A scanning electron-microscopic analysis of the morphology of equine lower lip sinus hair.

This research investigates the structure and morphology of sinus hairs in equine lower lips, with examination done by scanning electron-microscopy, highlighting features such as trabeculae, blood sinuses, and endothelial cells.

Methodology

• The study makes use of Karnovsky’s fixative to stabilize tissues and cells in the horse’s lower lips before analysis. This allows for more accurate depiction of the natural, undisturbed arrangement of cell structures.
• Scanning electron microscopy was then utilized to observe sinus hair characteristics and the layout of the sinus hairs. This technology allows for detailed and high-resolution images of the hair’s structure and the cells it’s composed of.

Findings

• The researchers describe an intricate network of trabeculae or supportive structures inside the sinus hairs. These trabeculae originate from the internal connective tissue layer, extending to the external layer to secure the hair root in place, giving a mesh-like appearance.
• Unique to the anatomy of the equine lower lip sinus hairs, there are distal trabeculae, or sections of the structural network farthest from the center, which don’t connect to the outer wall of the blood sinus. Instead, they end in a unique finger-like knob within the sinus, a characteristic shared with other, different type of tactile hair known as sinus type.
• Despite the presence of blood sinuses, where blood flow and exchange occur, this only happens via the internal wall of the sinus hairs. This suggests that the blood turnover within this region may be low.
• The entire structure of the sinus wall and trabeculae were all found to be lined with polygonal endothelial cells, which possess protruding nuclei, sparse microvilli — small projections that increase surface area — and very pronounced cell borders.
• The endothelial cells covering the trabeculae were found to be more elongated in comparison to cells at other regions. This adaptation could facilitate connection between trabeculae and surrounding structures.

Implications

• The unique form of the sinus hairs and their features, such as the finger-like trabeculae, are interpreted in terms of their presumed function. The research does not provide definitive answers but proposes hypothetical functions for these features, opening up new avenues for future research in tactile hair anatomy and its sensory roles in equine biology.