Spermiogenesis: Function, Tasks, Role & Diseases

Spermiogenesis is the term used to describe the remodeling phase of spermatids formed by spermatogenesis into mature spermatozoa capable of fertilization. During spermiogenesis, spermatids lose much of their cytoplasm and the flagellum forms, which serves active locomotion. On the head containing the nuclear DNA, opposite the point of attachment of the flagella, the acrosome is formed, which contains enzymes that allow penetration into the egg.

What is spermiogenesis?

Spermiogenesis is the term used to describe the remodeling phase of spermatids produced by spermatogenesis into mature sperm that are capable of fertilization. In contrast to spermatogenesis, in the course of which germ cells undergo mitosis and maturation division (meiosis) I and II, respectively, and are subsequently referred to as spermatids, spermiogenesis concerns exclusively the remodeling of spermatids into mature and fertilizable spermatozoa. Spermiogenesis of a spermatid takes about 24 days. The spermatids, which have only a haploid set of chromosomes due to prior meiosis, are transformed into a specialized cell for the sole purpose of penetrating a fertilizable female egg. The transformation of a spermatid into a sperm involves serious internal and external changes. The spermatid loses almost all of its cytoplasm, leaving essentially only the nucleus, which contains DNA. The greatly reduced cell is transformed into the head of the future sperm. Where the centriole is located, a flagellum, also called a tail, is formed, which serves for the active locomotion of the sperm. On the side opposite the flagellum, a cap is formed, the acrosome, which contains enzymes that allow penetration into the female egg. Mitochondria, including their mitochondrial DNA and RNA, originally located in the cytosol of the spermatid, attach to the midpiece of the flagellum and provide the energy necessary for locomotion.

Function and purpose

The spermatid, still recognizable as a haploid cell at the beginning of spermiogenesis, transforms into a spermatozoon that has undergone major external and internal changes. The haploid chromosome set is no longer changed. Only the mitochondria together with the mitochondrial DNA and RNA are removed in order to provide the flagella with the necessary energy for their movements. The sperm within an ejaculate differ genetically only in that 50 percent contain an X chromosome and the other 50 percent contain a Y chromosome. One distinctive feature is that when the sperm penetrates the female egg, it sheds the flagellum and thus the mitochondrial DNA from the male sperm cell no longer plays a role. The mitochondrial DNA of the fertilized egg, the later zygote, is derived exclusively from the mitochondria of the mother. Spermiogenesis serves to transform spermatids into purpose-built, optimized spermatozoa. Vigorous spermatids that can move as quickly as possible toward the fertilizable egg after ejaculation have the greatest chance of passing on their chromosome set. After docking with the membrane of the egg, a physiological process is triggered that prevents further sperm from docking. The motility and energy reserves of the individual sperm can play a decisive role in “winning the race”. This is not so much a matter of competition between genetically identical sperm within an ejaculate, but rather competition with sperm from a “foreign” ejaculate, since humans are not fundamentally monogamous. The possibilities of winning the competition against “foreign sperm” are not limited to “purely sporting competition”, but some of the sperm within an ejaculate are immobile and can block the way for foreign sperm. Within an ejaculate there are also “killer sperm” that can recognize foreign sperm and kill them with chemical means.

Diseases and complaints

Disorders, diseases, genetic abnormalities, overuse of alcohol or other drugs, and more can lead to impaired spermiogenesis, resulting in reversible or permanent infertility (infertility). In most cases, disorders of spermiogenesis should not be considered in isolation, as they are usually the result of impaired spermatogenesis.Basically, impaired spermiogenesis can be caused by diseases or lesions of the sperm-producing organs, the testes, or by malfunctions in hormone production. Various testicular anomalies such as undescended testis, testicular hypoplasia and infections of the prostate as well as mumps-related testicular inflammation (mumps orchitis) are typical causes of disorders in spermiogenesis and spermatogenesis, which usually lead to reduced fertility or even total infertility. Similar effects can be caused by diseases of the testicles such as varicoceles, spermatoceles, hydroceles or prostate tumors. Also within the scope of disruption of spermiogenesis by the producing organs is, for example, radiation therapy for cancer treatment, which can damage the testes. Extragenital causes are diseases that can have an impact on spermatogenesis and spermiogenesis. These are mainly febrile infections, which can lead to temporary impairment of spermatogenesis as a result of an increase in the temperature of the testicles. Environmental toxins and occupational exposure to toxic substances such as bisphenol A, organic solvents, pesticides, herbicides, heavy metals, plasticizers of plastics and many others pose risks for impaired spermiogenesis. The hypothalamus and pituitary gland, the main control center for hormonal processes in the body, also deserve special attention. If the pituitary gland is unable to provide control hormones such as FSH (follicle-stimulating hormone) and LH (luteinizing hormone) and some others in the necessary concentrations, the result is altered – usually decreased – production of sex hormones and consequent disruption of spermiogenesis.