Lecture 7Lecture 7 GenMed_2nd GenMed_2nd semestersemester

Epithelial tissue – definition, classification and histogenesis

Overview of covering and glandular epithelia. Characteristics of glandular cells

Absorptive, respiratory, and sensory epithelia

Epithelial tissue

is composed of cells that are in close apposition with one another; among cells only a small amount of intercellular substance is present

epithelial cells are usually of regular form without extensive cytoplasmic processes

adhesion between cells is very strong

epithelia derive from the all germ layers

epithelial tissue is avascular - it contains no blood capillaries

it exhibits a remarkable degree of physiologic regeneration

Classification of the epithelial tissue

a) according to the arrangement of cells -2 forms- epithelial membranes either sheets - composed of one or more cell layers in

thickness or - solid cords or tubules (rarely follicles) that have developed as out- growths

from an epithelial sheet

cords and tubulus especially occur in glands

b) according to the function of cells

covering or protective - cells cover external and internal surfaces of human body and protect underlying tissues against loss moisture and mechanical damage,secretory or glandular - cells are engaged in synthetic processes and product substances with defined functional destination,absorptive - cells transport substances from the alimentary canal and renal tubules into the systemic circulation,respiratory - cells take part in the transport of oxygen and carbon dioxide from alveoli into the blood,sensory - cells are specialized for reception of stimuli (olfactory epithelium, taste buds etc.)

a given epithelium may serve one or more functions, depending upon the cell types presented,eg. protective and secretory functions of the epithelium in the stomach or protective and

absorptive

CoveringCovering epithelia /eepithelia /epithelial membranespithelial membranes//

common features:common features:cells always rest upon a basement membrane it attaches them to underlying connective tissue

the basemenet membrane is thin and composed of delicate reticular fibers that are

embedded in a gel-like mucopolysaccharide substancein ordinary staining procedures (eg. using hematoxyline and eosin), the

membrane is not visible but it stains with PAS technique

epithelial cells are mostly polarized

lateral and apical aspects of cells are subjects of various modifications: microvilli, motile cilia

interdigitationsand intercellular junctionsspot and belt desmosome, gap junctions and occluding junctions)

two types:

simple and stratified epithelia

the simple epithelia consist of single layer of cells, all of which are in

contact with the basement membrane

the stratified epithelia are composed of two or more layers of cells, of

which only basal is in contact with the basement membrane and remainder

layer superimposed one upon other

the both are further subdivided according to the shape of cells:

the component cells may be

squamous - the width and depth of the cell is greater than its height;

cuboidal - the width, depth and height are approximately the same; and

columnar - the height of the cell appreciably exceeds the width and depth

the classification based on both criteria is:

Simple epithelia: - simple squamous epithelium - simple cuboidal epithelium - simple columnar epithelium - pseudostratified columnar epithelium

Stratified epithelia: - stratified squamous epithelium - stratified columnar epithelium- transitional epithelium

Simple squamous epithelium

cells are flattenedviewed from the surface they appear as fairly large cells with clear cytoplasm and an

ovalnucleus; cell boundaries are not visible in ordinary preparations but may be

demonstrated by the use of impregnation techniquein sections the cytoplasm is barely visible, but there is an enlargement of thecell at center where the nucleus is located

chief distribution: the parietal layer of capsule of Bowman and descending limb of the loop of Henle in the kidney inner surface of the wall of the membranous labyrinth and inner surface of the tympanic membrane the smallest excretory ducts of many glands

in two locations, simple squamous epithelium is modified and has special functionsthere is a mesothelium - lines serous cavities and is deriving from the mesoderm and the endothelium - lines the lumina of blood and lymph vessels and arises fromthe mesenchyme

Simple cuboidal epitheliumcells of cuboidal epithelium are smaller in surface view than those of the simple squamous type and are roughly hexagonalcell boundaries are often clearly visiblein vertical sections the cells are square with a spherical nucleus in the center of

eachthe square shape is modified to that of truncated prism when cells line small ducts

chief distribution: free surface of the ovary, the choroid plexus surface, the innersurface of the lens, the excretory ducts of many glands

Simple columnar epitheliumthe surface view of columnar epithelium is like that of the cuboidal epithelium in sections, the cells are seen to be taller than they are broad; their nucleus is near the base of the cell as in the case of the cuboidal epithelium, the rectangular shape of columnar cells

is changed to pyramidal when they are grouped around small lumen

in order to right diagnosis of this epithelium it is important to select a region, in which the section passes through the tissue in a plane perpendicular to the

surface

chief distribution: the stomach, larger excretory ducts, and uterus; in the case of oviduct and small bronchi, apices of some epithelial cells are ciliated

simple columnar epithelium with striated border (microvilli)

a simple columnar ciliated epithelium (with kinocilia)

Pseudostratified columnar epitheliumall cells are in contact with the basement membrane but only a part ofthem extends to the free surfacein pseudostratified epithelium cells are variable in shape and their nuclei lie at

different levels;

according to cell height, three different cell types are distinguished: small basal or supporting cells spindle-shaped cells tall superficial cells that are connected with the basement membrane via slender

processes, apices of tall superficial cells are often ciliatedgoblet cells producing mucous secretion are usually presentin this epithelium

chief distribution: the respiratory passages

Stratified epithelia

small cuboidal or columnar cells lie next to the basement membraneabove them one or more layers of polygonal cells arethe free surface is covered by a layer of cells that are important for diagmosis

Stratified squamous epithelium

the thickness of the epithelium and number of cells vary in different parts of the bodythe shape and arrangement of component cells, however, follow the same general plan:consists of small cuboidal or even columnar cells are are covered by a varying number of layers of more or less polygonal (polyhedral) cells they gradually begin to flatten out toward to the epithelium surfacethey become scale-like form, their nuclei are pyknotic but they may be yet considered as

viable

such epithelium is found in the mouth, the esophagus, the vagina, and on the surface

of the cornea

on the exposed outer surfaces, the stratified squamous epithelium is modified as cornified or

keratinized

the surface cells loose their nuclei and undergo a transformation into dry and dead scalesthe keratinized stratified squamous epithelium covers the skin as a epidermis

stratified squamous epithelium

stratified squamous epithelium (cornified) = epidermis

Stratified columnar epithelium

it differs from the pseudostratified epithelium in having a continuous layer of small, rounded

cells next to the basement membrane, a varying number (3-4) of more or less polyhedral cells

and by presence of superficial cells that are tall and prismatic the epithelium is truly stratifiedchief distribution: the fornix of the conjunctiva, the cavernous part of the male urethra, large excretory ducts of some glands

Transitional epithelium

it consists of several layers of cellsthe basal cells are like those of stratified columnar epitheliumabove them there is a varying number of layers of polygonal or pear-shaped cells, the

celllayer at the free surface is composed of large, rather flattened cells, usually described asdome shapedone of dome shaped cell often covers two or three pear-shaped cells that underlie it

cells show unusual degree the ability to change their position, sliding over each otherwhen an organ is distended, the epithelium is reduced to three or four layers and vice versa (by organ contraction the cells heap up forming several layers between the basal and surfacecells)

chief distribution: it covers the mucosa of the excretory passages from the renal calyces to the

urethra

transitional epithelium

Secretory /glandular/epithelium

the epithelium whose cells are able to synthesize special products - secretionssubstances that are for organism and right function of many organs very

important in many cases they cannot be substituted

secretory products are released either upon free surfaces or into blood (rarely lymphatic) vessels for distribution /hormones/

Histophysiology of secretionsecretory process involves 3 phases as follows:

ingestion of single substances or precursors from the surrounding or bloodsynthesis of secretory products including their segregation and accumulationrelease or extrusion of secretory products

Ingestion of precursors is usually realized by 3 ways:- passive diffusion down a concentration gradient,- active transport against a concentration gradient using ATP as an energy

source,- pinocytosis - the way providing of transport of macromolecules across the

plasma membrane

synthesis of secretory products under cooperation of many cell organelles - free ribosomes or unbounded polyribosomes and the rough endoplasmic reticulumsynthesized products are then transferred from this compartment to the Golgi apparatus, where they become final form and are directed to their final destination in each cellsynthetic processes need an energy - is produced by mitochondriaall the mentioned cell organelles are typical of glandular cells

release or extrusion of secretoryproducts

three modes have been distinguished: - merocrine- merocrine secretion secretion - apocrine- apocrine secretion secretion - - holocrineholocrine secretionsecretion

Release of secretion products

merocrine secretion

products are released through the cell membrane without the loss any cytoplasm (during the whole secretory cycle)

the cell does not change its shape

goblet cells

apocrine secretion

by a loss of the apical parts that contains an accumulated secretory material

glandular cells of follicles in thyroidapocrine sweet glands

holocrine secretion

the discharge of product is associated with a destruction of the whole glandular cells

cells of sebaceous glands

secretory cells constitute special organs called glands

Classification of glandsglands are classified in several different ways:

a) with respect to the mode of secretion:exocrine glands - discharge their product via ducts at an external or internal

surface,

endocrine glands release their product into the blood or lymph vesselshave no ducts (ductless glands)secretions are called hormones and regulate and coordinate the activities of

all thecells in the body in cooperation with the nervous system

b) with respect to the number of glandular cells:unicellular glands - the glandular components consist of a single cell among

other cells that are not glandular

multicellular glands - the glandular components consist of more than one cell

Exocrine glandsor glands of external secretion retain connections with surfaces

exocrine glands occur as uni - and multicellular ones unicellular glands are

- mucous cells or goblet cells and - enteroendocrine (GEP) cells

goblet cells are scattered among the columnarcells of the simple columnar orpseudostratified columnar epitheliacells secrete mucin

each cell has an expanded, oval apex filled with pale droplets of mucigen and a slender basal end containing a compressed nucleus and a small amount of deeply staining basophilic cytoplasm

enteroendocrine (GEP) cells occur in the wall of the stomach, small and large intestine, bile ducts (hepatic, cystic or common

bile) and islets of Langerhansproduce catechol amines or peptide hormones; for visualization of endocrine cells were formerly

used impregnation techniques, recently is used TEM or immunohistochemistry

goblet cells

multicellular glands

intraepithelial glands are relative rare and are composed of small accumulations of glandular cells that lie

whollywithin the thickness of an epithelial membranechief distribution: mucous glands of lacunae urethrales Morgagni in the male

urethra

extraepithelial glands are always in the connective tissue of respective organ or often more deeperglnds develop as invaginations of the epithelial membrane into the underlying tissues

the extraepithelial glands consist of secretory units and ductssecretory units are of three types:

tubules (unit is shaped like a tube)alveoli (acini) (unit is shaped like a flask)tuboalveolar portions (combination of the tube and alveolus which is located at one end of the tube)

ducts may be short and unbranched - simple glandsor ducts are long a branched - compound glands

the shape of secretory units and the length of ducts are used in the anatomical classification of glands as follows

Multicellular glandsMulticellular glands

besides the anatomical classification of glandsit may be taken in account the chemical properties of secreted product

the secretory units are divided into mucous units - the shape of straight tubules with a small lumina

cells are usually of cuboidal form or of truncated pyramids, nuclei are usually small, dark, and are flattened against the plasma membrane of the cellsthe cytoplasm stains light in HE preparation and may have a foamy appearance(alcian blue, mucicarmine, and the PAS reaction are used to demonstrate mucous tubules )

serous units - are the form of alveoli and are composed of pyramidal cells whose nuclei are located centrallysecretion granules are more or less acidophilic and are primarily protein in characterthe cytoplasm always stains in pink or pinkish purple, more darkly than that of mucous cellsthe lumen of alveoli (acini) is definite and and smaller in diameter than that of mucous tubules

seromucous units - mixed units are composed of both mucous and serous cellsa mixed unit is composed of mucous cells lining a tubular part (short tubule) to which one end is added a collection of serous cells arranged in a half-moon fashionthese serous cells collections are called as demilune complexes or lunulae of Giannuzziin a great number they are occurred in the human submandibular gland

acinar / alveoli /

tubuloacinar /tubules with serous demilunes/

tubule with serous demilune = demilune of Gianuzzi

Absorptive epitheliumcells whose apices are provided with microvilli

striated border (enterocytes) brush border (cells of proximal tubule of the nephron)

Respiratory epithelium

is extremely thin and lines alveoli membranous pneumocytes (type I) granular pneumocytes (type II)

Sensory epitheliumsensory organs

sensory areas (fields)supporting cellssensory cells - receipt stimuli from external environment, stimuli induce a special status in their bodies - irritation

sensory cells are of two types:a) primary sensory cells (neuroepithelial cells or unipolar neurons)

they consist of receptor area (can be photosensitive or registrate odoriferous substances), cell body and axon or neurite that conducts impulses to the next neuron

in the human - olfactory cells in the olfactory epithelium and by rod cells and cone cells in the retina

b) secondary sensory cells are only modified cells of covering epitheliacells have spindle-like or flask-like form and consist of only receptor area and bodythe process corresponding to the axon is always missing (signals from cells are conducted via dendritic processes of sensory nerves that endon the surface of these cells)

cells of taste buds, and hair cells in the membranous labyrinth

cone cell rod cell

taste bud with taste cellstaste bud with taste cells

hair cells hair cells (utricle)(utricle)