Development of the Placenta

During the follicular phase of the menstrual cycle, the endometrium thickens and gets ready for a fertilised egg to arrive. The myometrium sends off artery branches into the endometrium. Initially, these arteries grow straight outwards like plant shoots. As they continue to grow, they coil into a spiral. These thick-walled and coiled arteries are bunched together, making the endometrial tissue highly vascular. These are known as the spiral arteries


Placental and Umbilical Cord Development

When the blastocyst implants on the endometrium, the outermost layer, called the syncytiotrophoblast, grows into the endometrium. It forms finger-like projections called chorionic villi. The chorionic villi contain fetal blood vessels.

The chorionic villi nearest the connecting stalk of the developing embryo are the most vascular and contain mesoderm. This area is called the chorion frondosum. The cells in the chorion frondosum proliferate and become the placenta. The connecting stalk becomes the umbilical cord. Placental development is usually complete by 10 weeks gestation.


Development of the Lacunae

Trophoblast invasion of the endometrium sends signals to the spiral arteries in that area, reducing their vascular resistance and making them more fragile. The blood flow to these arteries increases, and eventually they break down, leaving pools of blood called lacunae (lakes). Maternal blood flows from the uterine arteries, into these lacunae, and back out through the uterine veinsLacunae form at around 20 weeks gestation. 

These lacunae surround the chorionic villi, separated by the placental membrane. Oxygen, carbon dioxide and other substances can diffuse across the placental membrane between the maternal and fetal blood. 

When the process of forming lacunae is inadequate, the woman can develop pre-eclampsia. Pre-eclampsia is caused by high vascular resistance in the spiral arteries. High vascular resistance in the spiral arteries results in a sharp rise in maternal blood pressure, and leads to a number of complications in the mother and fetus. 


Last updated May 2020
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