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• Epidermis
  • Regular acanthosis, 25
  • Lobular proliferation, 29
  • Reticulated proliferation, 35
  • Central pore, 42
  • Epidermal perforation, 46
• Dermis
  • Circular islands, 49
  • Cords/tubules and comma shapes, 53
  • Space with a lining, 59
  • Papillations, 70
  • Polypoid (dome-shaped), 77
  • Square/rectangular, 82
  • Palisading reactions, 88
  • Pseudoepitheliomatous hyperplasia above abscesses, 93
  • Pink ball (see Chapter 6)

Bowen disease

• Epidermal acanthosis
• Parakeratosis
• Full-thickness disorder of keratinocytes with atypical cells and mitoses
• Basal layer may focally appear normal ("eyeliner" sign) (arrow)

  Clear cell acanthoma
  

• Epidermal acanthosis
• Clear cells well-demarcated from the normal epidermis and adnexal keratinocytes
• Parakeratosis above clear cells

  Psoriasis
  

• Epidermal acanthosis
• Parakeratosis
• Neutrophils in stratum corneum (asterisk)
• Hypogranulosis
• Thinned suprapapillary plates (long arrow)
• Dilated vessels in papillary dermis (short arrows)

  Key differences
  

  (a)

  (b)

  (c)

  1. Bowen disease: disordered keratinocytes and atypical mitoses
  2. Clear cell acanthoma: pale/clear keratinocytes well-demarcated from normal epidermis
  3. Psoriasis: confluent parakeratosis above thickened epidermis, neutrophils in stratum corneum, normal keratinocytes, thin suprapapillary plates, dilated vessels

  Inverted follicular keratosis
  

• Lobular proliferation
• Normal-appearing keratinocytes with some arranged in squamous eddies

  Molluscum contagiosum
  

• Lobular proliferation
• Blue-gray hue to some of the keratinocytes
• Large, pink cytoplasmic inclusions (Henderson-Paterson bodies)

  Poroma
  

• Lobular proliferation (can be reticulated)
• Uniform blue cells with interspersed ducts (arrows)
• Fibrotic or hyalinized stroma with dilated vessels

  Seborrheic keratosis, acanthotic
  

• Lobular proliferation
• Acanthosis of epidermis
• Pseudohorn cysts
• No ducts

  Trichilemmoma
  

• Lobular proliferation
• Proliferation composed of pale/clear cells
• Peripheral palisading (long arrow) with thickened basement membrane (short arrow)

  Key differences
  

  (a)

  (b)

  (c)

  (d)

  (e)

  1. Inverted follicular keratosis: squamous eddies of normal keratinocytes
  2. Molluscum contagiosum: intracytoplasmic pink inclusions (Henderson-Paterson bodies)
  3. Poroma: uniform blue cells with interspersed ducts, hyalinized stroma
  4. Seborrheic keratosis: pseudohorn cysts
  5. Trichilemmoma: pale/clear keratinocytes with peripheral palisading and thickened basement membrane

  Fibroepithelioma of Pinkus
  

• Reticulated proliferation
• Strands of basaloid cells in a fibrovascular stroma
• Some hints of palisading of cells (arrow)

  Fibrofolliculoma
  

• Reticulated proliferation
• Fibrotic stroma adjacent to the hair follicle has reticulated strands of epithelium
• This entity has overlap with trichodiscoma (most consider these a spectrum of the same entity)

  Nevus sebaceus of Jadassohn
  

• Reticulated proliferation
• Sebaceous glands, basaloid proliferations (arrow) connect to the epidermis
• Apocrine glands may be seen deep
• Absent terminal hairs in mature stage

  Seborrheic keratosis, reticulated
  

• Reticulated proliferation
• Strands of banal keratinocytes, often pigmented
• Interspersed pseudohorn cysts

  Syringofibroadenoma
  

• Reticulated proliferation
• Strands of banal, round cells
• Interspersed ducts

  Tumor of the follicular infundibulum
  

• Reticulated proliferation
• Pale cells in columns with "windows" of dermis in between
• Peripheral palisading

  Key differences
  

  (a)

  (b)

  (c)

  (d)

  (e)

  (f)

  1. Fibroepithelioma of Pinkus: strands of basaloid epithelium in fibrovascular stroma
  2. Fibrofolliculoma: hair follicle with adjacent fibrotic stroma and reticulated epithelium
  3. Nevus sebaceus: proliferation of epidermis connecting to sebaceous lobules and basaloid proliferations
  4. Seborrheic keratosis, reticulated: reticulated strands of banal keratinocytes, often pigmented; interspersed pseudohorn cysts
  5. Syringofibroadenoma: strands of uniform, round cells with interspersed ducts
  6. Tumor of the follicular infundibulum: pale cells in columns with "windows" of dermis in between

  Dilated pore of Winer
  

• Central pore
• Invaginated epidermis is acanthotic

  Pilar sheath acanthoma
  

• Central pore
• Invaginated epidermis is acanthotic and has areas resembling outer root sheath with peripheral palisading around slightly pale cells

  Trichofolliculoma
  

• Central pore
• Invaginated epidermis connects to a primary hair follicle
• Multiple secondary hair follicles radiating away from the central follicle

  Key differences
  

  (a)

  (b)

  (c)

  1. Dilated pore of Winer: acanthotic epidermis
  2. Pilar sheath acanthoma: epidermal acanthosis and areas resembling outer root sheath
  3. Trichofolliculoma: primary follicle and surrounding secondary follicles

  Elastosis perforans serpiginosa
  

• Epidermal perforation
• Elongated claw (epidermal rete) gripping thin, glassy, eosinophilic elastic fibers and debris
• Altered elastic fibers are thinner than the collagen fibers in the dermis

  Reactive perforating collagenosis
  

• Epidermal perforation
• Shallow cup-shaped architecture
• Pink collagen fibers extend vertically through epidermis

  Key differences
  

  (a)

  (b)

  (c)

  (d)

  1. Elastosis perforans serpiginosis: elongated rete forming a claw (tortuous channel); case courtesy of Whitney High, MD, JD
  2. Reactive perforating collagenosis: shallow, broad cup-shaped architecture
  3. Calcinosis cutis, perforating: chunky, bluish material (see page 264) at the base of an ulcer

  4. Granuloma annulare, perforating: palisading histiocytes around collagen with mucin (see page 89)

     Note Other entities may also perforate the epidermis, for example, chondrodermatitis nodularis helicis, pseudoxanthoma elasticum.

  Adenoid cystic carcinoma
  

• Circular dermal islands
• Islands contain basaloid cells with a cribriform pattern of duct-like spaces filled with amorphous material

  Cylindroma
  

• Circular dermal islands
• Islands contain basaloid cells surrounded by a thick pink basement membrane (arrow)
• Islands arranged like a "jigsaw puzzle"

  Trichoadenoma
  

• Circular dermal islands
• Islands of epithelium with central flaky keratin (horn cysts)
• Interspersed basaloid cords

  Key differences
  

  (a)

  (b)

  (c)

  1. Adenoid cystic carcinoma: cribriform pattern of duct-like structures
  2. Cylindroma: puzzle-like arrangement, thick/pink basement membrane
  3. Trichoadenoma: numerous horn cysts

  Desmoplastic trichoepithelioma
  

• Cords/tubules and comma shapes in dermis
• Numerous horn cysts (long arrow) in fibrotic stroma
• Tubules of two-layered epithelium (short arrow)
• Calcification often present
• Confined to dermis

  Metastatic breast carcinoma
  

• Cords/tubules and comma shapes in dermis and below
• Tubules of single-layered ("Indian filing" - long arrow) and multi-layered epithelium
• Some cells forming gland-like structures (short arrow)
• Other metastatic carcinomas may look like this - need clinical history; immunohistochemistry may be helpful

  Microcystic adnexal carcinoma
  

• Cords/tubules and comma shapes in dermis
• Comma shapes with duct-like spaces
• Deeply infiltrative (fills dermis)
• Perineural involvement

  Morpheaform basal cell carcinoma
  

• Cords/tubules and comma shapes in dermis
• Tubules of epithelium composed of basaloid cells with hints of peripheral palisading
• New collagen forming around islands (arrow)
• Deeply...