Connective Tissue

Connective tissue is the widely distributed & most abundant in the body of complex animals. They are named so because of their unique feature of linking and supporting other tissue/organs of the body fibres. O.Hartwing called them mesenchyme because they originated from embryonic mesoderm.

1.0What is Connective Tissue?

• Its Primary role is to support organs & cells, protect the organs & other tissues of the body, store fat and repair damaged tissues.

• It consists of soft connective tissues to specialized types, which include cartilage, bone, adipose, and blood.

• In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or, elastin or reticulin.

• The fibres provide strength (collagen fibres), elasticity (elastin fibres) and flexibility to the tissue.

2.0Types of Connective Tissue (C.T)

On the basis of the matrix, connective tissue has three types.

1. C.T. Proper: - Matrix soft and fibrous

2. C.T. Skeleton: - Dense and mineralized Matrix. 

3. C.T. Vascular: - Liquid and fibres free matrix.

3.0Components of Connective Tissue

Connective Tissue is composed of three components.

(A) Different types of cells.

(B) Fibres. 

(C) Matrix.

(A) Different types of cells:

(1) Fibroblast:

• Largest cell of connective tissue proper

• Maximum number

• Branched cytoplasmic processes arise from these cells, so they appear irregular in shape

• The main function or primary function of these cells is to produce fibres. Fibres are composed of protein.

• Chief matrix-producing cell

(2) Macrophage (HISTEOCYTE):

• Modified monocyte (irregularly shaped)

• They are phagocytic in nature. They destroy bacteria & viruses by phagocytosis.

• Also known as scavenger cells of connective tissue because they damage or destroy cells to clean connective tissue

(3) Mast cell (MASTOCYTES):

• These are like basophils of blood in structure and function

• In their cytoplasm, basophilic granules are present, which can be stained with the basic dye methylene blue

(4) Adipocytes 

  (i) Monolocular Adipocyte - Single large fat globule is present.

(ii) Multilocular Adipocyte - Many fat globules are present.

(5) Lymphocyte

• Centrally located large nucleus 

• Cytoplasm is peripheral  

• Key cells of the immune system 

• Involved in the production of antibodies

(6) Plasma cells

• In these cells, a rounded nucleus is present in which chromatin material is arranged like spokes (radial rows) in a wheel, so they are also called as cartwheel cells

• Also called clones of lymphocytes as these are formed by division of lymphocytes

(B) Fibres : 

(1) Collagen fibres (White fibres)

• Bright & white fibres composed of collagen protein

• Present in maximum quantity in vertebrates 

• Wavy & tough fibres are always arranged in a bundle

• Bundle is called fascia

• Only collagen fibres constitute one-third Part of connective tissue fibres

(2) Elastic fibres (Yellow fibres)

• Yellow in colour and composed of elastin protein 

• Branched fibres but always arranged singly

• In these fibres, maximum elasticity is present

(3) Reticular fibres

• Also known as argyrophil fibre since they can be stained with silver salts

• Composed of reticulin protein

• Highly branched fibres, which always form a dense network

• Delicate fibres

• Elasticity is completely absent.

• Mainly distributed in lymphoid organs like spleen and lymph nodes

(C) Matrix:

• Connective Tissue also secrete modified polysaccharides or mucopolysaccharides like hyaluronic acid, which accumulate between cells and fibres and act as a matrix (ground substance).

• It may be fluid, gelatinous, semifluid, etc.

• It supports cells, binds them together, stores water, etc.

Loose Connective Tissue:

• It has more Matrix and fewer fibres.
•  cells & fibres are loosely arranged in a semifluid ground substance.
• It is of two types: - Areolar and adipose connective Tissue.

(A) Areolar Connective Tissue:

Also known as spongy connective Tissue.

• It is the most widely distributed Tissue in the body.

• In this Tissue, the maximum intercellular space and Matrix are present.

• Due to the irregular arrangement of a bundle of collagen fibres, many gaps are present. These gaps are called Areolae.

• In areolae, other components of connective Tissue like fibres, cells, & Matrix are distributed. 

• Few elastic fibres are present.

• It contains fibroblasts, mast and macrophage cells.

• Mast cells, macrophages & fibroblasts are more in number.

Function: - It serves as a support framework for epithelium.

e.g.: - Tela Subcutanea:- A thin continuous layer that connects skin with underlying skeletal muscles  (Panniculus carnosus). 

Submucosa of [Trachea, Bronchi, Intestine, Oesophagus, Stomach]

(B) Adipose Connective Tissue:

• It is located mainly beneath the skin.

• It contains adipocytes / fat cells abundantly.

• The cells are specialized to store fats; the excess nutrients that are not used immediately are converted into fats and stored in this tissue.

• On the basis of adipocytes, two types of fats are found in animals.

1. White fat

2. Brown fat

1. White fat: - It is composed of monolocular adipocytes in which a single large fat globule and a number of mitochondria are present. So, they produce less energy.

e.g., Panniculus adiposus: - A thin continuous layer of white fat under the dermis of the skin, which is also called hypodermis of skin. 

Yellow bone marrow, Fat bodies of frog, Blubber of whale, Hump of camel

2. Brown fat: - It is composed of multilocular adipocytes in which many fat globules are present, and due to the number of mitochondria, it produces 20 times more energy than white fat. 

e.g., cold resistance devices in newborn babies are due to the presence of brown fat.

Dense / Fibrous Connective Tissue:

• It contains more fibres and less Matrix.

• Fibres and fibroblast cells are compactly packed in dense connective Tissue.

• Orientation of fibres shows a regular or irregular pattern and is called dense regular and dense irregular Tissue.

Specialised Connective Tissue:

(i) Cartilage 

• The outermost covering of cartilage is called the perichondrium, which is composed of white fibres of connective tissue.
• Cartilage-producing cells, known as the chondroblasts, are arranged on the periphery of the cartilage.
• These are active cells & divide to form chondrocytes and synthesize the matrix of cartilage.
• Mature cells of cartilage are called chondrocytes.
• They are found in vacuole-like space in a matrix called lacuna, in which 1 – 4 chondrocyte are present.
• Chondroclasts are cartilage-destroying cells.
• The matrix of cartilage is called chondrin, which is composed of chondromucoprotein having chondrotin-6-sulphate and mucopolysaccharides (hyaluronic acid).
• Matrix of cartilage provides rigidity & elasticity to cartilage (matrix solid, pliable and resists compression).
• Blood circulation is absent in the matrix of cartilage, but blood supply is present in the perichondrium.

Hyaline Cartilage:

• It is the most common cartilage of the human body.

• Most of the parts of the embryonic skeleton are composed of this cartilage. So, most bones in our body are cartilaginous bones because they are developed from cartilage.

• The Matrix of this cartilage is glass-like clear or hyaline because fibres are completely absent.

• The colour of the Matrix is bluish & it is translucent (less transparent).

e.g. Nasal septum.

'C' shaped rings of trachea and bronchi. Sternal Part of ribs. (Costal cartilage)

Laryngeal cartilage: The largest part of the larynx is composed of hyaline cartilage.

Articular cartilage: - At the end of a long bone, the Periosteum is absent, and hyaline cartilage is present, known as Articular Cartilage

Elastic Cartilage:

In the Matrix of this cartilage, yellow fibres form a network, so it is a highly flexible cartilage of our body.

e.g. Tip of Nose

Ear Pinna

Larynx – Epiglottis and Cartilage of Santorini of larynx

Wall of Eustachian tube 

White Fibrous Cartilage:

• Perichondrium is absent because complete WFCT is converted into cartilage.

• In the matrix bundle of collagen, fibres are more numerous, so it is the strongest cartilage.

e.g., Pubic symphysis – Connects both the pelvic halves with each other.

Intervertebral disc – A pad of cushion-like structure that absorbs mechanical shock & jerks and protects the vertebral column.

• The central Part of the intervertebral disc is soft and called the Nucleus pulposus.

Calcified Cartilage:

• It is modified hyaline cartilage, but due to the deposition of calcium salts, its Matrix becomes hard like bones (hardest cartilage) 

• Ca++ salt deposits in the form of Hydroxy apatite. 

e.g. Pubis of frog's pelvic girdle.

Head of femur and humerus.

(ii) Bone 

Study of Bone – Osteology

• Process of bone formation – Ossification       

• Hardest Tissue – Bones

• Softest Tissue – Blood.

• Hardest substance – Enamel. 

• The outermost covering of bone is the Periosteum, which is composed of white fibrous connective Tissue.

• The producing cell is called an Osteoblast. They divide to form osteocytes & synthesize the organic Part of the Matrix. 

• A mature cell of bone is called an Osteocyte, which is found in a lacuna. Only one Osteocyte is found in one lacuna. 

• Bone-destroying cells are Osteoclast cells. 

• Bones have a hard and non-pliable ground substance rich in Ca-salts and collagen fibres, which gives bone its strength.

Matrix – Hard and Non-Pliable

It has two parts.

• Inorganic Part: - 65 – 68%

Ca3(PO4)2 – 80% max. rest 20% CaCO3 (10%),

Mg3(PO4)2 (10%), Fluorides (very less).

• Organic Part: - 32 – 35% Ossein, which is a bundle of   collagen fibres suspended in sulphated mucopolysaccharide.

Long bone has three region. 

(a)  Epiphysis

(b)  Diaphysis

(c) Metaphysis

Internal Structure Of Mammalian Bone :

Blood:

• Blood is a fluid connective Tissue containing plasma, red blood cells (RBC), white blood cells (WBC) and platelets. 

• It helps in the transport of various substances like gases, minerals, etc.

Lymph :  

Lymph is a colourless fluid that is formed by the filtration of blood at the capillary level.

• Lymph = plasma without large protein + WBCs mainly lymphocytes 

• Lymph is also called as tissue fluid ECF or interstitial fluid.

4.0Connective Tissue Disorder

Mixed connective tissue disease (MCTD) is a group of diseases that affects parts of the body that connect other tissues & organs together.

It can be classified into several types, including.

(1) Autoimmune Disorder:

Sometimes, due to genetic and other unknown reasons, the body attacks its cells. This results in damage to the body.

Examples: Systemic lupus erythematosus (SLE), Rheumatoid arthritis (R.A.), Systemic sclerosis (SSC) etc.

(2) Hereditary Disorder:

When genetic mutations like DNA, Genes RNA affect the structure and function of connective tissues.

Examples: Marfan syndrome and Ehlers-Danlos syndrome. 

(3) Inflammatory Disorder:

It involves inflammation of the muscle & skin and Immune system dysfunction.

Example: Polymyositis, dermatomyositis.