Flashcards in Histology 3: Skin and Associated Structures Deck (36):
What are the functions of the skin?
1. Protection against injury and bacterial invasion
2. Provides a cover for the underlying soft tissue
3. Sense organ (for touch, temperature and pain)
4. UV protection, carried out by melanocytes (skin pigment cells)
5. Thermoregulation via sweat glad evaporation and blood radiation
6. Excretion of water and heat via sweat glands
7. Absorption of solar UV radiation for Vitamin D synthesis
What are the three main areas of the skin?
What is the dermis?
A deeper connective tissue layer than the epidermis, composed of dense, irregular collagen that interdigitates (dermal papillae) with the epidermis.
What is the hypodermis?
Technically not part of the skin, it is superficial fascia that covers the entire body, immediately deep to the skin, and contains a loose connective tissue with large, varying amounts of fat
What is the name given to 'normal' skin?
Stratified, squamous, keratinised epithelium
What are the four types of cells that comprise the epidermis?
1. Keratinocytes (skin cells)
3. Langerhans cells
4. Merkel Cells
Skin can be classified as thick or thin depending on the thickness of the epidermis. What are some features of each?
Thick skin (covers the palms and soles)
-No hair follicles
-No arrector pili muscles
Thin Skin (covers most of the remainder of the body)
- Has a thin stratum corneum
- No definite stratum lucidum
- Has hair follicles, arrector pili muscles, sebaceous and sweat glands
What are the five epidermal layers?
1. Stratum Corneum
2. Stratum Lucidum (only in thick skin)
3. Stratum Granulosum
4. Stratum Spinosum
5. Stratum Basale
What are the features of Stratum Corneum?
1. The most superficial layer of skin
2. Composed of layers of flattened, keratinised cells
3. Cells lack nuclei and organelles but have numerous keratin filaments embedded in an amorphous matrix (non living tough shells) and are eventually shed
What are the features of Stratum Lucidum?
1. A clear later of flattened cells
2. Lack organelles and nuclei, but contain keratin filaments orientated parallel to the skin surface and eleiden (a transformation product of keratohyalin).
What are the features of Stratum Granulosum?
1. Consists of 3-5 layers of flattened keratinocytes
2. the most superficial (thin) layer of the epidermis
3. Cells possess many nuclei
4. Enzymes (release by lysosomes) digest the organelles and the nucleus of keratinocytes as they move through the layer
What are the features of Stratum Spinosum?
1. The thickest layer of the epidermis
2. Composed of more flattened cells containing bundles of tonofilaments, which attach adjacent cells to each other by desmosomes.
3. (ABOVE) causes cells to have a 'prickle-cell' appearance
What are the features of Stratum Basale?
1. The deepest layer of the epidermis
2. Keratinocytes are a single layer of mitotically active, cuboidal cells
3. When new cells are formed, they are pushed upwards to become the stratum spinosum, where the cells begin to flatten and the filaments accumulate
4. Process from basal layer to surface takes 20-30 days
What are the other three cells of the dermis?
2. Langerhan Cells
3. Merkel Cells
What are melanocytes?
1. Melanocytes sit in the stratum basale and produce melanin (a brown pigment giving skin colour). They put melanin inside the keratinocytes, by a melanocyte dendritic process whereby melanosomes transfer the melanin into the cytoplasm of keratinocytes.
2. Melanocytes absorb UV light and protect the skin, as the sun damages the collagen in the dermis
What are Langerhan cells?
Part of the immune system, and scattered throughout the epidermis (but found mainly in the SS) they are antigen-presenting cells, which 'creep' around the epidermal cells looking for antigenic invaders (bacteria etc.). They then migrate to nearby lymph nodes where they present epitopes of processed foreign antigens to T cell lymphocytes
What are Merkel cells?
Found among the keratinocytes of the SB and especially numerous in the fingertips, they function as mechanoreceptors and pressure receptors
What is the thickness of the dermis?
It varies from 0.6mm (in the eyelids) to 3mm (palms and soles)
What are some of the more important features of the papillary level of the dermis?
-Interdigitates with the epidermis, forming dermal ridges (papillae)
-It has loose connective tissue with thin type 3 collagen fibres (reticular fibres) and elastic fibres arranged in loose networks.
- Contains cells (fibroblasts, macrophages and mast cells as well as many capillary loops) which extend to the epidermis/dermis interface to regulate body temperature and nourish the cells of the avascular epidermis
What are some of the more important features of the reticular level of the dermis?
-Has dense, irregular collagen which are packed into large bundles lying mostly parallel to the skin surface
-It has networks of elastic fibres amongst the collagen fibres
Why are the collagen fibres in the dermis irregular?
Because the skin needs to move in many different directions
What is the ground substance in the reticular layer of the dermis?
A gel like liquid that fills the spaces, which contains many chemical (like glycoamino glycans) that are used so that, if the skin is grazed, the chemicals can bind to water and use it up so that the ground substance can become more viscous and slow the bacteria down so that it buys time for the WBCs to arrive
Are specialised cells found more in the reticular or papillary level?
What are the three main receptors in the dermis?
1. Meissner Corpuscles (Papillary Layer)
2. Pacinian Corpuscle (Reticular Layer)
3. Ruffini Corpuscle
What are Meissner Corpuscles?
Meissner corpuscles (shape like witchety grubs) are mechanoreceptors specialised to respond to fine, discriminative touch. They are found in the papillae of the dermis, and are common in areas sensitive to tactile stimulation (lips, external genitalia, nipples)
What is a Pacinian Corpuscle?
An encapsulated receptor that responds to pressure and vibrations- It is concerned with deep, meaningful touch
What is a Ruffini Corpuscle?
A receptor that responds to tension in the collagen
What do the glands of the skin include?
1. Eccrine and apocrine sweat glands
2. Sebaceous glands
3. Mammary glands
What are Eccrine sweat glands, and what are some of their features?
1. Eccrine sweat glands (normal sweat glands) are responsible for regulating temperature in production of fluid which evaporates on the body surface, thereby cooling that surface.
2. Around 0.4mm in diameter
3. Coiled, tubular glands are locate deep in the dermis or in the underlying hypodermis
4. Surrounded by myoepithelial cells on the basal lamina which produce contraction that help discharge secretion into the duct by squeezing them
5. Have small lumens in the secretors unit which make the sweat and the ducts which move them to the surface, both of which have an irregular stratified cuboidal appearance.
What are Apocrine sweat glands, and what are some of their features?
1. Specialised sweat glands found in certain locations (the axilla, the eyelid, the nipple areola and the anal region)
2. Larger than eccrine gland (3mm)
3. Secretion influenced by hormones
4. Restricted to the axillae and perineum and produce a more protein-rich secretion with pheromonal properties.
5. Lumen of secretory portion is much larger than in eccrine glands, and their ducts open into hair follicles rather than to the epidermal surface.
What are hairs?
Hard, keratinised structures that project from the epidermal surface of the skin.
What are hair follicles?
The organs from which hairs develop, arise from invaginations of the epidermis invading the dermis, hypodermis, or both.
They are surrounded by dense fibrous CT of the dermis.
The expanded part of the hair follicle, the hair root, is indented to conform to the shape of the dermal papilla pushing into it. These 2 structures together are the hair bulb.
Scattered among the matrix cells nearest the dermal papilla are melanocytes, which give hair a colour based on the amount of melanin present.
What are the Arrector Pili muscles?
Strands of smooth muscle running from the dermal sheath and insert into the hair follicles. Under sympathetic nervous stimulation, they contract and pull the hair erect when the body is cold or one is suddenly frightened.
What are nails?
Outgrowths of the epidermis (modification of the st. corneum) over the tips of the distal digits
What is the nail matrix?
A region of the nail root, located beneath the proximal nail fold. Cells of the nail matrix proliferate and become keratinized