Telocytes in neuromuscular spindles

Abstract

A new cell type named telocyte (TC) has recently been identified in various stromal tissues, including skeletal muscle interstitium. The aim of this study was to investigate by means of light (conventional and immunohistochemical procedures) and electron microscopy the presence of TCs in adult human neuromuscular spindles (NMSs) and lay the foundations for future research on their behaviour during human foetal development and in skeletal muscle pathology. A large number of TCs were observed in NMSs and were characterized ultrastructurally by very long, initially thin, moniliform prolongations (telopodes - Tps), in which thin segments (podomeres) alternated with dilations (podoms). TCs formed the innermost and (partially) the outermost layers of the external NMS capsule and the entire NMS internal capsule. In the latter, the Tps were organized in a dense network, which surrounded intrafusal striated muscle cells, nerve fibres and vessels, suggesting a passive and active role in controlling NMS activity, including their participation in cell-to-cell signalling. Immunohistochemically, TCs expressed vimentin, CD34 and occasionally c-kit/CD117. In human foetus (22-23 weeks of gestational age), TCs and perineural cells formed a sheath, serving as an interconnection guide for the intrafusal structures. In pathological conditions, the number of CD34-positive TCs increased in residual NMSs between infiltrative musculoaponeurotic fibromatosis and varied in NMSs surrounded by lymphocytic infiltrate in inflammatory myopathy. We conclude that TCs are numerous in NMSs (where striated muscle cells, nerves and vessels converge), which provide an ideal microanatomic structure for TC study.

Fig. 1

Cell components other than telocytes in normal NMSs. (A) An NMS, with external (ECA) and internal capsule (ICA), in routine H&E study (×220). (B) EMA-positive perineural cells in external capsule (×340). (C–F) Ultrastructural characteristics of perineural cells with presence of basement membrane, pinocytotic vesicles and junctions (arrows; C, ×9000; D–F, ×15,000). (G) Desmin-positive intrafusal striated muscle fibres (×300). (H) Positivity for anti-neurofilament protein in the axons of nerve intrafusal fibres (×300). (I) Typical ultrastructure of Schwann cells (SC; ×12,000). Insert: positivity in these cells for anti-S-100 protein (×220). (J) Sensory terminals (ST), with densely packed mitochondria, adjacent to a skeletal muscle cell (SM; ×20,000). (K) Intrafusal capillary with endothelial cells (EC) and pericytes (P; ×7500). Insert 1 Ka, positivity of the endothelial cells for anti-CD31 (arrow; ×220). Insert 1 Kb, expression of α-SMA in pericytes (×340).

Fig. 2

Telocytes (TCs—arrows) in normal NMSs. (A) Long, slender cytoplasmic processes (Telopodes: Tps—arrowheads) of TCs form a labyrinthine system surrounding other intrafusal structures. Striated muscle: SM (×5500). Insert 2 Aa: Expression of CD34 in TCs (arrowhead; ×220). Insert 2 Ab: Occasional positivity for anti-CD117 in a cell (arrow). (B) Detail of CD34-positive TCs forming the innermost layer (arrow) and part of the outermost layer (asterisks) of the external NMS capsule (ECA) and the internal capsule (ICA; ×640). (C) Ultrastructural detail of the body of a TC (arrow) and Tps (arrowheads) surrounding a striated muscle cell (SM; ×10,500).

Fig. 3

Telocytes (TCs—arrows) in normal NMs. (A) Telopodes (Tps—arrowheads) surrounding an intrafusal capillary, with endothelial cells (EC) and pericytes (P; ×9500). Insert: CD34-positive TCs (arrows) surrounding a capillary (C) and striated muscle cells (SM). Capillary endothelial cells are also heavily stained by CD34 (×420). (B and C) TC bodies (arrows) with triangular (B) or spindle (C) morphology and typically thin Tps (arrowheads) emerging from the cell body (asterisks), with ramifications, podomeres and podoms (×9500).

Fig. 4

Details of cytoplasmic body and Tps, and extracellular matrix in normal NMSs. (A) A centriole (arrow), a small Golgi apparatus (G) and few mitochondria (M) are observed in the scarce somatic cytoplasm of a TC (×15,000). (B–D) Pomoderes (P) and podoms (arrows) in Tps. Striated muscle: SM. Sensory terminal: ST (×10,000). (E–G) Exosomes (E) and shed vesicles (V) in vicinity of Tps. Some shed vesicles are observed being released from a Tp (G). (E and F: ×12,000; G: ×15,000). (H–J) Junctions between Tps (arrows; ×12,000). (K) PAS positivity of the extracellular matrix in the NMS external capsule (E) and Alcian Blue positivity in intrafusal area (I) PAS-Alcian (×380). (L) Collagen fibrils (arrows) in folds of Tps (arrowheads), which show a honeycomb-like structure (×12,000).

Fig. 5

TCs during development and in muscle pathology. (A) Early stage of NMS formation, in routine H&E study (arrows; ×220). (B and C) Immunohistochemical expression of EMA and CD34 in developing NMs (B) a primitive and incomplete external capsule is observed from extension of EMA-positive perineural cells (arrows). (C) CD34-positive TCs, forming an internal capsule (arrows; ×640). (D–F) Residual NMSs in musculoaponeurotic fibromatosis with infiltrative muscular pattern. (D) Vimentin expression (E) actin expression. (F) CD34-positive cells, forming most of the intrafusal components (×300). (G) Presence of a lymphocyte infiltrate around an NMS in inflammatory myopathy. Fewer CD34-positive intrafusal TCs (arrow) (×300).