Understanding Other Reproductive Physiology - AP Biology

The endometrium is a mucus membrane in the uterus of the female reproductive tract. The endometrium changes thickness based on hormone levels and is shed during the menstrual cycle if fertilization does not occur.

Human pregnancy can be divided into three trimesters, each categorized by different symptoms and stages of fetal development. During the second trimester of pregnancy, which occurs between 13 and 28 weeks, fetal movement can be felt. This is often called “quickening,” which is the stirring of the fetus.

The third trimester of pregnancy takes place during weeks 29 and 40. During this stage of pregnancy, women continue to gain weight, the fetus moves regularly, and the fetal head descends. The descent of the fetal head is important in easing maternal breathing and positioning the fetus for future delivery.

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.

Pregnancy related medical conditions and complications are incredibly common due to strain on the mother and extreme biological changes associated with pregnancy. Examples include postpartum depression, anemia, preeclampsia, and ectopic pregnancy.

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.

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.

The secondary oocyte travels from the ovary down the fallopian tube after ovulation, while sperm cells are deposited into the female vagina by the penis. The sperm cells then make their way through the cervix and uterus into the fallopian tube, where one sperm cell fertilizes the oocyte.

After sperm cell and oocyte plasma membrane fusion, the sperm pronucleus moves into the oocyte cytoplasm. The sperm and egg pronuclei migrate towards the center of the cell while performing DNA replication, where both nuclear membranes dissolve. The chromosomes are prevented from dispersing by a mitotic spindle that tethers the maternal and paternal genetic material together. The cell then begins mitosis, which fuses the genetic material into a single diploid genome.

The fast block to polyspermy is the initial method that prevents polyspermy by the large number of sperm cells surrounding the egg. The fast block is triggered by sperm binding, which initiates a change in membrane potential. There is a large influx of sodium ions into the egg, creating a depolarization event that prevents further sperm penetration.

Menstrual disorders include a heavy flow (menorrhagia), irregular cycles (amenorrhea, polymenorrhea, metrorhagia, and oligomenorrhea), and no ovulation (anovulation).

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.

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.