Rib Cage Morphogenesis in the Human Embryo: A Detailed Three‐Dimensional Analysis

Abstract

Formation of the skeletal structure in the human embryo has important consequences in terms of support, protection, and function of organs and other systems. We aimed to describe the formation of the rib cage during the embryonic period, in order to detect prominent features and identify the possible factors affecting rib cage morphology. We employed high‐resolution digitized imaging data (n = 34) obtained in human embryos with Carnegie stage (CS) between 17 and 23. The rib cage became detectable as cartilage formation at CS17, expanding outward from the dorsal side of the chest‐abdominal region. Ribs elongated progressively to surround the chest, differentiating into the upper and lower rib cage regions by CS20. The ends of corresponding ribs in the upper region elongated toward each other, leading to their joining and sternum formation between CS21 and CS23, while the lower region of the rib cage remained widely open. The rib cage area with the largest width shifted from the 5th rib pair at CS17 to the 9th pair at CS23. The depth of the rib cage was similar across the upper region at CS17, with the major portion remaining in the middle part after CS20. The heart was located beneath the rib pairs providing the largest depth, while the liver was located beneath the rib pairs providing the largest width. Formation of the sternum, development of spinal kyphosis, and organization of larger internal organs within the thoracic and abdominal cavity are possible factors affecting rib cage morphology. Anat Rec, 302:2211–2223, 2019. © 2019 American Association for Anatomy

Human embryonic ribs all progress through common morphological forms irrespective of their position on the axis

“…Notably, the data from CS17 was excluded in Procrustes and PC analyses although the cartilaginous ribs are already detectable at this point . The morphology of all rib pairs at CS17 appeared similar, and also appeared similar to that of the 12th rib pair between CS18 and CS21 (Figure C).…”

“…Thus, the rib morphology assessed in the present study may reflect the gross features of rib morphology but not reveal each rib precisely. Second, the cartilaginous ribs have been detectable at CS17 . Estimation of the starting point of shape for each rib pair on our [Shape] scale is important although we could not provide the data for technical reasons.…”

“…Five samples at CS17 were subjected to visualization of morphology and morphometric analysis only. The present study used the same primary data as our recent study, in which we described the timeline of rib cage formation during the human embryonic period.…”

“…Recently, we described the timeline of rib cage formation during the human embryonic period . The rib cage became detectable as cartilage formation at Carnegie stage (CS) 17, expanding outward from the dorsal side of the chest‐abdominal region.…”

“…The factors that may influence human rib cage morphology during embryonic periods may differ from those at the fetal and postnatal period. The size and organization of internal organs (eg, the heart and liver) occupying a large space within the thoracic cavity, along with the surrounding anatomical structures comprise possible factors affecting rib cage morphogenesis during embryonic development . In contrast, the kinetics of respiration might not contribute substantially to rib cage morphology .…”