Classification of Connective Tissues: Definition, Origin, Components, Types and Functions

Classification of Connective Tissue: A tissue is a clump of cells with similar structures that work together as a unit. The intercellular matrix, a nonliving substance, fills the spaces between the cells. This could be abundant in some tissues while being scarce in others. Salts and fibres may be found in the intercellular matrix, which is unique to a particular tissue and gives it distinct features.

Epithelial, connective, muscular, and nerve tissue are the four primary forms of tissue in the body. All tissues and organs in the body rely on connective tissue for support, protection, and structure. Additionally, connective tissue stores fat, aids in the movement of nutrients and other substances between tissues and organs, and aids in the repair of injured tissue. Read on more about the origin of connective tissue, basic components of connective tissues, types and functions.

Definition of Connective Tissues

Connective tissues hold structures together, provide a framework and support for organs and the entire body, store fat, transport substances, defend against disease, and aid in tissue repair. Cells, fibres, and a gel-like material(ground substance) make up connective tissue. They can be found all over the body. Bone, cartilage, fat, blood, and lymphatic tissue are all examples of connective tissue.

Origin of Connective Tissues

The most abundant and widely distributed tissue in the body is connective tissue. The embryo’s mesoderm is responsible for the formation of the connective tissues. It connects various tissues or organs and provides support to various structures of an animal body.

Fig: Connective tissue

Components of Connective Tissues

The connective tissues contain three components. Matrix, cells, and fibres are the three components.

(i) Matrix (ground substance):

Matrix is mostly a carbohydrate and protein blend. Mucopolysaccharides of numerous types have been identified. Hyaluronic acid is the most frequent mucopolysaccharide ground substance.

Fig: Different Types of Cells in the Connective Tissues

(ii) Connective Tissue cells:
The cells are of different types:
(a) Fibroblasts produce ECM’s structural proteins, i.e.,  fibrous collagen and elastin, adhesive proteins, i.e., laminin and fibronectin, and ground substance, i.e., glycosaminoglycans, such as hyaluronan and glycoproteins.

(b) Adipose cells are also known as Adipocytes or Lipocytes, basically, store fat.

(c) Plasma cells (Plasmatocytes) synthesise antibodies.

(d) Histamine, heparin, and serotonin are produced by mast cells (= Mastocytes). Blood basophils are linked to mast cells. Inflammatory and allergic reactions cause histamine to widen blood vessel walls, but heparin prevents blood from clotting (anticoagulant) inside the blood vessels. To stop bleeding and raise blood pressure, serotonin functions as a vasoconstrictor.

(e) Cell debris, microorganisms, and foreign substances are ingested by macrophages (= Histiocytes or Clasmatocytes). Monocytes give rise to macrophages.

(f) Lymphocytes ingest cell debris, microbes and foreign matter.

(g) Mesenchyme cells give rise to different types of cells of connective tissue.

(h) Chromatocytes (Pigment cells) are found in the dermis of the skin to impart colour to the animal.

(i) Reticular cells form reticular tissue and are phagocytic.

(iii) Connective Tissue Fibres:

These are of three types:

(a) Collagenous or collagen fibres (white fibres) are made up of collagen protein. These fibres occur in bundles. These are unbranched and inelastic.
(b) Elastic fibres (yellow fibres) are formed of a protein called elastin. These fibres are branched and elastic.
(c) Reticular fibres. These fibres are delicate, branched and inelastic. They are made up of reticulin protein. They always form a network.

Types of Connective Tissues

The connective tissue supports the body and binds the body’s various tissues and organs together.

Fig: Flow Chart of Classification of Connective Tissue

Connective tissues are divided into three groups:

A. Loose Connective Tissue
B. Dense Connective Tissue
C. Specialised Connective Tissue

A. Loose Connective Tissue

Loose connective tissues can be found all over the body, providing both support and elasticity. They, along with adipose tissues, form the subcutaneous layer beneath the skin, which connects muscles and other structures to the skin.

In the semi-fluid matrix, the fibres and cells are loosely organised. They operate as a shock absorber and a reservoir for salt and fluid and are located between several organs as a filler.

Areolar Tissue: It is found beneath the skin and helps to sustain the epithelium. Fibres, fibroblasts, mast cells, chromatocytes, adipocytes, and macrophages are spread irregularly. It wraps around blood and lymph veins, supports the organs in the abdominal cavity, and fills the gap between muscle fibres.

1. Adipose Tissue: They are specialised areolar tissue. They are present under the skin and store fat. It acts as a shock absorber and helps in maintaining body temperature in colder environments.
2. Reticular Connective Tissue: It is made up of reticular fibres. It supports the internal framework of organs such as the liver, lymph nodes and spleen

Fig: Connective Tissue

B. Dense Connective Tissue

In the dense connective tissue, fibroblast cells and fibres are compactly packed. Their main function is to support and transmit mechanical forces. They are somewhat less flexible than loose connective tissue. On the basis of the arrangement of collagen fibres, they are divided into two types:

1. Dense regular tissue: In this type of tissue, the orientation of fibres are regular. The collagen fibres are present between the running bundles of fibres parallelly. The regular arrangement enhances tensile strength and poses resistance to stretching in the direction of the orientation of fibre. Examples of dense regular tissue are tendons and ligaments. Tendons attach bones to skeletal muscles. Ligaments attach two bones together.
2. Dense irregular tissue: There are many fibres, including collagen, which are oriented irregularly or randomly. The irregular arrangement of tissue gives uniform strength in all directions. Fibres may form a mesh-like network. This type of tissue is present in the dermis of the skin.

C. Specialised Connective Tissue

Other than these, there are supportive connective tissues that help in maintaining correct posture and support internal organs, e.g. cartilage and bone.

1. Other than these, there are supportive connective tissues that help in maintaining correct posture and support internal organs, e.g. cartilage and bone.
2. Blood and lymph are fluid connective tissues that circulate in the body and help in interaction and communication among all the organs.

Structural connective tissue:

1. Cartilage is a rigid and elastic connective tissue that can resist compression and absorb mechanical stress. It consists of a ground substance or matrix, which is typically composed of an organic matter called chondrin. The chondrocytes (cartilage cells) secreting the matrix are embedded in spaces known as the lacunae (singular —lacuna). The cells usually occur in groups of two or four, and they have a tendency for multiplying. The entire cartilage is surrounded by a tough coat of fibrous areolar tissue called perichondrium.
2. There are several kinds of cartilage, the commonest and simplest being hyaline cartilage. In this, the matrix is clear and homogeneous present in the covering of joints.
3. In elastic cartilage, the matrix is permeated with innumerable elastic fibres present in the pinna of the ear, and white fibro-cartilage, the matrix is densely packed with way white fibres running in bundles.

Fig: Cartilages

4. Bone tissue is known as osseous tissue. The osseous tissue is relatively hard and lightweight in nature.
5. Bone is a rigid connective tissue with considerable strength that resists pressure and bending stress better than cartilage, but it is less flexible, meaning it does not spring back when stressed. Bone is made up of cells and an intercellular matrix, just like other connective tissues.
6. The hard part of the bone is called the ground substance or matrix, and it always contains calcium phosphate. The bone cells are arranged in a regular pattern inside the lacunae. There is a large central cavity containing bone marrow.

Structure of bone tissue

Bone tissue is divided into two types: compact and spongy. The names allude to the fact that the two varieties differ in density or how closely the tissue is packed together. Bone homeostasis is facilitated by three different types of cells. Osteoblasts form bone, osteoclasts break down or resorb bone, and osteocytes are mature bone cells. Bone tissue is maintained by a balance of osteoblasts and osteoclasts.

Fluid connective tissues

1. The liquid matrix or blood plasma in which blood corpuscles are suspended distinguishes blood from all other tissues. In the plasma, blood corpuscles float.
2. Haemoglobin is a red pigment found in red corpuscles or erythrocytes. Each red corpuscle in mammals is a biconcave, non-nucleated disc with a diameter of around 7.5 millimetres. 
3. A cubic millimetre of human blood contains roughly 5 million erythrocytes. 
4. The white corpuscles, also known as leucocytes, are fewer in number. They lack haemoglobin.
5. In human blood, the number of leucocytes fluctuates between 6 and 8 thousand per cubic millimetre. 
6. The thrombocytes, or blood platelets, are a large number of irregular cells that help in the clotting of blood.

Fig: Basic Types of Connective Tissue

Three types of connective tissues can be distinguished depending on the composition of the ground substance:
(1) Connective tissue proper
(2) Skeletal tissues
(3) Vascular tissues

(1) Connective Tissue Proper

Fig: Connective Tissue Proper

Areolar and fibrous tissues, both of which are primarily involved with binding components of the animal body, make up connective tissue. There are two types of fibre: slender yellow fibre that branchesly and connects up to form the elastic network, and wavy white fibres that are grouped in bundles and yield gelatin when boiled. Areolar tissue containing fat cells is referred to as adipose tissue.

(2) Skeletal Tissues: Skeletal tissues include cartilage and bone, which form the vertebrate endoskeleton. 

Fig: Skeletal Connective Tissue

(3) Vascular Tissues: Vascular tissue includes blood, lymph and the structures in which these fluids are formed, stored and destroyed.

Functions of Connective Tissues

1. The connective tissue sheath that surrounds muscle cells, the tendons that connect muscles to bones, and the skeleton that maintains the body’s positions are all examples of attachment and support.
2. Adipose cells store energy in the form of fat.
3. Transportation of nutrients and metabolites through direct diffusion between organs and connective tissue proper.
4. Defence and Scavenging- fights invading cells via inflammation
5. It serves as a shock absorber and aids in body temperature regulation in colder climates.
6. Haematopoiesis- Formation of Blood Corpuscles

Connective Tissue Disorder

Gene mutations or inherited defective genes can cause connective tissue disorders. Epidermolysis bullosa (EB) and Marfan syndrome are two hereditary connective tissue disorders, and Rheumatoid arthritis & Scleroderma are autoimmune connective tissue disorders.

1. Marfan syndrome is caused by faulty genes that produce the fibrillin protein. The condition is characterised by a body that is extremely thin and lengthy. The fingers and toes have a spider-like appearance.
2. Epidermolysis bullosa (EB) is characterised by skin oversensitivity by a gene mutation.
3. Rheumatoid arthritis (RA) –RA is due to the immune system attacking the synovium, the membrane between joints leading to damage in joints and pain.
4. Scleroderma– The connective tissue thickens and hardens, indicating this condition. It might be localised, affecting only a small area of skin, or systemic, affecting key organs.

Summary

All tissues and organs in the body rely on this tissue for support, protection, and structure. Additionally, connective tissue stores fat, aids in the movement of nutrients and other substances between tissues and organs, and aids in the repair of injured tissue. Cells, fibres, and ground substances make up connective tissue. Bone, cartilage, fat, blood, and lymphatic tissue are all examples of connective tissue. The connective tissues contain three components. Matrix, cells, and fibres are the three components.

Connective tissues are divided into three that are loose connective tissue, dense connective tissue and specialised connective tissue. Connective tissue proper, skeletal tissues, and vascular tissues are three categories of connective tissues that can be distinguished depending on the nature of the ground substance. Connective tissue abnormalities can be caused by gene mutations or inherited faulty genes. Marfan syndrome and epidermolysis bullosa (EB) are two inherited connective tissue illnesses.

Frequently Asked Questions (FAQs)

Q.1: What are connective tissues? Give two examples.
Ans: Bone, ligaments, tendons, cartilage, and adipose (fat) tissue are all kinds of connective tissue that differ only in density and cellularity, as well as more specialised and recognised varieties like bone ligaments, tendons, cartilage, and adipose (fat) tissue.

Q.2: Which are the 3 types of connective tissue?
Ans: There are three classes of connective tissues depending upon the nature of the ground substance are connective tissue proper, skeletal tissue, and vascular tissues.

Q.3: Which connective tissue does not contain collagen?
Ans: Blood is the fluid connective tissue that does not contain collagen. As well there are fewer collagen fibres in loose connective tissue, which is highly cellular and rich in proteoglycans.

Q.4: Is lymph a connective tissue?
Ans: Blood and lymph are fluid connective tissues.

Q.5: What is the function of connective tissue?
Ans: All tissues and organs in the body rely on connective tissue for support, protection, and structure. Additionally, connective tissue stores fat, aids in the movement of nutrients and other substances between tissues and organs, and aids in the repair of injured tissue.

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