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Summary Cell Biology-Histology
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A summary of all the lectures given during the course Cell-Biology Histology at the Vrije Unversiteit Amsterdam.
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Histology ResitEpithelial tissue
Able to recognize and name different types of epithelia
The characteristics of epithelia are:
1. Polarity. An epithelium has an exposed surface, either facing the external
environment or an internal space, and a base, which is attached to underlying
tissue (basal lamina). The term polarity refers to the presence of structural
and functional differences between the exposed and attached surfaces. The
apical surface (exposed) has microvilli; sometimes it has cilia. The polarity can
also be seen in distribution of organelles, in the lining of kidney tubules
mitochondria are concentrated near the base of the cell, where energy is in
high demand for the cell’s transport activities.
2. Cellularity. Epithelia are made almost entirely of cells bound closely together
by interconnections known as cell junctions. In other tissue types, the cells
are often widely separated by extracellular matrix.
3. Attachment. The base of an epithelium is bound to a thin, noncellular
basement membrane. This basement membrane is formed from the fusion
of several successive layers (the basal lamina and reticular lamina), a collagen
matrix, and proteoglycans (intercellular cement). The basement membrane
adheres to the basal surface and to the underlying tissues to establish the
cell’s border and resist stretching.
4. Avascularity. Epithelia are avascular (without vessels), which means that they
lack blood vessels. Epithelial cells get nutrients by diffusion or absorption
across either the exposed or the attached epithelial surface.
5. Regeneration. Epithelial cells that are damaged or lost at the exposed surface
are continuously replaced through stem cell divisions in the epithelium.
Regeneration is a characteristic of other tissues as well, but the rates of cell
division and replacement are typically much higher in epithelia than in other
tissues.
There are some different types of epithelia:
1. Simple and stratified squamous epithelia.
2. Simple and stratified cuboidal epithelia.
3. Simple and stratified columnar epithelia.
4. Transitional epithelia.
5. Glandular epithelia.
Able to explain the relation between shape and function of different types of epithelia
There are 2 types of ways that cells layer in epithelial cells:
1. Simple: Only one cell layer covers the basement membrane. All the cells have
the same distance from the nucleus to the basement. A simple epithelium
can provide much mechanical protection. They line internal compartments
and passageways and are also present where absorption and diffusion take
place.
, 2. Stratified: several layers of cells cover the basement membrane. Generally
located in areas that are exposed to mechanical or chemical stress. Surface of
the skin and lining of the mouth are examples.
There are many types of different specialized epithelia, you can categorize these on
the basis of their cell shape:
1. Squamous epithelia: the cells in a squamous epithelium are thin, flat and
somewhat irregular in shape, like pieces of a jigsaw puzzle. A simple
squamous epithelium is the body’s most delicate type of epithelium. It is
located where absorption or diffusion takes place, or where a slick, slippery
surface reduces friction. Examples are the alveoli, lining of the heart and
blood vessels. The simple squamous epithelium that lines the body cavities
and enclosing the lungs, heart and abdominal organs is called a mesothelium.
The simple squamous epithelium lining the inner surface of the heart and all
blood vessels is called endothelium.
A stratified squamous epithelium is generally located where mechanical
stress are severe. They protect against physical and chemical attacks, for
example the skin. Exposed body surfaces that can dehydrate, are keratinized.
2. Cuboidal epithelia: the cells of a cuboidal epithelium resemble hexagonal
boxes from their apical surface. The spherical nuclei are near the center of
each cell, and the distance between adjacent nuclei is roughly equal to the
height of the epithelium.
A simple cuboidal epithelium provides limited protection and occurs where
secretion or absorption takes place. Such an epithelium makes up glands and
lines portions of the kidney tubules.
Stratified cuboidal epithelium is relatively rare. They are located along the
ducts of sweat glands and in the larger ducts of the mammary glands.
3. A transitional epithelium is an unusual stratified epithelium because its cells
can change between being squamous and cuboidal in shape. It tolerates
repeated cycles of stretching without damage, while stretching the transition
occurs. It is found in the urinary bladder.
4. Columnar epithelial cells appear rectangular in a sectional view. In reality
they are hexagonal, the height of the epithelium is several times the distance
between adjacent nuclei.
A simple columnar epithelium is typically found where absorption or
secretion takes place, such as in the small intestine. In the stomach and large
intestine, the secretions of simple columnar epithelia protect against
chemical stress.
Portions of the respiratory tract contain a pseudostratified columnar
epithelium, a columnar epithelium that includes several types of cells with
varying shapes and functions. Pseudostratified epithelial cells typically have
cilia. Mostly found in the lining of the nasal cavity, the trachea, the bronchi
and portions of the male reproductive tract.
Stratified columnar epithelia are relatively rare, providing protection along
portions of the pharynx, epiglottis, anus and urethra. The epithelium has
either two layers or multiple layers. In the latter case, only the superficial
cells are columnar.
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