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Dr. Ahmed M. Almosaui
Humans are composed of billions of cells. A tissue is a collection of cells
with similar structural characteristics. Organs are groups of tissues. Most organs
are complex groupings of different tissue types. An organism is composed of
organs that are grouped together and functionally integrated.
The four types of human tissue
1. Epithelium. Epithelial cells form sheets of cells that are tightly joined to one
another. These cells cover surfaces, line cavities, and form the secretory
portions of many glandular structures, which are complex imaginations of the
body surface.
2. Connective tissues join epithelial structures to other parts of the body. They
exist under all epithelial layers and have a cellular component and an
extracellular matrix. Bone, cartilage, and blood are specialized connective
3. Muscle tissue
a- Skeletal muscle tissue is found in the gross muscles that cause skeletal
b- Cardiac muscle tissue is found in the heart wall and proximal portions of
the aorta.
c- Smooth muscle tissue is a prominent component of blood vessel walls and
visceral organs such as those of the gastrointestinal, urinary, and reproductive
4. Nerve tissue. The brain, spinal cord, autonomic ganglia, peripheral nerves,
and portions of sensory organs are composed of nerve tissue.
Tissue derivation.
The three primary germ layers in embryology are ectoderm (the outer germ layer),
mesoderm (the middle germ layer), and endoderm (the inner germ layer).
1. Epithelium is derived from each of these layers.
A. Ectoderm forms epidermis.
B. Mesoderm forms mesothelium.
Dr. Ahmed M. Almosaui
C. Endoderm forms the lining of the gastrointestinal tract.
2. Most connective tissue and muscle tissue is derived from mesoderm.
3. Most nerve tissue is derived from ectoderm.
Epithelial Tissue
Epithelia are specialized layers that line the internal and cover the external surfaces
of the body. An epithelium consists of a sheet of cells lying close together with
little intercellular space. These cells have strong adhesion and form cellular sheets
that cover the surface of the body and line its cavities.
General classifications
1. Epithelia are classified by the number of cell layers they contain.
a- Simple epithelia have one cell layer. All cells rest on the basement membrane
and reach the apical surface.
b- Stratified epithelia have more than one cell layer, consequently not all cells
rest on the basement membrane or reach the apical surface.
c- Pseudostratified epithelia are simple epithelia that appear to be stratified. In
these epithelia, all cells rest on the basement membrane; however, not all cells
reach the apical surface. The stratified appearance occurs because nuclei lie at
different levels in the epithelia.
2. Epithelia are further classified by cell shape.
a-Squamous cells are flat.
b-Cuboidal cells have approximately equal height and width.
c-Columnar cells are taller than they are wide.
Epithelial Tissue Types :
* Simple squamous epithelium is a single layer of flat cells.
Dr. Ahmed M. Almosaui
1- Simple squamous epithelium coats, or partially coats, the stomach, liver,
gallbladder, and other visceral organs. It also is found in respiratory system
alveoli and in the thin limbs of the loop of Henle in the kidney.
2- Mesothelium is the simple squamous epithelium that lines serous cavities
(peritoneal, pleural, and pericardial cavities) and coats many of the organs in
these cavities.
3- Endothelium is the simple squamous epithelium that lines the lumen of the
cardiovascular system.
* Simple cuboidal epithelium is a single layer of cells that are equal in height and
width. It typically exists in areas where ion transport occurs (e.g., in kidney
tubules, sweat glands, and some glandular ducts). It also covers the choroid
plexus the four clusters of capillaries in the walls of the ventricles of the brain
that help produce cerebrospinal fluid (CSF).
Simple cuboidal epithelium is abundant in the kidney, in the proximal and distal
tubules and parts of the collecting-duct system. Often, cuboidal epithelial cells
have many apical microvilli and mitochondria, which facilitate ion pumping and
fluid transport.
* Simple columnar epithelium is a single layer of tall cells and is present in areas
where absorption occurs.
1-Simple columnar epithelium composed of absorptive cells and goblet cells
covers the lining of the small and large intestines .
2- It is found in some glandular ducts and the gallbladder, and it lines papillary
collecting ducts in the urinary system.
3- Uterine epithelium is a simple columnar epithelium.
* Pseudostratified columnar epithelium is a layer of cells, in which all of the
cells rest on the basement membrane but only some extend to the apical surface
of the epithelium.
1- Cell height varies in pseudostratified epithelium, as does the position of the
nuclei with respect to the apical and basal limits of the epithelium. The apparent
layering of nuclei makes the epithelium appear stratified.
Dr. Ahmed M. Almosaui
2- Pseudostratified columnar epithelium exists in many sections of the respiratory
system, such as the nasal cavities, nasopharynx, trachea, and bronchi .
3. Pseudostratified columnar epithelium is abundant in the male reproductive
system. It is the luminal epithelium of the epididymis, vas deferens, prostate
gland, seminal vesicles, and the prostatic urethra.
* Stratified squamous epithelium contains many layers of cells, including an
apical layer of flat cells. These epithelia are classified according to the
characteristics of the apical layer.
1- Stratified squamous epithelium exists in locations subjected to chronic abrasion,
The apical layers are continuously sloughed and then replaced by cell division in
the basal layer.
2- The body is covered by a stratified squamous epithelium called epidermis. This
epithelium is keratinized because cells in the outer layer contain the protein
3- The digestive system contains some stratified squamous epithelium. Stratified
squamous epithelium on the tongue and lining the esophagus resists the abrasion
of mastication, swallowing, and the passage of food from the oral cavity to the
stomach. Stratified squamous epithelium in the anal canal resists the abrasion of
passing semi-solid feces.
4- In the female reproductive system, stratified squamous epithelium is abundant
in the vagina and covering the cervix
* Transitional epithelium is a stratified epithelium found exclusively in the
urinary passages of the urinary system. It contains many layers of polyhedral
cells and an outer apical layer of round pillow-shaped cells. It is found in the
minor calyces, major calyces, renal pelvis, ureters, urinary bladder, and p roximal
urethra. Transitional epithelium in the bladder undergoes a reversible
morphologic change during bladder distension and evacuation .
Dr. Ahmed M. Almosaui
Most epithelia rest on connective tissue. In the case of epithelia lining the cavity of
internal organs (especially in the digestive, respiratory, and urinary systems) this layer
of connective tissue is often called the lamina propria. The lamina propria not only
serves to support the epithelium but also provides nutrition and binds it to underlying
Function of Epithelial Tissue
1. Transcellular transport of molecules from one epithelial surface to another
2. Absorption occurs via endocytosis or pinocytosis in various organs (e.g., the
proximal convoluted tubule of the kidney).
Dr. Ahmed M. Almosaui
3. Secretion of various molecules (e.g., hormones, mucus, proteins) occurs by
4. Protection from abrasion and injury is provided by the epidermis, the epithelial
layer of the skin.
Apical protrusions : The epithelial apex often contains protrusions from the cell
surface. The protrusions may be scattered, as in mesothelium and endothelium, and
may take the form of microvilli.
a. Microvilli are cylindrical, cell-surface projections, 80 nm wide and 1-2 .m long, which
increase the cell surface area for absorbing materials from the lumen.
b. Stereocilia are long microvilli present in the male reproductive tract and in the
membranous labyrinth of the inner ear. They are similar to microvilli except that
they are longer and constricted at the point where they join the cell apex. little is
known about their movements in vivo. Sensory stereocilia contain many actin containing microfilaments and large amounts of myosin.
c. Cilia. Many epithelia apices are ciliated in some locations (see Chapter 5 III A
for information about ciliary ultrastructure).
1- In the trachea, certain columnar epithelial cells have many apical cilia that move
mucus along the apical surface of the tracheal lumen (airway).
2- Epithelial cells in the uterine tubes have apical cilia that help move the fertilized
ova into the reproductive tract.
1. Basement membranes
a- Most epithelia have a basement membrane (lymphatic capillaries are a
notable exception), which ranges from scant to thick, depending on the
b- Some basement membranes have an amorphous basement lamina adjacent to
the epithelium and a fibrous reticular lamina below the basement lamina.
c- The fine structure of the basement membrane varies considerably with the
location and function of the epithelia.
Dr. Ahmed M. Almosaui
Basement membrane functions
1-The basement membrane is the basal limit of an epithelium. In many luminal
epithelia, the basement membrane is the boundary between the epithelium and
subjacent connective tissue.
2-The basement membrane anchors the epithelium and may be a substratum for
epithelial cell and connective tissue cell attachment.
3-The basement membrane maintains the shape of acini and branched ducts
and tubules.
4-In some instances, the basement membrane is a selective barrier .
Dr. Ahmed M. Almosaui
Dr. Ahmed M. Almosaui
Glandular Tissue
Two types of glands exist: endocrine glands and exocrine glands. Both types of glands
are composed of and properly classified as epithelial tissues.
A. Endocrine glands
Endocrine glands secrete products into the bloodstream. They are derived from
surface epithelium during development; however, they soon lose their
connection to the body surface.
Endocrine tissue retains many epithelial characteristics. For example, thyroid
follicular epithelial cells form sheets, are surrounded by a basement membrane,
secrete hormone precursors apically, exhibit apical-basal polarity, and contain
junctional complexes.
B. Exocrine glands
Exocrine glands secrete products onto the body surface through ducts. The skin and
digestive tract, which are continuous with each other, receive exocrine secretions
from glands such as sweat glands (skin) and the liver and pancreas (digestive
Exocrine gland classification. Exocrine glands are classified in several ways
according to their mode of cellular secretion and arrangement of cells and ducts.
They can be simple glands or compound glands.
a) Simple tubular glands consist of a simple straight tubule and exist in the small
b) Simple Coiled tubular glands consist of a coiled tubule. Sweat glands are one
example of coiled tubular glands.
c) Simple Branched tubular glands have branches deep in the gland. This type of
gland exists in the stomach and endometrium.
d) Simple tubuloalveolar glands (Simple acinar ) have a single duct leading to a
cluster of alveoli, or acini. Examples of this type of gland include the small
salivary glands in the oral cavity and Brunner's glands in the duodenum.
Dr. Ahmed M. Almosaui
e) Simple alveolar glands (Simple Branched acinar ) have several acini attached to
a single duct. The sebaceous glands of the skin are this type of gland.
f) Compound tubular glands have numerous tubules connected to multiple ducts.
These glands exist in the testes.
g) Compound tubuloalveolar glands (Compound tubuloacinar ) have numerous
secretory acini that drain into numerous efferent ducts. These ducts typically merge
into a smaller number of main ducts. The parotid salivary glands and the pancreas
are examples of this type of gland.
h) Compound alveolar glands (Compound acinar ) are compound glands
because they have numerous draining ducts. However, they terminate in acini that
have flat squamous cells rather than acini with cuboidal or pyramidal cells. The lungs
are compound alveolar glands.
Modes of secretion
1- Merocrine secretion involves the release of membrane-bound packets of secretion
product. The packets are formed as membranes derived from the endoplasmic reticulum
and Golgi apparatus surround the secretion product. The thyroid gland and pancreas
exhibit merocrine secretion.
2- Apocrine secretion products include a portion of the apical cytoplasm from the
secretory cell. Mammary glands exhibit apocrine secretion.
3- Holocrine secretion is characterized by whole cells bursting open to become the
secretory product. Epidermal sebaceous glands exhibit holocrine secretion.
Dr. Ahmed M. Almosaui