The placenta is a fetomaternal organ that is meant for the nourishment of fetus. It grows during pregnancy in the uterus and has a disk-like shape, weighing 500g. Just like any organ in the body, the placenta also has several functions which includes the following:

• GASEOUS EXCHANGE: O2 and CO2 exchange takes place across placenta through simple diffusion. Fetal hemoglobin has high affinity for oxygen and high hemoglobin concentration in fetus facilitates transfer of oxygen from mother to the fetus.

• TRANSPORT OF NUTRIENTS: Nutrients such as glucose, fatty acids, amino acids, and electrolytes like sodium, potassium, and chloride.

• EXCRETION: Excretion of urea, uric acid and creatinine from fetal blood into maternal blood.

• PASSIVE IMMUNITY: Maternal antibodies (immunoglobulins) pass placental barrier by pinocytosis of syncytiotrophoblast. These antibodies provide passive immunity to fetus against diphtheria, measles, smallpox, and so on.

• PLACENTAL BARRIER: It prevents entry of many drugs and bacteria. However, almost all viruses can cross placental barrier. Some of fetal blood cells may cross placental barrier and circulate in maternal blood.

• STORAGE: It stores glycogen, calcium, and iron.

• ENDOCRINE FUNCTION: Placenta secretes following hormones
o Human chorionic gonadotropin
o Placental estrogen
o Placental progesterone
o Placental lactogen

In the illustration posted, it shows the histology of placenta (Right: Low magnification ; Left: High magnification) with the following parts:

• INTERVILLOUS SPACES; MATERNAL RBCs: Spaces surrounding the villi are called intervillous spaces. They are filled with maternal blood

• ANCHORING VILLUS: Branches of anchoring villus contains the fetal blood vessels.

• FLOATING VILLI: Usually, placental section shows floating/free villi surrounded by intervillous space.

• SYNCYTIOTROPHOBLASTS: Villi are covered by a layer of syncytiotrophoblasts that looks similar to cuboidal epithelium on H&E staining. There are no intercellular margins as syncytiotrophoblast are a protoplasmic multinucleated mass. Moreover, it may have striated border as it has projecting microvilli into intervillous space.

• SYNCYTIAL KNOTS: In some regions of placenta, nuclei of syncytiotrophoblast gather to form clusters called syncytial knots. Even in some areas, syncytiotrophoblast thin out and may be free of nuclei.

• FETAL BLOOD VESSELS: Fetal Blood Vessels and intervillous spaces show RBCs and other blood cells.

• FIBROBLASTS, MATERNAL BLOOD VESSELS, FETAL RBCs