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
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 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
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
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
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