This video, Part 2 of a series on normal skin histology and immunohistochemistry, delves into the characteristic staining patterns of several common immunostains within healthy skin structures. Dr. Gardner meticulously demonstrates the nuclear and cytoplasmic expression of P63, Epithelial Membrane Antigen (EMA/MUC1), Desmin, Smooth Muscle Actin (SMA), CD34, and Factor 13a. The discussion highlights each stain's utility in identifying specific cell types and adnexal structures like keratinocytes, hair follicles, sebaceous glands, eccrine glands, nerves, and muscles. Crucially, the video emphasizes important diagnostic considerations, including the specificity and non-specificity of certain markers, common pitfalls in interpretation (e.g., actin staining myofibroblasts, mast cells non-specifically staining with Desmin, or CD34's presence in normal dermis), and their application in differentiating various benign and malignant skin neoplasms.
Immunohistochemistry in Normal Skin (Part 2): Key Stains and Their Patterns
0:30 P63 (Nuclear Stain for Keratinocytes and Epithelial Structures):
Stains nuclei of keratinocytes, hair follicle epithelial cells, and the basal (germinated) layer of sebaceous glands (mature sebocytes are weakly positive or negative).
In eccrine glands, it stains the basal/myoepithelial layer of cells, particularly becoming more concentrated in the duct as it moves towards the surface. The inner secretory cells are mostly negative.
Key Takeaway: P63 (and its isoform P40) is highly useful in dermatopathology for diagnosing basal cell carcinoma, squamous cell carcinoma, and a wide variety of benign and malignant adnexal tumors due to its expression in most epithelial structures.
3:37 EMA (Epithelial Membrane Antigen / MUC1) - Beautiful Sebaceous and Luminal Staining:
Strongly stains the cytoplasm of sebaceous glands, uniquely highlighting the membranes around individual lipid vacuoles.
Epidermis is typically negative, but can show weak, patchy expression, which is an important consideration for distinguishing from squamous cell carcinomas.
Highlights parts of eccrine glands and ducts: in the glandular portion, it strongly stains the membrane lining the inner lumen (apical surface of secretory cells). In the ductal component, it begins to highlight the basal/outer layer of cells. This luminal highlighting can be a clue for sweat gland tumors.
Internal Control: EMA beautifully stains the perineurium (outer layer) of nerves, while Schwann cells and axons remain negative.
Strongly highlights erector pili muscles (responsible for "goosebumps").
Stains muscle in blood vessel walls much weaker or even negatively compared to actin.
Key Takeaway: Desmin is a more specific marker for muscle than actin, which can also stain myofibroblasts. Definitive diagnosis of smooth muscle neoplasms often relies on Desmin positivity.
Important Pitfall: Desmin non-specifically stains scattered mast cells in the dermis, which appear as granular cells with oval nuclei.
12:11 Smooth Muscle Actin (SMA) - Vascular and Myoepithelial Marker:
Stains erector pili muscles (similar to Desmin).
Strongly stains pericytes wrapping around small vessels and the full muscular layers of larger vessel walls.
Nicely stains myoepithelial cells in the basal layer of eccrine sweat coils, indicating their muscle-type protein components.
Key Takeaway: While useful for vascular smooth muscle and myoepithelium, actin is non-specific (e.g., myofibroblasts also stain), so it should not be used alone for definitive diagnosis of smooth muscle tumors without Desmin.
14:46 CD34 (Dermal and Endothelial Marker with Pitfalls):
Diffusely positive in spindled and stellate cells throughout the normal dermis (fibroblasts and dendritic cells).
Nicely stains endothelial cells lining the lumen of blood vessels.
Stains the outer root sheath of hair follicles (membranous staining with a "piano key" appearance) in the root portion of anagen hair follicles.
Key Takeaway: CD34 is widely expressed in normal dermis, making it difficult to use for margin assessment in tumors like dermatofibrosarcoma protuberans (DFSP). It is positive in various fibroblastic proliferations and vascular tumors.
Important Pitfall: Staining around the edge of a tumor might be compressed normal dermal cells; the tumor cells themselves might be CD34 negative.
19:02 Factor 13a (Dendritic Cell Marker with Unique Sebocyte Staining):
Stains dendritic cells scattered throughout the dermis, which are often increased in dermatofibromas.
Important Pitfall: Can non-specifically stain other tumors, such as spindle cell carcinoma.
Unique Feature (Clone aC1A1): This specific clone of Factor 13a strongly stains the nuclei of sebocytes, which can be useful for identifying sebaceous differentiation in tumors.