P::R::E::G::N::A::N::C::Y

WHAT IS PREGNANCY?

• Carrying of one or more fetus or embryo  inside the worm of a female
• there can be multiple gestation as the case of twins or triplet
• the most studied all mammalian pregnancies
• childbirth usually occurs about 38 weeks after conception
• the WHO defines normal term for delivery as between 37 weeks and 42 weeks

INITATION

HumanEmbryogenesis

Pregnancy occurs as the result of the female gamete or oocyte merging with the male gamete, spermatozoon, in a process referred to, in medicine, as fertilization, or more commonly known as "conception". After the point of fertilization, it is referred to as a zygote or fertilized egg.

The process of fertilization occurs in more than a single step, and the interruption of any of these can lead to a failure. Therefore, what is commonly known as "conception" is much more than the fusion between the female gamete and male spermatozoon. Through fertilization, the egg and sperm are saved: the egg is activated to begin its developmental program, and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism 

At the very beginning of the process, the sperm undergoes a series of changes which makes pregnancy likely to occur. As freshly ejaculated sperm is unable or poorly able to fertilize, the sperm undergoes the phenomenon called capacitation. It is estimated that during the ejaculation, 300,000,000 sperma is released, from which only 200 reach the oviduct. Capacitation is the process through which the spermatozoon is prepared for the merging with the egg. Capacitation occurs in 5 to 6 hours and it takes place once the sperm reaches the vagina. This is also the process through which the spermatozoon becomes hyperactivated and prepared for the acrosome reaction. In order to be able to fecundate the egg, the sperm must get through the coat surrounding the egg, the so called "zona pellucida". Once the zona pellucida is penetrated, the sperm is able to reach the oocyte. However in order to get through the egg's coat, the sperm undergoes an acrosome reaction that provides it with an enzymatic drill which is able to penetrate the zona pellucida. The acrosome itself is a modified lysosome, situated on the anterior part of the head of the sperm.

Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte. Binding occurs at the posterior (post-acrosomal) region of the sperm head. After binding occurs, the egg must also undergo a series of metabolic and physical changes which may influence the further development of the zygote. These changes are called in medicine egg activation, mainly because prior to fertilization, the egg is in a latent state.

Methods to assist reproduction also include intracytoplasmic sperm injection, gamete intrafallopian transfer (GIFT), zygote intrafallopian transfer (ZIFT), and embryo cryopreservation (frozen fertilized egg and sperm). These techniques are considered as alternatives to get pregnant by women who have tried unsuccessfully for at least one year. It is estimated that in the United States, more than 6 million adults, or 10% of the adult population, are affected by infertility.

to be continue.......