Embryogenesis

Embryogenesis is the process of cellular differentiation which leads to the development of an embryo.

Table of contents

1 Development of an embryo

1.1 From one cell to blastocyst
1.2 Blastocyst grows and invades
1.3 Differentiation of the inner cell mass (embryoblast)

Development of an embryo

From one cell to blastocyst

A whole human (or any other animal) starts as one single cell. This cell an oocyte (egg), that has been fertilised by a single sperm. The fertilised egg is called a zygote, and is surrounded by a strong membrane called the zona pellucida that the successful sperm managed to penetrate through.

The zygote divides by mitosis, often called cleavage, and the number of cells keeps doubling within the zona pellucida. When there are about 4-16 cells this is called the morula.

Cells keep dividing and a small cavity (blastocoele) forms. The zona pellucida is still present but is degenerating. This stage is the blastocyst.

Blastocyst grows and invades

In the blastocyst this cavity enlarges and two types of cells are apparent: the trophoblast and the inner cell mass (or embryoblast). The trophoblast can be thought of as a mostly single layer of cells forming a ball, with the inner cell mass inside this 'ball' attached to the inner wall. By this stage the conceptus (the embryo plus associated membranes) will be in the uterus and going to implant. The zona pellucida ultimately disappears.

The trophoblast then differentiates into two distinct layers. An inner cytotrophoblast, cuboidal cells that are the source of dividing cells, and the outer syncytiotrophoblast.

It is the syncytiotrophoblast that helps the blastocyst implant in the epithelium of the uterus. It forms finger-like projections that make their way into the uterus. This is assisted by hydrolytic enzymes that erode the epithelium. The syncytiotrophoblast also produces hCG, a hormone that 'notifies' the mother's body (though not necessarily the mother!) that she's pregnant.

The invading syncytiotrophoblast forms villi (the finger-like projections) and lacunae (spaces that will fill up with the mother's blood). The villi will branch as development continues, and will contain blood vessels (of the fetus) that allow gas exchange between mother and child.

Differentiation of the inner cell mass (embryoblast)

While the syncytiotrophoblast is starting to penetrate into the wall of the uterus, the inner cell mass (embryoblast) also develops.

The embryoblast forms a bilaminar (two layered) embryo, composed of the epiblast and the hypoblast. The epiblast is adjacent to the trophoblast and made of columnar cells, the hypoblast is closest to the blastocyst cavity, and made of cuboidal cells.

By separating from the trophoblast, the epiblast forms a new cavity, the amniotic cavity. This is lined by the amnionic membrane, with cells that come from the epiblast. The hypoblast forms the exocoelomic cavity, which will become the yolk sac. The primative yolk sac is lined by Heuser's membrane (also called the exocoelic membrane), and the cells forming this membrane come from the hypoblast.

The primative (or primary) yolk sac shrinks away from the cytotrophoblast. This creates a new cavity, called the chorionic cavity or extraembryonic coelom. The separated yolk sac is now called the secondary or definitive yolk sac.

The chorionic cavity fills up with fluid, and becomes the largest cavity in the developing conceptus.