ニワトリの脊髄固有ニューロンの大きさと軸索の長さについて

RELATIONSHIP BETWEEN NEURONAL SIZE AND AXONAL LENGTH IN CHICK PROPRIOSPINAL NEURONS

　抄録　ニューロンの大きさと軸索の長さの関係を脊髄固有ニューロンにおいて細胞体の断面積を指標として検討した。孵化後2-4日のニワトリ脊髄の頸膨大あるいは腰膨大に固形化HRPを刺入しTMB法で反応した。頸髄から仙髄までの完全連続横断切片を作成し，すべての標識細胞を写真を下じきにしてトレースした。そのうち頸髄，頸膨大，胸髄，腰髄，仙髄の高さで各々数切片について総数3,601個（3例）の標識細胞の細胞体の断面積を計測した。また標識細胞が主に分布するV-VIII層と運動ニューロンを含むIX層の細胞体の断面積をNissl標本で計測し比較対照した。Nissl標本ではV-VIII層の細胞は各髄節で150-250μm²の範囲に最も多く250μm²以下の面積を有するものが全体の半数を占めた。HRP刺入例では，いずれの膨大部へ刺入した例でも標識細胞は各髄節で小型から大型細胞まで分布し，軸索の長さと細胞体の大きさに明らかな相関はなかった。しかし標識細胞を250μm²以下の小型，250-400μm²の中型，400μm²以上の大型に分けその割合を髄節毎に比べると，小型細胞はHRP刺入部に近い髄節で高い割合を占める特徴があり，軸索の短いニューロンは小型細胞が多いことが示唆された。

In order to investigate the relationship between neuronal size and axonal length, we compared the size of chick propriospinal neurons in several seg-mental levels. As the index of neuron size, the cross sectional areas of somata were measured. After unilateral implantation of solidified HRP into the lumbar enlargement (2 cases) or the cervical enlarge-ment (1 case) in the 2-4 day post-hatch chick under Nembutal anesthesia, propriospinal neurons projec-ting to the enlargement were visualized by TMBmethod. Labeled cells found in complete serial trans-verse sections were all traced onto tracing papers put on photomicrographs under examination with the microscope (Fig. 2). In several successive sec-tions in the cervical cord, the cervical enlargement, the lumbar and the sacral cord, the cross sectional areas of their 3601 somata were measured on traced drawings of final magnification×243 by means of a computer system graphic analyzer (Cosmo Zone, Nikon) (Fig. 1). As a control case, cross sectional areas of somata were also measured in Nissl preparations in laminae V-VIII, where vast majo-rity of propriospinal neurons are located, and also lamina IX.

In Nissl preparations, the cross sectional areas of neurons in laminae V-VIII had a wide range distribution from 50 to 1600 μm². Over 90% of them were distributed from 50 to 600 μm². Among them, the neurons with somata of 150-250 μm² were most numerous. The distribution pattern was almost the same in all segments examind. The cross sectional areas of neurons in lamina IX were also distributed in a wide range from 150 to 1600 μm² (Fig. 3).

In the cases of HRP implantation into the spinal enlargement, the cross sectional areas of somata of labeled neurons were distributed in a range from 50 to 1500 μm², as was quite the same in Nissl preparations (Fig. 4). Labeled neurons were classified into 3 groups ; small neurons (<250 μm²), medium-sized neurons (250-400 μm²) and large neu-rons (>400 μm²). Comparison of the ratio of these 3 groups in each segment revealed no clear cor-relation between neuron size and distance from the HRP implantation level. So far as small neurons were concerned, they were more numerous in the neighboring segment to the implantation level. In the case of lumbar enlargement implan-tation, the ratio of small labeled neurons against the total labeled neurons in each segment was 53% in segment 24, 27% in segment 19, 25% in segment 14-15 and 9% in segment 9-12 (Fig. 4 right). In the case of the cervical enlargement implantation, the ratio of small labeled neurons was also more numerous in the neighboring seg-ments (segment 11, 15, 19) (Fig. 4 left). In both cases, however, the ratio was not linear to the axon length. Further, as for large neurons and medium-sized neurons, the ratio had no clear cor-relation to the axon length.