Question

Fertilization

Compare and contrast how the sperm penetrates the ovum in echinoderms and mammals. Why do you think echinoderm ova have more barriers in place for sperm penetration than mammalian ova?

Compare and contrast the roles of chemotaxis in abalone and human fertilization?

Answer 1

Ans:- Sperm penetrates the ovum in echinoderms are

Fertilization an essential proccess in sexual reproduction has sequential steps

(i) the acquistion of sperm motility and sperm chemotaxis in the direction of eggg

(ii) the introduction of the acrosomal reaction by egg surfacce components

(iii) sperm binding to the egg coat

(iv) sperm penetration through the egg coat

(v) sperm–egg fusion; and

(vi) egg activation to initiate the developmental program. These processes are conserved in invertebrates and vertebrates

The sperm enters and binds to the egg surface. This induces a calcium wave that causes the cortical granules, white, to fuse with the plasma membrane, releasing their contents and causing the fertilization membrane to rise. The sperm is then pulled into the egg where it moves to the egg nucleus and fuses.

In Echinodermata we see a example of Sea Urchin

Sea urchin gametes can be shed from adults by injecting 0.5 M KCl into the intracoelomic cavity. large quantities of gametes can be released in this manner, and the results gametes can then be used to produce large numbers of fertilized eggs. The orange colored gametes in this image are eggs; the white are sperm (Pacific purple sea urchin, Strongylocentrotus purpuratus).

Once an acrosome reacted sperm comes in contact with the egg, a series of events are initiated that culminate in the union of the genetic material. We can separate these steps into (1) species-specific sperm-egg binding, (2) the fast block to polyspermy, (3) the slow block to polyspermy, (4) sperm incorporation, and (5) pronuclear migration and fusion.

Fertilization in Mammals are

Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell, called a zygote, contains all of the genetic material needed to form a human—half from the mother and half from the father.

During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the female sexual part of body. Almost immediately, millions of these sperm are overcome by the acidity of the female sexual part of body(approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. Of those that do enter, thousands are destroyed by phagocytic uterine leukocytes. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the oocyte, is reduced to a few thousand contenders. Their journey—thought to be facilitated by uterine contractions—usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. Thus, fertilization can still occur if intercourse takes place a few days before ovulation. In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation. Intercourse more than a day after ovulation will therefore usually not result in fertilization.

During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called capacitation, or priming. The fluids improve the motility of the spermatozoa. They also deplete cholesterol molecules embedded in the membrane of the head of the sperm, thinning the membrane in such a way that will help facilitate the release of the lysosomal (digestive) enzymes needed for the sperm to penetrate the oocyte’s exterior once contact is made. Sperm must undergo the process of capacitation in order to have the “capacity” to fertilize an oocyte. If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s thick outer layer of cells.

Yes, the  echinoderm ova have more barriers in place for sperm penetration than mammalian ova because in some lower animal species the acrosomal process forms at the apex of the sperm head, supported by a core of actin microfilament. The membrane at the tip of the acrosomal process fuses with the egg's plasma membrane.

In some echinoderms, including starfish and sea urchins, a major portion of the exposed acrosomal content contains a protein that temporarily holds the sperm on the egg's surface.