Degenerative and regenerative changes in epidermal organ culture: a morphological study with reference to membrane-coating granules.

Membrane-coating granules (MCG) are poorly understood lamellate organelles unique to keratinized epithelia. This study provides data on a skin model for future in vitro investigations of MCG. Porcine ear epidermal organ cultures were used under standard cell culture conditions. This system was selected because it is easily established and, following a degenerative period in which MCG are lost, regenerates to form a highly differentiated epidermis. The epidermis appeared healthy during the first 2 d in vitro and contained MCG but lost keratohyalin granules (KHG). Overt degenerative changes were evident in the upper epidermis on Day 3, and MCG were now bloated. By Day 4 only one to three layers of viable undifferentiated cells remained. In the overlying necrotic epidermis MCG were rare, presumably due to the bursting of bloated MCG. Epidermal regeneration began around Day 5 and by Day 7 there were 8 to 13 layers, including a rudimentary parakeratotic stratum corneum (up to 4 layers). The stratum granulosum (two to three layers) now contained immature KHG and poorly lamellate MCG, but only amorphous material extracellularly. By Day 11 there were three to four layers of granular cells as in vivo, and an orthokeratotic stratum corneum (two to four layers). Improved cornification coincided with an increased number of mature KHG and cross-banded MCG, and lamellate MCG contents extracellularly. This model of epidermal regeneration will facilitate studies into the role played by MCG in keratinization because the epithelium initially lacked MCG but later expressed all the major morphologic features of epidermis. Furthermore the mechanisms by which MCG translocation and extrusion are effected may be probed by the inclusion of such agents as antimicrotubular drugs and calcium ionophores.