142 



EMBRYOLOGY 



Fig. 82. The relationship of the 

 developing limb to the neuroblasts 

 which form the limb nerves. The 

 neuroblasts are shown as small, 

 rounded cells in the neural tube and 

 in the neural crest. A broken line 

 indicates migration of neuroblasts 

 from the neural crest to the region 

 where the sympathetic ganglia will 

 form. The arrows connecting the 

 neural crest with the spinal cord 

 show the path of the developing 

 nerves. Similarly the arrows from 

 the neural crest to the limb indicate 

 the pathway of the limb nerves. 

 What directs or induces the nerves 

 to take these paths ? 



log oo%\ 

 SYMPATHETIC NEUROBLASTS 



neuroblasts, secondarily derived from the neural crests. Finally, there are 

 some neuroblasts which originate in the epidermis of the head region itself. 

 These neuroblasts give rise to the olfactory nerve, which has its origin 

 outside of the brain and then grows back into it. 



Consider now the developing limb which receives nerves from the neural 

 tube and neural crests (Fig. 82). The developing limb is essentially a very 

 rapidly growing bud of mesoderm which differentiates into the muscles and 

 bone of the limb. The problem is to determine the factors which bring about 

 the growth of nerves between the neural crests and the limb, between the 

 neural crest and the spinal cord, and between the ventral region of the spinal 

 cord and the limb. This nervous circuit, comprising the simplest type of 

 reflex arc, must be completed if the limb is to function. 



We shall begin by determining, first of all, how a nerve fiber develops. 

 In the human adult, nerve fibers may be as much as two or three feet long — 

 a tremendous length for a single cell. How do we derive these long fibers 

 from neuroblasts only a fraction of a millimeter in diameter? This problem 

 was solved by isolating neuroblasts and watching their development under 

 the microscope. If we isolate a small piece of the neural tube from a young 

 embryo in which the nerves have not yet developed, and transplant it to a 

 blood clot and seal the preparation off in a moist chamber, then we can follow 

 the development of the neuroblasts (Fig. 83). The first thing that happens 

 is that the isolated mass of tissue begins to spread out; the cells migrate and 

 form a very thin layer of cells, so that we can watch each individual cell. 

 Migration is usually the first activity observed in tissue cultures. 



