Fertilizationa process at the beginning of ourselves

•Sperm A human sperm. •Egg Overview The zona pellucida.

Structure of Sperm and Egg

Scanning electron micrograph of a human sperm contacting a hamster egg. For detail

Scanning electron micrograph of a clam egg with

sperm bound to its surface.

For detail

Block to polyspermy

• A depolarization of the egg plasma membrane a rapid, primary block to polyspermy

• Egg cortical reaction a slow, secondary block to polyspermy

primary block to polyspermy

• Na+ influx triggered by fusing with sperm make the membrane potential of egg shift from –60mV to +20mV within a few second.

• Low concentration of Na+ raise the incidence of polyspermy. Artificially depolarized an unfertilized egg prevent sperm from fusing with egg.

primary block to polyspermy

• It seems likely that the depolarization alter the conformation of a crucial protein in the egg plasma menbrane so that the sperm membrane can not fuse with the egg membrane.

• The egg membrane potencial returns to normal within a few minutes after fertilization.

• a second mechanism must provide a long-term barrier to polyspermy.

• In most egg, including those of mammals, this barrier is provided by Cortical Reaction.Back

Therefore

Second block to polyspermy

• When the sperm fuses with the egg plasma membrane, it activates the inositol phospholipid cell-signaling pathway in the egg. This, in turn, causes a local increase in cytosolic Ca2+, which spreads through the cell in a wave. The rise in Ca2+ activate the egg and initiate the cortical reaction.

Second block to polyspermy

• cortical granules located just under the plasma membrane of the egg release their contents by exocytosis. A,B

• The enzymes released by the cortical reaction change the structure of the zona pellucida, which becomes hardened so that sperm no longer bind to it, thereby providing a slow, secondary block to polyspermy. End

A mature Xenopus egg, ready for fertilization. For detail

The zona pellucida

The zona pellucida. For detail

A human sperm. It is shown in longitudinal section

Drawing of the midpiece of a mammalian sperm

Depolarization &

Cortical reaction

are the mechanics of

Block to Polyspermy

continue

Acrosome Reaction

• Sperm penetrate the zona pellucida by acrosome reaction.

• Process of acrosome reaction. Back

Process of Fertilization

• Acrosome reaction• Deplarization• Cortical Reaction• 2nd oocyte complete Meiosis Ⅱ• Pronucleus• Replication of DNA in pronucleus• First Mitosis