ch.12-skin.pdf

Skin Skin Lecture Objectives • Describe the functions of skin. • Describe the structure, location and function of the cell types found in epidermis: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. • Describe the structure and contents of the papillary dermis, reticular dermis, an...

Skin Skin Lecture Objectives • Describe the functions of skin. • Describe the structure, location and function of the cell types found in epidermis: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. • Describe the structure and contents of the papillary dermis, reticular dermis, and hypodermis. 2 Skin Lecture Objectives • Describe the structure and function of sebaceous glands, eccrine sweat glands, and apocrine sweat glands. • Describe the microscopic features of nails. 3 Skin Lecture Outline • Introduction • Epidermis • Dermis • Nail Skin Lecture Outline • Introduction Functions of Skin • Serves as a barrier protecting against physical and chemical injury and infection. • Prevents water entry and loss. • Helps regulate body temperature. • Receptor organ for sensory stimuli. • Involved in synthesis of vitamin D3 from precursors in skin. • Excretion of substances produced by glands. Embryologic Origins of Skin • Epidermis (the surface layer of skin) arises from ectoderm. • Dermis (the connective tissue component of skin) arises from somites of the paraxial mesoderm. Cranial and sensory ganglia and nerves Ectomesenchyme of face Adrenal medulla Meninges Melanocytes Central nervous system Posterior pituitary Neuroectoderm Urogenital system Neural crest Intermediate plate mesoderm Surface ectoderm Paraxial mesoderm Epidermis, hair and nails Anterior pituitary Tooth enamel Skeleton and trunk muscles Pharyngeal arches Connective tissue Dermis Endoderm Lining of GI tract Lateral plate mesoderm Pharyngeal arches Connective tissue Skin is composed of: • Epidermis: a surface of keratinized stratified squamous epithelium • Dermis: connective tissue • Dermal-epidermal junction: epidermal ridges, dermal papillae. Basement membrane separates epidermis from dermis. • Skin appendages: Merocrine (eccrine) sweat glands, apocrine sweat glands, sebaceous glands, hair follicles, nails. Layers of the skin Appendages of the skin Hair shaft Meissner corpuscle Sebaceous gland Arrector pili muscle Hair follicle 11 Pacinian corpuscle More Appendages of the Skin Subcutaneous tissue • Also called hypodermis • NOT considered part of the skin! • Consists of loose connective tissue and adipose tissue • Function: loosely bind the skin to underlying structures 13 Skin Lecture Outline • Introduction • Epidermis • Layers of the epidermis • Cells of the epidermis Skin Lecture Outline • Introduction • Epidermis • Layers of the epidermis The Five Layers of Epidermis • Corneum • Lucidum • Granulosum • Spinosum • Basale Five strata (layers) of skin corneum lucidum granulosum spinosum basale Stratum Basale (Basalis) • Bottom layer; just above basal lamina. • Cuboidal to columnar keratinocytes one layer thick. • Cells attached to each other by spot desmosomes and to basal lamina by hemidesmosomes. • Mitoses renew epidermis every 15-30 days. • Contains melanocytes and Merkel cells. Stratum basale Stratum Spinosum • Between stratum granulosum and basale. • Spot desmosomes connect cells. • Cells shrink during processing but remain attached at desmosomes, and look “spiny” or prickly. • Areas of skin subject to more mechanical pressure have more spot desmosomes. Stratum spinosum Keratinocytes in stratum spinosum are attached by spot desmosomes. Cells retract during fixation and appear to be connected by spines. Cytokeratin filaments Spot desmosomes Stratum spinosum cells Stratum Granulosum • Just above stratum spinosum. • Cells contain two types of granules. • Keratohyaline granules: large, basophilic; bind cytokeratin molecules together to make keratin. • Lamellar granules: small; contain lipid that is released into intercellular spaces (acts as a cement to prevent penetration of water and other materials). Stratum granulosum Stratum Lucidum • Under stratum corneum. • Keratinocytes have lost nuclei and organelles and appear as homogeneous, translucent cells. • Cells contain keratin. • Present only in very thick skin. Stratum lucidum Stratum Corneum • Most superficial layer. • Consists of flat, dead cells – basically keratin scales - that are continuously shed. • Keratin is composed of cytokeratin filaments (long intertwined protein chains) and keratohyalin (a substance that helps hold cytokeratin filaments together). Stratum corneum Thick vs. Thin Skin • Refers to thickness of epidermis and keratin layer • Thick skin is present on palms and soles • Epidermis has five cell layers + thick keratin layer. • Hair follicles and sebaceous glands are NOT present. • Thin skin is found everywhere else. • Epidermis has no stratum lucidum, and the stratum granulosum and corneum are much thinner. • May contain hair follicles and sebaceous glands. Thick skin Thick skin corneum lucidum granulosum spinosum basale corneum granulosum spinosum basale Thin skin Stratum lucidum not present. Stratum granulosum and corneum are much thinner. Skin Lecture Outline • Introduction • Epidermis • Layers of the epidermis • Cells of the epidermis Four Types of Cells in Epidermis • Keratinocytes are stratified squamous epithelial cells. Most common cell in epidermis. Function is to produce intermediate filaments called cytokeratins. Amount of cytokeratins increases as the cells move upward. • Melanocytes produce melanin. • Langerhans cells process antigen. • Merkel cells are involved in tactile sensation. keratinocyte melanocyte Melaninogenesis • Melanocytes synthesize melanin pigment in melanosomes and “donate” it to adjacent epidermal cells. • Melanosomes contain tyrosinase, which converts tyrosine to 3,4dihydroxyphenylalanine (DOPA). DOPA is metabolized into melanin and packaged in granules. More melanin is present in keratinocytes than in melanocytes. • The number of melanocytes per unit area varies from one part of the body to another but is independent of race. • Differences in skin color are due to differing numbers of melanin granules in melanocytes! Melanin and skin color Melanin granules accumulate above keratinocyte nuclei to protect genetic material from UV damage. Smart! Langerhans Cells and Merkel Cells Langerhans cells • A type of macrophage. • Arise in bone marrow, migrate to stratum spinosum. • Eat stuff (like bugs), present antigen to T cells. Merkel cells • Present in stratum basale. • Function as touch receptors. Skin Lecture Outline • Introduction • Epidermis • Dermis • Basic structure • Specialized receptors and structures Skin Lecture Outline • Introduction • Epidermis • Dermis • Basic structure Epidermis Dermis Hypodermis Dermal papillae Papillary dermis Reticular dermis Loose connective tissue with lots of capillary loops and thin elastic fibers Dermal papillae Papillary dermis Reticular dermis Dense connective tissue with thick collagen bundles and elastic fibers, larger blood vessels and glands. Skin Lecture Outline • Introduction • Epidermis • Dermis • Basic structure • Specialized receptors and structures Meissner’s and Pacinian Corpuscles Meissner’s corpuscle • Sensitive to light touch • Consists of an unmyelinated axon meandering back and forth between flattened Schwann cells. Pacinian corpuscle • Sensitive to vibration and pressure. • Consists of unmyelinated nerve terminal surrounded by layers of fibroblasts. Meissner’s corpuscle: sensitive to light touch Pacinian corpuscle: sensitive to vibration, pressure The Pilosebaceous Apparatus • Hairs: hair follicle and shaft • Sebaceous glands and ducts: empty into hair follicle • Arrector pili muscles: cause erection of hair shaft • Hairs and sebaceous glands derive from ectoderm Pilosebaceous apparatus and sweat gland Hair follicle Sebaceous gland Arrector pili muscle Pilosebaceous apparatus • The hair follicle is a tubular invagination of the epidermis extending deep into the dermis. • The hair bulb is the expanded terminal region of the hair follicle and is composed of epithelium. • The dermal papilla at the baste of the hair follicle is a connective tissue region with numerous capillaries. It maintains the viability of the hair follicle. Hair follicle The cuticle and cortex make up the hard keratin part of the hair shaft. Some hairs have a medulla as well. The glassy membrane is a thickened basal lamina that separates the hair follicle from the surrounding connective tissue. The internal root sheath contains cells with clear cytoplasm. The external root sheath is continuous with the epidermis. Layers of the hair follicle Skin glands • Three types: sebaceous glands, eccrine (merocrine) sweat glands, and apocrine sweat glands. • Arise from ectoderm. • Secretory portion of glands resides in the dermis. • Three different types of secretion: holocrine, merocrine, and apocrine. Types of secretion Types of glands Sebaceous glands • Present everywhere except palms and soles. • Secretory portion: peripheral, flattened undifferentiated cells. Central cells are large with foamy cytoplasm containing lipids. • Cells burst, releasing sebum (holocrine secretion). • Duct empties into hair follicle. • Become functional at puberty. Sebaceous gland Sebaceous glands emptying into hair follicle Sebaceous glands surrounding hair follicle Eccrine (Merocrine) Sweat Glands • Secretory portion has three cell types: • Clear cells (contain glycogen, produce a watery substance) • Darker cells (produce a proteinaceous substance) • Myoepithelial cells (surround gland) • Duct is lined by simple cuboidal epithelium and opens onto skin surface. Eccrine sweat glands and ducts Secretory cells Myoepithelial cells Eccrine sweat glands and ducts Apocrine Sweat Glands • Located only in axilla, areola of breast, and anal canal. • Have larger ducts and secretory units than eccrine sweat glands. • Ducts open into hair follicles. • Apocrine secretion is viscous and contains proteins, carbohydrates and lipids. Apocrine sweat glands Skin Lecture Outline • Introduction • Epidermis • Dermis • Nail Some words to know about nails Some more words to know about nails Eponychium (cuticle) (softer keratin) Nail matrix (generates hard keratin) Nail plate (hard keratin) Nail bed (epithelium) Hyponychium (protects nail bed) Nail histology Skin Lecture Outline • Introduction • Epidermis • Dermis • Nail