Animal Connective tissue
• Connective tissue, formed from mesoderm of the embryo, is the most abundant and widely distributed tissue of the body.
• This tissue provides the structural framework and support to different tissues and helps in body defense, repair, fat storage, etc.
Components of connective tissue
• Three components are present in the connective tissue, namely, matrix ground substances, cells (fixed and wandering) and fibres which are explained with the figure on the next page.
• The most common cell types are fibroblasts, which produce fibres and other intercellular materials. The two most common types of fibres are: collagen (collagenous) and elastic. Collagen fibres are for strength while the elastic ones are for elasticity of the tissue. Both the cells and the fibres are embedded in the intercellular substance.
• The proportion of the cell, fibres, and intercellular substance vary, depending on a particular nature and function of the connective tissue. For example, a strong connective tissue needs greater proportion of the collagen fibres and fewer cells.
– Reticular fibres are strong and form a branching network. They are made up of reticulin protein.
– Collagen fibres are thick, straight or wavy, and often form bundles. They are very strong, resist stretching and are made up of collagen protein.
– Elastic fibres are slender, unbranched, and very stretchy. They recoil to their original length after stretching or distortion. They are formed of elastin protein.
– A melanocyte is a fixed pigment cell that synthesizes melanin, a brownish-yellow pigment.
– A fixed macrophage is a stationary phagocytic cell that engulfs cell debris and pathogens.
– Mast cells are fixed cells that stimulate local inflammation and mobilize tissue defenses.
– Fibroblasts are fixed cells that synthesize the extracellular fibres of the connective tissue.
– Adipocytes (fat cells) are fixed cells that store lipid reserves in large intracellular vesicles.
– A plasma cell is an active, mobile immune cell that produce antibodies.
– Free marophages are wandering, phagocytic cells that patrol the tissue, engulfing debris or pathogens.
– Mesenchymal cells are mobile stem cells that participate in the repair of damaged tissues.
– Neutrophils are small, mobile, phagocytic blood cells that enter tissues during infection or injury.
– Lymphocytes are mobile cells of the immune system.
Ground substance fills the spaces between cells and surrounds connective tissue fibres. In all forms of connective tissue proper, ground substance is clear, colorless, and viscous due to the presence of proteoglycans and glycoproteins.