| Fetal Development |
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Prenatal development is the process in which an Embryo or Fetus (or ''foetus'') Gestates during Pregnancy , from Fertilization until Birth . Often, the terms '''fetal development''', '''foetal development''', or ''' Embryology ''' are used in a similar sense. After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of Gestational Age , the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age. FERTILIZATION See Also: Fertilization When Semen is deposited in the Vagina , the Spermatozoa travel through the Cervix and body of the uterus and into the Fallopian Tube s. Fertilization of the Ovum (egg Cell ) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its Genetic Material ( DNA ). The resulting combination is called a Zygote . The term "conception" refers variably to either fertilization or to formation of the Conceptus after uterine implantation, and This Terminology Is Controversial . Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information residing in the zygote's Nucleus comes from the mother's egg nucleus, and the other half from the nucleus of a single Sperm . However, the Mitochondria l genetic information of the zygote is in its totality contributed by the mother's egg. EMBRYONIC PERIOD See Also: Human embryogenesis The embryonic period in humans begins at Fertilization (2nd week of gestation) and continues until the end of the 10th week of gestation (8th week of development). The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a Morula . Further Cellular Division is accompanied by the formation of a small cavity between the cells. This stage is called a Blastocyst . Up to this point there is no growth in the overall size of the Embryo , so each division produces successively smaller cells. The Blastocyst reaches the Uterus at roughly the fifth day after fertilization. It is here that lysis of the Zona Pellucida , a glycoprotein shell, occurs. This is required so that the trophectoderm cells of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with Zona Hatching , an event that occurs '' In Vitro '' by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the Endometrial cell layer. This process is also called Implantation . In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and Placenta . Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception". Rapid growth occurs and the embryo's main external features begin to take form. This process is called Differentiation , which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or Miscarriage , in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the First Trimester ), the developing embryo is also susceptible to toxic exposures, such as:
Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase " Ontogeny Recapitulates Phylogeny ."1 For example, the backbone is a common structure among all vertebrates such as Fish , Reptile s and Mammal s, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The Cerebrum in humans, which is the most sophisticated part of the Brain , develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo. Changes by weeks of gestational age See Also: embryo Human embryogenesis , still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age).]] |
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