A-D, Low magnification confocal images of calbindin-immunoreactive (CB-IR) cells in the ventral horn of wild-type (A), Er81(−/−)(B), Egr3(−/−) (C) and mlcNT3(+/−) (D) animals at P20. The border between white and ventral gray matter is outlined and a white dotted line is drawn horizontally from the dorsal tip of the central canal (cc in A). CB-IR cells in the ventral horn of P20 mice are largely restricted to the Renshaw cell area in the ventral-most regions of LVII and LIX (arrow). A few additional CB-IR neurons (usually with weaker immunofluorescence) are found near the dorsal-ventral border and close to the central canal. Genetic alterations in proprioceptive inputs did not change the number or distribution of CB-IR neurons. E, The number of CB-IR cells per ventral horn in P20 mice remained largely unchanged. A small difference was detected in one Er81(−/−) mutant (asterisks, p<0.01, Bonferroni t-tests compared to average controls). F, Number of CB-IR cells in the ventral most 250 μm of LVII and LIX. Most of these CB-IR cells correspond with RCs and their numbers did not significantly change in any mutant. In E and F the wild-type bar (grey) represents the average estimate from ventral horns pooled from two animals per line (n = 23, 33 and 31 ventral horns in, respectively, the Er81(−/−), Egr3(−/−) and mlcNT3(+/−) samples), while averages in mutant animals (black bars) are represented individually (n = 10-18 ventral horns per animal). Error bars indicate SEM estimates. G-J, Parvalbumin-immunoreactivity (PV-IR) in the ventral horn of P20 wild-type (G), Er81(−/−) (H), Egr3(−/−) (I) and mlcNT3(+/−) (J) mice. PV-IR neurons in P20 mice are distributed throughout the whole ventral horn with a lower density in lateral lamina IX. Similar numbers and distribution of PV-IR cells are found in mutant animals. K, Number of PV-IR cells per ventral horn in P20 mice. Wild-type bar (grey) is the average estimate from control ventral horns from two animals (n = 22, 32 and 33 ventral horns in the Er81(−/−), Egr3(−/−) and mlcNT3(+/−) samples, respectively), while the average mutant animals (black bars) are represented individually (n = 8-22 ventral horns per animal). Error bars in all graphs indicate SEM estimates. Scale bars: 100 μm in A and G (A-D and G-J are at the same magnification).