Student Theses and Dissertations
Date of Award
1966
Document Type
Thesis
Degree Name
Doctor of Philosophy (PhD)
Thesis Advisor
Alfred Mirsky
Keywords
amphibian embryo, Xenopus laevis, RNA synthesis, early development, cytoplasmic factors, cell differentiation
Abstract
The amphibian embryo has been a fascinating object for study for a century or more. It is large in size, widely available, and easy to observe throughout embryogenesis. The control of early development and the role of the genes have also been investigated for a long time. The combination of older morphological and more recent biochemical descriptions of development under normal and experimental conditions are bringing us closer to an understanding of the mechanism of early differentiation. In the General Introduction to this thesis the role of the nucleus and cytoplasm in amphibian development are discussed. Other types of embryos are described when the evidence is clearer or more complete. The processes of cleavage and accompanying protein synthesis are independent of nuclear activity and therefore dependent on substances stored in the maternal cytoplasm. Nuclear function is first required for development beyond the late blastula stage. Gastrulation marks the beginning of rapid differentiation and diversification of cells to form the many tissues of the larval stage. The synthesis of messenger-like RNA is low during cleavage and increases from the late blastula or early gastrula stage. The establishment of different patterns of RNA synthesis represents the first step in cell differentiation. Since the nuclei of early embryonic cells are equivalent, extra-chromosomal factors must influence chromosomal activity. Prelocalization of cytoplasmic substances has been demonstrated in non-amphibian embryos. The formation of the grey crescent in the amphibian embryo is described. Experimental analysis of tissue differentiation has revealed that cytoplasmic factors have a radially diffuse localization in three main regions of the amphibian. The experimental section of this thesis describes the activation of RNA synthesis in the late blastula of Xenopus laevis embryos. A careful description of the onset of gene activity is fundamental to the understanding of the control of gene activity. An embryo half preparation is described which is permeable to the precursor uridine-H3. The time course of incorporation of uridine into total acid insoluble RNA shows a marked increase between the blastula and early gastrula stages. Autoradiographic studies demonstrated a sudden activation of nuclear RNA synthesis by the late blastula stage from an initially negligible level in the early blastula. Presumptive endoderm and mesoderm participate in this activation. RNA synthesis per nucleus remains constant through gastrulation. The ectoderm is activated at the mid gastrula stage. Analysis of phenol extracted RNA by sucrose gradient centrifugation and gel filtration on Sephadex G-100 demonstrates that 70% of the RNA synthesized by early gastrulae is a high molecular weight heterogeneous RNA. Its base composition approaches that of DNA. 30% of the label appears in transfer RNA. At the early blastula stage only a very small amount of heterogeneous RNA is synthesized.
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Recommended Citation
Bachvarova, Rosemary Faulkner, "Gene Activation in Early Amphibian Development" (1966). Student Theses and Dissertations. 563.
https://digitalcommons.rockefeller.edu/student_theses_and_dissertations/563
Comments
A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy