Epithelial Tissue pdf - Histology

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Epithelial ,Connective ,Muscular &

four basic types of tissue

only

Human body composed of

Nervous Tissues. These tissues, which are formed by cells and molecules of the extracellular

matrix, exist not as isolated units but rather in association with one another and in variable

proportions, forming different organs and systems of the body.

Table 4–1. Main Characteristics of the Four Basic Types of Tissues.

Tissue

Cells

Extracellular Matrix

Main Functions

Nervous

Intertwining elongated
processes

None

Transmission of nervous
impulses

Epithelial

Aggregated polyhedral
cells

Very small amount

Lining of surface or body
cavities, glandular secretion

Muscle

Elongated contractile
cells

Moderate amount

Movement

Connective

Several types of fixed
and wandering cells

Abundant amount

Support and protection

 Most organs can be divided into two components: parenchyma, which is composed of the cells
responsible for the main functions typical of the organ, and stroma, which is the supporting tissue.

Except in the brain and spinal cord, the stroma is made of C.T.
 The principal functions of epithelial tissues are the covering and lining of surfaces (eg, skin,
intestines), absorption (e.g., intestines), secretion (e.g., glands), sensation (e.g gustative and

olfactory neuroepithelium), & contractility (e.g., myoepithelial cells), everything that enters or

leaves the body must cross an epithelial sheet.

The Forms & Characteristics of Epithelial Cells

The forms and dimensions of epithelial cells range from high columnar to cuboidal to low

squamous cells. Epithelial cell nuclei have distinctive shapes, varying from spherical to elongated

or elliptic. The form of the nuclei of epithelial cells corresponds roughly to the cell shape; thus,

cuboidal cells have spherical nuclei and squamous cells have flattened nuclei. The long axis of the

nucleus is always parallel to the main axis of the cell.

Almost all epithelial cells, whether lining a surface or forming gland units, rest on a connective

tissue. In the case of epithelia that line the cavity of internal organs (especially the digestive,

respiratory, and urinary systems) this layer of connective tissue is often called lamina propria. The

lamina propria not only serves to support the epithelium but also provides nutrition and binds it to

neighboring structures. The area of contact between epithelium and lamina propria is increased by

irregularities in the connective tissue surface in the form of small evaginations

called papillae.

Papillae occur most frequently in epithelial tissues subject to stress, such as the skin and the tongue

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Basal Lamina & Basement Membrane

extracellular material by which Most epithelial cells are separated from

a sheet of

basal lamina

the connective tissue.This structure is visible only with the E.M. , where it appears as a dense

layer, 20–100 nm thick, consisting of a delicate network of very fine fibrils (lamina densa)

(Figure 4–2A). In addition, basal laminae may have an electron-lucent layer on one or both sides

of the lamina densa, called lamina rara or lamina lucida. Between cell layers without

intervening connective tissue, such as in lung alveoli and in the renal glomerulus (Figure 4–1),

the basal lamina is thicker as a result of fusion of the basal laminae of each epithelial cell layer.

connective tissue junction showing the basal

–

Section of skin at the epithelial

—

Figure 4

lamina (BL) and hemidesmosomes (arrows). The basal lamina together with part of the reticular

lamina (to the right of the basal lamina in this micrograph) forms a typical basement membrane

that can be seen with the light microscope.

Section of human skin showing hemidesmosomes (H), a basal lamina (BL), and anchoring

fibrils (arrows) that apparently insert into the basal lamina. The characteristic spacing of these

fibrils distinguishes them from the more common type I collagen fibrils.)

Basal laminae are found not only in epithelial tissues but also where other cell types come into

contact with connective tissue such as around muscle, adipose, and Schwann cells of nervous tissue.

Fig 4-1 :

A basal lamina always lies at the interface

between epithelial cells and connective tissue.
Two basal laminas can fuse in places where no
intervening connective tissue is present.

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The main components of basal laminae are type IV collagen, the glycoproteins laminin and

entactin, and proteoglycans (eg, the heparan sulfate proteoglycan called perlecan). The precise

molecular composition of these components varies between and within tissues. Basal laminae are

attached to the underlying connective tissues by anchoring fibrils formed by type VII collagen

(Figure 4–2B). These components are secreted by epithelial, muscle, adipose, and Schwann cells. In

some instances, reticular fibers are closely associated with the basal lamina, forming the reticular

lamina (Figure 4–2A). Connective tissue cells produce the reticular fibers.

Basal laminae have many functions. In addition to simple structural functions supporting the cells,

they provide a barrier that limits or regulates the exchange of macromolecules between connective

tissue and cells of other tissues. The basal lamina is also able to influence cell polarity, regulate cell

proliferation and differentiation by binding with growth factors, influence cell metabolism, and

serve as pathways for cell migration. The basal lamina seems to contain the information necessary

for certain cell-to-cell interactions, such as the reinnervation of denervated muscle cells. The

presence of the basal lamina around a muscle cell is necessary for the establishment of new

neuromuscular junctions.

basement membrane : This term is used to specify a periodic acid–Schiff (PAS)-positive layer,

visible with the light microscope, present beneath some epithelia. Figure 4–3 shows basement

membranes in a renal glomerulus and in renal tubules. The basement membrane is usually formed

by the association of either two basal laminae (Figure 4–1) or a basal lamina and a reticular lamina

(Figure 4–2A) and is therefore thicker.

Adhesion & Intecellular junction

Intercellular

Several membrane-associated structures contribute to cohesion and communication between cells.

They are present in most tissues but are prominent in epithelia, which is why they are described in

this chapter. Epithelial cells are extremely cohesive, and relatively strong mechanical forces are

necessary to separate them. Intercellular adhesion is especially marked in epithelial tissues that are

subjected to traction and pressure (eg, the skin). Adhesion is due in part to the binding action of a

family of transmembrane glycoproteins called cadherins. Cadherins lose their adhesive properties in

the absence of Ca

. Interdigitations between folds of membranes of neighbor cells also help to

increase intercellular adhesion (Figure 4-4)

Figure 4-3. Kidney section showing type IV collagen
of the glomerular and tubular basement membranes
(arrows). In the glomeruli the basement membrane,
besides having a supporting function, has an
important role as a filter. Picrosirius– hematoxylin
(PSH) stain. Medium magnification.

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1-Tight junction or Zonnula occludens
2- Zonula adherens
3- Gap or communicating junction
4- Desmosome or macula adherens
5- Hemidesmosomes (half desmosome)

the cell surface

Specialization of

Microvili

cytoplasm projections in absorptive cells lining epithelium of small intestine & cell of the

proximal renal tubule.

The complex of

In these absorptive cells the glycocalyx is thicker than it is in most other cells.

en with the light microscope and is called the brush, or striated,

microvilli and glycocalyx may be se

linked to each other and to

. Within the microvilli are clusters of actin filaments that are cross

border

the surrounding plasma membrane by several other proteins.

2. Stereocilia

Stereocilia are long, nonmotile extensions of cells of the epididymis and ductus

deferens that are actually long and branched microvilli and should not be confused with true cilia.

Stereocilia increase the cell surface area, facilitating the movement of molecules into and out of the

cell.

Figur4-4. The main structures that participate in
cohesion among epithelial cells. The drawing shows 3
cells from the intestinal epithelium. The cell in the
middle was emptied of its contents to show the inner
surface of its membrane. The zonula occludens and
zonula adherens form a continuous ribbon around the
cell apex, whereas the desmosomes and gap junctions
make spotlike plaques. Multiple ridges form the
zonula occludens, where the outer laminae of apposed
membranes fuse.

Figure 4—5. Electron micrograph of a section of
epithelial cells in the large intestine showing a
junctional complex with its zonula occludens
(ZO), zonula adherens (ZA), and desmosome
(D). Also shown is a microvillus (MV). x80,000.

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Cilia and Flagella

Figure 4—8. Apical region of an
intestinal epithelial cell seen with
transmission electron microscopy. The
terminal web is a network that contains
mainly actin filaments. Filaments that
constitute the core of the microvilli are
clearly seen. An extracellular cell coat
(glycocalyx) is bound to the
plasmalemma of the microvilli. x45,000.

Figure 4—9. Electron micrograph of a section from
the apical region of a cell from the intestinal lining
showing cross-sectioned microvilli. In their interiors,
note the microfilaments in a cross section. The
surrounding unit membrane can be clearly discerned
and is covered by a layer of glycocalyx, or cell coat.
x100,000.

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Glandular ep.

Covering ep.

Types of epithelia

Covering epithelia

Table 4–2. Common types of covering epithelia in the human body.

Number of Cell
Layers

Cell Form

Examples of
Distribution

Main Function

Simple (one layer)

Squamous

Lining of vessels
(endothelium).
Serous lining of
cavities; pericardium,
pleura, peritoneum
(mesothelium).

Facilitates the movement
of the viscera
(mesothelium), active
transport by pinocytosis
(mesothelium and
endothelium), secretion of
biologically active
molecules (mesothelium).

Cuboidal

Covering the ovary,
thyroid.

Covering, secretion.

Columnar

Lining of intestine,
gallbladder.

Protection, lubrication,
absorption, secretion.

Pseudostratified (layers
of cells with nuclei at
different levels; not all
cells reach surface but
all adhere to basal
lamina)

Lining of trachea,
bronchi, nasal cavity.

Protection, secretion; cilia-
mediated transport of
particles trapped in mucus
out of the air passages.

Stratified (two or more
layers)

Squamous
keratinized
(dry)

Epidermis.

Protection; prevents water
loss.

Squamous
nonkeratinized
(moist)

Mouth, esophagus,
larynx, vagina, anal
canal.

Protection, secretion;
prevents water loss.

Cuboidal

Sweat glands,
developing ovarian
follicles.

Protection, secretion.

Transitional

Bladder, ureters,
renal calyces.

Protection, distensibility.

Columnar

Conjunctiva.

Protection.

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The two other types of epithelium

Epithelial origin with specialized sensory function ( e.g. Cell of taste buds

cell:

Neuroepithelial

& cell of olfactory mucosa)

sin & actins filament specialized for

Are branched that contain myo

cells:

Myoepithelial

They are

mainly of secretory units of the mammary , sweat & salivary gland

contraction,

located between the basal lamina &the basal surface of secretory or ductal cells .They have

intermediate filaments .They embrace gland acini to help propel secretory products toward the

exterior.

2. Glandular epithelia

which is molecules stored in

secretion ,

Formed by cell specialized to produce

Glandular ep.:

small membrane – bound vesicles (secretory granules)

Glandular ep. Cell

1- Synthesize proteins

2- Store

3- Secret a- protein (pancreas) b- Lipid ( adrenal , sebaceous gland )

c- Carbohydrates & proteins (salivary glands )

 The mammary glands secret all a , b , & c

 Sweat glands ; have low synthesizing activity

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Types of Glandular Epithelia

 According to number of cell

1- Unicellular glands : Goblet cell of lining of small intestin

2- Multicellular glands a- Exocrine b- Endocrine

 Exocrine glands have a secretory

portion & Duct

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 According to how the secretory products leave the cell

) by Exocytosis

the pancreas

( e.g.

Merocrine

no loss of cellular material

) the

sebaceous glands

( e.g.

Holocrine

product of secretion is shed with the whole cell

) the

mammary glands

(e.g.

Apocrine

secretory product is discharged to gather with

parts of apical cytoplasm's.

 Some organs have both endocrine and exocrine

functions _e.g. , in the liver.

 In other organs, some cells are specialized in

exocrine secretion and others are endocrine ;

, the acinar cells secret digestive

pancreas

enzymes into the intestinal lumen , whereas

the islet cells secrete insulin and glucagon into

bloodstream.

General Biology of Epithelial Tissues

Polarity

 In many types of epithelial cells the distribution of organelles and membrane proteins is

different when comparing the basal and apical poles of the cell. This differential and stable

polarity

organization of cell components is called

 Because blood vessels do not normally penetrate an epithelium, all nutrients must pass out of the

capillaries in the underlying lamina propria. These nutrients diffuse across the basal lamina and are

taken up through the basal and lateral surfaces (basolateral surface) of the cell, usually by an

energy-dependent process.

membrane

the apical cell membrane may contain, as integral

In absorptive epithelial cells,



proteins, enzymes such as disaccharidases and peptidases, which complete the digestion of

molecules to be absorbed.

They are the sites where epithelial tissues receive a rich supply of sensory

Innervations:

nerve ending from nerve plexuses in the lamina propria ( e.g.. Sensitivity of cornea , the epi.

Covering the anterior surface of eye , is due to the great of sensory nerve fibers that ramify

between corneal epithelial cells

They are renewed continuously by mitotic activity . the renewal

Renewal of Epithelial Cells

rate can be fast in tissues (eg,.the intestinal ep.) or slow as in liver & pancreas . In stratified &

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pseudo stratified ep. , mitosis occurs within the germinal layer ,closest to the basal lamina ,

stem cells

which contains the

It is a reversible process which lead one type of ep. Tissue undergo

Meta plasia :

transformation into another type under abnormal conditions such as:

A. In heavy cigarette smoker ,the ciliated pseudo stratified ep. Lining the bronchi can be

transformed into stratified squamous epi.

B- In individuals with chronic vitamin A deficiency , epi. T. of bronchi & urinary bladder

,replaced by stratified squamous epithelium

Control of glandular activity

Both of Neural & Endocrine control the activity of the glands Ex.

Exocrine secretion of pancreas depend on stimulation by the hormones

holecystokinin

2. Salivary glands are principally under neural control .Both controls occur through the

action of ( Chemical messenger )

Cell that transport ions

 All cells have the ability to transport ions against a concentration & electrical –potential

gradient through ACTIVE TRANSPORT

In mammals , sodium concentration in extracellular fluid is 140 mm/ L. whereas the

intracellular concentration is 5-15 m mol / L The cell uses the energy stored in ATP to actively

extrude Na by mean of Mg activated Na / K –ATP ( SODIUM PUMP), thereby maintaining the

required low intracellular sodium concentration .

 Some cells transfer ions & fluid across epithelium ,from apex to its base or from its base to

its apex in (Tran cellular transport) .which involved absorption & secretion.

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Serous cells Ex. Acinar cells of Pancreas & Parotid salivary

glands

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secreting cells

Mucus

The most thoroughly studied mucus- secreting cell is goblet cell of intestines .the cell has

glycoprotein called mucins .The proteins are synthesized in the cell base where most R.E.R. is

located . When mucins are released from the cells , they become highly hydrated and form mucus

,viscous, elastic, protective lubricating gel. Other cells that synthesize mucins are found in salivary

glands, respiratory tract .Many of these mucous cells are organized as tubules .

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secreting cells

–

Steroid

The cells that secrete steroid are found in( testes, ovaries, adrenals They are endocrine cells

specialized for synthesizing and secreting steroids with hormonal activity ,and have following

characteristics

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Medical application

Both benign and malignant tumors can arise from most types of epithelial cells. A carcinoma (Gr.

karkinos, cancer, + oma, tumor) is a malignant tumor of epithelial cell origin. Malignant tumors

derived from glandular epithelial tissue are usually called adenocarcinomas (Gr. adenos, gland, +

karkinos); these are by far the most common tumors in adults. In children up to age 10 years, most

tumors develop (in decreasing order) from hematopoietic organs, nerve tissues, connective tissues,

and epithelial tissues. This proportion gradually changes, and after age 45 years,