Classification of Connective Tissue | Animal Tissues | Zoology

Based on the composition, organization of the extracellular matrix, cell types and their functions, the connective tissue is classified into following three major types: 1. Loose Connective Tissue 2. Dense Connective Tissue 3. Specialised Connective Tissue.

1. Loose Connective Tissue:

The loose connective tissue is the least specialized connective tissue in the body. This type of tissue has a relatively large proportion of ground substance or cells or both cells and ground substance. The fibers are very few.

The loose connective tissue is subdivided into the following:

(i) Areolar tissue.

(ii) Adipose tissue.

(i) Areolar Tissue:

The areolar tissue is the most simple and widely distributed connective tissue found in most parts of the vertebrate body as a packing tissue. It occurs between adjacent body tissues, as an external covering of blood vessels, nerves and muscles. It is also found beneath the skin and in the digestive, respiratory and urinary tracts.

Structure:

Areolar connective tissue is a loose connective tissue characterized by the presence of transparent, gelatinous, sticky matrix containing a mixture of mucin, hyaluronic acid and chondroitin sulphates. Embedded in the matrix there are a variety of cells and three types of fibres. Since the fibres are loosely arranged in a random manner, this tissue is also called loose fibrous connective tissue. The spaces found between the fibres are called areolae, hence also the name areolar tissue.

a. Fibres:

Basically there are two types of fibres-white and yellow. In addition a small number of reticular fibres are also seen.

i. White Fibres or Collagen Fibres:

White fibres are abundant longer, thinner, inelastic, flexible, unbranched and wavy fibres which occur in bundles called fascia. They are cemented together by mucin. These fibres are formed at the site of injury, thus helping in tissue repair. When boiled with water, some of the collagen of white fibres changes to a colloidal protein called gelatin. The collagen fibres provide tenacity (firmness) to the tissue. Collagen is digestible by pepsin.

ii. Yellow Fibres or Elastic Fibres:

Yellow fibres are fewer, smaller, thicker, elastic, flexible, branched and straight fibres which occur singly. The branches of yellow fibres join with one another and form an irregular network. These fibres are formed of a protein called elastin. It is resistant to boiling. The yellow fibres provide elasticity to the tissue and help to restore normal shape after distortion. Elastin is digestible by trypsin.

iii. Reticular Fibres:

Reticular fibres are very thin, inelastic, branched and occur singly. They form a tightly woven delicate network that joins the connective tissue to adjacent tissue. These fibres are mainly formed of a protein called reticulin. They resemble the white fibres in their structure, but are less common than the white and yellow fibres. Reticular fibres provide a delicate supporting framework for individual cells in the organs like liver or spleen.

b. Cells:

There are seven types of cells in the areolar tissue. They are fibroblasts, macrophages, mast cells, plasma cells, fat cells, chromatophores and mesenchyme cells.

i. Fibroblasts (Fibrocytes):

Fibroblasts are the most common and principal cells of the areolar tissue. Fibroblasts exhibit morphologically diverse appearances depending on their location and function. Generally they are large, flat, stellate cells with long processes and oval nucleus.

The main function of fibroblasts is to secrete the precursors of all the components of the extracellular matrix, primarily the ground substance and a variety of fibres. The fibroblasts provide a major role in wound healing. The fibrocytes, small inactive form of fibroblasts change into fibroblasts and secrete fibres during wound healing.

ii. Macrophages (Histiocytes):

Macrophages (Gr., macro = large, phagein = to eat) are large sized amoeboid cells with oval nucleus. They are the transformed monocytes of the blood. They are almost as numerous as the fibroblasts. These cells act as phagocytes. They engulf bacteria, damaged or dead cells and other microbes and foreign particles.

Although generally they are immobile, at times they move to the areas of bacterial invasion by amoeboid locomotion. When foreign body to be engulfed is too large, many macrophages fuse with one another and form multinucleate giant cell. Thus they serve a defensive function.

iii. Mast Cells (Mastocytes):

Mast cells are large cells with spherical nucleus and granular cytoplasm. They are secretory alarm cells. They produce three substances – heparin, histamine and serotonin. Heparin is an anticoagulant and prevents clotting of blood within the uninjured blood vessels. It neutralizes the action of the enzyme thrombin. Histamine is an important protein involved in many allergic and inflammatory reactions.

Whenever the histamine is released from the mast cells of the injured tissue, it attracts phagocytes to the site of the injury to help in body defense. Histamine causes vasodilatation and contraction of smooth muscles. The serotonin produced by the mast cells of some mammals, such as rat and mouse is a vaso-constrictor which stops bleeding and raises blood pressure.

iv. Plasma Cells:

Plasma cells are relatively large oval or rounded cells, smaller than macrophages. They possess non-granular cytoplasm and a small nucleus. They synthesize antibodies.

v. Fat Cells (Adipocytes):

The fat cells or adipose cells are spherical containing a large lipid droplet that occupies most of the cytoplasm. In fact, the cytoplasm and nucleus are pushed to the periphery in the fat cells.

vi. Chromatophores:

These pigment cells are present in specialized areas such as the skin and the eye. The much branched chromatophores are densely packed with melanin granules.

vii. Mesenchyme Cells:

Mesenchyme cells are the reserve undifferentiated cells which can be transformed to any one of the above cell types as and when needed.

(ii) Adipose Tissue:

Adipose tissue is a loose connective tissue, which is dominated by fat cells or adipocytes. Although most loose connective tissue contains scattered clusters of adipocytes, the term adipose tissue is usually applicable for large masses of adipocytes which are grossly visible.

Adipose tissue is found in specific locations which are referred to as ‘adipose depots’. In mammals it is found beneath the skin (panniculus adiposus), in the mesenteries around certain organs like heart, kidneys, eye­balls, palms, soles and in the inguinal region.

It also forms yellow bone marrow. In frogs the fat bodies are formed of adipose tissue. In whales, blubber, a thick layer of adipose tissue occurs largely composed of adipose tissue. It is mostly concentrated under the skin in the abdomen, buttocks, thighs and breasts in humans.

Structure:

Basically adipose tissue is an areolar tissue. It is packed with large, oval, fat cells or adipocytes. To begin with, a fat cell develops small fat droplets which later fuse to form a large fat globule. This so formed fat globule pushes the cytoplasm and nucleus to the periphery. This gives a ‘signet ring’ appearance to the adipocytes.

The matrix of the adipocytes also contains fibroblasts, macrophages, mast cells, white, yellow and reticular fibres. However, the fibres are almost indistinguishable. A fat cell may possess either a large fat globule (called monocular in white or yellow adipocytes) or a number of small fat droplets in the centre (called polyocular in brown adipocytes).

Functions:

Adipose tissue primarily helps as a food reserve. It enables the animal to sustain the body during starvation. It acts as a shock absorber around the organs like heart, kidneys and eye­balls. It prevents heat loss from the body and helps in homeothermy. It gives shape to the body.

2. Dense Connective Tissue:

Dense connective tissue is also called dense fibrous tissue. It has collagen fibres as the main element in its matrix. The fibroblasts are found arranged in rows between the collagen fibres.

Based on the orientation of fibres dense connective tissues are of two types:

(i) Dense Regular Connective Tissue:

This type of tissue provides strong connection between different tissues. In dense regular connective tissue, the collagen fibres are bundled in a parallel fashion. They are lined up in parallel, like the strands of a rope. Tendons which connect muscle to bone and ligaments which bind bone to bone derive their strength from the regular, longitudinal arrangement of bundles of fibres.

(ii) Dense Irregular Connective Tissue:

This tissue has fibres that are not arranged in parallel bundles as in dense regular connective tissue. This type of connective tissue produces the tough coverings that package organs, such as the capsules of the kidneys and adrenal glands. It also covers muscle as epimysium, nerves as perineurium and bones as periosteum. This tissue is also abundant in the dermal layer of skin.

3. Specialised Connective Tissue:

The specialized connective tissues include supportive connective tissues or skeletal tissues (cartilage and bone) and vascular or fluid tissues (blood and lymph). Either singly or in combination, the cartilage and bone form the endoskeleton of vertebrates.

I. Cartilage:

Structure:

Cartilage is a type of connective tissue which is tough, semi-transparent, elastic and flexible. The matrix or ground substance of cartilage consists of glyco-protein material, chondroitin. The cartilage cells called chondrocytes or chondroblasts occur scattered in the matrix inside the spaces called lacunae. The matrix also possesses collagen and elastin fibres.

Cartilage is covered by a dense fibrous membrane, the perichondrium from where the new chondroblasts are produced and released into the matrix. The cartilage has neither the nerves nor the blood vessels. The chondrocytes are fed by diffusion. Thus compared to other connective tissues, cartilage grows and repairs more slowly. Like all connective tissue, cartilage develops from mesoderm.

In some vertebrates like sharks, the entire skeleton is made of cartilage. In mammalian embryos, the skeleton first forms as cartilage tissue. The cartilage acts as a model and is gradually replaced by bone as the embryo grows. Such cartilage is known as temporary cartilage. The process by which bone tissue follows the cartilage model and slowly replaces it is known as ossification. Permanent cartilage, i.e., the cartilage which does not become ossified is found at the tip of the nose, in the external ear and in the wall of the trachea and the larynx.

Location:

Cartilage occurs in many places in the body such as the articular surface of the bones, the rib cage, the nose, the ear, the bronchial tubes and the intervertebral discs.

Types of Cartilage:

Based on the composition of matrix, the cartilage has been classified into following three types – hyaline (with more matrixes and less fibres), fibrocartilage (with more white fibres) and elastic cartilage (with more yellow fibres).

II. Bone Tissue or Osseous Tissue:

Bone is the hardest tissue in the body. Bone tissue occurs in the different bones of the skeleton. Bone forms the major part of the endoskeleton of bony fishes and tetrapods like amphibians, reptiles, birds and mammals. Bone is a hard, rigid, solid, inflexible and strong connective tissue. It consists of living cells with large amounts of ground substance or matrix.

It is impregnated with organic salts such as calcium carbonate (7%) and calcium phosphate (85 %). In addition to this, the matrix also contains small amounts of sodium and magnesium. The matrix also contains numerous collagenous fibres and a large amount of water. Collagen fibres together with the bone cells constitute the organic (living) matter in bone tissue.

Structure of a Bone:

i. A long bone like femur consists of a centre piece, the shaft (diaphysis) and a thickened head (epiphysis) at each end.

ii. The heads articulate with other bones in the joints and are covered with a thin layer of hyaline cartilage.

iii. The remainder of the bone is covered by a tough, strong membrane, the periosteum. The periosteum is composed of bundles of white fibrous tissue called Sharpey-Schafer fibres and is richly supplied with vessels.

iv. There is a small artery which penetrates the shaft near the centre and supplies the bone tissue with blood.

v. The periosteum also contains active bone forming cells called osteoblasts which produce new bone material.

vi. The periosteum provides surface for the attachment of tendons and muscles.

vii. Beneath the periosteum is a layer of compact bone which is thicker in the shaft than in the two heads.

viii. The shaft of the long bones such as humerus, femur, etc., encloses a cavity called marrow cavity or medullary cavity which is lined with a thin soft membrane called endosteum. But the small bones are solid.

ix. Like the periosteum, the endosteum also is composed of white fibrous tissue, blood vessels and bone forming cells.

x. The marrow cavity contains a soft and vascular fatty tissue called bone marrow or myelogenous or myeloid tissue.

xi. The bone marrow is of two types red marrow and yellow marrow. The red marrow occurs in the spongy bone of the epiphysis of long bones. The red bone marrow which is highly reticular vascular tissue forms erythrocytes and granular leucocytes. The yellow marrow which is less vascular occurs in the shaft. It is mainly concerned with the storage of fat. Only at the time of emergency it forms the blood corpuscles.

xii. The matrix of the bones (osteoid matrix) is formed of two types of matter – organic or animal matter and inorganic or mineral matter. The organic matter is formed of the protein, ossein (hence the name osseous tissue), secreted by the osteocytes. The inorganic matter is formed of phosphates, carbonates, sulphates and fluorides of calcium and magnesium. Of these, calcium phosphate predominates. These minerals are responsible for the hardness of the bone.

xiii. With the advancement of age, the inorganic matter increases and the organic matter decreases. As a result of which bones appear flexible in the young and brittle in the adults. That is why the aged people get bone fracture easily.

Under the microscope, the dense or compact bone shows a definite and a characteristic pattern of arrangement. The ground substance of bone is arranged in concentric layers, lamellae around the small canals called Haversian canals. Each Haversian canal is surrounded by 4-20 lamellae, which give the appearance of growth rings of a tree. Some lamellae occur concentrically around the marrow cavity. These are called circumferential lamellae and they lack Haversian canals.

Embedded in the lamellae, there are fluid filled cavities called lacunae. The lacunae occur at regular intervals in the concentric layers of ground substance. The lacunae are connected to one another and to the Haversian canals by a system of interconnecting canals called canaliculi.

Each lacuna is occupied by an irregular bone cell called osteoblast (inactive bone cells are called osteocytes). In the developing bone, each osteocyte gives off several protoplasmic processes called filopodia which extend through canaliculi, make contact with similar processes of the neighbouring cells and form a protoplasmic network throughout the matrix.

Each Haversian canal, its concentric lamellae, lacunae with osteocyte and canaliculi together form a cylindrical unit called Haversian system or osteon. Osteons are fundamental structural units of mammalian bone. The spaces between the Haversian systems are occupied by interstitial lamellae which are also without Haversian canals.

The Haversian canals run parallel to the long axis (shaft) of the bone. They are interconnected with one another via Volkmann’s canals. Each Haversian canal contains an artery, a vein, a lymph vessel and a nerve. The blood vessels of the Haversian canals are connected with those of periosteum and bone marrow.

The capillaries of the Haversian canal blood vessel are connected with the lacunae of the particular Haversian system through canaliculi. The capillaries supply oxygen and nourishment to and receive metabolic wastes from the osteocytes. The blood vessels of the Haversian system pass into the Volkmann’s canal.

Functions of Bone Tissue:

i. Support:

The skeleton which consists mainly of bone tissue forms a supportive frame work, giving shape and rigidity to the body. It provides attachment for most of the skeletal muscle.

ii. Locomotion/Movement:

The bone tissue forms a system of levers to which the voluntary muscles are attached. When the muscles contract, they pull on bones and movement is produced.

iii. Protection:

Bones protect many internal organs such as the brain and spinal cord from the injury. In addition the heart, lungs, kidney and reproductive organs are given some degree of protection.

iv. Mineral Homeostasis:

Bone tissue stores a number of minerals particularly calcium and phosphorus. Under the control of endocrine system, bone releases the minerals into the blood or stores the minerals in bone matrix to maintain critical mineral balance.

v. Blood Cell Production:

In all bones of infants and certain bones of the adult a connective tissue, red bone marrow produces blood cells by hematopoiesis process. Hematopoiesis process begins in the yolk sac in the first week of embryonic development. By the third month of gestation, stem cells migrate to the fetal liver and then to spleen (bet 3-7 months of gestation these two organs play a major role in hematopoiesis). Next the bone marrow becomes the major hematopoietic organ and hematopoiesis ceases in the liver and spleen.

vi. Storage of Energy:

In some bones, yellow bone marrow stores lipids, creating an important energy reserve for the body.

III. Vascular Tissue:

The vascular tissue is a special type of connective tissue with fluid matrix and free floating cells. Since fluid part is more in this type of connective tissue, it is also called fluid tissue. It differs from other connective tissue in that the matrix is motile and it lacks fibres. Therefore this tissue is also called circulating tissue.

But in vascular tissue, the matrix is not secreted by the cells it contains. The vascular tissue supplies nutrients to the cells, hence the name. It also transports the materials form one place to another within the body. As such, the vascular tissue is also called transporting tissue.

Types of Vascular Tissue:

The vascular tissue is of two types – blood and lymph.