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Epithelium

There are two basic types of epithelial tissues: covering and lining epithelia and glandular

epithelia

Diagrammatic illustration showing epithelial cells resting on basement membrane

Lining epithelial cells form a continuous layer over all the free surfaces of the body:

o The outer layer of the skin.

o The inner surface of the digestive and respiratory cavities.

o The inner surface of the heart and blood vessels.

o The walls and the organs of the closed ventral body cavities.

o The ducts of the exocrine glands.

Glandular epithelia make up most of the glands in the body.

General Features:

Maximum cell-to-cell contact.

Minimum extracellular material.

Cell junctions: Several types of junctional specializations unite adjacent epithelial

cells (tight junctions, desmosomes and gap junctions).

Supported by basement membrane (basal lamella).

Avascular: There are no blood vessels within the epithelial layer.

Derived from all embryonic germ layers, including endoderm, mesoderm and

ectoderm

With the exception of endocrine glands,

a. All epithelia have one free surface, called the apical surface, which is exposed

at the body surface or at the lumen (space) of the body cavity, duct, tube or

vessel.

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b. The basal surface of epithelium rests on a basement membrane: a non-living

adhesive material secreted by the epithelium and the underlying connective

tissue.

c. These cells are often characterized by frequent cell division because they are

exposed to wear and tear and injury, necessitating replacement.

Surface specializations or modifications (Cell polarity): Polarity (structural and functional

asymmetry) is characteristic of most epithelial cells. It is best seen in simple epithelia, where

each cell has 3 surfaces: an apical (Free) surface, lateral surfaces, and a basal surface

attached to the basal lamina.

1- Apical (Surface or luminal) modifications: It is specialized to carry out functions that

occur at these interfaces, including secretion, absorption, and movement of luminal contents.

Cilia: are membrane-covered extensions of the entire apical surface. They beat in

waves, often moving a surface coat of mucus and trapped materials. Ciliated epithelia

include ciliated pseudostratified columnar (respiratory) epithelium and the ciliated

simple columnar epithelium of the oviducts.

Flagella: are also concerned with movement. Spermatozoa, derived from seminiferous

epithelia, are the only flagellated human cells.

Microvilli (brush border or striated border): are plasma membrane-covered

extensions of the cell surface. Their cores are composed of parallel actin

microfilaments; these are anchored in a dense mat of filaments in the apical cytoplasm

called the terminal web. By interacting with cytoplasmic myosin, the microfilaments

can contract, shortening the microvilli. The apical surface of absorptive cells is usually

covered with microvilli, which greatly increase the apical surface area when extended.

Microvillus-covered epithelia, said to exhibit a striated border, or brush border,

include the absorptive simple columnar epithelium lining the small intestines and the

absorptive simple cuboidal epithelium lining the proximal tubules of the kidney.

Stereocilia: are not true cilia but very long microvilli. They are found in the male

reproductive tract (epididymis. ductus deferens). where they have an absorptive

function, and in the internal ear (hair cells of the maculae and organ of Corti). where

they have a sensory function.

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2- Lateral specializations (junctions and folding): cells attach tightly to one another by

specialized intercellular junctions. Several types of junctions can be seen, such as:-

o Zonula occludens (tight junctions, occluding junctions): are located near the cell apex

and seal off the intercellular space, allowing the epithelium to isolate certain body

compartments (they help keep intestinal bacteria and toxins out of the bloodstream).

Their structure, best seen in freeze-fracture preparations, results from the fusion of 2

trilaminar plasma membranes of adjacent cells to form a pentalaminar structure; this

fusion may require specific "tight-junction proteins." In some tissues, tight junctions

can be disrupted by removing calcium ions or treating with protease.

o Zonula adherens (sometimes called belt desmosomes): are usually just basal to the

tight junctions. The membranes of the adhering cells are typically 20-90 nm apart at a

zonula adherens. An electron-dense plaque containing myosin, tropomyosin, alpha

actinin, and vinculin is found on the cytoplasmic surface of each of the membranes

participating in the junction. Actin containing microfilaments arising from each cell's

terminal web insert into the plaques and appear to stabilize the junction.

o Macula adherens or desmosome, consists of 2 dense, granular attachment plaques

composed of several proteins and borne on the cytoplasmic surfaces of the opposing

cell membranes. Desmosomes, distributed in patches along the lateral membranes of

most epithelial cells. Transverse thin EM sections show dense arrays of tonofilaments

(cytokeratin intermediate filaments) that insert into the plaques or make hairpin turns

and return to the cytoplasm. The gap between the attached membranes is often over 30

nm. Sometimes fibrillar or granular material (probably glycoprotein) is seen as a dense

central line in the intercellular space

o Junctional complex: combination of zonula occludens, zonula adherens and

desmosomes.

o Communicating junctions (gap or nexus junctions): is a disk- or patch-shaped

structure, best appreciated by viewing both freeze-fracture and transverse thin EM

sections. The intercellular gap is 2 nm, and the membrane on each side contains a

circular patch of connexons, the connexons in one membrane link with those in the

other to form continuous pores that bridge the intercellular gap, allowing passage of

ions and small molecules (<800 daltons). As sites of electrotonic coupling (reduced

resistance to ion flow), gap junctions are important in intercellular communication and

coordination; they are found in most tissues.

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3- Basal specialization The basal surface contacts the basal lamina. Because it is the surface

closest to the underlying blood supply, it often contains receptors for blood borne factors

such as hormones.

1. A basal lamina underlies all true epithelial tissues. The basal lamina is a sheet-like

structure, usually composed of type IV collagen, proteoglycan, and laminin, a

glycoprotein that aids in binding cells to the basal lamina. The basal lamina exhibits

electron-lucent and electron-dense layers termed the lamina lucida (lamina rara) and

the lamina dense, respectively. Basal lamina components are contributed by the

epithelial cells, the underlying connective tissue cells, and (in some locations) muscle,

adipose, and Schwann cells. In some sites, a layer of type III collagen fibers (reticular

fibers), produced by the connective tissue cells and termed the reticular lamina,

underlies the basal lamina. Basal laminae accompanied by reticular laminae are often

thick enough to be seen with the light microscope as PAS- positive layers and are

sometimes termed basement membranes. The basal lamina forms a sieve-like barrier

between the epithelium and connective tissue. It aids in tissue organization and cell

adhesion and (through trans membrane linkages with cytoskeletal components) helps

maintain cell shape. it has a role in maintaining specific cell functions, probably

through its effect on shape. Muscle basal laminae are critical in establishing

neuromuscular junctions.

2. Hemidesmosomes: are located on the inner surface of basal plasma membranes in

contact with the basal lamina. They help to attach epithelial cells to the basal lamina.

The best examples are found in the basal layers of stratified squamous epithelium.

3. Sodium-potassium ATPase is a plasma membrane-bound enzyme localized

preferentially in the basal and basolateral regions of epithelial cells. It transports

sodium out of and potassium into the cell.

4- Intracellular Polarity: The nucleus and organelles are often found in characteristic

regions of epithelial cells, a feature particularly important to glandular cells. For example,

in protein secreting cells, the RER is preferentially located in the basal cytoplasm, the

nucleus in the basal to middle region just above the RER, and the Golgi complex just

above the nucleus. Mature secretory vesicles collect in the apical cytoplasm.

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Cell polarity: the upper, lower and lateral surface modification of epithelium.

Functions of epithelium

1. Protection: Epithelia protect underlying tissues against physical damage, drying out,

chemical injury and infection.

2. Epithelia allow and regulate the passage of materials (diffusion, absorption, filtration,

secretion, excretion) into and out of the tissues of the body which they cover or line.

3. Sensory reception: Specialized epithelia form sensory parts of organs such as the eye,

ear, mouth (taste buds), and nose (olfactory epithelium).

4. Most glands are derived from epithelial cells specialized for producing secretions.

The classification of the surface epithelia is based on:

1. The number of cell layers

- Simple epithelium: Being composed of one layer of cells only, they are very thin.

They are found in areas of minimum wear and tear. Their main function is to allow

passage of substances between the lumen and the surrounding tissues.

- Stratified epithelium: Being composed of several layers of cells, they are very

thick. Their main function is to protect the tissues that they cover. The shape of the

cells closest to the basement membrane is quite different from that of the cells at the

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top, near the lumen. The stratified epithelia are further classified according to the

shape of the cells at the free surface.

2. The shape of the cells

o Flat: squamous epithelium

o Square: cuboidal epithelium

o Rectangular: columnar epithelium

o If the shape changes depending of the degree of stretching of the tissue:

transitional epithelium.

Most often, it is very hard to distinguish the cell's boundary by light microscope. To figure out

what type of cells you are dealing with, look at the shape and position of the nuclei:

If the nuclei are flat and parallel to the free surface: squamous epithelium,

If the nuclei are oval and parallel to the axis of the cell and situated at its base:

columnar epithelium,

If the nuclei are round and situated in the middle of the cell: cuboidal epithelium.

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Types of epithelium and location

- Simple squamous: with flattened nuclei. Present in the alveoli of lungs, Kidneys, Lining

of visceral organs and all blood vessels. Function: selective diffusion, absorption or

secretion.

- Simple cuboidal: with central rounded nuclei. Present in liver, pancreas, acini of glands,

lines small ducts and tubules. Function: excretory, secretory or absorptive.

- Simple columnar: with basal oval nuclei. Present in the absorptive surfaces (intestine);

s e c r e t o r y s u r f a c e s ( s t o m a c h ) ; l i n i n g g a l l b l a d d e r ( a b s o r b s w a t e r ) .

- Simple columnar ciliated: Present in female reproductive tract (fallopian tube, uterus).

- Pseudostratified columnar ciliated: nuclei disposed at different levels; basal cells do

not extend to surface; Present in larger airways of respiratory system (trachea, bronchi).

- Stratified squamous keratinizing: Upper cell layer composed of squamous sells.

Present in surface of skin

- Stratified squamous non-keratinizing (mucous membrane): resists abrasion;

moistened by glandular secretions. Present in oral cavity, pharynx, esophagus, anal canal,

uterine cervix, and vagina.

- Stratified cuboidal: Upper cell layer composed of cuboidal cells. Only 2 to 3 cell layers;

lining large excretory ducts of salivary gland.

- Stratified columnar: The surface cell layer is columnar in shape, and it could be;

- Non-ciliated as in penile part of male urethra, large ducts of glands, recto-anal

junction and fornicies of the conjunctiva.

- Ciliated as in fetal esophagus, nasal surface of the soft palate, and the laryngeal

surface of the epiglottis.

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- Transitional: urinary tract; accommodates stretching and toxicity of urine; surface cells

larger, pale-staining, scalloped surface outline; luminal surface appears thickened; may

be binucleate; large, round, prominent nucleoli.

Illustration showing types of the surface epithelium

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Glandular epithelium (Parenchymous epithelium)

Generally formed by down growth of surface epithelium into underlying connective tissue,

and separated from connective tissue by basal lamina.

Illustration showing development of glandular epithelium

Classification of glands:

1- According to presence or absence of ducts: exocrine glands (have duct system) and

endocrine glands (ductless) secrete hormones glands.

2- According to Nature of secretion:

- Serous secretion: secret watery fluid rich in protein (parotid glands)

- Mucous secretion: secret mucus; poor in protein (goblet cells)

- Muco-serus secretion: as in mixed salivary glands

- Milky secretion: mammary gland

- Wax secretion: glands in external ear

- Fatty secretion: sebaceous glands

- Watery secretion: sweat glands

- Cellular secretion: ovary and testis.

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3- According to mode of secretion:

- Apocrine glands: a small portion of the apical cytoplasm discharged with

the secretory products. eg. Mammary glands and some sweat glands.

- Holocrine glands: discharge who cell; sebaceous glands (sebum).

- Merocrine glands: in which secretion occurs by exocytosis; i.e. no cellular

changes as parotid glands.

4- According to Number of cells:

- Unicellular glands (goblet cells) secrete mucus.

- Multicellular glands that can be further classified according to the shape of secretory

portion into:

Diagrammatic illustration of the types of glandular epithelium

A- Simple tubular gland: large intestine.

B- Simple coiled tubular gland: sweat glands.

C- Simple acinar (alveolar) gland: (rounded secretory unit) mucus-secreting glands of

penile urethra

D- Simple branched tubular gland: stomach.

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E- Simple branched acinar gland: sebaceous gland

F- Simple branched tubulo-alveolar glands: glands of oral cavity.

G- Compound tubular gland: liver, kidney

H- Compound acinar (alveolar) gland: mammary gland

I- Compound tubulo-acinar gland: pancreas

Striated ducts: striations due to mitochondria lined up along folds of basal membrane;

transport Na and bicarbonate; cells high cuboidal to columnar

Myoepithelial cells: Many exocrine glands have specialized type of epithelial cells embedded

in basement membrane between basal lamina and the basal part of secretory cells; may aid

secretion of acinus glands

Basement membranes (external lamina) is formed of 3 layers; lamina lucid, lamina densa

and lamina fibroreticularis. The first two layers (are produced by the epithelia)

Neuro-epithelium: special type of epithelium that has sensory functions. It is found in;

- Tongue: taste buds - Nose: olfactory mucosa

- Eye: retina

- Ear: organ of corti, crista ampullaris, macula sacculi, macula utriculi.

VI. Major types of epithelial cells

A. Epithelial Cells Specialized for Transport:

1. Ion-transporting cells. Some epithelial cells are specialized for trans-cellular transport;

ie. they can pump ions across their entire thickness, apex to base. Sheets of such cells form

active barriers that control ion and water concentrations in body compartments. Tight

junctions are often found between the cells and appear to restrict back-flow.

Ion-transporting cells typically have highly infolded basal plasma membranes that

interdigitate with numerous mitochondria. Commonly, the ion pump is specific for sodium

tie, it is Na/K+-ATPase), and chloride ions and water follow the sodium ion flow

passively. Important ion-transporting epithelia are found in the kidney tubules, striated

ducts of the salivary glands, gallbladder, choroid plexus and ciliary body of the eye.

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2. Cells that transport by pinocytosis. Epithelial cells specialized for pinocytosis have

tight junctions and abundant pinocytotic vesicles. The vesicles transport substances across

the cell from the luminal surface to the basal surface or vice versa. The best example is the

endothelial cells lining the blood vessels, where trans-cellular transport is rapid (2-3

minutes).

B. Epithelial Cells Specialized for Absorption:

Specialized absorptive cells lining the digestive tract (especially the small intestine) have

numerous microvilli on their apical surfaces to increase the exposed area. Small nutrient

molecules diffuse into the microvilli, and contraction of the microfilaments shortens the

microvilli, bringing the nutrients into the cytoplasm. Other nutrients are pinocytosed between

microvilli. Absorptive cells with similar specializations occur in the proximal tubules of the

kidney.

C. Epithelial Cells Specialized for Secretion:

1. Protein-secreting cells. Cells that synthesize proteins for segregation and secretion have

abundant basophilic RER, a well-developed Golgi complex, and, frequently, an

accumulation of secretory granules in the cell apex. Proteins secreted by epithelial cells

include the digestive enzymes, produced by pancreatic acinar cells and the chief cells of

the stomach: serum albumin, produced by liver hepatocytes; and protein hormones (eg,

parathyroid hormone, produced by the chief cells of the parathyroid gland).

2. Polypeptide-secreting cells. Secreted polypeptides have fewer amino acids than the

secreted proteins. Polypeptide-secreting cells have a small amount of RER, a supra-nuclear

Golgi complex, and an accumulation of secretory granules in their bases.

3. Mucous cells occur as unicellular, sheet, or solid glands. Histologic features include a

light staining, foamy appearance caused by numerous large mucus-containing vesicles

concentrated near the cell apex. PAS-positive staining from an abundance of

oligosaccharide residues, predominantly acidophilic staining with H&E; a large

supranuclear Golgi complex and sparse RER in the base of the cell.

4. Serous cells have characteristics of protein-secreting cells. They are usually smaller,

darker staining, and more basophilic than mucus-secreting cells. Serous cells include

pancreatic acinar cells and secretory cells of the parotid salivary glands.

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5. Steroid-secreting cells. Endocrine cells specialized to secrete steroid hormones are

polygonal or rounded, with a central nucleus and pale-staining, acidophilic cytoplasm that

often contains numerous lipid droplets. Their abundant SER contains enzymes for

cholesterol synthesis and for converting steroid hormone precursors (eg, progesterone) into

specific hormones leg, androgens, estrogens, and progesterone). Their mitochondria

typically have tubular cristae and contain enzymes that convert cholesterol to progesterone.

Steroid hormones include testosterone, produced by interstitial cells of the testes: estrogen,

from follicle cells of the ovaries; progesterone, from granulosa lutein cells of the corpus

luteum; and cortisone and aldosterone, from cells of the adrenal cortex.