Spermiogenesis versus spermiation
Spermiogenesis reshapes round spermatids, the cells left after meiosis, into sperm, and spermiation is the release of the finished cells into the lumen, the central channel of the tubule.
Remember
Spermiogenesis changes the shape of the cells and spermiation lets them go: shape first, release second, while motility (the ability to swim) and fertilising ability come later.
Memory aid
Spermiogenesis builds the sperm, spermiation lets it go.
Spermiogenesis is the shape change that builds the sperm, and spermiation is the later release that lets it go.
Sperm are made in the seminiferous tubules, the tightly coiled tubes that make up most of each testis. The developing cells are held in the tubule wall by Sertoli cells, the large supporting cells that surround and nourish them.
Meiosis (the special cell division that halves the chromosome number) leaves round spermatids, cells with half the usual number of chromosomes. No further division takes place. Instead, spermiogenesis reshapes each spermatid into a spermatozoon, which is a sperm cell. The nucleus is packed tightly into the head, an acrosome (a cap-like vesicle holding enzymes) forms over it, a flagellum (the tail) grows, and most of the spare cytoplasm is shed.
Spermiation is the outcome of spermiogenesis. The finished spermatids detach from the Sertoli cells and are released into the lumen. Until then, each spermatid is anchored to a Sertoli cell. If release fails, fully formed spermatids stay stuck in the epithelium while the earlier cells look normal.
Released cells are not yet fully functional. They pass through the rete testis (a fine meshwork of tubules) and the efferent ductules (small collecting channels) into the epididymis, the coiled duct where sperm mature. Motility is acquired during transit through the epididymis. Capacitation, the final preparation of sperm for fertilisation, comes later still, in the female reproductive tract.
Key facts
- Spermiogenesis
- reshapes round haploid spermatids into spermatozoa, with no further division
- Changes in the cell
- a tightly packed nucleus, an acrosome, a flagellum and loss of spare cytoplasm
- Spermiation
- the release of finished spermatids from the Sertoli cells, so that they reach the lumen of the seminiferous tubule
- Order
- spermiation is the outcome of spermiogenesis, and it comes before transit to the epididymis
- If spermiation fails
- fully formed spermatids stay in the epithelium, the earlier cells look normal and few sperm reach the lumen
- After release
- sperm pass through the rete testis and efferent ductules to the epididymis, where motility is gained
Exam traps
- Making haploid spermatids by meiosis is not spermiogenesis, because meiosis comes before it and spermiogenesis reshapes the spermatids rather than dividing them.
- Reshaping round spermatids into sperm describes spermiogenesis rather than spermiation, because spermiogenesis builds the sperm whereas spermiation sets it free from the Sertoli cells.
- Maturation in the epididymis does not describe spermiation, because sperm are first released into the tubule lumen and only then pass through the rete testis and efferent ductules to the epididymis, where they gain motility.
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