Constraints on the deformation of the vibrissa within the follicle
Fig 4
Images of horizontal (lengthwise) and parasagittal (cross-sectional) FSC sections permit estimation of relative tissue stiffness.
All images were taken at 4x. Left: The image shows a lengthwise cross-section of the A1 vibrissal FSC, assembled from multiple tiled images. Rostral is to the left. The slice does not pass exactly through the central axis, so the vibrissa (the continuous medial dark vertical line) is actually thinner than the full whisker diameter, and no medulla can be observed. The large space free of tissue observed around the vibrissa is an artifact of tissue preparation and was not represented in the model (see text for details). This panel serves only as a depth reference for the right panels and was not used for stiffness estimation. Horizontal black lines, labeled A–I, from superficial to deep represent the approximate levels at which the cross-sections (right 3x3 panel) were taken, on an equivalent follicle. Scale bar: 1 mm. Right: Cross sections of a single C3 vibrissa from superficial (section A) to deep (section I). Brightness, contrast, and magenta (R: 255, G: 0, B: 255) saturation were globally increased during post-processing. The vibrissa is present in sections D, F, G, and H, but has fallen out of the tissue in the remaining sections and is observed as a white oval. The whisker often falls out during tissue preparation because the keratin from which it is composed is highly cross-linked, and so does not have many available binding sites for fixative. The whisker in section H is diffuse and appears to be an empty space at 4x, but cells are observable at higher magnification, consistent with other descriptions of the HB and papilla [11]. A, B: A white oval where the vibrissa would be tightly held by the follicle wall (FW) is in the center of the image, and the follicle itself is also held tightly in the skin by the tissue. C: The outer root sheath (ORS), a membrane surrounding the vibrissa, is observed as a dark oval. Because the follicle has been sectioned at a slight angle, the RS (slightly deeper in the follicle) is observable on the rostral side. D: At the level of RS, most of the space inside the follicle is occupied by blood (brown) or empty space (white). The ORS and inner root sheath (IRS) can both be observed as dark ovals. The vibrissa (V) is held closely against the IRS, though the outer layer of the hair shaft is not pigmented so appears very light white/gray. E: Leaving the RS, all internal tissue becomes less stiff, appearing less darkly stained. At the level of CS, the FW is very thick compared to the vibrissa. MF: muscle fiber. F: Medially to the trabecula-dense region of the CS the internal membranes become denser (darker) but the vibrissa shaft becomes more diffuse, with melanocytes (pigmented portion of the whisker) no longer segregated to the center of the shaft (more apparent at higher magnification). G, H, I: All surrounding tissue is much less dense towards the end of the follicle, near the HB level. The whisker is also less dense. By comparing across all sections (A–I) it can be observed that the follicle wall is thin near the apex, thicker in the middle, and thin again deep in the tissue. Scale bar: 1 mm.