Fertilization and Embryology

Embryology is the study of how an organism develops from a fertilized egg into a fully formed individual. In humans, development begins with fertilization, the union of a sperm cell and an egg, or ovum. The fusion of these two cells creates a single cell called the zygote, which carries a complete set of genetic information drawn from both parents and contains all the instructions needed to build a new organism.

The zygote soon begins to divide, splitting into two cells, then four, and so on, in a process called cleavage. After repeated divisions it forms a compact ball of cells known as the morula. As division continues, a fluid-filled cavity appears inside, and the structure becomes a blastocyst, which has an outer layer of cells and an inner cell mass. The blastocyst travels to the uterus and implants in its lining, establishing the connection through which the developing organism will draw nourishment.

After implantation the developing human is called an embryo. During the embryonic period the basic body plan is laid down and the major organs and tissues begin to form from a small number of early cell layers. Once the principal structures are established, the developing organism is termed a fetus, and the remaining time before birth is largely a period of growth and maturation. Embryology is closely linked to the reproductive system and to the study of stem cells, the early, unspecialised cells that can give rise to the many specialised cell types of the body.

Frequently asked questions

What is a zygote?

A zygote is the single cell formed when a sperm fertilizes an egg. It contains a full set of genes from both parents and is the starting point of development.

What is the difference between a morula and a blastocyst?

A morula is a solid ball of cells produced by early division of the zygote. As division continues a fluid-filled cavity forms, turning it into a blastocyst.

When does an embryo become a fetus?

The developing human is called an embryo while its major organs are forming, and a fetus once the principal structures are established and growth and maturation dominate.