Usually, only a few hundred of the millions of spermatozoa in an ejaculate reach the site of fertilization, typically the ampulla of the uterine tube. On arrival, spermatozoa are confronted with the secondary oocyte surrounded by the corona radiata. Spermatozoa must penetrate the corona radiata to gain access to the zona pellucida. Although several spermatozoa may penetrate the zona pellucida, only one spermatozoon completes the fertilization process. Capacitation is complete when spermatozoa are able to bind to the zona pellucida receptors. Binding to the ZP-3 receptors on the zona pellucida triggers the acrosome reaction in which enzymes (mainly hyaluronidases) released from the acrosome enable a single spermatozoon to penetrate the zona pellucida. Penetration is accomplished by limited proteolysis of the zona pellucida in front of the advancing hypermotile spermatozoon.
After penetrating the zona pellucida, the spermatozoon enters the perivitelline space between the zona pellucida and the oocyte plasma membrane (or oolemma). Here the spermatozoon plasma membrane fuses with the oolemma, and the nucleus of the sperm head is finally incorporated into the oocyte. It forms the male pronucleus containing 23 paternal chromosomes. After the alignment and dissolution of nuclear membranes of the two pronuclei, the resulting zygote, with its diploid (2n) complement of 46 chromosomes, undergoes a mitotic division or first cleavage. This two-cell stage marks the beginning of embryonic development.
Before spermatozoa can fertilize the secondary oocyte, they must acquire more thrust to penetrate the corona radiata and zona pellucida.
When approaching a secondary oocyte, a spermatozoon becomes hyperactivated: It swims faster, and its tail movements become more forceful and erratic. Recent studies indicate that this hyperactivation reaction is caused by a sudden influx of Ca2 into the tails of spermatozoa. The plasma membrane of the sperm tail contains large numbers of transmembrane Ca2
FIGURE 23.12 • Photomicrograph of the corpus albicans of a human ovary. Large amounts of hyaline material can be seen among the degenerating cells of the former corpus luteum. The corpus albicans is surrounded by ovarian stroma. 125.
FIGURE 23.11 • Electron micrograph of theca lutein cells from the corpus luteum of a monkey. At this early implantation stage (day 10.5 of gestation), membrane-bounded dense bodies are clustered near the Golgi apparatus (G); most of the cytoplasm is packed with tubules of smooth endoplasmic reticulum (sER), lipid droplets (L), and mitochondria (M). Note the capillary (Cap) and the closely apposed cell membranes of the theca lutein cells (arrows). 10,000. (Courtesy of Dr. Carolynn B. Booher.) channel proteins called CatSpers (cation channels of sperm). CatSper proteins are expressed exclusively in membranes of the tail. Influx of Ca2 causes the tail to be more active and bend more forcefully, resulting in faster sperm movement through the viscous environment of the uterine tube. Together with limited proteolysis of the zona pellucida, hyperactivation is responsible for the physical penetration of the oocyte.
Sperm hyperactivity is necessary for breaking physical barriers that protect the secondary oocyte from fertilization. Thus, activation of CatSpers is required for male fertility.
Impregnation of the oocyte allows structures lying inside the spermatozoon to enter the cytoplasm of the oocyte.