Human Reproduction: One Mark Questions, Book Back Answers and Notes

About Human Reproduction

Hard ~150 min study

This chapter follows human reproduction step by step: first the organs, then the gametes they make, then fertilization, pregnancy and birth. If you learn it in that order, each topic sets up the next one.

Hormones run through the whole chapter. FSH and LH control the making of gametes, oestrogen and progesterone drive the changes of the menstrual cycle, hCG keeps the corpus luteum working in early pregnancy, oxytocin brings on labour and prolactin starts milk production. When you revise, keep asking which gland releases each hormone and what it acts on.

Questions from this chapter usually test labelled parts of the reproductive organs and sperm, the stages of spermatogenesis and oogenesis, the phases of the menstrual cycle and the order of events from fertilization to implantation. Many MCQ errors come from mixing up similar-sounding terms, so the common mistakes list below is worth reading carefully.

What you'll learn

• Label and describe the parts of the male and female reproductive systems.
• Compare the stages of spermatogenesis and oogenesis.
• Explain how pituitary and ovarian hormones control the menstrual cycle.
• Describe the events from fertilization through cleavage to implantation of the blastocyst.
• Explain the functions of the placenta and the foetal membranes during pregnancy.
• State the hormones that control parturition and lactation and what each one does.

Before you start

• Mitosis and meiosis.
• The main endocrine glands and the hormones they secrete.
• Sexual and asexual reproduction from Unit 1.

Human Reproduction explained

Human Reproduction: From Gametes to Birth

Male and Female Reproductive Systems

The male reproductive system produces, stores and delivers sperm. The testes lie outside the abdomen in the scrotum, which keeps them slightly cooler than the rest of the body, as sperm production needs. Accessory ducts carry the sperm, and accessory glands add the fluid that makes up semen. The female system has two jobs. The ovaries produce ova, and the uterus, with its thick muscular wall, holds and nourishes the developing foetus. Hormones from the hypothalamus, pituitary and gonads coordinate both systems.

Gametogenesis

Gametogenesis is the formation of haploid gametes by meiosis and differentiation. In males, spermatogenesis goes on continuously in the seminiferous tubules from puberty onwards and produces large numbers of small, motile sperm. In females, oogenesis begins before birth, stops partway through meiosis I, and resumes only after puberty. Normally one ovum, rich in stored food, is released in each cycle. Meiosis halves the chromosome number, so that fertilization restores the diploid number.

The Menstrual Cycle and Its Hormones

The menstrual cycle is the monthly series of changes in the ovary and the endometrium. FSH and LH from the anterior pituitary cause a follicle to mature, and a surge of LH triggers ovulation. Oestrogen and progesterone from the ovary build up the endometrium so it is ready for an embryo. If no fertilization takes place, the corpus luteum breaks down, hormone levels fall and the endometrium is shed as menstrual flow. The first day of bleeding is counted as day one of the next cycle.

Fertilization and Implantation

Fertilization happens in the fallopian tube, when a haploid sperm passes through the layers around the ovum and fuses with it to form a diploid zygote. The zygote divides repeatedly by mitosis, a process called cleavage, and becomes a morula and then a blastocyst as it moves down the tube. In the uterus, the blastocyst attaches to and sinks into the endometrium; this is implantation, and it marks the start of pregnancy.

Pregnancy and Embryonic Development

After implantation, the chorionic villi of the embryo and the uterine tissue together form the placenta. It exchanges nutrients, gases and wastes between mother and foetus and also acts as a temporary endocrine gland. The pregnancy is divided into three trimesters. The three primary germ layers give rise to all tissues and organs during organogenesis. The placenta secretes hCG, hPL, oestrogen and progesterone to keep the pregnancy going, and the fluid-filled amnion protects the foetus from shocks.

Parturition and Lactation

Parturition is the delivery of the baby by strong contractions of the myometrium. It is controlled by a neurohumoral reflex: the foetus presses on the cervix, the stretching signals the neurohypophysis to release oxytocin, and oxytocin makes the uterus contract harder. Stronger contractions cause more stretching and more oxytocin, which is a positive feedback loop. After birth, prolactin stimulates milk production and oxytocin causes milk ejection. The first secretion, colostrum, gives the baby nutrients and antibodies.

Common mistakes to avoid

• Using spermatogenesis and spermiogenesis as if they mean the same thing. Spermatogenesis is the whole process of sperm formation; spermiogenesis is only the last step, where spermatids change into sperm.
• Thinking the secondary oocyte finishes meiosis II before fertilization. It completes meiosis II only when a sperm enters it.
• Believing breast size decides how much milk a mother can produce. Breast size depends mostly on fat, not on the amount of milk-producing glandular tissue.
• Mixing up the endometrium and the myometrium. The endometrium is the inner lining that changes each cycle and is shed in menstruation; the myometrium is the muscle layer that contracts during childbirth.
• Believing an intact hymen proves virginity. The hymen can be stretched or torn by sports, cycling, horse riding or a sudden fall.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

Why are the testes located outside the abdominal cavity?

The scrotum holds the testes outside the abdomen, where the temperature is about 2 to 3 °C lower than inside the body. Normal sperm cannot be produced at internal body temperature, so the scrotum works as a temperature regulator for the testes.

What is the role of the acrosome during fertilization?

The acrosome is a cap over the head of the sperm. It contains enzymes called sperm lysins, including hyaluronidase. These enzymes break down the corona radiata and the zona pellucida around the ovum, so that the sperm can reach the egg membrane and fertilize it.

How does the body prevent multiple sperms from fertilizing an egg?

As soon as one sperm enters, the ovum releases the contents of its cortical granules. This forms a fertilization membrane around the ovum that blocks other sperm from entering. Stopping the entry of more than one sperm, called polyspermy, keeps the zygote diploid.

What causes menstruation to occur?

If the ovum is not fertilized, the corpus luteum degenerates into a scar called the corpus albicans. It stops making progesterone and oestrogen, so their levels drop sharply. Without these hormones the endometrium cannot be maintained, and it breaks down and is shed as menstrual bleeding.

What is colostrum and why is it important for a newborn?

Colostrum is the yellowish fluid produced by the mammary glands for the first few days after birth. According to the textbook it has more protein, vitamin A and minerals than ordinary milk, less lactose and almost no fat. It is also rich in IgA antibodies, which give the baby passive immunity.

What is the difference between identical and fraternal twins?

Identical (monozygotic) twins come from one zygote whose early embryo splits into two, so the twins have the same genes and the same sex. Fraternal (dizygotic) twins come from two ova released together and fertilized by two different sperm. They are no more alike than ordinary siblings and may be of different sexes.

What triggers uterine contractions during childbirth?

At the end of pregnancy the foetus moves down and stretches the cervix. Nerve signals from the cervix reach the brain, and the neurohypophysis releases oxytocin into the blood. Oxytocin acts on the myometrium and causes strong contractions that push the baby out through the birth canal.