Definition, Function, Reaction and Exocytosis

Definition: What is an Acrosome?

The acrosome is one of the main components of a sperm cell overlying the nucleus. It's located on the anterior part of the sperm head where it serves a number of important roles related to fertilization.

* Introduced in 1898 by Lenhossek, the term translates to apical body or tip body. 

The acrosome develops during spermiogenesis and is the product of the Golgi complex. Here, the biogenesis starts when proacrosomic vesicles generate from the Golgi complex belonging to early spermatids. The vesicles then combine forming a single, dense, acrosomic vesicle that ultimately extends to cover about two-thirds of the nuclear surface. 

In the sperm head, the acrosome is located between the plasma membrane and the nuclear membrane. As such, it is not directly exposed to the outer environment of the cell. It also has its own set of membranes consisting of the inner membrane (above the nuclear envelope) and the outer membrane which lies beneath the plasma membrane of the sperm cell head. 


* During acrosome reaction or exocytosis of the acrosome, the plasma membrane of the sperm cell fuses with the outer acrosomal membrane. 

* Sperm cells of some organisms, such as teleost fish, do not have an acrosome.


Some of the most important acrosome contents include:


  • Primary ligands - E.g. galactosyltransferase (primary ligands may be located on or near the acrosome)
  • Secondary ligands - Acrosin/ proacrosin (a serine protease) responsible for local lysis


* The acrosome also contains various other contents including various glycohydrolases.

Functions of the Acrosome

Transformation of the Sperm Head

The acrosome is involved in the transformation (shaping) of the sperm head through the Acrosome–Acroplaxome-manchette complex. Here, the two important structures involved are acroplaxone and manchette.

The acroplaxone (consisting of actin and keratin) overlays the acrosome and its complex with manchette contribute to the following:


  • Development of acrosomal sac
  • Anchoring acrosome to the nuclear envelope
  • Transformation of the sperm cell


* By shaping the acrosome, the complex allows the acrosome to shape the sperm head during spermiogenesis. 

Gamete Recognition

In such animal as frogs and sea urchin, proteins referred to as primary ligands have been shown to play an important role in gamete recognition. Although these ligands are usually located on the plasma membrane surface, some have been identified on the acrosome thus involving the organelle in gamete recognition.

Once the primary ligands identify specific proteins in the jerry surrounding the egg, they initiate binding. 

Acrosome Reaction and Exocytosis

When the spermatozoa (or primary ligands of the sperm) come into contact with the jerry layer that surrounds the female gamete (the egg) acrosome reaction/exocytosis is initiated. 

Before the sperm can undergo acrosome exocytosis, it goes through a process known as capacitation.

Here, the sperm undergoes a number of physiological changes that include:


  • The plasma membrane of the sperm loses cholesterol
  • Soluble acrosome reaction is activated 
  • The second messenger cyclic AMP is generated
  • Protein kinases activation 


Once the sperm cells have gone through these changes in the female reproductive tract, they are ready for exocytosis. 


When the sperm cell comes into contact with the external coat of the female gamete (the zona pellucida), some of the primary ligands (lectins) bind to the receptors molecules of ZP3 (O-linked oligosaccharides and glycans) thus binding the two gametes.

This process has been linked with ensuring that the sperm binds to the egg in a species-selective manner that allows for acrosome exocytosis (also referred to as acrosome reaction). Acrosome is calcium dependent and involves acrosome exocytosis.

Here, binding of the sperm to the egg is accompanied by the formation of numerous pores between the acrosome membrane and the plasma membrane of the sperm cell at the sperm head.

This reaction not only results in the fusion of the two membranes, the plasma membrane of the sperm cell and the acrosome membrane, but also allows contents of the acrosome to be released. Acrosome reaction has two main outcomes.

These include:


  1. Release of enzymes that degrade the zona pellucida surrounding the plasma membrane of the female gamete (at the zona pellucida)
  2. Exposing the sperm head (the nucleus) to the egg

Acrosome Secondary Ligands

Once the sperm cell binds to the zona pellucida surrounding the plasma membrane of the egg, pores are formed allowing acrosome components to be released. This is where secondary ligands (hydrolytic enzymes) come into play. 

One of the main components of the acrosome is a serine protease known as acrosin. In the acrosome, the protein is stored in an inactive form known as zymogen.

Once the acrosome membrane is compromised, allowing the protein to be released, it's converted into its active form (acrosin) by the coming in contact with the glycoproteins of zona pellucida.

In its active form, the enzyme plays an important role of degrading the zona pellucida of the egg in order to create an opening through which the sperm can penetrate through. This may take about 5 to 20 minutes. 


* Lysis of the egg zona pellucida is localized around the acrosome. As a result, not all parts of this surface are degraded to allow the sperm cell to penetrate the egg

* Based on a number of studies, acrosin has been shown to be lacking, or not being essential for fertilization. This has been shown to be the case among animals like mice.

* By comparing animals with acrosome acrosin to those without, it was discovered that during acrosome reaction, acrosin speeds up the dispersal of acrosome proteins. 


Following exocytosis of the acrosome, and thus acrosome reaction, degradation of the zona pellucida makes it possible for the head of the spermatozoon to fuse with the plasma membrane of the oocyte which in turn allows for cytoplasmic continuity between the gametes.

It is this particular fusion that results in fertilization that converts the oocyte into a zygote. Formation of the zygote prevents any other spermatozoon from penetrating the zona pellucida for fertilization.

Therefore, once the egg is fertilized, acrosome reaction or exocytosis no longer has any impact on the zona pellucida. This means that once the egg is fertilized, the acrosome of the other sperm cells no longer have a role to play in fertilization. 

Do plant cells have vesicles?

Do animal cells have vesicles?

Read about Sertoli Cells as well.

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