Tatton W G, Theriault E
Department of Physiology, University of Toronto, Ont., Canada.
Brain Res. 1988 Oct 1;471(2):191-206. doi: 10.1016/0165-3806(88)90099-5.
Cat muscle nerves and ventral roots for the hindlimbs show a unimodal distribution of axon diameter at birth, followed, at about 20 days postnatal (dPN), by a marked change to a bimodal distribution resembling that of the adult. However, volumes calculated for motoneuron somata retrogradely labeled with HRP have been reported to be divided into two size populations at birth in the kitten. In the literature it is suggested that a dissociation between axonal and somal growth appears at a very early age. This apparent dissociation, not present in adults, prompted us to examine the somal growth patterns of kitten lumbar motoneurons. In the present report we have examined somal size development in medial gastrocnemius (MG) motor nuclei in 18 cats aged 2 dPN to adulthood using retrogradely transported horseradish peroxidase to label the motoneurons. Measurements of minimum and maximum diameter somal size, volume calculations and a double circle technique relating the diameters of an estimated spherical volume contained within the soma to that of a second spherical volume enclosing the soma clearly distinguish two subpopulations in the adult, a small and a large population. In contrast, in the kitten we show there is a unimodal distribution of small motoneuron somata at birth which at 19-23 dPN differentiates into a bimodal population. This sudden differentiation of somal size coincides with that reported for MG motoneuron axonal calibre, ruling against a neonatal dissociation of somal and axonal size distributions, and appears to correspond to the time of onset of functional characteristics and the histochemical differentiation of fiber types in the MG muscle.
猫后肢的肌肉神经和腹根在出生时轴突直径呈单峰分布，随后在出生后约20天（dPN），明显转变为类似成年动物的双峰分布。然而，据报道，用辣根过氧化物酶（HRP）逆行标记的运动神经元胞体的体积在小猫出生时就分为两个大小群体。文献表明，轴突和胞体生长之间的分离在很早的年龄就出现了。这种在成年动物中不存在的明显分离促使我们研究小猫腰段运动神经元的胞体生长模式。在本报告中，我们使用逆行运输的辣根过氧化物酶标记运动神经元，研究了18只年龄从2 dPN到成年的猫的内侧腓肠肌（MG）运动核中胞体大小的发育情况。对胞体最小和最大直径的测量、体积计算以及一种双圆技术（将胞体内估计的球形体积的直径与包围胞体的第二个球形体积的直径相关联）清楚地将成年动物分为两个亚群，一个小群体和一个大群体。相比之下，在小猫中，我们发现出生时小运动神经元胞体呈单峰分布，在19 - 23 dPN时分化为双峰群体。胞体大小的这种突然分化与报道的MG运动神经元轴突管径的分化一致，排除了胞体和轴突大小分布在新生儿期的分离，并且似乎与功能特征的开始时间以及MG肌肉中纤维类型的组织化学分化相对应。
