Lecture 4 - Integumentary System

What are the cell types of epidermis?

• Keratinocytes (skin cells)

  • Produce fibrous keratin (protein that gives skin its protective properties) 

  • Most abundant cell type in epidermis 

  • Originate in basal layer and tied together with desmosomes (provide stability and prevent cells from being pulled apart) 

    • Migrate superficially and flatten over time 

• Melanocytes (pigment)

  • Spider shaped cells located in basal layer 

  • Each dendritic extension can supply melanin to around 30-40 keratinocytes

  • Produce melanin pigment which is packaged into melanosomes  

    • Transferred to keratinocytes - release melanin to shield and protect cells nucleus from UV damage 

• Dendritic (longerhans) cells (immunity)

  • Star shaped macrophages that are in the deep epidermis 

  • Key activators of immune system

  • Originate in bone marrow and migrate to epidermis through systemic circulation (blood supply) 

• Tactile epithelial (merkel) cells (sensory cells) 

  • Sensory receptors that sense touch/light

  • Found in basal layer and EACH is attached to sensory nerve endings 

What are the key features and functions of epidermis?

• Superficial region

• Keratinized stratified squamous epithelial tissue and avascular

• Function: protection (mechanical and chemical barrier), regulates water barrier (prevents dehydration + lysis of cells) 

What are the layers of the epidermis?

• Thick skin = five layers (strata) and found in high-abrasion areas (palms/soles)

• Thin skin = only four strata

• DEEP TO SUPERFICIAL 

• Stratum basal (basal layer) - youngest layer

  • Attaches to dermis through dermal papillae 

  • Single row of stem cells that are mitotic (can divide to produce two daughter cells) 

    • One daughter cell remains in this layer as stem cell

    • One daughter cell is pushed out of basal layer to become a keratinocyte (takes 25-45 days to reach skin's surface) 

  • 10-25% of layer composed of melanocytes

  • Contains tactile epithelial (merkle) cells

• Stratum spinosum (spinous/prinkly layer) 

  • Several cell layers thick

  • Keratinocytes attached to other keratinocytes by desmosomes - allow them to resist tension and pulling 

  • Also contains dendritic (langerhans) cells 

• Stratum granulosum (granular layer) 

  • Several layers of thick flattened cells 

  • Site where keratinization begins

    • Keratinocytes fill with keratin and causes changes in appearance 

      • Cells flatten → nuclei and organelles begging to disintegrate

      • Cells accumulate keratohyaline granules that help form keratin fibers in upper layers

      • Cells accumulate lamellar granules (water resistance glycolipid that slows water loss from skin) 

  • Last alive cell (far from dermal capillaries 

• Stratum lucidum (only thick skin) 

  • Only in palms and soles

  • Thin translucent band of 2-3 rows of clear, flat, and dead keratinocytes

• Stratum corneum (visible skin) - oldest layer 

  • Thickest layer of epidermis (¾ of epidermal thickness)

  • Flat anucleate keratinized (dead) cells

  • Functions: 

    • Protect deeper cells from the environment

    • Prevent water loss

    • Protect from abrasion and penetration

    • Act as a barrier against biological, chemical, and physical assault 

What are the key features (tissue type and location) and functions of dermis?

• Deeper region

• Mostly fibrous connective tissue and vascular

  • Strong dense irregular connective tissue (collagen fibers that cross each other for added strength + structure) and flexible areolar connective tissue 

• Function: connects epidermis to internal structures and creates cushions and creates resilience

What are the cell types of the dermis?

• Fibroblast - precursor/produces fibers (collagen and elastic fibers)

• Macrophages - phagocytes (engulf invaders and presents to WBC of immune system to signal it) 

• Occasionally mast cells (release histamine as part of inflammatory and immune response) and white blood cells (protection for immune response) 

What are sensory receptors in dermis?

• Free nerve endings - temp, itch, pain, pressure 

• Lamellar corpuscles - deep pressure, stretch, vibration

• Tactile (Meissner’s) corpuscles - light pressure, vibration, discriminative touch

• Bulbous corpuscles (Ruffini endings) - deep pressure, stretch

What are other structures besides cell types and sensory receptors that are found in dermis?

• epidermal hair follicle

• oil glands

• sweat gland

What are the layers of dermis?

• Papillary layer

  • 20% of dermal layer

  • Superficial layer of areolar connective tissue (collagen and elastic fibers) and blood vessels 

    • Provides nutrients and cushion

  • Dermal papillae - superficial region of dermis that sends fingerlike projections into the epidermis 

    • Separation of dermis and epidermis 

    • Contains: capillary loops, free nerve endings, touch receptors like tactile (Meissner’s) corpuscles to detect light pressure

• Reticular layer 

  • 80% of dermal thickness

  • Dense irregular fibrous connective tissue

    • Elastic fibers - provide stretch-recoil properties

    • Collagen fibers - provide strength and resiliency

      • Bind water and keep skin hydrates 

  • Contains

    • Dermal vascular plexus - network of blood vessels between reticular layer and hypodermis

    • Cleavage lines

      • Collagen fibers running parallel to skin surface (invisible) 

      • Lacerations cut parallel to cleavage lines heal more readily 

    • Flexure lines

      • Dermal folds at or near joints 

      • Visible on hands, wrist, fingers, soles, and toes 

    • Lamellar and bulbous corpuscles (sensory cells) 

• Dermal ridges = separation of epidermis and dermis 

What is the hypodermis?

aka subcutaneous tissue (superficial fascia)

• Subcutaneous layer deep to skin

• Not part of skin but shares some functions 

• Adipose tissue (absorbs shock and insulates) and connective tissue (anchors skin to underlying structures) 

Skin pigmentation

• Melanin 

  • Only pigment made in skin

  • Produced by melanocytes 

  • Packaged into melanosomes and sent to keratinocytes to protect their nucleus from sun/uv light

  • Freckles and pigmented moles = local accumulations of melanin 

• Carotene

  • Yellow to orange pigment

  • Most obvious in psalm and soles

  • Converted to vitamin A for vision and epidermal health

• Hemoglobin

  • Pinkish hue in fair skin 

Hair cell function and location

• Hair (aka pili) - flexible strands of dead keratinized cells

• Cover body EXCEPT for palms, soles, lips, nibbles, and mucous membranes

• Functions: 

  • Body - warmth, warning of touch (ex. Insects on skin) 

  • Hair on head - guard against physical trauma, protect from heat loss, shield skin from sunlight

  • Eyelashes - shield from light

  • Nose - filter large particles 

What creates pigmentation of hair?

• Determined by melanocytes in hair follicles 

• Combinations of different melanins 

  • Red hair has additional pheomelanin pigment

  • gray/white hair results when melanin production decreases and air bubbles replace melanin in shaft

What are the key structures of the hair

• Produced by hair follicles deep in dermis 

• Contains hard keratin (tougher and more durable) 

• Two main regions

  • Shaft (visible above skin) 

    • Area that extends above skin and where keratinization is complete

    • Medulla = central core

    • Cortex = several layers of flattened cells surrounding medulla

    • Cuticle = single outer layer of overlapping of cells (like shingles on roof) 

  • Root (under skin) 

    • Keratinization is still occuring 

  • Hair follicle - epidermal surface projects a pouch like shape into the dermis which encapsules and anchors a single hair root 

  • Hair bulb - base of follicle and the root of the hair 

  • Hair follicle receptor (or root hair plexus) - knot of sensory nerve endings that wrap around the bulb and makes each hair very sensitive to touch 

  • Hair matrix - actively dividing area of bulb that produces hair cells

    • As matrix makes new cells, old ones are pushed upward 

  • Hair papilla - dermal tissue containing a knot of capillaries that supplies nutrients to growing hair 

  • Arrector pili - small band of smooth muscle attached to hair follicle

    • Will contract in response to cold to form goose bumps (traps heat) 

What are the two types of hair?

• Vellus hair 

  • Pale fine body hair of children and adult females 

• Terminal hair 

  • Coarse long hair 

  • Found on scalp and eyebrows/lashes

  • At puberty - axillary and pubic regions and face/neck of males 

Nail functions and modifications and key features

• Scale-like modifications of epidermis that contain hard keratin

• Function: act as protective cover for distal dorsal surface of fingers and toes 

• Free edge

• Nail body/plate

• Proximal root (embedded in nail matrix) 

• Nail matrix = thickened portion of bed responsible for nail growth 

  • Lunule = thickened nail matrix and appears white 

• Nail bed = epidermal layer underneath keratinized nail plate 

• Nail fold = skin fold that overlap border of nail 

• Eponychium (cuticle) - nail fold that projects onto surface of nail body

• Hyponychium - ara under free edge of plate 

What are the three types of sweat glands? What is the other name for them?

sudoriferous glands

• eccrine sweat glands - smaller; secrete into pore on skin

• apocrine sweat gland - larger (axillary and anogenital regions); secrete into hair follicle

• Sebaceous (oil) glands - secrete into hair follicles (everywhere except palms and soles)

Eccrine sweat glands

• Merocrine gland (release secretion without damage to cells) 

• Contain myoepithelial cells 

• Ducts connect to pores through skin

• Function: thermoregulation (reduce temperature) - regulated by SNS, cold sweat under stress 

• Secretion: sweat (water with salts, dermicin - microbe killing peptide, and metabolic waste) 

• Location: palms, soles, and forehead 

• Structure: simpled coiled tubular glands 

Apocrine sweat glands

• Merocrine gland (release secretion without damage to cells) 

• Begin functioning at puberty 

• Function: secretions cause body odor 

• Secretion: viscous milky or yellowish discharge…sweat that contains fatty substances and proteins 

• Location: axillary and anogenital areas

• Structure: larger coiled tubular glands and empty right into hair follicle (deeper in epidermis) 

• Modified apocrine glands

  • Ceruminous glands: secrete cerumen (earwax) in lining of external auditory canal 

  • Mammary glands: secrete milk

Sebaceous (oil) gland

• Simple branched alveolar glands

• HOLOcrine glands - cell plasma membrane ruptures and releases cell’s cytoplasm into the lumen of the gland

• Secretion: sebum (can cause acne) 

• Functions: sebum contains bactericidal (bacterial killing) properties, softens hair and skins, and slows water loss from skin

• Secrete into hair follicles 

• Location: all over the body EXCEPT psalm and soles

• Activities during puberty via hormones

What are the 6 main functions of skin

1. protection (chemical, physical, and biological barriers)

2. body temperature regulation

3. cutaneous sensation

4. metabolic function

5. blood resevoir

6. excretion of waste

How is the skin offer protection?

• Chemical barrier - sweat (antimicrobial proteins), sebum and defensins (kill bacteria), melanin (against UV radiation)

• Physical barrier - keratinized cells and glycolipids (water loss)

• Biological barrier - epidermis has phagyoctic cells (dendritic cells or epidermis engulf foreign antigens), dermis contains phagocytic cells (macrophages)

How does skin regulate body temperature

• Insensible perspiration - sweat glands produce 500 ml/day of unnoticeable sweat

• Decreased body temperature - dermal blood vessels constrict 

• Increased body temperature - sensible perspiration (cool body), dilation of dermal vessels increase sweat gland activity

Skin cutaneous sensation

• Exteroreceptors respond to stimuli outside the body! 

  • FNE (epi and pap derm)

  • Tactile epithelial (merkel) cells (epi)

  • Hair follicle receptors (epi)

  • Tactile (Meissner’s) corpuscles (pap. derm)

  • Lamellar corpuscle (ret. derm)

  • Bulbous corpuscles (ret. derm)

What are the metabolic functions, blood resevoir, and excretion of wastes of skin

• Metabolic functions

  • Skin can synthesize vitamin D needed for calcium absorption (used for bone health) 

• Blood reservoir 

  • 5% of body’s total blood volume

  • Skin vessels constrict to shunt blood to other organs 

• Excretion of wastes 

  • Sweat → salt and water loss