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Meiotic entry is one of the earliest sex determination events of the germ cell in higher vertebrates. Although advances in meiosis onset have been achieved in mammals, birds and fish, how this process functions in reptiles is largely unknown. In this study, we present the molecular analysis of meiosis onset and the role of retinoic acid (RA) in this process in the red-eared slider turtle. Our results using Stra8 as a pre-meiosis indicator show that in the female embryonic gonad, meiosis commitment starts around stage 19. Additionally, signals of the meiosis marker Sycp3 could be detected at stage 19 and become highly expressed by stage 23. No expression of these genes was detected in male embryonic gonads, suggesting the entry into meiosis prophase I was restricted to female embryonic germ cells. Notably, RA activity in fetal gonads is likely to be elevated in females than that in males, as evidenced by the higher expression of RA synthase Aldh1a1 and lower expression of RA-degrading enzyme Cyp26a1 in female gonads prior to meiotic entry. In addition, exogenous RA treatment induced the expression of Stra8 and Sycp3 in both sexes, whether in vivo or in vitro. Together, these results indicate that high levels of RA in the embryonic female gonads can lead to the initiation of meiosis in the turtle. [ABSTRACT FROM AUTHOR]

The adenohypophysis consists of the anterior and intermediate lobes (AL and IL). The marginal cell layer (MCL), including the ventral region of the IL and the dorsal region of the AL lining the Rathke's cleft, acts as the primary stem/progenitor cell niches in adult adenohypophysis. The cells of the MCL on the IL side consisted of cluster of differentiation 9 (CD9)-positive stem/progenitor cells with or without motile cilia. However, any additional cellular properties of multiciliated CD9-positive cells are not known. The present study aimed to identify the character of the multiciliated cells in stem cell niche of the pituitary gland. We observed the fine structure of the multiciliated cells in the MCL of male Wistar rats at an early stage after birth and in adulthood (P60) using scanning electron microscopy. Since the previous study showed that the MCL cells of adult rats synthesize retinoic acid (RA), the present study determined whether the multiciliated cells are involved in RA regulation by the expression of retinal aldehyde dehydrogenase 1 (RALDH1) and CYP26A1, an enzyme synthesizing and degrading RA, respectively. Results showed that 96% of multiciliated cells in adult male rats expressed CYP26A1, while 60% expressed RALDH1. Furthermore, the isolated CD9-positive cells from the IL side MCL responded to RA and activated the degradation system of RA by increasing Cyp26a1 expression. These findings indicated that multiciliated cells are involved in RA metabolism in the MCL. Our observations provide novel insights regarding the stem cell niche of the adult pituitary. [ABSTRACT FROM AUTHOR]

Vitamin A (retinol) and its active metabolite, retinoic acid (RA), serve dual roles in the female reproductive tract. Cytochrome P450 26A1 (Cyp26a1), an RA-metabolizing enzyme, is involved in mammalian early pregnancy. In order to investigate the role of RA synthesis and metabolism during embryo implantation, we first investigated the spatiotemporal expression of RA-signal in the mouse uterus during the peri-implantation period. RA-signal-related molecules, including binding proteins, synthesizing enzymes, catabolizing enzymes and receptors, were all expressed in the mouse uterus during embryo implantation. The locations of the RA synthetic system (Aldh1a1, Aldh1a2, CRBP1) and catabolizing enzyme (Cyp26a1) were distinctive in the mouse uterus during the peri-implantation period. Aldh1a1 was located in the gland epithelium, whereas Aldh1a2 and CRBP1 were located in the stroma and Cyp26a1 was expressed in the luminal and glandular epithelium. These results demonstrate that RA synthesis occurs in the stroma, whereas RA metabolism takes place in the endometrial epithelium. When endometrial epithelial cells were isolated on day 4.5 of pregnancy and treated with E (17beta-estradiol) or a combination of E and progesterone, all-trans-RA (10 μM) significantly down-regulated the expression of LIF, HB-EF and CSF-1 in these cells in vitro. Taken together, these results suggest that the accumulation of RA in the stroma during mouse embryo implantation has an inhibitory effect on the expression of the three implantation-essential genes, LIF, HB-EGF and CSF-1. Therefore, the expression of Cyp26a1 in luminal and glandular epithelium might block the adverse effect of RA in order to promote successful embryo implantation. [ABSTRACT FROM AUTHOR]