Spatiotemporal cellular dynamics of the notochord shape intervertebral disc morphogenesis in the mouse embryo through apoptosis and proliferation

BACKGROUND: The notochord is a midline structure essential for vertebrate embryogenesis, contributing to the development of the nervous system, digestive tract, and vertebral column. In particular, notochord signaling is indispensable for proper patterning and coordinated development of alternating vertebrae and intervertebral discs (IVDs). Later, notochordal cells (NCs) mature and adopt a characteristic vacuolated morphology before giving rise to the core of the forming IVD, the nucleus pulposus (NP). Postnatally, NCs play a pivotal role in maintaining disc integrity through the secretion of specific factors and extracellular matrix (ECM). Despite its importance in disc formation and homeostasis, the morphogenetic mechanisms underlying the notochord's transformation into the NP are insufficiently characterized. RESULTS: We conducted a comprehensive histological and immunohistochemical analysis to investigate the cellular events governing NP formation in the mouse developing spine. Temporal analysis of intracytoplasmic vacuole formation using Lamp-1 marker revealed that vacuolation contributed to NP growth, while cell density progressively decreased. In addition, quantitative analyses demonstrated a notable proliferative capacity within notochordal cells coupled with region-specific apoptotic activity in sclerotome, at future disc sites. CONCLUSIONS: This study highlights the intricate balance of cellular proliferation, programmed cell death, matrix remodeling, and vacuolation dynamics as key determinants in shaping the NP along the rostro-caudal axis.