The control of cell number in the lumbar ventral horns during the development of Xenopus laevis tadpoles

Abstract

The developing limb has been amputated by many workers in several species and in each case the number of surviving motor neurones on the side of the operation was less than normal. This may be observed among the mammals (e.g. Barron, 1945), birds (e.g. Hamburger, 1934), urodeles (e.g. Stultz, 1942), and anurans (e.g. May, 1930). The loss of motor neurones after amputation in adults appears to have been first noticed by Vulpian (1868) and Johnson & Clarke (1868). The early evidence is reviewed by Sherrington (1893) and the later by Piatt (1948). The control that the developing leg has over proliferation, migration, maintenance and degeneration of ventral horn cells has been most completely analysed in the chick, notably by Hamburger (1934, 1939, 1958), Hamburger & Keefe (1944), Bueker (1943, 1944, 1945a), Barron (1946, 1948), Mottet (1952) and Mottet & Barron (1954). Less is known about this in Anura. May (1930, 1933) has recorded that in Discoglossus motor area hyperplasia can be produced by supernumerary limbs, and that the lateral motor horn (ventral horn) can be suppressed by replacement of the limb area with skin grafts. Bueker (1945b) found a frog (Rana) with three functional hind legs on one side; this had a hyperplasia of 22-7% in the ventral horn. Beaudoin (1955) ablated the early limb-bud in Rana pipiens and found a terminal hypoplasia of 82% in the ventral horn. Counts at intermediate stages were interpreted by him as indicating that the operation had delayed differentiation and the normal loss of cells on that side. In Bufo vulgaris, Perri (1956a) removed the hind-limb area at tailbud stages, and found that the ventral horn developed normally up to digit stages, when it regressed. The same happened when the isolated spinal cord was transplanted (Perri, 1956b). Three out of six animals with motile supernumerary limbs had hyperplastic spinal cords on the side of graft; this was not seen in any of the cases with non-motile grafts (Perri, 1957). Supernumerary limbs only became motile when innervated by limb plexus fibres, and never induced formation of new ventral horns in trunk regions. Perri therefore concluded that for the full development of the ventral horn, both regional intrinsic and also extrinsic limb factors were essential, the former necessarily acting before the latter.