El Liderazgo posibilita la construcción y transformación de la

Unfertilized sea urchin egg Suspension of sea urchin sperm Note the difference in size of egg vs. sperm (micrographs are at similar magnification)

To view fertilization, a drop of sperm is added to the slide containing eggs, and the eggs are observed under the microscope as shown in the following video. Note that shortly after a sperm has contacted the egg surface, the surrounding vitelline envelope rises creating what is called the "fertilization membrane". Prior to fertilization, the vitelline membrane adheres to the egg surface and is not visible as a separate structure. The fertilization membrane soon hardens to provide a protective covering for the developing embryo. The rising fertilization membrane also pushes other sperm away from the egg and forms a permanent block to polyspermy (i.e. no additional sperm can enter the egg).

You have learned from the topic Animal Development I that the sperm of one species canno usually t fertilize the eggs of a another species. We will now put this to the test. The following video shows a slide containing eggs from two species of sea urchin. The eggs of Lytechinus pictus are large and those of Strongylocentrotus purpuratus are small and brown. Sperm from Lytechinus pictus are added to the mixture at the beginning of the video.

To complete the fertilization process, the nucleus of sperm and egg must fuse. The compacted DNA within the sperm nucleus expands and can be seen within the egg. The fusing nuclei are called pronuclei and the resulting diploid cell is called a zygote. Observe pronuclear fusion in the following video.

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Several frog eggs are shown here. They are quite large, about 1 mm in diameter.Tthe brown part of the egg is mainly cytoplasm and the yellow part is mainly yolk. Frog eggs have relatively more yolk than sea urchin eggs, and the frog embryo develops longer before hatching. Thus the newly hatched larva of the frog embryo is more mature than that of the sea urchin. The frog egg is surrounded by a vitelline membrane which lies so close to the egg surface that it is not visible as a separate structure.

This zebrafish egg is physically smaller than the frog egg and must be viewed under a microscope.

However, the adult fish is smaller than a frog, so the egg is actually larger than the frog egg relative to animal size. Like all fish eggs, it contains a huge amount of yolk. In fact the entire egg, except for a small patch near the arrow, is yolk. With so much yolk, the embryo develops into a mature-looking larva before hatching. The covering around the egg is called a "chorion" but is analogous to the vitelline membrane of other species.

3.2 Internal Fertilization

Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother. This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth.

3.2.1Mammals and internal fertilization

To complete our activity on animal gametogenesis and fertilization, we will now examine these processes in mammals. Gametogenesis in mammals is unique in that the resulting eggs do not contain yolk (why?). Thus, they are smaller than the eggs of most other

animals. Since mammals live on land, gametes cannot be shed into water, so fertilization is internal. As compared to external fertilization, a relatively small number of sperm arrive at the location of the egg, so the block to polyspermy is less robust and slower to take effect.

Gametes

While all sperm have the same basic structure, there are often distinguishing features in different species. Note the different shape of the sperm head and/or acrosome in these sperm:

Human Hamster Opossum

Did you notice that sperm of the opossum have two tails? This condition is extremely rare and the reason for it is unknown. One might wonder if these sperm can swim. Watch the following video of opossum sperm and decide for yourself.

This light micrograph shows a human egg. It is typical of mammalian eggs and contains no yolk. The first meiotic division has been completed and the first polar body is visible.

As in many mammalian species, the second meiotic division will not occur until the egg is fertilized. The covering that surrounds the egg and polar body is called the zona pellucida. It bears sperm binding sites and is analogous to the vitelline membrane of non-mammalian eggs.

Fertilization in mammals occurs internally, as is typical of terrestrial animals. The sperm utilizes the acrosomal reaction to penetrate through the zona pellucida and is then pulled into the egg cytoplasm. After the second meiotic division is complete, the sperm and egg pronuclei fuse as in all animal species.

86 Sperm within uterus

Sperm will traverse the uterus and enter a fallopian tube

Fertilization

Red arrow: a polar body, white arrow: entry point of the sperm

Pronuclear fusion Circle surrounds the sperm and egg pronuclei

3.0 Conclusion

In this unit you were introduce to the term fertilization, types with specific examples to sea urchin , frog and zebra. Other example were also explain in mammal as internal fertilization. External fertilization is also found in aquatic organism e.g fish etc.

4.0 Summary

Fertilisation (also known as conception, fecundation and syngamy), is the fusion of gametes to produce a new organism. There are two types of fertilization, internal and external. In animals, the process involves a sperm fusing with an ovum, which

eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The fertilized egg cell is known as the zygote. Examples of internal and external fertilization in mammal and sea-urchin and frog are explained respectively.

5.0 Tutor-marked Assignment

1 Explain what you understand by fertilization

2 Explain with examples internal and external fertilization with example in animals 3 Explain fertilization with a specific example in (a) Mammals (b) Human

7.0 References

Regina Bailey, 1977. Sexual Reproduction: Fertilization About.com Guide, 1997 http://biology.northwestcollege.edu/biology/b1010lab/mitwfish.htm

Forgács, G. & Newman, Stuart A. (2005). "Cleavage and blastula formation". Biological physics of the developing embryo. Cambridge University Press. I

UNIT 2: CLEAVAGE AND BLASTOCYST FORMATION