The Reproductive System

The female reproductive system produces female gametes (eggs), provides a receptacle for male gametes (sperm), and provides structures for nourishing and protecting a baby. The male reproductive system produces sperm cells and provides a mechanism for delivering them to the female's body.

Primary oocyte: before birth (prophase 1) Secondary oocyte: ovulation ( metaphase 2) Egg cell: after fertilization (full meiosis) 1 egg cell (and 3 polar bodies) Primary spermatocyte: begins at puberty in the testis. Complete development in the epididymis. 4 sperm cells.

At the beginning, estrogen low. Low levels trigger the hypothalamus to secrete a hormone, causing the anterior pituitary to secrete FSH and LH. FSH causes a follicle to mature, low estrogen causes endometrium to break down. Maturing follicle secretes estrogen. The rise of estrogen signals the hypothalamus to slow secretion of hormone. So, the anterior pituitary secretes less FSH and LH. In the ovaries, the lower FSH and LH levels prevent additional follicles from developing. In the uterus, rising estrogen levels cause the endometrium to thicken.

Around day 12, the follicle reaches maturity, causing a rapid rise in levels of estrogen. This rapid rise in estrogen signals the hypothalamus to secrete high levels of releasing hormone. The anterior pituitary responds by secreting FSH and LH. In the ovaries, the high levels of LH cause the follicle to break open and release the egg into the oviduct in the process of ovulation.

After the follicle breaks open, the remaining follicle cells form a structure called the corpus luteum. It secretes estrogen and progesterone, which stimulate the endometrium to thicken further. corpus luteum If pregnancy does not occur, the corpus luteum begins to break down. It secretes less and less estrogen and progesterone. The lower levels of estrogen and progesterone signal the hypothalamus to secrete releasing hormone. This makes the anterior pituitary secrete FSH and LH. In the ovaries, high FSH levels signal a new follicle to mature. In the uterus, the falling estrogen and progesterone levels cause the endometrium to break down, initiating menstruation and starting the cycle all over again.

The same hormones exist in the male, but have different effects. Beginning at puberty, the hypothalamus starts secreting a releasing hormone which causes the anterior pituitary to secrete FSH and LH. In males, rising levels of LH signal the testes to produce and secrete testosterone. The combination of FSH and testosterone stimulates sperm production within the testes. Increases in testosterone levels signal the hypothalamus to decrease secretion of releasing hormone. Likewise, decreases in testosterone levels signal the hypothalamus to increase secretion of releasing hormone. In this way, the body regulates hormone levels and sperm production by a negative feedback loop.

Spermatazoa in testes are not yet capable of fertilization Epididymis- is actually over 20 feet long! spermatazoa complete maturation as they move through epididymis (about 2 weeks)  Ductus deferens (vas deferens)  Ejaculatory duct

The trophoblast contributes to the development of membranes that will nourish and protect the embryo. The inner cell mass will eventually form the organism itself.

Approximately three weeks after fertilization, a process called gastrulation takes place. Gastrulation forms three cell layers: ectoderm, endoderm, and mesoderm. The ectoderm (outer layer) forms the outer part of the embryo's skin and the central nervous system. The endoderm (inner layer) forms the digestive tract and lungs. The mesoderm (middle layer) forms most of the other organs.Gastrulation

During the first few weeks, membranes form that protect and nourish the embryo. One membrane, the amnion, forms a fluid-filled sac. The amniotic membrane protects the embryo from physical impact. The yolk sac membrane produces the first blood cells and is the source of cells that eventually form gametes. The chorion is a third membrane that becomes the embryo's portion of the placenta. Lastly, the allantois forms part of the umbilical cord that connects the embryo to the placenta.

This structure enables nutrients and waste products to be transferred between the mother and developing baby. By the end of the third month, the placenta is fully formed and functional. In the wall of the placenta, the mother's blood and baby's blood remain isolated in separate circulatory systems. However, the mother's blood vessels release pools of blood very close to the baby's blood vessels. Nutrients and waste products are able to diffuse back and forth through interstitial fluid from one blood system to the other Human placenta