Differential expression of calbindin and calretinin in the human fetal amygdala.

The distribution patterns of the calcium-binding proteins calbindin and calretinin, both expressed early during development within the various amygdaloid nuclei and areas, have been investigated. Anti-calbindin as well as anti-calretinin mark immature, partly migrating neurons in the 5th gestational month; the number of calretinin-immunoreactive neurons is distinctly higher. In the 8th month, calbindin and calretinin are found in a small proportion of presumed pyramidal cells and in various types of non-pyramidal neurons. Small and large bipolar and small and large multipolar neurons are shown to express calbindin and calretinin. Double-labellings show that calbindin and calretinin are largely contained in different subsets of these neuronal types, which are considered to represent interneurons. These nerve cell classes are widespread within the amygdala with mainly moderate to high packing densities. Diffuse immunoreactive structures, which are found in different intensities in the various amygdaloid nuclei, display distinct redistribution during fetal development. The results show that during early fetal development calbindin and particularly calretinin may be involved in the regulation of neuronal migration. In later development, definite subsets of interneurons, which are likely to be functionally different, are marked by anti-calbindin and -calretinin. Different diffuse immunolabelling at various developmental stages probably indicates the sequential arrival of afferent input from brain areas containing calbindin- or calretinin-immunoreactive nerve cells. With the exception that calretinin may be transiently expressed in pyramidal neurons, the distribution of calbindin- and calretinin-immunoreactive structures to a large degree corresponds to that in the adult. Thus, little reorganisation is to be expected during proceeding development.