Neurobiology of the basal platyhelminth Macrostomum lignano: map and digital 3D model of the juvenile brain neuropile

Abstract

We have analyzed brain structure in Macrostomum lignano, a representative of the basal platyhelminth taxon Macrostomida. Using confocal microscopy and digital 3D modeling software on specimens labeled with general markers for neurons (tyrTub), muscles (phalloidin), and nuclei (Sytox), an atlas and digital model of the juvenile Macrostomum brain was generated. The brain forms a ganglion with a central neuropile surrounded by a cortex of neuronal cell bodies. The neuropile contains a stereotypical array of compact axon bundles, as well as branched terminal axons and dendrites. Muscle fibers penetrate the flatworm brain horizontally and vertically at invariant positions. Beside the invariant pattern of neurite bundles, these "cerebral muscles" represent a convenient system of landmarks that help define discrete compartments in the juvenile brain. Commissural axon bundles define a dorsal and ventro-medial neuropile compartment, respectively. Longitudinal axons that enter the neuropile through an invariant set of anterior and posterior nerve roots define a ventro-basal and a central medial compartment in the neuropile. Flanking these "fibrous" compartments are neuropile domains that lack thick axon bundles and are composed of short collaterals and terminal arborizations of neurites. Two populations of neurons, visualized by antibodies against FMRFamide and serotonin, respectively, were mapped relative to compartment boundaries. This study will aid in the documentation and interpretation of patterns of gene expression, as well as functional studies, in the developing Macrostomum brain.

The evolutionary origins of glia

“…Major controversies have raged among taxonomists and evolutionary biologists over such issues. In the present context, for example, Morris et al (2007) opine that ''the absence of morphologically detectable glial cells in basal flatworms. .…”

Architecture of the nervous system in two Dactylopodola species (Gastrotricha, Macrodasyida)

“…The gross anatomy of the brain is a bilobed or trilobed (one lateral paired cluster plus an anterior cluster) cluster of neurons (e.g. for a review see Reuter et al 2001;Morris et al 2007). In comparison to these data, the brain of gastrotrichs has most similarities with the acoel brain and these may be interpreted as the ancestral conditions (see discussion below).…”

The nervous system of Neodasys chaetonotoideus (Gastrotricha: Neodasys) revealed by combining confocal laserscanning and transmission electron microscopy: evolutionary comparison of neuroanatomy within the Gastrotricha and basal Protostomia

“…The black circle refers to the stem species of Gastrotricha b Zoomorphology (2011) 130:51-84 79 dorsal or ventral of the dorsal commissure), which is more comparable to the conditions in many lophotrochozoans (e.g. Platyhelminthes, see Morris et al 2007 for Macrostomum lignano with a detailed description of the compact brain). Nevertheless, Remane (1926Remane ( , 1927Remane ( , 1936) described a massive dorsal commissure and excluded the presence of a ventral commissure.…”