Intervertebral discs are made of three main components: a gel-like center called the nucleus pulposus, a tough outer ring called the annulus fibrosus, and thin cartilage plates on the top and bottom called cartilaginous endplates. Together, these layers combine water, collagen, and specialized proteins into a structure that absorbs shock, allows your spine to bend and twist, and keeps your vertebrae from grinding against each other.
Three Parts Working Together
Each disc sits between two vertebrae like a cushion, and each one has the same basic architecture. The nucleus pulposus fills the center. The annulus fibrosus wraps around it in concentric layers, acting as a containment wall. And the cartilaginous endplates cap the disc above and below, anchoring it to the bone on either side. No single component works alone. The gel in the center pushes outward under pressure, the outer ring resists that force, and the endplates hold everything in place while feeding the disc nutrients.
The Nucleus Pulposus: A Water-Rich Gel Core
The nucleus pulposus is mostly water, somewhere between 66% and 90% depending on your age. Hydration peaks between ages 20 and 30, then gradually declines. That water isn’t just sitting there. It’s bound to large protein-sugar molecules called proteoglycans, which attract and hold water the way a sponge does. The main proteoglycan in the nucleus is aggrecan, which carries chains of sugar molecules that generate osmotic pressure, essentially pulling water into the disc and keeping it there.
This water-trapping system is what makes the disc work as a shock absorber. When you stand, sit, or lift something, the nucleus distributes that compressive force evenly in all directions, like a water balloon pressed between your hands. The collagen in the nucleus is primarily type II, the same kind found in joint cartilage, arranged in a loose, disorganized network that gives the gel its flexibility rather than structural rigidity.
The Annulus Fibrosus: Layered Collagen Rings
The annulus fibrosus is the disc’s structural backbone. It consists of 15 to 25 concentric layers (called lamellae) made primarily of type I collagen, the same tough protein found in tendons and ligaments. Within each layer, the collagen fibers run parallel to one another at roughly 30 degrees from vertical. In the next layer, they angle the opposite direction. This alternating pattern, similar to the cross-ply design of a car tire, gives the annulus remarkable resistance to forces coming from multiple directions: compression, twisting, bending forward, and bending backward.
Between each layer sits a thin matrix, less than 30 micrometers thick, packed with elastic fibers. These elastic fibers help the layers slide against one another during movement and snap back into position afterward. The collagen composition also shifts as you move from outside to inside. The outermost layers are rich in type I collagen, which is stiff and strong. Closer to the nucleus, type II collagen becomes more dominant, creating a gradual transition from rigid outer wall to flexible inner gel rather than an abrupt boundary.
Cartilaginous Endplates: The Nutrient Gateway
The cartilaginous endplates are thin layers of hyaline-like cartilage that cover the top and bottom of the disc, separating it from the vertebral bone. They serve two critical purposes: distributing mechanical load so the soft nucleus doesn’t bulge into the bone, and feeding the disc.
This second role is especially important because the intervertebral disc is the largest avascular tissue in the human body. It has no blood supply of its own. Instead, oxygen and glucose diffuse in from a network of tiny capillaries that sit just beneath the bony endplate (the layer of porous bone directly above and below). These nutrients pass through the bony endplate, through the cartilaginous endplate, and into the disc. Waste products travel the same route in reverse. Some nutrient exchange also happens through the outer annulus, but the endplates are the primary gateway. When this diffusion pathway gets disrupted, whether from calcification, smoking, or simply aging, the disc starves and begins to break down.
How Disc Composition Changes With Age
Disc degeneration is largely a story of losing water. Starting around age 20, the proteoglycans in the nucleus gradually break down. As aggrecan and other proteoglycans degrade, there are fewer binding sites to hold water in place. The nucleus dehydrates, shrinks, and loses its ability to distribute force evenly. Multiple studies using MRI have confirmed progressive water loss from approximately age 20 onward, continuing steadily into old age.
This isn’t just about the nucleus drying out. As it loses pressure, the annulus has to bear more of the load it wasn’t designed to handle alone. The collagen layers can develop small tears, particularly in the inner and posterior regions where the annulus is thinnest. Over time, repeated stress on weakened layers can lead to disc bulges or herniations, where the nucleus material pushes through the damaged annulus wall. Meanwhile, the cartilaginous endplates can calcify, further restricting nutrient flow and accelerating the cycle of degeneration.
None of this necessarily causes pain. Many people with significantly degenerated discs on imaging have no symptoms at all. But when degeneration does become symptomatic, understanding what the disc is made of helps explain why: a dehydrated nucleus can no longer cushion properly, a torn annulus can irritate nearby nerves, and a calcified endplate can choke off the disc’s only food supply.