MEDICAL BIOLOGY, ANATOMY, HUMAN PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL BASES OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.3. Biology of Individual Development

Mechanisms of Growth and Morphogenesis

Embryogenesis is characterized by intensive growth and morphogenesis. Growth is based on Cell Division, which increases cell number, and an accumulation of organic mass, primarily Proteins. Morphogenesis is manifested in The formation of Tissues and Organs, enabling the Organism as a whole and its individual parts to acquire the shape typical of adult stages. Morphogenesis is an irreversible process that occurs only under specific conditions. The fundamental Mechanisms of Morphogenesis are cell determination and differentiation. Experiments have demonstrated that isolated blastomeres at early Cleavage stages are totipotent (equipotential), meaning each of them can develop into a complete organism. In many hydromedusae, Cells remain totipotent up to the 32-cell stage, in newts up to the 16-cell stage, and in rabbits up to the 2-blastomere stage. The existence of a similar blastomere totipotency in humans is evidenced by the birth of two, three, four, and even seven monozygotic twins.

At the blastula stage, embryonic cells lose their totipotency. Differentiation begins—a process whereby differences arise among cells originating from a relatively homogeneous embryonic material. During differentiation, cells of embryonic primordia transform into specific Structure/83.html">Structural elements of the adult organism. As shown above, the myotome gives rise to skeletal musculature, the dermate gives rise to the dermis of the Skin, and so forth. Consequently, the ultimate developmental outcome of individual embryonic primordia is predetermined. The process whereby cell development is specified and restricted to a single pathway is termed determination. Although cells lose totipotency at the blastula stage, they retain the capacity for redirection (transdetermination) regarding their subsequent developmental pathway. Experiments on amphibian embryos have shown that cross-transplantation of certain regions leads to A change in subsequent development in accordance with their new Location. Results of analogous transplantations at the gastrula stage indicate a loss of this cellular capacity for transdetermination. Thus, the labile determination of developmental pathways observed at early stages is replaced by stable determination. As the embryo progresses through successive stages of embryogenesis, a restriction or even loss of the potential of individual primordia to develop in multiple directions occurs, a phenomenon known as developmental canalization. Differentiation and determination proceed against the Background of integration. Integration refers to the unification and coordination of the structures and Functions of a multicellular organism during embryogenesis. At all stages, the embryo functions as a single, integral organism in which all processes are interrelated and mutually dependent.

Using Methods of experimental Embryology, researchers have precisely mapped the Regions of the blastula that give rise to germ layers during Gastrulation, followed by embryonic primordia. Presumptive fate maps of embryonic primordia have been compiled for many animals, demonstrating that determination exists as early as the fertilized egg stage.