Van Dyke James U, Lindsay Laura A, Murphy Christopher R, Thompson Michael B
School of Biological Sciences, The University of Sydney, Sydney, New South Wales, Australia.
Department of Anatomy and Histology, The University of Sydney, Sydney, New South Wales, Australia.
J Exp Zool B Mol Dev Evol. 2015 Nov;324(7):636-46. doi: 10.1002/jez.b.22621. Epub 2015 Jun 7.
The evolution of viviparity requires the development of mechanisms that facilitate transport of respiratory gases between mother and developing embryo. Of particular importance is maternal excretion of embryonic carbon dioxide (CO2 ), which increases as the embryo grows in size during development. The carbonic anhydrases are a family of enzymes that convert CO2 to bicarbonate for transport throughout the cardiovascular system and which may also be important for CO2 transport from embryo to mother. We used immunohistochemistry to localize carbonic anhydrase II in the placental tissues of a viviparous and highly placentotrophic lizard, Pseudemoia entrecasteauxii. Carbonic anhydrase II is localized in the uterine component of the paraplacentome, presumably to facilitate transport of embryonic CO2 to the mother. Carbonic anhydrase II is also localized in both the uterine and embryonic components of the placentome, a region heavily involved in placental nutrient transport rather than respiratory gas exchange. In contrast, carbonic anhydrase II is not present in the uterine or embryonic components of the omphaloplacenta, another region responsible for nutrient transport. While carbonic anhydrase II in the paraplacentomal uterus is likely to be responsible for embryo-maternal CO2 transport, the distribution of carbonic anhydrase II throughout the placentome indicates a different function. Instead of transporting embryonic CO2 , placentomal carbonic anhydrase II appears to be responsible for transporting CO2 produced by energetically expensive nutrient transport mechanisms in both the uterus and the embryo, which implies that the mechanisms of nutrient transport in the omphaloplacenta may not be as energetically expensive.
胎生的进化需要发展出促进母体与发育中胚胎之间呼吸气体运输的机制。特别重要的是母体排出胚胎产生的二氧化碳（CO₂），随着胚胎在发育过程中体积增大，其产生的二氧化碳也会增加。碳酸酐酶是一类将CO₂转化为碳酸氢盐以便在心血管系统中运输的酶，对于CO₂从胚胎运输到母体也可能很重要。我们使用免疫组织化学方法在一种胎生且高度胎盘营养型蜥蜴——恩氏伪蜥（Pseudemoia entrecasteauxii）的胎盘组织中定位碳酸酐酶II。碳酸酐酶II定位于副胎盘的子宫部分，推测是为了促进胚胎产生的CO₂运输到母体。碳酸酐酶II也定位于胎盘的子宫和胚胎部分，这是一个大量参与胎盘营养物质运输而非呼吸气体交换的区域。相比之下，碳酸酐酶II不存在于脐胎盘的子宫或胚胎部分，脐胎盘是另一个负责营养物质运输的区域。虽然副胎盘子宫中的碳酸酐酶II可能负责胚胎与母体之间的CO₂运输，但碳酸酐酶II在整个胎盘的分布表明其具有不同的功能。胎盘的碳酸酐酶II似乎不是运输胚胎产生的CO₂，而是负责运输子宫和胚胎中能量消耗高昂的营养物质运输机制产生的CO₂，这意味着脐胎盘的营养物质运输机制可能在能量消耗上没那么大。
