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CONNECTIVE TISSUES

By Oba Mike



Introduction

Connective tissues are the most abundant tissues in the body. As the name suggests, the major function of connective tissue is to bind together, support and strengthen cells and other body tissues.

Functions of Connective tissues

1. They give structural and functional support to cells and other tissues.

2. Protection and defense against pathogens.

3. Stores energy.

4. Transport of substances within the body.

5. Insulation.

Structure of Connective Tissues

Connective tissues consist of two basic elements:

1. Cells
2. Extracellular matrix

Cells

Connective tissue cells originate from embryonic cells called mesenchyme. The cells vary according to the type of tissue and include the following:
A. Fibroblasts
B. Adipocytes (fat cells)
C. Macrophages
D. Mast cells
E. Blood cells ( erythrocytes, leukocytes, thrombocytes).

Fibroblasts

They are large flat cells with branching processes. They are the most numerous connective tissue cells and are present in all the general connective tissues. 
The function of fibroblasts is to secrete fibres and certain components of the ground substance of the extracellular matrix.

Adipocytes

Also called fat cells or adipose cells. They store triglycerides (fats) and insulate the body against heat loss. Located deep in the skin and around organs such as heart and kidneys.

Macrophages

They develop from monocytes (a type of white blood cell). They are involved in inflammatory response.

Blood cells

These include plasma cells, erythrocytes, leukocytes, and thrombocytes that have various functions.

Mast Cells

Mast cells are abundant alongside the blood vessels that supply connective tissue. They produce histamine, a chemical that dilates small blood vessels as part of the inflammatory response.  In addition, researchers have recently discovered that mast cells can bind to, ingest, and kill bacteria.

Extracellular Matrix

As the name suggests, the extracellular matrix is the substance found between the connective tissue cells. The matrix is responsible for the unique property of each tissue. It has two components:
a. Ground substance
b. Fibres.

Ground substance

The ground substance of the extracellular matrix is located in the spaces between the cells and the fibres. It may be fluid (blood plasma), semifluid, gelatinous ( jellylike) or calcified (as seen in bone).
It supports cells, bind them together, stores water, and provides a medium for exchange of substances.

Fibres

Apart from the ground substance, the extracellular matrix is made of fibres. 
There are three of these fibres:
i. Collagen fibres
ii. Reticular fibres
iii. Elastic fibres
Collagen fibres are the most abundant. The kind of fibres present and its abundance also determines the physical property of the tissue. For example the elasticity of the skin is due to numerous elastic fibres.

Classification of Connective Tissues

Connective tissues can be classified in various ways. Generally we have:

1. Embryonic Connective Tissue
2. Mature Connective Tissue

Embryonic connective tissues are present primarily in the embryo, the developing human from fertilization through the first two months of pregnancy and in the foetus (3rd month to birth).

Mature connective tissues are present in newborns and adults.
Based on these, we have:

1. Embryonic Connective Tissue
A. Mesenchyme
B. Mucous connective tissue

2. Mature Connective Tissue
A. Loose connective tissue
i. Loose areolar connective tissue
ii. Adipose tissue
iii. Reticular tissue

B. Dense Connective Tissue
i. Dense regular connective tissue
ii. Dense irregular connective tissue
iii. Elastic connective tissue.

C. Cartilage
i. Hyaline cartilage
ii. Fibrocartilage
iii. Elastic cartilage

D. Bone tissue

E. Liquid Connective Tissue
i. Blood
ii. Lymph

Details of location and functions of thess tissues will be highlighted in class.




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