Background: New models of the structural relationship between the arrector pili (AP) muscle and the sebaceous gland (SG) have been proposed recently.
Objectives: The purpose of the present study was to establish the actual morphological relationship between components of the follicular unit (FU) including the hair follicles, AP muscle and SG using 3D reconstruction of serially sectioned specimens so as to expand previous explanations of the secretory mechanism of the SG and to suggest other possible mechanisms based on newly proposed model.
Methods: Scalp skin specimens were processed using routine histological procedures, with serially sectioned tissue slides being stained with Masson's trichrome. 'Reconstruct' software was used to align, assemble and reconstruct the sections, with observations of the 3D-reconstructed FU [including hair follicles (HFs), AP muscle and SG].
Results: Fifty FUs were reconstructed. The AP muscle was curved and concave as it supported the basal portion of the sebaceous lobules in the perifolliculum. Sebaceous lobules were located between the AP muscle and HFs (angular area) and some sebaceous lobules located in the opposite (counter-angular) area.
Conclusions: We propose that the concave part of the AP muscle pushes up the basal portion of the sebaceous lobule between the HFs and AP muscle during AP muscle contraction and hair erection. In addition, the sebaceous lobule located at the counter-angular position is squeezed by the HF during AP muscle relaxation and hair repositioning. Combined with the previous mechanism of SG secretion, this newly established mechanism based on the 3D structure of the FU will improve our understanding of AP muscle function and SG secretion.