Acrosome is one of the most important structures of a sperm cell and plays a crucial role in fertilization, reproduction, and sperm–egg interaction. Many school exams, competitive exams, and NEET-level questions are frequently asked on this topic.
These notes explain what is acrosome, its structure, functions, origin, enzymes, importance in fertilization, acrosome reaction, and all related queries.
What is Acrosome?
- Acrosome is a cap-like structure present at the tip of the sperm head.
- Formed from Golgi bodies during sperm development (spermiogenesis).
- It contains hydrolytic enzymes that help the sperm fertilize the egg.
- Also known as the “sperm cap” or “Golgi cap” in early development stages.
Fact: The acrosome covers almost 70% of the sperm head.
Why is Acrosome Important?
- Without an acrosome, the sperm cannot penetrate the protective layers of the ovum.
- It enables the process of acrosome reaction, which is essential for fertilization.
- Helps the sperm recognize, attach, and enter the egg.
Acrosome Structure (Detailed & Pointwise)
Basic Structural Features:
- Located at the anterior tip of the sperm.
- Membrane-bound sac (vesicle).
- Semi-lunar / cap-shaped.
- Filled with digestive enzymes.
Layers of Acrosome:
- Outer acrosomal membrane
- Inner acrosomal membrane
These membranes are essential during the acrosome reaction when enzymes are released.
Size and Position:
- Covers the nucleus of sperm like a cap.
- Lies below the sperm plasma membrane.
Origin:
- Originates from Golgi apparatus during spermiogenesis.
- Initially forms as a proacrosomal granule → merges to become the acrosome.
Enzymes Present in Acrosome
The acrosome contains powerful enzymes that help sperm digest egg coverings.
Major Acrosomal Enzymes:
- Hyaluronidase – breaks down hyaluronic acid between follicular cells.
- Acrosin – a protease enzyme that digests zona pellucida.
- Zona lysin – helps in zona pellucida penetration.
- Neuraminidase – modifies egg surface molecules.
- Proteases – break protein layers of ovum coverings.
Fact: Acrosin works similarly to trypsin, a digestive enzyme.
Functions of Acrosome
Primary Function – Egg Penetration
The main job of acrosome is to release enzymes that allow the sperm to enter the egg.
Other Important Functions:
- Helps in sperm–egg recognition.
- Facilitates movement through corona radiata.
- Dissolves zona pellucida (glycoprotein layer).
- Triggers acrosome reaction, critical for fertilization.
- Ensures species-specific fertilization.
Also Read This : Human reproductive System
What is Acrosome Reaction?
Acrosome reaction is a biochemical process where the sperm releases enzymes to penetrate the ovum.
Steps of Acrosome Reaction
- Sperm reaches the corona radiata (outer layer of egg).
- Binding of sperm to zona pellucida (ZP3 receptor).
- Acrosomal membranes fuse with the sperm plasma membrane.
- Enzymes are released (hyaluronidase, acrosin, etc.).
- Sperm penetrates zona pellucida.
- Sperm reaches the perivitelline space.
- Fusion of sperm membrane with egg membrane occurs.
Importance of Acrosome Reaction
- Allows sperm to cross physical barriers.
- Enables fertilization.
- Prevents polyspermy because once one sperm enters, the egg activates a cortical reaction to block others.
Layers of Ovum the Acrosome Helps Penetrate
Acrosome helps sperm move through three important egg layers:
1. Corona Radiata
- Outermost layer of follicular cells.
- Hyaluronidase dissolves hyaluronic acid between cells.
2. Zona Pellucida
- Transparent glycoprotein layer.
- Acrosin digests this layer.
3. Vitelline Membrane / Egg Membrane
- Fusion occurs after penetration.
Development of Acrosome (Spermiogenesis Process)
Stages:
- Golgi Phase:
- Golgi forms proacrosomal vesicles.
- Vesicles fuse → form acrosomal vesicle.
- Cap Phase:
- Vesicle spreads over the nucleus like a cap.
- Acrosome Phase:
- Acrosome elongates.
- Nucleus condenses.
- Maturation Phase:
- Tail forms.
- Sperm becomes streamlined with a mature acrosome.
Also Read This : Male reproductive System
What Happens If Acrosome Is Missing? (Acrania or Globozoospermia)
A condition where sperm lack acrosomes is called Globozoospermia.
Consequences:
- Infertility.
- Sperm cannot penetrate the egg.
- Abnormal sperm morphology.
Acrosome vs Head of Sperm – Difference
| Feature | Acrosome | Sperm Head |
|---|---|---|
| Location | Tip of head | Entire front region |
| Composition | Enzymes, membranes | Nucleus + acrosome |
| Function | Egg penetration | Holds genetic material |
| Origin | Golgi bodies | Spermiogenesis |
Acrosomal Cap – What Does It Mean?
The term acrosomal cap refers to the expanded head-like covering formed during the cap phase of spermiogenesis.
It is the early form of the fully developed acrosome.
Role of Acrosome in Fertilization – Full Explanation
1. Binding to Egg
Acrosome recognizes ZP3 glycoproteins on the egg.
2. Enzyme Release
Acrosomal enzymes clear a path.
3. Sperm Entry
Penetrates zona pellucida into perivitelline space.
4. Membrane Fusion
Sperm and egg membranes fuse → leads to syngamy (fusion of nuclei).
5. Formation of Zygote
Fertilization completes → forms zygote → embryo.
Is Acrosome Found Only in Human Sperm?
No.
Acrosomes are found in almost all mammalian sperms and in many other species including:
- Humans
- Animals (dogs, cats, horses, cows, monkeys)
- Rodents
- Many invertebrates
Birds and reptiles also have sperm with acrosomes.
High-Yield Exam Questions on Acrosome (With Answers)
Explain acrosome.
A cap-like enzyme-filled structure on the sperm head helping in fertilization.
Where does acrosome originate?
From the Golgi apparatus.
Name one important enzyme in acrosome.
Hyaluronidase or Acrosin.
What is acrosome reaction?
The release of acrosomal enzymes to penetrate the egg.
Which layer does acrosin act on?
Zona pellucida.
What happens after acrosome reaction?
Sperm fuses with the egg membrane.
FAQs – Short and Sweet Answers
What is acrosome?
A cap-like structure on sperm containing enzymes for egg penetration.
Function of acrosome?
To dissolve the egg’s protective layers and help sperm enter.
What triggers the acrosome reaction?
Sperm binding to the zona pellucida (ZP3 receptor).
What enzymes does the acrosome contain?
Hyaluronidase, acrosin, proteases.
Where is acrosome located?
At the tip of the sperm head.
Is acrosome membrane-bound?
Yes, it is a membrane-bound vesicle.
Which organelle produces the acrosome?
Golgi apparatus.
Why is acrosome important?
Without it, fertilization cannot occur.
What is zona pellucida?
A glycoprotein layer surrounding the ovum.
What if acrosome is absent?
Sperm cannot fertilize the egg, leading to infertility.
Conclusion
The acrosome is a tiny but extremely important structure that ensures the successful union of sperm and egg. Formed from Golgi bodies and packed with powerful enzymes, it enables the sperm to break through barriers such as the corona radiata and zona pellucida. Without a functional acrosome or acrosome reaction, fertilization would not be possible.
Understanding the acrosome is essential from both biological and exam perspectives since it appears in Class 10, Class 11, Class 12, and NEET Biology chapters related to human reproduction, gametogenesis, and fertilization.
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