Histology 



321 



Fig. 263. {A) Mesenchymal tissue from embryo of the amphibian, Ambystoma. 

 (B) Pigment-cells from Ambystoma; (below) a cell with pigment dispersed in 

 numerous branched processes of the cell; (above) a "contracted" cell with pigment 

 concentrated, the transparent processes not shown. (Courtesy, Kingsley: "Com- 

 parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



gen, the source of gelatin and glue. These collagenous fibers are only 

 slightly elastic. They may be branched. Each fiber is a bundle of very 

 delicate fibrils. Exceedingly flattened cells with flat nuclei appear as if 

 clinging closely to the surface of a fiber. These connective-tissue cells 

 or fibrocytes are presumably the agencies which have brought about 

 the production of the fiber in the intercellular matrix. 



Elastic fibers are usually finer than collagenous fibers and differ 

 from them chemically in being composed of elastin, which is not a 

 source of gelatin. An occasional elongated connective-tissue cell, a 

 fibrocyte, may be seen stretching along the surface of a fiber (Fig. 264). 

 Elastic fibers commonly occur intermingled with collagenous fibers. 



Reticular tissue consists of cells whose branched processes join to 

 form a network. It resembles embryonic mesenchymal tissue (Fig. 

 263 A). It occurs in such organs as lymph-glands and the spleen. 



Areolar tissue is a loose, open meshwork of connective-tissue 

 fibers — e.g., the subcutaneous tissue lying between the skin and the 

 muscle of the body. 



Tendons and ligaments are connective-tissue structures highly 

 adapted to resisting tensile strain. They consist of connective-tissue 

 fibers arranged in compact bundles. Tendons are inelastic. 



Chromatophores, pigment-cells (Fig. 26W), may occur in con- 

 nective tissue, especially in the dermal layer of the skin. The specific 

 pigment appears as granules lying in the cytoplasm. Black pigment 

 (melanin) is most common, and cells containing it are called nielano- 



