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UNSW Embryology

Week 2 - Early Placentation

© Dr Mark Hill (2008)

Acknowledgements

Introduction

In week 2, the trophoblast shell cells proliferate and form a syncitiotrophoblast and cytotrophoblast layer around he conceptus. Syncitiotrophoblast cells migrate into the uterine wall, forming maternal blood-filled spaces (lacunae).

Placentation begins once the conceptus begins to implant in the uterine wall and the placenta will have both a fetal and a maternal component. The fetal component begins as villi form. The fetal chorion will form two regions: smooth chorion (chorion laeve) and villous chorion (chorion frondosum).

The maternal component is formed by the decidualization of the endometrium. (More? Placenta)

Primary Villi

Primary Villi are first formed by syncitiotrophoblast and cytotrophoblast cells.

Secondary Villi

Secondary Villi are first formed when primary villi are invaded by extraembryonic mesoderm.

Tertiary Villi

Tertiary Villi are first formed when secondary villi extraembryonic mesoderm differentiate to form blood vessels. These are the fetal blood vessels that will be the site of exchange within the placenta.

Maternal Decidua

The maternal uterine endometrium stromal cells (fibroblast-like) are transformed by steroid hormones (progesterone) and embryonic signals into the decidua.

The entire maternal decidua is divided into three regions: decidua basalis, decidua capsularis and decidua parietalis.

These 3 regions are named by their positional relationship to the conceptus.

Quick Links

Week 2 Pages:

Introduction | Timeline Week 2 | Carnegie stages 5 to 7 | Abnormalities | Blastocyst Implantation | Sites of Implantation | Bilaminar Embryo | Embryonic Cavitites | Early Placentation | Molecular | Stem Cells | References | Text only | WWW Links | Movies - Week 2

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G