Acrosome: Reaction, Function & Definition

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  • 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
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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.

Extension and Exposure

The G-actin in the sperm's cytosol polymerizes into F-actin, creating an extended projection at the tip of the sperm's head. This pointy projection is called the acrosomal process and is the part of the sperm membrane that is going to fuse with the egg's plasma membrane.

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