What is the Difference Between Annulus Fibrosus and Nucleus Pulposus

The key difference between annulus fibrosus and nucleus pulposus is that annulus fibrosus is the outer portion of the intervertebral disc, while the nucleus pulposus is the inner portion of the intervertebral disc.

The vertebral column provides protection to the spinal cord. It runs through the central cavity of the spinal cord. Generally, between each vertebra, there is an intervertebral disk. An intervertebral disc is made up of two main components: annulus fibrosus and nucleus pulposus. The annulus fibrosus is the outer portion of the intervertebral disc. It is composed of layers of collagen and proteins. This is called the lamellae. The nucleus pulposus is the inner gel material surrounded by the annulus fibrosus. It makes up about 40 percent of the disc.

CONTENTS

1. Overview and Key Difference
2. What is Annulus Fibrosus  
3. What is Nucleus Pulposus
4. Similarities – Annulus Fibrosus and Nucleus Pulposus
5. Annulus Fibrosus vs. Nucleus Pulposus in Tabular Form
6. Summary – Annulus Fibrosus vs. Nucleus Pulposus

What is Annulus Fibrosus?

Annulus fibrosus is a fibrocartilaginous tissue and is the outer portion of the intervertebral disc. It consists of layers of lamellae that are rich in highly cross-linked collagen fibrils. It makes up about 60 percent of the intervertebral disc. Annulus fibrosus is composed of a ring of ligament fibres that surrounds the inner core called the nucleus pulposus. Annulus fibrosus connects the spinal vertebrae above and below the disc. The composition of the nucleus pulpous varies by spinal level.

The annular fibrosus prevents the nucleus pulposus from herniating or leaking out of the intervertebral disc. This is done by hydraulically sealing the nucleus pulposus and evenly distributing any pressure and force usually imposed on the intervertebral disc. Furthermore, the annulus fibrosus resists the tensile and compressive stresses the intervertebral disc is subjected to.

What is Nucleus Pulposus?

The nucleus pulposus is the inner core of the intervertebral disc. This inner core is composed of jelly-like material. It consists mainly of water and a loose network of collagen fibers. This inner structure helps the intervertebral disc withstand compression and torsion forces.

When people are ageing, the intervertebral disc of the body can dehydrate and become stiffer. This makes the disc to be less capable of controlling the compression. With this, the nucleus pulposus can be herniated through the protective layer, annulus fibrosus. This leaking out of the disc space inflames the nerve roots next to the intervertebral disc. Furthermore, the nucleus pulposus plays a major role in providing shock absorption properties to the spine during movement. In addition, it prevents bone-to-bone contact, reducing the possibility of injuries and pain.

What are the Similarities Between Annulus Fibrosus and Nucleus Pulposus?

• The Annulus fibrosus and nucleus pulposus are the two main parts of the intervertebral disc.
• Both are located between two vertebrae.
• Both have collagen fibers.
• They perform very important functions, which help the spinal cord during movement.

What is the Difference Between Annulus Fibrosus and Nucleus Pulposus?

The Annulus fibrosus is the outer portion of the intervertebral disc, while the nucleus pulposus is the inner portion of the intervertebral disc. Thus, this is the key difference between annulus fibrosus and nucleus pulposus. Furthermore, annulus fibrosus makes up about 60 % of the intervertebral disc, while nucleus pulposus makes up about 40 % of the intervertebral disc.

The infographic below presents the differences between annulus fibrosus and nucleus pulposus in tabular form for side-by-side comparison.

Summary – Annulus Fibrosus vs. Nucleus Pulposus

The vertebral column is made up of 26 bones providing support to the body. Moreover, the vertebral column provides protection to the spinal cord, which runs through the central cavity of the spinal cord. Between the spaces of two vertebrae is a disk called an intervertebral disc. The intervertebral disk is filled with a gelatinous substance called the nucleus pulposus. It is the inner core. The annulus fibrosus is a fibrocartilaginous ring that surrounds the nucleus pulposus. It is the outer portion of the intervertebral disk. So, this summarizes the difference between annulus fibrosus and nucleus pulposus.

Reference:

1. “Nucleus Pulposus.” Spine Health.
2. “Annulus Fibrosus – An Overview.” ScienceDirect Topics.

Image Courtesy:

1. “Annulus Fibrosus” By BruceBlaus – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Cervical vertebra English” By user:debivort – Own work (CC BY-SA 3.0) via Commons Wikimedia