Structural and functional developmental perspectives of the placental barrier and its role in the fetal development during the first and second trimesters

Abstract

The term placenta has been derived from the Latin word meaning ‘Cake’ and the Greek word for ‘flat slab like structure’. The primary function of the placenta is to act as a fetomaternal organ and a barrier with two of its components, the fetal placenta also termed as the "Chorion frondosum" and the maternal placenta termed as "Decidua basalis". The fetal placenta develops from the blastocyst and the maternal placenta from the maternal uterine tissues. The first concept regarding the presence of a fetomaternal or a placental barrier was coined around 46 years ago by using transport physiology and ultrastructural analysis. However in terms of evolutionary trend it is observed that the human fetomaternal barrier which is 10 nm size is less selective than the epitheliochorial barriers in animals. The metabolic capacity of the trophoblast and macrophage along with the fetal blood cells plays an important role in the fetomaternal barrier in the first trimester and the early stages of second trimester by metabolizing and converting harmful toxins into less harmful substrates and by also forming a sequential barrier to the maternal activated immune cells. Also the placenta plays a very important role in providing a micro niche which supports the fetal growth through transfer of nutrients and immunological properties which helps in imparting protective immunological properties to the fetus. It also acts as a major fetomaternal barrier against pathogens and the maternal immune system. Placenta also undertakes the secretion of different hormones, cytokines, growth factors and other bioactive products essential for the fetus.