A scanning electron microscope study of the neck segment of the rabbit nephron.

The three-dimensional structure of the neck segment (NS) of the rabbit nephrons was studied by scanning electron microscopy (after fracture, micro-dissection, or after corrosion nephron casts), and by the air-cast method. The NS was observed at the glomerulotubular junction in 68.5% of all nephrons. In every case the NS appeared as a straight tube with its long axis oriented radially in relation to the glomerulus. Although the external diameter of the NS was smaller than that of the proximal tubule, its luminal diameter was greater. No valve-like structures were observed. Three cell types were observed in the NS: parietal-like, tubule-like, and intermediate. Parietal-like cells showed the same morphology as the parietal cells of the Bowman's capsule. Parietal-like cells constituted the only cell type in 25% of the NS. Tubule-like cells showed morphologic characteristics similar to proximal tubule cells; however, their microvilli were less numerous and exhibited an irregular pattern. Intermediate cells presented an intermediate morphology between tubule-like and parietal cells. In 75% of all NS, the three cellular types were present at the same time. The presence of tubule-like and intermediate cells is interpreted as the result of metaplasic transformation of the parietal cells. Our observations suggest that, in rabbits, the presence of the NS can be explained on the basis of phenotypical lability of the cells located at the glomerulo-tubular junction.