Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons

The postnatal development of length, branching structure, and spatial distribution of dendrites of triceps surae motoneurons, intracellularly stained with horseradish peroxidase, was studied from birth up to 44–46 days of postnatal (d.p.n.) age in kittens and compared with corresponding data from adult cats. The number of dendrites of a triceps surae motoneuron was about 12, and the arborization of each dendrite generated an average of 12–15 terminal branches. There was no net change in the number of dendrites of a neuron or in the degree of branching of the dendrites despite the occurrence of both a transient remodeling of the dendritic branching structure and changes of the spatial distribution of the dendritic branches during postnatal development. The perisomatic territory in the transverse plane occupied by the dendritic branches of a motoneuron increased in parallel with the overall growth of the spinal cord. Thus, the relative size of the dendritic territory in this plane was kept almost constant, whereas dendritic branches projecting in the rostrocaudal direction grew much faster than the spinal cord and also became more numerous. At birth the rostro‐caudal dendritic span of individual motoneurons bridged 1:6 to 1:5 of the L7 spinal cord segment length; this figure was 1:3 at 22–24 d.p.n. Hence, in this direction, the growing dendritic branches invaded novel dendritic territories. The change in dendritic branch length from birth to 6 weeks of age corresponded to an average growth rate of 2 to 4 μm per dendritic branch and day, which implies that the total increase in length of the dendrites of a neuron could amount to 1 mm/day. The increase in branch length did not occur in a uniform or random manner; instead, it followed a spatiotemporal pattern with three phases: From birth to 22–24 d.p.n., growth was particularly prominent in ⩾3rd order preterminal and 2nd through 6th order terminal branches. From 22–24 to 44–46 d.p.n., a large increase in branch length confined to terminal branches of ⩾3rd branch orders was observed. As indicated by topological analysis, this length increase was probably due in part to a resorption of peripheral dendritic branches during this stage of development. From 44–46 d.p.n. to maturity, the increase of dendritic branch length was restricted to preterminal branches of low (⩽4th) branch order. Topological analysis revealed that the arborization pattern of triceps surae motoneuron dendrites resembled most a tree structure