Perineurium

Structure/Morphology

The perineurium is a continuous layer that surrounds several primary fascicles within the nerve trunk. Note that several primary fascicles together constitute a single secondary fascicle. Perineurial septae extend into the (secondary) fascicle, subdividing it into several smaller bundles of fibers, or primary fascicles (Barral and Croibier, 2007).

The perineurium is smooth and transparent, unlike the epineurium which is tough and mechanically resistant. The perineurium is composed of layers of large, flat perineurial cells that are arranged concentrically. The number of layers of the perineurium varies, depending on the location of the nerve within the body. For example, the sciatic nerve may contain 15 layers; however, at the most distal part of the nerve, there may only be one or two layers (Peltonen et al., 2013). Each layer has a prominent basal lamina, up to 500 nm thick, and provides an additional barrier to the nerve fascicle (Standring, 2016).

The perineurium contains a thick extracellular matrix of collagen and fibronectin, which is capable of modulating external stretch forces.

Key Features/Anatomical Relations

Within the perineurium, primary fascicles are bundled together to form secondary fascicles. At the nerve’s proximal end, the perineurium is continuous with the leptomeningeal coverings (arachnoid and pia) of the spinal cord (Standring, 2016).

Function

The perineurium is a metabolically active diffusion barrier. Along with the endoneurium, the perineurium forms a blood—nerve barrier, which plays a role in maintaining osmotic balance and fluid pressure within the endoneurium. It also isolates the nerve fibers within the fascicles from pathogens and extraperineual fluids (Standring, 2016). The perineurium also has a role in protection of the nerve fibers contained within. The collagen fiber matrix of the perineurium prevents the fascicle from becoming compressed, or ‘kinked’, during movements, such as flexion of a joint. The perineurium is suspected to play a role in nerve fiber regeneration in the peripheral nervous system. There is remarkable somatotopic organization of fiber types within fascicles. Both myelinated and non-myelinated nerve fibers are segregated into fascicles or sub-segregated within fascicles based on their function.

List of Clinical Correlates

—Partial nerve lesions

References

Barral, J. P. and Croibier, A. (2007) Manual Therapy for the Peripheral Nerves. Churchill Livingstone/Elsevier.

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Peltonen, S., Alanne, M. and Peltonen, J. (2013) 'Barriers of the peripheral nerve', Tissue Barriers, 1(3), pp. e24956.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series: Elsevier Limited.