SN3
SHORT NOTE
5M
★ High-yield
Describe the histology of the skin. Differentiate thick from thin skin.
Layers of the Skin
The skin consists of a superficial epidermis of stratified squamous keratinised epithelium and a deeper dermis of connective tissue.
| Layer of epidermis | Features |
|---|---|
| Stratum basale (germinativum) | Single layer of columnar cells; mitotically active; contains melanocytes, Merkel cells |
| Stratum spinosum | Polyhedral cells joined by desmosomes giving a prickly appearance; contains Langerhans cells |
| Stratum granulosum | Keratohyalin granules; the cells begin to die |
| Stratum lucidum | Clear homogeneous layer; present only in thick skin — palms and soles |
| Stratum corneum | Dead anucleate keratinised squames; thickest in thick skin |
- Mnemonic from deep to superficial — "Baby Sam Got Little Chickenpox"; transit time from basale to corneum is about 4 weeks
Cells of the Epidermis
| Cell | Origin | Function |
|---|---|---|
| Keratinocyte | Ectoderm | 90% of cells; produces keratin |
| Melanocyte | Neural crest | Produces melanin and transfers it to keratinocytes in melanosomes |
| Langerhans cell | Bone marrow | Antigen presentation; dendritic, in the stratum spinosum |
| MERKEL cell | Neural crest | Mechanoreceptor for touch; associated with a nerve ending |
Dermis and Appendages
- Papillary layer — loose connective tissue forming dermal papillae, which interdigitate with the epidermal ridges and produce fingerprints
- Reticular layer — dense irregular connective tissue with thick collagen bundles arranged along Langer lines
- Appendages — hair follicles with arrector pili (smooth muscle, sympathetic), sebaceous glands, eccrine and apocrine sweat glands, nails
| Receptor | Location | Modality |
|---|---|---|
| Free nerve endings | Epidermis and dermis | Pain and temperature |
| Meissner corpuscle | Dermal papillae | Light (fine) touch |
| Pacinian corpuscle | Deep dermis and subcutaneous tissue; onion-like lamellae | Pressure and vibration |
| Merkel disc | Stratum basale | Sustained light touch and texture |
Thick and Thin Skin
| Feature | Thick skin | Thin skin |
|---|---|---|
| Sites | Palms and soles only | Everywhere else |
| Stratum lucidum | Present | Absent |
| Stratum corneum | Very thick | Thin |
| Hair follicles and sebaceous glands | Absent | Present |
| Sweat glands | Numerous | Fewer |
Applied Aspects
- Langer lines — incisions made along them heal with a fine scar; across them the wound gapes and scars badly
- Burns are assessed by depth: superficial burns spare the appendages and heal spontaneously; full-thickness burns destroy them and need grafting
- Psoriasis — greatly increased epidermal turnover with retention of nuclei in the corneum (parakeratosis) and loss of the granular layer
All questions in General Histology
- Q1Classify epithelium with sites and functions. Describe its surface specialisations, cell junctions and basement membrane. ★
- Q2Describe the cells, fibres and ground substance of connective tissue. Classify connective tissue with examples. ★
- Q3Compare the three types of muscle histologically. Describe the structure of skeletal muscle and the sarcomere. ★
- Q4Describe the structure of a neurone and classify neurones. Describe the neuroglia and add a note on nerve degeneration and regeneration. ★
- Q5Describe the histology of cartilage and bone. Add a note on the structure of blood vessels. ★
- SN1Classify glands with examples. Describe the modes and nature of secretion. ★
- SN2Describe the histology of the lymph node, spleen and thymus. ★
- SN3Describe the histology of the skin. Differentiate thick from thin skin. ★
- SN4Describe the preparation of a histological section and the common stains used. ★
- SN5Classify cell junctions and describe their structure and functions. ★
- SN6Describe the ultrastructure of a typical cell and the cytoskeleton.
- SN7Describe the histology of blood with its formed elements. ★