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What are the four layers of skin? What are they composed of?
- Epidermis - stratified keratinised squamous epithelium
- Basement membrane
- Dermis/corium - dense irregular connective tissue
- Subcutis/hypodermis - adipose tissue and losse connective tissue
What make up the adnexa associated with skin?
- Hair follicles (simple or compound)
- Sebaceous glands
- Sweat glands
List some functions of the common integument
- Protective - wear and tear
- Barrier - microbial penetration/impermeable to water
- Sensory perception
- Storage organ
- Synthesis of vitamin D3
- Glandular - sebum and sweat secretions
- Capture of prey - claws, talons, etc
Describe the embryological development of the skin
Primitive epidermis is of ectodermal origin and dermis is of mesodermal origin. Basal cells of the epidermis undergo proliferation, migration and differentiation resulting in cell death. Stratified keratinised squamous epithelium forms a physical and permeability barrier. Melanocytes from neural crest origin migrate to the dermal-epidermal border.
What are the four layers of the epithelium?
- Stratum corneum
- Stratum granulosum
- Stratum spinosum
- Stratum basale
What is the main cell type is the stratum basale?
Basal cells (keratinocytes)
What is the main function of basal cells?
What other cell types are found in the stratum basale? And what are their function?
- Melanocytes - photoprotection/photosensitisation
- Merkel cells - sensory (light and touch)
What other cell type is found in the epidermis? What is its function?
Langerhans cell - immunosurveillance
- Basal cell is the mitotically active cell. Tonofilaments appear in the daughter cells as they start to move outwards towards the surface. These tonofilaments are the first sign of keratinisation. Keratins are also produced by the cell; these differ between basal cells (5 and 14) and spinosum cells (1 and 10).
- Lamellar bodies appear at the border between the spinosum and granulosum layer. They are vesicles containing lipids. The most significant occurrence in the granulosum is the appearance of filaggrin protein. This is formed from keratohyalin granules. At this point the nucleus also breaks down. The role of filaggrin is to pull tonofilaments together.
- In the stratum corneum cells have no nuclei (they are dead). The lamellar bodies have released their lipid which has gone between the layers of squares and glued them together and water proofed them. Loricin and involucrin make the envelope more structurally sound.
What controls desquamation?
A balance of stratum corneum protease inhibitors and proteases. Also skin pH.
What is the main cell type that makes up the dermis?
What is the function of fibroblasts/fibrocytes?
They produce and maintain the extracellular matrix
What other cells are found in the dermis?
Mast cells, plasma cells, macrophages, adipocytes, melanocytes, lymphocytes, neutrophils
What are the different types of fibres found in the dermis?
Collagen, elastic and reticular fibres
Describe the blood supply to the skin
The epidermis is nourished by diffusion from blood vessels found in the dermis. There is a deep, middle and superficial plexus.
Describe how the blood supply to the skin helps with thermoregulation
In the skin there is a type of microcirculation which allows the blood supply to change depending on the needs of a surface. There is a special arrangement between the arteries and veins: the arteries can bypass the superficial plexus by returning via the venous system. This helps retain heat e.g. on a cold day. This is called an AV shunt (where arterial blood is shunted back through the venous system).
Hair completely covers the body of domestic species. Give some examples of some areas that are exceptions
Foot pad, hoof, glans penis, mucocutaneous junctions and teat of some species.
What are some functions of hair
- Mechanical protection
- Sensory perception
What are the two types of hair follicles?
Simple and compound hair follicles
Briefly describe the development of a simple hair follicle and associated gland
- This begins with a localised thickening of epidermis (ectodermal bud) which grows down into the underlying mesenchyme:
- The bud of the epidermis starts to invade downwards into the underlying dermal tissue
- Some of the cells associated with the wall of the follicle start to change. These eventually form sebaceous and sweat glands.
- Hair matrix cells have a close association with connective tissue (dermal papillae) which brings in a blood supply specifically to the matrix cells to keep the multiplying and differentiating.
What are the three main sections of a simple hair follicle?
The bulb, root and shaft
What type of cells are found in the bulb of a hair follicle?
Hair matrix cells
What type of cells form the a) inner b) outer root sheath?
- a) hair matrix cells
- b) outer epidermis
What are the outer, middle and central layers of the hair itself?
- Outer - cuticle
- Middle - cortex
- Inner - medulla
True or false: hair matrix cells are mitotically active?
True - they differentiate to form the hair and internal root sheath
What type of cells are associated with pigmentation?
Melanocytes - can see signs of pigmentation in hair because melanocytes associate with the highly mitotic cells of the hair matrix
Describe the process of a hair cycle
The relationship between the dermal papilla and hair matrix breaks down. The hair hangs stays put but there is no further growth. Then there is reformation of the relationship between the dermal papilla and hair matrix. They form a new hair which grows in the same follicle as the old hair. Over time the new hair pushes the old hair out and replaces it, so the number of hair follicles in an animal does not change.
True or false: keratinisation is continuous in hair follicles?
False - keratinisation is cyclic in hair follicles, continuous in skin.
Describe the development of a compound hair follicle
Development starts off with downward projection of the epidermis to get a simple follicle. Secondary follicles develop around the original (primary follicle) and within 6 months there is development into a compound follicle. Associated with each secondary follicle is usually a sebaceous gland.
What type of hair are whiskers?
Sinus or tactile hair
What is the function sinus/tactile hairs? How are they adapted to suit this?
They are specialised for tactile sense (spatial awareness). Between the layers of the root sheath is a blood filled sinus. The blood amplifies any signals/disturbances of the hair send down to the root.
What are sebaceous glands?
Sebaceous glands are a collection of secretory cells which have a duct leading into the hair follicle.
What product do sebaceous glands secrete? What is the function of this product?
Sebum. Its function is waterproofing, suppleness and spread of sweat
What type of secretion do sebaceous glands use: holocrine, apocrine or eccrine?
Holocrine - the cells are lost during the secretion process
What are the functions of sweat?
- Thermoregulation (some species e.g. horse)
- Scent signalling (some species)
What type of epithelium makes up sweat glands?
Simple cuboidal epithelium
What type of secretion do sweat glands use: holocrine, apocrine or eccrine?
- Apocrine secretion onto the hair follicle - the apex of the cell is lost
- Eccrine secretion directly onto the skin surface via sweat pores - none of the cell is lost
Give examples of localised accumulations of sebaceous/sweat glands in domestic species
- Supracaudal (or tail) gland in the dog - concentration of sebaceous glands on the dorsum of the tail
- The foot pads of dogs is another hairless area where sweat glands are present.