Mechanics of Fluid-Filled Interstitial Gaps. II. Gap Characteristics in Xenopus Embryonic Ectoderm

Abstract

The ectoderm of the Xenopus embryo is permeated by a network of channels that appear in histological sections as interstitial gaps. We characterized this interstitial space by measuring gap sizes, angles formed between adjacent cells, and curvatures of cell surfaces at gaps. From these parameters, and from surface-tension values measured previously, we estimated the values of critical mechanical variables that determine gap sizes and shapes in the ectoderm, using a general model of interstitial gap mechanics. We concluded that gaps of 1-4 μm side length can be formed by the insertion of extracellular matrix fluid at three-cell junctions such that cell adhesion is locally disrupted and a tension difference between cell-cell contacts and the free cell surface at gaps of 0.003 mJ/m² is generated. Furthermore, a cell hydrostatic pressure of 16.8 ± 1.7 Pa and an interstitial pressure of 3.9 ± 3.6 Pa, relative to the central blastocoel cavity of the embryo, was found to be consistent with the observed gap size and shape distribution. Reduction of cell adhesion by the knockdown of C-cadherin increased gap volume while leaving intracellular and interstitial pressures essentially unchanged. In both normal and adhesion-reduced ectoderm, cortical tension of the free cell surfaces at gaps does not return to the high values characteristic of the free surface of the whole tissue.

Figure 1

Transmission electron micrographs of ectoderm. (A) Overview of ectoderm, midsagittal section through a stage 11 Xenopus laevis gastrula with cell outlines indicated. Pixel dimensions are 3296 × 2568, with 53 nm/pixel. (B–F) Interstitial gaps of various shapes within the ectoderm. A tri-cellular gap stained for ECM using lanthanum nitrate is shown in (B). Black and white arrowheads in (A) denote bow-tie-shaped gaps and four-cell junctions, respectively. The white scale bar represents 10 μm and the black scale bars 1 μm. EC, ectoderm; EP, epithelium; BC, blastocoel; ME, mesoderm.

Figure 2

Gap structure and parameters. (A–A″) Three-dimensional illustration of an interstitial channel sectioned straight, obliquely, or at the branching zone to generate gaps that appear different in size and shape. (B) Geometric parameters of a gap. The mean side length of a gap was calculated from the three distances between adjacent corners of a gap and the mean angle from the three angles at each corner of a gap. (C) Mechanical parameters determining size and shape of a gap, including interstitial pressure, p_i, intercellular pressure, p_c, cortical tension, β_i at the free surface, and residual tension per cell, β^∗. Contributions are shown for the right cell only.

Figure 3

Gap size distribution. (A–C) Frequency of gap sizes based on their average side length. (A) Measured gaps (n = 120). (B) The same gaps as in (A) Steiner-ellipse corrected (n = 120). (C) Selection of gaps in (A) that appear equilateral in sections (n = 30).

Figure 4

Frequencies of contact angles. (A) Angles measured at each point of contact between adjacent cells at 120 tri-cellular junctions (n = 360). (B) mean angles per gap of the same gaps as in (A) (n = 120). (C) Angles between adjacent cells at the blastocoel surface of the ectoderm (n = 43).

Figure 5

Distribution of F-actin in the ectoderm. (A–D) Phalloidin 488 staining reveals increased F-actin density on the exposed, blastocoelic surface of the ectoderm (white arrowhead in A) in comparison to cell-cell interfaces (gray arrowheads) and gaps (dashed arrowheads). Occasionally, F-actin puncta can be seen on one corner of a gap (white arrows in C and D). (E) Fluorescence intensities for all conditions were normalized to exposed surface values (^∗∗∗p < 0.001, n.s., not significant). Error bars represent standard deviations from the mean intensity values for exposed (n = 12 cells), inner cell-cell interfaces (n = 24), and gaps without (n = 21) and with actin puncta (Gaps^∗). The white scale bar represents 10 μm and the gray scale bars 3 μm.

Figure 6

The gap domain of the gastrula ectoderm. Shown is the relationship between mean angles per gap $\left(\overline{2{\theta }_i}\right)$, corrected average side length per gap, $\overline{\mathit{l}}$, and radii of curvature, R_i. Each dot represents an observed gap with the measured mean of angles and side length, respectively. Lines of constant radius, R_i, are approximated by straight lines that intersect the abscissa at l = R_i. Two groups of gaps can be distinguished, as outlined (shaded areas). For the dark gray group of smaller gaps, the gap domain and the maximal and minimal contact angle sums were read off for each R_i value shown at the intersections of the respective R_i lines with the outline of the gap domain (e.g., dashed horizontal lines for R_i = 2 μm).

Figure 7

Deduction of cortical tension and adhesion tension variations from the maximal and minimal contact angles. (A–C) Adhesion tension, Γ/2, as a function of relative adhesiveness, α_i, at gaps. The slope of the linear function equals cortical tension, β_c, and its intersection with the horizontal line of constant Γ/2 of 0.003 mJ/m² defines respective α_i values and hence contact angles. (A) Example R_i = 2 μm, maximal β_c line (solid), and minimal β_c line (dashed) were chosen to match minimal and maximal α_i values, respectively, and hence contact angles, as derived from Fig. 4. (B and C) Maximal and minimal lines as in (A) are shown separately for clarity for several R_i values. (D) Maximal and minimal β_c values corresponding to the slopes in (B) and (C) plotted as a function of R_i.

Figure 8

Distribution of gap shapes in C-cad-MO-injected ectoderm. (A) Transmission electron micrograph of C-cad-MO ectoderm. Pixel dimensions are 2660 × 1662, with 53 nm/pixel. The scale bar represents 5 μm. EC, ectoderm; BC, blastocoel; EP, epithelium. (B) Angles in C-cad-MO ectoderm, measured at each point of contact between adjacent cells at 32 tri-cellular junctions (n = 96). (C and D) The frequency of gap sizes in C-cad-MO ectoderm, based on their average side length as measured (C) (n = 32) and Steiner ellipse corrected (D) (n = 32). (E) Interstitial areas within gaps at tri-cellular junctions in normal (uninjected) (n = 32) and C-cad-MO-injected ectoderm (n = 32).

Figure 9

Steiner ellipse correction for oblique sectioning. (A and C) A section perpendicular to the long axis of an interstitial gap channel results in an equilateral triangle circumscribed by a circle with a semi-major axis, a, equivalent to a semi-minor axis, b. (B and D) An oblique section of a gap channel results in a stretched triangle circumscribed by an ellipse with a semi-major axis longer than the semi-minor axis. (D and E) The Steiner ellipse correction can be used to determine the side length, l, of the equilateral triangle using the three side lengths d–f of the stretched triangle. This transforms all gaps with various configurations of unequal side lengths to symmetric gaps with equal side lengths (F–J).