Telocytes in mice bone marrow: electron microscope evidence.

Telocytes (TCs) are a novel type of interstitial cell of whom presence has been recently documented in many tissues and organs. However, whether TCs exists in bone marrow is still not reported. This study aims to find out TCs in mice bone marrow by using scanning electron microscope (SEM) and transmission electron microscope (TEM). SEM images showed that in mice bone marrow most of TCs have small spherical cell body (usually 4-6 μm diameter) with thin long telopodes (Tps; usually one to three). The longest Tp observed was about 70 μm, with an uneven calibre. Direct intercellular contacts exist between TCs. TEM shows mitochondria within dilations of Tps. Also, by TEM, we show the close spatial relations of Tps with blood vessels. In conclusion, this study provides ultrastructural evidence regarding the existence of TCs in mice bone marrow, in situ.

Telocytes (TCs) represent a newly discovered type of interstitial cells [1]. The defining ultrastructural feature of TCs is the presence of special thin, long and uneven calibre (moniliform) prolongations termed telopodes (Tps; for more details see www.telcytes.com). The presence of TCs has been documented in the interstitial compartments of various tissues and organs: skin [2-4], brain [5], eye [6], skeletal muscle [7,8], respiratory tract [9-12], heart [13-18], digestive system [19-22] and accessory glands of the digestive system [22-27], genital tract [28-31] and urinary tract [32-35]. Nevertheless, it was not established whether TCs reside in bone marrow. The aim of this study is to bring convincing evidence for the existence of TCs in mice bone marrow.

This study was approved by the Institutional Ethics Board of Fudan University, according to the generally accepted international standards. Ten male C57BL/6J mice aged 5 weeks (weight: 12–16 g) were killed in the research. The femurs were harvested, and then the soft tissues attached to the femurs were removed. The femurs were then cut into two halves along their longitudinal axis by microsurgical scissors in preparation for samples treatment. The specimens were handled according to scanning electron microscope (SEM) and transmission electron microscope (TEM) routine and observed under Philips XL30E SEM (Amsterdam, The Netherlands) and FEI TECAI SPIRIT TEM (Eindhoven, The Netherlands), respectively.

Scanning electron microscope images of mice bone marrow showed that TCs have round/oval small cell bodies (average diameter 4–6 μm; Figs1A, 2, 3). Also one or two (no more than three) very long Tps were observed in the studied specimens. SEM measurements showed that the longest observed Tp is of 66.5 μm (Fig.1A). Tps are of uneven calibre - consisting in an alternation of thin segments (podomers) and dilated segments (podoms; Figs1A and 2B). Figure2A shows TCs in the close vicinity of an arteriola, and also the direct contact between two TCs is seen in Figure2B. Under TEM (Fig.4), one TC with two long and thin Tps was observed in the vicinity of capillary. Mitochondria were seen in the dilated segment (podom) of one Tp.

Figure 1

Scanning electron microscope images of mice bone marrow. (A) It shows a telocyte (TC) with one visible telopode (Tp). Cellular body of TC is round (inset), having the measured sizes of 3.55/4.5 μm. Tp have the measured length of 66.5 μm, with the distal end partially covered. The inset shows the abrupt emerging on the Tp from the cellular body of TC; bar = 30 μm. (B) TC with two long and very thin Tps: Tp1 - 14.12 μm, and Tp2 - 6.48 μm. The measured size of the cellular body of TC is 2.65/4.55 μm. The uneven calibre (alternation of podoms and podomers) of the Tps is obvious; bar = 10 μm.

Figure 2

Scanning electron microscope images of mice bone marrow. (A) Telocyte (TC) and its Telopode (Tp) are located on the surface of an arteriola. TC cellular body size: 4.44/5.93 μm; Tp dimensions: length - 12.5 μm; diameter - 0.44 μm; bar = 5 μm. (B) TC with its Tp in close spatial relationship with surrounding interstitial cells. The moniliform aspect of the Tp is obvious: the alternation of dilated segments (A) with thin segments (B). TC cellular body size: 4.5 μm in diameter; Tp length - 15.12 μm (being which was partially sheltered by one cell); bar = 5 μm.

Figure 3

Scanning electron microscope images of mice bone marrow. (A) Telocyte (TC) with three telopodes. The cell body was oval which size was 3.53 μm in width and 4.85 μm in length. The lengths of three telopodes Tp1, Tp2 and Tp3 were 4.34, 8.12 and 8.58 μm, respectively. The terminal end of Tp2 formed contact with another cell (black dotted line circle); bar = 5 μm. (B) There was direct contact between two similar TCs in size and external appearance like dumbbell; bar = 5 μm. Telocyte-cell body, Tp-telopode.

Figure 4

Transmission electron microscope image of mice bone marrow (combined photo). A telocyte (TC) with two long telopodes (Tps) was close to a capillary. The cell body was not totally shown. The lengths of Tp1 and Tp2 were 19.98 μm and 12.99 μm, respectively. The calibre of Tps was uneven. Mitochondria (M) was observed within the podom. Cap: capillary; L: lumen; RBC: Red blood cells; End: Endothelial cell; bar = 5 μm.

The presence of TCs has been previously documented in many organs. They were ultrastructurally described in close contacts with stem cells within interstitium of several vital organs [3,6,10,14,16,36]. However, TCs in bone marrow (stem cell abundant hematolymphoid tissue) was never reported previously. We provide here for the first time ultrastructural evidence for the presence of TCs within bone marrow. Ultrastructurally, TCs found in mice bone marrow share the similar distinctive features with TCs described by Popescu's group within other organs [1]. TCs have small cellular body with 1–3 suddenly emerging telopodes of uneven calibre. Similar to the previous reports, within bone marrow, TCs appear interconnected by homocellular junctions, forming a network. In mice bone marrow TCs are observed in close spatial relationships with small blood vessels and/or capillaries. Also, the presence of TCs in bone marrow tissue characterized by the generous presence of stem cells and progenitor cells is somehow re-capitulate the condition of TCs in stem cell niches. This aspect could offer new insights regarding the presumptive role(s) of TCs in cellular (re)generation.