Learning aims

At the end of this module the student should be able to:

• name the fetal membranes and cavities together with their components and functions
• distinguish between the maternal and fetal parts of the placenta
• describe the macroscopic morphology of the placenta
• explain the development of the placental structures during pregnancy and their influence on the physiologic functions of the placenta
• name the structural and functional characteristics of the fetal blood circulation and the properties of the hemato-placental barrier
• list the endocrine functions of the placenta
• describe the peculiarities of twin pregnancies
• name the pathologies of embryonic development (ectopic pregnancy, hydatid mole, fetal erythroblastosis) in connection with the fetal membranes

What you should already know

• Preimplantation
• Implantation

Delving deeper

• When does the differentiation of the embryonic appendages begin?
• How is the 9 month long differentiation of the uterine mucosa triggered and maintained in order to permit the formation of the placenta?
• How does the structure of the placenta adapt to the evolution of the metabolic needs of the embryo, which initially is only a few thousand cells and, at birth, weighs several kilograms?
• How does the selectivity of the exchanges between the maternal and embryo-fetal blood circulation occur? Concept of the placental barrier.
• What happens with the embryonic covering and the placenta in multiple pregnancies?
• Which tissue can be used for prenatal diagnostics and at what point during the pregnancy can it be obtained?

Introduction

The human placenta is a transitional organ, a mediator between the mother and the fetus for physiologic exchange processes. It is genetically programmed to last for 9 months. Since it consists of maternal and fetal parts, its cells are of two different genotypes. This biologic situation has important immunologic consequences, since the feto-placental complex can be seen as a natural, allogenic transplant that is resistant to rejection.

Even though the maternal and fetal blood circulation systems are very close to each other in the placenta, they remain separated by tissue layers. This delimitation is termed the placental barrier. Oxygen and nutrients pass from maternal into fetal blood, and carbon dioxide and a multiplicity of metabolic waste products of the fetus are delivered into maternal blood. During pregnancy, this important exchange interface serves as the fetal lungs, kidneys and intestines. In addition, the placenta takes on an important role as an endocrine gland, which steers the hormone secretions of the hypothalamus, anterior hypophysis and ovaries of mother and child. The placenta is thus functionally autonomous and during pregnancy it takes over important regulatory functions.