Mechanical properties of growing melanocytic nevi and the progression to melanoma - PubMed (original) (raw)

Mechanical properties of growing melanocytic nevi and the progression to melanoma

Alessandro Taloni et al. PLoS One. 2014.

Abstract

Melanocytic nevi are benign proliferations that sometimes turn into malignant melanoma in a way that is still unclear from the biochemical and genetic point of view. Diagnostic and prognostic tools are then mostly based on dermoscopic examination and morphological analysis of histological tissues. To investigate the role of mechanics and geometry in the morpholgical dynamics of melanocytic nevi, we study a computation model for cell proliferation in a layered non-linear elastic tissue. Numerical simulations suggest that the morphology of the nevus is correlated to the initial location of the proliferating cell starting the growth process and to the mechanical properties of the tissue. Our results also support that melanocytes are subject to compressive stresses that fluctuate widely in the nevus and depend on the growth stage. Numerical simulations of cells in the epidermis releasing matrix metalloproteinases display an accelerated invasion of the dermis by destroying the basal membrane. Moreover, we suggest experimentally that osmotic stress and collagen inhibit growth in primary melanoma cells while the effect is much weaker in metastatic cells. Knowing that morphological features of nevi might also reflect geometry and mechanics rather than malignancy could be relevant for diagnostic purposes.

PubMed Disclaimer

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Computational model.

a) A two dimensional section of the skin (left image courtesy of Dr. Claudio Clemente) is simulated as a non-linear elastic layered material (right). The top layer is the stratum corneum (pink cells), resting on the epidermis (blue cell). The epidermis is separated from the dermis (green cells) by a basal membrane (grey). In a typical simulation we consider a melanocyte (yellow) dupicating inside the skin, either in the epidermis, or in the dermis. b) The connecting fiber have a non-linear elastic behavior with a fracture strength set at formula image. c) When in contact, the cells interact by a finite-thickness Hertz law. The corresponding zero-thickness law is reported for comparison.

Figure 2

Figure 2. Effect of osmotic pressure on cell proliferation in melanoma.

500/well were plated on 96 multiwells (IgR39 and IgR37). The day after plating the cells were submitted to different osmotic pressure (from 0.2 to 1 kPa) for 3 or 6 days. At the end of the incubation the cells were fixed with 50%TCA for 2 hours at 4C and air-dry at room temperature. Thus, the cells were incubated with 0.05% SBR solution for 30 minutes at room temperature and then quickly rinsed four times with 1% acidic acid to remove unbound dye. Finally, the protein-bound dye was solubized with 10 mM TRIS and OD was measured at 510 nm with microplate reader 550 (Bio-Rad). a) The growth of IgR39 (non-metastatic) cells is not affected by pressure after 3 days, b) but cells grow significantly less after 6 days. c) IgR37 (metastatic) cells are unaffected by pressure both after c) 3 or d) 6 days. Statistically significant results according to the KS test (formula image) are denoted with *.

Figure 3

Figure 3. Effect of osmotic pressure on colony formation in melanoma.

a) The cumulative distributions of colony size obtained from IgR39 cells under 1 kPa osmotic pressure with respect to the control (0 kPa). The curves are the fit with a continuous time branching process model (see Ref [42]) yielding a division of rate of formula image that is reduced to formula image under osmotic pressure. b) The average value of the colony size distribution with the associated standard error for IgR39 and IgR37 cells. Statistically significant results according to the KS test (formula image) are denoted with *. c) The images show two representative examples of the colonies for 0 kPa and 1 kPa conditions.

Figure 4

Figure 4. Effect of collagen coating on cell proliferation in melanoma.

500/well were plated on 96 collagen coated multiwells (IgR39 and IgR37) and submitted to 1 kPa osmotic pressure for 6 days. Cell growth was evaluated as in Fig.2. The graph shows that the presence of collagen slows cell growth both in primary (IgR39) and metastatic (IgR37) cells but the effect is much stronger for primary cells. Additional osmotic pressure does not significantly alter the results. Statistically significant results, according to the KS test (formula image), are denoted with *.

Figure 5

Figure 5. Morphology of nevi for different locations of the initiating cell for random growth.

Illustration of the results of numerical simulations for nevi grown from melanocytes located in different positions in the skin and for different mechanical properties of the basal membrane. Here growth occurs randomly and independently on compressive stress. Growing melanocytes are shown with a varying color that reflects their compressive stresses σ according to the color bar. The maximum formula image and minimum formula image for each configuration are equal to (in kPa): a) formula image, b) formula image, c) formula image, d) formula image, e) formula image, f) formula image. a) Nevi grown from melanocytes residing inside the epidermis tend to grow horizontally and do not spread towards the basal membrane. b) Nevi grown on the minima of a strong basal membrane tend to grow roughly parallel to the membrane itself. c) Nevi growing from the maxima of a strong basal membrane tend to grow vertically in the epidermis. d) Dermal nevi tend to have radial shape. When the basal membrane is weak, e) nevi growing from the minima of the basal membrane invade the dermis in a radial fashion, while f) when they start from maxima of the basal membrane the invasion of the dermis occurs more vertically.

Figure 6

Figure 6. Mechanical stresses in randomly growing nevi.

We report the evolution of the average compressive stress experienced by the melanocytes composing the nevus as a function of the size of the nevus, quantified by the total number of cells n, for the same conditions as in Fig. 5. The error bars represent the standard error of the mean. The stress is averaged over all the cells in a nevus and over at least ten statistically independent realizations of the growth process. The different panels represent different initial locations: a) in the middle of the epidermis, b) in the minima of a strong basal membrane, c) in the maxima of a strong basal membrane, d) in the dermis, e) in the minima of a weak basal membrane, f) in the maxima of a weak basal membrane.

Figure 7

Figure 7. Compressive stress distribution in nevi.

We report the distribution formula image of the compressive stresses σ (in kPa) experienced by melanocytes in a nevus. The results are sampled over different realizations of the process. Since the distribution also changes with time, we only consider the time at which the average compressive stress formula image is highest. The different panels represent different initial conditions corresponding to Fig. 5: a) in the middle of the epidermis, b) in the minima of the basal membrane (for strong and weak membranes) c) in the dermis, d) in the maxima of the basal membrane (for strong and weak membranes).

Figure 8

Figure 8. Role of adhesion forces and direct interactions with the ECM.

Morphology of a nevus grown in the dermis a) in presence of adhesive forces between nevi cells and b) considering direct interactions with the ECM modelled as discussed in the methods section. c) The evolution of the average compressive stress experienced by the melanocytes composing the nevus for cases a) and b) is compared with a simulation in which no adhesive forces and interactions with the ECM are present.

Figure 9

Figure 9. Model of MMP induced breaking of the basal membrane.

a) We report the typical morphology of a nevus starting from the epidermis with a strong basal membrane that can however be broken by MMPs. b) The evolution of the average compressive stress in presence of MMPs is compared with the case in which MMPs are not present.

Figure 10

Figure 10. Histological images of melanocytic nevi.

We illustrate the morphology observed by optical microscopy at different magnifications on histological sections of two types of nevi. a) Intraepithelial junctional nevi are confined in the epidermis and press against the basal membrane forming characteristic rounded pockets of densely packed cells (compare with Fig. 5b) b) Dermal nevi are confined in the dermis (compare with Fig. 5d). Cells are loosely packed and do not touch the basal membrane. The basal membrane is located at the separation between the upper (epidermis) and lower (dermis) skin layers and indicated by an arrow in panel a). Images courtesy of Dr. Claudio Clemente.

Similar articles

Cited by

References

    1. Baxter LL, Pavan WJ (2013) The etiology and molecular genetics of human pigmentation disorders. Wiley Interdiscip Rev Dev Biol 2: 379–92. - PMC - PubMed
    1. Kim JK, Nelson KC (2012) Dermoscopic features of common nevi: a review. G Ital Dermatol Venereol 147: 141–8. - PubMed
    1. Zalaudek I, Manzo M, Savarese I, Docimo G, Ferrara G, et al. (2009) The morphologic universe of melanocytic nevi. Semin Cutan Med Surg 28: 149–56. - PubMed
    1. Unna PG (1893) Naevi and naevocarcinome. Berl Klin Wochenschr 30.
    1. Cramer SF (1988) The melanocytic differentiation pathway in congenital melanocytic nevi: theoretical considerations. Pediatr Pathol 8: 253–65. - PubMed

Publication types

MeSH terms

Substances

LinkOut - more resources