Exercise practice 2: Human reproduction

6.1 E12 Oogenesis

Question. What is oogenesis? Give a brief account.

Answer. Oogenesis is the formation of the mature female gamete. It begins before birth, when oogonia form in the fetal ovary and enter prophase I of meiosis, stopping as primary oocytes. Each primary oocyte becomes surrounded by granulosa cells and forms a primary follicle. At puberty about 60,000 to 80,000 primary follicles remain in each ovary. These become secondary and then tertiary follicles, and in the tertiary follicle the primary oocyte completes its first meiotic division unequally, giving one large secondary oocyte and one small first polar body. Once the follicle has matured into a Graafian follicle, the secondary oocyte is set free at ovulation. The second meiotic division waits for the sperm to enter.

6.2 E13 An ovary in section

Question. Draw and label a diagram showing a section of the ovary.

Answer. The diagram should show the outer epithelium, the stroma with its cortex and medulla, and follicles at different stages. Label a primary follicle with one layer of granulosa cells and a primary oocyte, a secondary follicle with more granulosa layers and a theca, a tertiary follicle with the antrum, and a mature Graafian follicle close to the surface. Also label the corpus luteum in an ovary that has just ovulated.

6.3 E14 The Graafian follicle

Question. Draw and label a mature Graafian follicle.

Answer. Show a large follicle with a fluid-filled antrum. The secondary oocyte, surrounded by the zona pellucida and a cluster of granulosa cells, lies at one side. The wall has the theca interna and theca externa and the granulosa layer. Label the antrum, the oocyte and the theca layers, and note that this follicle ruptures at ovulation.

6.4 E15 Five structures and their jobs

Question. Give the job of each structure named here: the corpus luteum, the endometrium, the acrosome, the sperm tail and the fimbriae.

Answer. (a) The corpus luteum forms from the remains of the ruptured Graafian follicle and secretes progesterone, which maintains the endometrium for implantation. (b) The endometrium is the inner glandular lining of the uterus; it thickens each cycle and receives the blastocyst at implantation. (c) The acrosome is the enzyme-filled cap of the sperm head, and its enzymes digest a path through the zona pellucida and the egg membrane. (d) The sperm tail beats to move the sperm towards the ovum. (e) The fimbriae are finger-like projections at the infundibulum of the oviduct, and they sweep the ovum into the tube after ovulation.

6.5 E16 True or false

Question. Identify each statement as true or false and correct the false ones: (a) androgens are produced by Sertoli cells, (b) spermatozoa get nutrition from Sertoli cells, (c) Leydig cells are found in the ovary, (d) Leydig cells synthesise androgens, (e) oogenesis takes place in the corpus luteum, (f) the menstrual cycle stops during pregnancy, (g) whether the hymen is present tells nothing reliable about virginity.

Answer. (a) False. Androgens are produced by Leydig cells, which lie in the interstitial spaces of the testis. (b) True. (c) False. Leydig cells are found in the testis. (d) True. (e) False. Oogenesis takes place in the ovary, in the follicles, and not in the corpus luteum. (f) True. (g) True.

6.6 E17 The menstrual cycle and its hormones

Question. What is the menstrual cycle? Which hormones regulate it?

Answer. The menstrual cycle is the monthly cycle of changes in the ovary and uterus of female primates, beginning at menarche and ending at menopause, with one ovum released in the middle. The pituitary gonadotropins FSH and LH regulate follicular growth and ovulation, with the LH surge causing ovulation. The ovarian hormones oestrogen, from the growing follicle, and progesterone, from the corpus luteum, regulate the endometrium and feed back on the pituitary.

6.7 E18 Parturition

Question. Define parturition. Which hormones bring it about?

Answer. Parturition is the delivery of the baby at the end of pregnancy. The fully developed foetus and the placenta start the process by triggering mild uterine contractions called the foetal ejection reflex. This makes the maternal pituitary release oxytocin, which strengthens the contractions, and the stronger contractions release more oxytocin. Progesterone levels fall while oestrogen and oxytocin rise, and the loop of oxytocin and contraction continues until the baby is delivered.

6.8 E19 The sex of the child

Question. People sometimes blame the mother when a daughter is born. Explain why that blame is misplaced.

Answer. The mother contributes only an X chromosome, because every ovum carries X. The father's sperm may carry X or Y, in roughly equal numbers. A daughter forms when an X-bearing sperm fertilises the ovum, and a son forms when a Y-bearing sperm does. The sex of the child is therefore decided by the sperm and by the father, and blaming the mother has no biological basis.

6.9 E20 Eggs and twins

Question. How many ova does a human ovary normally release in one month? What number would have been released in the case of identical twins? Does the answer change for fraternal twins?

Answer. A human ovary releases one egg in a normal monthly cycle. Identical twins develop from a single fertilised egg that splits into two embryos, so only one egg was released. Fraternal twins develop from two separate eggs fertilised by two sperm, so two eggs were released.

6.10 E21 A dog with six puppies

Question. A female dog gave birth to six puppies. How many ova must her ovary have released?

Answer. At least six eggs. Each puppy develops from a separate zygote, and each zygote needs its own egg, so the ovary must have released at least six eggs. A dog normally releases several eggs in one oestrus cycle, and some of those eggs may not have been fertilised, so the true number could be more than six.