Human Reproduction: common doubts, answered
The questions students ask most often about Human Reproduction, each with a short answer. For the full chapter, read the Human Reproduction notes.
About the chapter
Who decides the sex of the baby, the father or the mother?
The father does, through the sperm. Every ovum carries an X chromosome, while a sperm carries either X or Y. If an X-bearing sperm fertilises the ovum the child is a girl (XX), and if a Y-bearing sperm does the child is a boy (XY). So the mother cannot be held responsible for the sex.
The male reproductive system
Read this section in the notes →Why are the testes outside the abdominal cavity?
The testes hang in the scrotum because sperm production needs a temperature about 2 to 2.5 °C lower than normal body temperature. The abdominal cavity is too warm for spermatogenesis. The scrotum therefore acts as a cooling arrangement, which is why a male can become less fertile if the testes are kept warm for long.
What is the difference between Sertoli cells and Leydig cells?
Sertoli cells lie inside the seminiferous tubules, nourish the developing germ cells and respond to FSH. Leydig cells lie in the spaces between the tubules, secrete androgens such as testosterone and respond to LH. A simple memory aid is that LH acts on Leydig cells, and FSH on the other type.
What do the seminal vesicles, prostate and bulbourethral glands add to semen?
Together these accessory glands make the seminal plasma, which is the fluid part of semen. It is rich in fructose, which gives sperm energy, and calcium and certain enzymes. The bulbourethral glands add a lubricating secretion for the penis. So sperm travel in a nourishing fluid rather than alone.
The female reproductive system
Read this section in the notes →What is the path of an ovum from the ovary to the uterus?
The ovum leaves the ovary and is caught by the funnel-shaped infundibulum, whose finger-like fimbriae help collect it. It then passes into the ampulla, then the narrow isthmus, which joins the uterus. Fertilisation occurs in the ampulla, so a fertilised egg reaches the uterus only after cleaving on the way.
What are the three layers of the uterine wall?
From outside to inside they are the perimetrium, myometrium and endometrium. The perimetrium is a thin protective membrane, the myometrium is thick smooth muscle that contracts strongly during childbirth, and the endometrium is the inner glandular lining that thickens each cycle and receives the embryo. It is the endometrium that is shed during menstruation.
Gametogenesis: spermatogenesis
Read this section in the notes →What is the difference between spermiogenesis and spermiation?
Spermiogenesis is the transformation of a round spermatid into a motile sperm with head, neck, middle piece and tail. Spermiation is the release of mature sperm from the Sertoli cells into the lumen of the seminiferous tubule. So one is a change of shape and the other is a release step that follows it.
How many sperms form from one primary spermatocyte?
Four sperms form. The primary spermatocyte divides by meiosis I into two secondary spermatocytes, and each of these divides by meiosis II into two spermatids, giving four in all. Each spermatid then becomes a sperm by spermiogenesis, with no further division. The primary spermatocyte is diploid and the later stages are haploid.
What hormones control spermatogenesis?
GnRH from the hypothalamus makes the anterior pituitary release LH and FSH. LH acts on Leydig cells to make androgens, which stimulate spermatogenesis. FSH acts on Sertoli cells, which secrete factors that help spermiogenesis. So the process depends on the hypothalamus, the pituitary and the testis acting together, beginning at puberty.
Gametogenesis: oogenesis
Read this section in the notes →Does a woman keep producing new oocytes throughout life?
No. Oogonia are formed only during foetal life, and no new ones are added after birth. At birth the ovary already holds primary oocytes, arrested in prophase I, and only a fraction of them mature over the reproductive years. This differs from spermatogenesis, where new sperms keep forming from puberty onward.
Is the cell released at ovulation an ovum or an oocyte?
It is a secondary oocyte, not a mature ovum. It has completed meiosis I and is arrested in metaphase of meiosis II. Meiosis II is completed only if a sperm enters, which releases the second polar body. So truly speaking the ovum is formed only at fertilisation.
What are the differences between primary, secondary and tertiary follicles?
The primary follicle is a primary oocyte surrounded by a single layer of granulosa cells. The secondary follicle has more layers of granulosa cells and a new outer covering, the theca. The tertiary follicle has a fluid-filled cavity, the antrum, and in it the primary oocyte completes meiosis I, giving a secondary oocyte. It then matures into the Graafian follicle, which ruptures at ovulation.
The menstrual cycle
Read this section in the notes →What happens on which phase of the menstrual cycle?
Menstruation lasts about the first 3 to 5 days. The follicular phase follows, with follicles growing and the endometrium regenerating under rising estrogen. Ovulation occurs around day 14 after an LH surge. In the luteal phase the corpus luteum secretes progesterone, which keeps the endometrium ready for implantation. If there is no pregnancy, the corpus luteum degenerates and menstruation starts again.
What triggers ovulation?
A sudden rise in LH, called the LH surge, around the middle of the cycle triggers ovulation. FSH also peaks then. The surge causes the Graafian follicle to rupture and release the secondary oocyte. After this the remains of the follicle turn into the corpus luteum, which secretes progesterone for the luteal phase.
Why does menstruation occur?
Menstruation occurs because, when the egg is not fertilised, the corpus luteum degenerates and progesterone falls. Without that hormone the thickened endometrium cannot be maintained, so it breaks down and is shed with blood through the vagina. This marks the start of a new cycle. During pregnancy there are no cycles and so no menstruation.
Fertilisation and implantation
Read this section in the notes →Where does fertilisation take place in humans?
Fertilisation takes place in the ampullary region of the fallopian tube, not in the uterus. The sperm and the secondary oocyte meet there, and the zygote starts cleaving as it moves towards the uterus. By the time of implantation it has become a blastocyst.
What are the morula and blastocyst, and which part forms the embryo?
The morula is a solid ball of 8 to 16 cells (blastomeres) formed by cleavage. The blastocyst is the later stage with an outer layer, the trophoblast, and an inner cell mass. The inner cell mass forms the embryo, while the trophoblast attaches to the endometrium and takes part in forming the placenta.
Pregnancy and embryonic development
Read this section in the notes →What is the function of the placenta?
The placenta is the structural and functional link between mother and foetus. It supplies oxygen and nutrients to the foetus and removes carbon dioxide and wastes. It also works as an endocrine organ, making hormones such as hCG, hPL, estrogens and progesterone. These keep the pregnancy going and prepare the mother for delivery.
Which hormones are made only during pregnancy?
hCG, hPL and relaxin are produced only in pregnancy. The placenta makes hCG and hPL, and relaxin is made by the ovary late in pregnancy. Estrogens and progesterone are also raised greatly, but they exist outside pregnancy too. Hence the earlier three are the ones asked as pregnancy-specific hormones.
Parturition and lactation
Read this section in the notes →What triggers childbirth?
Childbirth begins with signals from the fully developed foetus and the placenta that induce mild uterine contractions, called the foetal ejection reflex. This triggers the mother's pituitary to release oxytocin, which strengthens the contractions. The stronger contractions release even more oxytocin, a positive feedback loop that continues until the baby is delivered.
Why is colostrum important for a newborn?
Colostrum is the first milk produced after delivery, and it contains several antibodies. These give the newborn passive resistance against infection while its own immune system is still developing. For this reason breast-feeding during the initial period is strongly recommended. The antibodies are made by the mother, not the baby.
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