Influence of hypophysectomy during lactation on nucleic acids, ribosomes, and ultrastructure of the mammary gland of the ewe

Abstract Secretion of milk disappears very rapidly after removal of the pituitary gland in the ewe. The following modifications in mammary gland cells are associated with cessation of milk secretion: (1) a marked reduction in the number of secretory cells appears between 24 and 96 hr after hypophysectomy, as can be seen from the determination of the mammary gland DNA content and electron microscope examination; (2) an important decrease in the RNA content of the cells occurs before, and is greater than, the variation in the amount of DNA. It corresponds to a rapid inhibition of the 3 H-uridine incorporation into the RNA and results from the stimulation of RNA catabolism as measured by the increase in ribonuclease activity and the disappearance of the ribonuclease inhibitor. (3) there is a fall in the number of polyribosomes, particularly of those that are bound to the endoplasmic reticulum membranes, that is proportionately more pronounced than that of the total RNA. The polyribosomes maintain a synthetic activity that corresponds to their degree of aggregation.