"Rib" - what is it, definition of the term
The thoracic cage of mammals, including rodents such as rats and mice, consists of a series of curved skeletal elements attached posteriorly to the vertebral column and anteriorly to the sternum; each element shields the lungs, provides attachment points for intercostal muscles, and maintains the structural integrity of the chest wall.
Detailed information
Costal bones in rodents form the protective framework of the thoracic cavity. Each element consists of a hollow, mineralized shaft surrounded by a periosteal layer and articulates with adjacent vertebrae and sternum. The structure provides attachment points for intercostal muscles and supports the respiratory mechanics of small mammals.
In laboratory rats and mice, the rib series typically comprises twelve pairs on each side, though variations occur. The first pair is often reduced or fused with the sternum, creating a cranial costal cartilage. Posterior ribs become progressively shorter, terminating near the lumbar vertebrae. The following list summarizes common rib counts:
- Common laboratory rat: 12 pairs, with the first pair sometimes absent as a distinct bone.
- Common laboratory mouse: 12 pairs, occasional loss of the first rib in certain strains.
- Wild‑type variants: 11–13 pairs, reflecting genetic and developmental diversity.
Developmentally, rib formation initiates during embryogenesis from the sclerotome of somites. Chondrogenic condensation appears around day 10 in mouse embryos and day 12 in rat embryos, followed by ossification that completes by the end of the postnatal third week. Gene expression patterns, including Hox, Myf5, and Pax1, regulate segmental identity and growth rates.
Physiologically, the costal elements enable expansion of the thoracic cavity during inspiration. Intercostal muscles insert on the ventral surface, generating lateral and dorsoventral forces that alter lung volume. In small mammals, the high respiratory rate necessitates efficient rib movement, achieved through a combination of pivoting at the vertebral joints and slight sternal displacement.
Research applications frequently involve rib analysis. Skeletal imaging (micro‑CT) quantifies bone density and morphology, providing biomarkers for metabolic studies. Histological sections reveal marrow composition, useful in toxicology and endocrine investigations. Additionally, rib removal or fracture models serve to assess pain pathways and analgesic efficacy in rodents.
Overall, costal bones constitute a critical anatomical component in rats and mice, with defined morphology, developmental timing, and functional relevance that underpin both normal physiology and experimental methodologies.