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True or False
The hypodermis is considered a layer of skin
False
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Layers of skin
- Epidermis - 5 layers
- - strat. squamous keratinized
- Dermis - 2 layers
- - connective tissue
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Origin of the epidermis
Ectoderm
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Layers of the epidermis
- (Superficial to Deep)
- 1. Stratum Corneum
- 2. Stratum Lucidum
- 3. Stratum Granulosum
- 4. Stratum Spinosum
- 5. Stratum Germinativum
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Malphigian Layer
- Stratum germinaivum & stratum spinosum
- (2 deepest layers)
Region of keratinocyte proliferation!
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Stratum Germinativum
- Deepest layer (a.k.a. stratum basale)
- Single layer of cuboidal / columnar cells resting on a basal lamina
- Hemidesmosomes attach cells to basal lamina
- Desmosomes attach adjacent cells to each other
Mitosis is seen in this layer
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Stratum Spinosum
2 nd deepest layer
- Polygonal cells w/ spiny projections that form “intercellular bridges”
- Bridges contain desmosomes (contribute to cohesiveness of epidermis)
Mitoses seen in this layer as well
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Stratum Granulosum
Middle layer
- 1. basophilic (keratohyaline) granules - not membrane bound
- 2. membrane-coating granules
- contain glycosaminoglycans & phospholipids- - extruded into extracellular space
- - forms barrier to micro-organisms, foreign bodies, and water
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Stratum Lucidum
2 nd layer from top
Translucent layer of very acidophilic cells (devoid of nuclei / organelles)
- Prominent in thick skin (palms of hands / soles of feet)
- Often not identifiable in thin skin
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Stratum Corneum
Topmost layer
- Flat, keratin-filled cells - devoid of nucleus / organelles
- (a.k.a. horny cells / squams)
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Cells found in the epidermis
- 1. Keratinocytes
- 2. Melanocytes
- 3. Langerhans cells
- 4. Merkel cells
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Keratinocytes
- - Ectodermal origin
- - Predominent cell type in epidermis
- - undergo specialized differentiation
give rise to protective dead cell layer (stratum corneum)
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Langerhans Cells
- - Dendritic antigen presenting cells
- - Mainly in stratum spinosum
- - Can migrate out of epidermis and enter lymphatics
Don't stain well w/ H&E...use gold impregnation techniques
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Melanocytes
- - Neural crest origin
- - Found in stratum germinativum
- - Contain large amounts of Tyrosinase
- - Not connected to adjacent tissue via desmosomes
- - Can be attached to basal lamina via hemidesmosomes
- - Replicate slowly
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Merkel Cells
- Specialized keratinocyte - involved in touchMechanoreceptive cells
Found in thick skin where touch is accute
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Epidermal-melanin units
- One melanocyte associates w/ a fixed number of keratinocytes
- (Epidermal-melanin units vary regionally)
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Tyrosinase
- - Melanocyte enzyme
- - Converts tyrosine into melanin
- - Melanin granules injected into keratinocytes via cytocrine secretion
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Melanin
- Keratinocytes of malphigian layer
- Protect nucleus from UV radiation
- ! melanin seen in slides is predominantly in keratinocytes !
- (melanocytes have relatively fewer numbers of melanin granules)
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Layers of the Dermis
(Top to Bottom)
1. Papillary layer - 2. Reticular layer
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Papillary Layer
- Loose CTFibroblasts / mast cells / macrophages
- Some leukocytes (major part of dermal papillae)
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Reticular Layer
- Dense Irregular CTType I collagen
- Few cells
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Distinction between thick / thin skin is dependent on _________
Epidermis only!
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Thick Skin
- - contains all 5 epidermal layers
- - prominent stratum lucidum
- - thick startum corneum
- - hairless (no sebaceous glands)
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Thin Skin
- - typically without distinct startum lucidum
- - thin stratum corneum
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Hypodermis
- Not part of skin!
- - lies deep to dermis
- - loose CT w/ adipose cells
(a.k.a. superficial fascia / subcutaneous CT)
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Appendages of the skin
- During development, structures move from epidermis into dermis- hair follicles
- - sweat glands
- - sebaceous glands
- - mammary glands
- - nails
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Types of sweat glands
Develop as invaginations of epidermis into underlying CT
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Eccrine sweat glands
- Simple, coiled tubular glands
- Secrete non-viscous fluid (contains catabolites)
- Acini - simple cuboidal (stain lightly)
- Ducts - stratified cuboidal (stain darker)
Myoepithelial cells - squeeze secretions from acini (stain acidic - actin filaments)
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Apocrine sweat glands
Specialized glands (axillary, areolar, anal regions)
- - ducts open into hair follicles
- - secretes viscous, odorless fluid
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The mammary gland is considered a highly modified __________
apocrine sweat gland
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Sebaceous glands
- Typically associated w/ hair follicles
- - secrete sebum (holocrine secretion)
- Cells at the base of gland are germinal
- - as cells fill w/ sebum, nuclei become pyknotic then lost
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Skin as a sensory organ
- 1. Free nerve endings
- 2. Meissner's corpuscles
- 3. Pacinian corpuscles
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Free nerve endings
- Unmyelinated axons cross basal lam. & enter malphigian layer
- Temperature
- Pain
- (some crude touch)
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Meissner's corpuscles
- Encapsulated receptors (dermal papillae of thick skin)
- - also skin of lips / nipples
Discriminative touch
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Pacinian corpuscles
- Encapsulated receptor (dermis / hypoderm. of thick & thin skin)- abundant in skin of fingertips
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