52 



THE CONNECTIVE TISSUES 



gators have concluded that many are derived from lymphocytes 

 and monocytes escaping into the tissue from the capillaries, and 

 others are derived from mesenchymal cells in the tissue. The 

 nucleus is relatively large and the cytoplasm stains with basic dyes. 

 They vary in size and shape, so that gradations from those re- 

 sembling the leukocytes to those resembling typical histiocytes may 

 be found, this being especially true in inflamed tissue. 



Mast Cells.— This type of cell occurs in most vertebrates but 

 varies in distribution. It is characterized as having cytoplasmic 

 granules that stain selectively with basic aniline dyes. The cyto- 

 plasm of these cells is completely filled with granules, which in some 

 animals are water-soluble. In mammals the cells are usuallv 



Fig. 29. — Photograph of chromatophores in the connective tissue of a section of 

 cleared unstained king of the bullfrog. 



irregularly rounded, but in certain amphibians long cytoplasmic pro- 

 cesses are often present. When neutral red is used as a vital stain, 

 the cytoplasm appears filled with dark red granules, which methylene 

 blue stains purple. 



Pigment Celfe.— Elongated cells with irregular cytoplasmic pro- 

 cesses containing pigment granules occur commonly in loose fibro- 

 elastic connective tissue in certain regions and are known as chroma- 

 tophores. (Fig. 29.) In mammals such pigment cells occur below 

 the epidermis and in the choroid coat of the eye, but in lower verte- 

 brates similar cells are much more widely distributed and occur in 

 many internal organs. In fish and amphibians the ])igment cells 

 in the skin take part in the color changes exhibited by many forms. 

 Such changes are considered as due to alteration in tiie distribution 



