Fertilisation Part Three

Question 1

Why does the scrotum suspend the testis below the abdominal cavity?

Answer 1

Because sperm production requires a lower temperature than the internal body temperature of most animals.

Question 2

What are the accessory glands of the male?

Answer 2

1) . Seminal vesicles - contribute approximately 60% of the total semen volume.
2) . Prostate gland
3) . Bulbourethral glands

Prostate and bulbourethral glands add mucus, fructose for energy, prostaglandins and other substances needed for the movement of the sperm.

Question 3

The pH of the semen is acidic. True or false?

Answer 3

False. The pH of the semen is alkaline.

Question 4

Describe how the gonadotrophins stimulate sperm production.

Answer 4

Spermatogenesis, like oogenesis is FSH and LH dependent. FSH and LH are called gonadotrophins because they stimulate the gonads.

1) . FSH and LH are secreted from the anterior pituitary in response to GnRH.
2) . In the testis LH binds to leydig cells stimulating secretion of testosterone.
3) . Testosterone stimulates spermatogenesis. It also produces differentiation of male reproductive organs and maintains their function and induces secondary male sex characteristics.
4) . FSH is also critical for sperm production. It supports the function of serotoli cells where testosterone binds for stimulation of spermatogenesis and which in turn support many aspects of sperm cell maturation.

Question 5

Why is FSH vital in sperm production?

Answer 5

FSH is also critical for sperm production. It supports the function of serotoli cells where testosterone binds for stimulation of spermatogenesis and which in turn support many aspects of sperm cell maturation.

Question 6

Why is LH vital in sperm production?

Answer 6

In the testis LH binds o leydig cells stimulating secretion of testosterone. Testosterone stimulates spermatogenesis.

Question 7

Describe the steps in sperm maturation during spermatogenesis.

Answer 7

1) . Spermatogonia
2) . Primary Spermatocytes
3) . Secondary Spermatocytes
4) . Round Spermatids
5) . Mature spermatozoa

Mature spermatozoa are released from the protective serotoli cells into the lumen of the seminiferous tubules and a process of spermiation takes place which removes unnecessary cytoplasm and organelles from the cells.

Question 8

What does the process of spermiation do?

Answer 8

When mature spermatozoa are released from the protective serotoli cells in to the lumen of the seminiferous tubules spermiation takes place which removes the unnecessary cytoplasm and organelles from the cells.

Question 9

What did Yamanaka theorise regarding oocyte activating factor in humans?

Answer 9

In humans the soluble sperm activating factor for the oocyte may not be limiting in human round spermatids because identical activation responses were seen in sperm injected and spermatids injected into oocytes. This implies that sperm cells acquire their activating factors much earlier in spermatogenesis than mice as human round spermatids are still able to activate oocytes.

Question 10

What are some of the possible fates of ejaculated sperm?

Answer 10

1) . Leak out of the vagina immediately after deposition.
2) . Destroyed by the acidic vaginal environment.
3) . Fail to make it through the cervix.
4) . Dispersed into the uterine cavity or destroyed by pathogenic leukocytes.
5) . Reach the uterine tubes.

Question 11

What process must occur before the sperm can penetrate the oocyte?

Answer 11

Capacitation.

Question 12

Describe capacitation.

Answer 12

Capacitation is the attainment of full fertilising capacity of the sperm. It takes 4-5 hours and involves change in the movement of the sperm into whiplashing beats and hyperactivation. It also involves cleaving the sperm surface from glycoprotein molecules that were acquired after contact with seminal plasma. This results in a change in the membrane leading to the achrosome reaction.

Capacitation occurs in the uterus. In vitro it can be achieved by removing sperm from seminal fluid. Capacitated sperm have the ability to fertilise the egg.

Question 13

Describe how the sperm fertilises the egg.

Answer 13

1) . The sperm has to go through the cell surrounding the egg called the cumulus oocyte complex in order to fertilise the egg.
2) . This leads to the achrosome reaction whereby the achrosomal head content is released to allow sperm to penetrate the zona pellucida. The achrosome contains hyalurinidase, collaginase and achrosin allowing the spermatozoa to penetrate the zona pellucida and deposit the sperm inside the egg.
3) . Just after fertilisation the cortical granules move to the surface of the oocyte and then are released into the perivitelline space which will lead to biochemical changes in the zona that prevents further sperm from binding thus blocking polyspermy.

This will complete the process of fertilisation and allow the egg to complete meiosis and the cell cycle of the embryo to proceed.

Question 14

What is the window of fertilisation after ovulation?

Answer 14

The ovulated oocyte is viable for 6-24 hours. Sperm is viable for 24-72 hours.

For fertilisation to occur, coitus must occur between 3 days before ovulation and 24 hours after ovulation.

Question 15

Where on the Fallopian tube does fertilisation occur?

Answer 15

Fertilisation occurs in the ampullary-isthmus region of the Fallopian tube.

Question 16

How can you assess fertilisation?

Answer 16

You can’t do it in vivo.

In vitro you can check for the presence of pronuclei 15-18 hours post insemination or 5-6 hours post injection.

In normal fertilisation we usually see 2 parental pronuclei and 2 polar bodies in the perivitelline space. Sometimes the first polar body gets degenerated because it is a fragile structure.

Question 17

Describe the first cell cycle after fertilisation.

Answer 17

1) . Calcium oscillations continue. Formation of parental pronuclei and formation of membranes around both maternal and paternal nuclei needs to occur.
2) . There is a series of genome remodelling and reprogramming events that need to happen such as the replacement of protamines with histones in the sperm nucleus and a series epigenetic and imprinting steps.
3) . DNA needs to replicate to go into the second cell cycle.
4) . The zygotic genome activation that takes over the maternal dependence happens not at the first cell cycle but at later stages of development.
5) . Towards the end of the first cell cycle there is syngamy which is the fusion of both parental pronuclei leaving the first cleavage furrow and the division of the zygote into the 2-cell embryo.

Question 18

Describe the first few cell cycles of the embryo.

Answer 18

The first few cell cycles are similar in length to each other (1st cycle = 24 hours, next cycles = 16-18 hours).

The first few cycles take about 72 hours to complete and during the cell divisions of the early embryo there is a move in potency. The cells of the early embryo are totipotent. Once they proceed into later stages of development they become pluripotent. They eventually differentiate into one type of tissue.

Question 19

What happens between the 8 cell and 16 cell stage of the embryo.

Answer 19

Once the zygote has all its genes active (major zygote genome activation or ZGA) it has a change in metabolic requirements. This happens between the 8 and 16 cell stage.

Question 20

What happens after the 16 cell stage of embryo development?

Answer 20

After the 16 cell stage there is compaction at the morula stage which is Ca++ dependent and involves a number of proteins such as adhesion and e-cadherin molecules that are very important in this process.

The compaction of the cells happens by the formation of tight junctions or desmosomes between the blastomeres/embryonic calls and makes it hard to separate from each other after this stage.

Question 21

What happens after the compaction stage of the embryo?

Answer 21

The highly polarised morula cells within the outer cells and the inner cells with different properties start actively pumping glucose, salts and water into the intracellular space. This process is called cavitation. This is now termed the early blastocyst because we can see a cavity at this stage.

Question 22

What happens in the early blastocyst stage just after cavitation has occurred?

Answer 22

EGF and TGF-a stimulate blastocyst expansion. The blastocyst formation is distinguishable by the formation of two populations of cells within the embryo, the inner cell mass which forms the embryo proper and the trophoectodermal cells which form the placenta.

Blastocyst expansion and hatching then take place.

Question 23

At what stage does the embryo enter the uterus?

Answer 23

The embryo enters the uterus at 4-5 days when the blastocyst should hopefully implant.

Question 24

What are the main events in embryo development?

Answer 24

1) . First few cell cycles = 72 hours.
2) . Zygote Genome Activation between 8 and 16 cell stage.
3) . Compaction after the 16 cell stage.
4) . Morula cavitation to form early blastocyst.
5) . Blastocyst expansion.
6) . Blastocyst hatches and enters the uterus at 4-5days.

Question 25

Describe implantation of the blastocyst. What do the cytotrophoblast and syncytiotrophoblast cells do?

Answer 25

Implantation happens via the position of the uterus on the uterine lining, with the inner cell mass facing the uterine endometrial epithelium.

Shortly before the blastocyst comes into contact with the endometrium the trophoblast starts to differentiate into two different cell masses, the outer syncytiotrophoblast and the inner cytotrophoblast.

The inner cytotrophoblast cells consist of a regular layer of single ovoid cells and in the periphery the outer syncytiotrophoblast cells form the synctium, a multinucleate layer without cell boundaries that arise from the fusion of cytotrophoblast cells.

The syncytiotrophoblast cells produce lytic enzymes and secrete factors that cause apoptosis of the endometrial epithelial cells. They then cross the basal lamina and penetrate into the stroma that lies below, eroding the walls of the capillaries.

With the implantation of the blastocyst in the endometrium he syncytiotrophoblast develops quickly and will entirely surround the embryo as soon as it completely embeds in the endometrium.

Question 26

Describe placentation. What is the function of the placental barrier?

Answer 26

As the placenta forms, HCG levels decline and the placenta starts secreting its own progesterone.

The placental barrier is composed of structures that separate the maternal and foetal blood. The makeup of that barrier changes of the course of pregnancy but the role of this barrier is the foetal nutrient and waste exchange in maternal and foetal blood.

Question 27

Where is the sperm produced?

Answer 27

The sperm is produced in the coiled seminiferous tubules in the testis.