Epidermis - outermost superficial region (thin)

Dermis- middle region (leathery, fibrous connective tissue)

tough, insoluble proteins

stable & permanent

form internal structure (keratin in epithelials)

look like ropes with tensile

diameter between MF & MT

• produce the fibrous protein keratin

• Melanocytes

• produce the brown pigment melanin
• Bound into granules called melanosomes
• Transported to keratinocytes
• Accumulate on the “sunny” side of the nucleus
• Makes a pigment shield protecting the nucleus

• Langerhans’ cells

• epidermal macrophages that help activate the immune system
• “Epidermal dendritic cells” (not neurons, dumb name cause it makes you think of neurons but they are not.  They have a lot of branches.  Should have been called “Double Tree”, or, “Weeping Willow”)
• Phagocytes, activate the immune system
• Lots of ‘em, form a network.

• Merkel cells

• function as touch receptors in association with sensory nerve endings
• Cell plus nerve ending = Merkel Disc

• Keratinocytes 

• produce the fibrous protein keratin

•  produce the fibrous protein keratin

• Millions rub off daily – new skin every 45 days

• House dust is mostly you floating around

Stratum Basale 

• Deepest epidermal layer firmly attached to the dermis
• Consists of a single row of the youngest keratinocytes
• Cells undergo rapid division, hence its alternate name, stratum germinativum

Stratum Spinosum 

• Cells contain a weblike system of intermediate filaments attached to desmosomes
• Melanin granules and Langerhans’ cells are abundant in this layer

Layers of the Epidermis: Stratum Granulosum
(Granular Layer)

• Thin; three to five cell layers in which drastic changes in keratinocyte appearance occurs                                         
• Keratohyaline and lamellated granules accumulate in the cells of this layer

Stratum Lucidum
(Clear Layer)

• Thin, transparent band superficial to the stratum granulosum
• Consists of a few rows of flat, dead keratinocytes

• Visible only in thick skin

Layers of the Epidermis: Stratum Corneum
(Horny Layer)

• Outermost layer of keratinized cells
• Accounts for three quarters of the epidermal thickness
• Functions include:

• Waterproofing – glycolipids between cells
• Protection from abrasion and penetration - keratin
• Rendering the body relatively insensitive to biological, chemical, and physical assaults

Dermis

• Second major skin region containing strong, flexible connective tissue
• Cell types include fibroblasts, macrophages, and occasionally mast cells and white blood cells
• If skinned and tanned = leather
• Composed of two layers – papillary and reticular

Layers of the Dermis: Papillary Layer

• Papillary layer

• Areolar connective tissue with collagen and elastic fibers
• Its superficial surface contains peglike projections called dermal papillae
• Dermal papillae contain capillary loops, Meissner’s corpuscles, and free nerve endings
• In fingers and feet, on top of “dermal ridges” they form epidermal ridges = fingerprints
• Epidermal ridges on top of dermal papillae or dermal ridges in fingers and feet plus a little sweat oil = fingerprints

Layers of the Dermis: Reticular Layer

• Reticular layer

• Accounts for approximately 80% of the thickness of the skin
• Collagen fibers in this layer add strength and resiliency to the skin
• Elastin fibers provide stretch-recoil properties

Hypodermis

• Subcutaneous layer deep to the skin

• Composed of adipose and areolar connective tissue

Three pigments that contribute to skin color

• Melanin 
• Carotene
• Hemoglobin

• Melanin

• yellow to reddish-brown to black pigment, responsible for dark skin colors

• Freckles and pigmented moles 

• result from local accumulations of melanin

• Carotene 

• yellow to orange pigment, most obvious in the palms and soles of the feet

• Hemoglobin

• reddish pigment responsible for the pinkish hue of the skin

Skin appendages

• Sweat Glands
• Sebaceous Glands, (Oil)
• Hairs and hair follicles
• Nails

Sweat (sudoriferous) Glands

• Different types 

• 1) prevent overheating of the body 
• 2) secrete cerumen 
• 3) secrete milk

• Up to 3 million glands per person

• Eccrine sweat glands

• found mostly in palms, soles of the feet, and forehead but really are everywhere.

• 99% water, some salts (NaCl), vit C, antibodies, and dermicidin (a microbicide) (What the heck is a microbicide?), some wastes

• Apocrine sweat glands – 

found mostly in axillary and anogenital areas and wherever there is body hair

• Same content as eccrine + fats and proteins
• Odorless, but smell when decomposed by bacteria
• Empty onto hair follicles
• Could be human equivalent of animal sexual scent glands

• Ceruminous glands

• modified apocrine glands 

• One place, or, rather, two places, each ear canal
• Secrete cerumen

• Mammary glands

•  specialized sweat glands that secrete milk

Sebaceous Glands

• Found all over the body
• Holocrine type (cells fill up and burst)
• Soften skin when stimulated by hormones
• Become active at puberty

• Secrete an oily secretion called sebum

Hair

• Cover entire body surface EXCEPT

• Palms, soles, lips, nipples and some external genitalia
• Not much use, now, mostly decorative
• Dead, keratinized cells
• Harder keratin than skin type
• Tougher
• Does not flake off

Structure of a Nail

• Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes
• Corresponds to a hoof or claw
• Useful to pick up stuff or scratch
• Hard keratin