Theodosiou Nicole A, Hall Daniel A, Jowdry Andrea L
Department of Biology, Bowdoin College, 6500 College Station, Brunswick, Maine 04011, USA.
J Exp Zool B Mol Dev Evol. 2007 Jul 15;308(4):442-53. doi: 10.1002/jez.b.21170.
The digestive tract of vertebrates is a complex organ system required for the digestion of food and the absorption of nutrients. The colon evolved as a water absorption organ essential for vertebrates to survive on land. In contrast to land vertebrates, the Chondrichthyes (sharks, skates and rays) are nearly iso-osmotic with their ocean environment and do not reabsorb water from food waste. To understand the origin of the vertebrate colon, we examined the distribution of sulfated and sialyated mucus-producing cells in the little skate, Raja erinacea, as an indication of water absorption function in the chondrichthian digestive tract. The percentage of acid mucin producing goblet cells was analyzed in the spiral valve and hindgut of little skate and the small intestine and colon of mouse embryos. Levels of acid mucins in the hindgut of the little skate was comparable to that of the small intestines of terrestrial vertebrates, whereas the distal region of the spiral valve contained high levels of acid mucin producing cells similar to the colon of mouse and chick. The low numbers of acid mucins in the little skate hindgut confirms that a functional colon for water absorption is absent in the Chondrichthyes. Interestingly, the presence of high levels of acid mucins in the posterior spiral valve provides evidence for a possible primordial water-absorbing organ in the elasmobranchs. Hoxd13 patterns acid mucins in the colons of terrestrial vertebrates. Expression of Hoxd13 and Hoxa13 in R. erinacea suggests conserved roles for Hox genes in patterning the early hindgut.
脊椎动物的消化道是一个复杂的器官系统，用于食物消化和营养吸收。结肠进化成为一个对脊椎动物在陆地上生存至关重要的水分吸收器官。与陆地脊椎动物不同，软骨鱼类（鲨鱼、鳐鱼和魟鱼）与其海洋环境几乎等渗，不会从食物残渣中重新吸收水分。为了了解脊椎动物结肠的起源，我们研究了小斑鳐（Raja erinacea）中硫酸化和唾液酸化黏液产生细胞的分布，以此作为软骨鱼类消化道中水分吸收功能的指标。分析了小斑鳐螺旋瓣和后肠以及小鼠胚胎小肠和结肠中产生酸性黏蛋白的杯状细胞的百分比。小斑鳐后肠中的酸性黏蛋白水平与陆地脊椎动物小肠的相当，而螺旋瓣的远端区域含有大量产生酸性黏蛋白的细胞，类似于小鼠和鸡的结肠。小斑鳐后肠中酸性黏蛋白数量较少，证实软骨鱼类中不存在用于水分吸收的功能性结肠。有趣的是，后螺旋瓣中高水平酸性黏蛋白的存在为板鳃亚纲动物中可能存在原始水分吸收器官提供了证据。Hoxd13在陆地脊椎动物结肠中调控酸性黏蛋白。Hoxd13和Hoxa13在小斑鳐中的表达表明Hox基因在早期后肠模式形成中具有保守作用。
