11.4 Sexual reproduction

Essential Questions: Why are egg cells so much bigger than sperm? Why are there millions of sperm present in human fertilisation, and how is it controlled so only one sperm fertilises an egg?

Lesson one: Comparing oogenesis and spermatogenesis

 

Objective: Compare and contrast the formation of human egg and sperm

Keywords:

  • Primary / Secondary oocyte / ovum (a): A developing egg cell before the first division of meiosis, present from birth / A developing egg cell after first division of meiosis (also produces the polar body, a tiny cell which dies) / Mature egg cell (s).
  • Ovary: organ where oogenesis occurs.
  • Primary follicle / Secondary follicle: both a central oocyte surrouinded by a single layer of follicle cells and fluid, difference is primary follicle contains primary oocyte; and secondary follicle contains secondary oocyte. Follicles produce estrogen.
  • Corpus luteum: the remains of the follicle after ovulation has occured (the oocyte leaves the follicle). Produces progesterone.
  • Spermatogenesis / Oogenesis: the development of sperm / egg cells
  • Primary / Secondary spermatocyte / Spermatid / Sperm or spermatozoa: A developing sperm cell before first division of meiosis (called spermatogonium when immature, produced from outer layer germinal epithelium) / A developing sperm cell after first division of meiosis / A developing sperm cell after second division of meiosis but without a tail / A mature sperm cell that has grown a tail and a mid-piece with the help of nurse cells (sertoli cells)

Oogenesis:

By the time of birth, hundreds of thousands of primary follicles already exist in the ovary of a human female. These primary follicles are produced by mitosis. Every menstrual cycle, a few primary follicles start to develop, and the first division of meiosis occurs.

Uneven meiosis:

When primary oocytes divide my meiosis (first division), the cytoplasm is shared unevenly. A large cell is produced – the secondary oocyte, and a tiny cell – the polar body. The polar body dies. In this way, the secondary oocyte is much larger than the primary oocyte.

 

The same thing happens again with the second division of meiosis, which actually happens after fertilisation. A large cell is produced, a mature ovum, and a tiny cell – the second polar body. Once again the polar body dies. In this way, ovum are enlarged cells.

The human ovum is the largest (volume, not length) cells in the human body, and can be seen as a speck with the naked eye (o.1mm).

 

egg

 Development of spermatazoa (sperm)

Sperm not produced in appreciable numbers until human males reach sexual maturity, then according to national geographic (https://news.nationalgeographic.com/news/2010/03/100318-men-sperm-1500-stem-cells-second-male-birth-control/) 1,500 sperm cells are produced every second by the average adult male.

This takes place in seminiferous tubules within the testes. The answer to questions such as causes of testicular cancer, and the reasons for declining male fertility, may lie in studying germ cells (germinal epithelium cells). These cell are in the outer lining of a seminiferous tubule. As you move towards the centre of a seminiferous tubule, you encounter developing sperm in a more mature phase of development. In the center free swimming fully developed sperm are found. Mature sperm are also stored in the epididymis.

Germ cells become primary spermatocytes, by mitosis. Primary spermatocytes become secondary spermatocytes after the first division of meiosis. Secondary spermatocytes will become associated with nurse cells (also called sertoli cells). These will provide nourishment for the secondary spermatocytes so that they can develop a tail and a mid-piece. They are then called spermatids. Once they are fully developed, they detach from the nurse cells and become free swimming spermatozoa.

Skill:  Labelled egg and sperm

imagecredit:bioninja

imagecredit:bioninja

 

You should also be familiar with microscope images of ovaries and seminiferous tubules. The images below are helpful. Please note Leydig cells are not in the syllabus and spermatogonium is another name for the germ cells (germinal epithelial).

 

image credit:montgomeryedu

image credit:montgomeryedu

 

image credit:animalscience

image credit:animalscience

 

Fertilisation

Fertilisation may be internal or external. Internal fertilisation has a higher success rate, but demands more proximity and specially adapted sex organs. External fertilisation is less successful and therefore many more eggs must be produced. However, there is lesser need for specialised sex organs. Externally fertilised embyros may have to develop independently.

During fertilisation, polyspermy is avoided. (the fertilisation of an ovum by more than one sperm, after all there may be 2 million in an ejaculation). This diagram illustrates the mechanisms clearly.

Figure 1: This image, from LIFE: The Science of Biology, Purves et al, 1998

Figure 1: This image, from LIFE: The Science of Biology, Purves et al, 1998

 

The development of an embryo

A fertilised egg will divide to form a ball of unspecialised cells called an embryo. An embryo quickly uses up the food resources available, and a successful pregnancy relies on implantation of the embryo in the wall of the uterus (endometrium).

The endometrium is so important during early pregnancy that the embryo secretes a hormone called human chorionic gonatotrophin (hcg) which stimulates the corpus luteum to keep producing estrogen and progesterone (these maintain the lining of the uterus). After about nine weeks the placenta takes over and secretes enough estrogen and progesterone to maintain the endometrium.

Human daily birth control pills work because they mimic the action of the corpus luteum during pregrancy, as they contain the hormones estrogen and progesterone. This prevents the maturation of new follicles. The rise of use of these chemical birth control methods have led to a detectable rise of estrogens in drinking water of major cities. Although this has not been proved, it is postulated this may be the cause of the decline in male fertility being experienced in many parts of the world today.

Humans are placental mammals, the placenta is a development of the fetal tissues that allows the mothers blood to pass very close to the blood of the fetus. This arrangement is achieved by placental villi, which contain the blood of the fetus (in capillaries). The mother’s blood pools around the placental villi, and the exchange of substances occurs.

 

figure_2_6_the_placental_circulation_500

 

Birth

Another hormone, oxytocin, is involved in stimulating the contractions of the muscular walls of the uterus that cause labor and ultimately, birth. Oxytocin is kept in check by progesterone and estrogen, but when the placenta stops secreting them (after about 9 months), oxytocin is produced. Oxytocin also stimulates the production of mother’s milk in mammary glands.

Getting ready, different strategies.

In some mammals, such as mice, with short gestation periods (19 days), babies are relatively undeveloped and helpless (baby mice, or pups, have closed eyes, cannot move and are totally helpless).

Mammals with longer gestation periods tend to have newborn who have hair, open eyes and can move and begin to show some independence right away. An elephant would be a good example with a 22month gestation period.

Humans occupy the middle ground, with a nine month gestation period and a moderately helpless newborn with open eyes and some hair.

 

 

 

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