The secretory granular cell: the outermost granular cell as a specialized secretory cell

Area of Science:

• Dermatology
• Cell Biology
• Skin Barrier Research

Background:

• Epidermal lamellar bodies (LB) are crucial for skin barrier formation, releasing contents at the stratum granulosum (SG)-stratum corneum (SC) interface.
• The precise subcellular mechanisms governing LB secretion and their rapid response to barrier disruption remain incompletely understood.

Purpose of the Study:

• To elucidate the subcellular basis of epidermal lamellar body (LB) secretion at the stratum granulosum (SG)-stratum corneum (SC) interface.
• To characterize the cellular structures and processes involved in polarized LB release and the response to barrier perturbation.

Main Methods:

• Confocal microscopy using fluorescent lipid dyes (Nile red, NBD-ceramide) to visualize membrane systems.
• Ultrastructural analysis (electron microscopy) to examine subcellular details of LB and associated membrane networks.
• Assessment of LB secretion following acute epidermal permeabilization (acetone, sonophoresis, iontophoresis).

Main Results:

• A tubulo-reticular membrane system, identified as a trans-Golgi-like network (TGN), is present in the apical cytosol of outermost SG cells.
• This TGN network associates with lamellar bodies (LB) and deep invaginations of the apical plasma membrane, forming a honeycomb structure.
• LB fusion with these invaginations occurs rapidly, especially after barrier disruption, and nascent LB appear to bud from the TGN.

Conclusions:

• The outermost SG cell possesses a specialized secretory apparatus, termed 'secretory granulocyte,' facilitating polarized LB secretion via TGN and plasma membrane invaginations.
• This unique cellular structure enables a rapid and efficient response to epidermal barrier damage.
• The findings provide novel insights into the cellular mechanisms underlying skin barrier homeostasis and repair.