Ultrastructure of a specialized neck region in the rabbit nephron.

The transition between the glomerulus and the proximal tubule was studied by light and electron microscopy in perfusion-fixed rabbit kidneys. In most nephrons a transitional segment of tubule occurred between the Bowman's capsule and the proximal tubule proper. This initial part of the tubule was lined by flattened cells and formed a cytologically distinct neck segment. The nephron population was heterogeneous with respect to the length of this neck segment regardless of the level of the cortex. The neck cells contained an abundance of thin filaments often oriented at right angles to the long axis of the tubule and comparable in thickness to actin filaments. The presence of actin-like filaments suggests that neck cells may effect changes in the diameter of the neck segment and, thus, influence single nephron glomerular filtration rate.