Chapter I

Abstract

Endocrinology could be defined in the broadest context as a field dealing with chemical integrations in individuals. The integrating substances may act within the same cell they originate from, in the neighborhood of this cell passing through the cellular membranes from one cell to the other, or in some remote cells where the substance is transported by blood or by another extracellular fluid. The exchange of substances between adjacent cells represents, phylogenetically, the oldest system of intercellular communication. All the cellular functions are strictly genetically determined. At the beginning of ontogenic development, after fertilization of a highly specialized cell known as the oocyte, all the genes are in a repressed stage. During the oocyte cleavage, only some basic genes involved in the mitosis are operating, and most of the genes are repressed. The differentiation is a process based on the differential switching on and off of genes. The early differentiation (during blastomere formation) is influenced by intracellular cytoplasmic gradients and intercellular contacts. Later on, organizing centers develop; organizers are released and mor­phogenetic movements take place producing the basic embryonal layers: ectoderm, endoderm, mesoderm and notochord. Organizers of a higher degree (secondary, tertiary, etc.), inducing and repressing substances are involved in the differentiation of the primordia of different organs. Differentiation based on specific genes is related to the synthesis of specific peptides, proteins, glycoproteins or lipids and to their recognition. In morphological and physical terms, differentiation means the build-up of complicated systems of membranes keeping the chemical substances apart to prevent them from mixing freely.