HISTOLOGY 



i6i 



between the parenchymatous elements. The appearance of the epithehal 

 cells of the alveoli is not uniform, indicating different phases of secretory 

 activity. In some cells the nucleus is in the middle of the cell and the cyto- 

 plasm is homogeneous. In others the cytoplasm appears foamy or contains 

 large protruding fat droplets. 



If the litter is small and all the nipples are not suckled, some of the 

 glands may undergo partial regression while others are still functioning 

 (16,35). The young mice usually suckle for about 21-23 days. Thelength 

 of the suckling period depends somewhat on the size of the litter, large 



^jjg. --.s;^l-g^'»'"'««^'^ 



^ 



Fig. 86. — Mammary gland 24 hours after lactation stopped. 



for 22 days. (X 100.) 



Lactation had continued 



litters usually suckling longer than small litters. About three weeks after 

 parturition the glands begin to show signs of regression. 



Twenty-four hours after suckling ceases, milk has accumulated in the 

 ducts and alveoli, which are distended. Epithelial cells have become 

 detached and are lying loose in the lumen. These degenerate, the cytoplasm 

 becoming swollen and the pyknotic nuclei fragmenting (Fig. 86). In some 

 epithelial cells the swollen cytoplasm forms globules which are discharged 

 into the lumen, but the nuclei with small amounts of cytoplasm remain 

 intact. The shrunken alveoli lose their close contact with the capillaries. 

 The lack, of blood supply hastens the process of regression. During this 

 process the space between the shrinking alveoli is being filled by adipose 

 cells. Some of these cells seem to develop from fibroblasts which are in 



