18 hours after the sperm has entered the egg two spherical objects can be seen within the egg. These are called the pronuclei. One contains the genetic material from the egg, the other from the sperm. The female pronucleus contains 22 chromosome + the X chromosome and the male pronucleus 22 chromosome plus either an X or Y chromosome (but not both). Soon after this stage the pronuclei will fuse, become invisible & cell division (cleavage) begins. The polar bodies can be seen beneath the zona pellucida at about 4 O'clock.
Left: Photomicrograph of a
pronucleate embryo taken by Dr. Phil Matson using interference microscopy -
there is a three dimensional effect which reverses if the image is rotated
through 180 degrees.Right:
Rotation through 180 degrees
changes the optical illusion of three dimensions - sperm now appear 'raised'
and pronuclei 'sunken'. Note that sperm can still be seen attached to the
outside of the zona pellucida. Usually only one sperm penetrates the zona
pellucida and eventually enters the cytoplasm of the egg - the zona pellucida
prevents further sperm penetration - occasionally however two sperm may
fertilize the egg - then three pronuclei will be visible. Such tripronucelate
(or multi pronucleate embryos cannot develop in a normal embryo.