Neuronal migration and the evolution of the human brain

Migration Pathways of Thalamic Neurons and Development of Thalamocortical Connections in Humans Revealed by Diffusion MR Tractography

The thalamus plays an important role in signal relays in the brain, with thalamocortical (TC) neuronal pathways linked to various sensory/cognitive functions. In this study, we aimed to see fetal and postnatal development of the thalamus including neuronal migration to the thalamus and the emergence/maturation of the TC pathways. Pathways from/to the thalami of human postmortem fetuses and in vivo subjects ranging from newborns to adults with no neurological histories were studied using high angular resolution diffusion MR imaging (HARDI) tractography. Pathways likely linked to neuronal migration from the ventricular zone and ganglionic eminence (GE) to the thalami were both successfully detected. Between the ventricular zone and thalami, more tractography pathways were found in anterior compared with posterior regions, which was well in agreement with postnatal observations that the anterior TC segment had more tract count and volume than the posterior segment. Three different pathways likely linked to neuronal migration from the GE to the thalami were detected. No hemispheric asymmetry of the TC pathways was quantitatively observed during development. These results suggest that HARDI tractography is useful to identify multiple differential neuronal migration pathways in human brains, and regional differences in brain development in fetal ages persisted in postnatal development.

Stress-induced localization of HSPA6 (HSP70B') and HSPA1A (HSP70-1) proteins to centrioles in human neuronal cells

The localization of yellow fluorescent protein (YFP)-tagged HSP70 proteins was employed to identify stress-sensitive sites in human neurons following temperature elevation. Stable lines of human SH-SY5Y neuronal cells were established that expressed YFP-tagged protein products of the human inducible HSP70 genes HSPA6 (HSP70B') and HSPA1A (HSP70-1). Following a brief period of thermal stress, YFP-tagged HSPA6 and HSPA1A rapidly appeared at centrioles in the cytoplasm of human neuronal cells, with HSPA6 demonstrating a more prolonged signal compared to HSPA1A. Each centriole is composed of a distal end and a proximal end, the latter linking the centriole doublet. The YFP-tagged HSP70 proteins targeted the proximal end of centrioles (identified by γ-tubulin marker) rather than the distal end (centrin marker). Centrioles play key roles in cellular polarity and migration during neuronal differentiation. The proximal end of the centriole, which is involved in centriole stabilization, may be stress-sensitive in post-mitotic, differentiating human neurons.

Scaling of neuron number and volume of the pulvinar complex in New World primates: comparisons with humans, other primates, and mammals

The lateral posterior nucleus and pulvinar (LP-pulvinar complex) are the principal thalamic nuclei associated with the elaborate development of the dorsal and ventral streams of the parietal cortex in primates. In humans, a novel site of origin for a subpopulation of pulvinar neurons has been observed, the ganglionic eminence of the telencephalon. This additional site of neuron origin has been proposed to contribute to the pulvinar's evolutionary expansion (Letinic and Rakic [2001] Nat Neurosci 4:930-936). Studies of neuron number in the LP-pulvinar complex in gibbon, chimpanzee, and gorilla compared to humans, however, did not show that the human LP-pulvinar was unexpectedly large (Armstrong [1981] Am J Phys Anthropol 55:369-383). Here we enlarge the allometric basis for comparison by determining neuron number in the LP-pulvinar complex of six New World primates (Cebus apella, Saimiri ustius, Saguinus midas niger, Alouatta caraya, Aotus azarae, and Callicebus moloch) as well as measuring LP-pulvinar volume in a further set of 24 species including additional primates, carnivores, and rodents. The volume of the LP-pulvinar complex scaled with positive allometry with respect to brain volume across all species examined. The scaling of the number of neurons in the LP-pulvinar complex was extremely similar in New World primates and anthropoid apes, with the human LP-pulvinar value close to the regression line. Comparison of the relative volumes of the LP-pulvinar in the larger sample confirmed this observation, and further demonstrated that both primates and carnivores showed a "grade shift" in its size compared to rodents, with the pulvinar comprising a greater proportion of total brain volume across the board. Diurnal, nocturnal, or crepuscular niche did not discriminate LP-pulvinar size across taxa.

Neuronal migration and molecular conservation with leukocyte chemotaxis

Cell migration is essential in species ranging from bacteria to humans (for recent reviews, see Lauffenburger and Horwitz 1996; Mitchison and Cramer 1996; Montell 1999). In the amoebae Dictyostelium discoideum, cell migration is involved in chemotaxis toward food sources and in aggregation (for review, see Devreotes and Zigmond 1988; Parent and Devreotes 1999; Chung et al. 2001). In higher vertebrates, cell migration plays crucial roles in multiple physiological and pathological processes. During embryonic and neonatal development, cell migration is crucial in morphogenetic processes such as gastrulation, cardiogenesis, and the formation of the nervous system (for review, see Hatten and Mason 1990; Rakic 1990; Hatten and Heintz 1998; Bentivoglio and Mazzarello 1999). In adult animals, cell migration is required for leukocyte trafficking and inflammatory responses (for review, see McCutcheon 1946; Harris 1954; Devreotes and Zigmond 1988). In tumoriogenesis, tumor-induced angiogenesis and tumor metastasis both involve cell migration. Although it is well known that cell migration is necessary for all these processes, our understanding of mechanisms controlling cell migration is still limited. Here we briefly review the significance of neuronal migration and focus on recent studies on the directional guidance of neuronal migration, discussing the possibility that guidance mechanisms for neurons are conserved with those for other somatic cells.

Ganglionic eminence of the human fetal brain--new vistas

This review deals with recent findings concerning the complex functions of the ganglionic eminence (GE), which represents a conspicuous domain of the telencephalic proliferative zone and persists nearly throughout fetal life. The GE not only contains precursor neurons of the basal ganglia, it also contributes significantly to the population of interneurons in the cerebral cortex and to a population of thalamic neurons. The latter migrate through a distinct transient structure, the gangliothalamic body (GTB). The GE also represents an intermediate target for growing thalamic axons (on their way to the cerebral cortex) and cortical axons (on their way to the thalamus). In developmental neuropathology the GE plays an important role in prematurely born infants. The pathogenesis of GE bleedings is discussed with regard to the abundant expression of interleukin-6 (IL-6) receptors on GE cells. The consequences of such bleedings are discussed in view of cellular responses, such as the induction of leukemia inhibitory factor (LIF) expression in GE cells after hemorrhage.