Placenta 

The placenta is a temporary, disk-shaped organ that forms during pregnancy in the uterus. It serves as a critical interface between the mother and the developing fetus, enabling nutrient exchange, gas exchange, and waste removal. The placenta also produces hormones necessary for maintaining pregnancy and supporting fetal development.

1.0Introduction

• The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta. 
• The embryo depends upon maternal tissues for shelter, nutrition, respiration etc. 
• Placenta is found in viviparous animals. 
• Placenta is a temporary endocrine structure which partially derives from maternal and embryonic tissue. 

2.0Hormones of Placenta 

3.0Functions of Placenta

• Exchange of important materials between foetal and maternal blood.  
• The essential materials are exchanged by diffusion, pinocytosis or active transport. 
• The small molecules like O₂, CO₂, H₂O etc. are diffused through placenta. 
• Placenta serves as a respiratory medium for exchange of O₂ and CO₂ between embryo and mother. 
• The nutritional substances are supplied to the embryo from the mother through placenta. The nitrogenous and metabolic wastes from the fetus are released into the blood of the mother by diffusion through placenta. 
• The antibodies (IgG) for measles, chickenpox, polio etc. present in the blood of the mother reach the embryo through placenta. 
• If a female takes some harmful chemicals (teratogens), liquor, drugs etc. during pregnancy, these may cross the placenta and on reaching into the fetus may cause deformity during organogenesis. (e.g. Thalidomide) 
• Placenta acts as a temporary endocrine structure. 
• It secretes some hormones like progesterone, estrogen, inhibin, hCG, hCS/HPL etc. HCG maintains the corpus luteum. Progesterone, maintains and supports the fetus during the whole pregnancy period. 
• At the time of parturition, relaxin is secreted by placenta which lubricates, and widens the birth canal to facilitate childbirth. Note: During pregnancy the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood. 

4.0Parturition And Lactation

• The average duration of human pregnancy is about 9 months which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the fetus (childbirth) is called parturition. 
• The process of childbirth is called parturition which is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin. Many hormonal changes take place during it. 
• By the end of pregnancy mammary glands start producing milk by a process called lactation.

Foetal Ejection Reflex 

• The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex.

Parturition 

• As a result of mild uterine contractions, the maternal pituitary gland secretes Oxytocin. 
• This hormone induces intense contractions in the uterus. Due to these contractions, the fetus starts moving towards the vagina. 
• The labour pain during child-birth, is due to this hormone. Oxytocin is the main parturition hormone. 
• After parturition, Oxytocin stimulates milk-let down by milk ejection reflex. 
• During parturition, Relaxin hormone is secreted by the placenta and the ovary of pregnant females. This hormone relaxes the pubic-symphysis i.e. the joint between the pelvic-girdles. So more space is available to the fetus to move out. 

Mechanism

Lactation

• The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation. 
• This helps the mother in feeding the new- born. The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance for the new-born babies. 
• Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.