![]() ![]() There is also greater diffusion in the inner annulus compared with the outer annulus, consistent with the higher water content found in the inner annulus. ![]() 36 Because of the higher water content in the nucleus, this region shows greater diffusion coefficients, especially for smaller solutes. 3, 26 Conversely, a hydrated matrix demonstrates greater diffusion. Low hydration inhibits the movement of molecules into tissue. Smaller molecules, such as oxygen, sulfate, and glucose, experience less resistance diffusing, whereas larger ones face greater friction. 13 Pores affect the solute transport in the matrix. 34, 35 A 10% decrease in porosity may be enough to completely remove glucose from the nucleus and inner annulus. 13 Greater water content corresponds with greater pore size. Glycosaminoglycan content is highest in the nucleus.ĭiffusion relies on a certain level of porosity. Uncharged methyl glucose (similar chemical nature and size of glucose), on the other hand, is more easily incorporated. 1 The sulfate incorporation mechanism is of note: Because of a sulfate ion’s negative charge, it is partially repelled from the negatively charged matrix, making its uptake a slow process (160-600 days). Sulfate ion incorporation into the disk matrix precedes GAG synthesis. Cations such as Na + and Ca 2+ and positively charged antibiotics such as gentamycin and aminoglycosides are more readily transported, whereas anions such as the negatively charged antibiotics penicillin and cefuroxime are repelled to a greater extent. 26 High aggrecan levels have the potential to limit the transport of even small, uncharged molecules, as shown in the instance with glucose partially excluded from the nucleus, which has high aggrecan concentration. The fixed negative charge from proteoglycans, notably aggrecan, attracts cations and repels anions. Uncharged solutes, such as oxygen and glucose, enter the disk via the end plate and annulus anions such as sulfate ions diffuse mostly via the annulus and cations such as Ca 2+ via the end plate. 3 Only the outer annulus depends on peripheral vasculature, whereas the inner annulus receives its nutrients from the end plates. Metabolic waste is removed via this system in a reverse route. 3, 4, 8, 11, 12, 21, 22, 23, 24 From there, it diffuses across 7 to 8 mm of end plate and matrix into the cells of the disk. ![]() Nearly the entire disk receives its nutrients from its end plate’s blood supply. The dorsal aspect is also more saturated with nutrients. 20 A greater amount reaches this destination via the annulus, with a greater nutrient concentration found toward the periphery of the disk and decreasing amounts toward the nucleus pulposus (NP). 19 The cartilaginous end plate absorbs nutrients rapidly and delivers them into the disk. The disk is largely avascular, but there are many arterioles and capillaries in the outer annulus, limited to the marrow space of vertebrae on the end plate side. Convection is powered by hydraulic pressure gradients influenced by mechanical loading, and diffusion is mainly influenced by solute concentration gradients. Transport of these nutrients mainly depend on the size of nutrient solutes the larger solutes (ie, GAGs, growth factors, proteins, enzymes, and hormones) 2, 3, 4, 5 rely on convection as a transport mechanism whereas smaller ones (ie, glucose, oxygen, and lactate) rely on diffusion. Impeding any of these paths deprives the disk of vital nutrients. The predominant route is via the capillary beds of the cartilaginous end plate, and the other is via the peripheral annulus. 2 There are 2 blood delivery routes to supply nutrients to the avascular disk. Oxygen is needed to produce glycosaminoglycans (GAGs), 1 which provide mechanical strength and integrity for the disk’s extracellular matrix (ECM). Glycolysis is the energy pathway of choice and consists of breaking down glucose to produce adenosine triphosphate (ATP), with lactate as a byproduct. Glucose is a major nutrient necessary for cell survival in the disk. The IVD cells require energy supplied through vital nutrients. ![]()
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