Human Chorionic Gonadotropin (hCG) is secreted by the syncytiotrophoblast cells of the placenta during early pregnancy. It maintains the corpus luteum, which in turn continues to secrete progesterone to sustain the pregnancy until the placenta takes over progesterone production.

After ovulation, the theca interna cells, along with granulosa cells, form the corpus luteum. The theca interna cells specifically contribute to the formation of theca lutein cells, which secrete progesterone to support early pregnancy.

The endometrium, the inner lining of the uterus, undergoes cyclic changes during the menstrual cycle to prepare for implantation. It becomes thick and vascularized under the influence of progesterone during the secretory phase, providing a suitable environment for the blastocyst to implant.

Relaxin, secreted by the corpus luteum and later by the placenta, helps in softening the cervix and relaxing the pelvic ligaments during the later stages of pregnancy. This facilitates easier passage of the baby through the birth canal during parturition.

During oogenesis, the first polar body is formed as a result of the unequal division of the primary oocyte during meiosis I, which occurs before ovulation. The first polar body is a small, non-functional cell that eventually degenerates.

During parturition, the pressure of the fetal head against the cervix triggers a neuroendocrine reflex, leading to the release of oxytocin from the posterior pituitary gland. Oxytocin stimulates strong uterine contractions to facilitate delivery.

The morula is an early stage of embryonic development characterized by a solid ball of 16-32 cells formed by rapid cleavage divisions of the zygote. It lacks a cavity and precedes the blastocyst stage, where a fluid-filled cavity forms.

The acrosome, located at the tip of the sperm head, contains hydrolytic enzymes such as hyaluronidase and acrosin. These enzymes help in breaking down the zona pellucida, the protective layer surrounding the ovum, allowing the sperm to penetrate and fertilize it.

The corpus luteum degenerates into corpus albicans if fertilization does not occur due to the absence of human Chorionic Gonadotropin (hCG). hCG, secreted by the developing embryo, is necessary to maintain the corpus luteum; without it, progesterone levels drop, leading to menstruation.

In human females, the second meiotic division of the secondary oocyte is completed only after fertilization. When the sperm penetrates the ovum, it triggers the completion of meiosis II, resulting in the formation of a mature ovum and the second polar body.