The three villus types

Introduction

While the embryo is nourished in the first weeks through simple diffusion, later, due to its rapid growth, it needs a more powerful gas and nutrient exchange system. This is made possible by the development of the utero-placental circulation system in which the circulation systems of the mother and of the embryo get closer together, thus allowing an exchange of gases and metabolites via diffusion.
It must be always kept in mind, though, that maternal and fetal blood never come into direct contact with each other.

This system decays after the ninth day in the lacunar stage (stage 5b).

Through the lytic activity of the syncytiotrophoblast (Fig. 18 and 19) the maternal capillaries are eroded and anastomose with the trophoblast lacunae, forming the sinusoids. At the end of the pregnancy the lacunae communicate with each other and form a single, connected system that is delimited by the syncytiotrophoblast and is termed the intervillous space.

Between the 11th and 13th day cytotrophoblast cells penetrate into the cords of the syncytiotrophoblast creating the primary trophoblast villi (stage 5b).

After the 16th day the extra-embryonic mesoblast also grows into this primary trophoblast villus, which is now called a secondary villus (stage 5c) and expands into the lacunae that are filled with maternal blood. As was already mentioned, the ST forms the outermost layer of every villus.

At the end of the 3rd week the villus mesoblast differentiates into connective tissue and blood vessels. They connect up with the embryonic blood vessels. Villi that contain differentiated blood vessels are called tertiary villi (stage 6).

From this time on gases, nutrients, and waste products that diffuse through the maternal and fetal blood must pass through a total of four layers:

• Capillary endothelium of the villus
• Loose connective tissue that surrounds the endothelium
• Cytotrophoblast
• Syncytiotrophoblast

These four elements together form the placental barrier.

Note! The endothelium that surrounds the maternal blood vessels never penetrates into the trophoblast lacunae, but comes just to their boundaries.
Numerous "daughter" villi arise out of the tertiary villi. These remain either free and project into the intervillous space (free villi), or they anchor themselves to the basal plate (anchoring villi). (Interactive diagram)

After the 4th month the cytotrophoblast in the tertiary villi disappear slowly, the villi divide further and become very thin, whereby the distance between the intervillous space with maternal blood and the fetal vessels gets smaller. The villi that arise in this way are called free villi.