Postnatal maturation of gonadotropes in the male rat pituitary

The postnatal maturation of gonadotropes was studied with the use of electron microscopic-immunocytochemical strains for the beta-chains of LH and FSH on serially sectioned fields. A third serially sectioned field was stained with antiserum to ACTH-(17-39). Morphometric studies on semithin and ultrathin plastic sections stained for LH and FSH showed a 200-300% increase in the percentages of LH and FSH cells during the first week of postnatal life. In the 1- to 2-week-old rats, the percentage of gonadotropes in the population was greater than that in the adult. Analysis of the hormone content of serially sectioned gonadotropes showed that storage patterns similar to those in the adult were reached by the seventh day of life. In the neonatal group, 66% of the serially sectioned cells contained both FSH and LH, 20% contained only LH, and 14% contained only FSH. In the 7- to 15-day-old rats, the percentages of gonadotropes containing both hormones increased to 79% while 10-11% of the cells contained only LH or FSH. This is similar to the findings in the adult rat population (75% LH and FSH cells, 14.2% LH cells, and 11% FSH cells). The fields stained for ACTH showed that 35-80% of the gonadotropes contained ACTH-like immunoreactivity during neonatal development, with the highest values seen in the 7- to 15-day-old rats. In the normal adult rats, 2-10% of the gonadotropes contain ACTH activity. Cells containing only ACTH were also seen in all age groups examined. The morphological analysis showed that immature gonadotropic cells contained scattered storage granules, arranged either at the cell periphery or concentrated near the nucleus. They were small, irregularly shaped, and often exhibited a high nucleo-cytoplasmic ratio. They were difficult to distinguish from corticotropes. More mature cell types were found after 1 week of age. These were ovoid and contained numerous large and small granules, features similar to the adult type I cell. The granules, however, were more irregularly shaped (pleomorphic). Immature forms were observed in the gonadotrope population as late as 15 days of age. Most gonadotropes from the 20- to 25-day-old rats resembled those in the adult population. Our findings support the physiological studies which show that the first week of life is an important time for functional and morphological maturation of the gonadotrope population. They also support the finding that immature gonadotropes are larger, more numerous, and more heterogeneous than those in the adult male rat population. Finally, we propose that a subpopulation of gonadotropes may serve as a stem cell or may be involved in the development of adrenal-gonadal interactions.