Sperm-Head Formation and Factors Affecting It

Abstract

Various types of sperm-head anomalies can be observed in patients with teratozoospermia or oligozoospermia. The presence of such abnormalities is one of the primary causes of infertility in males. Here, I discuss these abnormalities in relation to spermatogenesis.

Prior to their entry into the first meiosis, germ-line stem cells proliferate at the spermatogonial stage, undergoing self-renewal as type A spermatogonia (A1–A3 in the rat). Type A spermatogonia differentiate into type B spermatogonia, and these in turn differentiate into primary spermatocytes that subsequently undergo meiosis (Figs. 3.1 and 3.2). Prior to the first meiotic division, primary spermatocytes duplicate their DNA/chromosomes. A typical feature of the first meiosis is genetic recombination; at this stage, a synaptonemal complex containing recombination nodules is formed in the pachytene-phase spermatocytes. Immediately after meiosis (within approximately 8 h in the case of humans), the secondary spermatocytes immediately develop into haploid spermatids. Thus, almost all of the molecules/proteins required for sperm-head formation are presumed to be prepared at the gene level in primary spermatocytes. The gene expression of spermatids is referred to as postmeiotic gene expression. These steps are followed by four major events of spermiogenesis: nuclear modification, acrosome biogenesis, cytoplasmic trimming, and flagellum formation. The first three events are involved in head formation.

Sperm-head formation is triggered by the initiation of acrosome biogenesis during spermiogenesis. Head formation involves acrosome biogenesis, nuclear condensation and elongation (flattening), and the formation of cytoplasmic layers. Cytoplasmic molecules/substances gradually accumulate to form the perinuclear theca. Some of these essential molecules are transported along manchettes (a manchette is an organized microtubular structure that girdles the posterior region of the spermatid nucleus) and are organized into the various structural components. This organization occurs most actively in elongating spermatids. In particular, drastic changes occur from the mid-stage to the final step of spermiogenesis. The cytoplasm released by the germ cells is phagocytosed by the Sertoli cells before spermiation. Thus, germ cells produce proteins essential for fertilization, and these proteins are organized into the head and tail structures or their components in a time-sensitive manner.