The connections formed between sensory and motor neurons (MNs) play a critical role in the control of motor behavior. During development, the axons of proprioceptive sensory neurons project into the spinal cord and form both direct and indirect connections with MNs. Two ETS transcription factors, ER81 and PEA3, are expressed by developing proprioceptive neurons and MNs, raising the possibility that these genes are involved in the formation of sensory-motor connections. Er81 mutant mice exhibit a severe motor discoordination, yet the specification of MNs and induction of muscle spindles occurs normally. The motor defect in Er81 mutants results from a failure of group Ia proprioceptive afferents to form a discrete termination zone in the ventral spinal cord. As a consequence there is a dramatic reduction in the formation of direct connections between proprioceptive afferents and MNs. ER81 therefore controls a late step in the establishment of functional sensory-motor circuitry in the developing spinal cord.

SUMMARY The reciprocal activation of flexor and extensor muscles constitutes the fundamental mechanism that tetrapod vertebrates use for locomotion and limb-driven reflex behaviors. This aspect of motor coordination is controlled by inhibitory neurons in the spinal cord; however, the identity of the spinal interneurons that serve this function is not known. Here we show that the production of an alternating flexor-extensor motor rhythm depends on the composite activities of two classes of ventrally-located inhibitory neurons, V1 and V2b interneurons (INs). Abrogating V1 and V2b IN-derived neurotransmission in the isolated spinal cord results in a synchronous pattern of L2 flexor-related and L5 extensor-related locomotor activity. Mice lacking V1 and V2b inhibition are unable to articulate their limb joints and display marked deficits in limb-driven reflex movements. Taken together, these findings identify V1- and V2b-derived neurons as the core interneuronal components of the limb central pattern generator (CPG) that coordinate flexor-extensor motor activity.

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SUMMARYWild tomato species in Solanum Section Lycopersicon often exhibit two types of reproductive barriers: selfincompatibility (SI) and unilateral incompatibility or incongruity (UI), wherein the success of an inter-specific cross depends on the direction of the cross. UI pollen rejection often follows the 'SI · SC' rule, i.e. pistils of SI species reject the pollen of SC (self-compatible) species but not vice versa, suggesting that the SI and UI pollen rejection mechanisms may overlap. In order to address this question, pollen tube growth was measured after inter-specific crosses using wild tomato species as the female parents and pollen from cultivated tomato (Solanum lycopersicum). Two modes of UI pollen rejection, early and late, were observed, and both differed from SI pollen rejection. The structure and expression of known stylar SI genes were evaluated. We found that S-RNase expression is not required for either the early or late mode of UI pollen rejection. However, two HT family genes, HT-A and HT-B, map to a UI QTL. Surprisingly, we found that a gene previously implicated in SI, HT-B, is mutated in both SI and SC S. habrochaites accessions, and no HT-B protein could be detected. HT-A genes were detected and expressed in all species examined, and may therefore function in both SI and UI. We conclude that there are significant differences between SI and UI in the tomato clade, in that pollen tube growth differs between these two rejection systems, and some stylar SI factors, including S-RNase and HT-B, are not required for UI.

Background. Although thrombosis associated with a fissured atherosclerotic plaque is believed to be the most common cause of acute coronary syndromes, the underlying factors that trigger plaque rupture are currently unknown. However, the mechanical behavior of the plaque is probably of critical importance.Methods and Results. To test the hypothesis that the mechanical properties of a plaque are dependent on its composition and, in particular, that the stiffness of fibrous caps changes within the range of frequencies carried by a physiological pressure wave, the stress-strain relation was studied in 27 fibrous caps and related to the underlying histological structure of the fibrous cap. Fibrous caps were obtained during 14 autopsies from the abdominal aorta and