Chapter 27. General Mechanisms of Normal Development - Review of Medical Embryology Book

Normal development involves growth and differentiation, both under very strict coordination and rigorous and precise organization. The processes that accomplish this development depend on 2 major overall controls

GENETIC CONTROL determines the inherent potential of the organism
EPIGENETIC CONTROL assures progressive formation of the primordia, then of the definitive organs, and includes a series of complex mechanisms all acting simultaneously or successively

Cellular movements are based entirely on migration and invagination of primary ectoderm cells: gastrulation is a good example; another is migration of sclerotome cells toward the notochord region to furnish material for the future spinal axis
Induction represents reciprocal influences between cellular groups. It is the essential determinant of embryonic development and is the process by which a cellular group, the inductor or organizer, influences or induces differentiation of another cellular group, the competent or induced tissue

The primary inductor or organizer: for a period of time in early development, certain embryonic tissue is capable of inducing the development of adjacent tissu The best-known inductive tissues are the primitive streak, the notochord, and paraxial mesoderm which act as primary organizers of the CNS; , development of the forebrain; and the notochord as an inductor of sclerotome which will form the vertebral primordia
Secondary induction: once the basic embryonic plan has been established by the primary organizers, a chain of secondary inductions takes plac Development of the eye and ear is an example of the forebrain reacting to the secondary induction of its adjacent mesenchyme
Nature of the inductor: induction has its effect by provoking cellular differentiation (at cellular level) or synthesis of a new type of protein (at the molecular level). Some substance, probably protein in nature, passes from the inducing tissue to the tissue being induced

When the sclerotome begins to form the cartilaginous matrix of the vertebral primordium, this cellular morphologic change is preceded by synthesis in the mesenchymal cells of the constituent proteins of cartilage

Regression: the notochord again serves as a prime example since it disappears or regresses almost entirely after having contributed to the formation of the vertebral column. It persists only at the level of the intervertebral disk
Regulation is shown by an egg reconstituting a harmonious whole when part of its substance is remove Monozygotic twins represent an apparent demonstration of the existence of the phenomenon of regulation in the human species