Intracellular dynamics in pituitary mammotropes throughout the rat estrous cycle. I. Morphometric methodology and hormonal correlations with cellular and nuclear volumes

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Abstract
Serum prolactin (PRL) levels display regular patterns throughout the 4‐day rat estrous cycle. A morphometric analysis of mammotropes (PRL‐secreting cells) in the anterior pituitary was performed in an effort to quantify some of the ultrastructural events occurring prior to and during the secretion of PRL throughout the estrous cycle. In this study, a systematic sampling protocol applicable to the rat adenohypophysis was developed, which took into account the lack of synchrony in estrous cycles among the animals, the heterogeneity of the gland, and the number of cells required to yield a representative sample. These techniques were used to determine the cell, nuclear, nucleolar, and cytosol volumes, which were correlated with the changing levels of serum estradiol and PRL and pituitary PRL. Estradiol concentrations rose during late diestrus‐1 and continued rising in a pulsatile manner, reaching maximal levels during proestrus. The elevations of estrogen were accompanied by increases in nucleolar and nuclear volumes. However, at early estrus and on the afternoon of diestrus‐2, times at which it has been reported that progesterone levels are elevated, there was a dampening effect and these volumes declined. During early diestrus‐2, when estradiol levels began to rise, there were increases in nucleolar, nuclear, cellular and cytosol volumes, as well as increases in pituitary PRL; but at late diestrus‐2 there was a marked increase in nuclear and nucleolar volumes which was not accompanied by increases in cellular and cytosol volumes. During proestrus, two peaks of serum PRL were found, at 6 pm and 12 m, which were concomitant with increased nucleolar, nuclear and cellular volumes. The 6‐pm proestrous peak of PRL was preceded by elevated pituitary PRL; however, the 12‐m proestrous peak occurred when the pituitary PRL was minimal, even though nuclear and cellular volumes were elevated. This suggests that the 6‐pm release of PRL was derived from previously synthesized and stored hormone, while the 12‐m peak was the result of the immediate release of PRL, with little storage.