Quantitative ultrastructural study of prolactin cells from lactating mice during a 24-hour period

A quantitative ultrastructural morphometric study was performed on the prolactin (PRL) cells of lactating mice, at different times in a circadian period. The results showed significant time fluctuations in the percentages of cytoplasmic volume occupied by several organelles. The endoplasmic reticulum appeared more developed during the dark period, indicating a higher synthesis of secretory material in this phase. The Golgi complex, including an increased number of immature granules, was found remarkably enlarged during the light period. Moreover, the release of PRL would be increased during the dark phase, as judged by the diminution of the secretory granules and the higher degree of emiocytotic activity. The present morphometric approach in the detection of circadian variations of PRL secretion in lactating mice is emphasized.