van Mier P, Armstrong J, Roberts A
Department of Anatomy & Embryology, University of Nijmegen, The Netherlands.
Neuroscience. 1989;32(1):113-26. doi: 10.1016/0306-4522(89)90111-5.
The development of the axial musculature, its innervation and early locomotion in Xenopus laevis embryos are described. Between stages 17 and 40 some 45 myotomes are formed on each side of the body. During this period the animals develop from non-motile to free swimming embryos. Using fluorescein-conjugated bungarotoxin the acquisition of acetylcholine receptor-sites was studied. At stage 25 (early flexure stage) bound bungarotoxin was confined to the first seven intermyotomal clefts, in free swimming embryos (stage 33) to the first 20 clefts. Application of horseradish peroxidase to the intermyotomal clefts in embryos ranging from stages 25 to 37/38 revealed that primary motoneurons were usually situated 100-400 microns, i.e. 0.5-1.5 myotomes, rostral to the cleft they innervated. The motor axons left the spinal cord at the caudal side of each spinal segment where neural crest was present between the cord and the myotomes. At stage 25 ventral root activity could be recorded extracellularly from only the first three intermyotomal clefts, at stage 32/33 from the first 16 clefts. The first spontaneous rhythmic swimming-like activity could be recorded around stage 28. Between stages 27 and 32/33 the initial swimming frequency and the swimming episode duration increased at least three-fold. Comparable results were obtained with high-speed cinematography and measurements with a photoelectric transducer. Between stages 17 and 40 the number of myotomes increased by 0.9 myotome h, approximately 11.4 h later followed by the innervation of the myotomes at 0.7 cleft/h. About 3.6 h after this, ventral root activity appeared at the rate of 0.6 cleft h. This study shows that the early swimming pattern generating neuronal network, located within the rostral spinal cord, reaches a state of "critical mass" around stage 27, at which the first rhythmic swimming activity occurs. At least 6-10 functional spinal segments and adjacent myotomes are required for early swimming.
本文描述了非洲爪蟾胚胎中轴肌肉组织的发育、其神经支配以及早期运动情况。在第17至40阶段，身体两侧大约形成了45个肌节。在此期间，动物从不能运动的胚胎发育为自由游泳的胚胎。使用荧光素偶联的银环蛇毒素研究了乙酰胆碱受体位点的获得情况。在第25阶段（早期弯曲阶段），结合的银环蛇毒素局限于前七个肌节间裂，在自由游泳胚胎（第33阶段）中则局限于前20个裂。将辣根过氧化物酶应用于第25至37/38阶段胚胎的肌节间裂，结果显示初级运动神经元通常位于它们所支配裂的前方100 - 400微米处，即0.5 - 1.5个肌节。运动轴突在每个脊髓节段的尾侧离开脊髓，此处脊髓和肌节之间存在神经嵴。在第25阶段，仅能从最初的三个肌节间裂细胞外记录到腹根活动，在第32/33阶段能从最初的16个裂记录到。大约在第28阶段可首次记录到自发的节律性类似游泳的活动。在第27至32/33阶段，初始游泳频率和游泳发作持续时间至少增加了两倍。高速摄影和光电换能器测量也得到了类似结果。在第17至40阶段，肌节数量以每小时0.9个肌节的速度增加，大约11.4小时后肌节开始接受神经支配，速度为每小时0.7个裂。在此之后约3.6小时，腹根活动以每小时0.6个裂的速度出现。这项研究表明，位于脊髓前部的早期游泳模式产生神经网络在第27阶段左右达到“临界质量”，此时首次出现节律性游泳活动。早期游泳至少需要6 - 10个功能性脊髓节段和相邻的肌节。
