Ultrastructure of the mammary gland in lactating rats after local implantation of oestrogen.

Cannulae containing oestrogen were implanted in the mammary glands of rats on the first day after parturition and lactation assessed by daily measurements of the litter weight. Sham-implanted and intact lactating rats were used as controls. A clear-cut inhibition of the milk yield was observed in the oestrogen-implanted group. Mammary tissue was processed for fine structural study after 4, 8 or 10 days. The initial phase showed milk stasis and secretion of colostrum progressively involving the alveoli and culminating in regression of the mammary tissue. Disappearance of myofilaments from the myoepithelial cells was consistently observed. Secretion of colostrum and mammary regression could be the result of the stasis produced by an abnormal dynamic state of the myoepithelial cell elicited by the steroid.