Porter William Ruger, Witmer Lawrence M
Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, Ohio, United States of America.
PLoS One. 2015 Oct 14;10(10):e0139215. doi: 10.1371/journal.pone.0139215. eCollection 2015.
Squamates use the circulatory system to regulate body and head temperatures during both heating and cooling. The flexibility of this system, which possibly exceeds that of endotherms, offers a number of physiological mechanisms to gain or retain heat (e.g., increase peripheral blood flow and heart rate, cooling the head to prolong basking time for the body) as well as to shed heat (modulate peripheral blood flow, expose sites of thermal exchange). Squamates also have the ability to establish and maintain the same head-to-body temperature differential that birds, crocodilians, and mammals demonstrate, but without a discrete rete or other vascular physiological device. Squamates offer important anatomical and phylogenetic evidence for the inference of the blood vessels of dinosaurs and other extinct archosaurs in that they shed light on the basal diapsid condition. Given this basal positioning, squamates likewise inform and constrain the range of physiological thermoregulatory mechanisms that may have been found in Dinosauria. Unfortunately, the literature on squamate vascular anatomy is limited. Cephalic vascular anatomy of green iguanas (Iguana iguana) was investigated using a differential-contrast, dual-vascular injection (DCDVI) technique and high-resolution X-ray microcomputed tomography (μCT). Blood vessels were digitally segmented to create a surface representation of vascular pathways. Known sites of thermal exchange, consisting of the oral, nasal, and orbital regions, were given special attention due to their role in brain and cephalic thermoregulation. Blood vessels to and from sites of thermal exchange were investigated to detect conserved vascular patterns and to assess their ability to deliver cooled blood to the dural venous sinuses. Arteries within sites of thermal exchange were found to deliver blood directly and through collateral pathways. The venous drainage was found to have multiple pathways that could influence neurosensory tissue temperature, as well as pathways that would bypass neurosensory tissues. The orbital region houses a large venous sinus that receives cooled blood from the nasal region. Blood vessels from the nasal region and orbital sinus show anastomotic connections to the dural sinus system, allowing for the direct modulation of brain temperatures. The generality of the vascular patterns discovered in iguanas were assessed by firsthand comparison with other squamates taxa (e.g., via dissection and osteological study) as well as the literature. Similar to extant archosaurs, iguanas and other squamates have highly vascularized sites of thermal exchange that likely support physiological thermoregulation that "fine tunes" temperatures attained through behavioral thermoregulation.
有鳞目动物在加热和冷却过程中利用循环系统调节身体和头部温度。该系统的灵活性可能超过恒温动物，提供了多种生理机制来获取或保留热量（例如，增加外周血流量和心率，冷却头部以延长身体晒太阳的时间）以及散热（调节外周血流量，暴露热交换部位）。有鳞目动物还能够建立并维持与鸟类、鳄鱼和哺乳动物相同的头部与身体的温差，但没有离散的 rete 或其他血管生理装置。有鳞目动物为推断恐龙和其他已灭绝的主龙类的血管提供了重要的解剖学和系统发育证据，因为它们揭示了基干双孔类的状况。鉴于这种基础定位，有鳞目动物同样为恐龙可能存在的生理体温调节机制的范围提供了信息并加以限制。不幸的是，关于有鳞目动物血管解剖学的文献有限。使用差分对比双血管注射（DCDVI）技术和高分辨率 X 射线显微计算机断层扫描（μCT）对绿鬣蜥（Iguana iguana）的头部血管解剖结构进行了研究。对血管进行数字分割以创建血管路径的表面表示。由口腔、鼻腔和眼眶区域组成的已知热交换部位因其在大脑和头部体温调节中的作用而受到特别关注。对进出热交换部位的血管进行了研究，以检测保守的血管模式，并评估它们将冷却血液输送到硬脑膜静脉窦的能力。发现热交换部位内的动脉直接并通过侧支途径输送血液。发现静脉引流有多种途径，可能影响神经感觉组织温度，以及绕过神经感觉组织的途径。眼眶区域有一个大的静脉窦，从鼻腔区域接收冷却的血液。来自鼻腔区域和眼眶窦的血管显示与硬脑膜窦系统有吻合连接，从而可以直接调节脑温。通过与其他有鳞目类群（例如，通过解剖和骨学研究）以及文献进行第一手比较，评估了在鬣蜥中发现的血管模式的普遍性。与现存的主龙类相似，鬣蜥和其他有鳞目动物有高度血管化的热交换部位，可能支持对通过行为体温调节获得的温度进行“微调”的生理体温调节。
