Coordination of spermatogenic processes in the testis: lessons from cystic spermatogenesis

The effects of fine particulate matter on the blood-testis barrier and its potential mechanisms

With the rapid expansion of industrial scale, an increasing number of fine particulate matter (PM2.5) has bringing health concerns. Although exposure to PM2.5 has been clearly associated with male reproductive toxicity, the exact mechanisms are still unclear. Recent studies demonstrated that exposure to PM2.5 can disturb spermatogenesis through destroying the blood-testis barrier (BTB), consisting of different junction types, containing tight junctions (TJs), gap junctions (GJs), ectoplasmic specialization (ES) and desmosomes. The BTB is one of the tightest blood-tissue barriers among mammals, which isolating germ cells from hazardous substances and immune cell infiltration during spermatogenesis. Therefore, once the BTB is destroyed, hazardous substances and immune cells will enter seminiferous tubule and cause adversely reproductive effects. In addition, PM2.5 also has shown to cause cells and tissues injury via inducing autophagy, inflammation, sex hormones disorder, and oxidative stress. However, the exact mechanisms of the disruption of the BTB, induced by PM2.5, are still unclear. It is suggested that more research is required to identify the potential mechanisms. In this review, we aim to understand the adverse effects on the BTB after exposure to PM2.5 and explore its potential mechanisms, which provides novel insight into accounting for PM2.5-induced BTB injury.

Aspects of spermatogenesis in immature and mature specimens of the long-lived Greenland shark: Novelties concerning the germinal compartment's assembly, complement of Sertoli cells and demise

Cystic spermatogenesis in the subadult, maturing and adult Greenland shark (Somniosus microcephalus) displays multiple novel features, characterized early on by an unorganized internal cellular environment of the spermatocysts (anatomically discrete follicle-like units containing a single germ cell stage and its complement of co-developing Sertoli cells). These typically show polar asymmetries due to asymmetrically distributed germ and Sertoli cells. These arise from several novel cellular rearrangements at the immature pole, including fusion of a cluster of somatic cells with newly formed cysts containing only one to three spermatogonia and that already display an excess of Sertoli cells. The subadult's germinative zone revealed an additional novelty, namely numerous previously formed somatic cell-lined rings into which spermatogonia were incorporated. A striking finding was the conspicuous rarity of the routinely discernible Sertoli mitotic figures in the hallmark cyst stage of diametric elasmobranch spermatogenesis that is known for the peak display of the latter. Scrutiny of sequentially unfolding phenomena in the linearly arranged spermatogonial generations revealed that the cellular developments at the most common type of cyst-duct transition area (comprising slender to spindle-like basophilic cells with pointed ends) were concurrent with the discreet appearance of a second dark Sertoli nucleus, a development that persisted in spermiated cysts. Spermatogenically active mature males displayed vigorous meiotic divisions. However, a scattering of their spermatid cysts also displayed shark-atypical asynchronous passage through spermiogenesis, phenomena which were exacerbated as arrested spermiogenesis in an archival collection of tissues from 13 maturing specimens. Subadult specimens revealed meiotic arrest, and foci of infiltration of leukocytes that originate from a mass of eosinophilic, granule-laden immune cells dorsally under the testis capsule. This tissue was identical to the testis-affixed bone marrow equivalent in other shark species. This tissue is likely developmentally regulated in the Greenland shark as it is absent in adults.

Revealing Spermatogenesis in Smooth-Hound Sharks Mustelus mustelus: Insights into the Morphological and Macromolecular Composition of Spermatogenic Cells

Elasmobranchs have an ancestral reproductive system, which offers insights into vertebrate reproductive evolution. Despite their unchanged design over 400 million years, they evolved complex mechanisms ensuring reproductive success. However, human activities induced a significant decline in elasmobranch populations worldwide. In the Mediterranean basin, the smooth-hound shark (Mustelus mustelus) is one of the species that are considered vulnerable to human activities. Conservation efforts necessitate a thorough understanding of its reproductive strategy. This study focused on mature male specimens of smooth-hound sharks that were captured in the Adriatic area and successively analyzed to provide, for the first time, a histologically detailed description of testicular development in the species. Seven phases of the spermatogenesis process were identified, along with the macromolecular characterization of cells obtained using Fourier-transform infrared imaging. Histological analysis showed structural and cellular features similar to those documented in the spermatocysts of other elasmobranchs. The examination of the evolution and migration of both germinative and Sertoli cells at each phase revealed their close connection. Furthermore, different expression levels of lipids, proteins, and phosphates (DNA) at each spermatogenesis stage were observed. This research provided new information on spermatogenesis in the common smooth-hound shark, which is crucial for conservation efforts against population decline and anthropogenic pressures.

Comparative Proteome Analysis of Four Stages of Spermatogenesis in the Small-Spotted Catshark (Scyliorhinus canicula), Using High-Resolution NanoLC-ESI-MS/MS

Spermatogenesis is a highly specialized process of cell proliferation and differentiation leading to the production of spermatozoa from spermatogonial stem cells. Due to its testicular anatomy, Scyliorhinus canicula is an interesting model to explore stage-based changes in proteins during spermatogenesis. The proteomes of four testicular zones corresponding to the germinative niche and to spermatocysts (cysts) with spermatogonia (zone A), cysts with spermatocytes (zone B), cysts with young spermatids (zone C), and cysts with late spermatids (zone D) have been analyzed by nanoLC-ESI-MS/MS. Gene ontology and KEGG annotations were also performed. A total of 3346 multiple protein groups were identified. Zone-specific protein analyses highlighted RNA-processing, chromosome-related processes, cilium organization, and cilium activity in zones A, D, C, and D, respectively. Analyses of proteins with zone-dependent abundance revealed processes related to cellular stress, ubiquitin-dependent degradation by the proteasome, post-transcriptional regulation, and regulation of cellular homeostasis. Our results also suggest that the roles of some proteins, such as ceruloplasmin, optineurin, the pregnancy zone protein, PA28β or the Culling-RING ligase 5 complex, as well as some uncharacterized proteins, during spermatogenesis could be further explored. Finally, the study of this shark species allows one to integrate these data in an evolutionary context of the regulation of spermatogenesis. Mass spectrometry data are freely accessible via iProX-integrated Proteome resources (https://www.iprox.cn/) for reuse purposes.

Several routes of cell death to secondary necrosis in the elasmobranch testis

The process of spermatogenesis features significant germ cell loss through apoptosis. Routine histology of the testes of well-studied animal models hardly discloses any trace of their phagocytic clearance by the supporting Sertoli cells. This review highlights lessons learnt from the cystic, diametric testes of some seasonally migrating elasmobranchs (e.g., spiny dogfish and blue sharks) that offer unconventional investigative paradigms to study these phenomena as these organs readily disclose a pronounced apoptosis gradient affecting exclusively spermatogonial clones that each are enclosed with their own Sertoli cells in spherical structures called spermatocysts. This gradient is visible at a certain time of year in the spermatogenically active shark, and peaks in mature spermatogonial cysts as clustered deaths with sporadic, and not massive secondary necrosis. Conversely, immature spermatogonial cysts in blue sharks reveal a characteristic periluminal display of single apoptotic deaths. Tracing aberrations in the immunostaining patterns of the conserved cell cycle marker, proliferating cell nuclear antigen, the gradual progression of the death process in individual or coalesced spermatogonia in contiguous cysts becomes clear. The multiple apoptotic nuclear fragmentation morphologies inform also of a protracted death process involving three different morphological routes of nuclear fragmentation (of which some are TUNEL-positive and other TUNEL-negative) and concomitant chromatin compaction that culminate in freed apoptotic bodies (i.e., secondary necrosis). It is discussed that the staggered spermatogonial deaths and accompanying intermittent secondary necrosis in mature blue shark spermatogonial cysts may well relate to the low phagocytosis capacity of cyst’s Sertoli cells that are still functionally naïve.

Action and Interaction between Retinoic Acid Signaling and Blood–Testis Barrier Function in the Spermatogenesis Cycle

Spermatogenesis is a complex process occurring in mammalian testes, and constant sperm production depends on the exact regulation of the microenvironment in the testes. Many studies have indicated the crucial role of blood–testis barrier (BTB) junctions and retinoic acid (RA) signaling in the spermatogenesis process. The BTB consists of junctions between adjacent Sertoli cells, comprised mainly of tight junctions and gap junctions. In vitamin A-deficient mice, halted spermatogenesis could be rebooted by RA or vitamin A administration, indicating that RA is absolutely required for spermatogenesis. Accordingly, this manuscript will review and discuss how RA and the BTB regulate spermatogenesis and the interaction between RA signaling and BTB function.

Simple, once-off mapping of various, recurrent immunostaining patterns of proliferating cell nuclear antigen in spermatogonia at the immature pole of the testis of adult wild-caught blue shark, Prionace glauca: Correlations with changes in testicular status

This study was a single time-point mapping of various immunostaining patterns revealed with the proliferating cell nuclear antigen (PCNA) PC10 antibody in spermatogonia at the immature pole of the testis of the Blue shark (Prionace glauca). Scattered in the stroma of the germinal ridge that demarcates the immature pole's outer boundary were nests of variously immunoreactive A-spermatogonia, each flanked by a fusiform cell. Spermatocysts were assembled from niche-derived stromal cells, displaced A-progenitors, and their progeny, which showed one of two main immunostaining patterns (i.e., an uneven light brown/globular and homogeneous dark [hod] brown appearance). The testes of wild-caught Prionace showed two conditions, namely, extensive multinucleate cell death (MNC) near the mitosis-meiosis transition or an early recovery phase from the latter showing vacuolated areas. Both the proportion of cysts with immature B^hod -spermatogonia and the frequency of mitotic figures in such cysts in the early recovery testis condition were significantly higher than the comparable parameters in MNC testis condition. Moreover, the post-MNC recovery phase revealed a decrease in the proportion of immature cysts with uneven light brown/globular-like spermatogonia. The protracted spread of a cell cycle signal in an anatomically discrete, syncytially connected spermatogonial clone manifests as different PCNA immunoreactivities.

Multiple Sources for Sertoli Cells and Two Sertoli Phenotypes in the Adult Elasmobranch Testis: Insight from Two Species Belonging to Different Orders

Findings presented here for the chondrichthyans Prionace glauca and Isurus oxyrinchus show that the assembly of the spermatocyst and it's attached collecting duct are seamlessly connected developmental phenomena. The cyst's somatic cell component (i.e., Sertoli cells, SCs) and the duct's constituent cells derive both from a common precursor cell type (typically a large oblong cell) found among the A-spermatogonia in the folliculogenic region. Novel findings show that the co-developing collecting duct itself serves also as a source of either normal-looking or basophilic atypical SCs (aSCs), depending on whether the duct-cyst transition remains mitotically active and open, or is sealed. The aSCs arise from accumulating slender basophilic cells at the duct-cyst interface after which the newly formed cyst is sealed. Quantitative analysis of the latter in P. glauca revealed a correlation between the appearance of this aSC in immature cysts and the degenerated testicular condition that displays a gradient of multinucleate cell (MNC) death among spermatogonial cysts. Findings seem to implicate these aSCs in the life-death balance in mature spermatogonial cohorts downstream in the spermatogenic sequence, rather than in newly formed cysts that exhibit low rates of apoptosis. Photomicrographs of developing spermatogonial cysts showing several aSCs interspersed among cytoplasmically linked spermatogonia that are proliferating or have died, seem to suggest that these small SCs may be involved in confining MNC death to a given cyst region under conditions of subthreshold levels of apoptosis such as to ensure cyst recovery in immature spermatogonial cysts. Anat Rec, 301:1944-1954, 2018. © 2018 Wiley Periodicals, Inc.

Germ cell desquamation-based testis regression in a seasonal breeder, the Egyptian long-eared hedgehog, Hemiechinus auritus

Testes of seasonally breeding species experience a severe functional regression before the non-breeding period, which implies a substantial mass reduction due to massive germ-cell depletion. Two alternative mechanisms of seasonal germ-cell depletion have been described in mammals, apoptosis and desquamation (sloughing), but their prevalence has not been determined yet due to reduced number of species studied. We performed a morphological, hormonal, and molecular study of the mechanism of seasonal testicular regression in males of the Egyptian long eared-hedgehog (Hemiechinus auritus). Our results show that live, non-apoptotic, germ cells are massively depleted by desquamation during the testis regression process. This is concomitant with both decreased levels of serum testosterone and irregular distribution of the cell-adhesion molecules in the seminiferous epithelium. The inactive testes maintain some meiotic activity as meiosis onset is not halted and spermatocytes die by apoptosis at the pachytene stage. Our data support the notion that apoptosis is not the major testis regression effector in mammals. Instead, desquamation appears to be a common mechanism in this class.

Prespermatogenesis and early spermatogenesis in frogs

Spermatogenesis in frogs was for the first time divided into two phases: prespermatogenesis, when gonocytes proliferate in developing tadpole testes, and active spermatogenesis when spermatogonial stem cells (i.e. descendants of gonocytes), either self-renew or enter into meiotic cycles within cysts formed by Sertoli cells. We argue that amphibian larval gonocytes are homologues to mammalian gonocytes, whereas spermatogonial stem cells (SSCs) in adult frogs are homologous to mammalian single spermatogonia (A_s). Gonocytes constitute sex cords, i.e. the precursors of seminiferous tubules; they are bigger than SSCs and differ in morphology and ultrastructure. The nuclear envelope in gonocytes formed deep finger-like invaginations absent in SSCs. All stages of male germ cells contained lipid droplets, which were surrounded by glycogen in SSCs, but not in gonocytes. Mitochondria in gonocytes had enlarged edges of cristae, and in SSCs also lamellar mitochondria appeared. Minimal duration of prespermatogenesis was 46days after gonadal sex differentiation, but usually it lasted longer. SSCs give rise to secondary spermatogonia (equal to mammalian A, In, and B). Their lowest number inside a cyst was eight and this indicated the minimal number of cell cycles (three) of secondary spermatogonia necessary to enter meiosis. We sorted them according to the number of cell cycles (from 8 to 256 cells). This number is similar to that recorded for mammals as the result of a single A_s proliferation. The number of secondary spermatogonia correlates with the volume of a cyst. The general conclusion is that spermatogenesis in amphibians and mammals follows basically the same scheme.

From cyst to tubule: innovations in vertebrate spermatogenesis

Although vertebrates share many common traits, their germline development and function exhibit significant divergence. In particular, this article focuses on their spermatogenesis. The fundamental elements that constitute vertebrate spermatogenesis and the evolutionary changes that occurred upon transition from water to land will be discussed. The life-long continuity of spermatogenesis is supported by the function of stem cells. Series of mitotic and meiotic germ cell divisions are 'incomplete' due to incomplete cytokinesis, forming syncytia interconnected via intercellular bridges (ICBs). Throughout this process, germ cells are supported by appropriate microenvironments established primarily by somatic Sertoli cells. In anamniotes (fish and amphibians) spermatogenesis progresses in cysts, in which developing germ cell syncytia are individually encapsulated by Sertoli cells. Accordingly, Sertoli cells undergo turnover with germ cells that they nourish. This mode of cystic spermatogenesis is also observed in nonvertebrates as insects. In amniotes (reptiles, birds, and mammals), however, Sertoli cells do not turn over but comprise a persistent structure of seminiferous tubules. Sertoli cells nourish different stages of germ cells simultaneously in distinct regions of their surface. This function of Sertoli cells is spatiotemporally orchestrated, and the seminiferous epithelial cycle and spermatogenic wave make the seminiferous tubules a high-throughput factory for sperm production. Furthermore, contrary to the organized differentiating cells, undifferentiated spermatogonia that comprise the stem cell compartment exhibit active motion over the basal layer of seminiferous tubules and the frequent breakdown of ICBs. Thus, amniote seminiferous tubules represent a typical facultative (or open) niche environment without a stem cell tethering anatomically defined niche. WIREs Dev Biol 2016, 5:119-131. doi: 10.1002/wdev.204 For further resources related to this article, please visit the WIREs website.

Circannual Testis Changes in Seasonally Breeding Mammals

In the non-equatorial zones of the Earth, species concentrate their reproductive effort in the more favorable season. A consequence of seasonal breeding is seasonal testis regression, which implies the depletion of the germinative epithelium, permeation of the blood-testis barrier, and reduced androgenic function. This process has been studied in a number of vertebrates, but the mechanisms controlling it are not yet well understood. Apoptosis was assumed for years to be an important effector of seasonal germ cell depletion in all vertebrates, including mammals, but an alternative mechanism has recently been reported in the Iberian mole as well as in the large hairy armadillo. It is based on the desquamation of meiotic and post-meiotic germ cells as a consequence of altered Sertoli-germ cell adhesion molecule expression and distribution. Desquamated cells are either discarded alive through the epididymis, as in the mole, or subsequently die by apoptosis, as in the armadillo. Also, recent findings on the reproductive cycle of the greater white-toothed shrew at the meridional limits of its distribution area have revealed that the mechanisms controlling seasonal breeding are in fact far more plastic and versatile than initially suspected. Perhaps these higher adaptive capacities place mammals in a better position to face the ongoing climate change.

Biology and identity of fish spermatogonial stem cell

Although present at relatively low number in the testis, spermatogonial stem cells (SSCs) are crucial for the establishment and maintenance of spermatogenesis in eukaryotes and, until recently, those cells were investigated in fish using morphological criteria. The isolation and characterization of these cells in fish have been so far limited by the lack of specific molecular markers, hampering the high SSCs biotechnological potential for aquaculture. However, some highly conserved vertebrate molecular markers, such as Gfra1 and Pou5f1/Oct4, are now available representing important candidates for studies evaluating the regulation of SSCs in fish and even functional investigations using germ cells transplantation. A technique already used to demonstrate that, different from mammals, fish germ stem cells (spermatogonia and oogonia) present high sexual plasticity that is determined by the somatic microenvironment. As relatively well established in mammals, and demonstrated in zebrafish and dogfish, this somatic environment is very important for the preferential location and regulation of SSCs. Importantly, a long-term in vitro culture system for SSCs has been now established for some fish species. Therefore, besides the aforementioned possibilities, such culture system would allow the development of strategies to in vitro investigate key regulatory and functional aspects of germline stem cells (ex: self-renewal and/or differentiation) or to amplify SSCs of rare, endangered, or commercially valuable fish species, representing an important tool for transgenesis and the development of new biotechnologies in fish production.

Cell biology solves mysteries of reproduction

Reproduction and fertility have been objects of keen inquiry since the dawn of humanity. Medieval anatomists provided the first accurate depictions of the female reproductive system, and early microscopists were fascinated by the magnified sight of sperm cells. Initial successes were achieved in the in vitro fertilization of frogs and the artificial insemination of dogs. Gamete and embryo research was in the cradle of modern cell biology, providing the first evidence of the multi-cellular composition of living beings and pointing out the importance of chromosomes for heredity. In the 20th century, reproductive research paved the way for the study of the cytoskeleton, cell signaling, and the cell cycle. In the last three decades, the advent of reproductive cell biology has brought us human in vitro fertilization, animal cloning, and human and animal embryonic stem cells. It has contributed to the development of transgenesis, proteomics, genomics, and epigenetics. This Special Issue represents a sample of the various areas of reproductive biology, with emphasis on molecular and cell biological aspects. Advances in spermatology, ovarian function, fertilization, and maternal-fetal interactions are discussed within the framework of fertility and diseases such as endometriosis and diabetes.