¹Department of Obstetrics and Gynecology, American University of Beirut Medical Center, Lebanon ²Division of Gynecologic Oncology, American University of Beirut Medical Center, Lebanon

^*Address for Correspondence: Ghazeeri G, Division of Reproductive Endocrinology and Infertility, Department of Obstetrics & Gynecology, American University of Beirut Medical Center, Hamra, Beirut, PO BOX: 113-6044 Lebanon, Telephone: 01-350000, E-mail: gg02@aub.edu.lb

Submission: 25 September, 2019 Accepted: 9 October, 2019 Published: 18 October, 2019

Copyright: © 2019 Dalia K. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The multiplication and development of germ cells in the seminiferous tubules of the testiclesoccur through a complex series of cellular events that are controlled by multiple signals. It is composed of 6 stages in humans (Figure 1).

Spermatogonial stem cells are self-renewed via mitosis, meiosis and contribute to the formation of haploid spermatids from diploid spermatocytes. Through the process of spermiogenesis, spermatids undergo maturation and are transformed into functional spermatozoa which are released at spermiation after the breakage of intercellular bridges attaching the spermatids to Sertoli cells. Spermatogenesis is a continuous process requiring the contribution of numerous cell and regulatory factors. Its understanding is essential in order to advance research for treatment of male infertility. The different stages of spermatogenesis along with the main roles of Sertoli cell and BTB will be reviewed. Some emerging fields in research regarding the new classification was briefly examined for a better understanding of the complexity of the process.

Spermatogenesis; Spermiation; Spermiogenesis; Sertoli cells

Spermatogenesis is a process occurring in the Seminiferous Tubules (ST) of the testicles. The multiplication and development of germ cells occur through a complex series of cellular events that are controlled by multiple signals. While fourteen stages are found in rats, the seminiferous epithelial cycle is composed of 6 stages in humans [1]. Spermatogonial stem cells are self-renewed via mitosis, meiosis (I and II) and contribute to the formation of haploid spermatids from diploid spermatocytes. Through the process of spermiogenesis, spermatids undergo maturation and are transformed into functional spermatozoa which are released at spermiation after the breakage of intercellular bridges attaching the spermatids to Sertoli cells [2,3]. Intra-testicular and extra-testicular regulatory hormones involving the release of Follicle Stimulating Hormone (FSH) from pituitary and testosterone from leydig cells are the prime components of a wellorganized spermatogenesis [4]. Sertoli cells control the milieu within the seminiferous tubules in order to facilitate the progression of germ cells to spermatozoa. Hense, spermatogenesis is regulated through the control of FSH on Sertoli cells and LH on Leydig cells.

The testis is composed of 400 to 600 ST, which is the functional unit of the test is where 300 million sperms per day are produced after puberty [5]. Sertoli cells have a significant role in supporting the growth of the gonadal cells and thus are known as the ‘nurse’ cells [6]. Leydig cells in return, under the influence of LH, supports the production of testosterone necessarily for the step by step process of spermatogenesis [7].

The Blood-Testis Barrier (BTB) is situated within the basement membrane of the ST, providing the microenvironment for the emergence of spermatids [8]. This barrier is well established at puberty, concomitantly with the onset of meiosis [9]. The basal and the ad-luminal compartments are the compartments found in the Seminiferous Epithelium (SE) which are separated by the BTB. The basal compartment includes undifferentiated spermatogonia (A and B) and preleptotene spermatocytes. The ad-luminal compartment is inhabited by the primary, secondary spermatocytes, haploid spermatids and spermatozoa. During the 6 stages of spermatogenesis, the junctions between Sertoli cells and reproductive cells are in a constant remodeling process to allow transportation through the SE. It is in the ad-luminal compartment that meiosis I and II, the formation up to the spermatozoa stage and spermiation take place [10].

In humans, spermatogonia enter spermatogenesis every 16 days, divide in a continuous way through mitosis to produce different variety of cells. This is entitled “cycle of the seminiferous epithelium” [11]. The detail inspection of cross sections of tubules over the years uncovered 6 main stages of spermatogenesis or cellular associations [12].

The epithelial cycle is composed of 6 different stages along the SE of the testis [13]. A stage refers to a specific segment of the SE over time [14]. The duration of spermatogenesis is around 64 days in which a type A spermatogonia is transformed into multiple haploid spermatozoa [15]. In this context, an epithelial cycle takes 16 days to be completed, yet it takes four cycles for a spermatogonia to become a spermatid along the section of a tubule (74 days).

Two types of spermatogonia (A and B) are present in the basal compartment of the ST. Spermatogonia Ad (A dark type) and Ap (A pale type) represent type A spermatogonia. Spermatogonia Ap have a self-renewal property and thus are predominant while spermatogonia Ad are observed as the regenerative reserve of stem cells [12,16]. On the other hand, B type spermatogonia characterize the beginning of reproductive cell growth up to the spermatids stage. The multiplication of spermatogonia is synchronous through mitosis where the divided cells communicate via cytoplasmic bridges that dissolve at the spermatids stages [17].

The multiplication and differentiation of these cells is regulated by the tyrosine kinase receptor (cKIT) protein which is only manifested in spermatogonia, round spermatids and spermatozoa. Its ligand (cKIT ligand) is found at the level of Sertoli cells [18]. Retinoic acid is involved in the commencement of meiosis and the conversion of undifferentiated spermatogonia into type A spermatogonia [19].

Data in the literature is inconsistent about the stage of appearance of spermatogonia B. Clermont documented that that these cells were formed between stages VI and I, evident in stages I and II and divide into preleptotene spermatocytes in the late stage II [11]. After the ultimate spermatogonial division, cells undergo 2 stages of meiosis. A pair of spermatogonia Ap produces 8 preleptotene spermatocytes [20].

Stage III of the epithelial cycle represents the differentiation of spermatogonia B into preleptotene spermatocytes, which are shifted past the BTB while converting to leptotene spermatocytes. Thus, after the initiation of meiosis at puberty, spermatocytes are located in the SE at different stages: leptotene, zygotene, pachytene, and diplotene [21]. During meiosis I, diplotene spermatocytes are transformed into secondary spermatocytes (1N:2C) which are then transformed into two round spermatids (1N:1C) via meiosis II. To note that primary spermatocytes start their meiosis in the basal compartment of SE at the leptotene stage and reach the adluminal compartment through the BTB to proceed with further stages of division into secondary spermatocytes. Meiosis I involve DNA replication, condensation of chromosomes, pairing and crossing over of homologous chromosomes [17]. Lactate is the main energy source of spermatocytes which is metabolized by the Sertoli cells via glucose transporters (GLUT1). This mechanism is controlled by a close regulation between gonadotrophins, steroids and paracrine factors [22].

Four haploid spermatids are the products of the two meiotic divisions of each spermatocyte.

Spermiogenesis is the process during which spermatids are transformed into spermatozoa. Different techniques of staining were used over the years to describe the formation of human spermatids [11]. Nuclear characteristics of spermatids were described by Holstein & Roosen by using glutaraldehyde fixation and toluidine blue stain. Subsequently, they were able to detect 6 main stages (S_a, S_b1, S_b2, S_c,S_d1, and S_d2) including morphological changes during which an acrosomic granule is produced in the Golgi apparatus and grows over the nucleus. Additional changes happen, including: acrosome formation, condensation of nuclear chromatin, detachment of the cytoplasm forming the residual body, which is subsequently digested by Sertoli cells via phagocytosis, and finally the formation of a mature spermatozoon [23]. Any disruption at the level of the acrosome formation, nucleus and/or flagellum maturation may lead to maturational arrest or hypospermatogenesis [24]. All these stages are still within the premises of the human testis.

Spermiation is the final process during which mature spermatozoa are delivered into the lumen of ST for their subsequent maturation in the epididymis. The delivery of matured spermatids is achieved via the Sertoli cells during which they separate from their connections and become spermatozoa [25]. Residual bodies including parts of the spermatids are digested into the Sertoli cells. The molecular mechanism is still poorly understood yet, it is dependent on FSH and testosterone [26].

The final product of spermatogenesis is the spermatozoa. The unique shape of spermatozoa permits its precise contact with the oocyte: nucleus is condensed, protected by an acrosome and attached to a flagellum to allow progressive motility. The motility is acquired during the transport in the epididymal ducts, depending on the normal development of axonemes, presence of mitochondria and implantation of flagellum at the nucleus [23]. To note that the highly condensed spermatozoa has a particular form of DNA packaging in the sperm nuclei where loops are packed as doughnuts, thus allowing the transfer of the genome in a compacted form to the zygote [27].

Generations of differentiated germ cells are illustrated from the basement membrane upward to the tubule lumen. Spermatogonia are established in stage V, present in all cellular associations. The preleptotene spermatocytes appear during stage III, are transported through the BTB, to undergo meiosis during the stage IV of the cycle. Newly formed spermatids are present in stage I with spherical nucleus. It is till the end of stage II that spermiation is noted.

Different methods were used to precisely characterize the morphology of germ cells within the SE. The present descriptions are based on initial studies done during the early 60s [11]. Although, these were innovative studies, they had flaws in describing the nuclear features of germ cells and the organization among the different stages. For a better understanding of the dynamics of spermatogenesis, it has been recently revisited by several investigators using high resolution light microscopy method on open testes biopsies from adult and elderly patients. They proposed new cellular associations in order to enable a more advanced and consistent source of reference [14]. The number of six stages originally proposed was maintained, yet the stages were better defined.

Spermatogenesis is a continuous process requiring the contribution of numerous cell and regulatory factors. Its understanding is essential in order to advance research for treatment of male infertility. Numerous areas of the ST are occupied by different stages of spermatogenesis; hence several stages can be discerned in an isolated tubule section. The different stages of spermatogenesis along with the main roles of Sertoli cell and BTB were reviewed. Some emerging fields in research regarding the new classification was briefly examined for a better understanding of the complexity of the process.