Background Spermatogenesis in mammals is organized in a manner that maximizes sperm production. The central aspect of this organization is the cycle of the seminiferous epithelium that is characterized by an asynchronous repeating series of germ cell associations. These cell associations are the result of a fixed point of entry into the cycle at regular short time intervals and the longer time required for cells to fully differentiate and exit the cycle. Objective This review will examine the current information on the action and metabolism of retinoic acid in the testis, the interaction of retinoic acid (RA) with the cycle and the spermatogenic wave, and the mechanisms that can lead to synchronous spermatogenesis. Finally, the unique applications of synchronous spermatogenesis to the study of the cycle and the mass isolation of specific germ cell populations are described. Materials and methods Retinoic acid metabolism and spermatogonial differentiation have been examined by gene deletions, immunocytochemistry, chemical inhibitors, and mass spectrometry. Results, discussion, and conclusion Both the Sertoli cells and the germ cells have the capacity to synthesize retinoic acid from retinol and in the mouse the entry into the cycle of the seminiferous epithelium, and the subsequent conversion of undifferentiated spermatogonia into differentiating spermatogonia is governed by a peak of RA synthesis occurring at stages VIII‐IX of the cycle. Normal asynchronous spermatogenesis can be modified by altering RA levels, and as a result the entire testis will consist of a few closely related stages of the cycle.

Mammalian sexual development commences when fetal bipotential progenitor cells adopt male Sertoli (in XY) or female granulosa (in XX) gonadal cell fates. Differentiation of these cells involves extensive divergence in chromatin state and gene expression, reflecting distinct roles in sexual differentiation and gametogenesis. Surprisingly, differentiated gonadal cell fates require active maintenance through postnatal life to prevent sexual transdifferentiation and female cell fate can be reprogrammed by ectopic expression of the sex regulator DMRT1. Here we examine how DMRT1 reprograms granulosa cells to Sertoli-like cells in vivo and in culture. We define postnatal sex-biased gene expression programs and identify three-dimensional chromatin contacts and differentially accessible chromatin regions (DARs) associated with differentially expressed genes. Using a conditional transgene we find DMRT1 only partially reprograms the ovarian transcriptome in the absence of SOX9 and its paralog SOX8, indicating that these factors functionally cooperate with DMRT1. ATAC-seq and ChIP-seq show that DMRT1 induces formation of many DARs that it binds with SOX9, and DMRT1 is required for binding of SOX9 at most of these. We suggest that DMRT1 can act as a pioneer factor to open chromatin and allow binding of SOX9, which then cooperates with DMRT1 to reprogram sexual cell fate.

Semen liquefaction changes semen from a gel-like to watery consistency and is required for sperm to gain mobility and swim to the fertilization site in the Fallopian tubes. Kallikrein-related peptidases 3 (KLK3) and other kallikrein-related peptidases from male prostate glands are responsible for semen liquefaction by cleaving gel-forming proteins (semenogelin and collagen). In a physiological context, the liquefaction process occurs within the female reproductive tract. How seminal proteins interact with the female reproductive environment is still largely unexplored. We previously reported that conditional genetic ablation of Esr1 (estrogen receptor α) in the epithelial cells of the female reproductive tract (Wnt7a) causes female infertility, partly due to a drastic reduction in the number of motile sperm entering the oviduct. In this study, we found that post-ejaculated semen from fertile wild-type males was solidified and the sperm were entrapped in Wnt7aCre/+ ;Esr1 f/f uteri, compared to the watery semen (liquefied) found in Esr1 f/f controls. In addition, semenogelin and collagen were not degraded in Wnt7aCre/+ ;Esr1 f/f uteri. Amongst multiple gene families aberrantly expressed in the absence of epithelial ESR1, we have identified that a lack of Klks in the uterus is a potential cause for the liquefaction defect. Pharmacological inhibition of KLKs in the uterus replicated the phenotype observed in Wnt7aCre/+ ;Esr1 f/f uteri, suggesting that loss of uterine and seminal KLK function causes this liquefaction defect. In human cervical cell culture, expression of several KLKs and their inhibitors (SPINKs) was regulated by estrogen in an ESR1-dependent manner. Our study demonstrates that estrogen/ESR1 signaling in the female reproductive tract plays an indispensable role in normal semen liquefaction, providing fundamental evidence that exposure of post-ejaculated semen to the suboptimal microenvironment in the female reproductive tract leads to faulty liquefaction and subsequently causes a fertility defect. Author summarySemen liquefaction has been considered to be solely modulated by prostate-derived kallikrein-related peptidases (KLKs), especially KLK3 (or prostate specific antigen). However, our research demonstrated that female mice lacking estrogen receptor alpha (ERα) in the uterine epithelial cells had a drastic decrease in Klk transcripts and semen from fertile males fails to liquefy within the uteri of these females. Therefore, our results provide a novel aspect that, due to an interplay between semen and female reproductive tract secretions, the physiology of semen liquefaction is more complicated than previously assumed. This information will advance research on semen liquefaction in the female reproductive tract, an area that has never been explored, and could lead to the development of diagnostic tools for unexplained infertility cases and non-invasive contraception technologies.

Spermatogonial development is a key process during spermatogenesis to prepare germ cells to enter meiosis. While the initial point of spermatogonial differentiation is well‐characterized, the development of spermatogonia from the onset of differentiation to the point of meiotic entry has not been well defined. Further, STRA8 is highly induced at the onset of spermatogonial development but its function in spermatogonia has not been defined. To better understand how STRA8 impacts spermatogonia, we performed RNA‐sequencing in both wild‐type and STRA8 knockout mice at multiple timepoints during retinoic acid (RA)‐stimulated spermatogonial development. As expected, in spermatogonia from wild‐type mice we found that steady‐state levels of many transcripts that define undifferentiated progenitor cells were decreased while transcripts that define the differentiating spermatogonia were increased as a result of the actions of RA. However, the spermatogonia from STRA8 knockout mice displayed a muted RA response such that there were more transcripts typical of undifferentiated cells and fewer transcripts typical of differentiating cells following RA action. While spermatogonia from STRA8 knockout mice can ultimately form spermatocytes that fail to complete meiosis, it appears that the defect likely begins as a result of altered messenger RNA levels during spermatogonial differentiation.

Male meiosis is a complex process whereby spermatocytes undergo cell division to form haploid cells. This review focuses on the role of retinoic acid (RA) in meiosis, as well as several processes regulated by RA before cell entry into meiosis that are critical for proper meiotic entry and completion. Here, we discuss RA metabolism in the testis as well as the roles of stimulated by retinoic acid gene 8 (STRA8) and MEIOSIN, which are responsive to RA and are critical for meiosis. We assert that transcriptional regulation in the spermatogonia is critical for successful meiosis.