Abstract
For decades, the mechanism of skeletal patterning along a proximal-distal axis has been an area of intense inquiry. Here, we examine the development of the ribs, simple structures that in most terrestrial vertebrates consist of two skeletal elements-a proximal bone and a distal cartilage portion. While the ribs have been shown to arise from the somites, little is known about how the two segments are specified. During our examination of genetically modified mice, we discovered a series of progressively worsening phenotypes that could not be easily explained. Here, we combine genetic analysis of rib development with agent-based simulations to conclude that proximal-distal patterning and outgrowth could occur based on simple rules. In our model, specification occurs during somite stages due to varying Hedgehog protein levels, while later expansion refines the pattern. This framework is broadly applicable for understanding the mechanisms of skeletal patterning along a proximal-distal axis.
Highlights
During evolution, a number of changes in vertebrate body plan allowed terrestrial tetrapod species to thrive on land and take advantage of new habitats
Previous studies have demonstrated the importance of Hedgehog (Hh) signaling for sclerotome induction and specification
(Charrier et al, 2001; Thibert et al, 2003) and in the absence of Sonic hedgehog (Shh) or Shh-producing cells in the floor plate and notochord the number of somite cells going through apoptosis is greatly increased (Teillet et al, 1998)
Summary
A number of changes in vertebrate body plan allowed terrestrial tetrapod species to thrive on land and take advantage of new habitats. Current tetrapod species typically have ribs that are subdivided into two segments, a proximal endochondral bony segment connected to the vertebrae, and a distal permanent cartilage segment that articulates with the sternum (Fig. 1A, B). The rib bones support the body wall and protect the internal organs; the costal cartilage maintains thoracic elasticity, allowing respiration while still enclosing the thoracic cage. Clues from the fossil record are beginning to reveal when the enclosed rib cage arose during evolution (Daeschler et al, 2006; Pierce et al., 2013), little is known about what changes occurred during embryogenesis to extend the ribs around the body and to connect the ribs to the sternum via a costal cartilage element (Brainerd, 2015). Using genetic and computational approaches, we generate a plausible model for how two rib segments form during development
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