Sperm Cell Structure Fertilization Early Development

1. Sperm Cell StructureFertilizationEarly Development

5. Fertilization

6. Sperm Capacitation • Freshly ejaculated sperm are unable or poorly able to fertilize. • They must first undergo a series of changes known collectively as capacitation. • Capacitationis associated with removal of proteins, reorganization of plasma membrane lipids and proteins.

7. Sperm-ZonaPellucida Binding • Binding of sperm to the zonapellucida is a receptor-ligand interaction • The carbohydrate groups on the zonapellucidaglycoproteins function as sperm receptors. • The sperm molecule that binds this receptor is not known with certainty, and indeed, there may be several proteins that can serve this function.

8. The Acrosome Reaction • The sperm then faces the daunting task of penetrating the zonapellucida to get to the oocyte. • The acrosome - a huge modified lysosome that is packed with zona-digesting enzymes • The acrosome reaction provides the sperm with an enzymatic drill to get throught the zonapellucida. • Leakage of acrosomal enzymes from the sperm's head. • As the acrosome reaction progresses and the sperm passes through the zonapellucida, more and more of the plasma membrane and acrosomal contents are lost. • Some sperm that lose their acrosomes and are not effective

9. Penetration of the ZonaPellucida • The force from the sperm's flagellating tail+ acrosomal enzymes, allow the sperm to create a tract through the zonapellucida. • These two factors allow the sperm to traverse the zonapellucida. • Sperm motility is important to zona penetration, allowing the sperm to basically cut its way through the zona

10. Sperm-Oocyte Binding • Once a sperm penetrates the zonapellucida, it binds to and fuses with the plasma membrane of the oocyte. • The molecular nature of sperm-oocyte binding is not completely resolved. • A leading candidate in some species is a dimeric sperm glycoprotein called fertilin, which binds to a protein in the oocyte plasma membrane and may also induce fusion.

11. The Zona Reaction • The zona reaction refers to an alteration in the structure of the zonapellucida catalyzed by proteases from cortical granules.  • blocks polyspermy in most mammals. • The zonapellucida hardens. • Runner-up sperm that have not finished traversing the zonapellucida by the time the hardening occurs are stopped in their tracks. • Sperm receptors in the zonapellucida are destroyed. • Therefore, any sperm that have not yet bound to the zonapellucida will no longer be able to bind, let alone fertilize the egg.

12. Cleavage and Blastocyst Formation

14. The one cell embryo undergoes a series of cleavage divisions, progressing through 2-cell, 4-cell, 8-cell and 16 cell stages. A four cell embryo is shown here. The cells in cleavage stage embryos are known as blastomeres.

15. Early on, cleavage divisions occur quite synchronously. In other words, both blastomeres in a two-cell undergo mitosis and cytokinesis almost simultaneously.

16. Soon after development of the 8-cell or 16-cell embryo the formation becomes a mass of cells called a morula. • this embryo shown here probably has between 20 and 30 cells. • It is difficult to count the cells in a morula;

17. Formation of an accumulation of fluid inside the embryo, signals formation of the blastocyst.

18. Implantation