True or False The hypodermis is considered a layer of skin False Layers of skin Epidermis - 5 layers - strat. squamous keratinized Dermis - 2 layers - connective tissue Origin of the epidermis Ectoderm Layers of the epidermis (Superficial to Deep) 1. Stratum Corneum 2. Stratum Lucidum 3. Stratum Granulosum 4. Stratum Spinosum 5. Stratum Germinativum Malphigian Layer Stratum germinaivum & stratum spinosum (2 deepest layers) Region of keratinocyte proliferation! 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 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 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 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 Stratum Corneum Topmost layer Flat, keratin-filled cells - devoid of nucleus / organelles (a.k.a. horny cells / squams) Cells found in the epidermis 1. Keratinocytes 2. Melanocytes 3. Langerhans cells 4. Merkel cells Keratinocytes - Ectodermal origin - Predominent cell type in epidermis - undergo specialized differentiation give rise to protective dead cell layer (stratum corneum) 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 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 Merkel Cells Specialized keratinocyte - involved in touch Mechanoreceptive cells Found in thick skin where touch is accute Epidermal-melanin units One melanocyte associates w/ a fixed number of keratinocytes (Epidermal-melanin units vary regionally) Tyrosinase - Melanocyte enzyme - Converts tyrosine into melanin - Melanin granules injected into keratinocytes via cytocrine secretion 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) Layers of the Dermis (Top to Bottom) 1. Papillary layer 2. Reticular layer Papillary Layer Loose CT Fibroblasts / mast cells / macrophages Some leukocytes (major part of dermal papillae) Reticular Layer Dense Irregular CT Type I collagen Few cells Distinction between thick / thin skin is dependent on _________ Epidermis only! Thick Skin - contains all 5 epidermal layers - prominent stratum lucidum - thick startum corneum - hairless (no sebaceous glands) Thin Skin - typically without distinct startum lucidum - thin stratum corneum Hypodermis Not part of skin! - lies deep to dermis - loose CT w/ adipose cells (a.k.a. superficial fascia / subcutaneous CT) Appendages of the skin During development, structures move from epidermis into dermis - hair follicles - sweat glands - sebaceous glands - mammary glands - nails Types of sweat glands 1. Eccrine 2. Apocrine Develop as invaginations of epidermis into underlying CT 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) Apocrine sweat glands Specialized glands (axillary, areolar, anal regions) - ducts open into hair follicles - secretes viscous, odorless fluid The mammary gland is considered a highly modified __________ apocrine sweat gland 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 Skin as a sensory organ 1. Free nerve endings 2. Meissner's corpuscles 3. Pacinian corpuscles Free nerve endings Unmyelinated axons cross basal lam. & enter malphigian layer Temperature Pain (some crude touch) Meissner's corpuscles Encapsulated receptors (dermal papillae of thick skin) - also skin of lips / nipples Discriminative touch Pacinian corpuscles Encapsulated receptor (dermis / hypoderm. of thick & thin skin) - abundant in skin of fingertips Pressure Vibrations