Spatial-temporal characteristics of intercellular junctions in early zebrafish and loach embryos before and during gastrulation.

Injections of lucifer yellow and fluorescein dyes into loach (Misgurnus fossilis) and zebrafish (Danio rerio) embryos were used to analyse the intercellular communication via gap junctions (GJs) and their role in morphogenetic processes during the period from early blastula to late gastrula. It is shown that the efficiency of dye transfer between the superficial blastomeres increases by the late blastula stage. Blastomeres of the basal layer, on the other hand, become gradually uncoupled from the yolk cell (YC). This process is spatially uneven and finishes by the late gastrula stage. Prior to it, at the early epiboly stage, a local increase in dye transfer is observed in the circular zone of the blastoderm margin. During gastrulation, GJ communication between blastomeres and the YC in this zone and also in the newly-formed germ ring region (the prospective mesoderm domain) persists for a longer period of time (up to the stage of 60-70% epiboly) than in the remaining part of the basal layer (the prospective ectoderm domain). Taking into account the data on changes in the adhesive properties of blastomeres during normal development and observations on embryos with retarded epiboly, we hypothesize that changes in GJ communication between superficial blastomeres, on one hand, and between basal blastomeres and the YC, on the other, are the consequences of the same, more general morphogenetic process of compaction occurring within the blastoderm, which supports epiboly and is probably responsible for the distinction between mesodermal and ectodermal fates of cells differently located within the forming epithelioid sheet.