6. Connective tissue. Histogenesis. General characteristics. Classification.

Connective tissue is a combination of cells and has lots of intercellular substance. It derives from mesenchyme and has productive, protective and trophic functions. Fibroblasts, osteoblasts and chondroblasts are productive cells. Macrophages, mast cells and plasma cells are protective cells whilst pigment cells and adipocytes are trophic cells. 

Fibroblasts originate from mesenchyme, have a spindle-like shape and a large nucleus. Their cytoplasm is basophilic-stained and well-developed RER and Golgi complex can be seen with an electron microscope. 

Macrophages arise from monocytes and have lots of RER, SER, lysosomes, Golgi apparatus and mitochondria. Involved in phagocytosis. 

Plasma cells originate from B-lymphocytes and produce antibodies. Its nuclei are eccentrically positioned with heterochromatin and the cytoplasm consists of well-developed RER and Golgi apparatus. 

Mast cells arise from basophil granulocytes and they play a role in allergic reactions. Their granules contain histamine, heparin and serotonin.

Pigment cells contain melanin and can be found on the skin, choroid of the eye, iris of the eye and meninges of the brain. Melanin is produced by melanocytes and stored in melanophores.

Adipose tissues is derived from mesenchyme. There are two types; white adipose tissue and brown adipose tissue. 

White adipose tissues are unilocular adipocytes and they contain a single large lipid droplet. Brown adipose tissues are multilocular adipocytes containing many lipid droplets and they regulate body temperature.

Connective tissue has a good blood supply.There are two types of connective tissue; embryonic and mature connective tissue. 

Mesenchyme and mucous are types of embryonic connective tissue. 

Mature connective tissue is split into two categories; with fibrous intercellular substance and with solid intercellular substance. 

Fibrous intercellular substance includes loose connective tissue, collagenous connective tissue, elastic connective tissue, reticular tissue, adipose tissue and pigment tissue, whereas solid intercellular substance includes bone, cartilage and dentin.

Connective tissue is found as packing between the structures in the organism.

Its basic components are ground substance & extracellular matrix which contains cells and numerous fibres.

There are 3 types of fibres:

Collagen fibres are the most numerous and arranged in bundles of both straight and branched fibrils. They are formed in all types of connective tissue and consist of collagen. They are formed by fibroblasts. 

Elastic fibres are fewer in number and are arranged in bundles to form a 3D network. They are found in ligaments. 

Reticular fibres are made of type III collagen and are finer than the collagen fibres. They are uneven in thickness and form a network. 

Found in lymphoid organs, lymph nodes, bone marrow, and spleen. 

Fibres produced by fibroblast and functions includes hydrophilia, metachromasia and optical homogeneity. 

There are 3 types of cartilage; hyaline cartilage is the most common type. It is found in the ribs, respiratory system and at the articular cartilage. Hyaline cartilage consists of 3-5 isogenous groups with lots of chondrocytes and it has a metachromatic-stained intercellular matrix due to proteoglycans. 

Fibrocartilage has thick bundles of collagen fibres and many rows of isogenous groups. It is the strongest type of cartilage and it is found in the invertebral discs, joint capsule and the cruciate ligament of the knee. 

Elastic cartilage has isogenous groups of 1-2 chondrocytes and it has a perichondrium. It is found in the external ear, epiglottis and larynx.

Bone is a connective tissue because its matrix is mineralised by calcium phosphate. The osteon is the main structural unit of the bones and it consists of bone lamella, osteocytes and osteoclasts. There are two types of mature bone: compact bone and spongy bone.

Osteoblasts elaborate bone matrix that arise from mesenchyme and have a basophilic-stained cytoplasm. 

Osteocytes maintain bone tissue and are involved in remodelling, growth and repair of the bone. Osteoclasts aremulti-nucleated, and involved in the breakdown of bone matrix. They are derived from the fusion of monocytes and have lots of lysosomes and microvilli on the surface