Understanding Other Reproductive Physiology - AP Biology

A woman is said to be “full term” when she is at 39 to 41 weeks of pregnancy. Medical intervention to induce labor during this time is not ideal.

When a sperm and egg fuse to form a zygote, the nucleus of the sperm enters the cytoplasm of the egg. As a result, the father's genome is passed onto the offspring, but no cellular organelles from the sperm are transferred. Any DNA contained in the mitochondria must come from the mother's egg, and could not have come from the cytoplasm of the sperm. The offspring will inherit all mitochondrial DNA from the mother.

This allows geneticists to trace mitochondrial lineages to find distant ancestors and track the evolution of species.

Following ovulation, the remaining follicle previously containing the egg is called the corpus luteum. This structure will release progesterone, and continue to do so if the egg is fertilized and a zygote is formed. If no pregnancy occurs, the corpus luteum will degrade into the corpus albicans.

The luteal surge is characterized by a sharp increase in estradiol (estrogen) levels, which then causes an increase in luteinizing hormone levels. This event causes ovulation to take place.

The release of the secondary oocyte from the follicle is called ovulation. During this process, a hole called the “stigma” is formed and it allows the secondary oocyte to leave the follicle surrounded by a layer of cells called the cumulus oophorus. After its release, the secondary oocyte enters the fallopian tube.

Leading up to ovulation, the developing follicle secretes estrogen. Over time, this secretion increases estrogen concentration. This high concentration of estrogen triggers the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. The spike in LH and FSH concentrations lead to the release of the secondary oocyte from the follicle. Progesterone, on the other hand, is only present at low concentrations at the time of ovulation. Progesterone levels rise after ovulation.

The slow increase in estrogen concentration leading up to ovulation triggers the secretion of luteinizing hormone (LH) from the anterior pituitary gland. The spike in LH initiates signal transduction pathways that release proteolytic enzymes. These enzymes create a hole, or stigma, in the follicle that allows the secondary oocyte to exit.

During the follicular phase of the estrous cycle, estrogen, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) are in a positive feedback loop. High concentrations of estrogen stimulate the anterior pituitary gland to secrete LH and FSH.

The increase in luteinizing hormone (LH) concentration during the follicular stage of the estrous cycle leads to the release of proteolytic enzymes from the follicle. These enzymes degrade the follicle tissue and create a hole called the “stigma.” The secondary oocyte exits the follicle from the stigma in a process called ovulation.

Semen is a fluid that contains sperm cells, proteolytic enzymes, lipids, and fructose. The prostate gland, bulbourethral gland, and seminal vesicles all contribute to the makeup of semen. Semen protects and transports sperm cells inside the female reproductive tract as they seek out the egg cell.

Semen is the fluid that contains and transports sperm cells to the female reproductive tract. The female vagina is acidic due to lactic acid output from normally occurring bacteria in the vagina. In order to protect sperm cells from the acidic environment, semen is alkaline.

The bulbourethral gland is a part of the male reproductive system that contributes protein-rich fluid to semen and lubricates the urethra.

Prior to ejaculation, secretions from the bulbourethral gland, prostate gland, epididymis, and seminal vesicles enter the ejaculatory duct. The ejaculatory duct is the place where semen mixes and is stored until ejaculation.

Seminal vesicles secrete a large portion of the components that make up semen. A high concentration of fructose is included in these secretions. The fructose provides energy rich molecules for sperm cell movement.

In males, the urethra carries semen from the ejaculatory duct out of the penis. The urethra also carries urine from the bladder through the penis.

During copulation and sexual intercourse, the penis enters the female vagina. Ejaculation—the release of semen from the penis—also occurs in the vagina.

After ejaculation into the female vagina, capacitation of sperm cells takes place. During this process, sperm undergo molecular and biochemical changes that allow them to fertilize the oocyte. In this final stage of maturation, sperm cells lose many membrane proteins (including glycoproteins and steroids to ease binding to the oocyte), undergo an influx of intracellular calcium, and change their tail movement pattern. The changes that occur during sperm activation put the sperm cells into a state of hyperactivity, or heightened motility that aids fertilization.

During capacitation, sperm cells undergo biochemical changes that allow for fertilization of the oocyte. One of these changes is an influx of intracellular calcium, which leads to an increase in intracellular cAMP levels. The high levels of calcium and cAMP promote hyperactivation—increased motility—through deeper tail bends. This change in sperm tail movement is accompanied by the adoption of a swinging movement by the head of the cell. These changes in motility patterns help sperm cell movement within the female reproductive tract.

Sperm cells bind to ZP3 glycoproteins in the zona pellucida—a layer surrounding the oocyte that is composed of glycoproteins. This binding triggers the acrosomal reaction in the sperm cell. Once inside the female reproductive tract, sperm cells undergo capacitation. The subsequent state of hyperactivity allows sperm cells to successfully move up the fallopian tubes towards the oocyte. Once a sperm cell encounters the cell layers surrounding the oocyte, the sperm binds and penetrates the layers of cells surrounding the oocyte until it reaches the zona pellucida.

The acrosomal reaction in sperm cells is another process that aids in fusing the sperm and oocyte. After binding to glycoproteins in the zona pellucida, the acrosome—cap of the sperm cell—fuses with the sperm plasma membrane. This fusion causes the release of acrosomal contents, which include enzymes that allow the sperm cell to move further towards the oocyte.