Acrosome is the term used to describe a vesicular organelle that precedes the nucleus of the sperm cells (spermatozoa) of vertebrate and invertebrate animals and is composed of specially configured proteins and enzymes.
Sperm are the gametes or male sex cells. They have half the genetic load of the organism that gives rise to them, that is, they are haploid cells, and their main function is to fertilize the ovum produced by a female, in order to form a new genetically different individual.
In most animals, sperm are mobile cells whose body is divided into two well-defined regions: a head and a tail, both covered by the same plasma membrane. The head is the portion that contains the nucleus and much of the cytosol, while the tail is a flagellar structure that serves for motility.
The acrosome is located in the head of the sperm cells, specifically at the distal end, covering practically the entire cell surface, and the proteins contained in this vesicle have special functions during the fertilization process.
Acrosome functions
Schematic of the structure of a sperm and the location of the acrosome (Source: Gevictor via Wikimedia Commons)
The acrosomes have a primary function during the fertilization process at the site of attachment of the sperm with the zona pellucida of the ovum (which is the outer covering of this female gametic cell), which has been shown by some related infertility studies with defects in this vesicular structure.
In some scientific articles it is possible to find descriptions of these organelles where they are referred to as "similar to cellular lysosomes", since they are saccule-shaped structures that serve different intracellular digestive and defense purposes.
Thus, the function of these sperm vesicles is to degrade the components of the zona pellucida while the sperm makes its way towards the ovum to fuse with its membrane and fertilize it.
Training
The morphology of the acrosome varies greatly between species, but it is almost always a vesicular structure derived from the Golgi complex, which is synthesized and assembled during the early stages of spermiogenesis (differentiation of spermatids into sperm).
The acrosomal vesicle is delimited by two membranes known as the acrosomal membranes, which are one internal and one external. These membranes contain different structural and non-structural components, proteins and enzymes of different types, which are important for the establishment of an internal matrix.
These internal components participate in the dispersion of the acrosomal matrix, in the penetration of the sperm through the zona pellucida of the ovum (extracellular covering) and in the interaction between the plasma membranes of both gametic cells.
How is the acrosome formed?
At the beginning of spermiogenesis, when meiosis is complete, the rounded haploid cells change their shape to that which is characteristic of sperm.
During this process, the Golgi complex is a prominent system of densely packed tubules and vesicles that are distributed in the regions near the poles of the nucleus. Some vesicles derived from the Golgi complex increase in size and increase their concentration of fine granular components.
Each fine granule releases its rich glycoprotein content inside these larger vesicles, and this is what some authors call the “acrosomal system in formation”, from which the sperm head hood and the acrosome are subsequently formed.
Concomitant with the granule “loading” process, these vesicles also receive multiple glycoproteins that are synthesized and actively transported into them.
In rodents, the process of formation and evolution of the acrosomal sperm system occurs in four phases during spermiogenesis. The first is known as the Golgi phase and is when “pro-acrosomal” granules are formed from the saccules of the trans face of the Golgi complex.
Subsequently, these granules fuse to form a single acrosomal granule, which is lengthened thanks to the translocation of new proteins from the Golgi complex (second phase). The third phase is known as the acrosomic phase and consists of the hemispherical structural conformation of the acrosome.
The fourth phase, also known as the maturation phase, has to do with different changes that occur in nuclear morphology (the acrosome in formation is in close proximity to the nucleus) and with the migration of the acrosome and its distribution throughout the cell..
Reaction
As mentioned, the acrosome is a vesicle that differs from the Golgi complex of the sperm. The process by which the luminal content of this vesicle is released prior to fusion between the ovum and the sperm during sexual reproduction is known as the acrosome reaction.
This reaction, as well as the morphology of acrosomes, varies widely from one species to another, especially between vertebrates and invertebrates; however, in both cases it is a highly regulated event.
Acrosomic reaction (Source: Cremaster via Wikimedia Commons)
Background
The acrosomal reaction only takes place when sperm are released by a male into the genital tract of a female and travel to the ovaries, where the eggs are located, which implies that these cells have previously undergone two maturation processes:
- The transit through the epididymis (in male gonads)
- Training (during transit through the female's genital tract)
Only trained sperm are capable, molecularly speaking, of "recognizing" the zona pellucida and joining it, since it is a process mediated by carbohydrates that are recognized by specific receptors on the sperm membrane.
When a sperm unites with the zona pellucida of an egg, calcium-dependent signaling pathways are activated that trigger acrosome exocytosis, which begins with the fusion of the outer acrosomal membrane with the sperm's plasma membrane.
Fertilization, that is, the fusion of the female and male nuclei in the cytosol of the ovule is only possible through the acrosomic reaction, since the sperm uses the enzymes contained in this vesicle to cross the zona pellucida and reach the membrane plasma of the ovum.
Enzymes
There are several enzymes contained in the acrosomal lumen; Similar to those in lysosomes are some acid glycohydrolases, proteases, esterases, acid phosphatases, and arylsulfatases.
Among the acrosomal proteinases and peptidases are acrosin, the most studied enzyme in the acrosome and which is an endoproteinase with properties similar to those of pancreatic trypsin. Its presence has been confirmed at least in all mammals. It is present in its inactive form, proacrosin.
Some of the literature suggests that this enzyme can also be found on the surface of spermatozoa, where the proacrosin / acrosin complex seems to be one of the receptors necessary for the recognition of the zona pellucida.
Acrosomes are also rich in glycosidase enzymes and the best known is hyaluronidase, which is associated with the outer acrosomal membrane and the plasma membrane of the sperm.
Among the lipase enzymes present in acrosomes, phospholipase A2 and phospholipase C stand out. They also have phosphatases such as alkaline phosphatase and some ATPases.
References
- Abou-Haila, A., & Tulsiani, DR (2000). Mammalian sperm acrosome: formation, contents, and function. Archives of biochemistry and biophysics, 379 (2), 173-182.
- Berruti, G., & Paiardi, C. (2011). Acrosome biogenesis: Revisiting old questions to yield new insights. Spermatogenesis, 1 (2), 95-98.
- Dan, JC (1956). The acrosome reaction. In International review of cytology (Vol. 5, pp. 365-393). Academic Press.
- Dan, JC (1967). Acrosome reaction and lysins. In Fertilization (pp. 237-293). Academic Press.
- Khawar, MB, Gao, H., & Li, W. (2019). Mechanism of Acrosome Biogenesis in Mammals. Frontiers in Cell and Developmental Biology, 7, 195.
- Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.
- Zaneveld, LJD, & De Jonge, CJ (1991). Mammalian sperm acrosomal enzymes and the acrosome reaction. In A comparative overview of mammalian fertilization (pp. 63-79). Springer, Boston, MA.