An electron microscopical study of developing sympathetic neurons in man

Summary An electron microscopic study has been made of the sympathetic ganglia of a 15 and a 17 week old male human fetus. The fetal sympathetic neurons were densely packed in a scanty connective tissue matrix which also contained blood vessels. The fetal sympathetic neurons had a large, electron-light nucleus with one or two nucleoli, and was of a somewhat mottled appearance due to irregularly dispersed aggregates of fine and coarse granules. The perikaryon usually formed a thin envelope around the nucleus and contained, except for large pigment granules, all intracytoplasmic structures which were also found in mature sympathetic neurons. Adjacent sympathetic cells were either in immediate contact, or slightly separated by a wedge of electron-light satellite expansions, or lined by primitive axons. The satellite cells were in the early state of development. Electron-dense axons either stood side by side with, or were slightly engulfed by light Schwann cell expansions and formed distinct bundles surrounded by a common basement membrane. There was practically no trace of myelin formation or Schwann cell wrapping characteristic for unmyelinated fibers as seen in the adult.This investigation was supported (in whole) by United States Public Health Service Grant NB-01879-05, Institute for Nervous Diseases and Blindness.Grateful acknowledgment is made to Professor Dr. John Lind who madea vailable the fetal material through the Laboratory of Prenatal Growth and Development, Karolinska Institutet, Stockholm, Sweden.The authors wish to thank Docent Dr. Gunnar Bloom who provided the facilities necessary to prepare the fetal material for electron microscopical examination, in his laboratory for Cell Research, Karolinska Institutet, Stockholm, Sweden.