Question

Describe the process of implantation. Identify the placenta and embryonic membranes on anatomical models and describe their functions. d. e. f. Describe the fetal circulation and how it differs from adult circulation.

Answer 1

d. Soon after fertilization, the zygote divides repeatedly by mitotic division to form a morula. A morula ia an embryo with 8 to 16 cells. The blastomeres [cells of morula] divide further to form a large ball of cells called the blastocyst which passes into the uterus.

The cells of the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells called the 'inner cell mass'.

The trophoblast layer gets attached to the endometrium of the uterus. The endometrium cells divide rapidly and cover the blastocyst. This is implantation.

e. The cells of the trophoblast differentiate into an inner and an outer layer. The outer layer is called the chorion and forms the chorionic villi, which are finger-like projections that grow into the endometrium. This results in an intimate connection between the chorionic villi and the uterine tissue and forms the placenta.

The placenta is a structural and functional unit between the developing embryo and the maternal body. The functions of the placenta include providing nutrient and oxygen to the fetus and removing wastes from the fetal blood. It also produces certain hormones like, estrogen, progesteron, human chorionic gonadotropin and human placental lactogen which are essential for the normal maintenance of pregnancy.

An umbilical cord connects placenta with the embryo which helps in transport of substances to and from the developing fetus.

f. In fetal circulation, the blood bypasses the lungs and liver by flowing in a different pathway and through special openings called 'shunts'. Oxygen and nutrients from the mother's blood are transferred across the placenta to the fetus through the umbilical cord.

The fetal circulatory system uses 3 shunts, which are actually small passages that direct blood that needs to be oxygenated. The blood vessel called ductus arteriosus moves blood from the pulmonary artery to the aorta. In fact, fetal circulation is more complicated than after the baby is born. Ductus venosus is the fetal blood vessel connecting the umbilical vein to the IVC. Foramenovale shunts highly oxygenated blood from right atrium to the left atrium. As soon as the baby is born, the foramen ovale, ductus arteriosus, ductus venosus and the umbilical vessels are no longer needed. Due to a sequence of events, blood pressure in the aorta becomes high and the systemic circulation begins to work.

The mother's circulation does not have any shunts and the lungs and the liver are richly supplied with blood vessels.