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What percent of body weight does the skin make up?
What are three general functions of the integuement?
- Protective, waterproof barrier
- Regulates body temperature
- Sensory receptors for temperature, touch, and pain
What are the two layers of the skin?
- Epidermis: outerlayer, stratified squamous keratinized epithelium from ectoderm. Includes skin appendages ie. sweat and sebaceous glands, hair, nails
- Dermis: thicker, underlying layer of CT from mesoderm. Within CT are muscle and nervous tissue
What is the hypodermis?
CT below dermis, mainly adipose tissue. Connects skin to other tissues and organs.
What are the four cell types that make up the epidermis?
- Keratinocytes (major cell type)
- Langerhans cells
- Merkel cells
What are the four layers of the epidermis?
- Stratum basale: most basal layer, undifferentiated, mitotic (after mitosis, one cell remains while the other migrates up to differentiate)
- Stratum spinosum: intermediate filaments mechanically join cells, cytokeratin, desmosomes
- Stratum Granulosum: keratohyalin granules (eventually come together to form keratin in the next layer). Lipid in secretory vesicles released into ECM to form the waterproof barrier
- Stratum Corneum: layers of dead cells with protective function due to highly insoluble protein keratin. Dead cells are sloughed and replaces, generally every 3-4 weeks
What layer of the epidermis is the most differentiated and which is the least differentiated?
- most- stratum corneum
- least- stratum basale
How is skin classified?
- According to the thickness of epidermis (amount of keratin)
- There is thin skin and thick skin.
What is thick skin, and where is it found?
- Mainly thicker layer of Stratum corneum, but also has no hair.
- Found on the palms of hands, soles of feet, etc
Describe the structure and function of melanocytes.
They are found in the neural crest. They synthesize the pigment, melanin, which protects against UV radiation by absorbing UV rays. Prolonged exposure to sun increases the synthesis of melanin. Difference in skin pigmentation is due to the amount of melanin synthesized, not the number of melanocytes.
How are keratinocytes involved in the spreading of melanin?
Melanocytes have cytoplasmic processes containing melanin which extend between keratinocytes. The keratinocytes phagocytose these processes, effectively spreading the melanin.
What are Langerhans cells?
Macrophages in the basal layers of keratinocytes. They have an immune function- phagocytize foreign material invading the skin.
What are Merkel cells?
These cells differentiate from epidermal epithelial cells in the basal layers of keratinocytes. They have sensory receptors that associate with free nerve endings in the CT allowing one to feel temperature and pain.
What are the two layers making up the dermis?
Papillary layer and Reticular layer
Describe the structure of the papillary layer.
- loose CT
- Allows movement of epidermis
- Contains capillaries which help to regulate body temperature
- Meissner's corpuscles- touch receptors which associate with nerve ending for temp and pain
Describe the structure of the reticular layer.
- dense irregular CT
- supports epithelium
- Contains blood vessels, lymphatics, nerves, Pacinian corpuscles (pressure receptors)
- Also contains epidermally derived sweat glands, hair follicles, and sebaceous glands
What are the three types of skin appendages, and how do they arise?
- Eccrine Sweat Glands
- Sebaceous Glands
- Arise as epidermal invaginations into dermis
Describe the structure of the eccrine sweat glands.
- Found throughout body
- Simple, coiled, tubular glands
- Secretory cells release product by merocrine secretion into a duct
What is the function of the eccrine sweat glands?
Regulate body temperature- respond to heat and nervous strain by cooling effect of secretions (electrolytes ie Na, K, Cl) and wastes (ie. ammonia, urea, lactic acid)
Describe the structure of the sebaceous glands.
- Found only in thin skin
- Associated with hair follicles
- Simple, branched, acinar glands
- clusters of acini release lipid and cellular debris by holocrine secretion into a short single suct continuous with the canal at the hair follicle
What is holocrine secretion?
Lipid fills the cell and causes lysis of the cell organelles. The cell dies. The lipid as well as cellular debris are secreted into a duct.
What is merocrine secretion?
secretory vesicles merge with PM releasing contents into ducts
What is the function of sebaceous glands?
- Keep skin and hair soft and water-repellent by the production of a lipid, sebum
- Secretory activity increased at puberty in response to male hormones, androgens
Describe the structure of hair.
- Epidermis invaginates into the dermis to form the hair follicle.
- Keratin-containing cells of the S. corneum aren't shed, but rather form the hair shaft which is embedded in the other layers.
What is the function of hair?
Tactile sensation- any stimuli that deforms hair is translated down the shaft to sensory nerves surrounding the hair follicle
What are arrector pili muscles?
Smooth muscle cells attached to hair follicle at an oblique angle. They contract in response to cold and fear, causing elevation of the hair shaft and skin around it forming 'goose bumps'. They are stimulated as a part of sympathetic response.
Name two differences between thick skin and thin skin.
- Thick skin has several layers of stratum corneum, while thin skin has one.
- Thin skin has hair, while thick skin does not.