Pre-natal nutrition, placenta and foetal membranes (7.3)

Question 1

Describe the process and timescale of implantation, and outline the normal and abnormal sites of blastocyst implantation.

Answer 1

Normal site: Endometrial lining of the uterine body

Abnormal sites of implantation: Abdomen, fallopian tube, cervix, vagina, uterine fundus

Question 2

Describe the processes involved in formation of the placenta and be able to explain / sketch the structure of the placental membrane between maternal and fetal blood.

Answer 2

Syncytiotrophoblast mediated invasion of the endometrial lining of the uterus. Maternal blood vessels of the uterus are invaded, leading to the filling of trophoblast-derived lacunae with blood - allows for cotyledon formation.

Formation of primary (cytotrophoblast grows into the syncytiotrophoblast), secondary (extraembryonic mesoderm grows into the villi) and tertiary (blood vessels develop in the extraembryonic mesoderm) chorionic villi.

The syncytiotrophoblast secretes hCG to allow maintenance of the corpus luteum until the placenta is established (well formed by week 12, assumes role at week 14).

Formation of the embryonic amnion and maternal chorion - amniochorionic membrane.

Question 3

Describe the structure and functions of the mature placenta and consider how these functions may be compromised.

Answer 3

Chorion: Formed from cyto- and syncytiotrophoblasts and the extraembryonic mesoderm

Amnion: Formed from the extraembryonic mesoderm and ectoderm

A potential space (chorionic cavity) is seen between the membranes initially. As the embryo grows the membrane fuse ans the chorionic cavity is obliterated.

The uterine cavity is also obliterated eventually - with the decidua capsularis becoming continuous with the parietalis.

Mechanisms of compromise:

• Pre-eclampsia
• PPROM
• Placenta praevia
• Placenta accreta

Question 4

Explain the position of the 3 main decidual layers of the endometrium.

Answer 4

Basalis: Seen beneath the developing foetus

Capsularis: Surrounding the developing foetus (adjacent to the foetal membranes)

Paretarlis: Seen around the remainder of the uterus

Question 5

Describe the fetal membranes, their origin, and how their arrangement changes during embryonic disc folding and fetal growth.

Answer 5

Origin:

Amnion: Trophoblast (cyto- and syncytio-) and extraembryonic mesoderm

Chorion: Extraembryonic mesoderm and ectoderm

Changes seen during embryonic disc folding: -

Change seen during in fetal growth: The chorionic cavity (a potential space existing between the amnion and chorion) is obliterated as the foetus grows

Question 6

Outline the potential arrangements of placenta and fetal membranes in both dizygotic and monozygotic twins.

Answer 6

Question 7

Explain the basis of conjoined / parasitic twins.

Answer 7

In monozygotic twins failure of the embryonic disc to split can lead to conjoint twins

Question 8

Outline the functional the yolk sac in the early stages of pregnancy

Answer 8

• Initial blood cell synthesis (begins week 3 - blood islands form)
• Nutrient source in the first 3 weeks
• Primordial germ cell source

The vitelline duct allows continued communication of the yolk sac with the embryo, following its diminishment. The vitelline duct is usually obliterated. Persistence of the vitelline duct leads to Meckel’s diverticulum (mostly asymptomatic)