John Jellies

Muscle assembly in simple systems

Muscle development has been the subject of intense scrutiny at cellular, biochemical and molecular levels, yet little is known about the factors that generate anatomically distinct muscles during embryogenesis. We now know that at least some muscles are initially organized by interactions with particular cells that appear early in development, the muscle organizers. These organizers have been described in both arthropods and annelids, and serve similar functions in both groups: they provide cellular scaffolding during the early and relatively simple anatomical stages of embryogenesis, which is then used to pattern the assembly of large numbers of pre-myocytes. Thus, single cells provide an early framework that is retained even as the embryo becomes vastly more complex. Furthermore, studies have shown that the muscle organizer is necessary for motor neuron growth cones to locate (or recognize) the appropriate target region. In other words, the motor neurons extend toward muscles during muscle histogenesis and can use the muscle organizer for guidance, rather than the definitive muscle which has not yet emerged. The discovery of these identifiable tissue organizers has opened several intriguing avenues by which the roles of cell-cell interactions in development can be further addressed. Additionally, the discovery of these cells implies that in order to understand the ways in which motor neurons are initially matched to particular muscle targets we should also study the organizers that may provide positional introductions between future synaptic partners.

Tract formation and axon fasciculation of molecularly distinct peripheral neuron subpopulations during leech embryogenesis

In leech, the central projections of peripheral sensory neurons segregate into specific axonal tracts, which are distinguished by differential expression of surface antigens recognized by the monoclonal antibodies Lan3-2 and Lan4-2. Lan3-2 recognizes an epitope expressed on axons that segregate into three distinct axon fascicles. In contrast, the Lan4-2-positive axons selectively project into only one of the Lan3-2-positive axon tracts. These observations provide evidence for a hierarchy of guidance cues mediating specific pathway formation in this system. Since the Lan3-2 antibody has been shown to perturb this process and since, as shown here, the Lan3-2 and Lan4-2 antigens are closely molecularly interrelated, these antibodies may help define molecules and epitopes mediating neuronal recognition and axonal guidance.

Hierarchical guidance cues and selective axon pathway formation

Publisher Summary This chapter explores the developmental mechanisms of navigation and selective fasciculation of peripheral sensory neurons in the embryo of the leech. It shows that central nervous system (CNS)-derived guidance cues are necessary for correct pathway formation of these neurons into the CNS. In the absence of these cues, the growing axons lose their orientation and default to fasciculation onto each other, often forming circular paths. In normal development, when the peripheral neurons reach the CNS, they segregate into four well-defined axonal tracts. The chapter also demonstrates that at least two antigenically distinct subpopulations of peripheral neurons can be defined which have selective affinity for two different pathways. These results provide compelling evidence for the existence of a hierarchy of guidance cues mediating specific tract formation in this system. The chapter proposes that different subsets of peripheral neurons differentially express other molecules or epitopes that can interact with CNS-derived guidance cues, possibly through heterophilic mechanisms and signal transduction events. These interactions override the common fasciculation and mediate selective segregation and tract formation.

Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech, Hirudo medicinalis

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.