Calcium-Binding Proteins in the Human Developing Brain

Abstract

Recent studies have demonstrated that antibodies against the calcium-binding proteins (CaBPs) parvalbumin (PV), calbindin (CB), and calretinin (CR) are appropriate tools for demonstrating transient features and developmental changes of human fetal brain organization as well as for detecting specific alterations in pathologically altered specimens. CB and CR are abundantly expressed in various nerve cell types of the subplate in the second half of gestation. The subplate being an outstandingly wide zone subjacent to the cortical plate, it is a "waiting compartment" for various cortical afferents that reside here prior to entering the cortical plate. The cortical plate (future layers II-VI of the cerebral cortex) contains only CR-ir neurons until the 6th gestational month. In the 7th and 8th month, cortical CB- and PV-ir interneurons are observed in deeper portions of the cortical plate. Cajal-Retzius cells of layer I are CR-immunolabeled from the 4th month onwards. Fetal hydrocephalus causes severe alterations of CB- and PV-ir neurons in the subplate and the cortical plate: shrinkage of ir neurons, loss of process labeling and in most severe cases, entire loss of immunolabeling. Such alterations, which cannot be detected in Nissl-stained sections, indicate distinct impairment of neuronal function. The ganglionic eminence being a prominent part of the telencephalic proliferative zone persists nearly throughout the entire fetal period. Between 16 and 24 weeks of gestation, CR-ir cells are found in the center and, in a higher number, in the periphery, i.e., the mantle zone, of the ganglionic eminence. The mantle zone also exhibits CB-ir cells. These observations support experimental data showing that CR-ir precursor cells leave the ganglionic eminence to migrate towards the cerebral cortex. The CR- and CB-ir neurons of the mantle zone most probably represent an intermediate target for outgrowing axons. This notion is supported by the observation that SNAP (synaptosomal associated protein) 25-ir fibers coming from the intermediate zone terminate upon CR-ir cells in the mantle zone. Within the amygdaloid complex, immature, migrating CR- or CB-ir neurons are observed in the 5th and 6th gestational month. In the 8th and 9th month, anti-CR and anti-CB mark different subsets of interneurons as well as a small proportion of pyramidal projection neurons. The different subsets of interneurons are likely to be functionally different with regard to their connectivities. Considering studies in the literature, it is obvious that CR is transiently expressed in pyramidal cells. Moreover, diffuse (neuropil) CB and CR immunolabeling, which is found in different intensities in the various amygdaloid subdivisions, displays distinct redistribution during development, an observation indicating reorganization of afferent inputs. The sequential arrival of various afferent fiber systems in the two compartments of the striatum (patch and matrix compartment) is reflected by changing patterns of diffuse CB immunolabeling: During the second half of gestation, the patches are labeled and postnatally a changeover to matrix labeling is seen. The thalamic reticular complex reveals prominent transient features seen in PV and CR immunopreparations. Four subdivisions become obvious: the main portion, the perireticular nucleus, the medial subnucleus, and the pregeniculate nucleus. The PV- and CR-ir perireticular nucleus, not visible in the mature brain, is a distinct fetal entity located within the internal capsule. The main portion of the reticular complex is much more prominent in the fetus than in the adult and displays transitory CR expression. The most probable developmental role of the reticular complex is to provide guiding cues for outgrowing axons from or into the dorsal thalamus. The basal nucleus of Meynert and the hypothalamic tuberomamillary nucleus both provide extrathalamic projections to the cerebral cortex. The sequential differentiation of the two nuclei can be demonstrated using anti-CB and anti-PV. The basal nucleus strongly expresses CB and appears to be mature distinctly earlier than the PV-ir tuberomamillary nucleus. Antisera against CaBPs clearly demonstrate that the magnocellular part of the red nucleus located in the mesencephalic tegmentum is outstanding in the fetal and perinatal brain and inconspicuous in the adult. In particular, CB is the most abundant CaBP in this portion of the red nucleus. The dominance of the magnocellular part over the parvocellular part may be a substrate for a specific transitory pattern of motor behavior. On the whole, CaBPs mark the transient architectonic organization of the brain, which is involved in the establishment of transitory neuronal circuitries. The latter are essential for the formation of mature projections. Detailed data on the normal organization of the transient structures are required for the evaluation of alterations occurring in the fetal and perinatal brain. The transient structures are sites of predilection for alteration caused by hypoxia-ischemia, hemorrhage, or hydrocephalus.