HISTOLOGY (Study of tissues)
A tissue is a group of similar cells organized into a structural and functional unit. It is a group of cells of
similar structure organized for carrying out a particular function(s).
Characteristics of tissues
Cells of a tissue are physically linked.
The cells of a tissue may be interspersed with intercellular substances.
A tissue may comprise one or more types of cells.
A tissue is specialized to perform a particular function(s).
ANIMAL TISSUES
Early in development, the cells of the growing embryo differentiate into three fundamental embryonic
tissues called germ layers.
Ectoderm which forms the outer layer of the skin and the nervous system.
Mesoderm which forms muscles, connective tissues, skeleton, kidneys and circulatory and
reproductive organs.
Endoderm which forms the lining of the respiratory tract and urinary bladder. It also forms the
glands associated with the guts and respiratory tract.
Therefore the germ layers in turn differentiate into different cell types and tissues that are characteristic of
the vertebrate’s body. Tissues are joined to each other by proteins. The point of connection between two
cells is a junction.
In adult vertebrates, the principle kinds of tissues include; epithelial tissue, connective tissue,
muscular tissue ,nervous tissue and reproductive tissue(associated with ovaries and testes; concerned
with production of gametes: eggs and sperm respectively).
EPITHELIAL TISSUE
This is a collection of closely packed single and multilayered compound sheets of cells covering the
external and internal surface of the body of an organism.
Characteristics of epithelial tissue
1. Epithelial tissue consists of tightly packed cells that are firmly attached to each other with little
intercellular material between them. Epithelial cells are held firmly together by small amounts of
carbohydrate cementing substances and by special intercellular junctions between the cells.
2. The bottom of epithelial cells rest on a basement membrane composed of a network of fibres
which include collagen. The portion of epithelial cells attached to the basement membrane is
called the basal surface, the opposite end facing the external environment or the lumen of the
body cavity is called the apical surface/free surface.
3. There are no blood vessels in the epithelial tissues hence the tissue lacks vascularity. As the
epithelial cells are not supplied with blood vessels, they rely on diffusion of nutrients and
oxygen from lymph vessels which run through nearby intercellular spaces. However, nerve
endings may occur in the epithelium.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 1
, 4. Regeneration- Epithelial cells have a high regeneration capacity due to rapid cell division. This
gives the epithelial tissue quick recovery after any injury or abrasions.
CLASSIFICATION OF EPITHELIAL TISSUES
Epithelial tissues are classified according to the number of cells/layers and the shape of the individual
cells.
SIMPLE EPITHELIAL TISSUES
A. Simple squamous/pavement epithelium
It consists of a single layer of cells. They are so thin that the nucleus causes a bulge and it is centrally
placed, with a disc shape. Cells are irregular with tapering edges. Simple squamous epithelium is found in
the following areas: Renal corpuscles of the kidney, lining of the alveoli of the lungs, lining of the blood
vessels where it is referred to as the endothelium, blood capillary walls and lining of lymphatic vessels.
Functions
Diffusion of materials
Exchange of gases
Adaptations
i. Thin flattened cells to reduce the distances across which materials diffuse.
ii. Possesses smooth lining to allow relatively friction free passage of fluids and materials
through the hollow structures.
B. Simple cuboidal epithelium
The cells are roughly cube shaped and
possess a central spherical nucleus. The
upper surface of cuboidal cells is either
pentagonal or hexagonal in outline. It is
the least specialized of all the epithelial
tissues.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 2
,Distribution
Lining of salivary ducts, pancreatic ducts, convoluted tubules and collecting ducts of the
nephrons of the kidney.
Lining of the salivary glands, sweat glands and thyroid glands
Lining of the retina
NB: The simple cuboidal epithelial tissue is non-secretary in the proximal convoluted tubule, distal
convoluted tubule and pancreatic ducts.
The functions include: Protection, excretion, absorption and secretion
Adaptations
i. Cells are tightly packed together with little intercellular spaces between them to offer
protection from injury and infection.
ii. Possess many Golgi bodies which perform functions of secretion of hormones and enzymes.
iii. Some possess microvilli which increase the surface area for example reabsorption of
materials from the renal fluids in the kidney tubules.
iv. Cells have numerous mitochondria for energy production to be used in active reabsorption of
materials e.g from renal filtrate back into the bloodstream.
C. Simple columnar epithelium
It is a single layer of column like narrow elongated cells at right angles to the basement membrane. Each
cell possesses a nucleus situated at the basal end and it is oval in shape. The epithelium is often
interspersed with goblet cells. The free surface of each columnar cell has microvilli forming a brush
border.
Distribution
Lining of the stomach, small intestines, gall bladder, and kidney ducts.
Lining of the gastric glands, intestinal glands, mammary glands, thyroid glands, salivary glands.
The functions include: Secretion, protection, absorption and brush border increasing surface area by
having microvilli at the cell free surface.
Adaptations
i) Possess fingerlike projections called microvilli which increase the surface area for absorption
such as digested food in the intestines.
ii) Possess mucus secreting cells which secrete mucus. The mucus protects the gastric walls
from hydrochloric acid and digestive enzymes.
iii) Mucus from goblet cells also lubricates the passage of food in the intestines.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 3
, Illustratiion of secretory columnar/glandular epithelium
D. Ciliated epithelium
Cells of this tissue are usually columnar in shape but bear numerous cilia at their free surfaces. The cells
are usually associated with mucus secreting goblet cells.
Distribution
Lines the oviduct
Ventricles of the brain, spinal canal and respiratory passages (trachea, bronchi and bronchioles).
Functions
Mucus protects lining and lubricates the passage of materials.
Cilia set up currents to move materials in a certain direction.
Adaptations
i) Interspersed with goblet cells which secrete mucus to protect the lining of the gut from
enzyme and acidic action.
ii) Possess cilia which set up currents that move materials from one direction to another.
iii) Possess goblet cells which lubricate the passage.
Illustration
E. Pseudostratified epithelium
This is a simple epithelium since all cells rest on a basement membrane, but some do not reach the free
surfaces. This gives an appearance of the epithelium to be on different levels and the nuclei at different
layers. Nevertheless, the epithelium is one layer of cells thick with each cell attached to the basement
membrane.
Most cells are columnar, thus usually named pseudostratified columnar epithelium. Where the cilia
appear at the free surface (trachea, bronchi, bronchioles), it is called pseudo stratified columnar ciliated
epithelium.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 4
A tissue is a group of similar cells organized into a structural and functional unit. It is a group of cells of
similar structure organized for carrying out a particular function(s).
Characteristics of tissues
Cells of a tissue are physically linked.
The cells of a tissue may be interspersed with intercellular substances.
A tissue may comprise one or more types of cells.
A tissue is specialized to perform a particular function(s).
ANIMAL TISSUES
Early in development, the cells of the growing embryo differentiate into three fundamental embryonic
tissues called germ layers.
Ectoderm which forms the outer layer of the skin and the nervous system.
Mesoderm which forms muscles, connective tissues, skeleton, kidneys and circulatory and
reproductive organs.
Endoderm which forms the lining of the respiratory tract and urinary bladder. It also forms the
glands associated with the guts and respiratory tract.
Therefore the germ layers in turn differentiate into different cell types and tissues that are characteristic of
the vertebrate’s body. Tissues are joined to each other by proteins. The point of connection between two
cells is a junction.
In adult vertebrates, the principle kinds of tissues include; epithelial tissue, connective tissue,
muscular tissue ,nervous tissue and reproductive tissue(associated with ovaries and testes; concerned
with production of gametes: eggs and sperm respectively).
EPITHELIAL TISSUE
This is a collection of closely packed single and multilayered compound sheets of cells covering the
external and internal surface of the body of an organism.
Characteristics of epithelial tissue
1. Epithelial tissue consists of tightly packed cells that are firmly attached to each other with little
intercellular material between them. Epithelial cells are held firmly together by small amounts of
carbohydrate cementing substances and by special intercellular junctions between the cells.
2. The bottom of epithelial cells rest on a basement membrane composed of a network of fibres
which include collagen. The portion of epithelial cells attached to the basement membrane is
called the basal surface, the opposite end facing the external environment or the lumen of the
body cavity is called the apical surface/free surface.
3. There are no blood vessels in the epithelial tissues hence the tissue lacks vascularity. As the
epithelial cells are not supplied with blood vessels, they rely on diffusion of nutrients and
oxygen from lymph vessels which run through nearby intercellular spaces. However, nerve
endings may occur in the epithelium.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 1
, 4. Regeneration- Epithelial cells have a high regeneration capacity due to rapid cell division. This
gives the epithelial tissue quick recovery after any injury or abrasions.
CLASSIFICATION OF EPITHELIAL TISSUES
Epithelial tissues are classified according to the number of cells/layers and the shape of the individual
cells.
SIMPLE EPITHELIAL TISSUES
A. Simple squamous/pavement epithelium
It consists of a single layer of cells. They are so thin that the nucleus causes a bulge and it is centrally
placed, with a disc shape. Cells are irregular with tapering edges. Simple squamous epithelium is found in
the following areas: Renal corpuscles of the kidney, lining of the alveoli of the lungs, lining of the blood
vessels where it is referred to as the endothelium, blood capillary walls and lining of lymphatic vessels.
Functions
Diffusion of materials
Exchange of gases
Adaptations
i. Thin flattened cells to reduce the distances across which materials diffuse.
ii. Possesses smooth lining to allow relatively friction free passage of fluids and materials
through the hollow structures.
B. Simple cuboidal epithelium
The cells are roughly cube shaped and
possess a central spherical nucleus. The
upper surface of cuboidal cells is either
pentagonal or hexagonal in outline. It is
the least specialized of all the epithelial
tissues.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 2
,Distribution
Lining of salivary ducts, pancreatic ducts, convoluted tubules and collecting ducts of the
nephrons of the kidney.
Lining of the salivary glands, sweat glands and thyroid glands
Lining of the retina
NB: The simple cuboidal epithelial tissue is non-secretary in the proximal convoluted tubule, distal
convoluted tubule and pancreatic ducts.
The functions include: Protection, excretion, absorption and secretion
Adaptations
i. Cells are tightly packed together with little intercellular spaces between them to offer
protection from injury and infection.
ii. Possess many Golgi bodies which perform functions of secretion of hormones and enzymes.
iii. Some possess microvilli which increase the surface area for example reabsorption of
materials from the renal fluids in the kidney tubules.
iv. Cells have numerous mitochondria for energy production to be used in active reabsorption of
materials e.g from renal filtrate back into the bloodstream.
C. Simple columnar epithelium
It is a single layer of column like narrow elongated cells at right angles to the basement membrane. Each
cell possesses a nucleus situated at the basal end and it is oval in shape. The epithelium is often
interspersed with goblet cells. The free surface of each columnar cell has microvilli forming a brush
border.
Distribution
Lining of the stomach, small intestines, gall bladder, and kidney ducts.
Lining of the gastric glands, intestinal glands, mammary glands, thyroid glands, salivary glands.
The functions include: Secretion, protection, absorption and brush border increasing surface area by
having microvilli at the cell free surface.
Adaptations
i) Possess fingerlike projections called microvilli which increase the surface area for absorption
such as digested food in the intestines.
ii) Possess mucus secreting cells which secrete mucus. The mucus protects the gastric walls
from hydrochloric acid and digestive enzymes.
iii) Mucus from goblet cells also lubricates the passage of food in the intestines.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 3
, Illustratiion of secretory columnar/glandular epithelium
D. Ciliated epithelium
Cells of this tissue are usually columnar in shape but bear numerous cilia at their free surfaces. The cells
are usually associated with mucus secreting goblet cells.
Distribution
Lines the oviduct
Ventricles of the brain, spinal canal and respiratory passages (trachea, bronchi and bronchioles).
Functions
Mucus protects lining and lubricates the passage of materials.
Cilia set up currents to move materials in a certain direction.
Adaptations
i) Interspersed with goblet cells which secrete mucus to protect the lining of the gut from
enzyme and acidic action.
ii) Possess cilia which set up currents that move materials from one direction to another.
iii) Possess goblet cells which lubricate the passage.
Illustration
E. Pseudostratified epithelium
This is a simple epithelium since all cells rest on a basement membrane, but some do not reach the free
surfaces. This gives an appearance of the epithelium to be on different levels and the nuclei at different
layers. Nevertheless, the epithelium is one layer of cells thick with each cell attached to the basement
membrane.
Most cells are columnar, thus usually named pseudostratified columnar epithelium. Where the cilia
appear at the free surface (trachea, bronchi, bronchioles), it is called pseudo stratified columnar ciliated
epithelium.
UACE Biology NOTES- Histology. ©John Kakooza2018 Page 4
