Prenatal or antenatal 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
Main article: Human fertilizationWhen semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. 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 egg then polarizes, repelling any additional sperm. 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.
Prior to fertilization, each ovum contains a complete human genome, including a single X but no Y chromosome. Likewise, each spermatozoon contains a complete set of autosomes and a single sex chromosome, either X or Y. The resulting zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome, the Y chromosome contains very little genetic information. However it does contain a gene, SRY, which will switch on androgen production at a later stage, leading to the development of a male body type. In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum.
Embryonic period
Main article: Human embryogenesisThe embryonic period in humans begins at fertilization (12-24hrs after ovulation) and continues until the end of the 10th week of gestation (8th week by embryonic age).
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, which give rise to extra-embryonic structures such as the placenta, 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:
- Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome
- Infection (such as rubella or cytomegalovirus)
- Radiation from x-rays or radiation therapy
- Nutritional deficiencies such as lack of folate which contributes to spina bifida
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." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, 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 gestation
See also: Embryo and Human embryogenesisGestational age vs. embryonic age
Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age , in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.
Thus, the first week of embryonic age is already week three counting with gestational age.
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.
The following table summarizes the various expression systems during week number x of gestation.
Week 1-2
Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.
Embryonic age: -2 to -1 weeks old. That is, week 1-2 of gestational age are merely theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.
Week 3
Gestational age: 2 (whole) weeks old. 15–21 days from last menstruation.
Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.
- Fertilization of the ovum to form a zygote. (day 1 of fert.)
- The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3 of fert.)
- The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells.
- The embryo hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5-6 of fert.)
- If separation into identical twins occurs, 1/3 of the time it will happen before day 5.
Week 4
Gestational age: 3 weeks old. 22–28 days from last menstruation.
Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.
- Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert.
- Formation of the yolk sac.
- The embryonic cells flatten into a disk, two cells thick.
- If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
- Primitive streak develops. (day 13 of fert.)
- Primary stem villi appear. (day 13 of fert.)
Week 5
Gestational age: 4 weeks old. 29–35 days from last menstruation.
Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
- A notochord forms in the center of the embryonic disk. (day 16 of fert.)
- Gastrulation commences. (day 16of fert.)
- A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert.)
- Somites, the divisions of the future vertebra, form. (day 20 of fert.)
- Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.)
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