What is the Connective Tissue?
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Connective Tissue
The connective tissues are most abundant and widely distributed
in the body of complex animals. They are named as connective
tissues because of their special function of linking and
supporting other tissues/organs of the body.
Generally, connective tissues are made up of three
components
l. Matrix
It is a clear and viscous substance. It’s consistency may vary
from liquid (e.g., blood) to semisolid (e.g., cartilage) and
solid (e.g., bone) form.
2. Cells Embedded in the Matrix
These are responsible for secreting the matrix and other
substances.
The cells of connective tissue are of different types
(i) Fibroblasts produce fibres and matrix.
(ii) Adipose cells store fat.
(iii) Plasma cells synthesise antibodies. These are also
called ‘cart wheel cells’ because thin chromatin in their
nucleus forms four or five clumps giving the nucleus a
resemblance of a cart wheel.
(iv) Mast cells produce histamine, heparin and
serotonin. These are related to basophils of the blood.
(a) Histamine dilates the walls of blood vessels in
inflammatory and allergic reactions.
(b) Heparin checks clotting of blood inside the blood
vessels.
(c) Serotonin acts as a vasoconstrictor to check
bleeding and to increase the blood pressure.
(v) Mesenchyme cells produce various types of
connective tissue cells.
(vi) Macrophages ingest cell debris, bacteria and foreign
matter.
(vii) Chromatophores (pigment cells) are found in the
dermis of the skin which impart colour to the animals.
Reticular cells form reticular tissue and are
phagocytic in nature.
3. Fibres
In all connective tissues except blood, the cells secrete
fibres of structural proteins called collagen or elastin.
They provide strength, elasticity and flexibility to the
tissues. These are non-living products of the cells.
These are of three types
(i) Collagen or Collagenous fibres (white fibres) are
made up of collagen protein. When boiled in the
water, collagen changes into gelatin.
(ii) Elastic fibres (yellow fibres) are formed of a protein
called elastin. These fibres are branched and elastic
(iii) Reticular fibres are delicate, branched and
inelastic. They are made up of reticulin protein•
They always form a network.
—Types of Connective Tissues
The connective tissues are mainly of following three types
l. Loose Connective Tissue
Loose connective tissue has cells and fibres loosely
arranged in a semifluid ground substance.
These tissues are of two types, i.e., areolar tissues and
adipose tissue.
i. Areolar Tissue
It is found under the epithelial tissue of the skin, visceral
organs like stomach, trachea and the walls of the blood
vessels, etc. Its matrix is made up of glycoproteins. It
contains two types of fibres, i.e., the white collagen fibres
made up of collagen and the yellow elastic fibres made up
of elastin.
The different cells of areolar tissue are fibrocytes, macro-
phages and mast cells.
Functions The tensile strength of collagen fibres and the
elasticity of the yellow fibres protect the various organs
from mechanical injuries.
This tissue also provides rapid diffusion of the materials and
migration of wondering cells towards the areas of microbial
infection and injured tissues.
ii. Adipose Tissue
It is a modified type of areolar tissue. Its matrix contains
large number of adipose cells along with fibrocytes and
macrophages. White and yellow fibres are present in the
matrix. The cells of this tissue are specialised to store fats.
The excess of nutrients which are not used immediately are
converted into fats and are stored in this tissue. The adipose
tissues are found in the subcutaneous region, around the
heart, kidneys, eyeballs, etc.
It is also found in the blubber of whales and elephants,
hump of camel, fat bodies of frog and yellow bone marrow.
Functions The adipose tissues mainly store fat in the
form of oil droplets. It forms a shock-absorbing cushion
around the eyeballs and kidneys also. The tissue also helps
In the production of blood corpuscles.
2. Dense Connective Tissue
Fibres and fibroblasts are found compactly packed in the
dense connective tissues. This tissue is of two types, i.e.,
dense regular and dense irregular connective tissue.
i. Dense Regular Connective Tissue
In this tissue, the collagen fibres are present in rows between
many parallel bundles of fibres.
It is further of two types
(a) White Fibrous Connective Tissue
It mainly consists of white fibres arranged in bundles. The
fibroblasts are present in rows between the bundles.
It is of two types
Tendons The white fibrous connective tissue forms
the cords called tendons. These join the skeletal
muscles with the bones.
Sheets The white fibrous connective tissue also forms
flat plates or sheets. It occurs in the dermis of the skin,
periosteum of the bone, perichondrium of cartilage,
pericardium of heart, etc . The white fibrous
connective tissue has great strength however its
flexibility is limited.
(b) Yellow Elastic Connective Tissue
It mainly consists of yellow elastic fibres. The fibres are
thicker. The fibroblasts and a few white fibres are found in
between the yellow fibres. It is also of two types
Ligaments The yellow elastic connective tissue forms
the cords called ligaments. These join bones to bones.
• Sheets The yellow fibrous sheets formed by this tissue
occur in the walls of blood vessels, lungs and
bronchioles, true vocal cords, cartilage of larynx,
trachea, etc.
The yellow elastic connective tissue has considerable
strength and remarkable elasticity. Thus, it allows the
stretching of various organs, specially joints in between the
bones.
ii. Dense Irregular Connective Tissue
It has fibroblasts and many fibres (mostly collagen) that are
oriented in different pattern. This tissue is present in the skin.
3. Specialised Connective Tissues
The specialised connective tissues are of following types
i. Skeletal Tissues
These tissues form the endoskeleton of the vertebrates. They
form a rigid framework which supports the body, protects the
vital organs and helps in locomotion.
The two types of skeletal tissues, i.e., cartilage and bone.
(a) Cartilage
It is a tough, semitransparent, elastic and flexible tissue. The
cartilage cells (chondrocytes) lie in groups of 2-3 in fluid-filled
spaces called lacunae. The cartilage is bounded externally by a
stiff sheath called perichondrivm containing white fibrous
tissue. Cartilage is present in the tip of nose, outer ear joint,
between adjacent bones of the vertebral column, limbs and
hand in adults.
The cartilages are of three types, i.e., hyaline, fibrous and
calcified.
Hyaline Cartilage It has a clear, translucent, bluish green
matrix. It is flexible and forms articular surfaces at the
joints of long bones, where it is called articular cartilage
• Fibrous Cartilage It has well-developed fibres in the
matrix. It is of two types i.e., white fibrous cartilage and
yellow elastic cartilage.
Calcified Cartilage When matrix of cartilage contains
granules of calcium carbonate, the cartilage is called
calcified cartilage.
(b) Bone
It is a hard and rigid connective tissue. These are non-pliable
ground substance rich in calcium salts and collagen fibres
providing strength to the bone. The cells of bone are found in a
calcified matrix made up of ossein. The bone cells known as
osteocytes lodged in the spaces called lacunae.
They also interact with skeletal muscles attached to them to
bring about movements. The bone consists of four parts, i.e.,
• Periosteum It is a thick and tough sheath that forms an
envelope around the bone. It is composed of collagen
fibrous tissue. The periosteum contains blood vessels. It
also contains bone-forming cells, the osteoblasts, which
produce new bone material.
• Matrix It is composed of a protein called ossein.
The Haversian canals, a characteristic feature of
mammalian bones are present in the matrix. Each
Haversian canal contains an artery, a vein, a lymph
vessel, a nerve and some bone cells.
• Endosteum It is present outer to the bone marrow
cavity. It comprises white fibrous tissue and the
bone forming cells called osteoblast. The latter
produces new bone material.
• Bone Marrow It is the vascular, soft pulpy
connective tissue found in the bone marrow cavity
of long bones like humerous, femur, etc.
Bone marrow is of two types
• Yellow marrow (rich in fat cells called
adipocytes).
• Red marrow (blood cells are formed in this
marrow).
Note
In fætus, red marrow cmurs in all bones. After birth, it is
restricts to limited places like humerus and fernur bones.
The bones can be spongy or compact on the basis of
density and texture.
(a) Spongy (Cancellate) Bone It contains a network
of thin and irregularly longitudinal and
transverse bony bars called trabeculae covered by
the endosteum. It is found at the ends of long
bones (epiphyses).
(b) Compact (Dense) Bone It is hard and compact
and found in the shaft of long bones. It contains
yellow bone marrow and has Haversian systems.
ii. Vascular Tissues
These are motile connective tissues consisting Of fluid
matrix and free cells. The matrix is without fibres.
vascular tissue helps in the transport of materials from
one place to another.
(a) Blood
It is a mobile, watery fluid with a slightly salty taste ed
composed of plasma (a fluid matrix) and the cells call
when
blood corpuscles. It is bright red in colour
oxygenated and purple when deoxygenated.
The volume of blood in an adult is about 5L It circulates
within the blood vessels in higher animals. It is slightly
alkaline (pH 7.4) in nature.
Plasma is a yellowish, straw-coloured liquid which is
composed mainly of water (92%). About 55% of the total
blood volume is plasma. The solid materials in plasma
include plasma proteins, nutrients (glucose, amino acids,
fatty acids and vitamins), hormones, antibodies, enzymes,
lactic acid, cholesterol, dissolved gases (oxygen, carbon
dioxide), mineral salts and waste products (urea, uric acid
and creatinine).
Functions It helps in transport of substances, provides
y immunity, prevents the blood loses, retains fluid in
blood, maintains blood pH and conducts heat to skin for
dissipation.
Blood Cells
The blood cells or blood corpuscles form about 45% of the
blood volume. These cells are formed in the marrow of
the long bones and the lymph nodes. The process of blood
cells formation is called haemopoiesis and the tissues where
these are formed is called haemopoietic tissue.
The blood cells are of following types
Erythrocytes or Red Blood Grpuscles (RBCs) are the
most abundant elements in blood. These carry
red-coloured oxygen carrying pigment called
haemoglobin. They are 7-8 prn in diameter.
The human RBCs are smaller than the white blood
corpuscles. In mammals, they are non-nucleated,
biconcave and circular (exception camel). The
formation of erythrocytes is called erythropoiesis.
• Leucocytes or White Blood Cells (WBCs) lack
haemoglobin and are colourless. They are nucleated
with rounded or irregular shape. They can change their
shape and are capable of amoeboid movement.
• Thrombocytes (Blood Platelets) These are small,
colourless, plate-like discs having size of about 2-3 gm.
Their number ranges between 0.15-0.4 million/mm3 of
blood. Their normal lifespan is about a week. No nucleus
is visible in these cells.
Functions of Blood
The blood performs following functions in the animal
body
• Blood transports oxygen from the respiratory organs
to the tissues and carbon dioxide from the tissues to
the respiratory organs.
• It transports nutrients to all parts of the body.
Blood maintains the constant body temperature by
distributing the heat throughout the body.
Lymphocytes and eosinophils produce antitoxins to
neutralise the toxins, released by the microbes.
Blood helps to maintain water balance to a constant
level by bringing about constant exchange of water
between circulating blood and tissue fluid.
• It helps to regulate the pH of the body fluids as it
contains buffer materials such as proteins and
mineral salts.
• Blood helps in healing of injuries by maintaining
necessary supplies for the repair of damaged tissues.
(b) Lymph
It is a mobile connective tissue comprising lymph plasma
(fluid) and lymph corpuscles (cells). It is pale yellow in
colour and its composition is similar to plasma without
the plasma proteins. It is present in the vessels called
lymph vessels.
Lymph is formed of liquid components and formed
elements or cells. It contains about 94% water and 6% of
organic and inorganic substances. The organic part
includes protein, fat droplets, carbohydrates, nitrogenous
wastes and hormones.
Lymph performs the following functions in animal body
(i) It plays an important role in the defence of the
body especially against invading organisms.
(ii) The digested products of fat digestion enter the
lymph vessels present in the villus of the small
intestine.
(iii) Lymph helps to maintain the blood volume by
returning the interstitial fluid back to the blood
during circulation.
(iv) The lymph nodes produce lymphocytes.
(v) It keeps the tissue cells moist.
4. Reticular Connective Tissues
Tissues consist of star-shaped reticular cells whose
protoplasmic processes join to form a cellular network.
The reticular fibres are present on the reticular cells
(composed of a protein called reticulin.)
The reticular connective tissue is present in the liver, spleen,
lymph nodes, thymus, tonsils, bone marrow and lamina
propria of the gut wall.
Functions This tissue provides strength and support as it
forms the supporting framework of many organs. It also helps
to bind together the cells of smooth muscles. The reticular
cells are phagocytic and forms the defence mechanism of the
body.
5. Pigmented Connective Tissue
The cells of pigmented connective tissue are irregular and are
called pigment cells (chromatophores or melanophores).
These cells contain yellowish brown, black or blue melanin
pigment granules. Melanin is produced by other cells called
melanocytes.
This tissue is present in the choroid, ciliary body and iris of
the eye and dermis of the human skin.
Function It gives colour to the structures.
6. Mucoid Connective Tissue
This tissue occurs as a foetal or embryonic connective tissue
as it is present in the umbilical cord.
The mucoid tissue contains a jelly-like substance called
Whartson’s jelly and some delicate collagen fibres and
primitive type of fibroblasts. It occurs as embryonic
connective tissue in the foetus and vitreous chamber of the eye.
Functions of Connective Tissue The connective tissue
performs following main functions
(i) The connective tissue mainly joins one tissue to
another in the organs.
(ii) The adipose tissue stores fat.
(iii) The cartilage and bones form a supporting framework
for the body.
(iv) Blood and lymph carry materials from one part to
another in the body.
(v) The cells of connective tissues like macrophages,
monocytes, neutrophils ingest bacteria, cell debris and
foreign materials. Thus, they protect and clean the body.
(vi) The adipose tissue acts as shock absorber around some
organs, such as eyeballs and kidneys. It also acts as
packing material in various organs.
(vii) Bone marrow is the source of blood corpuscles.
(viii) The collagen fibres help in the repair of injured tissues.
Knowledge Plus
• In old age, the bone marrow of the cranial bones undergoes
degeneration and is called gelatinous marrow.
• Bone marrow is a special kind of myeloid (myelogenous) tissue.
• Prothrombin and fibrinogen are the largest blood proteins and
albumins are the smallest one.