Abstract:

This video, presented by Jared Gardner, provides a comprehensive guide to interpreting immunohistochemistry (IHC) staining patterns in normal human skin. It emphasizes the critical importance of utilizing internal normal tissue controls within a patient's own sample for accurate pathological diagnosis and validation of stain efficacy, rather than solely relying on external control slides. The presentation meticulously demonstrates the distinct immunoreactivity profiles of various cytokeratin markers, including CK 903, CK 5/6, Cam 5.2, and pan cytokeratin, across the diverse epithelial structures of normal skin (epidermis, hair follicles, sebaceous glands, and eccrine glands). Key distinctions between high and low molecular weight keratin expression are detailed. A significant segment addresses a common and crucial pitfall: the non-specific granular cytoplasmic staining of mast cells by certain pan cytokeratin and other antibodies, highlighting the necessity to recognize this artifact to prevent diagnostic errors, especially in the context of metastatic disease.

Understanding Immunohistochemistry in Normal Skin

• 0:00:04 Introduction to Immunohistochemistry (IHC) in Normal Skin: The video emphasizes the crucial role of understanding IHC patterns in normal tissue for accurate tumor diagnosis and validating stain efficacy using internal controls.
• 0:01:05 CK 903 (High Molecular Weight Keratin) Staining: This stain, targeting keratins 1, 5, 10, and 14, shows positive cytoplasmic staining in the epidermis (nuclei remain negative), hair follicle epithelium, basal layer of sebaceous glands, and eccrine glands (myoepithelial cells stain darker, apical cells lighter).
• 0:06:01 Keratin Globules in Sun-Damaged Dermis: The video highlights amorphous, keratin-positive globules in sun-damaged dermis. These are not tumor cells but are believed to be breakdown products of keratinocytes from the epidermis, an important artifact to recognize.
• 0:08:00 CK 5/6 (High Molecular Weight Keratin) Staining: This marker, also a high molecular weight keratin cocktail, presents a slightly different pattern from CK 903, more strongly highlighting the basal layer of the epidermis and the basal/myoepithelial layer of eccrine coils, while inner apical secretory cells are negative.
• 0:09:19 Diversity of Keratin Stains: Different keratin "cocktails" (mixtures of antibodies) can yield varying staining reactions depending on the specific antibodies included and their individual strengths.
• 0:10:51 Cam 5.2 (Low Molecular Weight Keratin) Staining: This stain, primarily targeting cytokeratins 8 and 7, is notably negative in the epidermis but strongly positive in eccrine coils (especially the inner apical secretory cells) and parts of the hair follicle's outer root sheath.
• 0:12:58 Pan Cytokeratin Variability: An example of a pan cytokeratin stain (from an external lab) is shown that disproportionately favors low molecular weight keratins, strongly staining eccrine coils but only weakly the epidermal basal layer. The ideal pan cytokeratin should strongly stain all epithelial components.
• 0:15:08 Non-Specific Mast Cell Staining: A critical artifact is identified where scattered mast cells in the dermis non-specifically stain with certain antibodies (e.g., pan cytokeratin AE1/AE3, Desmin, Mart-1, Myogenin). This granular cytoplasmic staining is a false positive and must not be misinterpreted as tumor cells, particularly in the context of metastatic workups.
• 0:18:11 Importance of Controls: The video concludes by reinforcing the necessity of carefully reviewing both external and internal normal tissue controls to understand subtle staining nuances, avoid false positives, and ensure accurate diagnoses in pathology.