Spinal discs are made of water, collagen, and sugar-protein molecules called proteoglycans, arranged in three distinct layers. The center is a soft, water-rich gel. The outer shell is a tough, layered ring of collagen fibers. And thin cartilage plates cap the top and bottom, connecting each disc to the vertebrae above and below. This layered design lets the discs act as shock absorbers and flexible spacers between your vertebrae.
The Three Parts of a Spinal Disc
Every spinal disc has the same basic architecture: a nucleus pulposus in the center, an annulus fibrosus surrounding it, and cartilaginous endplates on the top and bottom. Each part has a different composition tailored to its mechanical job.
The nucleus pulposus is the gel-like core. It is 70% to 90% water, which is what gives it the ability to absorb compressive forces when you stand, walk, or lift. The remaining solid material is mostly a loose network of collagen fibers and large proteoglycan molecules, especially one called aggrecan. Aggrecan carries a strong negative electrical charge that attracts and holds water, essentially inflating the nucleus like a water balloon under pressure. More than 85% of the collagen in the nucleus is type II, the same kind found in joint cartilage.
The annulus fibrosus is the outer ring that contains and protects the nucleus. It is made of 15 to 25 concentric layers called lamellae, somewhat like the rings of an onion. Within each layer, collagen fibers run parallel to one another at roughly 60 degrees from vertical. In the next layer, the fibers angle the opposite direction. This crisscross pattern gives the annulus remarkable tensile strength, allowing it to resist bending, twisting, and internal pressure from the nucleus pushing outward. The collagen composition shifts gradually across the annulus: the outermost layers are almost entirely type I collagen (the same tough variety found in tendons and ligaments), while the innermost layers transition to mostly type II collagen as they approach the softer nucleus.
The cartilaginous endplates are thin horizontal plates of hyaline cartilage, each roughly 1 millimeter thick, that sit between the disc and the vertebral bone above and below. They serve two purposes: distributing mechanical loads evenly and acting as the disc’s supply line. Because spinal discs have no blood supply of their own, nutrients and oxygen must diffuse from tiny blood vessels in the vertebral bone, through the endplate, and into the disc. Waste products travel the reverse route. This makes the endplate one of the most important structures for long-term disc health.
How Proteoglycans Keep Discs Hydrated
The proteoglycan aggrecan is the single most important molecule for disc function. It is abundant in the nucleus, the inner annulus, and the endplates. Aggrecan molecules are studded with chains of sugar molecules called glycosaminoglycans, which carry a dense negative charge. That charge pulls water into the disc through osmosis, creating internal swelling pressure. This pressure is what allows the disc to resist the compressive weight of your body.
When aggrecan breaks down, the consequences are dramatic. The disc loses its negative charge density, can no longer hold water effectively, and the swelling pressure drops. What remains is a stiffer, collapsed matrix dominated by collagen fibers that can no longer distribute forces evenly. This is a central feature of disc degeneration.
The Cells That Build the Disc
Spinal discs are living tissue, maintained by specialized cells embedded within the matrix they produce. The nucleus pulposus contains two cell types: notochordal cells (left over from embryonic development) and chondrocyte-like cells, which resemble the cells found in joint cartilage. These cells produce the type II collagen and proteoglycans that form the gel matrix. The annulus fibrosus contains fibroblast-like cells that produce both type I and type II collagen fibers.
Cell density in the disc is low compared to most tissues, and because there is no direct blood supply, these cells operate in a low-oxygen, low-nutrient environment. Their survival depends entirely on diffusion through the endplates. When endplate permeability drops, as it does with age and degeneration, the cells receive less nutrition and produce less matrix, accelerating the cycle of breakdown.
How Water Content Shifts Throughout the Day
Your discs are not static structures. They gain and lose water in a daily cycle driven by gravity and movement. During the day, compressive loading from standing and walking squeezes fluid out of the nucleus. Cervical disc height drops by about 10% over the course of a day, and MRI measurements show that hydration levels in the disc fall by roughly 13% between morning and evening. At night, when you lie down and spinal loading decreases, the proteoglycans draw water back in, rehydrating the nucleus. This is why you are measurably taller in the morning than at bedtime.
Healthy discs rebound well from this daily cycle. Degenerating discs do not. Research on cervical discs found that healthy discs showed a 16% hydration swing between morning and evening, while more degenerated discs showed a smaller swing of around 13%, reflecting their reduced capacity to absorb and release water.
What Changes as Discs Age
The composition of a spinal disc shifts over a lifetime. The most consistent change is a loss of water and proteoglycan content, particularly in the center of the disc. Collagen levels stay relatively stable, but without enough proteoglycans to attract water, the nucleus becomes drier and less gel-like. The boundary between the soft nucleus and the firmer annulus, which is distinct in a young disc, gradually blurs.
The endplates play a key role in this process. Studies show that endplate permeability decreases by 50 to 60% with degeneration, choking off the nutrient supply to the disc’s interior. With fewer nutrients reaching the cells, the cells produce less aggrecan and collagen, and the matrix deteriorates further. This creates a self-reinforcing loop: less permeability leads to less nutrition, which leads to less matrix production, which leads to a structurally weaker disc that is more vulnerable to tears, bulging, and herniation.
Understanding what discs are made of helps explain why they are so vulnerable to degeneration. Their mechanical genius, a pressurized gel core held in place by layered collagen rings and fed through thin cartilage plates, depends on a precise balance of water, proteoglycans, and collagen. When any part of that balance shifts, the entire system is affected.