Pulmonary anatomy and a case of unilateral aplasia in a common snapping turtle (Chelydra serpentina): developmental perspectives on cryptodiran lungs

The common snapping turtle (Chelydra serpentina) is a well studied and broadly distributed member of Testudines; however, very little is known concerning developmental anomalies and soft tissue pathologies of turtles and other reptiles. Here, we present an unusual case of unilateral pulmonary aplasia, asymmetrical carapacial kyphosis, and mild scoliosis in a live adult C. serpentina. The detailed three-dimensional (3D) anatomy of the respiratory system in both the pathological and normal adult C. serpentina, and a hatchling are visualized using computed tomography (CT), microCT, and 3D digital anatomical models. In the pathological turtle, the right lung consists of an extrapulmonary bronchus that terminates in a blind stump with no lung present. The left lung is hyperinflated relative to the normal adult, occupying the extra coelomic space facilitated by the unusual mid-carapacial kyphotic bulge. The bronchial tree of the left lung retains the overall bauplan of the normal specimens, with some minor downstream variation in the number of secondary airways. The primary difference between the internal pulmonary structure of the pathological individual and that of a normal adult is a marked increase in the surface area and density of the parenchymal tissue originating from the secondary airways, a 14.3% increase in the surface area to volume ratio. Despite this, the aplasia has not had an impact upon the ability of the turtle to survive; however, it did interfere with aquatic locomotion and buoyancy control under water. This turtle represents a striking example of a non-fatal congenital defect and compensatory visceral hypertrophy.

拟鳄龟（Chelydra serpentina）是龟鳖目（Testudines）中被广泛研究且分布范围极广的类群，但目前针对龟类及其他爬行类的发育异常与软组织病变的研究仍较为匮乏。本研究报道了一例罕见的活体成年拟鳄龟病例，该个体存在单侧肺发育不全、不对称甲壳后凸畸形及轻度脊柱侧凸。本研究通过计算机断层扫描（CT）、显微计算机断层扫描（microCT）及三维（3D）数字化解剖模型，对患病成年拟鳄龟、正常成年拟鳄龟以及刚孵化幼体的呼吸系统三维解剖结构进行了可视化呈现。该患病个体的右侧支气管为肺外支气管，末端仅为盲端，无肺组织存在。相较于正常成年个体，该个体的左肺出现过度充气，借由异常的甲壳中部后凸隆起所腾出的额外体腔得以扩张。左肺的支气管树整体保留了正常个体的基本构型，仅在次级气道的数量上存在轻微的远端差异。患病个体与正常成年个体肺部内部结构的主要差异在于：源自次级气道的肺实质组织的表面积与密度显著提升，其表面积体积比增加了14.3%。尽管存在上述结构异常，单侧肺发育不全并未影响该拟鳄龟的存活能力，但却干扰了其水下运动与浮力调控功能。该个体为非致命性先天性缺陷伴代偿性内脏肥大的典型研究案例。