The Placenta & Twinning

Professor A. Cuschieri

Department of Anatomy

University of Malta

Objectives:

o Name the structures that form the placenta

o Distinguish between the foetal and maternal contributions to the placenta

o List the main functions of the placenta

o Distinguish between identical and fraternal twins and how they arise

o Explain how identical twins can share a common amnion and chorion

Introduction

When the blastocyst embeds itself in the maternal endometrium it is covered with chorionic villi derived from the syncitiotrophoblast and cytotrophoblast. The distribution and size of the villi are not uniform throughout the surface of the chorion.

The chorion frondosum consists of numerous villi over the embryonic pole. This will contribute to the formation of the placenta .

The chorion laeve contains very sparse villi over the abembryonic pole. The villi will eventually disappear, and here the chorionic membranes are formed.

The Placenta is derived from two sources:

• The foetal chorion

• The maternal deciduas