IB Biology/Human Reproduction
Topic 9: Human Reproduction
Production of Gametes
Draw the structure of testis tissue as seen using a light microscope.
Outline the processes involved in spermatogenesis including mitosis, cell growth, the two divisions of meiosis, and cell differentiation.
- The outer layer of cells, called spermatogonia (2n) divides by mitosis, producing more spermatogonia (also 2n).
- Spermatogonia grow into larger cells called primary spermatocytes (2n).
- Primary spermatocytes (2n) carry out the first stage of meiosis, producing 2 secondary spermatocytes (n).
- Each secondary spermatocyte carries out the second division of meiosis, producing four spermatids (n).
- Spermatids become associated with nurse cells called sertoli cells, which help the spermatids develop into spermatozoa (n).
- Mature sperm (the spermatozoa) detach from sertoli cells and eventually are carried out of the testes by fluid in the center of the seminiferous tubule.
Outline the origin and the role of the follicle stimulating hormone (FSH), testosterone, and lutenizing hormone (LH) in spermatogenesis.
- Follicle Stimulating Hormone: Released by the pituitary gland; stimulates primary spermatocytes to undergo the first division of meiosis.
- Testosterone: Released by testes; stimulates development of secondary spermatocytes into mature sperm.
- Lutenizing Hormone: Released from the pituitary gland; stimulates secretion of testosterone by testes.
Draw the structure of the ovary as seen using a light microscope.
Outline the processes involved in oogenesis including mitosis, cell growth, the two divisions of meiosis, the unequal division of cytoplasm, and the degeneration of polar bodies.
- In the ovaries of a female fetus, cells called oogonia (2n) divide by mitosis to make more oogonia cells.
- Oogonia grow into larger cells called primary oocytes (2n).
- Primary oocytes start meiosis but stop at prophase I. Primary oocytes and a single layer of follicle cells form a primary follicle.
- When a baby girl is born, the ovaries contain 400,000 primary follicles.
- Every menstrual cycle, a few primary follicles start to develop -- During this time, the Primary oocyte completes meiosis I, forming two haploid (n) daughter nuclei - cytoplasm of the primary oocyte develops unequally, forming a secondary oocyte (n) and a small polar body (n).
- Then, the Secondary oocyte starts second division of meiosis but stops at prophase II. The follicle cells, meanwhile, are reproducing quickly and follicle fluid is forming.
- When the mature follicle bursts at the time of ovulation, the released egg is still a secondary oocyte (n).
- After fertilization, the secondary oocyte completes the second division of meiosis to form an ovum with the sperm nucleus already inside.
- Polar bodies do not develop into eggs; they degenerate.
Draw the structure of a mature sperm and egg.
Outline the role of the epididymis, seminal vesicle, and prostate gland in the production of sperm.
- Epididymus - where mature sperm are stored, waiting to be released into the vas deferens to become part of ejaculatory fluid.
- Seminal vesicle - releases a sugary solution to feed sperm nutrients during their journey.
- Prostate gland - releases an alkaline (very basic/high pH) solution that constitutes seminal fluid; alkaline solution helps provide the proper environment for the sperm while they're in the acidic environment of the female reproductive system.
Compare the processes of spermatogenesis and oogenesis, including the number of gametes and the timing of the formation and release of gametes.
- Number of gametes - Spermatogenesis: 4; Oogenesis: 1.
- Timing of formation - Spermatogenesis: constantly, beginning at puberty; Oogenesis: up to prophase I by time of birth, up to prophase II every menstrual cycle, completion only at time of fertilization.
- Release of gametes - Spermatogenesis: variable starting at puberty; Oogenesis: monthly starting at puberty.
Fertilization and Pregnancy
Describe the process of fertilization including the acrosome reaction, penetration of the egg membrane by a sperm, and the cortical reaction.
- Acrosome reaction - When a sperm comes in contact with the protein coats surrounding the egg, the acrosome (an enzyme-filled vacuole at the tip of the sperm cell) bursts open and releases enzymes that digest the jelly-like coating of the egg.
- Penetration of egg membrane - When the sperm membrane comes in contact with the egg membrane, the two membranes fuse, and the sperm nucleus enters the cytoplasm of the egg. This, as implied, happens after the acrosome reaction which digests the jelly-like coating of the egg to allow the sperm to penetrate the egg membrane.
- Cortical reaction - Once the membranes have fused, a chemical reaction occurs in the egg membrane, making it impossible for other sperm cells to enter the egg. This prevents other sperm from attempting to fertilize the egg.
Outline the role of human chorionic gonadotrophin (HCG) in early pregnancy.
- HCG is an embryonic hormone that maintains secretion of progesterone and estrogen by the uterus lining through the first trimester. The level of HCG in maternal blood is so high that it is present in the urine, which allows it to be detected for pregnancy tests.
Describe the structure and function of the placenta, including its hormonal role in the maintenance of pregnancy (secretion of estrogen and progesterone).
- During the first 2-4 weeks of development, the embryo obtains nutrients directly from the endometrium (the uterus lining).
- Tissues grow out of the developing embryo and mingle with the endometrium to form the placenta.
- Diffusion of material between the maternal and embryonic circulatory systems via the placenta provides nutrients, exchanges respiratory gases, and disposes of metabolic wastes from the embryo.
- Blood from the embryo travels to the placenta through the arteries of the umbilical cord and returns though the umbilical vein.
- The embryo secretes hormones that signal its presence and controls the mother's reproductive system.
- HCG acts like lutenizing hormone to maintain secretion of progesterone and estrogen.