The embryonic period begins during the middle of the second week and concludes at the end of the eighth week, at which time the physical appearance of the embryo is clearly human (see Figure 3.1). It is during this time that all the major organs of the body begin to form. It is a time of specialization where cells divide and differentiate to form specific organs, e.g., the heart and lungs.
One of the mysteries of development is how cells ‘know’ to become a heart or lung cell, given that they are all identical at the start of the differentiation process. The local environment of surrounding cells and chemical messages is undoubtedly important, but exactly how one cell becomes a toenail, another a hair, is unknown. During this period the individual is called an embryo. The heart, although only two-chambered, begins to beat and blood is circulated around the embryo by the end of the third week. This enables the removal of waste and the acquisition of nutrients. As all the body’s organs begin to form during this period, it is considered the most critical stage of development.
The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age).
The embryo spends the next few days traveling down the Fallopian tube. It starts out as a single cell zygote and then 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, as it is confined within a glycoprotein shell, known as the zona pellucida. Instead, 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 occurs. This process is analogous to zona hatching, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial cells of the uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as the placenta and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation (Wilcox et al. 1999). The embryo, the extra-embryonic membranes, and the placenta are collectively 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[4] 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.
Source: http://en.wikipedia.org/, http://www.oup.com, http://www.youtube.com
No comments:
Post a Comment
Terimakasih atas perhatiaanya