Objective. The nucleus pulposus (NP) of the intervertebral disc develops from the notochord. Humans and other species in which notochordal cells (NCs) disappear to be replaced by chondrocyte-like mature NP cells (MNPCs) frequently develop disc degeneration, unlike other species that retain NCs. The reasons for NC disappearance are unknown. In humans, the change in cell phenotype (to MNPCs) coincides with changes that decrease nutrient supply to the avascular disc. We undertook this study to test the hypothesis that the consequent nutrient stress could be associated with NC disappearance.Methods. We measured cell densities and metabolic rates in 3-dimensional cultures of porcine NCs and bovine MNPCs, and we determined survival rates under conditions of nutrient deprivation. We used scanning electron microscopy to examine end plate porosity of discs with NCs and those with MNPCs. Nutrientmetabolite profiles and cell viability were calculated as a function of cell density and disc size in a consumption/ diffusion mathematical model.Results. NCs were more active metabolically and more susceptible to nutrient deprivation than were MNPCs. Hypoxia increased rates of glycolysis in NCs but not in MNPCs. Higher end plate porosity in discs with NCs suggested greater nutrient supply in keeping with higher nutritional demands. Mathematical simulations and experiments using an analog disc diffusion chamber indicated that a fall in nutrient concentrations resulting from increased diffusion distance during growth and/or a fall in blood supply through end plate changes could instigate NC disappearance.Conclusion. NCs demand more energy and are less resistant to nutritional stress than MNPCs, which may shed light on the fate of NCs in humans. This provides important information about prospective NC tissue engineering approaches.The intervertebral discs are cartilaginous structures interspersed between the vertebral bodies, providing flexibility to the spinal column. They consist of 3 regions: the outer annulus fibrosus (AF) surrounding the inner nucleus pulposus (NP) and a thin hyaline cartilaginous end plate lying between the disc and the adjacent vertebral bodies. The AF and NP differ in developmental origin, with the annulus arising from the mesenchyme and the nucleus from the notochord (1,2). During development the highly hydrated NP is populated by clusters of large vacuolated notochordal cells (NCs) of distinct molecular phenotype (2,3). In humans and some other species (e.g., cattle, chondrodystrophoid dogs) but not in others (e.g., rodents, pigs), NCs disappear before maturity to be replaced by chondrocyte-like cells of unknown provenance (here called mature NP cells [MNPCs]), which synthesize a more collagenous and less hydrated matrix (1,4-6).

The surface morphology of synchronized P815Y mastocytoma cells has been examined by scanning electron microscopy. Early G~ cells are comparatively smooth or lightly villated, whereas at later stages the surface becomes progressively more villated. In G~ cells most microvilli have a uniform diameter, whereas in S and G2, cells, many microvilli show branching and often originate from much larger surface protuberances. Small "blebs" are seen on the surface of many cells but these structures do not appear to be a characteristic feature of cells at any one stage of the cell cycle. The presence of microvilli increases the total surface of the cell to such an extent that the ratio of volume to surface area remains constant throughout the cell cycle. The mechanism of cytokinesis is thus a physical one, involving the unfolding of previously accumulated microvilli.

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The expression of H–2 histocompatibility antigens, assayed by immune cytolysis, is minimal during the S period of the cell cycle. In order to investigate this phenomenon, we have measured the amount of H–2 antigens on P815Y cells at different stages of the cell cycle by two different techniques. The first involves the binding of [125I] labelled antibody, the second the inhibition of immune cytolysis of indicator cells by bound and salt‐extractable antigens. Each method shows that the amount of H–2 antigens increases during the G1 period and then remains constant. On the other hand, the fragility of cells assayed by cytolysis caused by detergents, hypotonicity or freeze thawing shows the same minimum in S as does the expression of H–2 measured by immune cytolysis. It is concluded that cell surface antigens are inserted into the plasma membrane during G1 and remain accessible to combination with antibody thereafter. Because of a decrease in cellular fragility during interphase, cytolytic techniques should be used with caution.