Spermatogenesis Fertilization

3. Spermatogenesis, Oogenesis, fertilization and implantation.

Spermatogenesis, the origin and development of the sperm cells within the male reproductive organs, the testes. The testes are composed of numerous thin, tightly coiled tubules known as the seminiferous tubules; the sperm cells are produced within the walls of the tubules. Within the walls of the tubules, also,are many randomly scattered cells, called sertoli cells, that function to support and nourish the immature sperm cells by giving them nutrients and blood prodcts. As the young germ cells grow, the sertoli cells help to transport them from the outer of the seminiferous tubule to the central channel of the tubule. One immature germ cell takes as long as 74 days to reach final

These move from the outer portion of the seminiferous tubule to a more central location and attach themselves around the sertoli cells. The primary sperm cells then develop somewhat by increasing the amount of cytoplasm and organelles within the cytoplasm. After a resting phase the primary cells divide into a form called a secondary sperm cell. During this cell division there is a splitting of the nuclear material. In the nucleus of the primary sperm cells there are 46 chromosomes; in each of the secondary sperm cells there are only 23 chromosomes, asthere are in the egg. When the egg and sperm combine and their chromosome unite, the characteristics of both individuals blend and the new organism starts to grow. The secondary sperm cell still must mature before it can fertilize an egg; maturation entails certain changes in the shape and form of the sperm cell. The head is covered partially by a cap, called the acrosome, which is important in helping the sperm to gain entry into the egg. Attached to the opposite end of the head is the tailpiece. The tail is derived from the secondary sperm cells cytoplasm. In the mature sperm, it consists of a long, slender bundle

Fertilization of the egg usually occurs after the egg has left the ovary and is being transported through the fallopian tubes. Once fertilized, the egg begins to undergo a series of cell divisions. The egg takes up to seven days to reach the uterus; by this time the single-celled egg has divided numerous times, so that it is a ball of approximately 200 cells. The uterus has thick walls suitable for egg attachment and growth. A female hormone known as progesterone, secreted by the corpus luteum in the ovary, influences the redness of the uterine wall for egg implantation. It increases the blood supply in the wall, water content, and secretion of glycogen, a nutrient for the surrounding tissue and developing egg. If the uterus is not first prepared by progesterone, the egg will not attach itself. Progesterone also inhibits muscular contractions in the uterine wall that would tend to reject the adhering egg. When the egg reaches the uterus, it usually remains free in the uterine cavity for about a day. It then attaches to the uterine lining. Cells in the outer surface of the egg grow rapidly once contact is made with the uterine wall. The egg disrupts the surface of the endometrium and actively burrows into the deeper tissue. By