- The stratified squamous epithelium of the skin, specifically the epidermis, is chiefly involved in disorders that affect keratinocyte proliferation and differentiation.

- Psoriasis is a chronic inflammatory skin disease characterized by hyperproliferation of keratinocytes in the stratified squamous epithelium of the epidermis.
- This leads to thickened, scaly plaques typically seen on extensor surfaces.
- Histologically, psoriasis shows acanthosis (epidermal hyperplasia) and parakeratosis due to the rapid turnover of epidermal cells.

- Lichen planus involves an immune-mediated attack primarily targeting the basal layer of the squamous epithelium, but it is more focused on interface dermatitis rather than hyperproliferation of the stratified squamous epithelium.

- Pemphigus vulgaris is an autoimmune blistering disorder where antibodies target desmoglein-3 and desmoglein-1 in the epidermal desmosomes, leading to acantholysis and intraepidermal blisters. Although it involves the epidermis, the key pathology relates to cell-cell adhesion disruption rather than primary hyperproliferation or thickening of the stratified squamous epithelium.

- Bullous pemphigoid is another autoimmune blistering disorder, but it involves antibodies against hemidesmosomal components in the basement membrane zone between the basal keratinocytes and the dermis, leading to subepidermal blisters. Thus, it affects the dermo-epidermal junction rather than the stratified squamous epithelium itself.

Key Points:
- Psoriasis primarily involves the hyperproliferation of keratinocytes in the stratified squamous epithelium of the skin.
- Lichen planus involves immune-mediated damage mainly at the basal layer.
- Pemphigus vulgaris causes intraepidermal blistering by disrupting desmosomal adhesion.
- Bullous pemphigoid causes subepidermal blistering by targeting the basement membrane zone.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th Edition, Chapter 8: Diseases of the Skin and Subcutaneous Tissue, Pages 115-120.