Histology
All human structures are composed of just four basic
types of tissue:
Epithelial tissue
Connective tissue
Muscular tissue
Nervous tissue
These tissues, which are formed by cells and molecules of the
extracellular matrix
The main characteristics of these basic types of tissue are shown
in table - A
Tissue
Cells
Nervous
Intertwining
Elongated
Processes
Aggregated
Epithelial
Muscle
Connective
Extracellular
matrix
None
Very small
polyhedral cells
amount
Elongated
contractile cell
Several types of
Moderate
amount
Abundant
fixed and
wondering cells
amount
Main
function
Transmission
of nervous
impulses
Lining of
surface or
body cavities
glandular
secretion
Movement
Support and
protection
Epithelial tissues
Are composed of closely aggregated polyhedral cells with very little
extracellular substance. These cells have strong adhesion due to adhesion
molecules, membrane interdigitation, and intercellular junctions.
These features allow the cells to form cellular sheets that cover the surface
the body and line it cavities or are arranged as three-dimensional secretory
units.
The principal functions of epithelial tissues are
1. Covering and lining of surfaces
e.g. skin
2. Absorption
e.g. intestines
3. Secretion
e.g. glands
4. Sensation
e.g. neuroepithelial cells
5. Contraction
e.g. myoepithelial cells
6. Protection
skin
Basal lamina and basement membrane:
most epi cells are separated from the c.t. by sheet of extracellular material
called basal lamina, this structure is visible only with E.M. where it
appears as a dense layer, consisting of a delicate network of very thin
fibrils lamina densa, in addition, basal lamina may have an electronlucent layer on one or both sides of the lamina densa, called lamina
Lucida.
Between cell layers without intervening C.T. such as in lung alveoli, renal
glomerulus. The basal lamina is thicker as a result of fusion of the basal
lamina of each epithelial cell layer.
The main components of basal lamina are type IV collagen, the
glycoprotein [lamina and entactine] and proteoglycans.
Basal laminae are attached to the underlying C.T. by anchoring
fibrils formed by type VII collagen.
These components are secreted by epithelial, muscle, adipose and
Schwann cells.
Podocyte
Lamina densa
Laminin
Basal lamina
Lamina lucid
Endothelium
Basal lamina
Anchoring fibril
Reticular lamina
Basement membrane
Basement membrane
Is usually formed by the association of either two basal laminae or a basal lamina and
a reticular lamina and is therefore thicker.
This layer visible with L.M. when used to specify a periodic acid-Schiff (PAS)positive layer.
Basal lamina
Types of epithelia
Epithelia are divided into two main groups according to their
structure and function
covering epithelia
Glandular epithelia
In covering epithelia the cells are organized in layers that cover
the external surface or line the cavities of the body.
They can be classified according in the number of cell layers and
the morphological features of the cells in the surface layer.
Simple epithelium contains only one layer of cells stratified
epithelium contains than one layer based on cell shape, simple
epi. can be squamous, cuboidal, columnar, pseudostratified
endothelium: squamous that lines blood and lymph v.
mesothelium: that lines certain body cavities such as the pleural
and peritoneal cavities and covers the viscera.
Psudostratified: the nuclei appear to lie in various layers all the
cells attached to the basal lamina.
Common types of covering epithelia in the human body
Type
Cell form
Example of
Main function
distribution
Simple
Squamous
Lining of vessels
Facilitates the
lining of body
movement of the
cavities. lining
viscera, diffusion
bowman’s capsule
cuboidal
Thyroid, covering the Covering, secretion
ovary, tubules of
kidney
Columnar
Lining of intestine,
Protection,
gall bladder
lubrication,
absorption, secretion
Pseudostratified Some columnar Lining. Trachea,
Protection, secretion
some cuboidal
bronchi, nasal cavity cilia-mediated
transport of particles
trapped in mucus
Stratified
Squamous
Epidermis
Protection, prevent
keratinized
water loss
Squamous
Mouth, esophagus
Protection, secretion
nonkeratinized
vagina, larynx
prevent water loss
Cuboidal
Duct of sweat gland
Protection, secretion
ovarian follicles
Transitional
Bladder, ureter, part
Protection,
of urethra
distensibility
Columnar
Conjunctiva large
Protection
duct of salivary gland
Mesothelium : simple squamous epithelium lining the cavities of the
body.
Endothelium: simple squamous epithelium lining the blood vessels.
Stratified epithelial
1. Stratified squamous epi.:
a. Keratinize
b. Non keratinize
Keratinize: The surface cells have died after having secreted a
large amount of the tough protein keratin.
This type of st. epi. Resists abrasion there for well suited to the skin
surface and the passages subject to abrasion by the swallowing of
food and passage of feces.
Exfoliation: The separation of or desquamation surface cells from the
surface.
Exfoliate cytology:The study of exfoliated cells .
St. cuboidal epi.: has cuboidal or rounded surface cells. It lines
follicles in the ovaries and sperm-producing ducts called
seminiferous tubules in the testes , sweat gland duct.
St. Columnar. Epi.: Is a rare type in which col. Surface cells rest
on cuboidal basal cell it is found in short transitional zones where a
st. Epi. Grades into a columnar or pseudst. Type as in limited region
of the pharynx, larynx, anal canal, and male urethra.
Transitional: This type of epi. Is adapted to stretching when the
bladders empty the epi. Is up to six cells thick, However as the
bladder becomes distended with urine the epi. cells slide over each
other the epi. Becomes thinner (only 2 or 3 cells thick) and the
surface cells flatten.
Two other of epi
Neuroepithelial cells: Are cell of epi. Origin with specialized
sensory function (cell of taste buds)
Myoepithelial cells
Metaplasia
Under certain abnormal conditions are type of epi. T. may undergo
transformation in to another type. This eversible process is called mataplassia
in heavy cigarette smokers. The ciliated pseudostratified epi. Lining the
bronchi can be transformed into stratified sq. epi. Metaplasia is not restricted to
epi t. it also occurs in c.t.
Benign and malignant tumors can arise from most type of epi cells. Carcinoma
is a malignant. Malignant tumors derived from glandular epi. Called
adenocarcinoma.
Neuroepithelial cell: Are cells of epithelial origin with specialized sensory
function cells of taste buds and of the olfactory mucosa.
Myoepithelial cells: Are branched cells that contain myosin and a large number
of action filaments. They are specialized for contraction mainly of the secretary
units of the mammary, sweat and salivary glands.
Specialization of the cell surface
1. Microvili: found in absorptive cell, proximal renal tubule. The
glycocalyx is thicker than it is in most other cell.
The complex of microvillus and glycocalyx may be seen with L.M. and is
called brush or striated border.
>
Normal microvilli (right) in contrast of microvilli structure
in coeliac disease (left).
2. Cilia and flagella: cilia are cylindrical motile structure, surrounded by
the cell membrane, cilia inserted into basal bodies at the apical pole of the
cell. Cilia beat in waves that sweep across the surface of an epithelium.
Always in the same direction, they bend forward producing a power
stroke that pushes a long the mucus. In some organs, cilia have lost their
motility and assumed sensory cell (retina of the eye) modified cilium
specialized for an absorbing light.
flagella: long structure the only
functional flagellum in humans is the tail of sperm.
3. Stereocilia: are long non-motile extensions of cells found in
epididymis and ductus differences.
Intercellular junctions
The cells are firmly attached to each other by various kinds of intercellular
junctions
1. desmosomes : junction provides a mechanism for communication
between
adjacent cells.
2. Tight junction: junction serves as sites of adhesion and as seals to
prevent the flow of material through the space between epi. Cells.
3. Gap junction : found in nearly all mammalian tissues, skeletal muscle
being a major exception
4. hemidesmosmes : observed in the contact zone between epi cells and
basal lamina that bind the epithelial cells to the subjacent basal lamina
tight junction (zonula occludens) Her the most apical of the junctions
zonula refers to the fact the junction forms a band completely encircling
the cell the principal function of the tight junction is to form a seal that
prevents the flow of material between epi. Cells.