Sexual Reproduction

Oogenesis and Spermatogenesis

The Process of Oogenesis

Hormones Involved

Follicle Stimulation Hormone

Referred to as FSH, this hormone stimulates the primary follicle development. With more follicular cells comes more estrogen production. FSH also produces follicular fluid and triggers the development of the oocyte in the follicle.

Progesterone

This hormone maintains the endometrium lining along the walls of the uterus. Progesterone provides negative feedback of FSH and another hormone, Luteinising Hormone (LH). This means as progesterone levels rise, FSH and LH levels drop.

Stages

This process begins with primary follicles in the ovary. These are in prophase I of meiosis I. They develop into secondary follicles through growth of the follicle and then it becomes a mature follicle, also known as the oogonium. The diploid oogonium goes through mitosis, producing four oogonia. These are called primary oocytes. Through the process of meiosis I, the primary oocyte is divided into the secondary oocyte and the polar body, which happens because division of the cytoplasm is not even, so this cell is stored.

The Process of Spermatogenesis

Steps

The production of sperm happens in the seminiferous tubule inside the testes. The process begins with a spermatogonium, which goes through mitosis to produce diploid spermatogonia. These grow into diploid primary spermatocytes, and then meiosis I happens to produce two haploid secondary spermatocytes. These each go through meiosis II resulting in two spermatids. Cell differentiation occurs, making each spermatozoa carry a different set of DNA.

Differences Between Oogenesis & Spermatogenesis

Although both oogenesis and spermatogenesis begin with mitosis and involve meiosis, the two processes vary from each other. While oogenesis produces only one gamete per meiosis, spermatogenesis produces 4 cells. There is only 1 cell produced per cycle by oogenesis, however, spermatogenesis produces millions per day. In males, spermatogenesis begins at puberty, but for females, their process begins as their own life begins. The fertility lasts lifelong in spermatogenesis although it reduces, but in females, fertility is limited until menopause.

Fetilisation

The process of fertilization begins when millions of sperm cells are ejaculated into the vagina and make their way through the cervix and uterus towards the oviducts. Next, the sperm must penetrate outer layers of follicle cells around the secondary oocyte, which is produced during the process of oogenesis in the ovaries of of the female. The sperm continues to pass through the follicle cells of the secondary oocyte and eventually reach the zona pellucida, the layer under the follicular cells, and binds to it. After binding, an acrosome reaction takes place where the enzymes in the acrosome ar released and proteases digest an pathway through the zona pellucida in the oocyte. The acrosome, the cap-like structure on the anterior head of the sperm, fuses with the membrane of the secondary oocyte, which triggers a cortical reaction, allowing the sperm nucleus to enter the cytoplasm of the secondary oocyte. Then, the cortical granules in the cytoplasm of the oocyte fuse with the plasma membrane and releases its contents. Enzymes then cause the zona pellucida, the second innermost layer of the secondary oocyte, to harden, preventing any other sperms from attaching to the oocyte. This completes the cycle of meiosis II for the secondary oocyte as well as expelling the secondary bipolar body. Finally, the sperm and ovum nuclei merge forming a fertilized ovum, also known as a zygote.


Pregnancy

24 hours after the zygote has been formed, the process of mitosis begins, where the zygote starts to divide. After 5 days of division has passed, the zygote divides into 100 balls of cells also known as blastocysts. The blastocyst has an outer layer of cells, which form part of the placenta, that also encloses an inner mass of cells, which develop to become the body of the embryo, as well as a fluid-filled space. As the division continues, the blastocysts moves through the oviducts towards the uterus, where after about 7 days, the blastocysts reach the uterus and attach to endometrium lining and continues to divide and eventually develop into an embryo. Once the blastocyst attaches to the endometrium, it starts to secrete Human Chorionic Gonadotrophin (HCG). HCG is a hormone that helps to nourish the egg after it fertilizes on the endometrium lining. Another fun fact about HCG is that it is excreted through the urine of a pregnant woman, which is the hormone that is detected during a pregnancy test. After being secreted, HCG travels through the bloodstream towards the ovaries, where it helps to maintain the corpus luteum. The corpus luteum is the mass of cells that developed from the empty follicle during oogenesis. It is important the the corpus luteum is maintained is because it produces the hormones progesterone and estrogen, which helps to maintain the endometrium during the menstrual cycle in non-pregnant women. The endometrium lining needs to stay in place during the pregnancy, so the HCG keeps the corpus luteum growing and producing estrogen and progesterone for the first three months of pregnancy. Later, when the placenta is formed and can produce the estrogen and progesterone, the corpus luteum degenerates. The embryo continues to grow and develop and at about after two months it is classified as a fetus. At this point, the uterus is providing nourishment to the early embryo as it is developing, but after about 12 weeks, the fetal membranes are functioning. In order to receive this nourishment, the fetus is attached to the placenta through the umbilical cord. This is surrounded by a fluid sac called amnion, which is filled with amniotic fluid. The amniotic fluid supports the fetus during its development and acts as a shock absorber from bumps. It also helps the fetus to move and develop its muscles.


Placenta

Overall, the placenta allows for the exchange of materials between the mother and fetus and acting as an endocrine gland, producing estrogen and progesterone, which maintains the pregnancy. The placenta is a disc-shaped structure and is made up of maternal endometrium and villi from the outer layers of the chorion, which surround the embryo. These chorionic villi grow out of the endometrium, and contain capillaries, increase the surface area of the embryo so that it can receive as many materials from the placenta as possible. The capillaries contained in the chorionic villi contain the fetus’s blood. This blood penetrates the endometrium until surrounded by the maternal blood. Now the mother’s blood is close to the fetus blood in order for diffusion to occur without the two mixing together. Overall, the placenta allows for the exchange of materials between the mother and fetus and acting as an endocrine gland, producing estrogen and progesterone, which maintains the pregnancy. The placenta is a disc-shaped structure and is made up of maternal endometrium and villi from the outer layers of the chorion, which surround the embryo. These chorionic villi grow out of the endometrium, and contain capillaries, increase the surface area of the embryo so that it can receive as many materials from the placenta as possible. The capillaries contained in the chorionic villi contain the fetus’s blood. This blood penetrates the endometrium until surrounded by the maternal blood. Now the mother’s blood is close to the fetus blood in order for diffusion to occur without the two mixing together.

Exchange of Materials

The fetal blood is carried through two umbilical arteries, which divide into capillaries in the villi, to the placenta. Nutrients diffuse into the capillaries and travel to the fetus through the umbilical cord. The fetus’s waste products and carbon dioxide are carried through two umbilical arteries and diffuse into the mother’s blood.


Materials Exchanged from Mother to Fetus:

- oxygen

- glucose, amino acids, vitamins and minerals

- water

- hormones

- alcohol / drugs

- viruses


Materials Exchanged from Fetus to Mother:

- carbon dioxide

- urea

- water

- hormones (HCG)

Hormone Levels & Birth

First 12 weeks, the corpus luteum produces estrogen and progesterone, but then transitions to the placenta producing the estrogen and progesterone. Towards the end, progesterone levels drop, indicating an onset of uterine contractions known as labor. During this time, the pituitary gland secretes the hormone oxytocin, which stimulates uterine muscles to contract and is controlled by positive feedback. Small contractions of the uterus muscle stimulates the release of oxytocin, which stimulates stronger contractions. While the uterus is contracting, the cervix widens, the amniotic sac breaks, releasing the amniotic fluid. Contractions continue as the baby is pushed through the cervix and out towards the vagina. Gentle contractions occur until the placenta, known as the afterbirth, is expelled from the uterus. Once the birth is complete blood levels of the hormone prolactin from the pituitary gland increase. Prolactin is an important hormone that helps to regulate the immune system. In this case, the prolactin hormone stimulates milk production by the mammary glands. Prolactin secretion is maintained while oxytocin is released from the pituitary gland. The oxytocin causes the milk to released from the milk ducts.