A possible transepithelial pathway via endoplasmic reticulum in foetal sheep choroid plexus

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Abstract
Choroid plexuses from early (30-60 days gestation) and late (125 days) sheep fetuses were examined by various ultrastructural techniques in order to investigate possible explanations for the greater penetration of protein and non-electrolytes from blood into CSF, which occurs in the early fetus in contrast to later stages. The greater penetration occurs despite the presence of well-formed tight junctions between epithelial cells and development of some of the characteristic ion gradients between CSF and plasma. A tubulocisternal system of endoplasmic reticulum appears to connect the basolateral and apical cell surfaces in early but not in late fetuses. Several types of connection between the endoplasmic reticulum and cell membrane were present in early fetuses; these may account for some of the different permeability properties of the immature choroid plexus.