Following their maturation in the epididymis, spermatozoa must be activated within the female reproductive tract. During this activation process, called capacitation, structural and functional changes take place in the spermatozoon that result in its increased affinity to bind to zona pellucida receptors. Successful capacitation is confirmed by hyperactivation of the spermatozoa, which manifests as a vigorous, whiplash-like beating pattern of their flagella.
Capacitation involves several biochemical changes and modifications to the spermatozoon and its plasma membrane. They include:  Increased activity of adenylyl cyclase leading to increasing levels of cAMP  Increased rate of tyrosine phosphorylation (measurement of tyrosine phosphorylation is used clinically as a biochemical marker of capacitation)  Activation of Ca2 channels resulting in increased intracellular Ca2 levels  Release of seminal fluid glycoconjugates from the surface of the head of the spermatozoon. These surface glycosides (also called decapacitation factors) added during sperm maturation in the epididymis inhibit binding to the zona pellucida receptors  Extensive modification of the plasma membrane by removal of cholesterol, the predominant inhibitor of capacitation, and redistribution of phospholipids and carbohydrate moieties
FIGURE 23.9 • Diagram illustrating changes that occur during growth, maturation, and fertilization of the oocyte. The primary oocyte remains arrested in prophase I of meiosis. The first meiotic or reductional division is completed only after the oocyte progresses to ovulation. The second meiotic or equatorial division is not completed unless the secondary oocyte is impregnated by a spermatozoon. Note the formation of first and second polar bodies. In some mammals, the first polar body divides (as shown on this drawing) so that there are four total meiotic products. However, in humans, the first polar body does not divide but persists for about 20 hours; therefore, the fertilized egg can be recognized by the presence of two polar bodies.
migration of nucleus to cell surface hyperactivation of spermatozoa impregnation of oocyte and resumption of second meiotic division alignment of male and female pronuclei
FIGURE 23.10 • Photomicrograph of human corpus luteum. a. The corpus luteum is formed from the collapsed follicle wall that contains the granulosa and theca cells. The granulosa lutein cells form a thick, folded layer around the former follicular cavity (Cav). Within the folds are cells of the theca interna (arrows). 12. b. This photomicrograph shows the wall of the corpus luteum at higher magnification. The main cell mass is composed of granulosa lutein cells (GLC). These cells have a large spherical nucleus and a large amount of cytoplasm. The theca lutein cells (TLC) also have a spherical nucleus, but the cells are considerably smaller than the granulosa lutein cells. 240.
Fertilization normally occurs in the ampulla of the uterine tube.