Ontogenesis of microtubule-associated protein 2 (MAP2) in embryonic mouse cortex.

The developing neocortex in mice from embryonic day 13 (E13) until birth (E19) was immunoreacted with a monoclonal antibody for microtubule-associated protein 2 (MAP2) that is highly specific for neuronal somata and dendrites. In E13 neocortex there was no detectable MAP2 immunoreactivity on tissue sections or on gel blots. From E14 to birth the MAP2 immunoreactivity was present in both tissue sections and immunoblots of homogenized cortex. In the neocortex the staining pattern was lamina-specific. The molecular layer and the cortical subplate contained the most dense staining of dendrites and cell somata. The cortical plate showed weak to moderate staining at these ages while the intermediate and ventricular zones were not stained above background control levels. Gel blots correspondingly did not show detectable levels of MAP2 until E14. Ultrastructural data suggest that MAP2 is present in dendrites in each of the laminae. The laminar pattern of MAP2 immunoreactivity may be due to either the higher density of differentiating dendrites in the molecular and subplate layers or to compartmentalization of MAP2 within individual cortical neurons.