Unit 18 — Skin & Appendages · Question Bank

The epidermis is the ectodermal, keratinised, stratified squamous epithelium that forms the outermost wear–and–tear surface of the body. Its design is essentially an assembly line: a single basal layer of mitotically active cells gives rise to daughter keratinocytes that migrate slowly outward over roughly four weeks, switch on a precise sequence of structural proteins, and are finally shed as dead keratin–filled plates. Five histologically distinct strata mark stages of that journey, and in thick skin all five can be identified, while in thin skin the stratum lucidum is absent or unrecognisable.

Stratum basale (germinativum)

The deepest stratum is a single row of cuboidal to low–columnar keratinocytes resting on the basement membrane and anchored to it by hemidesmosomes. This is the only mitotically active layer; its p63–positive stem cells supply both renewal and the upward–migrating differentiation pool. It expresses the K5/K14 keratin pair and houses two non–keratinocyte residents, melanocytes (neural crest, pigment) and Merkel cells (touch).

Stratum spinosum

Several layers of polyhedral keratinocytes lie above the basale. They are joined by abundant desmosomes anchored to bundles of K1/K10 tonofilaments, and during routine fixation the cells shrink but stay tethered at every desmosome, giving the layer its characteristic “prickly” or spinous appearance. Langerhans cells, the dendritic antigen–presenting cells of the epidermis, lurk between the keratinocytes here.

Stratum granulosum

Three to five layers of flattened keratinocytes packed with two organelles. Basophilic keratohyalin granules contain profilaggrin (the precursor of filaggrin, which aggregates keratin into a dense matrix above) plus loricrin and involucrin for the cornified envelope. Lamellar bodies exocytose glycolipids and sterols into the intercellular space; that lipid is the actual waterproof barrier of the skin. The cells then lose their nuclei and most organelles as they ascend.

Stratum lucidum

A thin, translucent, eosinophilic band of dead flattened cells filled with eleidin (a transformation product of keratohyalin). It is present only in thick skin of the palms and soles, sitting between the granulosum and a very thick corneum, and acts as an additional cushion against mechanical wear.

Stratum corneum

The outermost layer is many sheets of dead, flat, anucleate corneocytes whose K1/K10 keratin has been bundled by filaggrin and whose cornified envelopes are cross–linked under the plasma membrane by transglutaminases. Lipid mortar from the lamellar bodies fills the spaces between these “bricks”. Serine proteases chew the surface desmosomes loose so the outermost squames flake off (desquamation) at a rate that matches new keratinocyte production below.

Clinical link

Pemphigus vulgaris attacks desmoglein in the spinosum, producing intra–epidermal blisters; bullous pemphigoid attacks hemidesmosomes at the BM, producing sub–epidermal blisters; psoriasis dysregulates basal turnover and gives a parakeratotic corneum. The order of the strata is therefore not academic — it predicts the level at which a given disease cleaves the skin.

Although keratinocytes form the bulk of the epidermis and supply its mechanical barrier, three minority resident populations carry out the rest of the integumentary work: pigmentation, immune surveillance and fine–touch sensation. Each has a different embryological birthplace, a different home in the epidermis, and a different histological signature you should be able to recognise on a stained section.

Melanocyte

Melanocytes are dendritic cells of neural–crest origin that migrate during embryonic life to lodge in the stratum basale, sitting on the basement membrane between basal keratinocytes. On H&E they appear as small rounded cells with a clear perinuclear halo because their long dendritic processes do not stain; DOPA reaction, S100, HMB–45 and Melan–A highlight them well. Inside the melanocyte, tyrosinase oxidises tyrosine through DOPA into melanin, which is packaged in lysosome–related organelles called melanosomes. Mature melanosomes are pushed out along the dendrites and phagocytosed by surrounding keratinocytes — a unique process called cytocrine secretion — where the pigment piles up on the sun–facing side of the nucleus and absorbs UV photons before they damage DNA. Skin–colour differences across human populations reflect the size and distribution of melanosomes rather than the number of melanocytes.

Langerhans cell

Langerhans cells are bone–marrow–derived dendritic cells located mainly in the stratum spinosum, where they form a network reaching almost up to the granulosum. They sample antigens that breach the corneum, migrate down through the dermis to draining lymph nodes, and present antigen on MHC class II to naive T cells. On electron microscopy they carry the unique tennis–racquet–shaped Birbeck granule. They are the front line in contact hypersensitivity (nickel, poison ivy), and are also the cell of origin of Langerhans cell histiocytosis.

Merkel (tactile) cell

Merkel cells sit in the stratum basale, often clustered into “touch domes” on fingertips, lips and the bases of hair follicles. They form synapse–like contacts with a flattened terminal of a sensory nerve (the Merkel disc); together they act as a slowly–adapting mechanoreceptor exquisitely tuned to steady pressure and fine spatial detail — the cell you depend on to read Braille or recognise the shape of a coin in your pocket. Their neuroendocrine cousin, Merkel cell carcinoma, is a rare but aggressive skin malignancy linked to Merkel cell polyomavirus.

Clinical link

Vitiligo (autoimmune destruction of melanocytes), albinism (tyrosinase deficiency), melanoma (malignancy of melanocytes, prognosis tied to Breslow depth), contact dermatitis (Langerhans–driven hypersensitivity) and Merkel cell carcinoma between them illustrate why each of these minority cells matters far beyond its small numbers.

If the epidermis is the wallpaper of the body, the dermis is the wall. It is the thick, vascular, nerve–rich connective–tissue layer of mesodermal origin that gives skin its tensile strength, holds the appendages, feeds the avascular epidermis by diffusion across the basement membrane and houses every encapsulated sensory receptor of the body wall. Two sublayers can be told apart at low power by the size of their collagen bundles.

Papillary layer

The superficial papillary dermis is a thin sheet of loose connective tissue dominated by fine type–III collagen and a delicate elastic meshwork. It throws up finger–like dermal papillae that interdigitate with epidermal rete ridges, a clever way to lock the two layers together mechanically and to maximise the surface area for diffusion of nutrients up into the avascular epidermis. Each papilla contains a capillary loop that feeds the epidermis above and, on touch–sensitive sites such as fingertips, an encapsulated Meissner corpuscle.

Reticular layer

The deeper reticular dermis is a much thicker layer of dense irregular connective tissue dominated by interwoven bundles of type–I collagen plus elastic fibres. The dominant fibre directions are Langer's lines; surgeons cut along them because incisions parallel to Langer's lines gape less and scar more cosmetically. This layer carries the larger vessels, the bases of hair follicles and sweat glands, and the deep mechanoreceptors.

Cutaneous receptors

Free nerve endings (unmyelinated Aδ and C fibres) ascend into the epidermis itself and mediate pain, temperature and itch. Meissner corpuscles in the dermal papillae are rapidly–adapting touch receptors that look like a stack of pancakes — flattened Schwann–like cells stacked around a coiled axon — and detect light, low–frequency touch and texture. Pacinian (lamellated) corpuscles in the deep reticular dermis and hypodermis are large, ovoid, onion–like receptors whose concentric capsular lamellae filter out everything except deep pressure and high–frequency vibration. Ruffini endings, also in the reticular layer, are slowly–adapting receptors for sustained stretch and skin distortion, and Krause end–bulbs at mucocutaneous junctions are classically taught as cold receptors.

Vessels, nerves & appendages

The dermis also carries a superficial and a deep vascular plexus connected by perforators, autonomic nerves to vessels and sweat glands, and the basal portions of the skin appendages — hair follicles, sebaceous glands, sweat glands, arrector pili muscles — all of which are epidermal in origin but live mostly in the dermis.

Skin appendages are epidermal invaginations that have grown down into the dermis during development and have specialised into glands or hair–making organs. All of them are lined by epithelium continuous with the surface epidermis, which is why they double as reservoirs of stem cells able to reseed the epidermis after a partial–thickness burn. Four appendage types matter for histology.

Hair follicle

A hair follicle is a cylindrical down–growth of epidermis ending in a bulb that sits on a small vascular knob of dermis called the dermal papilla. From outside in, the follicle wall is built of a connective–tissue sheath, a glassy basement membrane, an external root sheath (which contains the bulge stem–cell niche where the arrector pili muscle attaches), and a three–layered internal root sheath (Henle's, Huxley's and the IRS cuticle). The hair shaft itself has a cuticle, a hard–keratin cortex pigmented by melanocytes in the bulb, and a soft vacuolated medulla. Hairs grow in a three–stage cycle: anagen (growth), catagen (regression) and telogen (rest). The arrector pili, a slip of smooth muscle running from the bulge to the papillary dermis, erects the hair on sympathetic stimulation and helps express sebum.

Sebaceous gland

Sebaceous glands are typically branched acinar glands opening into the upper part of a hair follicle (forming the pilosebaceous unit). They secrete by the holocrine mechanism: the entire central cell fills up with foamy, lipid–rich cytoplasm, then disintegrates and becomes the secretion (sebum). A continuous peripheral basal layer replaces the lost cells. Sebum lubricates the hair and skin and has mild antimicrobial activity. These glands are absent from palms and soles and most abundant on the scalp, face and upper trunk — the “acne distribution”.

Eccrine sweat gland

Eccrine glands are simple coiled tubular merocrine glands distributed almost all over the body and most densely on palms, soles and forehead. The deep secretory coil contains pale clear cells (mitochondria–rich, watery secretion) and darker mucoid cells, surrounded by contractile myoepithelial cells. The duct is two–layered stratified cuboidal epithelium that reabsorbs Na⁺ and Cl⁻ as the primary secretion travels up, so the final sweat reaching the skin surface is hypotonic. Their job is thermoregulation under sympathetic cholinergic drive; cystic fibrosis (defective CFTR) leaves sweat salty — the basis of the sweat–chloride test.

Apocrine sweat gland

Apocrine glands are found in axillae, areolae and the anogenital region. They are larger than eccrine glands, have very wide lumina, open into the upper hair follicle rather than directly onto the surface, and become active at puberty. Their viscid, lipid–rich secretion is odourless until skin bacteria act on it, giving rise to body odour. Despite the name, in humans much of the actual secretion is in fact merocrine; the term “apocrine” reflects developmental and classical observations rather than the dominant mode in life.

Clinical link

Acne (blocked pilosebaceous units + Cutibacterium acnes), hidradenitis suppurativa (chronic apocrine inflammation), cystic fibrosis (salty sweat) and alopecia areata (autoimmune attack on the hair bulb) each map onto one of these appendages.

Skin comes in two functional flavours that differ in epidermal architecture, appendage content and surface relief, and once you know that thick skin exists because palms and soles must take heavy mechanical wear and produce a lot of cooling sweat, every other difference falls out logically. The two terms refer specifically to the epidermis — the dermis underneath can be thick or thin independently.

Epidermal architecture

Thick skin has all five strata: basale, spinosum, granulosum, lucidum and a very heavy corneum. The lucidum — a translucent eosinophilic band of dead flattened cells filled with eleidin — is the histological tell–tale of thick skin and is essentially absent from thin skin. The corneum in thick skin can be ten or more times the thickness of the rest of the epidermis put together, while in thin skin it is a modest layer sitting directly on the granulosum.

Location

Thick skin covers only the palms of the hands and the soles of the feet (the glabrous, weight–bearing or grip–bearing surfaces). Thin skin covers everywhere else, including scalp, face, trunk and limbs.

Appendages

Thick skin has no hair follicles and no sebaceous glands — hair would interfere with grip and oil would make the surface slippery — but it is extremely rich in eccrine sweat glands for thermoregulation and friction control. Thin skin carries hair follicles, sebaceous glands, eccrine and (regionally) apocrine sweat glands, and arrector pili muscles.

Surface ridges & sensory receptors

Thick skin shows prominent epidermal ridges and matching dermal papillae that produce the friction ridges responsible for fingerprints and footprints; these enhance grip and lock the epidermis to the dermis under shearing forces. The papillary dermis of thick skin is densely populated with Meissner corpuscles for fine touch, and the deep dermis contains many Pacinian corpuscles for pressure and vibration. Thin skin has less pronounced ridging and fewer (though still functional) receptors per unit area.

Clinical link

Persistent mechanical stress on thin skin can convert it to a thicker, lichenified or callous–like form; conversely, scars and burns destroy the appendage–rich layer and may leave reseeding to surviving deep follicles. Forensic identification by fingerprints relies on the friction ridges of thick skin remaining patterned for life.