Skin Anatomy - Layers and Appendages
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Direct answer
Skin anatomy comprises three layers, the epidermis, dermis and subcutaneous fat, together with appendages that develop from the epidermis: hair follicles, sebaceous glands, sweat glands and nails. The epidermis is a keratinising stratified squamous epithelium, and its basal layer holds stem cells and melanocytes. The dermis supplies collagen, elastic fibres and the blood vessels that the avascular epidermis lacks.
What you must remember
- Epidermal strata: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum and stratum basale, listed from the surface inwards.
- Stratum lucidum: a clear layer of eleidin present only in the thick skin of palms and soles.
- Basal layer: contains stem cells and melanocytes; each melanocyte with about 36 neighbouring keratinocytes forms the epidermal melanin unit.
- Langerhans cells: dendritic antigen-presenting cells in the spinous layer, identified by CD1a and Birbeck granules.
- Merkel cells: touch receptors in the basal layer, and the cell of origin of Merkel cell carcinoma.
- Dermoepidermal junction: hemidesmosomes anchor basal cells through BP180 and BP230, while laminin-332 and type VII collagen anchoring fibrils bind the basement membrane to the dermis.
- Dermis: a papillary layer of loose collagen beneath the epidermis, and a deeper reticular layer of dense collagen.
- Appendages: apocrine glands in the axillae and anogenital skin, eccrine glands spread over most of the body for thermoregulation, and sebaceous glands that secrete sebum by holocrine secretion.
Tracing a keratinocyte from the basement membrane to the surface
A keratinocyte is born in the basal layer and takes roughly four weeks to reach the surface in healthy skin. Each stage has a characteristic structure, and histology questions often ask you to name the layer where a change occurs.
- Basal birth: stem cells in the stratum basale divide, and daughter cells leave the basal layer to begin differentiating.
- Spinous assembly: cells in the stratum spinosum form numerous desmosomes, which give the prickly appearance seen on histology, and Langerhans cells sit here.
- Granular stage: keratohyalin granules bind keratin filaments, lamellar granules begin releasing lipids, and the cells flatten as their nuclei break down.
- Cornification: anucleate corneocytes in the stratum corneum sit in lipid lamellae, forming the epidermal barrier.
- Desquamation: corneocytes are shed as their corneodesmosomes break down, so the epidermis renews itself continuously.
Disease that disrupts adhesion between spinous cells produces acantholysis, whereas faulty cornification produces hyperkeratosis or parakeratosis. Naming the layer where the defect lies is therefore the first step in reading a skin biopsy.
Where anatomy questions catch candidates
Anatomy questions reward precise layer names and the right cell at each level.
- Trap: placing melanocytes in the spinous layer Correct: melanocytes sit in the basal layer, while Langerhans cells occupy the spinous layer.
- Trap: treating Merkel cells as pigment or immune cells Correct: Merkel cells are slowly adapting touch receptors in the basal layer, and Merkel cell carcinoma arises from them.
- Trap: assuming every site has a stratum lucidum Correct: the layer is present only in thick palmar and plantar skin, not on the trunk or face.
- Trap: linking palmar sweating to apocrine glands Correct: palmar sweating is eccrine, whereas apocrine glands lie in the axillae and anogenital skin and become active at puberty.
Frequently asked questions
Which cells sit in the stratum basale of the epidermis?
The stratum basale contains stem cells that renew the epidermis and melanocytes that supply melanin to neighbouring keratinocytes. Merkel cells, the touch receptors of the skin, are also found in this basal layer.
Which epidermal layer is present only in the palms and soles?
The stratum lucidum is a thin, clear layer of eleidin found only in thick skin of the palms and soles. Its absence from a biopsy taken from the trunk or face is therefore normal and does not indicate disease.
What anchors the epidermis to the dermis?
The dermoepidermal junction anchors the epidermis to the dermis through hemidesmosomes, which contain BP180 and BP230, along with laminin-332 and type VII collagen anchoring fibrils. Autoantibodies against BP180 cause bullous pemphigoid, while inherited defects in laminin-332 or BP180 cause junctional epidermolysis bullosa and defects in type VII collagen cause dystrophic epidermolysis bullosa.
Which sweat glands are found in the axillae and anogenital region?
Apocrine sweat glands are concentrated in the axillae, anogenital region and areolae, and they become active at puberty. Their secretion is initially odourless, and bacterial breakdown of it produces body odour. Eccrine glands, by contrast, cover most of the body and mainly serve thermoregulation.
What is the function of Merkel cells in the skin?
Merkel cells are specialised mechanoreceptors in the basal layer of the epidermis, forming contacts with slowly adapting type I nerve endings. They respond to light touch and sustained pressure. Merkel cell carcinoma, an aggressive neuroendocrine skin tumour, arises from these cells.