Endoneurial Fibroblast-Like Cells

J Neuropathol Exp Neurol Copyright Ó 2012 by the American Association of Neuropathologists, Inc. Vol. 71, No. 11 November 2012 pp. 938Y947 REVIEW ARTICLE Endoneurial Fibroblast-Like Cells Laurence Richard, MSc, Piotr Topilko, PhD, Laurent Magy, MD, PhD, Anne-Valérie Decouvelaere, MD, PhD, Patrick Charnay, PhD, Benoı̂t Funalot, MD, PhD, and Jean-Michel Vallat, MD Ultrastructural Features Abstract Endoneurial fibroblast-like cells (EFLCs) have been described for more than 60 years, but the embryology, functions, and pathology of these cells are not well defined. Several hypotheses of their origin have been proposed. A previous study suggesting that they were of neural crest origin is supported by our data in humans. This lineage might account for EFLCs having multiple biologic functions and involvement in pathological processes. Here, we review what is known about the origin; functions in collagen synthesis, phagocytosis, inflammatory responses, and immune surveillance; and the pathological alterations of EFLCs based on the literature and on our personal observations. Key Words: Cajal cells, Electron microscopy, Endoneurial fibroblasts, Immunoelectron microscopy, Neural crest, Nerve biopsy, Perineurial cells, Peripheral nerve. INTRODUCTION In addition to myelinating and nonmyelinating Schwann cells, a variety of cell types are present within peripheral nerves, including perineural cells, mast cells, pericytes, endothelial cells, and endoneurial fibroblast-like cells (EFLCs). Conventional fibroblasts constitute a pervasive, but diverse, population of cells the primary function of which is to establish, maintain, and modify connective tissue stromata that functionally interact with other tissues, such as epithelial tissue. The main defining characteristics of fibroblasts are their shape and ability to secrete extracellular matrix molecules such as type I collagen. The EFLCs seem to have more potential activities than regular fibroblasts, but their biologic roles have not been completely explored. The embryology, functions, and pathology of EFLCs are still not understood (1). Therefore, it is appropriate to review this issue, taking both the recent literature and our personal observations into account. National Referral Center for Rare Peripheral Neuropathies, Departments of Neurology (LR, LM, BF, J-MV), and Biochemistry and Molecular Genetics (BF), Centre Hospitalier Universitaire, Limoges; IBENS, Ecole Normale Supérieure, INSERM, Paris (PT, PC); and Cancer Center Léon Bérard, Department of Pathology, Lyon (A-VD), France. Send correspondence and reprint requests to: Jean-Michel Vallat, MD, Service et Laboratoire de Neurologie, CHU de Limoges, 2 avenue Martin Luther King, 87042 Limoges, France; E-mail: 938 The EFLCs are diffusely scattered between nerve fibers in the endoneurium but are not easily seen in routinely stained paraffin sections (Fig. 1). However, they are clearly distinguishable by electron microscopy (EM) (Fig. 2) and are frequently located near blood vessels and under the perineurium where they are usually arranged parallel to perineurial cells (PCs) (Fig. 3). Numerous fibroblasts can be seen in the epineurium, and most of these are also orientated parallel to PCs. In normal peripheral nerves of mice and rabbits, fibroblasts are very rarely detected by EM within the perineurium (2). We have confirmed this in normal human nerves. The EFLCs may represent approximately 2% to 9% of the endoneurial cells (3, 4). They appear as spindle-shaped cells on EM, with triangular or rectangular cell bodies when seen in transverse sections. They have long slender cytoplasmic processes that extend either along the nerve trunk or laterally between nerve fibers; these processes loosely interdigitate with those from neighboring EFLCs and also encircle one or several Schwann cells. Compared with the Schwann cell nuclei, their nuclei seem paler. Their cell membranes show many smooth invaginations of micropinocytotic vesicles (Fig. 4) but no finger-like expansions as in typical macrophages (5). The cytoplasm of EFLCs contains scattered mitochondria, Golgi apparatus, and prominent endoplasmic reticulum, which is often orientated in more or less parallel arrays and is dilated in places; the cisternae contain granular material. In normal nerves, these cells contain only a few lysosomes. Nevertheless, some are known as ‘‘giantly vacuolated.’’ Midroni and Bilbao (6) indicate that it is very difficult to differentiate between intracytoplasmic vacuolation and extremely convoluted cells observed in cross section. Fine intracellular filaments may also be present in their cytoplasm (Fig. 4). Occasionally, the cytoplasm of EFLCs is invaginated by bundles of collagen that lie in direct contact with the plasma membrane (7). The lack of a continuous basal lamina around EFLCs distinguishes them from Schwann cells, PCs, and pericytes, all of which have continuous basal laminae. During Wallerian degeneration, it seems that EFLCs show 2 types of modifications at the level of the plasma membrane. The first is characterized by subplasmalemmal cytoplasmic condensations, the thickness of which is relatively constant, whereas their length varies. Similar adjacent plasma membrane specialization may be a feature of the cells of the mononuclear phagocytic system (8) or may be consistent with the known J Neuropathol Exp Neurol  Volume 71, Number 11, November 2012 Copyright © 2012 by the American Association of Neuropathologists, Inc. Unauthorized reproduction of this article is prohibited. J Neuropathol Exp Neurol  Volume 71, Number 11, November 2012 FIGURE 1. Normal human nerve biopsy. It is not possible to identify endoneurial fibroblast-like cells in this section stained with hematoxylin and eosin (longitudinal paraffin-embedded section). Endoneurial Fibroblast-Like Cells FIGURE 3. Normal human nerve. Endoneurial fibroblast-like cells (arrows) are arranged parallel to perineural cells (P). Electron microscopy (transverse section). hemidesmosome-like membrane specializations of activated fibroblasts (9). The second modification is the presence of small fragments of basal lamina membranes that are located outside the plasma membrane. Fibroblasts of various tissues show several forms of intercellular contact with each other. These include gap junctions in the mouse phrenic nerve (10) or close contacts in the rat mandible (11). Paired or odd subsarcolemmal linear condensations of variable lengths are detected in the apposing fibroblasts by EM. Three or more fibroblasts may be linked by several symmetrical junctions, and multiple junctions may occur between 2 fibroblasts. In conclusion, the morphological characteristics of EFLCs based on EM include their endoneurial location, spindle-shaped morphology, failure to associate with axons, lack of a continuous basal lamina, and long angular processes that are usually so narrow that they cannot be clearly observed by light microscopy (12). In a normal nerve, EFLCs se (...truncated)