Ureter Smooth Muscle Cell Orientation in Rat Is Predominantly Longitudinal

In ureter peristalsis, the orientation of the contracting smooth muscle cells is essential, yet current descriptions of orientation and composition of the smooth muscle layer in human as well as in rat ureter are inconsistent. The present study aims to improve quantification of smooth muscle orientation in rat ureters as a basis for mechanistic understanding of peristalsis. A crucial step in our approach is to use two-photon laser scanning microscopy and image analysis providing objective, quantitative data on smooth muscle cell orientation in intact ureters, avoiding the usual sectioning artifacts. In 36 rat ureter segments, originating from a proximal, middle or distal site and from a left or right ureter, we found close to the adventitia a well-defined longitudinal smooth muscle orientation. Towards the lamina propria, the orientation gradually became slightly more disperse, yet the main orientation remained longitudinal. We conclude that smooth muscle cell orientation in rat ureter is predominantly longitudinal, though the orientation gradually becomes more disperse towards the proprial side. These findings do not support identification of separate layers. The observed longitudinal orientation suggests that smooth muscle contraction would rather cause local shortening of the ureter, than cause luminal constriction. However, the net-like connective tissue of the ureter wall may translate local longitudinal shortening into co-local luminal constriction, facilitating peristalsis. Our quantitative, minimally invasive approach is a crucial step towards more mechanistic insight into ureter peristalsis, and may also be used to study smooth muscle cell orientation in other tube-like structures like gut and blood vessels.在输尿管蠕动(ureter peristalsis)过程中，收缩状态的平滑肌细胞(smooth muscle cell)的取向至关重要，但目前针对人类及大鼠输尿管平滑肌层(smooth muscle layer)的取向与组成的相关描述仍存在不一致之处。本研究旨在提升大鼠输尿管中平滑肌细胞取向的量化分析水平，以此作为阐明蠕动机制的基础。本研究方法的关键步骤之一，是采用双光子激光扫描显微镜(two-photon laser scanning microscopy)与图像分析技术，获取完整输尿管平滑肌细胞取向的客观定量数据，从而规避常规切片操作带来的切片伪影(sectioning artifacts)。我们对36段大鼠输尿管标本进行了分析，这些标本分别取自输尿管的近端、中段或远端位点，以及左侧或右侧输尿管。研究发现，在外膜(adventitia)附近区域，平滑肌细胞呈现出清晰明确的纵向取向。而向固有层(lamina propria)方向延伸时，细胞取向的分散程度逐渐小幅增加，但主要取向仍保持纵向。综上，大鼠输尿管平滑肌细胞的取向以纵向为主，但朝向固有层侧时，取向的分散程度会逐渐升高。上述研究结果不支持将输尿管平滑肌层划分为独立分层的结论。观测到的纵向取向表明，平滑肌收缩更易引发输尿管的局部缩短，而非管腔缩窄(luminal constriction)。然而，输尿管壁的网状结缔组织可将局部纵向缩短转化为协同性的管腔缩窄，从而助力蠕动过程的进行。我们所采用的定量、微创研究方法，是进一步阐明输尿管蠕动机制的关键一步，同时该方法也可用于研究肠道、血管等其他管状结构中的平滑肌细胞取向。