Human reproduction

Question 1

What does the male system consist of?

Answer 1

A pair of testes, contained in an external sac, the scrotum; the penis, which is an intromittent organ; genital ducts connecting the two; and various accessory glands, which provide constituents for the semen. Each testis consists of about a thousand seminiferous tubules which produce the spermatoa.

Question 2

What happens to the sperm once produced?

Answer 2

When sperm have been produced they collect in the vas deferens and then pass to the head of the epididymis where they mature. They then pass along the coiled tube to the base of the epididymis where they are stored for a short time before passing via the vas deferens to the urethra during ejaculation. Before the vas deferens joins the urethra it combines with the duct leading from the seminal vesicle to form the ejaculatory duct. The seminal vesicle produces a mucus secretion which helps the mobility of the sperm. The ejaculatory duct then passes through the prostate gland which produces an alkaline secretion that neutralises the acidity of any urine in the urethra as well as aiding sperm mobility.

Question 3

What happens to ovaries?

Answer 3

There are two ovaries each of which produces ova or eggs. They are produced in the germinal epithelium where they develop into follicles. Mature follicles migrate to the surface when their development is complete do that the ova can be shed. Ova are passed to the Fallopian tube or oviduct which conveys them to the uterus (womb). The uterus has muscular walls and is lined internally by a mucus membrane called the endometrium. It is well supplied with blood and is part of the womb into which the embryo implants during pregnancy and which is shed during menstruation. The uterus opens into the vagina through a ring of muscle, the cervix.

Question 4

What is gametogenesis?

Answer 4

The production of gametes in sex cells.

Question 5

What is spermatogenesis?

Answer 5

The formation of the sperm in testis.

Question 6

What is oogenesis?

Answer 6

The formation of eggs or ova in the ovary.

Question 7

Why is it important that gametes are haploid?

Answer 7

So that at fertilisation the diploid number is restored.

Question 8

Where does spermatozoa get produced?

Answer 8

Germinal epithelium of the seminiferous tubule

Question 9

What happens in spermatogenesis?

Answer 9

1) The diploid spermatogonia divide many times by mitosis to produce primary spermatocytes
2) These then undergo meiosis and after the first meiotic division form haploid secondary spermatocytes
3) After the second meiotic division they form spermatids which differentiate into mature spermatozoa.

Question 10

Where are Sertoli cells and what’s their function?

Answer 10

They are in the wall of the seminiferous tubule and secrete a fluid which nourishes the spermatids and protects them from the immune system of the male.

Question 11

What are interstitial cells function?

Answer 11

They secrete the male sex hormone

Question 12

Where is the haploid nucleus contained?

Answer 12

The head

Question 13

What is the middle piece got in it?

Answer 13

It’s packed with mitochondria to provide ATP for movement using tail.

Question 14

What happens in oogenesis?

Answer 14

1) Oogonia (formed before birth) undergo mitosis to form primary oocytes
2) The primary oocytes start to divide by meiosis but the process stops at Prophase I.
3) The germinal epithelium also divides to form follicle cells which surround primary oocytes to form primary follicles
4) The primary oocytes do no mature until just before ovulation
5) At puberty, hormones stimulate the follicles to develop further. Each month several follicles start to develop but only one matures into a fully developed Graafian follicle
6) First the primary oocyte completes the first meiotic division to form the haploid secondary oocyte and a small polar body
7) The mature graafian follicle migrates to the surface of the ovary where it bursts and the secondary oocyte is released (Ovulation)
8) The secondary oocyte begins the secondary meiotic division but this is arrested at metaphase unless fertilisation takes place. On fertilisation the division is completed to form a large ovum and a second polar body. Once the division has taken place the nucleus of the ovum fuses with that of the sperm to forma zygote which will then develop into an embryo.
9) After ovulation the graafian follicle becomes the corpus luteum. This produces hormones during pregnancy but regresses if fertilisation doesn’t take place.

Question 15

Sexual intercourse

Answer 15

The sperm travel in semen from the seminiferous tubules to the oviduct of the female. Secretion in the seminal vesicles, Cowper’s glands and then the prostate gland are added to the sperm to form semen. During sexual intercourse the penis is inserted into the vagina. Movements of the penis result in the ejaculation of semen into the vagina. The force of the ejaculation is sufficient to propel some sperm through the cervix into the uterus with the remainder being deposited at the top of the vagina. The sperm swim through the uterus into the oviducts by the lashing movements of their tails. Only a few from several millions released reach the site of fertilisation in the oviduct and surround ovum.

Question 16

How long can the sperm remain viable?

Answer 16

2-5 days

Question 17

How long does the egg or ovum survive for?

Answer 17

24 hours

Question 18

What is the secondary oocyte surrounded by?

Answer 18

Follicle cells and a clear membrane called zona pellucida

Question 19

What process has to take place before the sperm can fertilise an ovum?

Answer 19

Capacitation

Question 20

How does fertilisation take place?

Answer 20

When the sperm reaches the oocyte, contact with zona pellucida results in the acrosome membrane rupturing an protease enzymes are released. The enzymes digest the layers of cells surround the oocyte. inversion of the acrosome results in a fine needle-like filament developing at the top of the sperm and this pierces the already softened portion of the membrane. The whole process is called the acrosome reaction and it enables the sperm to penetrate the egg. This entry stimulates reactions of the oocyte that brings about the formation of the fertilisation membrane preventing the entry of further sperm. Entry of the sperm stimulates the completion of the second meiotic division of the oocyte nucleus. The nuclei of the ovum and sperm are drawn together and fuse to form a diploid nucleus.

Question 21

What is implantation?

Answer 21

The ovum or zygote begin to divide after fertilisation by mitosis until a hollow ball of cells the blastocyst is produced. The development of the zygote continues during its passage down the fallopian tube . After about 3 days the blastocyst reaches the uterus and embeds in the endometrium

Question 22

What is the outer layer of the blastocyst called?

Answer 22

Trophoblast which later develops into two membranes - the amnion and chorion.

Question 23

What does chorionic villi do?

Answer 23

Increase the surface are for the absorption if nutrients from the wall of uterus. Eventually it forms part of the placenta which attaches to foetus by umbilical cord.

Question 24

What does the chorion membrane do?

Answer 24

produces chorionic villi and secretes a hormone called human chorionic gonadotrophin hCG which prevents the degeneration of the corpus luteum.

Question 25

What is sub-fertility?

Answer 25

Difficulty in conceiving naturally for reasons affecting the male, female or both partners.

Question 26

What is infertility?

Answer 26

Complete inability to conceive a child.

Question 27

What are the cause of infertility?

Answer 27

1) Failure to ovulate usually associated with absence of, or an irregular menstrual cycle- 95% treatable with drug clomiphene
2) A blockage of the fallopian tube - prevents passage of the ovum to the site of fertilisation in the fallopian tubes. This blockage could be caused by an infection and treated usually involves microsurgery.

Question 28

Pregnancy testing

Answer 28

Use monoclonal antibodies (responds to one foreign antigen and specific to hCG) to test for presence of hCG which is produced in placenta during early stages of pregnancy. This is excreted in the urine and high levels act as a confirmation of pregnancy. The test relies on reaction between antibodies bound to the coloured latex beads and hCG. It causes the hCG molecules to bind together to produce a colour change.

Question 29

Describe IVF

Answer 29

1) Ovulation is stimulated using hormones at a specific dosage which aims to cause several follicles to develop at the same time
2) The oocytes are collected from the female using a tube inserted through the vagina and into the oviducts. Ultrasound is used to guide the tube
3) On the same day, the male’s semen is collected and placed in liquid containing nutrients.
4) Each oocyte is placed in a separate dish and about 100,000 sperm are added to each (or inject sperm DNA into oocyte)
5) Three days later the oocytes are examined to see which one shave been fertilised. Two are selected to increase the chance that at least one will implant and inserted into the uterus using a tube.