Download Glandular epithelium

Lecture (2) :
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:
1- The outer layer of the skin.
2- The inner surface of the digestive and respiratory cavities.
3- The inner surface of the heart and blood vessels.
4- The walls and the organs of the closed ventral body cavities.
5- The ducts of the exocrine glands.
Glandular epithelia make up most of the glands in the body.
Surface specializations or modifications : 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.The apical surface of
absorptive cells is usually covered with microvilli, which greatly
increase the apical surface area when extended. Microvilluscovered 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.
Aanother by specialized intercellular junctions. Several
types of junctions can be seen, such as:-
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).
Zonula adherens ( or 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 electrondense 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.
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.
Junctional complex: combination of zonula occludens, zonula
adherens and desmosomes.
Communicating junctions (gap or nexus junctions): is a diskor patch-shaped structure.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.
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.
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.
B. 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.
C. 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.
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 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
Flat: squamous epithelium
Square: cuboidal epithelium
Rectangular: columnar epithelium
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.
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); secretory surfaces (stomach);
lining gall bladder (absorbs water).
- 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.
- 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
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.
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.
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 ICompound 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