Reproduction Index Glossary

Structure of the Gametes Before Fertilization

Fertilization presents some major challenges to both sperm and egg:

  • The fertilizing sperm must somehow recognize, bind to and ultimately traverse the zona pellucida surrounding the egg. It then must bind to the plasma membrane of the egg.
  • The egg must not only respond to the fertilizing sperm in a number of ways, but actively prevent more than one sperm from fertilizing it. Fertilization by more than one sperm is bad.

In their mature form, both sperm and egg possess structures that allow them to fulfill these mission objectives.

Structure of the Sperm

Mature sperm, know formally as spermatozoa, have a morphology that most people over the age of ten would recognize immediately. The nucleus is contained within the head, which, for most mammals, has a flattened, oval shape. During spermiogenesis, the haploid sperm cell develops a tail or flagellum, and all of its mitochondria become aligned in a helix around the first part of the tail, forming the midpiece. The entire cell is, of course, enveloped by a plasma membrane. The image to the right shows these structures at the light microscopic level with a bull sperm.

The other structure in the mature sperm that plays a critical role in fertilization is the acrosome. The acrosome is, in essence, a gigantic lysosome that forms around the anterior portion of the nucleus. It is bounded by a membrane that is considered to have two faces - the inner acrosomal membrane faces the nucleus, while the outer acrosomal membrane is in close contact with the plasma membrane.

The image to the right shows the front end of a stallion sperm, viewed with an electron microscope. The ruffled appearance of the plasma membrane is an artifact of fixation. The acrosome is the dark band of material between the plasma membrane and nucleus - inner and outer acrosomal membranes are not clearly visible at this magnification (image courtesy of Carol Moeller)

The function and fate of the acrosome is discussed in the next section on fertilization.

Structure of the Egg

Most mammals ovulate an "egg" that has matured into a secondary oocyte; it is always the secondary oocyte that is fertilized. The secondary oocyte is produced along with the first polar body as a result of the first meiotic division. Both of these cells are encased in a thick glycoprotein shell called the zona pellucida. The image to the right shows a secondary oocyte from a mouse; residual follicle cells have been stripped away.

Genetically, the secondary oocyte that arrives in the oviduct is in metaphase of the second meiotic division. The metaphase plate is located inside the oocyte immediately below the first polar body.

The final structural feature of the egg that serves a critical function during fertilization is a set of cortical granules. During oogenesis, the oocyte develops thousands of small membrane-bound granules that accumulate in the cortical cytoplasm, just beneath the plasma membrane.

The image to the right shows a mouse oocyte that has been stained to show cortical granules (all the small red dots). In this preparation, genomic DNA in the metaphase plate of the secondary oocyte (top right) appears bluish-white.

(From Biology of Reproduction 57:743-750, 1997 with the kind permission of the publisher and authors Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)

Cortical granules are the egg's homolog to the acrosome in sperm. Their function is described in the following section on fertilization.

Index of: Fertilization and Early Embryonic Development
Gamete Transport Fertilization

Last updated on April 9, 2000
Author: R. Bowen
Send comments via form or email to