The development of the gracile nucleus in the rat: the time of ingrowth of ascending primary sensory fibres and effect of early deafferentation.
An investigation was carried out of the time of ingrowth of primary sensory fibres in the medulla and of their penetration into the gracile nucleus, and of the effect of an early loss of these fibres upon the development of the nucleus in rats. After injection of the conjugate horseradish peroxidase-wheat germ agglutinin in the hind limbs of fetuses, a bundle of labelled fibres was seen in close proximity of the gracile nucleus at embryonic day 17. However, fibres did not appear to leave the bundle until embryonic day 19, when they were seen to project ventrally and penetrate the nucleus which, on embryonic day 20 and thereafter, contained an increasing number of labelled fibres. Synaptic contacts within the gracile nucleus were found at all stages of the observation; the presynaptic processes consisted of an electron-lucent matrix which contained round vesicles. Although no mature glomeruli were observed, an occasional terminal appeared to be presynaptic to more than one process. After transection of the primary sensory afferents at embryonic day 18 and 19, no degeneration was seen within the gracile nucleus; degenerated boutons were occasionally seen after deafferentation at embryonic day 20 and became more numerous thereafter; nerve cells in various stages of degeneration could also be seen. Removal of primary afferents to the gracile nucleus at the time they reach the nucleus or soon after was followed by a severe loss of nerve cells and a reduced increment in size of the remaining ones. Moreover, the results of the present investigation show that penetration of primary sensory fibres into the gracile nucleus takes place approximately 2 days after they have been seen in the medulla and are in keeping with observations made in other pathways of the nervous system of the rat as well as in other animals. The findings that mature glomeruli, previously described in 1-day-old rats, are not present shortly before birth, suggest a fast rate of maturation of these synapses.
Chimelli
L
L
Department of Neuropathology, National Hospital, Queen Square, London, U.K.
Scaravilli
F
F
D000344:Afferent Pathways / Q000254:growth & development; D000818:Animals; D002452:Cell Count; D003714:Denervation; D006735:Horseradish Peroxidase; D008526:Medulla Oblongata / Q000254:growth & development*; D009410:Nerve Degeneration; D009412:Nerve Fibers / Q000254:growth & development*; D009474:Neurons / Q000166:cytology; D051381:Rats; D011919:Rats, Inbred Strains; D012677:Sensation / Q000502:physiology*
D006735:Horseradish Peroxidase