After penetrating the zona pellucida, the spermatozoon enters the perivitelline space between the zona pellucida and the oolemma (oocyte’s plasma membrane). Here, after docking to the oolemma, the spermatozoon plasma membrane fuses with the oolemma. This process, called the impregnation of the oocyte, allows the nucleus of the sperm (containing highly concentrated DNA), the centrosome, the midpiece with the mitochondria, and the kinocilium to be incorporated into the cytoplasm of the oocyte. The tail plasma membrane remains as an appendage to the oolemma.
An impregnating spermatozoon generates a molecular signal for resumption and termination of the second meiotic division. This division transforms the secondary oocyte into a mature oocyte and triggers the expulsion of the second polar body into the perivitelline space.
The male genetic material lying within the nucleus of incorporated sperm head is unpacked and used for building the male pronucleus, which contains 23 paternal chromosomes. Nuclear membranes of both female and male pronuclei dissolve (without fusion), and the chromosomes align themselves within the common mitotic spindle. The resulting zygote contains a diploid (2n) complement of 46 chromosomes and later undergoes the first mitotic division or cleavage. The male centrosome is essential for the alignment of the mitotic spindle that divides chromosomes into the first two cells of the embryo. Only centrosomes from the father are used in building first and subsequent mitotic spindles. The incorporated kinocilium is finally dissolved, and all sperm mitochondria are eliminated from the cytoplasm of the oocyte. Note that all mitochondria in human cells normally derive from the mother, but all centrosomes originate from the father’s sperm cell.
Several spermatozoa may penetrate the zona pellucida, but only one spermatozoon completes the fertilization process.
As the fertilizing spermatozoon penetrates the ooplasm, at least three types of postfusion reactions occur to prevent other spermatozoa from entering the secondary oocyte (polyspermy). These events include the following.
 Fast block to polyspermy. A large and long-lasting (up to 1 minute) depolarization of the oolemma creates a transient electrical block to polyspermy.
 Cortical reaction. Changes in the polarity of the oolemma then trigger release of Ca2 from the ooplasmic stores. The Ca2 propagates a cortical reaction wave in which cortical granules move to the surface and fuse with the oolemma, leading to a transient increase in surface area of the ovum and reorganization of the membrane. The contents of the cortical granules are released into the perivitelline space.
 Zona reaction. The released enzymes (proteases) of the cortical granules not only degrade the glycoprotein oocyte plasma membrane receptors for sperm binding but also form the perivitelline barrier by cross-linking proteins on the surface of the zona pellucida. This event creates the final and permanent block to polyspermy.
The corpus luteum of pregnancy is formed after fertilization and implantation.
If fertilization and implantation occur, the corpus luteum increases in size to form the corpus luteum of pregnancy. The existence and function of the corpus luteum depends on a combination of paracrine and endocrine secretions, collectively described as luteotropins.
Paracrine luteotropins are locally produced by the ovary. They include: