The neuroarchitecture of Acoela has been at the center of morphological debates. Some authors, using immunochemical tools, suggest that the nervous system in Acoela is organized as a commissural brain that bears little resemblance to the central, ganglionic type brain of other flatworms, and bilaterians in general. Others, who used histological staining on paraffin sections, conclude that it is a compact structure (an endonal brain; e.g., Raikova 2004; von Graff 1891; Delage Arch Zool Exp Gén 4:109-144, 1886). To address this question with modern tools, we have obtained images from serial transmission electron microscopic sections of the entire hatchling of Symsagittifera roscoffensis. In addition, we obtained data from wholemounts of hatchlings labeled with markers for serotonin and tyrosinated tubulin. Our data show that the central nervous system of a juvenile S. roscoffensis consists of an anterior compact brain, formed by a dense, bilobed mass of neuronal cell bodies surrounding a central neuropile. The neuropile flanks the median statocyst and contains several types of neurites, classified according to their types of synaptic vesicles. The neuropile issues three pairs of nerve cords that run at different dorso-ventral positions along the whole length of the body. Neuronal cell bodies flank the cords, and neuromuscular synapses are abundant. The TEM analysis also reveals different classes of peripheral sensory neurons and provides valuable information about the spatial relationships between neurites and other cell types within the brain and nerve cords. We conclude that the acoel S. roscoffensis has a central brain that is comparable in size and architecture to the brain of other (rhabditophoran) flatworms.

Considerable progress has been made in the understanding of molecular and cellular mechanisms controlling the development of the mammalian cortex. The proliferative and neurogenic properties of cortical progenitors located in the ventricular germinal zone start being understood. Little is known however on the cis-regulatory control that finely tunes gene expression in these progenitors. Here, we undertook an in silico-based approach to address this question, followed by some functional validation. Using the Eurexpress database, we established a list of 30 genes specifically expressed in the cortical germinal zone, we selected mouse/human conserved non-coding elements (CNEs) around these genes and we performed motif-enrichment search in these CNEs. We found an over-representation of motifs corresponding to binding sites for Pax, Sox, and Lhx transcription factors, often found as pairs and located within 100bp windows. A small subset of CNEs (n=7) was tested for enhancer activity, by ex-vivo and in utero electroporation assays. Two showed strong enhancer activity in the germinal zone progenitors. Mutagenesis experiments on a selected CNE showed the functional importance of the Pax, Sox, and Lhx TFBS for conferring enhancer activity to the CNE. Overall, from a cis-regulatory viewpoint, our data suggest an input from Pax, Sox and Lhx transcription factors to orchestrate corticogenesis. These results are discussed with regards to the known functional roles of Pax6, Sox2 and Lhx2 in cortical development.

XLF/Cernunnos is a component of the ligation complex used in classical non-homologous end-joining (cNHEJ), a major DNA double-strand break (DSB) repair pathway. We report neurodevelopmental delays and significant behavioral alterations associated with microcephaly in Xlf-/- mice. This phenotype, reminiscent of clinical and neuropathologic features in humans deficient in cNHEJ, is associated with a low level of apoptosis of neural cells and premature neurogenesis, which consists of an early shift of neural progenitors from proliferative to neurogenic divisions during brain development. We show that premature neurogenesis is related to an increase in chromatid breaks affecting mitotic spindle orientation, highlighting a direct link between asymmetric chromosome segregation and asymmetric neurogenic divisions. This study reveals thus that XLF is required for maintaining symmetric proliferative divisions of neural progenitors during brain development and shows that premature neurogenesis may play a major role in neurodevelopmental pathologies caused by NHEJ deficiency and/or genotoxic stress.