Читать книгу Bovine Reproduction - Группа авторов - Страница 54

Spermatogenesis starts at puberty and human beings produce 1000 spermatozoa with each heartbeat. This enormous spermatogenesis is maintained by spermatogonial stem cells (SSCs) at the basement of the ST [4]. Alkaline phosphatase positive epiblast cells of ectoderm are the embryonic origin of SSCs. About a hundred such cells, which had been lineage restricted from day 7.2 post mating, develop to primordial germ cells (PGCs) in the mouse embryo [5]. The PGCs multiply and reach the genital ridge which is prominent by day 11.5 post coitum [6]. About 10 000 unipotent PGCs are found in each mice gonad. In the gonadal ridge, PGCs harbor the seminiferous cords and become mitotically arrested at day 13.5 post coitum, thereafter termed as gonocytes or prespermatogonia or prospermatogonia. Prospermatogonia, morphologically different from PGCs, undergo extensive epigenetic reprogramming for their later functioning as male gametes [7]. Once prospermatogonia reach basement peripartum, they are morphologically and biochemically different from prospermatogonia and are termed as A_{undifferentiated} spermatogonia. A_{undifferentiated} spermatogonia proliferate but remain connected as A_single, A_paired, and A_aligned through intercellular bridges. Intercellular bridges play a role in cell to cell communication and synchronization of these mitoses. A_aligned are syncytia of 4, 8, and 16 undifferentiated spermatogonia. Currently, no method distinguishes among different types of A_{undifferentiated} spermatogonia [1]. An undifferentiated spermatogonia irreversibly differentiates to A₁ spermatogonia under the repeated stimulation of retinoic acid. In rodents, chain identity is maintained upon this transition and it has been estimated that 64% of A_pair, 94% of A_aligned4, and 100% of A_{aligned8‐16} transition to the differentiating A₁ state following a retinoic acid pulse [8]. A₁ spermatogonia undergo six synchronized mitoses through A₂, A₃, A₄, intermediate (In), and B spermatogonia to form primary spermatocyte [4]. In the classical Huckins and Oakberg model, SSCs are contained within the A_single population [9, 10]. However, one current model of SSC renewal suggests that syncytia of A_paired and A_aligned spermatogonia can break to yield SSCs [11].

All undifferentiated spermatogonia other than SSCs are called progenitors. A_single undergo symmetric divisions forming syncytia of undifferentiating spermatogonia, or there is asymmetric division forming an SSC and a progenitor A_single, thus maintaining steady‐state spermatogenesis [4]. Total number of A_single in adult mouse testis is estimated at 35 000 and each adult mice testis has about 3000 SSCs, which is about 0.01% of total testis cells [12], and 0.03% of the entire testicular germ cell population. Furthermore, 33 × 10⁴ undifferentiated spermatogonia and 280 × 10⁴ differentiating spermatogonia populate each testis in the mouse [8]. SSCs as well as epiblast and PGCs from days 6–16.5 post coitum embryo can regenerate spermatogenesis upon transplantation to appropriate donor testes [13].

SSCs, capable of self‐renewal and generating progenitor cells through symmetrical and/or asymmetrical divisions, reside in a specific microenvironment within the ST termed as a germ line stem cell niche [14]. Within this microenvironment of the niche, SSCs are intermingled with other undifferentiated spermatogonia. Sertoli cells in communication with other somatic cells of the testes secrete factors to maintain the microenvironment of the niche. Within the niche, SSCs are located toward areas of the ST closer to interstitial vasculature [15].

Development of preleptotene spermatocyte from type B spermatogonia marks a cell’s detachment and migration away from the basement membrane. Preleptotene spermatocytes cross the tight Sertoli cell junctions and reach the adluminal portion of the ST. Primary spermatocytes first divide mitotically and then form short‐lived haploid secondary spermatocytes through meiosis I. During prophase of meiosis, preleptotene spermatocytes differentiate into leptotene, zygotene, pachytene, and diplotene, respectively. Haploid spermatids are formed by equational meiosis II of secondary spermatocytes. Round spermatids mature into spermatozoa through various steps of spermiogenesis without any increase in cell number. Spermatozoa are released into the lumen of the ST through the process of spermiation [16]. Figure 3.1 shows the steps in spermatoogenesis.

Figure 3.1 Steps in spermatogenesis.

Spermatogenesis can be divided into spermatocytogenesis, meiosis, and spermiogenesis. During spermatocytogenesis, germ cells divide by several mitoses to increase the yield of spermatogenesis, renew spermatogonial stems cells, produce more undifferentiated spermatogonia, and finally produce primary spermatocytes. During meiosis, recombination of genetic material happens, homologous chromosomes move apart, and chromosome number is reduced by half to yield haploid round spermatids. Spermiogenesis involves differentiation of haploid round spermatids into mature haploid elongated spermatozoa without mitosis or meiosis [16].