Regional biosynthetic markers in the early amphibian embryo

Abstract

A search has been made for regional differences in macromolecular synthesis in the axolotl embryo at a stage when the principal regions have become determined but terminal differentiation has not yet begun. The epidermis of the neurula makes a number of abundant proteins which are not made elsewhere. Some of these are identified by immunoprecipitation as cytokeratins (relative molecular masses (Mr) 62,59,54,51 and 46×103). At the same stage a network of tonofilaments becomes visible by electron microscopy and is also confined to the epidermis. There is no significant incorporation of 3H-sugars into glycoproteins before neurulation. During neurulation specific species begin to be made by the notochord (Mr 265 and 185 ×103), the mesoderm (315×103) and the epidermis (170×103). The latter species is water soluble, has a native Mr of 470×103, is somewhat resistant to trypsin and has a high sugar content. Since these characteristics do not relate to any known glycoprotein it has been given a new name: epimucin. Several neutral glycolipids and gangliosides are present in the early embryo, the most abundant of which is identified as galactocerebroside. Synthesis occurs from fertilization onwards, however even by the neurula stage the qualitative pattern is the same in all parts except for the epidermis which shows two substantial enhancements and one reduction compared to other regions. The differences reported probably relate to physiological functions rather than to the mechanism of determination itself, so their chief importance is as markers of early embryo regions in experiments on induction.