The process of formation of an embryo is called embryogenesis. Embryogenesis starts from a single embryogenic cell, that can be a zygote (the product of the fusion of an egg and a sperm during fertilization), or an undifferentiated callus cell. Embryos developing from zygotes are called zygotic embryos, while those derived from somatic cells are called somatic embryos. During the embryonic development, the polar axis of the plant is established, domains that set up the organization of the plant body are defined, and the primary tissue and organ systems are delineated. Somatic embryogenesis is another important way to regenerate new plants in plant tissue culture.
Embryo development occurs through an exceptionally organized sequence of cell division, enlargement and differentiation. Zygotic and somatic embryos share the same gross pattern of development. Both types of embryos develop as passing through typical developmental stages . Embryo development is bipolar, having a shoot pole and a radicular pole at opposite ends. Embryos are not organs because they are structurally independent from their parent body (i.e. they do not have a vascular system connecting them with their parent plant body).
The physical, observable transition from a nonembryogenic cell to an embryogenic cell in somatic embryogenesis appears to occur when the progenitor cell undergoes an unequal division, resulting in a larger vacuolate cell and a small, densely cytoplasmic (embryogenic) cell . The embryogenic cell then either continues to divide irregularly to form a proembryonal complex or divides in a highly organized manner to form a somatic embryo.
Somatic embryos usually do not mature properly. Instead, due to environmental factors such as constant contact with inducing medium, somatic embryos often deviate from the normal developmental pattern by bypassing embryo maturation producing callus, undergoing direct secondary embryogenesis and/or germinating precociously. Somatic embryos growing from proembryonal complexes tend to develop asynchronously so that several stages are present in culture at any given time.