Colloid in the pituitary pars distalis of viscacha (Lagostomus maximus maximus): Ultrastructure and occurrence in relation to season, sex, and growth

Randomly distributed extracellular colloidal accumulations were observed ed in the pars distalis of viscacha (Lagostomus maximus maximus). They were p referentially located in the peripheral zone of the gland and showed variab ility in shape and size. Two different types of colloidal accumulations wer e found by electron microscopy: 1) those surrounded by non-granulated folli cular cells that correspond to characteristic follicles, and 2) those surro unded by granulated cells. In the follicles lined by non-granulated follicu lar cells, long, prominent microvilli and cytoplasmic processes protruded i nto the lumen. The frequency of these accumulations varies during the year in adult male animals, showing an increase in number during summer and a de crease during winter. The lowest value was registered in August (winter). T he mean follicular diameter did not vary seasonally. The number of colloida l accumulations did not vary seasonally in adult female viscachas, but a si gnificant difference in the mean follicular diameter between pregnant and n on-pregnant females was observed. Pituitaries of immature animals contain f ewer colloidal accumulations than those of adults. In fetuses, these accumu lations were absent. The administration of melatonin provoked a decrease in the number of these structures. The numeric changes of the colloidal accum ulations observed in this study are associated with: 1) the seasonal reprod uctive activity in adult males, and 2) the reproductive condition, body wei ght and sexual maturity in males and females. The fact that melatonin admin istration decreases the population of colloidal accumulations in males sugg ests participation of the pineal gland in these changes. Anat Rec 258:252-2 61, 2000. (C) 2000 Wiley-Liss, Inc.