Ontogeny of calbindin immunoreactivity in the human hippocampal formation with a special emphasis on granule cells of the dentate gyrus.

Calbindin (CB) is a calcium-binding protein that is present in principal cells as well as in interneurons of the hippocampal formation of various species including humans. Studies with transgenic mice revealed that CB is essential for long-term potentiation and synaptic plasticity which are the cellular basis of learning and memory. In a previous study we have shown that CB expression in granule cells of the dentate gyrus correlates with the functional maturation of the hippocampal formation in the rat. In the present study we examined the ontogeny of CB using immunohistochemistry in the human hippocampal formation paying special attention to the granule cells of the dentate gyrus. As early as the 14(th) week of gestation (GW), CB was being expressed by pyramidal cells of CA1-3 regions in the deepest cell rows of the pyramidal layer towards the ventricular zone. Later, CB sequentially appears in more superficial cell rows. After midgestation, CB disappears from CA3 pyramidal neurons. Expression of CB by granule cells starts at the 22(nd)-23(rd) GW, first by the most superficial neurons of the ectal end of the dorsal blade. At the 24(th) GW, CB is expressed by granule cells of the crest and medial portion of the ventral blade whereas later the entire ventral blade revealed CB immunoreactivity. At term, and in the first few postnatal months, CB-immunoreaction is detected in granule cells of both blades except for those neurons in the deepest cell rows at the hilar border. At around 2-3 years of age, all granule cells of the entire cell layer are CB-immunoreactive. Axons of granule cells, the mossy fibers, start to express CB around the 30(th) GW in stratum lucidum of CA3a. With further development, CB is expressed in CA3b and c, as well as in the hilus. An adult-like pattern of CB-immunoreactivity could be observed at 11 years of age. Our results indicate that (i) CB is expressed by hippocampal pyramidal cells a few weeks before midgestation; (ii) similarly to rodents, migration of postmitotic human hippocampal pyramidal cells follows the inside-out gradient; (iii) CB was expressed transiently in pyramidal cells of the CA3 area of the human hippocampus; (iv) granule cells of the dentate gyrus start to express CB as early as midgestation; (v) maturation and migration of human granule cells follow the outside-in migrational gradient described in rodents and non-human primates; (vi) CB-immunoreactivity in the axon terminals of granule cells could be observed a few weeks before birth with a long-lasting increase in staining intensity postnatally; (vii) the maturation pattern of the CB-positive mossy fiber system suggests that the development of connectivity and the mature topographical termination pattern between dentate gyrus and the CA3 area of Ammon's horn in humans resembles that previously described for rodents; (viii) the dorsal-ventral delay in development may explain the topography of neuropathologic alterations of the granule cell layer found in temporal lobe epilepsy related to febrile seizures.