Embryonic development

Embryonic development or embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers primarily to the earliest stages of prenatal development, while the terms fetus and fetal development describe later stages.

Embryogenesis begins with the fertilization of the ovum (ovum) by a sperm cell (sperm). Once fertilized, the egg is known as a zygote, a single diploid cell. The zygote undergoes mitotic divisions without significant growth (a process known as cleavage) and cell differentiation, leading to the development of a multicellular embryo.

Although embryogenesis occurs in both animal and plant development, this article addresses common features among different animals, with some emphasis on vertebrate and mammalian embryonic development.

The ovum is generally asymmetrical, with an “animal pole” (future ectoderm and mesoderm) and a “vegetable pole” (future endoderm). It is covered with protective envelopes, with different layers. The first shell – the one in contact with the egg membrane – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). Different taxa show different cellular and acellular envelopes enclosing the vitelline membrane.

Fertilization (also known as ‘conception’, ‘fertilisation’ and ‘syngamy’) is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an egg cell, eventually leading to the development of an embryo. Depending on the animal species, the process can occur inside the female’s body in internal fertilization, or outside in the case of external fertilization. The fertilized egg is known as the zygote.

At some point after the different layers of the embryo are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds to form the neural tube (see above). Other common organs or structures that arise at this time include the heart and somites (also above), but as of now embryogenesis does not follow a common pattern among the different taxa of the animal kingdom.

In most animals organogenesis together with morphogenesis will result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.