Present only in the reticular tissue and stellate in appearance.In fact they are modified fibroblast producing reticular fibres.
Mast cells were discovered by Paul Echrlich.They are large,irregular ovoid cells found in areolar tissue.and their number increase during allergies.It produces or secretes histamine (vasodilator), serotonin(vasoconstrictor) and heparin(anticoagulant). Histamine dilate the blood vessels in allergic and inflammatory conditions.Heparin checks the clotting of blood inside the blood vessels.Serotonin act as vasoconstrictor to arrest bleeding.
These are the smallest ,less numerous and spherical or ovoid cells resembling lymphocytes of blood and lymph.These actively move about by pseudopodia.Their function is to form and carry antibodies.That is why,they are seen in large numbers at sites of inflammation.
These are usually small and rounded ,superficially resembling lymphocytes but are sluggishly amoeboid and short-lived(only 2 or 3 days).These are the most potential antibody-forming cells of body presumably ,mature lymphocytes(B-lymphocytes form antibody) transform into plasma cells or proliferate to form plasma cells.Plasma cells are also known as cart wheel cells.
Fat or Adipose cells (Adipocytes or Lipocytes):
A few,large and spherical cells occur in areolar tissue,singly or in clusters around small blood vessels.Each cell contains a large globule of fat surrounded by a thin peripheral layer of cytoplasm having a nucleus.
These cells closely resemble the eosinophilic leucocytes of blood.These probably play a phagocytic role in inflammatory and allergic reactions.
These are pigment cell present in specialized areas such as skin and eye.They are much branched and packed with pigment granules.They are stellate(star like) cell,which are phagocytic in nature.They phagocytes melanin producing cells and retain melanin hence they provide colour to the skin and other organs.Melanin is black pigment which protects body from ultraviolet rays of sun.
These are reserve undifferentiated cell which can be transformed into other types of cells when needed.
(ii)Fibres of areolar tissue:
These are made up with protein and non living structures of protein produced by fibroblasts and present in matrix of connective tissue and are of three types_
These are the most abundant fibrous element of areolar and other connective tissue. These are long,unbranched fibres of a soluble and shining collagen protein(tropo collagen). These fibres are most strengthful and provide maximum tensile strength.These are colourless and hyaline,yet called white fibres to distinguish them from yellow elastin fibres.Collagen protein is the most abundant protein of the body constitutes 25% the total body protein. Collagen fibre can be stained by eosin.When collagen fibres are removed from the areolar tissue they become loose and elastic.e.g.,Bone ,Cartilage,Ligement and tendon.
Yellow elastin fibres:
Formed of elastin protein ,these fibres are less numerous ,thinner,branched ,anastomosing, and of a pale yellow colour.These are very elastic and remain stretched due to tension in the areolar tissue,when broken in teased preparations,these coil and curl like tense wires. Elastin is probably the most resistant of all body proteins to chemical changes. Thousands of years old 'mummies' still have their arteries intact due to well-preserved elastin fibres.They are the orceinophilic i.e.,stained by orcein.
These are delicate,freely branching and inelastic fibres of reticulin protein,found interwoven ,to form networks.These are very abundant in embryos,new born babies and in healing and regenerating wounds.In areolar tissues of adults,these are mostly replaced by collagen fibres,but remain abundant in lymphoid adn blood forming tissues and in the stroma of pancreas,liver etc.They are stained with AgBr and AgNO3 hence are called Argentophillic.On boiling collagen and reticular fibres both convert in glue.