Growth and Development of the Fertilised Ovum
During the first 8 weeks of pregnancy, embryonic tissues and the surrounding supportive structures are formed simultaneously. It is during this period that the embryo is at greatest risk for malformation. From the 8th week through the end of pregnancy, the embryo is known as the FETUS. The supportive structures that nourish and maintain the growing fetus are called the fetal membranes. These include the yolk sac, amnion, chorion, decidua and the placenta.
As development continues small projections begin to appear all over the surface of the blastocyst known as the tropoblast, becoming most prolific at the area of contact – are a of inner cell mass. The trophoblast differentiates into layers.
1. The outer syncytiotropoblast (syncitium): it is capable of breaking the decidua tissue during embedding. It erodes the wall of the blood vessels, making nutrient in the maternal blood accessible to the developing embryo. It acts as a protective layer between the chorionic villi.
2. Cytotrophoblast: This is a well defined single layer of cells which produce Human Chorionic gonadotrophin (HCG). It informs the corpus Luteum that pregnancy has begun, so as to continue to produce progesterone and oestrogen. The progesterone maintain the integrity of the decidua so that shedding does not take place (menstruation is suppressed), while the high level of oestrogen suppresses the production of FSH. The HCG is produced in high level in the first trimester and it is the basis for pregnancy test.
3. The Mesoderm: Consist of loose connective tissue. It is continuous with that in the inner cell mass where they join in the body stalk which later develops into the umbilical cord.
The trophoblast later form finger like process called –Primitive villi which develop into placenta and the chorion.
As the trophoblast is developing into the placenta which will nourish the fetus, the inner cell mass is forming the fetus itself, umbilical cord and the amnion. The cells differentiate into three layers each of which will form particular parts of the fetus.
The Ectoderm: Mainly forms the skin, nervous system,mammary glands salivary glands, Pharynx, nasal passage and crystalline lens of the eyes, certain lining of the mucosa, hair, nails, and enamel of the teeth.
The Mesoderm: Forms the bones muscles, circulatory system oldvessels Reproductory system (ovary and testes), kidneys, ureters, connective tissues, lymphatic system.
The Endoderm: Lines the yolk sac. It forms the Alimentary tract,liver, pancreas, lungs, Bladder thyroid glands.
The fetus develops it’s own blood like other organs in the body. The maternal and the fetal blood never mix. During the later weeks (4 wks) the organs like the liver and heart start to function.
The three layers together are known as the embryonic plate. Two cavities appear in the inner cell mass one on either sides of the embryonic plate.
1. The Amniotic Cavity: this lies on the side of theectoderm. The cavity which is filled with fluid gradually enlarges and fold round the embryo to enclose it the lining forms the amnion. It later enlarges in the chronic cavity and comes in contact with the chorionic membrane.
2. The Yolk Sac: Lies on the side of the endoderm andprovides nourishment for the embryo until the placenta(alimentary tract
After birth the remnants of the yolk sac is the vestigial structure in the base of the umbilical cord, known as vitelline duct.
The developing of spring is referred as EMBRYO after fertilization up to 8 weeks after which the conceptus is known as FETUS until birth.