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Day postconception

Figure 6 Differential maturation of the appendages, epidermis, and free nerve endings of the star at different postnatal stages. a, The proportion of the star taken up by the tactile fovea (appendage 11) over the course of development. b, Thin section of the epidermis of appendage 11 on postnatal day 1 showing the differentiated Eimer's organs consisting of keratinocytes in separate papillae (arrows). c, Thin section of appendage 6 at the same stage showing the undifferentiated epidermis. d, The adult configuration of free nerve endings in the epidermis at the apex of a mature Eimer's organ. The fibers form a remarkable hub-and-spoke configuration of nerve terminals just below the skin surface. e, On postnatal day 18, the fibers at the apex of each Eimer's organ on the 11th appendage have formed the hub-and-spoke configuration, though short extensions of the neurites remain. f, The fibers at the apex of each Eimer's organ on the lateral appendages remain disorganized, although they have reached the appropriate location. Thus, the 11th appendage leads the development of the star, not only in size, but also in the rate of sensory organ maturation. Scale bars: b and c, 50 mm; d-f, 5 mm. Reproduced from Catania, K. C. 2001. Early development of a somatosensory fovea: A head start in the cortical space race? Nat. Neurosci. 4, 353-354, with permission.

competitive nature of afferent distributions in cortical maps provides a developmental mechanism for skewing the allocation of cortical territory in favor of some inputs - namely those that lead the development of a sensory surface and are therefore most active, in the greatest numbers, at the earliest time.

It is clear that much larger-scale changes to brain organization have occurred in many mammalian lineages. Obvious examples include the Primate and Carnivore orders that have brains with more cortical subdivisions than smaller-brained rodents and insectivores. In this regard it is significant that star-nosed moles have more cortical subdivisions in their soma-tosensory system (S1, S2, and S3) than other moles and shrews, which have only two areas (S1 and S2). It seems likely that the additional area facilitates processing of the high volumes of complex sensory information from the star. Such changes in the number of cortical areas require corresponding changes in gene expression in the CNS, and recent studies suggest potential mechanisms by which these more global changes can occur (Fukuchi-Shimogori and Grove, 2001). However, it is possible that a modified sensory periphery often sets the stage for subsequent adaptive modifications to the CNS in the course of evolution (see Evolution of the Somatosensory System - Clues from Specialized Species). In support of this possibility, Bush et al. (2004) have found fossil evidence that suggests high-acuity vision preceded brain expansion in anthropoid evolution.

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