Schneeberger E E, Karnovsky M J
Circ Res. 1976 May;38(5):404-11. doi: 10.1161/01.res.38.5.404.
The purpose of this study was to examine by freeze-fracture the ultrastructure of intercellular junctions between mouse pulmonary endothelial and epithelial cells, and to relate this fine structure to that deduced from previous physiological and ultrastructural studies using tracer techniques. Junctions between capillary endothelial cells consist of one to three interconnected rows of particles which show occasional discontinuities. Small gap junctions are associated with these rows of particles in the arteriolar end of the capillary bed. At the venular end, the junctions consist of low profile ridges on the protoplasmic fracture (PF) face or complimentary grooves on the exoplasmic fracture (EF) face some of which have a sparse number of associated particles. The vascular junctions are similar to those of vessels in rat omentum and mesentery, and resemble "leaky" junctions described in renal proximal convoluted tubular cells. Tight junctions (zonulae occludentes) between type I pneumocytes or between types I and II pneumocytes consist of a band of interconnecting ridges on the PF face and complimentary interconnecting grooves on the EF face. These continuous epithelial junctions have a structure that is typical of tight occluding junctions. Occasionally zonulae occludentes between type I and type II pneumocytes are discontinuous; this may be the result of cell translocation as pneumocytes are shed into the alveolus. Intravascular perfusion fixation at high pressure (140 cm H2O) had no discernible effect on the structure of endothelial or epithelial junctions. The appearance in our study of freeze-fractured pulmonary endothelial and epithelial junctions reveals clearly the physical basis for the results of ultrastructural tracer and physiological studies which have suggested that it is the alveolar epithelium rather than the endothelium that is the chief permeability barrier to small, water-soluble molecules.
本研究的目的是通过冷冻断裂技术检查小鼠肺内皮细胞和上皮细胞之间细胞间连接的超微结构，并将这种精细结构与先前使用示踪技术进行的生理学和超微结构研究推断出的结构相关联。毛细血管内皮细胞之间的连接由一到三排相互连接的颗粒组成，这些颗粒偶尔会出现间断。小间隙连接与毛细血管床小动脉端的这些排颗粒相关。在小静脉端，连接由原生质断裂（PF）面上的低轮廓嵴或外质断裂（EF）面上的互补凹槽组成，其中一些有少量相关颗粒。血管连接与大鼠网膜和肠系膜中的血管连接相似，类似于肾近端曲管细胞中描述的“渗漏”连接。I型肺泡上皮细胞之间或I型和II型肺泡上皮细胞之间的紧密连接（闭锁小带）由PF面上的一条相互连接的嵴带和EF面上的互补相互连接的凹槽组成。这些连续的上皮连接具有典型的紧密闭锁连接结构。偶尔，I型和II型肺泡上皮细胞之间的闭锁小带是不连续的；这可能是由于肺泡上皮细胞脱落进入肺泡时细胞移位的结果。高压（140 cm H2O）血管内灌注固定对内皮或上皮连接的结构没有明显影响。我们对冷冻断裂的肺内皮和上皮连接的研究结果清楚地揭示了超微结构示踪和生理学研究结果的物理基础，这些研究表明，对于小的水溶性分子，主要的渗透屏障是肺泡上皮而不是内皮。
