Earth Science: Biology – Humans – Embryogenesis
A developing child goes through several phases as an embryo. Embryogenesis begins with the first cell divisions while the zygote travels to the uterus where the embryonic and fetal phases then occur.
Once an egg cell has been fertilized, it travels as a zygote through the fallopian tube into the uterus. At this time, the cell has already divided several times. The resulting cell cluster is similar to a mulberry (Moms spp.) and, hence, called a morula.
The morula differentiates into an outer layer of cells which functions as nutritional tissue and an inner cell cluster from which the embryo develops. The inside of the morula fills with fluid, creating a germinal vesicle or blastula, which then plants itself into the uterine lining or endometrium.
This process is called implantation or nidation. The placenta develops during this phase and creates a life-supporting connection between mother and child.
Embryonic phase
Within only a few days after implantation, two adjacent layers of cells develop from the interior cell cluster. These will later develop into three germ layers: ectoderm, endoderm, and mesoderm. The organ systems will emerge from these layers.
The nervous system, the skin, and the mammary and sweat glands, for example, develop from the ectoderm, and the thyroid gland, liver, and pancreas develop from the endoderm.
Starting in week four, the spine, heart, and eyes begin to appear. The body shape becomes visible after about eight weeks when the transition to the fetal phase happens.
Fetal phase
The fetal phase is characterized by major growth of the unborn child. At the beginning of month five, the fetus weighs about 1.6 pounds
(700 g) and noticeably moves around inside the amniotic sac.
It can already move its fingers, toes, and mouth. The unborn child may be able to survive outside the womb from week 28. However, this very much depends on the individual progress made in developing the respiratory and nervous system.
At birth, about 266 days following fertilization of the egg cell, a newborn weighs on average about 7.7 pounds (3,500 g) and measures almost 20 inches (about 50 cm).
AMNIOCENTESIS
An amniocentesis is a prenatal test often recommended for older pregnant women, whose children tend to experience a slightly elevated risk of chromosome defects. During this test, a sample of amniotic fluid is taken from the uterus and necrotic cells from the fluid are tested for potential defects.
Before the amniotic sac is punctured through the abdominal wall with a local anesthesia, the precise position of the fetus is determined by ultrasound to prevent injury to the unborn child.
BASICS
THE BABY’S DUE DATE is said to be 38 weeks after conception or 40 weeks after the first day of the last menstrual period.
But 85 per cent are born a few days earlier or later.