Effigy iii: (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells. (b) The arrows in this micrograph point to the mucous-secreting goblet cells. LM × 1600. (Micrograph provided past the Regents of University of Michigan Medical Schoolhouse © 2012)

The second image is a micrograph of the innermost lining of the small intestine. This innermost lining is a simple columnar epithelium, with a single layer of rectangular cells oriented in a line. Occasionally, the line of epithelial cells is interrupted by a goblet cell. Goblet cells are thinner than the epithelial cells and appear roughly pill shaped. In this micrograph, the cells did not stain as darkly as the epithelial cells. — Effigy iii: (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells. (b) The arrows in this micrograph point to the mucous-secreting goblet cells. LM × 1600. (Micrograph provided past the Regents of University of Michigan Medical Schoolhouse © 2012)

Stratified Epithelium

A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical article of clothing and tear. The stratified epithelium is named past the shape of the nigh apical layer of cells, closest to the free space. Stratified squamous epithelium is the well-nigh common type of stratified epithelium in the human body. The upmost cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The peak layer may be covered with dead cells filled with keratin. Mammalian skin is an case of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an case of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can too exist institute in certain glands and ducts, but are uncommon in the human body.

Another kind of stratified epithelium is transitional epithelium, and so-called because of the gradual changes in the shapes of the upmost cells as the bladder fills with urine. It is institute only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the upmost cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. Figure 4 summarizes the different categories of epithelial jail cell tissue cells.

This figure is a table with three columns and eight rows. The leftmost column is titled cells, and contains a drawing in each row showing how epithelial cells are arranged above a basement membrane. The middle column is titled location, while the rightmost column is titled function. In a simple squamous epithelium, the cells are flattened and single-layered. Simple squamous cells are found in the air sacs of the lungs, in the lining of the heart, blood vessels and lymphatic vessels. Their function is to allow materials to pass through by diffusion and filtration, as well as to secrete lubricating substances. In a simple cuboidal epithelium, the cells are cube shaped and single layered and located in ducts and secretory portions of small glands as well as in the kidney tubules. The function of simple cuboidal epithelium is to secrete and absorb. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. Simple columnar epithelium is found in ciliated tissues including the bronchi, uterine tubes, and uterus, as well as in smooth, nonciliated tissues such as the digestive tract bladder. The function of simple columnar epithelium is to absorb substances but also to secrete mucous and enzymes. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. Pseudostratified columnar epithelium lines the trachea and much of the upper respiratory tract. The function of pseudostratified columnar epithelium is to secrete mucous and also move that mucus using the hair like cilia projecting from the top of each cell. A stratified squamous epithelium contains many layers of flattened cells. Stratified squamous epithelium lines the esophagus, mouth, and vagina. The function of stratified squamous epithelium is to protect against abrasion. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified cuboidal epithelium is found in the sweat glands, salivary glands, and mammary glands. The function of stratified cuboidal epithelium is to protect other tissues of the body. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells. Stratified columnar epithelium is located in the male urethra and the ducts of some glands. The function of stratified columnar epithelium is to secrete and protect. Transitional epithelium consists of many layers of irregularly shaped cells with diverse sizes. Transitional epithelium is found lining the bladder, urethra and ureters. The function of transitional epithelium is to allow the urinary organs to expand and stretch.

Watch the video linked to beneath notice out more about the anatomy of epithelial tissues. Where in the trunk would i discover non-keratinizing stratified squamous epithelium?

Glandular Epithelium

A gland is a construction made up of i or more cells modified to synthesize and secrete chemic substances. About glands consist of groups of epithelial cells. A gland can exist classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = "inside"), or an exocrine gland whose secretions exit through a duct that opens directly, or indirectly, to the external environment (exo- = "outside").

Endocrodine Glands

The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that accept receptors to bind the hormones. The endocrine system is part of a major regulatory system analogous the regulation and integration of trunk responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.

Exocrine Glands

Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.

Glandular Structure

Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine.

The multicellular exocrine glands known as serous glands develop from unproblematic epithelium to form a secretory surface that secretes straight into an inner crenel. These glands line the internal cavities of the belly and chest and release their secretions direct into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The duct is single in a simple gland but in chemical compound glands is divided into one or more than branches (Figure v). In tubular glands, the ducts tin be directly or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) chemical compound glands. In a branched gland, a duct is continued to more than than one secretory group of cells.

This table shows the different types of exocrine glands: alveolar (acinar) versus tubular and those with simple ducts versus compound ducts. Each diagram shows a single layer of columnar epithelial cells with a line of cells travelling along the surface of a tissue (surface epithelium) and then dipping into a hole in the tissue. The cells travel down the right side of the hole until they reach the bottom, then curve around the bottom of the hole and then travel up the left side. Finally, the cells emerge back onto the surface of the tissue. The surface epithelial cells are those that are on the surface of the tissue; the duct cells are those that line both walls of the hole. The gland cells are those that line the bottom of the hole. The shape of the hole differs in each gland. In the simple alvelolar (acinar) gland, the duct and gland cells are bulb shaped with the gland cells being the larger end of the bulb. Simple alveolar glands are not found in adults, as these represent an early developmental stage of simple, branched glands. In simple tubular glands, the duct and gland cells are U shaped. Simple tubular glands are found in the intestinal glands. In simple branched alveolar glands, the gland cells form three bulbs at the end of the duct, similar in appearance to a clover leaf. The sebaceous (oil) glands are examples of simple branched alveolar glands. In simple coiled tubular glands, the duct and gland cells form a U, however, the bottom of the U, which is all gland cells, is curved up to the right. Merocrine sweat glands are examples of simple coiled tubular glands. In simple branched tubular glands, the duct is very short and the gland cells divide into three lobes, similar in appearance to a bird's foot. The gastric glands of the stomach and mucous glands of the esophagus, tongue and duodenum are examples of simple branched tubular glands. Among the glands with compound ducts, compound alveolar (acinar) glands have three sets of clover leaf bulbs, for a total of six bulbs. Two of the clover leaf shaped structures extend parallel to the surface epithelium in opposite directions to each other. The third clover leaf extends down into the tissue, perpendicular to the surface. The duct is cross-shaped. The mammary glands are an example of compound alveolar glands. Compound tubular glands have a similar structure to compound alveolar glands. However, instead of three cloverleaf shaped bulbs, the compound tubular gland has three bird's foot shaped bulbs. The duct is also cross-shaped in the compound tubular gland. The mucous glands of the mouth and the bulbourethral glands of the male reproductive system are examples of compound tubular glands, which are also found in the seminiferous tubules of the testis. Compound tubuloalveolar glands are a hybrid between the compound alveolar gland and the compound tubular gland. The two sets of bulbs that run parallel to the surface are bird-foot shaped; however, the set of bulbs that runs perpendicularly below the surface is cloverleaf shaped. The salivary glands, glands of the respiratory passages and glands of the pancreas are all compound tubuloalveolar glands. — Effigy 5: Exocrine glands are classified by their construction.

Methods and Types of Secretion

Exocrine glands tin be classified by their way of secretion and the nature of the substances released, as well as past the construction of the glands and shape of ducts (Figure 6). Merocrine secretion is the most common blazon of exocrine secretion. The secretions are enclosed in vesicles that move to the upmost surface of the prison cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are some other example.

These three diagrams show the three modes of secretion. All three diagrams show three orange cells in a line with attached to a basement membrane. Each cell has a large nucleus in its lower half. The upper half of each cell contains a Golgi apparatus, which appears like an upside down jellyfish. Yellow secretory vesicles are budding from the top end of the Golgi apparatus. Each vesicle contains several orange circles, which are the secreted substance. In merocrine secretion, the secretory vesicles travel to the top edge of the cells and release the secretion from the cell by melding with the cell membrane. In apocrine secretion, the top third of the cell, which contains the secretory vesicles, pinches in at the sides and then completely disconnects above the Golgi complex. The pinched off portion of the cell is the secretion, as it contains the majority of the secretory vesicles. In holocrine secretion, the upper third of the cell, just above the Golgi complex, forms many finger like projections. Each projection contains several vesicles. The tips of the projections that contain secretory vesicles bud off from the cell. In this method of secretion, the mature cell eventually dies and becomes the secretory product. — Figure half-dozen: (a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell is released, as well. (c) In holocrine secretion, the jail cell is destroyed equally it releases its product and the prison cell itself becomes part of the secretion.

Apocrine secretion accumulates well-nigh the apical portion of the jail cell. That portion of the jail cell and its secretory contents pinch off from the prison cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break downwards; this causes torso odor. Both merocrine and apocrine glands keep to produce and secrete their contents with fiddling damage caused to the cell because the nucleus and golgi regions remain intact later secretion.

In dissimilarity, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the pare and hair are holocrine glands/cells (Effigy seven).

Image A depicts a cross section of the skin layers. The surface of the skin is at the top of the diagram, with the outer layer occupying about one fifth of the cross section. The outer layer has an irregular border with the inner skin layer, which occupies the remainder of the cross section. A hair follicle is embedded within the inner layer. However, the outer layer actually invaginates into the inner layer around the outside of the follicle, completely sheathing the follicle. The follicle has a bulb at its bottom that is connected to blood vessels. The hair projects from the bulb and travels through the sheath to erupt from the skin surface. The sebaceous gland is an irregular, yellow structure attached at the midpoint of the hair shaft near the border between the inner and outer layers of skin. Its duct actually connects into the side of the hair follicle. Image B shows a micrograph of a sebaceous gland connected to a hair follicle. The bulb of the hair follicle is evident in the micrograph as a bundle of cell surrounding the growing hair at its center. The sebaceous gland is connected to the right of the follicle bulb. The gland appears as an oval shaped mass of pink staining, cube shaped cells with purple nuclei. — Figure 7: These glands secrete oils that lubricate and protect the skin. They are holocrine glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical Schoolhouse © 2012)

Glands are likewise named after the products they produce. The serous gland produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth. Mixed exocrine glands contain both serous and mucous glands and release both types of secretions.

Chapter Review

In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the surround, coverage, secretion and excretion, absorption, and filtration. Cells are spring together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow costless exchange of soluble molecules between cells, and anchoring junctions, which adhere cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers class simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.

Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones straight into the interstitial fluid and blood stream. Glands are classified both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized. Apocrine glands release secretions by pinching off the upmost portion of the cell, whereas holocrine gland cells store their secretions until they rupture and release their contents. In this case, the prison cell becomes function of the secretion.

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