To understand why the water channel aquaporin-0 (AQP0) replaces aquaporin-1(AQP1) during lens development, we studied its spatial arrangement and interactions with proteins in the plasma membrane of equatorial fibers. We used freeze-fracture-labelling; a method that can identify the individual intramembrane particle representing the AQP0 channel. We found that AQP0 was arranged in micro-domains that extended along the long axis of the equatorial fiber cell. One micro-domain consisted of AQP0 channels intermingled with the normal complement of integral proteins of the fiber plasma membrane. We found that the density of AQP0 channels varied along the long axis of the fiber. At the apical end of the fiber, the density was barely above background noise (approximately 50 microm(-2)). It increased first to 345=109 microm(-2) and then to 719+/-35 microm(-2) in the region of the plasma membrane facing adjacent fibers (the lateral surface). Another micro-domain, located at the apical end of the fiber, was composed of AQP0 channels within gap junctions ('mixed' junctions). This micro-domain contained approximately 1.5 x 10(5) cell-to-cell channels and approximately 3500 AQP0 channels. A third micro-domain, located exclusively in the lateral surface of the fiber, was composed of clusters of channels abutted against an opposing, particle-free plasma membrane (AQP0 junction). In equatorial fibers, the intramembrane particles in the AQP0 junctions were densely packed (6747+/-1007 microm(-2)), but were not arranged in orthogonal arrays that are characteristic of equaporins. This micro-domain occupied 20-25% of the lateral surface of equatorial fibers and, more importantly, it was arranged in 'ribbons' that extended for long stretches (30-40 microm) along the apical-basal axis. We concluded that the ability of AQP0 to arrange itself in micro-domains conferred functional properties that might contribute to the maintenance of lens transparency and homeostasis.