cellularization

Cellularization is a morphogenetic process in early embryogenesis by which a multinucleate cell, or syncytium, is partitioned into individual cells by the formation of plasma membrane. It is most extensively described in arthropods, particularly in Drosophila melanogaster, where development begins with rapid nuclear divisions without cytokinesis, creating a syncytial blastoderm. At a certain stage, cortical actin and other cytoskeletal components drive the inward growth of membrane furrows that envelope each nucleus.

These cellularization furrows extend from the periphery toward the center, progressively separating nuclei into distinct cells. The process relies on coordinated membrane trafficking, cytoskeletal remodeling, and, later, zygotic transcription to synthesize membrane components and polarity determinants. Once completed, a single layer of somatic cells surrounds a yolk-rich interior, forming a cellular blastoderm that will undergo gastrulation.

Although best studied in insects, cellularization-like partitioning occurs in other species with syncytial embryos, and the timing and mechanics can vary. In some organisms, membranes begin forming around nuclei after several rapid divisions, whereas in others, cellularization is incomplete or replaced by other morphogenetic strategies.

Significance: cellularization establishes basic cell boundaries and epithelial polarity, setting the stage for subsequent germ layer formation and body plan development. It is a model system for studying cytoskeletal dynamics, membrane trafficking, and the regulation of cleavage in early embryos.