Wicht H, Northcutt R G
Klinikum, Johann Wolfgang Goethe-Universität, Frankfurt/Main, F.R.G.
Philos Trans R Soc Lond B Biol Sci. 1995 Aug 29;349(1328):119-34. doi: 10.1098/rstb.1995.0098.
The head of adult hagfishes (jawless craniates, Myxinoidea) of the family Eptatretidae displays a number of skin grooves of uncertain origin. These grooves have been homologized to the neuromast lines of other craniates, and they are innervated by two ganglionated cranial nerves that have been interpreted as lateral line nerves. The grooves do not, however, contain the compound receptors that are typical of a lateral line (i.e. neuromasts or electroreceptors), and both their development and function have remained enigmatic. To elucidate the embryonic origin of the grooves (which should develop from placodes if they are homologues of the lateral line system), embryos of Pacific hagfish were examined by means of three-dimensional reconstructions from serial sections. Because of the scarcity of specimens of embryonic hagfishes, only two embryos were reconstructed, but these reconstructions clearly show that a number of placodes and placodal derivatives (i.e. sensory ridges, receptor primordia, and cranial ganglia) occur in the head of embryonic eptatretid hagfishes. Some of these placodes correspond to the lens and epibranchial placodes of other craniates, but there are also three other placodes which represent possible homologues of lateral line placodes. The topology of the placodes in this latter group corresponds to the topology of the grooves of adult hagfishes, and we therefore reach three conclusions: (i) that an embryonic lateral line system is present in hagfishes; (ii) that the grooves of adult hagfishes in all probability derive from lateral line placodes; and (iii) that the presence of lateral line placodes is a primitive character of craniates.
盲鳗科成年盲鳗（无颌有头类，黏盲鳗目）的头部有许多来源不明的皮肤沟。这些沟已被认为与其他有头类的神经丘线同源，并且由两条带神经节的脑神经支配，这两条神经被解释为侧线神经。然而，这些沟中并不包含侧线典型的复合感受器（即神经丘或电感受器），其发育和功能一直成谜。为了阐明这些沟的胚胎起源（如果它们是侧线系统的同源物，应该由基板发育而来），通过对连续切片进行三维重建，对太平洋盲鳗的胚胎进行了检查。由于胚胎盲鳗标本稀少，仅重建了两个胚胎，但这些重建结果清楚地表明，在胚胎期的盲鳗科盲鳗头部出现了一些基板和基板衍生物（即感觉嵴、感受器原基和脑神经节）。其中一些基板与其他有头类的晶状体基板和鳃上基板相对应，但还有另外三个基板，它们可能是侧线基板的同源物。后一组基板的拓扑结构与成年盲鳗皮肤沟的拓扑结构相对应，因此我们得出三个结论：（i）盲鳗存在胚胎期侧线系统；（ii）成年盲鳗的皮肤沟很可能源自侧线基板；（iii）侧线基板的存在是有头类的一个原始特征。
