Question

There are two major events in fetal development. Block to polyspermy and the transition of the corpus luteum control to the placenta. Explain how each of these events are important to child reproduction. (Please give detailed answers)

Answer 1

Polyspermy should be prevented because if  multiple sperm fertilize an egg, the embryo inherits multiple paternal centrioles. This causes competition for extra chromosomes and results in the disruption of the creation of the cleavage furrow, thus causing the zygote to die.

prevented by:-

1) modest sperm number

2) Fast block

3) Slow block

Fast Block

After the sperm’s receptors come into contact with the egg’s jelly layer and the acrosomal enzymes are released and break down the jelly layer, the sperm head comes into contact with the vitelline and plasma membranes of the egg. When the two plasma membranes contact one another, signals in the egg are initiated.

First, Na+ channels in the egg open, allowing Na+ to flood into the egg. This causes a depolarization of the egg from it’s normal resting potential of -70 mV.

While depolarization is occurring, the remainder of the jelly layer is dissolving. With the dissolution of the jelly layer and the depolarization of the plasma membrane, the first block to preventing fertilization by multiple sperm is put into place.

These two simple changes are part of the first block to polyspermy, known as the fast block. Within 1/10th of a second of contact, the fast block to polyspermy is initiated.

Slow Block

While the first fast block is relatively simple and happens quickly, the second block to polyspermy, the slow block, takes place 10-60 seconds after the initial penetration of the egg’s plasma membrane by the sperm. In this pathway, the physical layout of the membranes change.

The corpus luteum is essential for establishing and maintaining pregnancy in females. The corpus luteum secretes progesterone:- , which responsible for the development of the endomatrium  and maintenance. It also produces Relaxin, a hormone responsible for softening of the pubic symphysis which helps in parturition.

When egg is fertilized :-

syncytiotrophoblast (derived from trophoblast) cells of the blastocyst secrete the hormone human chorionic gonadotropin (hCG, or a similar hormone in other species) by day 9 post-fertilization.

Human chorionic gonadotropin signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus and providing an area rich in blood vessels in which the zygote(s) can develop. From this point on, the corpus luteum is called the corpus luteum graviditatis.

The introduction of prostaglandins at this point causes the degeneration of the corpus luteum and the abortion of the fetus. However, in placental animals such as humans, the placenta eventually takes over progesterone production and the corpus luteum degrades into a corpus albicanswithout embryo/fetus loss.

Luteal support refers to the administration of medication (generally progestins) for the purpose of increasing the success of implantation and early embryogenesis, thereby complementing the function of the corpus luteum.