Morphometric analysis of thyrotropes in developing and cycling female rats: studies of intact pituitaries and cell fractions separated by centrifugal elutriation.

Abstract

The development and morphology of immunocytochemically stained thyrotropes were studied in sections of intact pituitaries and dissociated cell fractions separated by centrifugal elutriation. In the initial cell suspension from six elutriation experiments, adult female rat thyrotropes were 4.8 +/- 0.5% (+/- SE) of the cell population. Correlative morphometric studies of Araldite- or glycol methacrylate-embedded pituitaries showed that there were no changes in the percentage of thyrotropes with the stage of the estrous cycle and that the percentage of thyrotropes (5.14 +/- 0.4%) was not significantly different from the percentages in the initial cell suspension. TSH cell area fraction (a) measurements (with a 10,000-microns 2 ocular grid) showed that adult rat thyrotropes covered 171 +/- 5 microns 2 of the grid area and averaged 15 microns in diameter. In neonatal rats, thyrotropes were half the a of those in the adult for the first 9 days of life. Thereafter, they expanded, and the a reached adult levels by 20-21 days of development. TSH cell percentages remained 2-3 times adult levels throughout postnatal development (2-22 days). This developmental pattern contrasted with that in the male, which showed adult percentages of thyrotropes by 15 days of age. In the elutriation experiments from adult rats, thyrotropes were 2- to 4-fold more concentrated in the fractions eluted at 37.0 ml/min or greater, which contained the largest cells. Fraction 7 (39.5 ml/min) showed a 2-fold enrichment of thyrotropes to 11.6 +/- 1.4%, and fraction 8 showed a 4-fold enrichment to 19.6 +/- 2.7%. A few small TSH cells were found in the fractions 1-3, eluted at 11.8-19.5 ml/min. Electron microscopic studies showed that some of these small TSH cells were poorly granulated and difficult to distinguish from small gonadotropes, whereas the large thyrotropes resembled those described previously in the adult or developing male rat. These studies, thus, combine techniques of immunocytochemistry, morphometrics, and cell separation by elutriation to describe TSH cells in the female rat pituitary. Our findings agree with those reported by Denef et al., who showed that thyrotropes in the adult male rat are among the largest cells in the pituitary. The few small thyrotropes in the female rat may be equivalent to the prolific cells described by Leuschen et al. in the male rat that enrich monolayers from these fractions after 7 days in culture.