东方对虾有髓神经纤维的个体发生的电镜观察

ELECTRON MICROSCOPIC OBSERVATION ON ONTOGENESIS OF THE MYELINATED NERVE FIBER IN THE SHRIMP (Penaeus orientalis)

用电镜技术观察了东方对虾（Ｐｅｎａｅｕｓｏｒｉｅｎｔａｌｉｓ）有髓神经纤维的髓鞘化，以及位于其髓鞘与轴突间被命名为髓鞘下间隙和微管鞘这两个侍有结构的个体发生过程。结果如下：（１）在无节幼体（ｎａｕｐｌｉｕｓ）已有由神经历细胞和神经胶质母细胞构成的神经成分出现；（２）在状幼体（ｚｏａｅａ）神经细胞发生裸露的突起，其直径不等，走向不一；（３）在糠虾幼体阶段（ｍｙｓｉｓｓｔａｇｅ）已形成了与成体虾相似的腹神经索，其中的一对内侧巨大纤维已出现，并开始髓鞘化。在此发育阶段后期这两根巨大纤维的髓鞘开始脱离轴突，形成间隙，并在轴突周围开始出现由微管束构成的鞘。在此发育阶段虽已可辨认出一对外侧巨大纤维，但通常尚未髓鞘化；（４）在仔虾（ｐｏｓｔ－ｌａｒｖａ）阶段神经索中许多较粗神经纤维开始髓鞘化，也是由新生的髓鞘直接包绕轴突，然后与之脱离形成髓鞘下间隙和形成直接复盖轴突的微管鞘；（５）髓鞘的形成不是如在脊椎动物髓鞘通常所见到的那样，由一许旺细胞的浆膜片以螺旋方式包绕轴突而形成的，对虾髓鞘的形成则是由许旺氏细胞向两侧伸出许多浆膜片以同心方式包绕轴突，当两侧的浆膜片在某处相遇时，均形成内含２至数根微管的终扣（ｔｅｒｍｉｎａｌｌｏ?

Using electron microscopic technique the ontogenetic development of myelin sheath and two unique structures named as submyelinic space and microtubular sheath located be.ween the myelin sheath and the axon of Penaeus orientalis were studied. The results were as follows : (1)In stage of naupius a piece of neural element composed of neuroblasts and spongioblasts was observed;(2)in stage of zoaea the neroe cells began to give off many naked processes with various diameter and to every direction; (3) In mysis stage the abdominal nerve cord composed of two symmetrical parts similar to that of adult shrimp was formed. In the cord a pair of medial giant axons of the largest diameter clearly appeared and began to myelinize. In the late peried of mysis stage the unique structures just consequently began to appear in the two giant fibers. As to a pair of lateral giant fibers, they could already be identified . but were without sign of myelinizntion yet: (4)In stage of post-larva myelinization took place in many thick axons; (5)It was commonly observed that during the process of myelinization of the shrimp axons the new-generated myelin sheath first directly covered the axon, and then gradually dissociated itself from the axon, leaving a space, at the same time the microtubular sheath dirertly envelopased the axon appeared; (6)The shrimp myelin sheath was clearly shown to be formed not by a cytoplasmic sheet wrapped the axon in a spiral way, as it was commonly seen in vertebrate myelin. Actually, numbers of laminae given off by a Schwann cell concentrically enveloped the axon from two opposite sides. Each lamina terminated with a terminal loop with 2 or more microtubules in it. Two terminal loops from opposite sides met each other somewhere and connercted tip to tip, the connected two loops from esch layer were radially arranged more or less regularly in a line, giving the appearance of coarser laminae zone in the shrimp myelin as it was reported before[1].