The cellular 3D printer of a marine bristle worm—chaetogenesis in Platynereis dumerilii (Audouin & Milne Edwards, 1834) (Annelida)

Abstract

Annelid chaetae are extracellular chitinous structures that are formed in an extracellular epidermal invagination, the chaetal follicle. The basalmost cell of this follicle, the chaetoblast, serves like a 3D-printer as it dynamically shapes the chaeta. During chaetogenesis apical microvilli of the chaetoblast form the template for the chaeta, any structural details result from modulating the microvilli pattern. This study describes this process in detail in the model organism Platynereis dumerilii and clarifies some aspects of chaetogenesis in its close relative Nereis vexillosa, the first annelid in which the ultrastructure of chaetogenesis had been described. Nereid species possess compound chaetae characteristic for numerous subgroups of errant annelids. The distal most section of these chaetae is movable; a hinge connects this part of the chaeta to the shaft. Modulation of the microvilli and differences in their structure, diameter and number of microvilli, and their withdrawal and reappearance determine the shape of these compound chaetae. Chaetal structure and pattern also change during life history. While larvae possess a single type of chaeta (in addition to internal aciculae), juveniles and adults possess two types of chaetae that are replaced by large paddle-shaped chaetae in swimming epitokous stages. Chaetogenesis is a continuous process that lasts during the entire lifespan. The detailed developmental sequence of chaetae and their site of formation are very similar within species and species groups. We expect that similarity results from a conserved gene regulatory network making this an optimal system to test the phylogenetic affinity of taxa and the homology of their chaetae.

Dynamic microvilli sculpt bristles at nanometric scale

“…In other contexts, such as the vertebrate hair cells or lateral line neuromasts, elaborate cellular geometries rely on the occurrence of specialised apical microvilli 22 . Consistent with the role of microvilli in polychaete bristle biogenesis, classical ultrastructural work has found cellular protrusions to be associated with bristles in the annelid Nereis vexillosa 7 , and recent analyses have confirmed largely similar geometries in chaetoblasts of Platynereis individuals fixed at different stages of development 9 . To systematically probe for a correlation of microvillar geometry at the cell cortex of Platynereis chaetoblasts and the biogenesis of diagnostic bristle features, we performed a systematic analysis of chaetoblast microvilli at distinct times of development associated with the production of characteristic bristle features.…”

“…Subsequent disassembly of actin and concomitant retraction is accompanied by displacement of the microvillus to the distal edge of the blade, with a total of 10–12 microvilli present in the entire diameter of the blade at a given time. This model explains both the polarity of the blade and its slender shape, and differs from previous assumptions that teeth would be sculpted as separate structures and fused to the blade 7 , or that microvilli would fuse to maintain constant numbers 9 .…”

“…Spatial reconstructions thus precisely inform about the spatiotemporal changes in the chaetoblast surface geometry over the course of hours. Whereas a recent study has suggested a link between individual microvilli and the formation of teeth, the precise steps of how this would be achieved have remained elusive, and the concept of merging/fusing microvilli has been inferred to explain the retention of a stable blade width despite continuous addition of teeth 9 . Combined with the analysis and manipulation of F-actin geometries, our 3D reconstructions allowed us to derive a distinct mechanistic model for tooth formation: For each tooth, a new microvillus is initiated at the serrated edge of the blade, and extension of F-actin fibres over several cycle lengths is needed for the synthesis of an apical chitin cap.…”

“…Moreover, our model also implies the chaetoblast microvilli themselves as relevant sites of tooth synthesis. As outlined before, a long-standing assumption is that microvilli would merely act as moulds for a chitin matrix cast by basal parts of the chaetoblast or adjacent follicle cells 7 – 9 , possibly by shedding of N-acetylglucosamine monomers that spontaneously polymerise in the extracellular space 9 . By contrast, the chaetoblast-specific expression of cs3 , and the observed similarity of cs3 crispants with the phenotype obtained by general inhibition of chitin synthases—combined with the localisation of chitin synthases to tips of microvilli in the insect gut 27 , 28 , and the recent validation that chitin synthases act as transmembrane proteins capable of directional synthesis 19 —collectively rather support the alternative model in which the chaetoblast works like the biological correlate of a 3D printer relying on a material jetting principle 10 .…”

“…Due to the lack of a suitable experimental model, however, it has never been tested if the chaetoblast surface is instructive for bristle shape. Likewise, it has remained unclear if the chaetoblast just provides a 3-dimensional mould for extracellular material secreted from the neighbouring follicle cells—a long-standing model 7 – 9 that we refer to as a mould/cast mechanism; or if, alternatively, chaetoblasts themselves actively secrete bristle material, in a biological mechanism resembling 3D printing 10 . To solve these fundamental questions, we explored the bristleworm Platynereis dumerilii , a laboratory model amenable to molecular experimentation 11 .…”