Development of a Three-Layered Fascial Architecture in the Rabbit Carotid Sheath: Histological Findings Consistent With Mechanical Influences.

Current understanding of the fascial structures within the human carotid sheath (CS) is largely based on static anatomical descriptions, with limited knowledge of their dynamic developmental process. This study aimed to characterise the developmental progression of the fascial architecture surrounding neurovascular components within the CS across four stages in the New Zealand White rabbit. Twenty New Zealand White rabbits were divided into four groups (n = 5 each): early foetal (Gestational Day 10), late foetal (Gestational Day 20), newborn (3 days postnatal) and adult (3-4 months). Whole transverse cervical sections were processed with H&E staining and α-SMA immunohistochemistry. In the rabbit, the common carotid artery, internal jugular vein and vagus nerve within the CS consistently exhibited a three-layered fascial organisation comprising an inner fascial layer, an intermediate zone and an outer fascial layer. The arterial inner fascia showed the earliest onset and most pronounced thickening, forming a dense multilayered fibrous structure, while the venous and neural inner fasciae remained comparatively thin. This differential thickening pattern corresponds to the varying levels of intrinsic pulsatile haemodynamic stress among the three structures. The outer fascial layers of adjacent structures showed potential fusion, likely reflecting extrinsic mechanical interactions from surrounding tissues. This study demonstrates a consistent three-layered fascial organisation within the CS across developmental stages, and these histomorphological findings are consistent with a possible role of dual mechanical influences in CS development.