GENERAL HISTOLOGY
NOTES
MEANING AND
SCOPE OF HISTOLOGY
Animal and plant bodies are made up of
several cells, known generally as the basic unit of life, which aggregate and
coordinate together as tissues. The study of these tissues is referred to as
histology. The word Histology is derived from two Greek words (i) “Histos” and (ii) “logia”. The Greek word “Histos” means “Tissues” and “Logia” means “Study
of”. These two put together means “Study of Tissues”. Thus Histology is described as the study of tissues. Hickman et al
(2001) describes a tissue as “an aggregation of cells and cell products of
similar structure and embryonic origin performing common functions”. In a
similar manner that cells make tissues, so also organs and organ systems are
made up of tissues. The scope of histology therefore consists of the following:
i.
Cytology – the study of cells.
ii.
Histology – the study of tissues.
iii.
Organology – the study of organ and organ systems.
iv.
Gross Anatomy – the study of the whole or component part of
animals.
The study of cells (Cytology) has been dealt
with extensively in another course (BIO 201 – Cell Biology). The student is
advised to review that course for a good grounding for the basis of histology.
These lectures may therefore focus more on tissues and their higher organizations
including organs and organ systems especially of the mammalian body. This makes
it extremely important for the student of histology to also review courses that
have dealt with mammalian body especially ZOO 102 (Organization of Mammalian
Body) and ZOO 303 (Comparative Vertebrate Anatomy).
TISSUES
The body of animals consists of different
types of tissues. These mainly include the following:
(i)
Epithelial Tissues.
(ii)
Connective Tissues.
(iii)
Muscle Tissues
(iv)
Nervous Tissues
Epithelial
Tissues – Structure, Types
and Functions
Epithelial
tissue cover the surfaces various
structures in the body. They form both internal and external coverings. An
epithelial tissue that cover surfaces on the interior of the structure is known
as endothelium while the one covering
external surface of a structure is called epithelium. Each epithelial
Tissue is made up of cells that are
arranged in closely packed fashion. They may be arranged as one or more layers. Epithelial cells are
packed tightly together, with almost no
intercellular spaces and only a small
amount of intercellular substance. An epithelial tissue, irrespective of
its type, is usually separated from the underlying tissue by a thin sheet of
connective tissue called basement
membrane. The basement membrane provides
structural support for the epithelium and also binds it to adjoining structures.
The types of epithelial
tissues are identified on the following bases:
i.
The shape and
function of their cells (Squamous, cuboidal, columnar or transitional)
ii.
The number of
layers they have (Simple if only one layer and Stratified if more than one
layer).
Simple
epithelium
As stated
above, simple epithelia can be subdivided according to the shape and
function of their cells. The following can therefore be recognized:
Squamous cells
have the appearance of thin, flat
plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells,
for example, tend to have horizontal
flattened, elliptical nuclei because of the thin flattened form of the
cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.
As their name
implies, cuboidal cells are roughly cuboidal
in shape. Each cell has a spherical
nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also
constitute the germinal epithelium
which produces the egg cells in
the female ovary and the sperm cells in the male testes.
The cells that
form columnar epithelia are elongated
and column-shaped since they are taller
than they are wide. The nuclei
are elongated and are usually located near the base of the cells.
Columnar epithelium forms the lining of
the stomach and intestines. Some columnar cells are specialised for sensory reception such
as in the nose, ears and the taste buds
of the tongue. Goblet cells
(unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus or slime, a lubricating
substance which keeps the surface smooth.
Some Columnar Epithelia may be ciliated, the presence of cilia being a
surface specialization. Ciliated
columnar epithelia cells are usually simple. These cilia are
capable of rapid, rhythmic, wavelike
beatings in a certain direction. This movement of the cilia in a certain
direction causes the mucus, which is secreted by the goblet cells, to move
(flow or stream) in that direction. Ciliated epithelium is usually found in the
air passages like the nose. It
is also found in the uterus and
Fallopian tubes of females. The movement of the cilia propels the ovum to the uterus.
Some columnar epithelia may be glandular in function. Columnar epithelium with goblet cells is called glandular epithelium. Some parts of
the glandular epithelium consist of such a large number of goblet cells that there are only a few normal
epithelial cells left. Columnar and cuboidal epithelial cells often become specialised as gland cells which are
capable of synthesising and secreting
certain substances such as enzymes,
hormones, milk, mucus, sweat, wax and saliva. Most glands are multicellular including salivary glands.
In places where
body linings have to withstand wear and tear, the epithelia are composed of several layers of cells and are therefore
called stratified epithelium. The top cells are flat and
scaly and it may or may not be keratinized.
Keratinzed cells have deposit of a tough, resistant protein called
keratin. The mammalian skin is
an example of dry, keratinised,
stratified epithelium. Conversely, the lining of the mouth cavity is an example of an unkeratinisied, stratified epithelium.
1. Protection – Some epithelial
cells such as those from the skin protect
underlying tissue from mechanical injury, harmful chemicals and invasion by
microorganisms such as bacteria. They also protect the skin from excessive loss
of water.
2. Sensation – Some epithelia
function in the perception of stimuli by their possessors. Specialised
epithelial tissues containing sensory nerve endings are found in the skin,
eyes, ears, nose and on the tongue.
- Secretion – As already mentioned, some epithelia are specialized to secret specific chemical substances such as enzymes, hormones and lubricating fluids. These epithelia are found in glands.
- Absorption – Some epithelial cells that line the small intestine, for example, absorb nutrients from the digestion of food.
- Excretion – Epithelia tissues found the kidney and the skin are examples of those that functions in excretion. The epithelial tissues found in the kidney, for instance excrete waste products from the body and reabsorb needed materials from the urine. Also, sweat is also excreted from the body by epithelial cells in the sweat glands.
- Diffusion - Simple epithelium are functionally ideal for the diffusion of gases for example those found in the walls of capillaries and lungs. Some epithelia are therefore important in the exchange of respiratory gases. This is because of their characteristic thing nature.
- Cleaning – Some ciliated epithelia assist in removing dust particles and foreign bodies that found their ways into air passages.
- Reduction of Friction - Smooth, tightly-interlocking, epithelial cells lining the entire circulatory system of mammals and reduce friction between the blood and the walls of the blood vessels.
References
- Hickman, C. P., Hickman, F. M., Kats, L. B. (2001). Laboratory Studies in Integrated Principles of Zoology. Tenth Edition. Mc Graw Hill Publishers, New York. 440pp.
