The development of the human amygdala during early embryonic life ,

The development of the human amygdaloid complex is described beginning at the time that it first appears, when the cerebral hemispheres begin to evaginate (approximately 8‐‐9 mm), and including a fetus of 27.4 mm CR length Both Nissl and protargol silver series, transversely and sagittally sectioned, were used. The earliest cell migration representing the striatal complex is from the germinal epithelium in the region of the interventricular foramen, lateral to the primordial hippocampal formation. This is the characteristic topographic location of the amygdala, which, therefore, is the first portion of the human striatal complex to appear embryologically. All of the amygdaloid nuclei identified develop by the migration of neuroblasts from the germinal epithelium. The three main subdivisions of the amygdaloid complex (anterior amygdaloid area, corticomedial complex and basolateral nuclear group) are identifiable almost immediately after the primordial amygdala appears (9.5 mm in the material studied). In this embryo the cortical and medial nuclei are identifiable, but the central nucleus was not seen until 22.2 mm and the nucleus of the lateral olfactory tract was not identified even in the oldest fetuses. The basolateral complex differentiates much later than the corticomedial complex and all of the nuclear components became identifiable at the same time (20.7 mm embryo). However the basal nucleus is best represented, the accessory basal next in size, and the lateral only just appearing. In the oldest fetus the subdivisions of the basal and accessory basal nuclei seen in the adult brain are recognizable. Even before the individual nuclei appear, the basolateral complex is much larger than the corticomedial complex. The lateral to medial rotation of the amygdaloid complex has not yet begun in the oldest fetuses, but the posterior end of the lateral ventricle is beginning to turn anteriorly. During early development, all cells of the amygdala are derived from the lateral striatal ridge, the only one present until 14.0 mm. At 20.7 mm and later neuroblasts are contributed from the medial striatal ridge also. Although the greater part of the basolateral amygdaloid complex is derived from the lateral ridge, some cells are contributed from the medial ridge, particularly to the accessory basal nucleus. By contrast, the corticomedial complex is derived in part from the medial striatal ridge, after it appears, even though the early development is from the lateral ridge. Comparisons between the amygdala at different developmental stages and the amygdala in fishes, in amphibians, in reptiles and in lower mammals are made. The amygdala in fishes, in amphibians, in reptiles and in lower mammals are made. The topographic relationships between the amygdala and the hippocampal formation, the piriform cortex and the caudate, the putamen and the globus pallidus are considered.