Fig. 1.

Fertilization in mammals under natural conditions, in vitro, and by means of sperm microinjection. (A) Fertilization in vivo, for which a single sperm cell interacts with the oocyte and its coats, the cumulus oophorus and zona pellucida (ZP). (B) Fertilization in vitro. Many sperm cells interact with the oocyte. (C) Intracytoplasmic sperm injection. The cumulus-free oocyte is held by a pipette, and the sperm cell is aspirated and microinjected with a capillary micropipette. PB, oocyte's polar body; MII: oocyte's metaphase II; PN: pronucleus.