脊椎动物中的一种解剖学新模型——黄鳝外周嗅觉系统

PERIPHERY OLFACTORY SYSTEM IN RICE FIELD EEL, MONOPTERUS ALBUS (PISCES: SYNBRANCHIDAE),A NEW MODEL IN ANATOMY IN VERTEBRATE

除单鼻型的圆口类外,脊椎动物的左、右两侧嗅觉器官和嗅神经皆互为独立地分布于头前端,而且它们的前鼻孔(外鼻孔)、嗅腔、嗅觉副囊腔(部分鱼具嗅觉副囊)与后鼻孔(或内鼻孔)也都互为相通,且多呈开放状态。它们还通常具有一个体积相对较大且较稳定的嗅腔,而嗅上皮则多位于嗅腔的一侧。此外,鱼类的嗅囊与鼻窝之间通常也无明显间隙。然而,运用常规的解剖学方法发现,黄鳝(Monopterus albus)外周嗅觉系统(嗅觉器官和嗅神经)在解剖结构上已发生如下重大变化:(1)虽然具有前、后鼻孔,但两者互不相通,而嗅腔仅靠前鼻孔通至外界;(2)两侧嗅囊的末端及两侧嗅神经的前段均分别发生了合并。此外,在该鱼上还发现:(1)嗅囊为一柔软而扁塌的长管囊结构,其唯一的开口(即位于前鼻孔球上的前鼻孔)却常呈关闭状,故此时该嗅腔实际上是一个体积被压扁到最小且暂时被封闭的空间;(2)嗅囊纵向地贴附于长鼻窝的内侧壁上,它仅占鼻窝的一小部分空间,故鼻窝显得相对很宽敞;(3)嗅觉副囊不与嗅腔相通,而与鼻窝共同经后鼻孔通至外界;(4)两侧嗅囊的末端相向地穿越鼻窝内侧壁,进入筛骨与额骨之间的"筛-额横管",在那里发生嗅囊合并;(5)嗅囊壁周缘几乎都内衬着嗅上皮,且具数个褶窝(说明该嗅囊有扩张的可能)。因此,黄鳝的这套解剖学特征不同于包括鱼类在内的所有脊椎动物的外周嗅觉系统。研究所发现的黄鳝这套形态学特征不仅为脊椎动物外周嗅觉系统的研究提供了一个独特的解剖学新模型,同时也为动物进化研究提供了一个有关前、后鼻孔互不相通的进化特例。此外,研究还依据上述发现提出嗅囊扩张-压缩假说以解释气味媒质进出于黄鳝这种特殊嗅腔的动力学机制。

Generally,olfactory organs and olfactory nerves of the left and right side in vertebrate are separate completely.Even for single nose-type cyclostomes,such as lampreys,their olfactory nerves are also separate completely.Furthermore,their anterior and posterior nostril（or choana）,olfactory cavity,and cavity of olfactory accessory sac all communicate with each others,and the organs are usually open in whole.They usually have olfactory cavities,whose volume is relatively larger and steadier,and their olfactory epithelia primarily locate on one side of the cavities.Moreover,there is usually no lacuna in fish between their olfactory sacs and nasal pits.However,some great changes were found in the periphery olfactory system of rice field eel,Monopterus albus,with some conventional anatomy methods.The details were as follows： 1） although it possessed anterior and posterior nostrils,but they did not pass through each others,and its olfactory cavity connected the environment only via anterior one;2） the posterior part of two olfactory sacs and the anterior part of two olfactory nerves were combined together,respectively.Besides,there were some unique characteristics in the rice field eel： 1） the olfactory sac was a long soft tubular-sac,its only opening（the anterior nostril） was often shut.2） the olfactory sac attached longitudinally to the medial wall of each nasal pit,occupying only a small part of the pit cavity,so the pit was rather spacious.3） the olfactory accessory sac did not communicate with the olfactory cavity and anterior nostril,but accessed to the outside via nasal pit and posterior nostril.4） the terminals of two olfactory sacs get vis-à-vis across medial wall of each nasal pit,and enter oppositely into the transverse-tube between ethmoid and frontal bone,where the two olfactory sacs combined.5） there were olfactory epithelium lined around on the inner edge of olfactory sac wall,with several puckers between the rugae,indicating that the sac might dilate.Therefore,the anatomical characteristics of periphery olfactory system in M.albus were greatly different from all vertebrates including fishes.This study provided not only a novel model of periphery olfactory system in vertebrate,but also a specific evolutionary example,and the anterior and posterior nostrils were not interlinked.Furthermore,we proposed a hypothesis of olfactory sac expanding-compress to explain the dynamic mechanism of odor sampling medium in and out the special olfactory cavity of the rice field eel.