How to complete boundary in masked image?

Question

I am trying to generate a mask of the elliptical boundaries in this object. Any ideas on how I could complete the edge of a partially filled elliptical object? Thus far, I've obtained a mask that is fairly close to the desired result, which would enclose the boundaries of all cells in an edged, but unfilled border.

tD = Import["https://i.sstatic.net/7x8EB.png"];

gaussKernel = 0.;
neighborhood = 5.;
mean = .77;
sd = .61;
size = 100;
cellMask =
ColorNegate@ColorNegate[Erosion[Dilation[SelectComponents[
  ColorNegate@
   LocalAdaptiveBinarize[
    GaussianFilter[ImageAdjust[tD], gaussKernel], 
    neighborhood, {mean, sd}], {"EnclosingComponentCount", 
   "Count"}, #2 > size && #1 == 0 &], 2], DiskMatrix[1.5]]]

Answer 1

Don't very sure what you want,this is my thinking about your question

m = DeleteSmallComponents[
  LaplacianGaussianFilter[tD, 1.4] // Binarize, 60]

SelectComponents[
 Colorize[MorphologicalComponents[m, Method -> "ConvexHull"], 
  ColorRules -> {0 -> Black, _ -> White}], Small]

If you get the unfilled border,you can use MorphologicalPerimeter to the result.