Lecture 7
GenMed_2nd semester
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
Covering epithelia /epithelial membranes/
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 substance
in 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
interdigitations
and intercellular junctions
spot 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 flattened
viewed from the surface they appear as fairly large cells with clear cytoplasm and an oval
nucleus; cell boundaries are not visible in ordinary preparations but may be demonstrated by
the use of impregnation technique
in sections the cytoplasm is barely visible, but there is an enlargement of the
cell 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 functions
there 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 from
the mesenchyme
Simple cuboidal epithelium
cells of cuboidal epithelium are smaller in surface view than those of the simple
squamous type and are roughly hexagonal
cell boundaries are often clearly visible
in vertical sections the cells are square with a spherical nucleus in the center of each
the 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 inner
surface of the lens, the excretory ducts of many glands
Simple columnar epithelium
the 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 epithelium
all cells are in contact with the basement membrane but only a part of
them extends to the free surface
in 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 ciliated
goblet cells producing mucous
secretion are usually present
in this epithelium
chief distribution:
the respiratory passages
Stratified epithelia
small cuboidal or columnar cells lie next to the basement membrane
above them one or more layers of polygonal cells are
the 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 body
the 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 surface
they 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 scales
the 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 stratified
chief 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 cells
the basal cells are like those of stratified columnar epithelium
above them there is a varying number of layers of polygonal or pear-shaped cells, the cell
layer at the free surface is composed of large, rather flattened cells, usually described as
dome shaped
one 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 other
when 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 surface
cells)
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 - secretions
substances 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 secretion
secretory process involves 3 phases as follows:
ingestion of single substances or precursors from the surrounding or blood
synthesis of secretory products including their segregation and accumulation
release 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 reticulum
synthesized 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 cell
synthetic processes need an energy - is produced by mitochondria
all the mentioned cell organelles are typical of glandular cells
release or extrusion of secretory
products
three modes have been distinguished:
- merocrine secretion
- apocrine secretion
- holocrine secretion
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
thyroid
apocrine 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 glands
glands 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 vessels
have no ducts (ductless glands)
secretions are called hormones and regulate and coordinate the activities of all the
cells 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 glands
or glands of external secretion retain connections with surfaces
goblet cells
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 columnar
cells of the simple columnar or
pseudostratified columnar epithelia
cells 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 Langerhans
produce catechol amines or peptide hormones; for visualization of endocrine cells were formerly used
impregnation techniques, recently is used TEM or immunohistochemistry
multicellular glands
intraepithelial glands
are relative rare and are composed of small accumulations of glandular cells that lie wholly
within the thickness of an epithelial membrane
chief 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 deeper
glnds develop as invaginations of the epithelial membrane into the underlying tissues
the extraepithelial glands consist of secretory units and ducts
secretory 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 glands
or
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 glands
besides the anatomical classification of glands
it 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 cells
the 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 centrally
secretion granules are more or less acidophilic and are primarily protein in character
the cytoplasm always stains in pink or pinkish purple, more darkly than that of
mucous cells
the 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
cells
a 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 fashion
these serous cells collections are called as demilune complexes or lunulae of
Giannuzzi
in 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 epithelium
cells 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 epithelium
sensory organs
sensory areas (fields)
supporting cells
sensory 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 epithelia
cells have spindle-like or flask-like form and consist of only receptor area and body
the 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 cells
hair cells (utricle)