Question

When and how does placenta develop in a human female?

Answer 1

Hint: Placenta is a disc shaped protective layer developed during pregnancy, covering the embryo. It provides oxygen and nutrients and removes waste through villi.

Complete answer:
The placenta is defined as a life-supporting network that consists of disk-shaped tissue inside which small blood vessels from the mother and offspring intertwine but don’t join. After the fertilization that is the fusion of sperm and ovum from the male and female respectively; Then the prenatal development period begins which lasts for 266 to 280 days that is 38 to 40 weeks. It is divided into three periods: germinal, embryonic and fetal.
 Germinal period is the period of prenatal development that takes place within the first fortnight after conception. It includes the production of the fertilized egg, called a zygote, cell division and the attachment of the zygote to the uterine wall, Rapid cell division by the zygote continues throughout the germinal period.

Embryonic period is the period of prenatal development that occurs from two to eight weeks after conception. During this period, the rate of cell differentiation intensifies, support systems for cells form, and organs appear. This period begins as the blastocyst attaches to the uterine wall. The mass of cells is now called an embryo, and three layers of cells form. The layers are -inner layer endoderm, the middle layer mesoderm and the outer layer ectoderm. During the period in which the embryo's three layers form, life-support systems for the embryo develop rapidly. This life-support system includes the amnion, the umbilical cord (both of which develop from the fertilized egg, not the mother’s body), and the placenta. After implantation, finger-like projections appear on the trophoblast and chorionic villi, which are surrounded by the uterine tissue and the maternal blood. During the embryonic stage, these finger-like projections along with the uterine tissue become interdigitated with each other and jointly form a structural and functional unit between the fetus and maternal blood.

Mother’s blood flows through the uterine arteries to the spaces housing the placenta, and it returns through the uterine veins to the maternal circulation. Fetal blood flows through the umbilical arteries into the capillaries of the placenta and returns through the umbilical vein to the fetal circulation. The exchange of materials takes place across the layer separating the maternal and fetal blood supplies, so the blood never comes into contact. Very minute molecules—oxygen, water, salt, food from the mother’s blood, as well as carbon dioxide and digestive wastes from the offspring’s blood—pass back and forth between the mother and embryo or fetus. Large molecules cannot pass through the placental wall; these include red blood cells and harmful substances, such as most bacteria and maternal wastes. The mechanisms that govern the transfer of substances across the placental barrier are complex.



Note: Placenta also acts as an endocrine tissue and produces several hormones like human Chorionic Gonadotropin (hCG), human Placental Lactogen (hPL), estrogen, progesterone, etc.
In later stages of pregnancy, a hormone called Relaxin is produced by the ovary. It is important to note that hCG, hPL and Relaxin are produced only during pregnancy in women.