선린대 로고 선린대학교 물리치료과

1. Basic mechanism of Morphogenesis
   • A. Stage of morphogenesis. For convenience, morphogenesis can be divided in to three stage: Cytogenesis, Histogenesis, and Organogenesis
2. Embryonic development
   • The human embryonic disk and the embryo that develops from it show the sequence fo cytogenesis, histogenesis, and organogenesis and the formation of the basic cell configurations
3. Origin of the embryonic disk (cytogenesis to histogenesis)
   • A. The fertilized ovum multip lies to form a berry-like mass, a MORULA
   • B. The morula forms a fluid-filled ball, a BLASTOCYST
   • C. Cells at one pole of the blastocyst multiply to from a polar mass, inner cell mass

1. Cytogenesis
   • Formation of gametes by mitosis and meiosis, followed by fertilization
   • Cell multiplication at proper time and site
   • Cell differentiation into various parenchymal and supporting cell
   • Programmed death of certain cell population
2. Histogenesis
   • Orientation of cells to each other and their supporting tissues
   • Establishment of intercellular contacts (synapse)
   • Migration cell population
   • Arrangement of migrating, multiplying, and differentiating parenchymal and connective tissue cells, and blood vessels, into tissues
3. Organogenesis
   • Blending of tissues into organs
   • Shaping of the external contour of organs
   • Shaping of the internal contour of organs
   • Growth to complete the somatotype of the individual

A. Major organogenic events of the neuraxis (Central nervous system)

1. The fundamental event in organogenesis of the neuraxis is the rolling up of the monolayer of ectodermal cells to form the neural tube
2. The organogenic events consist of
   • Closure of the neural tube ( neurulation)
   • Transverse segmentation of the neural tube
   • evagination of the neural tube walls
   • Flexion of the neural tube
   • Protrusion of masses
   • Fissuring and Sulcation of the cerebrum and cerebellum
   • Growth to adult size of the part already formed

   Neural Plate →Neural groove →Neural folds →Neural tube

A. As the neural tube tube closes, multiplying cells in the periventricular zone produce Nreuroblast and Glioblast (Although most neuron and glia arise from the periventricular zone , some CNS cells derive from the neural crest)

Apolar Neuroblast →Bipolar Neuroblast →Unipolar Neuroblast →Multipolar Neuroblast → Muitipolar Neuron

1. Disposition of neuronal perikarya in gray matter
   • Neuclei, reticular formation, cortex in CNS. Ganglia, plexus, in the wall of vicera
2. Migration of neuroblast
   • In the CNS, neuroblast undergo mitosis in the periventricular matrix zone
   • They migrate further outward to the surface, where they form cortex
3. Disposition of nuclei
   • The nucleate arrangement extends from the caudal tip of the spinal cord through the brain stem, diencephalon, and basal nuclei to the basal region of the cerebrum
4. Disposition of cortex and its relationship to underlying nuclei
   • Cortex consist of layer of neuronal perikarya with layer of dendrites and axon. In the Cbr and Cbll, Neuroblast form the cortex by migrating to the surface. By contrast, Neuroblast that remain close to the ventricular lumina form nuclei. In Cbll, deep nuclei of the Cbll, form of the diencephalon and basal ganglia)

1. Three concentric layer of the spinal cord
   • The ependymal layer : monocallular epithelium surrounding the central canal
   • The mantle zone : become nucleated gray matter as neuroblast differentiate within it
   • The marginal zone : become the white matter as axons invade it from the dorsal root ganglia, the spinal cord nuclei, and the brainstem and cerebral cortex
2. Neural Crest : The neural crest consists of two longitudinal bands of cells with nodular thickening
   • Rostal (cranial) neural crest : adjacent to the brain
   • Caudal (spinal) neural crest : adjacent to the spinal cord
   • Neural derivatives of the neural crest
     • Sensory ganglia
     • Autonomic ganglia
     • Supporting cells

The spinal cord has 31pairs of spinal nerves. Each derives from the dorsal and ventral roots

1. Formation of the dorsal roots
   • the central branch pierces the dorsolateral aspect of the spinal cord, forming the dorsal roots
   • the peripheral branch extends to a receptor in the skin or vicera
2. Formation the ventral roots
   • Neuroblasts in the vental horn gray matter differentiate and produce axons that exit from the ventrolateral aspect the spinal cord
   • Two types of axons enter the ventral roots- axons destined for skeletal muscles and axon s destined for autonomic ganglia
     ( The axons going to skeletal muscles issue from the motoneurons in the ventral horn. They travel directly to the muscle without further synapses)

A. Division into sensory and motor axons

1. Since dorsal roots conduct sensory impulses to the CNS and ventral roots conduct motor impulse away from the CNS. The fact that the
2. The fact that the dorsal roots are afferent, or sensory, while the ventral roots are efferent, or motor, is called ( Law of Bell and Magendie )
   • The spinal cord gray matter reflects the division of function between dorsal and ventral roots
     • the anal plates or dorsal horns, are primarily sensory in function
     • the basal plates, or ventral horns, are primarily motor in function

B. Formation of somatic nerve plexuses

• the axon of the individual nerve trunks intermingle in the plexus, but each axon retains its own identify and does not anastomose with axons of another segment
• Three Plexus : Nerve trunk from three plexuss along the spinal cord -CERVICAL, BRACHIAL, LUMBOSACRAL

Jelly - roll hypothesis of myelin formation
The axons of the CNS and PNS initially grow out as naked cytoplasmic extension Many then receive a myelin sheath, while others remain unmyelinated.

• In the PNS
  • Schwan cell, derived from neural crest, from myelin sheach
• In the CNS
  • Oligodendroglial cells, derived from the neural ectodermal, from the myelin sheath