Distribution of MAP1A, MAP1B, and MAP2A&B during layer formation in the optic tectum of developing chick embryos.

The expression patterns of three microtubule-associated proteins (MAP1A, MAP1B, and MAP2A&B) were investigated in the developing optic tectum. Expression of MAP1B and middle-molecular-weight peptide of neurofilament (NF-M) was first observed in the same mesencephalic cells on day 3 of incubation, indicating that neuroblasts had been produced. At day 5, MAP1A and MAP2A&B expression appeared in the cellular layer containing the first neuroblasts that differentiate into large multipolar cells. The NF-M+ neurites in the striatum album centrale (SAC) and the striatum opticum (SO) were MAP1B+ up to day 19, but the intensity of MAP1B immunoreactivity decreased with development. All three MAPs were expressed in large multipolar neurons in the developing stratum griseum centrale from the beginning of maturation. Stratum griseum et fibrosum centrale cellular layers, containing radially arranged piriform neurons, were MAP1A-/MAP2A&B- on day 11 but became MAP1A+/MAP2A&B+ during later stages. These results suggest that the timing of MAP expression in neuronal maturation of large multipolar cells differs from that of piriform cells. The expression of MAPs has revealed specific cellular events in the developing optic tectum. Based on our observations, the development of the optic tectum can be divided into four periods.