Smodlaka H, Khamas W, Tkalcic S, Golub T, Palmer L
College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA 91766-1854, USA.
Anat Histol Embryol. 2010 Jun;39(3):178-85. doi: 10.1111/j.1439-0264.2010.00994.x. Epub 2010 Mar 11.
Phocid seals exhibit vascular adaptations that allow them to undertake prolonged deep dives. These vascular adaptations are either unique to phocids, or are modified vascular equivalents to those present in terrestrial mammals. One such adaptation, the aortic bulb, is a spherical enlargement of the ascending aorta specific to phocid seals. Its histological make-up consists of a reinforced tunica media with circular and longitudinal layers of elastic fibres. This reinforcement enables multi-axial deformation of the aortic bulb, thus complementing its function as a prominent elastic reservoir or 'windkessel'. A second adaptation, the hepatic sinus, is an asymmetrical dilation of the abdominal portion of the caudal vena cava and accompanying hepatic veins. The hepatic sinus is comprised of a relatively thin tunica media, with a scant smooth muscle component. The bulk of the sinus wall is comprised of tunica adventitia. A third vascular adaptation distinctive to the phocids is the pericardial venous plexus, composed of convoluted veins circumnavigating the perimeter of the heart. Microscopically, these veins have a thick tunica media and also contain valves. Smaller arteries, venules and distinct capillary beds are observed interspersed in-between these veins. It can be hypothesized, that in seals, certain vascular embryonic development may be arrested at an earlier embryonic stage, resulting in these unusual vascular formations. These modifications play a vital role in blood pressure regulation and distribution of oxygenated blood during prolonged deep diving. The purpose of this work was to elucidate the histological aspects of these unique vascular modifications and relate them to specific function.
海豹表现出血管适应性，使其能够进行长时间的深潜。这些血管适应性要么是海豹特有的，要么是对陆生哺乳动物血管的改良。一种这样的适应性结构是主动脉球，它是海豹特有的升主动脉的球形扩张。其组织学构成包括一层强化的中膜，有环形和纵向排列的弹性纤维层。这种强化使主动脉球能够进行多轴变形，从而补充其作为一个突出的弹性贮器或“风箱”的功能。第二种适应性结构是肝窦，它是尾腔静脉腹部部分和伴随的肝静脉的不对称扩张。肝窦由相对较薄的中膜组成，平滑肌成分很少。窦壁的大部分由外膜组成。海豹特有的第三种血管适应性结构是心包静脉丛，由环绕心脏周边的盘绕静脉组成。在显微镜下，这些静脉有厚厚的中膜，并且还含有瓣膜。在这些静脉之间可以观察到较小的动脉、小静脉和独特的毛细血管床。可以推测，在海豹中，某些血管的胚胎发育可能在更早的胚胎阶段就停止了，从而导致这些不寻常的血管形成。这些改良在长时间深潜过程中的血压调节和含氧血液的分布中起着至关重要的作用。这项工作的目的是阐明这些独特血管改良的组织学方面，并将它们与特定功能联系起来。
