Skin (Anatomy)

Question 1

Describe the histological features of skin

Answer 1

Three layers: epidermis, dermis, and hypodermis (not true skin)

Epidermis

• Keratinised stratified squamous epithelium.
• Avascular
• Contains stem cells, keratinocytes, melanocytes, tactile cells, and dendritic cells.
• Stratum corneum consists of up to 30 layers of dead, keratinised cells that form a durable surface layer resistant to abrasion, penetration, and water loss.
• Stratum lucidum is densely packed keratinocytes containing the protein eleidin. The cells have no nuclei or organelles.
• Stratum granulosum consists of 3-5 layers of flat keratinocytes which contain keratohyalin granules.
• Stratum spinosum consists of several layers of keratinocytes. Thickest stratum in most skin. Cells appear flatter and flatter the higher you look in this layer.
• Stratum basale consists mainly of a single layer cuboidal to low columnar stem cells and keratinocytes resting on the basement membrane.

Dermis

• Dense irregular connective tissue abundant in collagen with less elastin and reticular fibres.
• Nuclei seen within are fibroblasts (make collagen).
• Vascular with rich supply of nerve fibres, sensory receptors, and lymphatic vessels.
• Epidermis and dermis separated by dermal papillae and dermal ridges.
• Papillary layer of dermis is a thin zone of areolar tissue in and near the dermal papillae.
• Reticular layer of dermis is deeper and thicker. The collagen forms thicker bundles here. Attached to underlying hypodermis.

Hypodermis

• Also known as subcutaneous tissue and not true skin.
• Composed of areolar and adipose tissue.

Question 2

Describe the differences between thick and thin skin

Answer 2

• Thick skin is found on the palms of hands and soles of feet. Thin skin is found everywhere else.
• Thick skin has sweat glands but no hair follicles or sebaceous glands. Thin skin has sweat glands, hair follicles, and sebaceous glands.
• Thick skin has a very thick epidermis due to a thick stratum corneum. Thin skin has a thin epidermis due to a thin stratum corneum.
• The stratum granulosum is thicker in thick skin than in thin skin.
• The stratum lucidum is only present in thick skin.

Question 3

Describe the relationship between epidermis, dermis and underlying structures

Answer 3

• The epidermis is avascular and so depends on the diffusion of nutrients from the underlying connective tissue in the dermis.
• The dermal papillae and dermal ridges prevent slippage of the epidermis across the dermis.

Question 4

Describe and explain the importance of cutaneous sensory receptors

Answer 4

Unencapsulated receptors

• Merkel cells are each associated with expanded nerve endings and function as tonic receptors for sustained light touch.
• Free nerve endings are in the papillary dermis and extend into lower epidermal layers. They respond to extreme temperatures, pain, and itching. Also function as tactile receptors.
• Root hair plexuses form a web of sensory fibres surrounding the base of hair follicles in the dermis that detects movements of the hairs.

Encapsulated receptors

• Meissner (tactile) corpuscles initiate impulses when light touch or low-frequency stimuli temporarily deform their shape. Numerous in fingertips, palms, and soles.
• Pacinian (lamellar) corpuscles are specialised for sensing coarse touch, pressure, and vibrations, with distortion of the capsule amplifying a mechanical stimulus to the axonal core where an impulse is initiated. Found in the dermis and hypodermis.
• Krause end bulbs are found primarily in the skin of male and female genitalia where they sense low frequency vibrations.
• Ruffini corpuscles are stimulated by stretch (tension) or twisting in the skin.

Question 5

Describe skin appendages and glands

Answer 5

Hair follicles

• Extend from the epidermis to dermis.
• Associated with sebaceous glands.
• Each hair has a piloerector muscle (bundle of smooth muscle) which makes the hair stand on end when contracted.

Sweat glands

• Apocrine sweat glands are present in axillary and anode it’s regions. Their ducts lead into nearby hair follicles. They are scent glands that respond to stress and sexual stimulation. Their function is to secrete sex pheromones.
• Merocrine (eccrine) sweat glands are distributed over the entire body but are especially abundant on the palms, soles, and forehead. Primary function is to cool the body. The sympathetic nervous system causes myoepithelial cells at the base of the gland to contract and squeeze perspiration up the duct.

Sebaceous glands

• Produce sebum.
• Flask-shaped with short ducts that open into a hair follicle.
• Holocrine glands.
• Sebum keeps the skin and hair from becoming dry, brittle, and cracked.