Blastoderm

Blastoderm is an early embryonic structure in certain animals, referring to a single layer of cells that forms after fertilization during the cleavage stages. It represents the initial epithelial sheet from which the germ layers and much of the embryo will arise and marks a transition from a more primitive cellular product to a defined cellular layer.

In insects such as Drosophila, the zygote undergoes rapid nuclear divisions without cytokinesis, producing a multinucleate syncytial blastoderm. Nuclei migrate to the periphery, and subsequent cellularization partitions the cytoplasm around each nucleus to form a cellular blastoderm. This layer gives rise to the future ectoderm and establishes the framework for gastrulation.

In birds and other yolk-rich reptiles, the blastoderm is a small disc of cytoplasm on the yolk, known as the blastodisc. It consists of two layers—the epiblast, which contributes to the embryo proper, and the hypoblast, which contributes to extraembryonic membranes. Gastrulation begins within the blastoderm, notably through the formation and movement of a primitive streak.

In mammals, the term blastoderm is used less commonly. Early development is more often described in terms of a blastocyst containing an inner cell mass and trophoblast, though the concept of a bilaminar embryonic disc derived from the inner cell mass can be described in similar terms in certain contexts. Overall, the blastoderm represents an early, essential stage from which the body plan and germ layers are organized across different vertebrate and invertebrate lineages.