What are the stages of early embryonic development?

The stages of early embryonic development are fertilisation, cleavage, blastulation, gastrulation, and organogenesis.

Fertilisation is the initial stage of embryonic development. It involves the fusion of a sperm cell from the male and an egg cell from the female to form a single cell called a zygote. This process occurs in the fallopian tubes of the female reproductive system. The zygote contains all the genetic information necessary for the development of a new individual.

Following fertilisation, the zygote undergoes a series of rapid cell divisions in a process known as cleavage. This results in a multicellular structure called a morula, which is a solid ball of cells. Cleavage is a unique type of cell division because it does not increase the size of the embryo, but rather increases the number of cells.

The next stage is blastulation, where the morula develops into a hollow ball of cells known as a blastocyst. The blastocyst consists of an outer layer of cells, the trophoblast, and an inner cell mass. The trophoblast will eventually form the placenta, while the inner cell mass will develop into the embryo.

Gastrulation follows blastulation. During this stage, the cells of the blastocyst rearrange themselves to form three distinct layers: the ectoderm, mesoderm, and endoderm. These three layers are known as germ layers and each will give rise to different parts of the developing organism. For example, the ectoderm will form the skin and nervous system, the mesoderm will form muscles, bones and the circulatory system, and the endoderm will form the digestive and respiratory systems.

The final stage of early embryonic development is organogenesis. This is when the germ layers develop into the various organs and structures of the body. This process involves cell differentiation, where cells become specialised to perform specific functions. For example, some cells will become nerve cells, while others will become muscle cells. This stage continues until the embryo develops into a fully formed organism.