Extracellular matrix distribution during neocortical wall ontogenesis in "normal" and "Reeler" mice

The spatio-temporal distribution of extracellular spaces (ECS) was analysed during prenatal corticogenesis in the normal mouse or in the recessive mutant Reeler. Colloidal iron was used to visualize extracellular anionic substances (containing glycosaminoglycans) which constitute an extracellular matrix (ECM). The spatio temporal distribution of the ECM appeared closely related to ECS localization. Before the cortical plate formation, the ECM occurred only in the marginal zone. The plate appearance resulted in a redistribution of anionic sites in the subplate of the normal mouse in addition to the marginal zone. Although, in the Reeler, the ECM remained mainly located in a broad outer cortical zone. Until birth, the distribution of the ECM in the normal mouse, followed closely the inside-out gradient of cortical neuropile maturation, while in the Reeler and outside-in gradient was observed. With EM, following standard fixation and staining an extracellular matrix (formed of 5 to 10 nm fibrils) occurred in sites where the ECM was already detected with light microscopy. The ECM fibrils were seen mainly in the vicinity of young neurons showing characters of secretory functions, such as dilated RER cisternae filled with a fibrillar material and well developed golgi complexes. Young Cajal-Retzius cells appeared especially well suited to elaborate the ECM in the marginal zone of the normal mouse or in the outer cortical zone of the Reeler. The potential role of ECM for the differentiation and formshaping of young neurons is discussed.