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Research Papers

Endogenous Sheet-Averaged Tension Within a Large Epithelial Cell Colony

[+] Author and Article Information
Sandeep P. Dumbali, Lanju Mei, Shizhi Qian

Mechanical and Aerospace Engineering,
Old Dominion University,
Norfolk, VA 23529

Venkat Maruthamuthu

Mechanical and Aerospace Engineering,
Old Dominion University,
4635 Hampton Boulevard,
238e Kaufman,
Norfolk, VA 23529
e-mail: vmarutha@odu.edu

1Corresponding author.

Manuscript received March 16, 2017; final manuscript received July 21, 2017; published online August 18, 2017. Assoc. Editor: Jeffrey Ruberti.

J Biomech Eng 139(10), 101008 (Aug 18, 2017) (5 pages) Paper No: BIO-17-1112; doi: 10.1115/1.4037404 History: Received March 16, 2017; Revised July 21, 2017

Epithelial cells form quasi-two-dimensional sheets that function as contractile media to effect tissue shape changes during development and homeostasis. Endogenously generated intrasheet tension is a driver of such changes, but has predominantly been measured in the presence of directional migration. The nature of epithelial cell-generated forces transmitted over supracellular distances, in the absence of directional migration, is thus largely unclear. In this report, we consider large epithelial cell colonies which are archetypical multicell collectives with extensive cell–cell contacts but with a symmetric (circular) boundary. Using the traction force imbalance method (TFIM) (traction force microscopy combined with physical force balance), we first show that one can determine the colony-level endogenous sheet forces exerted at the midline by one half of the colony on the other half with no prior assumptions on the uniformity of the mechanical properties of the cell sheet. Importantly, we find that this colony-level sheet force exhibits large variations with orientation—the difference between the maximum and minimum sheet force is comparable to the average sheet force itself. Furthermore, the sheet force at the colony midline is largely tensile but the shear component exhibits significantly more variation with orientation. We thus show that even an unperturbed epithelial colony with a symmetric boundary shows significant directional variation in the endogenous sheet tension and shear forces that subsist at the colony level.

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Copyright © 2017 by ASME
Topics: Tension , Traction
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Figures

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Fig. 1

(a) Schematic depiction of the epithelial cell colony on the PAA hydrogel. (b) Phase image of the circular MDCK cell colony with the traction stress vectors overlaid. Scale bar for distance is 50 μm and for traction vector is 1000 Pa. (c) Heat map representation of the traction stress under the colony.

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Fig. 2

(a) Distribution of the magnitudes of traction stresses exerted under the central, medial, and distal regions within the colonies. Only above average traction stresses (used as a proxy for significant/large traction stresses) are considered in this plot. Data are pooled from n = 6 colonies. (b) Traction forces under each epithelial cell colony are balanced. The vector sum for each of the colonies is close to zero relative to the scalar sum of the traction forces that are shown for comparison. n = 6 colonies.

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Fig. 3

(a) Schematic depiction of the physical force balance used to determine the intrasheet force at the colony midline. (b) Variation of the intrasheet force at the colony midline as a function of the orientation of the midline for the colony shown in Figs. 1(b) and 1(c). (c) The maximum and minimum sheet force in a colony normalized by its average sheet force across all orientations (n = 6 colonies).

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Fig. 4

(a) Variation of the tensile and shear components of the endogenous sheet force at the colony midline with the orientation of the midline, shown for three representative colonies with similar average sheet force. (b) Normalized variation in the sheet tension and shear within each colony in % (n = 6 colonies).

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Fig. 5

Traction and sheet forces are Rho-kinase sensitive. (a) Heat map representation of the traction stress under the MDCK cell colony before and after 1 h of treatment with 50 μM of the Rho-kinase inhibitor Y27632. (b) Variation of the intrasheet force at the colony midline as a function of the orientation of the midline before and after 1 h treatment with Y27632.

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Fig. 6

(a) Heat map representation of the traction stresses under the colony computed using a finite element model of the substrate as a linear, isotropic elastic medium of finite thickness (of 150 μm). Compare with the Boussinesq solution in Fig. 1(c). (b) Comparison of the intrasheet force at the midline obtained using the Boussinesq solution and the FEM result considering the finite thickness of the substrate. Estimated errors (not shown in the figure) in the Boussinesq solution sheet force are 310 nN and in the FEM results sheet force are 670 nN.

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