11.4 – Reproduction
11.4.1 – Annotate a light micrograph of testis tissue to show the location and function of interstitial cell (Leydig cells), germinal epithelium cells, developing spermatozoa and Sertoli cells
11.4.2 – Outline the processes involved in spermatogenesis within the testis, including mitosis, cell growth, the two divisions or meiosis and cell differentiation
In the testes of the male embryo, the primordial germ cells will differentiate to form the spermatogonia. These are still diploid cells.
Endless mitosis of the germinal epithelium cells produces many more spermatogonia which are found in the outer wall of the seminiferous tubule. This is a process that continues into adulthood, allowing for the continuous production of sperm.
These diploid cells grow larger to become primary spermatocytes (2n)
They will then enter meiosis I, dividing to become secondary spermatocytes (2n)
Following this, they enter meiosis II, during which they will divide to become two spermatids (n), making a total of four spermatids produced from each germinal epithelium cell.
The spermatids are then nurtured by the Sertoli cells to develop and differentiate into spermatozoa (n)
The sperm then detach from the Sertoli cells. The fluid of the seminiferous tubule carries them out of the testis and into the epididymis.
The final product of spermatogenesis is four sperm cells from each germinal epithelium cell.
11.4.3 – State the role of LH, testosterone and FSH in spermatogenesis
Luteinising Hormone – Secreted from the pituitary gland, and stimulates the secretion of testosterone by the testes.
Testosterone – Secreted from the interstitial cells in the testes. It stimulates the development of secondary spermatocytes into mature sperm.
Follicle Stimulating Hormone – This is secreted from the pituitary gland. It stimulates the primary spermatocytes to undergo meiosis I, forming secondary spermatocytes.
11.4.4 – Annotate a diagram of the ovary to show the location and function of germinal epithelium, primary follicles, mature follicle and secondary oocyte
The oogonia are formed by mitosis from the germinal epithelium and will then grow to become primary oocytes. Each menstrual cycle, a few primary follicles are formed from an oocyte and a layer of follicle cells. Meiosis I then occurs to produce the secondary oocyte and the first polar body.
The secondary oocyte will continue to mature in the follicle, or Graafian follicle. The oocyte will continue to mature, entering meiosis II, but stopping at prophase II. It will not develop further until a sperm enters the ovum.
The follicle burst open to release the secondary oocyte, with follicle cells, into the fallopian tube. This is ovulation. The remaining follicle becomes the corpus luteum, which temporarily produces progesterone. If the ovum is not fertilised, then this will degenerate.
11.4.5 – Outline the processes involved in oogenesis within the ovary, including mitosis, cell growth, the two divisions of meiosis, the unequal division of cytoplasm and the degeneration of polar body
In oogenesis, ova are produced in the ovaries.
When the female is still a foetus, diploid cells divide by mitosis in the germinal epithelium, creating more diploid cells (2n).
These cells then grow to become primary oocytes (2n)
The primary oocytes will then start meiosis I, but will stop during prophase I. The primary follicle consists of the primary oocyte and a layer of follicle cells around it.
In general, a female is born with about 400 000 primary follicles.
During each menstrual cycle, a primary follicle will develop. In this time, the primary oocyte completes meiosis I to form two haploid nuclei. However, due to unequal cytokinesis, the cytoplasm in divided unequally and the result is a large secondary oocyte and a small polar cell (n)
The secondary oocyte will then enter meiosis II, stopping in prophase II. Simultaneously, the surrounding follicle cells will proliferate and form follicle fluid.
At ovulation, the follicle will burst and release the secondary oocyte into the fallopian tubes.
The remaining follicle then becomes the corpus luteum, which secretes progesterone.
If the ovum is fertilised, then it will complete meiosis II with the sperm nucleus inside it and a second polar body is formed due to unequal cytokinesis. The first and second polar bodies will degenerate.
11.4.6 – Draw and label a diagram of a mature sperm and egg
11.4.7 – Outline the role of the epididymis, seminal vesicle and the prostate gland in the production of semen
Epididymis – The sperm reach the epididymis after the leave the testes. At this stage, they are unable to swim. The sperm are stored here to continue to mature and become able to swim.
Seminal Vesicles – Produce and store fluids that are released for ejaculation, mixed with the sperm to increase the total volume of the ejaculate. This fluid contains nutrients such as fructose, providing energy for the sperm, as well as mucus to protect the sperm when they reach the vagina.
Prostate Gland – Produce and store fluids that are released for ejaculation, mixed with the sperm to increase the total volume of the ejaculate. This fluid contains mineral ions and is alkaline to protect the sperm from the acidic environment of the vagina.
11.4.8 – Compare the processes of spermatogenesis and oogenesis, including the number of gametes and the timing of the formation and release of gametes
11.4.9 – Describe the process of fertilisation, including the acrosome reaction, penetration of the egg membrane by a sperm and the cortical reaction z
Fertilisation takes place in the fallopian tubes. A secondary oocyte is released from the ovaries at about day 14 of the menstrual cycle and enters the oviducts. The sperm enter the female body during sexual intercourse through the vagina, and then travel up into the fallopian tubes. Only a few sperm will reach this stage, as many will die due to the high acidity of the female system, along with other factors.
When the sperm and the ovum meet, the sperm must pass between the follicle cells that surround it. They will then arrive at the jelly coat or the zona pellucida. In the acrosome of the sperm, there are digestive enzymes that break down the coat to form a path.
The process of changing the head of the sperm is called capacitation.
The head of the sperm will then fuse with the plasma membrane of the ovum, allowing the nucleus to enter.
Once a sperm has entered the ovum, the cortical granules are then released across the membrane through exocytosis to prevent any more sperm from entering. If another sperm does enter, then the zygote would not survive.
At this point, the secondary oocyte will recommence meiosis II, forming a second polar body. Both the first and second polar bodies are then released and degenerate.
Fertilisation is complete once the nuclei of the ovum and the sperm fuse together. The cytoplasm will divide to form diploid cells of the embryo.
11.4.10 – Outline the role of HCG in early pregnancy
Human Chorionic Gonadotrophin (HCG) is secreted once the embryo is implanted in the wall of the uterus. This prevents the corpus luteum from degenerating, but causes it to grow to continue producing oestrogen and progesterone. This maintains the pregnancy. Eventually, the placenta will take the place of the corpus luteum in secreting oestrogen and progesterone.
11.4.11 – Outline early embryo development up to the implantation of the blastocyst
The fertilisation of the ovum happens in the fallopian tube. When the zygote (fertilised ovum) is formed, it travels down the fallopian tube, dividing by mitosis as it does so.
When the zygote first begins to divide, it does so through cleavage of the cell. At this stage, the mass and size of the embryo do not change. The result is that, by the time it reaches the uterus, the embryo has undergone several mitotic divisions to become a hollow ball of small cells called blastomeres, which organise themselves to form the blastocyst, which is also filled with fluid.
The embryo will usually implant in the endometrium at about days 7-14, at which point the blastocyst contains approximately 100 cells. This is called implantation. A few blastomeres will group together to form the inner cell mass, which later become the foetus. The endometrium provides nutrients to embryo.
11.4.12 – Explain how the structure and functions of the placenta, including its hormonal role in secretion of oestrogen and progesterone, maintain pregnancy
The placenta is made up of maternal and fetal membrane tissues, allowing the blood of the mother and child to come close enough together to allow for exchange. The umbilical cord is formed from an artery and a vein, and connects the foetus to the placenta. The placenta also protects the baby from bacteria; however some viruses are still able to cross it. Exchange takes place through both active transport and diffusion.
In addition to the exchange of materials, the placenta also has important functions in the production of hormones. In early pregnancy, it produces HCG in order to maintain the corpus luteum. The placenta will later replace the corpus luteum to produce progesterone and oestrogen. This is important for ensuring that the pregnancy is maintained.
11.4.13 – State that the foetus is supported and protected by the amniotic sac and amniotic fluid
During gestation, the embryo is protected by the amniotic sac and fluid. The embryo is a very small, delicate structure; hence such protection from injury is essential. It is able to float in the amniotic fluid to support it and protect it from shock.
11.4.14 – State that materials are exchanged between the maternal and foetal blood in the placenta
The placenta is attached to the foetus, allowing for the exchange of materials between the mother and child. This occurs through diffusion and active transport. These materials include:
- Respiratory gases – Including oxygen diffuses across the membrane to the foetus, with carbon dioxide diffusing back.
- Water – Enters the placenta by osmosis
- Glucose – Enters by facilitated diffusion
- Ions and Amino Acids – Transported across the membrane using active transport
- Excretory Products – This includes urea, which leave the foetus
- Antibodies – These enter the foetus’ bloodstream from the mother to protect it from the same diseases, called passive immunity.
The placenta acts as a barrier to bacteria, but not all viruses
11.4.15 – Outline the process of birth and its hormonal control, including the changes in progesterone and oxytocin levels and positive feedback
Just before birth, the concentration of progesterone decreases dramatically. Since progesterone inhibits the contraction of the muscles in the uterus, this effect is stopped and the contractions can begin.
Oxytocin is released from the pituitary gland to relax the fibres of the bones of the pelvic girdle. The cervix begins to dilate. Oxytocin will also cause contractions.
A positive feedback loop is established: as the cervix stretches further, more oxytocin is released to continue the contractions. This stops once birth is complete and the cervix stops stretching.
Contractions move down towards the cervix to push the baby out, becoming more and more powerful and frequent as time goes on.
The placenta and the remaining umbilicus are discharged afterwards in a period called the afterbirth.