Embryonic development

Embryonic development is the sequence of changes by which a fertilized egg divides, organizes, and differentiates into an embryo with developing tissues and organs.

Cleavage (embryology): Rapid early cell divisions that partition an egg into smaller cells without substantially increasing its overall size. These divisions begin development by multiplying cells while the embryo remains enclosed within the egg’s original volume.
Fertilization: The union of sperm and egg that produces a zygote and combines their genetic material. It creates the single cell from which the developmental sequence begins.
Congenital disorder: A structural or functional condition that is present at birth and may arise during development. Errors in embryonic formation can produce disorders that are evident at birth.
Fetal development: Growth and maturation after the embryonic period, leading toward birth. It follows embryonic development, with growth and maturation replacing much early body-plan formation.
Developmental plasticity: The capacity of developing cells or organisms to produce different outcomes in response to conditions or signals. It raises questions about how reliably cell fates are fixed during embryonic development.
Gastrulation: A coordinated process that rearranges embryonic cells into germ layers and establishes the body plan. It converts an early cell assembly into an embryo with foundational tissue layers and axes.
Zygote: The diploid cell formed when an egg and sperm unite. The zygote is the starting cell that undergoes the earliest embryonic divisions.
Teratology: The study of abnormal development and the causes of congenital abnormalities. It examines how genetic and environmental disruptions alter embryonic development.
Metamorphosis: A post-embryonic transformation in body form through distinct life stages. Unlike embryonic development, metamorphosis reshapes an organism after it has hatched or been born.
Epigenetics: The study of heritable changes in gene activity that do not alter the DNA sequence. Epigenetic regulation may help explain how developmental states persist as cells specialize.

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