Placenta Development

The embryo enters the uterus 4 days after fertilisation and transitions from a morula to a blastocyst.

– It then undergoes hatching where the outer zona pellucida is removed allowing it to attach to the endometrium.

– It undergoes invasive interstitial implantation, where the cells of the blastocyst invade into the uterine lining.

– This invasion, and the merging of the syncitiotrophoblasts and endometrial cells allows the placenta to form. 

The placenta is a discoid organ which is composed of two plates:

i.) Chorionic plate –> faces the foetus and has the umbilical cord attached

ii.) Basal plate –> opposed to the decidua basalis and maternal blood enters through it

The placenta is formed in a number of steps, which are key to its role in achieving efficient nutrient exchange

i) Trophoblast differentiation:

The trophoblast (outer layer of blastocyst) differentiates into the:

– Syncitiotrophoblast (STB) –> outer multinucleated layer

– Cytotrophoblast (CTB) –> inner deeper layer

– The Syncitiotrophoblast is non-proliferative and is generated

by continual fusion of the Cytotrophoblast cells

ii) Erosion of decidua:

The Syncitiotrophoblast erodes into the decidua (endometrium)

– This breaks the endometrial glands and superficial capillaries

– Spaces appear within the STB and coalesce to form lacunae

– Glandular secretions and maternal blood fill the lacunae.

iii) Early villus formation:

Cytotrophoblast cells and extra-embryonic mesoderm from the embryo penetrate into the trabeculae of the Syncitiotrophoblast between the lacunae

– This forms the earliest placental villi

– A vascular network develops in the mesoderm

– This connects back to the foetus via the connecting stalk

iv) Formation of villous tree:

Side branches extend from the early villi into the lacunae

– They gradually branch to become more complex

– Repeated branching forms the placental villous tree

– The lacunae are then referred to as the intervillous space

 v) Regression of villi:

In early pregnancy, placental villi form over the entire chorionic sac

– Later however, the villi regress over the superficial pole to leave the discoid placenta. (i.e., they only remain in one pole, forming the placenta)

– The remainder of the sac forms the placental membranes (these rupture to a provide a route of exit at birth)

vi) Remodelling of circulation:

The general anatomical organisation of the placenta is achieved by 3-4 weeks

– However, it is not fully functional until maternal circulation has been remodelled which occurs by 10-12 weeks.

– CTB cells, known as extravillous trophoblasts (EVTs),  invade the maternal spiral arteries which supply the endometrium 

– The EVTs replace the endothelium of the arteries,  resulting in vessel dilation and loss of vasoreactivity 

– This results in blood flow to the intervillous spaces that is low pressure and low velocity

– Failure to convert the spiral arteries is associated  with complications like growth restriction and pre-eclampsia

Umbilical cord

The umbilical cord is formed from the connecting stalk, which connects the blastocyst to the placenta.

– It is made of 3 blood vessels that carry foetal blood:

i) 2 umbilical arteries:

– These are branches from the internal iliac arteries

– Carry deoxygenated blood from the fetus to the placenta

ii) 1 umbilical vein:

– This joins the inferior vena cava via the ductus venosus

– Carries oxygenated blood from the placenta to the fetus

Amniotic fluid

This describes the protective fluid within the amniotic sac that cushions the fetus and serves as a transport medium for nutrients and metabolites.

– It is formed by maternal plasma diffusing through placenta and foetal urine

– It is around 850–1500 mL by the end of pregnancy

– The amniotic fluid is completely exchanged every 3 hours

– It is drained by the fetus (swallows fluid) + reabsorbed by umbilical cord into maternal circulation

– It contains proteins, glucose, urea, hair, dead skin, sebum and foetal urine