Fig. 4.

High-power electron micrograph of choroid plexus of wild-type mouse compared with that of Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1) knockout mouse. A: choroid plexus epithelial cells (CPECs) of NKCC1 wild-type mouse (NKCC1+/+) show normal ultrastructure. Their apical (cerebrospinal fluid-facing) surface exhibits abundant microvilli (mv), and the cells are joined together at their apical ends by an intercellular junctional complex: a zonula occludens or tight junction (tj) and, beneath this, a zonula adherens (za) or adherens junction. Beyond this junctional complex, the intercellular space exhibits modest dilations that gradually expand toward the basal pole, at which point the basolateral membrane forms a series of large infoldings. Basal end of the cells (e.g., left bottom corner) is relatively smooth and is anchored to the basal lamina (bl) of the basement membrane. Cytoplasm contains abundant mitochondria (mit), a nucleus (Nuc), and some vesicular bodies (vb). B: CPECs of NKCC1 knockout mouse (NKCC1−/−) are shrunken, and their mv appear less abundant and smaller than wild-type mouse. Shrunken CPECs remain joined through the tj and the za. Beneath the za, there is a large dilation of the intercellular space (asterisk) resulting from cell shrinkage. Basolateral membrane is retracted, and the extracellular spaces in between the basal infoldings are greatly enlarged (stars). Basolateral membrane forms fingerlike extensions that remain attached to the bl of the basement membrane (bm).