A coenocyte (from Greek: κοινός (koinos) = "common" + κύτος (kytos) = "box, i.e. cell") is a multinucleate cell which can result from multiple nuclear divisions without their accompanying cytokinesis, in contrast to a syncytium which results from cellular aggregation followed by dissolution of the cell membranes inside the mass. The word syncytium in animal embryology is used to refer to the coenocytic blastoderm of invertebrates.
Physiological examples 
Filamentous fungi may contain multiple nuclei in a coenocytic mycelium. A coenocyte functions as a single coordinated unit composed of multiple cells linked structurally and functionally, i.e. through gap junctions. Fungal mycelia in which hyphae lack septa are known as "aseptate" or "coenocytic".
Coenocytic cells are present in diverse and unrelated groups of algae, including Xanthophyceae (e.g., Vaucheria), red algae (e.g., Griffithsia) and green algae. The green alga Chara has internodal cells that are coenocytic. The green algal order Cladophorales is characterized by multicellular thalli composed of coenocytic cells. In the siphonous green algae Bryopsidales and some Dasycladales the entire thallus is a single multinucleate cell, which can be many meters across (e.g. Caulerpa). In contrast to the Cladophorales where nuclei are organized in regularly spaced cytoplasmic domains, the cytoplasm of Bryopsidales exhibits streaming, enabling transportation of organelles, transcripts and nutrients across the plant. 
Slime molds 
Sporozoa and Cnidospora 
Many insects, such as the model organism Drosophila melanogaster, lay eggs that initially develop as "syncytial" blastoderms, i.e. early on the embryos exhibit incomplete cell division. The nuclei undergo S-phase (DNA replication) and sister chromatids get pulled apart and re-assembled into nuclei containing full sets of homologous chromosomes, but cytokinesis does not occur. Thus, the nuclei multiply in a common cytoplasmic space.
The early embryo "syncytium" of invertebrates such as Drosophila is important for "syncytial" specification of cell differentiation. The egg cell cytoplasm contains localized mRNA molecules such as those that encode the transcription factors Bicoid and Nanos. Bicoid protein is expressed in a gradient that extends from the anterior end of the early embryo, whereas Nanos protein is concentrated at the posterior end. At first, the nuclei of the early embryo rapidly and synchronously divide in the "syncytial" blastoderm and then migrate through the cytoplasm and position themselves in a monolayer around the periphery, leaving only a small number of nuclei in the center of the egg, which will become yolk nuclei. The position of the nuclei along the embryonic axes determines the relative exposure of different amounts of Bicoid, Nanos, and other morphogens. Those nuclei with more Bicoid will activate genes that promote differentiation of cells into head and thorax structures. Nuclei exposed to more Nanos will activate genes responsible for differentiation of posterior regions, such as the abdomen and germ cells. The same principles hold true for the specification of the dorso-ventral axis – higher concentration of nuclear Dorsal protein on the ventral side of the egg specify the ventral fate, whereas absence thereof allows dorsal fates. After the nuclei are positioned in a monolayer underneath the egg membrane, the membrane begins to slowly invaginate, thus separating the nuclei into cellular compartments; during this period, the egg is called a cellular blastoderm. The pole cells – the germline anlage – are the first cells to separate fully.
Pathological examples 
Certain mutations and the activation of certain cell-cycle control genes can lead to bacteria forming "filament-like" cells with multiple chromosomes but without cellular division. These mechanisms or mistakes may lead to a similar structure to a coenocyte, though bacteria do not possess nuclei. This fact has been used in certain synthetic biology applications, for example to create fibers for an organically grown concrete.
See also 
- Daubenmire, R. F. (1936). "The Use of the Terms Coenocyte and Syncytium in Biology". Science 84 (2189): 533–534. doi:10.1126/science.84.2189.533. PMID 17806554.
- Willmer, P. G. (1990). Invertebrate Relationships : Patterns in Animal Evolution. Cambridge University Press, Cambridge.
- Mine, I.; Menzel, D.; Okuda, K. (2008). "Morphogenesis in giant-celled algae". Int. Rev. Cell Mol. Biol. 266: 37–83. doi:10.1016/S1937-6448(07)66002-X.
- Leliaert, F.; Smith, D.R.; Moreau, H.; Herron, M. D.; Verbruggen, H.; Delwiche, C.F.; De Clerck, O. (2012). "Phylogeny and Molecular Evolution of the Green Algae". Crit. Rev. Plant Sci. 31: 1–46. doi:10.1080/07352689.2011.615705.