Date Published: April 29, 2018
Publisher: Impact Journals
Author(s): Xin Lin, Xingting Liu, Yanfen Ma, Yuling Mi, Weidong Zeng, Jian Li, Caiqiao Zhang.
After ovulation in mammals, rupture of mature follicles is reorganized into the corpus luteum that secrets progesterone (P4) to stimulate endometrial development. The situation in birds differs considerably. Beyond ovulation the ruptured avian follicle forms a postovulatory follicle (POF) that is not considered analogous to mammalian corpus luteum. The function and regression mechanisms of avian POFs remain poorly understood. Here we investigated the changes in apoptotic and autophagic activities that were involved during POF degradation. Results showed that the structure and secretory function of POF3 manifested the most apparent deterioration during whole processes of regression. A TUENL assay revealed that the granulosa layer maintained longer viability than the theca layer. Importantly, mitochondrial apoptosis and endoplasmic reticulum (ER) stress-associated genes and proteins reached their highest levels in the granulosa cells of POF3. Beclin1 was distributed mainly in theca cells and coupled with LC3β-II accumulation, Sequestosome-1 (p62) degradation and Beclin1 elevation confirmed that autophagic activity had increased dramatically in the theca layer of POFs. These results indicate that the apoptosis of the granulosa cells from POFs occurs by mitochondrial apoptosis and ER stress and that a coherence of Beclin1-induced autophagy and caspase-induced apoptosis results in regression of theca layers of avian POFs.
In the chicken, the postovulatory follicle (POF) forms from the largest preovulatory follicle (F1). Although the POF, containing both granulosa and theca layers, plays an important role in oviposition and nesting behavior , there is a consensus that it has no structure or function directly analogous to that of the corpus luteum in mammals . The POFs suffer structural and functional regression after ovulation and this process is almost entirely completed within a 4-6 day period [3–5]. Caspase-induced apoptosis is known to impose a pivotal impact on the regression of POFs [4,6]. However, beyond apoptosis, few studies have reported any further details regarding any other aspects of the molecular mechanisms involved in the regression of the avian POFs.
The mechanisms of chicken POF regression remain poorly understood. Traditionally, structural regression of the POFs has been simply considered a result of programmed cell death . Using HE staining, we observed the loss of the majority of granulosa cells by POF4, and, consistent with the previous study, that the size of theca layer shrank dramatically after POF3 . However, in our results the accumulation of discrete lipid droplets in granulosa cells appeared at a later stage than was reported in the previous study . The TEM observation and Oil Red O staining verified that the accumulation of lipid droplets in the cytoplasm and such accumulation first appeared in POF3. This suggested that steatosis-like morphology actually began at POF3, despite the more obvious and typical steatosis-like morphology occurring in the coalescence of POF6. Consistent with the previous report, the caspase3 level did ascend significantly with POFs regression . On the other hand, functional regression of POFs embodies a gradual loss of secretion capacity. The secretory function of POFs, especially the most recent ruptured follicle, must not be overlooked in the chicken ovary, as such secretions exert effects on oviposition and nesting behavior . Previous studies showed that the level of P4 in the granulosa and theca layer descended significantly within 52 h . Our studies revealed that the level of CYP11A1 decreased sharply at POF3 stage and that the biosynthesis of P4 is rapidly terminated in POF3. These results suggest that POF3 suffers that maximal apoptosis degree, at structural and functional levels, than any other POF level.