Apoptosis in pituitary pars distalis of male viscachas: quantitative analysis during the reproductive cycle

Abstract

Apoptosis, or programmed cell death, is a genetically controlled cell process whereby cells induce their own death in response to certain stimuli. The balance between this process and cell proliferation is key to maintaining tissue homeostasis in pituitary gland during physiological endocrine events. A series of cysteine-dependent proteases, called caspases, participate in the intracellular mechanisms involved in apoptosis. The aim of this work was to localize and quantify apoptotic cells that express cleaved caspase 3 (CASP3) in different regions of pituitary pars distalis (PD) of adult male viscachas during their annual reproductive cycle (reproductive, gonadal regression and gonadal recovery periods). In each period, four pituitary glands were collected and processed for light microscopy. CASP3 was detected by immunohistochemistry and morphometrically quantified by image analysis. The immunoreactive(-ir) cells were counted and expressed as percentage of the total number of cells per microscopic field. The values were statistically analyzed and expressed as mean ± SEM. The immunostaining pattern was mainly cytoplasmic, although nuclear labeling was also observed. Abundant CASP3-ir cells were located around blood vessels. The total percentage of these cells did not differ significantly among the three periods studied (P>0.05). However, there were significant differences in CASP3-ir cells between different PD zones during reproductive cycle. In reproductive period, numerous CASP3-ir cells were mainly located at the rostral end, in the ventral and dorsal regions (P<0.01). Few positive cells were found at the ventral region of PD (P<0.05) during gonadal regression period. In recovery gonadal period, the percentage of CASP3-ir cells did not differ significantly (P>0.05) in different PD zones. In addition, each of different zones of PD was compared throughout the reproductive cycle. The percentage of CASP3-ir cells increased significantly at the rostral end, and it decreased significantly at the caudal end and dorsal region in reproductive period compared with gonadal recovery period (P<0.05). In ventral region, the percentage of CASP3-ir cells was significantly higher (P<0.01) in gonadal recovery period compared with gonadal regression period. The results obtained demonstrated that there are regional variations in the expression of CASP3 throughout the reproductive cycle in the PD parenchyma of male viscachas. These differences might be due to a differential distribution of proapoptotic factors that reach the PD through the blood vessels of the pituitary hypothalamic portal system. It is likely that in the reproductive period, these factors mainly affect the rostral end and then be distributed to other PD zones. The changes in apoptosis rates are probably related to the maintenance and cellular renewal processes and to the variation hormonal secretion of different cell populations depending of the physiological demands of this rodent throughout the year.