Acrosome: Reaction, Function & Definition

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:00 A Sperm's Journey
  • 0:45 Anatomy Of A Sperm
  • 2:05 The Acrosome Reaction
  • 3:20 Release Of Proteolytic…
  • 4:40 Extension And Exposure
  • 5:50 Lesson Summary
Create an account to start this course today
Try it free for 5 days!
Create An Account

Recommended Lessons and Courses for You

Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

When a sperm finds an egg, the complex process of fertilization has only just begun. In this lesson, you'll learn about the acrosome, a specialized vesicle in the head of the sperm, and the important roles it plays in fertilization.

A Sperm's Journey

Let's be honest: sperm have a tough job. First, they have to find an egg to fertilize. And even after swimming long distances through treacherous conditions, the vast majority of these tiny, short-lived cells don't find an egg.

Those few sperm that do find an egg aren't finished; they still have to cross the egg's protective coat and fuse their own membrane with the plasma membrane of the egg, a process that requires an acrosome reaction (more on that in a minute).

In almost all cases, only one sperm out of the millions released manages to do this. This lucky sperm's nucleus and genetic material enter the egg, and the development of a new organism begins.

Anatomy of a Sperm

To understand the acrosome reaction, we first need to understand the different components involved.

Here's a basic diagram of the anatomy of a sperm. The tail is, of course, for swimming and the head contains a nucleus, a pool of actin, and an acrosome. So, what are these parts for?

The acrosome is a vesicle positioned close to the plasma membrane at the tip of the sperm's head. This vesicle contains soluble proteolytic enzymes (depicted in the diagram as pink contents) and inner membrane proteins, such as bindin (depicted as cyan-colored dots). The nucleus (depicted in green) contains the genetic material that the sperm is trying to pass on, a haploid genome because it contains only one copy of each chromosome.

In front of the nucleus and behind the acrosome, the sperm stores actin (depicted in purple). Actin is a cytoskeletal protein that exists in two major forms in cells: globular, or G-actin, and filamentous, or F-actin. G-actin proteins are monomers that can polymerize into long F-actin filaments, which can change the shape of the cell. As we'll see, this is important during the acrosome reaction.

The Acrosome Reaction

The acrosome reaction is the exocytosis of the acrosome, or the fusion of the acrosomal membrane with the sperm's plasma membrane. This process releases the acrosomal contents to the outside of the sperm and exposes the inner acrosomal membrane proteins on the sperm's outer plasma membrane.

The function of the acrosome reaction is to help the sperm get through the egg's protective coat and to allow the plasma membranes of the sperm and egg to fuse. This places both haploid nuclei (one from the sperm and one from the egg) into the same cell, where they form the diploid genome of the new organism.

The acrosome reaction is initiated by the binding of the sperm to molecules in the egg's protective coat. This binding causes a calcium influx into the sperm's cytosol, which stimulates the exocytosis of the acrosome. This has three important consequences: the release of proteolytic enzymes, extension of the acrosomal process, and exposure of membrane proteins. Let's look at each of these in further detail.

Release of Proteolytic Enzymes

When the acrosome's proteolytic enzymes are released, they begin to degrade the egg's protein-rich protective coat. This creates a path for the sperm to swim through on its way to the egg's plasma membrane.

What is this protective egg coat we keep referring to? It's different in mammals than in marine organisms, such as sea urchins, whose external fertilization process has often been studied. In sea urchins, the egg plasma membrane is surrounded by a thin, porous vitelline envelope made of extracellular matrix and a thicker jelly coat surrounding that. In mammals, the egg plasma membrane is surrounded by a thick zona pellucida layer, made of extracellular matrix, and a jelly-like cumulus layer surrounding that.

In both marine organisms and mammals, the egg's coat protects the egg from damage and helps to prevent cross-species fertilization, which could result in inviable offspring. In a marine environment, this is especially important because there are lots of eggs and sperm in the water that have been released from different organisms. The egg's coat also plays a role in preventing polyspermy, or fertilization by more than one sperm, which would also result in inviable offspring.

To unlock this lesson you must be a Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher
What is your educational goal?

Unlock Your Education

See for yourself why 10 million people use

Become a member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 49 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Click "next lesson" whenever you finish a lesson and quiz. Got It
You now have full access to our lessons and courses. Watch the lesson now or keep exploring. Got It
You're 25% of the way through this course! Keep going at this rate,and you'll be done before you know it.
The first step is always the hardest! Congrats on finishing your first lesson. Go to Next Lesson Take Quiz
Way to go! If you watch at least 30 minutes of lessons each day you'll master your goals before you know it. Go to Next Lesson Take Quiz
Congratulations on earning a badge for watching 10 videos but you've only scratched the surface. Keep it up! Go to Next Lesson Take Quiz
You've just watched 20 videos and earned a badge for your accomplishment! Go to Next Lesson Take Quiz
You've just earned a badge for watching 50 different lessons. Keep it up, you're making great progress! Go to Next Lesson Take Quiz
You just watched your 100th video lesson. You have earned a badge for this achievement! Go to Next Lesson Take Quiz
Congratulations! You just finished watching your 200th lesson and earned a badge! Go to Next Lesson Take Quiz
Congratulations! You just finished watching your 300th lesson and earned a badge! Go to Next Lesson Take Quiz
You are a superstar! You have earned the prestigious 500 video lessons watched badge. Go to Next Lesson Take Quiz
Incredible. You have just entered the exclusive club and earned the 1000 videos watched badge. Go to Next Lesson Take Quiz
You have earned a badge for watching 20 minutes of lessons.
You have earned a badge for watching 50 minutes of lessons.
You have earned a badge for watching 100 minutes of lessons.
You have earned a badge for watching 250 minutes of lessons.
You have earned a badge for watching 500 minutes of lessons.
You have earned a badge for watching 1000 minutes of lessons.