This video provides a detailed description of seborrheic keratosis (SK), a common benign skin lesion, covering its macroscopic and microscopic characteristics, as well as various subtypes. Macroscopically, SK typically presents as multiple, brown, hyperkeratotic papules with a "stuck-on" appearance and greasy, friable scales, often found in elderly individuals. Microscopically, the most common acanthotic subtype is characterized by a thickened epidermis composed of basaloid keratinocytes, abundant melanin pigment, and distinctive horn cysts (pseudocysts). A crucial diagnostic feature is the abrupt keratinization within these cysts, devoid of parakeratosis, which helps differentiate SK from squamous cell carcinoma. The video further explores specific variants including the deeply pigmented melanoacanthotic SK, the interconnected epithelial bands of reticular SK, the intercellular cohesion loss in acantholytic SK, and the basaloid cell nests indicative of clonal SK, providing key distinguishing features for each.
Summarizing Seborrheic Keratosis: Features and Subtypes
0:02 Seborrheic Keratosis (SK) Introduction: SK is a very common skin lesion, particularly in elderly individuals, often appearing as multiple, hyperkeratotic papules measuring several millimeters to 1-2 centimeters.
0:29 Macroscopic Features: SK typically has a brown color due to melanin pigment, a "pebbled" surface with greasy, friable scales, and a classical "stuck-on" appearance.
0:43 Microscopic Appearance (Acanthotic Subtype): The most common acanthotic subtype is a superficial, elevated tumor with a thickened epidermis (acanthotic means thicker epidermis) consisting of anastomosing rete ridges. It is composed of basaloid keratinocytes (cells resembling those in the basal layer of the epidermis), which are roundish, slightly paler, regular, and have relatively larger nuclei.
2:04 Melanin Pigment & Melanoacanthotic SK: A lot of melanin pigment is present in the epidermis, contributing to the brown macroscopic color. The "melanoacanthotic" subtype has particularly prominent melanin pigmentation and a dark brown color.
2:23 Differential Diagnosis: The main differential diagnosis for pigmented SK is a melanocytic nevus.
2:37 Characteristic Horn Cysts: SK features multiple "horn cysts" (or pseudocysts, as they communicate with the surface). These cysts are characterized by abrupt keratinization, where basaloid cells suddenly mature into regular keratin lamellae without parakeratosis (no visible nuclei in the keratin).
3:19 Differentiation from Squamous Cell Carcinoma (SCC): The absence of parakeratosis in SK's horn cysts is a very important feature, differentiating it from SCC, where cysts would typically show parakeratosis (unless SK is irritated). The keratin in SK horn cysts often appears bluish.
4:06 General Microscopic Features: The surface of classical SK is usually smooth, and the acanthotic epidermis does not protrude into the deeper dermis. The cells are bland, without mitotic figures or atypia.
5:10 Reticular Subtype: This variant consists of multiple interconnected epithelial bands, often only two cells thick, forming thin anastomosing strands, frequently with prominent pigmentation and the classical abrupt keratinization.
6:05 Acantholytic Subtype: Characterized by prominent acantholysis β the loss of intercellular connections (desmosomes) leading to clear spaces and loss of cohesion between keratinocytes throughout the tumor.
6:38 Clonal Subtype: This form is defined by multiple, sharply outlined nests of regular basaloid cells within the epidermis, representing an early stage of seborrheic keratosis, typically with very mild acanthosis and few to no horn cysts.