Neurilemma

Neurilemma

Primary Disciplinary Field(s): Neuroscience, Anatomy, Histology, Pathology

1. Core Definition and Anatomical Context

The neurilemma, also known as the neurolemma or sheath of Schwann, constitutes the outermost cytoplasmic layer of a Schwann cell that encases the axon of peripheral nerve fibers. This protective and regenerative covering is exclusive to the Peripheral Nervous System (PNS). In myelinated peripheral nerves, the neurilemma forms the outermost boundary of the entire nerve fiber, enveloping the myelin sheath that insulates the axon. Its presence is a distinguishing feature that sets peripheral nerves apart from their central nervous system counterparts.

Functionally, the neurilemma serves as a crucial protective envelope, safeguarding the delicate underlying axon and the myelin sheath from mechanical stress and potential pathogens. Beyond its protective role, it plays an indispensable part in the repair and regeneration processes of damaged peripheral nerves. This capability is largely attributed to the resident Schwann cells, whose preserved neurilemmal tubes provide a scaffold and a guiding pathway for regenerating axons. This regenerative capacity is a hallmark of the PNS, contrasting sharply with the limited regenerative potential observed in the central nervous system.

While the term often refers specifically to the Schwann cell’s plasma membrane and its thin layer of cytoplasm, it is more broadly understood to encompass the entire outer cellular investment provided by the Schwann cell around the axon. This includes the nucleus and the majority of the Schwann cell’s cytoplasm, which remains external to the myelin layers in a myelinated fiber, or directly surrounds the unmyelinated axons. Thus, the neurilemma is not merely a passive covering but an active cellular component integral to the structural and functional integrity of peripheral nerves.

2. Etymology and Historical Understanding

The term neurilemma is derived from Greek roots: “neuron,” meaning nerve, and “lemma,” meaning sheath or husk. This etymology directly reflects its anatomical description as the sheath surrounding a nerve fiber. The concept of a distinct outer covering for nerve fibers dates back to early microscopic observations in the 19th century, a period marked by intense research into the microscopic structure of tissues following advancements in microscopy.

The comprehensive understanding of the neurilemma became intimately linked with the discovery and characterization of Schwann cells, named after Theodor Schwann, who described them in 1839. Schwann’s pioneering work in cellular biology, particularly his contributions to the cell theory, laid the groundwork for understanding the cellular components of nerve fibers. It was gradually recognized that these cells, rather than simply being a connective tissue layer, were active participants in nerve structure and function, specifically responsible for forming the myelin sheath in the PNS and providing the outer neurilemmal layer.

Over time, further histological and ultrastructural studies using electron microscopy in the mid-20th century provided a more refined view of the neurilemma, distinguishing it as the living outer cytoplasmic layer of the Schwann cell. These detailed observations solidified its role not just as a structural boundary but as a vital component involved in metabolic support, protection, and the remarkable regenerative capabilities unique to the peripheral nervous system, thereby refining the initial conceptualization of a simple “nerve sheath.”

3. Relationship with Schwann Cells

The neurilemma is an intrinsic part of the Schwann cell, which are the principal glial cells of the Peripheral Nervous System (PNS). These cells play a fundamental role in supporting neuronal function, insulating axons, and facilitating nerve repair. Each peripheral nerve fiber, whether myelinated or unmyelinated, is invariably associated with Schwann cells, and it is the outermost cytoplasmic expanse of these cells that forms the neurilemma.

In the case of myelinating Schwann cells, a single Schwann cell wraps its plasma membrane multiple times around a segment of a single axon, forming the lipid-rich myelin sheath. During this process, the majority of the Schwann cell’s cytoplasm and its nucleus are displaced to the periphery, forming the outermost layer, which is the neurilemma. This outer sheath is crucial because it contains the cell’s nucleus and metabolic machinery necessary for maintaining the integrity of the myelin and supporting the underlying axon. The neurilemma, in this context, effectively acts as the protective outer covering for the entire myelinated segment.

Conversely, non-myelinating Schwann cells typically envelop multiple small-diameter unmyelinated axons. In this arrangement, each axon invaginates into the Schwann cell’s cytoplasm, and while no extensive myelin sheath is formed, the Schwann cell’s plasma membrane still surrounds these axons. The entire outer boundary of this Schwann cell, enclosing the bundles of unmyelinated axons, also constitutes the neurilemma. Therefore, whether myelinated or unmyelinated, the presence of the neurilemma is a consistent anatomical feature, always denoting the outer cellular envelope provided by a Schwann cell.

4. Key Characteristics and Composition

The neurilemma, as the outermost layer of a Schwann cell, is essentially composed of the Schwann cell’s plasma membrane, its cytoplasm, and the centrally located nucleus (though the nucleus may not always be visible in a given cross-section). This cellular investment provides metabolic support to the underlying axon and the myelin sheath, ensuring their health and functionality. The plasma membrane of the Schwann cell directly interfaces with the extracellular matrix, which typically includes a basal lamina (or basement membrane) that further encases the entire nerve fiber, providing additional structural support and a barrier function.

In myelinated nerve fibers, the neurilemma is the most peripheral layer of the Schwann cell, encompassing the tightly wrapped layers of myelin. This outer cytoplasmic layer is rich in organelles, reflecting its active metabolic role in lipid synthesis for myelin maintenance, protein synthesis for axonal support, and cellular waste removal. It also contains various adhesion molecules and receptors that mediate interactions with the extracellular environment and other cells, which are crucial during development and regeneration. The continuous nature of the neurilemma along the length of the axon, interrupted only at the Nodes of Ranvier where adjacent Schwann cells meet, ensures comprehensive coverage and protection.

For unmyelinated nerve fibers, the neurilemma forms the direct outer boundary of the Schwann cell that invaginates to enclose multiple axons. Although these axons lack a thick myelin sheath, they are still protected by the Schwann cell’s cytoplasm and plasma membrane, which collectively form the neurilemma. The integrity of this outer sheath is paramount for maintaining the microenvironment around both myelinated and unmyelinated axons, regulating ion concentrations, and facilitating nutrient exchange, all of which are essential for proper nerve signal transmission and overall axonal health.

5. Functional Significance: Protection and Regeneration

The neurilemma’s functional significance extends far beyond mere structural enclosure; it is pivotal for both the protection and the remarkable regenerative capacity of peripheral nerve fibers. As the outermost cellular layer of a Schwann cell, the neurilemma acts as a physical barrier, shielding the delicate axon and its myelin sheath from external mechanical forces, chemical insults, and potentially harmful pathogens. This protective role is essential for maintaining the integrity and function of peripheral nerves, which are often exposed to physical trauma.

Perhaps its most critical function lies in facilitating peripheral nerve regeneration following injury. When a peripheral nerve axon is severed, the distal part (separated from the cell body) undergoes Wallerian degeneration. However, the neurilemmal tubes formed by the Schwann cells in the distal stump remain largely intact. These residual neurilemmal tubes, often referred to as the “bands of Bungner,” provide a crucial scaffold and a guiding pathway for the regenerating axon sprouts originating from the proximal stump. The Schwann cells within these tubes proliferate, express growth factors, and lay down an extracellular matrix, all of which are conducive to axonal regrowth.

Without the guidance provided by the continuous neurilemmal sheath, regenerating axons would struggle to find their appropriate targets, leading to disorganized growth and impaired functional recovery. The neurilemma thus ensures that the growing axon sprouts are directed towards their original innervation targets, allowing for the re-establishment of functional connections. This intrinsic regenerative capacity of the PNS, heavily reliant on the preserved neurilemma and active Schwann cells, is a fundamental difference compared to the typically limited regeneration observed in the central nervous system.

6. Clinical Relevance and Pathologies

The clinical significance of the neurilemma is underscored by its role in both nerve integrity and certain pathological conditions. The most prominent example is the neurilemoma, also known as a schwannoma. This is a benign tumor that originates directly from the Schwann cells themselves, specifically from the neurilemma. Neurilemomas can occur along any peripheral nerve in the body, including cranial nerves (e.g., vestibular schwannomas on the eighth cranial nerve, affecting hearing and balance) and spinal nerves. They typically grow slowly and are encapsulated, compressing the nerve rather than invading it, which distinguishes them from malignant nerve sheath tumors.

Symptoms of neurilemomas vary depending on their location, but often include pain, numbness, tingling, or weakness due to compression of the affected nerve. While benign, their growth can lead to significant functional deficits if they impinge on critical nerves. Treatment typically involves surgical removal, which can be challenging depending on the tumor’s size and proximity to vital structures. Understanding the cellular origin of these tumors in the neurilemma is crucial for accurate diagnosis and effective management, highlighting the direct clinical implications of this anatomical structure.

Beyond tumors, the health of the neurilemma and its associated Schwann cells is critical in various peripheral neuropathies. Conditions such as Guillain-Barré syndrome, an acute inflammatory demyelinating polyneuropathy, or Charcot-Marie-Tooth disease, a group of inherited neuropathies, primarily affect the Schwann cells and their ability to form or maintain the myelin sheath. While these conditions directly target myelin or Schwann cell function rather than the neurilemma itself, the ultimate protective and regenerative capacity of the nerve fiber, mediated by the neurilemma, is often compromised, leading to axonal damage and neurological deficits. Therefore, the integrity of the neurilemma is a key factor in the overall resilience and recovery potential of the peripheral nervous system.

7. Comparison to Central Nervous System Myelination

A critical distinguishing feature of the neurilemma is its exclusive presence in the Peripheral Nervous System (PNS), setting peripheral nerve fibers apart from those in the Central Nervous System (CNS). In the CNS, the myelin sheath is formed by oligodendrocytes, a different type of glial cell. Unlike Schwann cells, which typically myelinate only a single segment of one axon and retain a significant outer cytoplasmic layer (the neurilemma), a single oligodendrocyte can myelinate multiple segments of several different axons.

The structural organization of oligodendrocytes means that they do not form a distinct, continuous outer cytoplasmic sheath like the neurilemma around each myelinated axon in the CNS. The bulk of the oligodendrocyte’s cell body and nucleus remain separate from the axons it myelinates, connected by slender processes that wrap around the axons. Consequently, CNS myelinated axons lack the protective and regenerative neurilemmal tube that is characteristic of the PNS. This structural difference has profound implications for nerve repair.

The absence of a neurilemma in the CNS, combined with other factors such as the presence of inhibitory glial scar tissue, lack of neurotrophic factors, and intrinsic neuronal properties, contributes significantly to the limited capacity for axon regeneration following injury in the brain and spinal cord. While the PNS benefits from the guiding scaffold provided by the neurilemmal tubes of surviving Schwann cells, CNS axons face a far more hostile environment for regrowth. Thus, the neurilemma is not merely an anatomical detail but a key biological differentiator explaining the contrasting regenerative potentials between the two major divisions of the nervous system.

Further Reading

• Neurilemma – Wikipedia
• Schwann cell – Wikipedia
• Myelin sheath – Wikipedia
• Peripheral nervous system – Wikipedia
• Schwannoma – Wikipedia
• Neurilemoma – NCI Dictionary of Cancer Terms
• Neuroanatomy, Myelin Sheath – StatPearls – NCBI Bookshelf