Development

DevelopmentAnimal Implantation

As the zygote passes down the oviduct it divides by successive nuclear and cell divisions into a small ball of cells by a process called cleavage. Cleavage involves cells division without growth in size, because the cells continued to be retained within the zona pellucida. The cells just get smaller and smaller at this stage. Nuclear division is by mitosis. The cells formed are called blastomeres and they form a hollow ball of cells whose central cavity is called the blastocoel. This fills with liquid from the oviduct. The outer layer of blastomeres is called the trophoblast and this thickens at one point to form a mass of cells called the inner cell mass (ICM). This stage is called the blastocyst, about 100 cells and is reached about 4-5 days after fertilisation.

When the blastocyst arrives in the uterus the zona pellucida gradually disappears over about two days, allowing the cells of the trophoblast to make contact with the cells of the endometrium. The term tropho- means feeding and the trophoblast starts the process of invading the uterus wall and gaining nutrients from the endometrium. The trophoblast cells multiply in the presence of these nutrients and between the sixth and ninth days after fertilisation the blastocyst becomes embedded within the endometrium. The process is called implantation.

The cells of the trophoblast differentiate into an inner layer and an outer layer. The outer layer is called the chorion, and forms the chorionic villi, finger-like processes which grow into the endometrium. The areas of the endometrium between these villi form interconnecting spaces which give this region of the endometrium a spongy appearance. Hydrolytic enzymes released by the trophoblast cause the arterial and venous blood vessels in the endometrium to break down and blood from them fills the spaces/sinuses called lacunae. In the early stages of development of the blastocyst, exchange of nutrients, oxygen and excretory materials between the cells of the blastocyst and the maternal blood in the uterus wall occurs through the chorionic villi. Later in development his function is taken over by the placenta.

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