When the sperm of a male successfully enters the ovum (egg) of a female fertilization takes place. The fertilized egg is called the zygote (developing baby). After fertilization the zygote undergoes rapid cell division. Initially they divide into two’s and then four’s, finally dividing into quadruplicates only.
By the end of day-four it forms a small solid cluster of cells called morula that resembles a mulberry. The morula then travels down the fallopian tube and reaches the uterus about three to four days after fertilization.
Blastocyst Stage Of Embryonic Development
The morula then grows to form a Blastocyst before the end of the first week.The blastocyst is made up of two distinctive layers of cells with a cavity at the center.
- The inner layer of cells will develop into an embryo.
- And the outer layer of cells will develop into Placental Membranes comprising the amnion and the chorion.
The blastocyst will then find a convenient place and just lie along nutrient rich lining of the uterus ‘the endoterium’. The process of implantation also begins around the same time (about one week after conception).
The chorionic layer (outermost layer of the blastocyst) will develop into finger like tissues called chorionic villi.These (chorionic villi) will anchor the blastocyst firmly to the uterine wall.
The villi are the start of the placenta. They will then build up a network of blood vessels. The villi feed the fetus with oxygen and nutrients through the umbilical chord from the mother throughout pregnancy.
Embryo Development In First Week Of Pregnancy
The inner layer of the blastocyst develops into an embryo about fifteen days after conception. Next to the embryo floats a cluster of blood vessels (yolk sac) that provides blood to the embryo at this early stage until the placenta takes over. The embryo is surrounded by a water-tight sac called amnion or amniotic sac. Amniotic fluid in the sac provides a warm and weightless environment to the developing baby. It also protects the baby against bumps and injuries from other routine work. The amniotic sac is surrounded by membrane called the chorion. The chorion provides double protection to the embryo.
The embryo now is about the size of a raisin and about quarter inch long. The embryo has started the development of most major organ system by the end of four weeks. The lung buds appear, heart is beating, and gastrointestinal system and liver are developing well. The basis of Central Nervous System (neural tube) has developed. The fore, mid and hind brain are defined. First layer of skin appears, facial features come into existence. The arm and leg buds are seen.