LOOSELY ORCAMZEI) Fl Bh'OELASTlC TISSUE 53 



of the jjranules. rignuMit cells containing the black melanin graiuiles 

 are called melanophores; those with yellow-red granules are called 

 xanth()])h()res; and those with brown-red granules are designated 

 erythrophores. No single species necessarily otters examples of all 

 three types of cells. In some fishes guanine crystals contained in 

 the pigment cells are responsible for the silvery color of the scales. 



Undifferentiated {Mesenchymal) Cells.— Msiny mesenchyme cells 

 remain undift'erentiated and scattered throughout this type of con- 

 nective tissue, usually near the blood vessels. Such cells are smaller 

 than the fibrocytes but not otherwise sharply distinguishable from 

 them in appearance. However, these cells are capable of dift'erenti- 

 ating into various types of connective tissue. 



Fat Cells. — Certain cells of this tissue are characterized by a 

 storage of fat droplets until the cell becomes distended to a spherical 

 shape, with its nucleus and thin layer of cytoplasm pushed to the 

 periphery. (Fig. 30.) Such cells occur singly or in groups along 



Fig. .30. — Diagram showing afrumiilation of fat inside of a fibroblast and the develop- 

 ment of this into a fat cell. 



capillaries, and organizations of them constitute adipose tissue, 

 which we will consider separately. 



Fibers. The fibrous elements which give the name to this tissue 

 are divided into two tyi)es, a white or collagenous and a yellow or 

 elastic fiber. 



Collagenous Fibers. (Fig. 31.)— These fibers are almost trans- 

 parent in the living tissue and are called collagenous because 

 they yield a gelatin when boiled in water. Fixed and stained 

 spreads show a network of fibers extending in all directions. 

 Study shows that the strands are bundles of microscopic 

 fibrils, the larger strands containing more fibrils than the smaller 

 ones. These fibrils are presumably held together in bundles by a 

 cement substance which dissolves when fresh material is soaked 

 in lime water. The fibrils do not branch but their organization into 

 bundles is such that the bundles branch and form networks. In 

 dilute alkalies like potassium hydroxide, and such acids as acetic, 



