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epithelial tissue
• The
epithelial
tissue,
or
epithelium, consists of sheets of
cells that cover the external
surfaces of the body, line the
internal cavities and the organs,
form various organs and glands,
and line their ducts.
• Classification of Epithelium
• Epithelium is classified according to the
number of cell layers and the morphology
or structure of the surface cells.
• 1-Simple Epithelium
• 2-Stratified Epithelium
• 1-Simple Epithelium
• A-Simple squamous epithelium
• that covers the external surfaces of the digestive
organs, lungs, and heart is called mesothelium.
Simple squamous epithelium that lines the lumina
of the heart, chambers and all blood and
lymphatic vessels is called endothelium.
• B-Simple cuboidal epithelium
• lines small excretory ducts in different
organs. In the proximal convoluted tubules of
the kidney, the apical surfaces of the simple
cuboidal epithelium are lined with a brush
border consisting of microvilli
• C-Simple columnar epithelium
• covers the digestive organs (stomach, small and
large intestines, and gallbladder). In the small
intestine, simple columnar absorptive cells that
cover the villi also exhibit microvilli.
• In uterine tubes and the uterine cavity of the
female reproductive tract, the simple columnar
epithelium is lined with motile cilia.
• D-Pseudostratified columnar epithelium
• lines the respiratory passages and lumina of
the epididymis and vas deferens. In the
trachea, bronchi, and larger bronchioles, some
surface cells are lined with motile cilia
• 2-Stratified Epithelium
• A-Stratified squamous epithelium
• contains multiple cell layers. The basal cells
are cuboidal to columnar; these cells
produce cells that migrate toward the
surface and become squamous. There are
two types of stratified squamous epithelia:
nonkeratinized and keratinized.
• Nonkeratinized epithelium exhibits live
surface cells and covers moist cavities, such
as the mouth, pharynx, esophagus, vagina,
and anal canal.
• Keratinized epithelium lines the external
surfaces of the body. The surface layers
contain nonliving, keratinized cells that are
filled with the protein keratin. The exposed
epithelium that covers the palms and soles
• B-Stratified cuboidal epithelium
• C-Stratified columnar epithelium
• Both types B and C have a limited distribution in
the body. Both types of epithelia line the larger
excretory ducts of the pancreas, salivary glands,
and sweat glands. In these ducts, the epithelium
exhibits two or more layers of cells
• Transitional epithelium
• lines the minor and major calyces, pelvis,
ureters, and the bladder of the urinary system.
This type of epithelium changes shape and can
resemble either stratified squamous or
stratified cuboidal epithelium, depending on
whether it is stretched or contracted.
Glandular tissue
The body contains a variety of glands.
These glands develop from epithelial cells
that extend from the surface into the
underlying connective tissue. They are
classified as either:
1-Exocrine Glands
2-Endocrine Glands
• Exocrine Glands
• are connected to the surface epithelium by
excretory ducts, into which they secrete
their secretory products that pass to the
external surface.
• Unicellular
• Unicellular glands consist of single cells . The
mucus secreting goblet cells found in the
epithelia of the small and large intestines and
in the respiratory passages.
• Multicellular
• Multicellular glands are characterized by a
secretory portion, an end piece where the
epithelial cells secrete a product, and an
epithelium-lined excretory ductal portion,
through which the secretion from the secretory
regions is delivered to the exterior of the gland.
• Multicellular exocrine glands
• are divided into two major categories depending
on the structure of their ductal portion.
• A simple exocrine gland
• exhibits an unbranched duct, which may be
straight or coiled. Also, if the terminal secretory
portion of the gland is shaped in the form of a
tube, the gland is called a tubular gland.
Unbranched Simple
Tubular Exocrine
Glands: Intestinal
Glands
Simple Branched
Tubular Exocrine
Glands: Gastric
Glands
Coiled Tubular
Exocrine Glands:
Sweat Glands
• compound exocrine gland
• An exocrine gland that shows a repeated
branching pattern of the ducts that drain the
secretory portions.
Compound Acinar
(Exocrine) Gland:
Mammary Glands
Compound Tubuloacinar
(Exocrine) Gland:
Salivary Glands
• Exocrine glands may also be classified on the
secretory portions of the gland
1-Acinar (alveolar) glands
2-Tubular glands
3-Tubuloacinar glands
• Exocrine glands may also be classified
on the basis of the secretory products of
their cells.
• 1-Mucous glands
• Glands with cells that produce a viscous secretion
that lubricates or protects the inner lining of the
organs.
• 2-Serous glands
• Glands with cells that produce watery secretions
that are often rich in enzymes.
• 3-Seromucous glands.
• Certain glands in the body contain a mixture of
both mucous and serous secretory cells.
Serous glands
Seromucous glands
• Exocrine glands may also be classified
according to the method by which their
secretory product is discharged.
• 1- Merocrine glands
• Such as pancreas and sweat glands, release their
secretion by exocytosis without any loss of cellular
components.
Pancreas
Sweat glands
• 2- Holocrine glands
• Such as the sebaceous glands of the skin, the cells themselves
become the secretory product that accumulates in the glands.
Here, gland cells accumulate lipids, die, and degenerate to
become sebum, the secretory product
• 3- Apocrine glands
• Discharges part of the secretory cell as the secretory product.
However, almost all glands that were once classified as apocrine
are now regarded as merocrine glands.
Sebaceous glands
Mammery glands
• Endocrine glands
• Endocrine glands have lost their
connection to the surface epithelium and
their secretory products are delivered
directly into the capillaries of the
connective tissue that surrounds the
circulatory system.
• Endocrine glands differ from exocrine
glands in that they do not have excretory
ducts for their secretory products.
• The endocrine glands can be individual
cells (unicellular glands) as seen in the
digestive organs as enteroendocrine
cells; endocrine tissue in mixed glands
(both endocrine and exocrine) as seen
in the pancreas and male and female
reproductive organs; or separate
endocrine organs as the pituitary
gland, thyroid glands, parathyroid
glands, and adrenal glands.
Endocrine and Exocrine Pancreas