286 



HISTOGENESIS 



Adipose Tissue. Certain of the mesenchymal cells give rise, not 

 to fibroblasts, but to fat cells. They secrete within their cytoplasm 

 droplets of fat which increase in size and become confluent (Fig. 292). 

 Finally, a single fat globule fills the cell, and the nucleus and cytoplasm 

 are pressed to the periphery. The fat cells are most numerous along the 

 course of the blood vessels in areolar connective tissue and appear first 

 during the fourth month. 



Nucleus 

 Fat globules 

 b 



Mes. Precartilage Cartilage 



FIG. 292. Developing fat cells, 

 the fat blackened with ' osmic acid 

 (after Ranvier). 



FIG. 293. Diagrams of the development of 

 cartilage from mesencyhma (Lewis and Stohr). A, 

 Based upon Studnicka's studies of fish; B, upon 

 Mall's study of 'mammals. Mes., mesenchyma. 



CARTILAGE 



Cartilage has been described as developing in two ways: (i) The 

 mesenchymal cells increase in size and form a compact, cellular precarti- 

 lage. Later, the hyaline matrix is developed between the cells from their 

 cytoplasm (Fig. 293 A}. The matrix may in this case be regarded as 

 the ectoplasm of the cartilage cells. (2) According to Mall, mesenchymal 

 cells give rise first to an ectoplasm in which fibrillae develop. Next, 

 the cells increase in size and are gradually extruded until they lie in the 

 spaces of the ectoplasmic matrix (Figs. 291 C and 2 93 B}. Simultaneously, 

 the ectoplasm undergoes both a chemical and structural change and is 

 converted into the hyaline matrix peculiar to cartilage. About the car- 

 tilage cells the endoplasm produces capsules of hyaline substance. 



The interstitial growth of cartilage is due: (i) to the proliferation of adult cartilage 

 cells; (2) to the production of new matrix. Appositional growth also takes place, through 

 the mitotic activity of the connective-tissue sheath, the perichondrium. Its inner cells 

 are transformed into young cartilage cells. 



