Changes in Historic Patterns during Amphibian Embryonic Development

Abstract

As gastrulation procedes in the amphibian embryo, the pattern of differentiation for the major organ systems becomes irreversibly determined, yet their histological and physiological characteristics will not be apparent until later developmental stages. The expression of differentiation can be viewed as activation of certain genes, and, presumably, the repression of others. Since specific histones appear to play a key role in chromosome structure, it is possible that histones are involved in the process of embryonic differentiation. The present study demonstrates that embryonic histone patterns differ markedly from those of adult tissue and progressively change to the adult pattern during embryogenesis. Histones isolated from the Rana pipiens embryo were identified and characterized by analysis of acid-soluble crude chromatin protein on low pH-urea and SDS polyacrylamide gels and by incorporation of lysine but not tryptophan. Analysis of the acid-soluble crude chromatin protein of the gastrula revealed a deficiency in the slightly lysine-rich histones H2A and H2B and an enrichment of the arginine-rich histone H4. Between the gastrula and tadpole stage, the histone profile progressively changed to the adult histone pattern. Analysis of histone from a mixture of gastrulae and tadpoles showed that the presence of the gastrula did not interfere with isolation of the usual quantity of tadpole H2A and H2B histone, and is evidence against significant proteolytic degradation during isolation of gastrula histone. Incomplete extraction of gastrula H2A and H2B was ruled out by 8 M urea-2 M LiCl extraction of gastrula crude chromatin which failed to yield additional H2A and H2B histone. Analysis of supernatant fractions discarded during isolation of gastrula crude chromatin failed to reveal the presence of H2A and H2B histones. It is therefore unlikely that these histones are selectively lost during the isolation of gastrula crude chromatin. The unusual pattern of histones present in the early amphibian embryo may indicate that the highly conserved histone-DNA structure of the repeating chromosomal subunit (nucleosome) found in differentiated cells may be altered in the early embryo.