Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 3: Developmental changes in spermatid flagellum and cytoplasmic droplet and interaction of sperm with the zona pellucida and egg plasma membrane

Abstract

Spermiogenesis constitutes the steps involved in the metamorphosis of spermatids into spermatozoa. It involves modification of several organelles in addition to the formation of several structures including the flagellum and cytoplasmic droplet. The flagellum is composed of a neck region and middle, principal, and end pieces. The axoneme composed of nine outer microtubular doublets circularly arranged to form a cylinder around a central pair of microtubules is present throughout the flagellum. The middle and principal pieces each contain specific components such as the mitochondrial sheath and fibrous sheath, respectively, while outer dense fibers are common to both. A plethora of proteins are constituents of each of these structures, with each playing key roles in functions related to the fertility of spermatozoa. At the end of spermiogenesis, a portion of spermatid cytoplasm remains associated with the released spermatozoa, referred to as the cytoplasmic droplet. The latter has as its main feature Golgi saccules, which appear to modify the plasma membrane of spermatozoa as they move down the epididymal duct and hence may be partly involved in male gamete maturation. The end product of spermatogenesis is highly streamlined and motile spermatozoa having a condensed nucleus equipped with an acrosome. Spermatozoa move through the female reproductive tract and eventually penetrate the zona pellucida and bind to the egg plasma membrane. Many proteins have been implicated in the process of fertilization as well as a plethora of proteins involved in the development of spermatids and sperm, and these are high lighted in this review.

KATNB1 in the human testis and its genetic variants in fertile and oligoasthenoteratozoospermic infertile men

“…1). The major microtubule-mediated event occurring in round spermatids is the assembly of the axoneme from the distal centriole of the round spermatid centrosome (Hermo et al, 2010). KATNB1 was also present in spermatids as they entered the elongation phase ( Fig.…”

“…In parallel with axoneme development, the secondary structures of the tail are assembled, including outer dense fibres, fibrous sheath and the mitochondrial sheath. This assembly of accessory structures involves cargo transport pathways, including microtubule-mediated transport of key proteins to the developing flagella (Kierszenbaum, 2002;Hermo et al, 2010;O'Donnell & O'Bryan, 2014). Sperm head shaping is initiated during mid-spermiogenesis, when the nucleus begins compaction.…”

Mechanisms of spermiogenesis and spermiation and how they are disturbed

“…4,5 Another major event occurring during spermiogenesis is the assembly of the sperm flagellum, reviewed in. [7][8][9] The central component of the flagellum, the microtubule-based axoneme, is assembled soon after the completion of meiosis. As spermatids elongate, the accessory structures needed for flagella function (outer dense fibers, fibrous sheath, mitochondrial sheath) are assembled around the central axoneme.…”

CFAP157 is a murine downstream effector of FOXJ1 that is specifically required for flagellum morphogenesis and sperm motility

“…Concomitantly, the sperm head reshapes and the acrosome, which is a sperm-specific secretory vesicle, forms (reviewed by Fléchon, 2016). Most of the cytoplasm is removed via a residual body that detaches from the neck region and, via a cytoplasmic droplet that is shed at the midpiece-principal piece boundary, the annulus (reviewed by Hermo et al, 2010). Spermatozoa are stored in the cauda epididymis, where they undergo further maturation (reviewed by Buffone et al, 2012;Cornwall, 2009).…”