Connective Tissue in Animals

INTRODUCTION
• Tissues of animal are classified into four major categories:
epithelial, connective, muscle and nervous tissues. All these
categories of tissues are organized to form organs and structures of
animal body.
• Although each of the animals usually has these four kinds of tissues,
the appearance and expression of all these tissues types may vary.
• Connective tissue is one of the four major categories of animal
tissues that supports, joins and separate different forms of tissues
and organs of the body.
• Cells of the connective tissue are more broadly apart from each
other as compared to those in epithelial tissues.
• Connective tissues contains larger amount of extra-cellular matrix
which is considerably absent in epithelial tissue

GENERAL FEATURES OF CONNECTIVE
TISSUE
• Connective tissues are present in all part of the body excluding central nervous
system.
• These tissues are never exposed to the outer environment.
• In their various forms, connective tissues have a variety of functions.
• Most of the connective tissues contain a lot of blood vessels (means they are
highly vascular).
• They have receptors which can perceive pain, pressure, temperature, and other
sensations.
• Connective tissue fills spaces between organs and tissues, and provides structural
and metabolic
• support for other tissues and organs.
• Connective tissues consist of two chief components: extra-cellular matrix and cells.
• Extra-cellular matrix in connective tissues is the substances located in spaces
between cells.
• The cells of connective tissues secrete extra-cellular fibres and account for the
many of the functional properties of the tissue in addition to controlling the
surrounding watery environment via specific proteoglycan molecules.

• Various cells of connective tissues produced
from embryonic cells called mesenchyme.
• Mesenchyme arises from mesoderm, an
embryonic germ layer, and is composed of
stellate or fusiform cells embedded in a
gelatinous ground substance. Mesenchyme is
the stem tissue of all the connective tissues of
the body.

FUNCTIONS OF CONNECTIVE TISSUES
• Support and protection: The cells of connective tissue
produce minerals and fibers that make up the bony
structural framework in the body. As a result, it protects
delicate organs, and covers and connects the other types of
tissues.
• Transport: Fluid connective tissues such as blood and
lymph efficiently carry substances from one area to
another area of the body.
• Storage: Adipose cells in the connective tissues store fats a
form of energy until it is required.
• Defense: It involves defense against microbes through the
intercellular interactions and antibodies production by
specialized cells of connective tissues.

COMPONENTS OF CONNECTIVE TISSUE
• Main components of connective tissue are (I)
specialized cells and (II) extracellular matrix.
• The extracellular matrix is made up of fibres in a
protein and polysaccharide matrix, secreted and
organized by cells in the extracellular matrix.
• Variations in the composition of the extracellular
matrix, determines the properties of the connective
tissue.
• The cells sit in a matrix made up of glycoproteins,
fibrous proteins and glycosoaminoglycans, which have
been secreted by the fibroblasts, and the major
component of the matrix, is in fact, water.

(I) Specialized cells in Connective
Tissue
1. Fibroblasts: These are bulky and flat cells containing
branching projections. Fibroblasts contain large and oval
shaped nuclei with one or two conspicuous nucleoli. The
active fibroblast contains a lot of rough endoplasmic
reticulum necessary for production of collagen and elastin
fibers.
2. Macrophages: It arises from a type of white blood cell
termed as monocytes. Macrophages are not in regular
shape and have short branching processes. At first, these
cells are not active. After inflammation, they intensively
turn into amoeboid and phagocytic stage. They directly
engulf blood cells, bacteria, dead cells and debris digesting
this material with powerful enzymes

• Fixed macrophages exist in in a particular tissue;
example includes alveolar macrophages in the
lungs or spleenic macrophages in the spleen.
Wandering macrophages have the ability to
move throughout the tissue and gather at sites of
infection or inflammation to carry on
phagocytosis.
3. Plasma cells: These are small cells which arise
from B lymphocyte, a type of white blood cell.
Plasma cells release antibodies and proteins that
attack or reduce the effect of foreign materials in
the body.

4. Mast cells: These are large cells (20-30μm)
packed with intensely basophilic granules that
most of the time obscure the nucleus. These cells
also have mediators e. g. histamine, heparin and
serotonin for instantaneous hypersensitivity.
• 5. Adipocytes: These are also called fat cells or
adipose cells. These cells store triglycerides the
form of fat. They are prevalent underneath to the
skin and around organs for example heart and
kidneys.
6. Leukocytes: These are white blood cells that
move along with the connective tissues around
blood vessels.

(II) Extracellular Matrix in Connective
Tissue
• The cells of connective tissue are interspersed in
a large volume of extra-cellular matrix. This extra-
cellular matrix (ECM) is secreted by the cells. It
constitutes protein fibers interpose in an
amorphous mixture of large protein-
polysaccharide molecules termed as ground
substance.
• The followings are description of two major
components of extracellular materials: (i) ground
substance and (ii) fibres.

Ground Substance: Ground substance is the part of the
extracellular matrix that occupies the spaces between
the cells and fibers. It may be fluid, semi fluid, jellylike
or calcified. Functions of ground substance are
followed as:
• It supports cells and helps in connecting cell to cell.
• Helps in water storage.
• Works as an exchanging medium between the blood
and cells.
• It has an important role in which ways tissues develop,
migrate, proliferate, and change shape, and in what
ways they perform their metabolic functions.

CONNECTIVE TISSUE FIBERS
• Collagen fibres: These fibres are very strong and resist pulling
forces, but they are not stiff, which allows tissue flexibility. The
properties of different types of collagen fibres vary from tissue to
tissue. Chemically, collagen fibres consist of the protein collagen,
which is the most abundant protein in our body.
• Elastic fibres: Elastic fibers contain the protein elastin surrounded
by a glycoprotein named fibrillin, which adds strength and stability.
They are branched and wavy and after stretching will return to their
original length.
• Reticular fibres: Reticular fibers (reticulum, a network), the least
common of the three, are thinner than collagen fibers and
commonly form a branching, interwoven framework in various
organs. give support in the walls of blood vessels and form a
network surrounding the cells in some tissues, such as areolar
connective tissue, adipose tissue, nerve fibres, and smooth muscle
tissue.

COLLAGEN FIBERS RETICULAR FIBERS
ELASTIC FIBERS

CLASSIFICATION OF CONNECTIVE
TISSUES
• Basically, Connective tissue can be classified
on the basis of their maturity i.e.
(A) Embryonic and (B) Mature connective
tissue. Followings are details about each type
of Connective tissue.

EMBRYONIC CONNECTIVE TISSUES
• Embryonic connective tissues are present
basically in the embryo (human which is being
developed during initial two months of
pregnancy), and in the fetus (human which is
being developed during pregnancy period of
third month till birth). These are of two types,
mesenchyme and mucous connective tissue.

Mesenchyme
• These cells are not regular in shape and
embedded in semifluid ground substance
which is composed of delicate reticular fibers.
• Location: It is located almost exclusively under
skin and along developing bones of embryo;
some in adult connective tissue, especially
along blood vessels.
• Function: It forms almost all other types of
connective tissue.

Mucous (or mucoid) connective tissue
• It is widely scattered fibroblasts embedded in
gelatinous and characteristically abundant
ground substance consists of fine collagen
fibers. It also is transitorily encountered as a
stage in the differentiation of mesenchyme
into mature connective tissue.
• Location: It is chiefly located in fetus.
• Function: It forms the umbilical cord.

Loose Connective Tissues
• Loose connective tissues are the packing
material in human body. The fibres of loose
connective tissues are loosely arranged
between cells. Loose connective tissue forms a
layer that separates the skin from underlying
muscles, providing both padding and a
considerable amount of independent
movement.

Areolar connective tissue
• It is one among the most abundant connective tissues in
the body. It consists of fibers (collagen, elastic, reticular)
arranged randomly and several kind of cells such as
fibroblasts, macrophages, plasma cells, adipocytes, mast
cells, and a few white blood cells, embedded in semi-fluid
ground substance viz. hyaluronic acid, chondroitin sulfate,
dermatan sulfate, and keratan sulfate.
• Location: It is known as packing material of the body as it is
found in nearly every body structure. It is present in
subcutaneous layer deep to skin, papillary region of dermis,
lamina propria of mucous membranes, near blood vessels,
nerves, and body organs.
• Function: It provides strength, elasticity and support to the
different body parts.

Adipose connective tissue
• Adipose tissue or fat is a loose connective tissue containing large numbers of fat cells, or
adipocytes. It contains cells or adipocytes arise from fibroblasts which are modified to store
triglycerides as conspicuous and centrally located fat droplets.
• (a) White Adipose tissue: This composes 20 to 25 % of the body weight in healthy adults. The
amount of adipose tissue in an individual is determined the balance between energy intake
and expenditure. It is found supporting the kidneys and the eyes, between muscle fibres and
under the skin, where it acts as a thermal insulator and energy store.
• (b) Brown Adipose tissue: This (BAT) is characteristically found in the fetus and newborn. It
includes most abundant capillary network and several pigmented mitochondria that are
responsible for aerobic cellular respiration. When brown tissue is metabolised, it produce
less energy and considerably more heat than other fat, contributing to the maintenance of
body temperature. In some adults it is present in small amounts.
• Location: It is present in subcutaneous layer inner to skin, surrounding heart (Fig. 6) and
kidneys, yellow bone marrow, stuffing around joints. It fills bony socket behind the eyeball. It
dominates extensive areas of loose connective tissue in the pericardial and peritoneal
(abdominal) cavities.
• Function: It provides insulation that slows heat loss through skin.It provides another source
of padding and shock absorption for the body. It stores energy in the form of fat; maintains
and protects organs. In newborns, BAT produces heat to maintain proper body temperature.

Reticular connective tissue
• It consists of fine interlocking network of reticular
fibres (thin form of collagen fiber) and reticular cells.
• Location: It is present in liver, spleen, lymph nodes; red
bone marrow; reticular lamina of basement
membrane; around blood vessels and muscles.
• Function: It forms stroma (supporting framework) of
organs; binds smooth muscle tissue cells; filters and
eliminates damaged blood cells on spleen and
microorganisms in lymph nodes.

Dense Connective Tissues
• It contain more fibres, which are thicker and
more densely packed, but have noticeably less
number cells than loose connective tissues.
Dense connective tissues consist mostly of
collagen fibers; they may also be called
fibrous, or collagenous tissues. Dense
connective tissues are tough, strong, and
durable. They resist tension and distortion and
interconnect bones and muscles.

Dense regular connective tissue
• In dense regular connective tissue, the collagen fibers are parallel to
each other, packed tightly, and aligned with the forces applied to
the tissue. Tendons are cords of dense regular connective tissue
that attach skeletal muscles to bones. Their collagen fibers run
along the length of the tendon and transfer the pull of the
contracting muscle to the bone. Ligaments resemble tendons but
connect one bone to another. Ligaments often contain elastic fibers
as well as collagen fibers and thus can tolerate a modest amount of
stretching. Aponeuroses are layers of tendon like materials which
join muscle to muscle and muscle to bone.
• Location: It is present as Tendons, Ligaments and Aponeuroses in
the body.
• Function: It provides strong attachment between various
structures. Tissue structure withstands pulling (tension) along long
axis of fibers.

Dense irregular connective tissue
• Dense irregular connective tissue contains an interwoven
meshwork of collagen fibers with a few fibroblasts. This
structural pattern provides support to areas subjected to
stresses from many directions and gives skin its strength.
• Location: It occurs in sheets, such as fasciae (tissue beneath
skin and around muscles and other organs), reticular
(deeper) region of dermis of skin, fibrous pericardium of
heart, periosteum of bone, perichondrium of cartilage,
joint capsules, membrane capsules around various organs
(kidneys, liver, testes, lymph nodes, valves of heart.
• Function: It provides pulling (tensile) strength in many
directions.

Elastic connective tissue
• It is predominantly elastic fibers with fibroblasts
between fibers and yellowish in colour.
• Location: It is found in lung tissue, walls of elastic
arteries, trachea, bronchial tubes, vocal cords,
suspensory ligaments of penis and some
ligaments between vertebrae.
• Function: It allows stretching of various organs. It
is sturdy and move back to original shape after
being stretched. Elasticity is important for normal
functioning of lung tissue and elastic arteries.

SUPPORTING CONNECTIVE
TISSUE
• Supporting connective tissues includes
cartilage and bone. These tissues constitute a
strong framework which supports the rest
parts of the body. These connective tissues
are composed of the extra-cellular matrix rich
in fibers. In some cases, it contains deposits of
insoluble calcium salts.

CARTILAGE
• Cartilage composed of a dense network of collagen and
elastic fibers which are embedded in the chondroitin
sulfate, a jelly like ground substance. Cartilage can tolerate
noticeably more stress than loose and dense connective
tissues. The strength of cartilage is owing to its collagen
fibers, and its resilience is owing to chondroitin sulfate.
• Cells of mature cartilage, termed chondrocytes, prevail
singly or in groups within spaces termed as lacunae in the
extracellular matrix.
• A covering of dense irregular connective tissue termed as
perichondrium surrounds the surface of most cartilage.
Perichondrium has blood vessel and nervous system and is
the source of cartilage cells. Because cartilage contains no
blood supply, it repairs slowly subsequent to an injury.

Hyaline cartilage
• It consists of a resilient gel as ground substance and
appears in the body as a bluish- white, shiny substance. It
constitutes very thin collagen fibers are not easily visible in
microscope. Prominent chondrocytes are found in lacunae
surrounded by perichondrium.
• Location: Most abundant cartilage in body. It is prevalent at
ends of long bones, anterior end of ribs, nose, parts of
larynx, trachea, bronchi, bronchial tubes, embryonic and
fetal skeleton.
• Function: It forms smooth surfaces for movement at joints
It provides flexibility and support. It is weakest type of
cartilage.

Fibrous cartilage
• Cells of fibrous cartilage are chondrocytes that
are scattered between visibly thick bundles of
collagen fibers within the extracellular matrix. It
lacks perichondrium.
• Location: It is prevalent in pubic symphysis,
intervertebral discs, menisci (cartilage pads) of
knee, portions of tendons that insert into
cartilage.
• Function: It support and joins structures together.
Strength and rigidity compose it as the strongest
type of cartilage.

Elastic cartilage
• Chondrocytes are embedded in fine network
of elastic fibers within extracellular matrix. It
has perichondrium. It is also termed as yellow
cartilage.
• Location: It forms lid on top of larynx, parts of
external ear (auricle), auditory tubes.
• Function: It gives strength and elasticity and
maintains shape of certain structures.

BONE TISSUE
• Cartilage, joint, and bones make up the skeletal system. The
skeletal system supports soft tissues, protects delicate
structures, and works with skeletal muscles to generate
movement. Bones store calcium and Phosphorus, contain
red bone marrow, which generates blood cells and contain
yellow bone marrow, a storage site for triglycerides. Bones
are organs comprised of many different connective tissues.
These include bone or osseous tissue, periosteum, red and
yellow bone marrow and endosteum.
• Location: Compact and spongy bones tissue constitute the
various parts of bones of the body.
• Function: It provides support, protection and storage. It
contains blood forming tissue. It provides levers that act
with muscle tissue to enable movement.

Compact Bone
• Compact bone comprised of tightly packed osteons or haversian
systems. The osteon contains a central canal termed as osteonic
(haversian) canal that is enclosed by concentric rings (lamellae) of
matrix.
• Mature bone cells (osteocytes) are placed in spaces between the
rings of matrix termed as lacunae. Small channels termed as
canaliculi radiate from the lacunae to the central osteonic
(haversian) canal to give passageways by the hard matrix.
• The osteonic canals constitute blood vessels and nerves which are
parallel to the long axis of the bone. These blood vessels interlinked
through the way of perforating canals with vessels on the surface of
the bone.
• In compact bone, the haversian systems are packed closely together
to form a solid mass.

Spongy (Cancellous) Bone
• Spongy bone is lighter and less dense than
compact bone. It contains plates (trabeculae)
and bars of bone neighbouring to small and
irregular cavities which have red bone
marrow. The canaliculi attach to the
neighbouring cavities, instead of a central
haversian canal, to receive their blood supply.
The trabeculae are organized to give
maximum strength. The trabeculae of spongy
bone follow the lines of stress and can realign
if the direction of stress changes.

FLUID CONNECTIVE TISSUE
• Fluid connective tissues include blood and
lymph which are characteristic collections of
cells in a fluid extracellular matrix. Under
normal conditions, the proteins dissolved in
this watery matrix do not form large insoluble
fibers. In blood, the watery matrix is called
plasma.

Blood Tissue
• Blood tissue is a connective tissue having liquid extra-
cellular matrix and formed elements. The extra-cellular
matrix is termed as blood plasma.
• The blood plasma is pale yellow fluid which consists
generally of water with a wide variety of dissolved
materials (e.g. nutrients, wastes, enzymes, plasma proteins,
hormones, respiratory gases, and ions).
• Suspended in the blood plasma are formed elements—red
blood cells, white blood cells and platelets. Red blood cells
carry oxygen to various body cells and remove some carbon
dioxide from them. White blood cells are responsible for
phagocytosis, immunity, and allergic responses. Platelets
participate in clotting of blood

Red Blood Cell
Platelets
White Blood Cell

• Location: It is located within blood vessels,
within chambers of heart
• Function: (i) Red blood cells carry oxygen and
few carbon dioxide.
(ii) White blood cells carry on phagocytosis
and mediate allergic reactions and immune
system responses.
(iii) Platelets are essential for blood clotting.

Lymph
• Lymph is the extra-cellular fluid that moves in lymphatic vessels.
This is the connective tissue which comprised of various forms of
cells in a transparent liquid extracellular matrix that resembles to
blood plasma although contains much less protein.
• The composition of lymph varies from one part of body to another.
For example, lymph leaving lymph nodes includes many
lymphocytes, a type of white blood cell, in contrast to lymph from
the small intestine, which has a high content of newly absorbed
dietary lipids.
• The three main forms of lymphocyte are thymus cells (T cells), B
cells (bursa-derived cells) and natural killer (NK) cells. Lymphocytes
have characteristically large.
• Location: It is located within lymphatic vessels.
• Function: It facilitates the immune system to the body.

Basophil
Neutrophil
Monocyte
Eosinophil

Connective Tissue in Animals

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