10.7 The various placenta types

• Introduction
• Outer form
• Feto-maternal interdigitation
• Feto-maternal separation layers

Introduction

The development of the human placenta can be better understood by comparison with other placental forms that are encountered in mammals.
One can classify the allanto-chorial placenta according to various criteria:

• the outer form
• the type of connection = interdigitation between the maternal and embryonic tissues
• the type and number of the layers that separate the maternal and embryonic bloods
• the geometrical relationship of the maternal and embryonic blood streams with each other in the placenta. (This will not be discussed in this chapter.)

Outer form

Fig. 53
In the human placenta the exchange contact surface is restricted to a discoid region.

Feto-maternal interdigitation

The limited contact surface between mother and child, as occurs with a discoid placenta, is compensated by an intensive interdigitation between the two surfaces. In humans one finds the villus placenta that is constructed out of a highly complex system of interdigital folds (septa). The septa are thin, long and branched. The section comprised between two septa is called a cotyledon. Since in a human placenta these septa do not divide up the placenta completely, one also speaks of the pseudocotyledon in the human placenta.

Fig. 54
The villi that make up the feto-
maternal interdigitation are incompletely separated in cotyledons. This is why this kind of placenta is termed a
pseudo-cotyledon placenta.

Feto-maternal separation layers

Fig. 55
In a hemo-chorial placenta the villi, which are bounded by the syncytium, swim in maternal blood.

For this reason one distinguishes among three types of placentas:

• Epithelio-chorial placenta (swine, horse)
• Endothelio-chorial placenta (dog, cat)
• Hemo-chorial placenta (human, rodents, primates)

On the fetal side fetal vessel endothelium, villus connective tissue, and trophoblasts are thus always present. On the maternal side these vary. Either the entire uterine epithelium or only the maternal vessel endothelium is intact or, as in humans, both are absent.