spermiation

Spermiation is the final stage of spermatogenesis, the process by which mature spermatozoa are released from Sertoli cells into the lumen of the seminiferous tubules. It follows spermiogenesis, during which spermatids develop into morphologically mature sperm. Spermiation is distinct from ejaculation, which is the expulsion of semen from the male reproductive tract.

During spermiation, adhesions between developing sperm and the Sertoli cell are dynamically remodeled. This involves the breakdown of Sertoli–germ cell junctions and the coordinated withdrawal of the spermatid's cytoplasm from the Sertoli cell into the tubular lumen. Tubulobulbar complexes form to facilitate detachment, and the residual cytoplasm surrounding maturing sperm is shed and later phagocytosed by Sertoli cells as residual bodies. The result is the release of free spermatozoa into the lumen, ready to be transported toward the rete testis and epididymis.

Regulation of spermiation is tightly coordinated with the seminiferous epithelium cycle and relies on hormonal signals acting on Sertoli cells, particularly testosterone and follicle-stimulating hormone (FSH), along with local paracrine factors within the tubules. The process is species-conserved in mammals and can be influenced by physiological factors such as temperature, health, and environmental exposures.

Clinically, failure or disruption of spermiation can contribute to male infertility, even when earlier stages of spermatogenesis are intact. Because spermatozoa express unique antigens, the testicular environment normally maintains immune privilege to prevent autoimmune reactions. Spermiation thus represents a critical, tightly regulated step ensuring the production of motile, fertilization-capable sperm.