Spermatogenesis: The Commitment to Meiosis

Abstract

Mammalian spermatogenesis requires a stem cell pool, a period of amplification of cell numbers, the completion of reduction division to haploid cells (meiosis), and the morphological transformation of the haploid cells into spermatozoa (spermiogenesis). The net result of these processes is the production of massive numbers of spermatozoa over the reproductive lifetime of the animal. One study that utilized homogenization-resistant spermatids as the standard determined that human daily sperm production (dsp) was at 45 million per day per testis (60). For each human that means ∼1,000 sperm are produced per second. A key to this level of gamete production is the organization and architecture of the mammalian testes that results in continuous sperm production. The seemingly complex repetitious relationship of cells termed the "cycle of the seminiferous epithelium" is driven by the continuous commitment of undifferentiated spermatogonia to meiosis and the period of time required to form spermatozoa. This commitment termed the A to A1 transition requires the action of retinoic acid (RA) on the undifferentiated spermatogonia or prospermatogonia. In stages VII to IX of the cycle of the seminiferous epithelium, Sertoli cells and germ cells are influenced by pulses of RA. These pulses of RA move along the seminiferous tubules coincident with the spermatogenic wave, presumably undergoing constant synthesis and degradation. The RA pulse then serves as a trigger to commit undifferentiated progenitor cells to the rigidly timed pathway into meiosis and spermatid differentiation.

Spermatogenesis associated 22 is required for DNA repair and synapsis of homologous chromosomes in mouse germ cells

“…Interestingly, sexual dimorphism in timing and execution of meiotic events has been observed in many species, most notably in mammals. In the male mouse, meiotic prophase first begins around postnatal day 8 and spermatogenesis proceeds continuously throughout the lifespan of the individual (Bellve et al , 1977; Griswold, 2016). In the female mouse, cells enter meiosis in the fetal ovary, around embryonic day 13.5, but completion of prophase I is halted prior to birth; meiosis remains arrested until hormonal stimulation triggers resumption of the first meiotic division in a small cohort of oocytes (Pepling, 2006).…”

Identification and characterization of human testis derived circular RNAs and their existence in seminal plasma

“…Spermatogenesis is a complex process which includes the following: differentiation of spermatogonia stem cell into spermatogonia cell, spermatocyte, round spermatid and elongated sperm1. During spermatogenesis, sequential and coordinated expression of stage-specific genes is essential for the normal development of male gametes2.…”

GLIS1–3 transcription factors: critical roles in the regulation of multiple physiological processes and diseases

“…In the mouse testis at PND1, nondividing gonocytes migrate to the basal compartment of the seminiferous tubule where they differentiate into self-renewing spermatogonial stem cells (SSCs), which are distinguished by the high expression of inhibitor of DNA binding 4 ( Id4 ) [131–134]. The SSCs sequentially differentiate into several subsets of spermatogonial progenitor cells (SPCs) that are classified as A-single (A s ), A-paired (A pr ), and A-aligned spermatogonia (A al4 , A al8 , A al16 ) consisting of 1, 2, 4, 8 and 16 cell(s), respectively (Fig.…”

“…The SSCs and SPCs together are referred to as undifferentiated spermatogonia and share the expression of a number of genes implicated in SSC maintenance and self-renewal, and PGC development, including Pax7 , Etv5 , Gfra1 , Zbtb16 ( Plzf ), Lhx1 , Sall4 , Lin28 , and Nanos2 . The A pr and A al SPCs are transient amplifying cells that become irreversibly committed to differentiation into type B spermatogonia [131–134]. This is accompanied by down-regulation of the expression of various genes, including Gfra1 , Zbtb16 , E-cadherin ( Cdh1 ), Nanos2 , and FoxO1 and induction of cKit .…”