**2. The concept of telocytes**

In 2005, we described a new cell type which we called interstitial Cajal-like cells (ICLC) due to their similarity with canonical gastrointestinal interstitial cells of Cajal (ICC). By using electron microscopy, immunohistochemstry and cell cultures, we revealed that ICLC have particular features that distinguish and separate them from the ICC and/or other interstitial cells. Given these new findings, Popescu renamed ICLC to TELOCYTES (TCs) (Popescu & Faussone-Pellegrini, 2010) by using the Greek affix 'telos', meaning "goal", "end", and "fulfilment", suggesting cells with a particular goal, accomplished through their extremely long prolongations. The new term aims to avoid any confusion between these cells and other interstititial cells such as fibroblasts, mesenchymal cells, and myofibroblasts. The very long and thin prolongations emitted by TCs were re-defined as telopodes (TPs). TPs are built of alternating thin segments known as podomers (≤ 200 nm, below the resolving power of light microscope) and dilated segments called podoms (with a mean width of 462.31 nm), which accommodate mitochondria, rough endoplasmic reticulum and caveolae.

Telocytes in Human Fallopian Tube and Uterus Express Estrogen and Progesterone Receptors 93

Fig. 3. Human term placenta. Telocyte 1 (blue) has few organelles in the perinuclear area and three emerging TPs (red arrows); black arrowheads mark the dichotomic branching points. Note the podoms and podomeres. The black arrow indicates the junction between TPs and a smooth muscle cell (SMC, coloured in brown). Reproduced, with permission,

Fig. 4. Human resting mammary gland stroma. One TC hallmark, namely TPs, appears quite long and convoluted. Note homocellular junctions marked by red circles, as well as shed vesicles (blue) and an exosome (violet). Reproduced, with permission, from Gherghiceanu &

 **Length**: tens to hundreds of µm, as measured on EM images (Figure 5). However, under favorable cell culture conditions, their entire length can be captured in several

from Suciu et al., 2010.

Popescu, 2005.

successive images (Figure 6);

Fig. 1. Graph showing the ascending trend of the number of articles retrieved from www.pubmed.gov using key words "telocytes", "interstitial Cajal-like cell", or "ICC-like". The number of published papers is increasing exponentially.

TPs are a distinctive feature of TCs and are characterized by the following main features:

 **Number**: can vary between 1 and 5. Frequently, only 2–3 telopodes are observed on a single section, depending on site and angle of section (Figure 2, 3), since their 3D convolutions prevent them from being observed at their full length in a very thin 2D section (Figure 4);

Fig. 2. Non-pregnant myometrium. Digitally coloured TC (blue) with 3 TPs that encircle bundles of cross-cut smooth muscle cells (SMC, Sienna brown); N - nuclei. Reproduced, with permission, from Ciontea et al., 2005.

Fig. 1. Graph showing the ascending trend of the number of articles retrieved from

The number of published papers is increasing exponentially.

section (Figure 4);

with permission, from Ciontea et al., 2005.

www.pubmed.gov using key words "telocytes", "interstitial Cajal-like cell", or "ICC-like".

TPs are a distinctive feature of TCs and are characterized by the following main features: **Number**: can vary between 1 and 5. Frequently, only 2–3 telopodes are observed on a single section, depending on site and angle of section (Figure 2, 3), since their 3D convolutions prevent them from being observed at their full length in a very thin 2D

Fig. 2. Non-pregnant myometrium. Digitally coloured TC (blue) with 3 TPs that encircle bundles of cross-cut smooth muscle cells (SMC, Sienna brown); N - nuclei. Reproduced,

Fig. 3. Human term placenta. Telocyte 1 (blue) has few organelles in the perinuclear area and three emerging TPs (red arrows); black arrowheads mark the dichotomic branching points. Note the podoms and podomeres. The black arrow indicates the junction between TPs and a smooth muscle cell (SMC, coloured in brown). Reproduced, with permission, from Suciu et al., 2010.

Fig. 4. Human resting mammary gland stroma. One TC hallmark, namely TPs, appears quite long and convoluted. Note homocellular junctions marked by red circles, as well as shed vesicles (blue) and an exosome (violet). Reproduced, with permission, from Gherghiceanu & Popescu, 2005.

 **Length**: tens to hundreds of µm, as measured on EM images (Figure 5). However, under favorable cell culture conditions, their entire length can be captured in several successive images (Figure 6);

Telocytes in Human Fallopian Tube and Uterus Express Estrogen and Progesterone Receptors 95

**Thickness**: uneven caliber, mostly below 0.2 µm (below the resolving power of light

 **Moniliform aspect**: podoms and podomeres (Figures 7, 8); average caliber of podomeres: 0.1 µm ± 0.05 µm, min. = 0.003 µm; max. = 0.24 µm; Podoms accommodate: mitochondria, (rough) endoplasmic reticulum, caveolae, a trio called 'Ca2+-

Fig. 7. Rat jejunum. A typical TP (blue) located between smooth muscle cells (SMC) and nerve endings. Note a large podom and the corresponding podomeres. TC body is not

Fig. 8. A. Human pregnant myometrium. Primary confluent cultures (day 8) showing a telocyte with at least seven 'beads' per process. B. Human fallopian tube, preconfluent primary cell cultures. Conventional light microscopy, Giemsa staining. Original

magnification 40x (A), 100x, oil immersion (B). Reproduced, with permission, from Ciontea

microscopy), visible under electron microscopy;

uptake/release units' (Figure 9);

captured in the image.

et al., 2005 and Popescu et al., 2005a.

Fig. 5. Digitally coloured electron micrograph of mouse ventricular endocardium (burgundy). TCs (blue) form an interstitial network in the heart. A subendocardial telocyte (TC1) sends TPs between cardiomyocytes (CM) and communicates with TC2. Cap, blood capillary. Scale bar 5 μm. Reproduced, with permission, from Gherghiceanu et al., 2010.

Fig. 6. Non-pregnant human myometrium in cell culture, day 3, the first passage. Giemsa staining. TC establishing contacts with a myocyte by a TP of about 65 µm long. Photographic composition of 4 serial phase contrast images; original magnification 40x. A higher magnification of TP (rectangles) clearly shows a moniliform aspect: at least 40 specific dilations (podoms) connected by thin segments (podomers) are visible in a 'beadlike' fashion. Reproduced, with permission, from Ciontea et al., 2005.

Fig. 5. Digitally coloured electron micrograph of mouse ventricular endocardium

(burgundy). TCs (blue) form an interstitial network in the heart. A subendocardial telocyte (TC1) sends TPs between cardiomyocytes (CM) and communicates with TC2. Cap, blood capillary. Scale bar 5 μm. Reproduced, with permission, from Gherghiceanu et al., 2010.

Fig. 6. Non-pregnant human myometrium in cell culture, day 3, the first passage. Giemsa

Photographic composition of 4 serial phase contrast images; original magnification 40x. A higher magnification of TP (rectangles) clearly shows a moniliform aspect: at least 40 specific dilations (podoms) connected by thin segments (podomers) are visible in a

staining. TC establishing contacts with a myocyte by a TP of about 65 µm long.

'beadlike' fashion. Reproduced, with permission, from Ciontea et al., 2005.


Fig. 7. Rat jejunum. A typical TP (blue) located between smooth muscle cells (SMC) and nerve endings. Note a large podom and the corresponding podomeres. TC body is not captured in the image.

Fig. 8. A. Human pregnant myometrium. Primary confluent cultures (day 8) showing a telocyte with at least seven 'beads' per process. B. Human fallopian tube, preconfluent primary cell cultures. Conventional light microscopy, Giemsa staining. Original magnification 40x (A), 100x, oil immersion (B). Reproduced, with permission, from Ciontea et al., 2005 and Popescu et al., 2005a.

Telocytes in Human Fallopian Tube and Uterus Express Estrogen and Progesterone Receptors 97

Organization in a labyrinthine system, forming a 3D network anchored by hetero- and

The concept of TC was soon embraced by other laboratories as well (Bani et al., 2010; Cantarero et al., 2011; Carmona et al., 2011; Eyden et al., 2010; Kostin, 2010; Zhou et al.,

The "sex hormones"— estrogens, progesterone, and androgens—are a special category of steroids. Their actions are mediated by intracellular receptors, generally known as nuclear receptors, acting as ligand regulating transcription factors (slow genomic mechanisms) as well as by membrane-associated receptors and signaling cascades (fast nongenomic mechanism) (Giretti & Simoncini, 2008; Tetel et al., 2009). Sex steroids are involved in the regulation of many functions in human organism, including reproduction and behaviour. Female genital organs, especially those directly involved in ovum fertilization and embryo

implantation - fallopian tubes and uterus - are highly influenced by sex steroids.

Fig. 11. Electron microscopy of non-pregnant human uterus. Note the telocyte covering smooth muscle cells (M). The telopode is digitally coloured in blue, marked with asterisks. Image obtained in 2006. Courtesy of Prof. M. Taggart (Newcastle University, UK) and Dr.

Connective tissue filling the space between epithelial, muscular and nerve tissue, found in the walls of these organs, abound in different cells. They are generally referred to as stromal

Carolyn J.P. Jones (Manchester University, UK).

homocellular junctions.

**3. Sex steroids and TCs** 

2010).

Fig. 9. The schematic drawing of a podom (blue), the dilated portion of a telopode. Note the podomic endoplasmic reticulum in yellow and the mitochondria in red.

**Branching**, with a dichotomous pattern (Figure 10);

Fig. 10. Digitally coloured TEM image shows TC (blue) in human subepicardium, bordering the peripheral cardiomyocytes (CM, highlighted in brown). The TC has three telopodes, illustrating: a) the distinctive dichotomous pattern of branching (arrows); b) Tp are very thin at the emergence from the cell body; c) alternating podoms and podomeres. Note that some portions of podomeres have the same thickness as collagen fibrils, which makes observation under light microscopy impossible. E – elastin. Scale bar - 2 µm. Reproduced, with permission, from Popescu et al., 2010b.

 Organization in a labyrinthine system, forming a 3D network anchored by hetero- and homocellular junctions.

The concept of TC was soon embraced by other laboratories as well (Bani et al., 2010; Cantarero et al., 2011; Carmona et al., 2011; Eyden et al., 2010; Kostin, 2010; Zhou et al., 2010).
