Seasonal variations in pituitary LH-gonadotropes of the hibernating bat Myotis lucifugus lucifugus: an immunohistochemical study.

Pituitary gonadotropes were identified throughout the year in the seasonally breeding, hibernating bat Myotis lucifugus lucifugus by means of light microscopic immunohistochemistry. In both male and female bats, these cells were immunoreactive with an antiserum directed to the beta subunit of luteinizing hormone. Some gonadotropes were aggregated near a portion of the infundibular stalk which crosses the anterior lobe, while most were scattered singly in a uniform manner throughout the rest of the pars distalis. This cell population exhibited seasonal variations in both sexes. In males, the proportional volume of the pars distalis occupied by immunoreactive gonadotropes (volume fraction) was significantly reduced in late July, when plasma testosterone levels were approaching their seasonal peak. In females, the volume fraction declined in April, following ovulation, and remained low during pregnancy and lactation. The size and shape of gonadotropes appeared relatively constant throughout the annual reproductive cycle in male bats; the immunoreactive cells were irregular in shape, with cytoplasmic extensions insinuating between and often "cupping" other secretory cell types. In females, the gonadotropes resembled those of males throughout most of the year, except during pregnancy, when these cells became enlarged and ovoid. No evidence of involution was observed in these anterior pituitary cells in either males or females during hibernation.