Chapter 6 – Integument

 

1)      The skin – integument

a)      Boundary layer of body (interface) between internal and external environment

i)        ~ 15% of body weight in humans

b)      Composed of many tissues

i)        Epithelium, connective tissue, fat, and smooth muscle

ii)       Contains blood vessels, glands, sensory receptors, and nerves

c)      Functions – maintain internal environmental conditions

i)        Protect

ii)       Reduces undue changes in water and salts, UV radiation

iii)     Sensation

iv)     Regulates temperature (birds and mammals), water, ions, and gases

v)      Vitamin D synthesis

vi)     Blood pressure regulation

vii)   Camouflage and/or warning

viii)  Ecosystem of viruses, bacteria, fungi, yeasts, mites, etc.

2)      General structure and development of skin (Fig 6-1)

a)      Epidermis (upon skin)

i)        Composed of stratified, squamous epithelium

ii)       Develops from ectoderm

iii)     Rests on basal lamina

iv)     Layers of epidermis (thin in deep fishes and thick in mammals)

(1)   Stratum germinativum

(a)    Located just above basal lamina

(b)   Cuboidal (or columnar) cells that multiply mitotically

(2)   Cells move toward body surface

(a)    Differentiate, flatten and slough off in sheets

(b)   Produce keratin

(i)      Water-insoluble, horny protein that replaces cellular organelles

(3)   Stratum corneum

(a)    Dead, keratin-filled cells on the skin surface

b)      Dermis (distinctive part of vertebrate skin)

i)        Underlying fibrous connective tissue composed of an extracellular matrix

(1)   Collagen and elastic fibers embedded in a ground substance of proteoglycans (bind to water)

ii)       Cellular elements

(1)   Fibroblasts – synthesize fibers

(2)   Fat cells, smooth muscle cells, white blood cells

(a)    Macrophages – phagocytic protective cells

(3)   Nerves and sense organs

iii)     Develops from mesenchyme (mesodermal dermatomes of somites and neural crest)

iv)     Layers of dermis (dermis with high collagen content is processed as leather

(1)   Stratum laxum – collagen fibers are loosely arranged

(2)   Stratum compactum – collagen fibers are tightly packed in this deeper segment

(3)   Dermal papillae – protrusions of dermis into the epidermis (fingerprints)

c)      Skin derivatives

i)        Teeth, bony scales, feathers, hair

(1)   Develop from epithelial-mesenchymal interactions between dermis and epidermis

(a)    Inductive influences of adjacent cells

(i)      Neural crest cells migrate between epidermis and dermis

(2)   Keratinized structures (feathers and hair) are composed of epidermal cells

(3)   Scales and teeth are composed of epidermal and dermal products

d)      Dermal (integumentary) skeleton

i)        Develops within or just beneath the skin (not to be confused with endoskeleton or exoskeleton of arthropods or other invertebrates)

3)      Skin coloration and its functions

a)      Skin contains pigments within cells known as chromatophores (neural crest origin)

i)        Located in dermis of fishes, amphibians and reptiles

(1)   Penetrate into epidermis in birds and mammals

ii)       Melanophores – dark pigment melanin (most mammals)

(1)   Star-shaped cells with long, branching processes (Fig 6-2A)

(2)   Melanosomes – organelles where pigment is synthesized

(a)    Pigment is contained in melanosomes in anamniotes and reptiles

(b)   Pigment is transferred to feather, hair, and epidermal cells in birds and mammals

iii)     Iridophores – brighter pigment cells that contain reflective platelets of purine (guanine) – silvery appearance

iv)     Xanthophores and erythrophores – yellowish or reddish pigments (respectively)

(1)   Composed of pteridines and carotenoids

v)      Dermal chromatophore unit (Fig 6-2B)

(1)   Three or more chromatophores

(a)    Iridophores in center of unit are surrounded by processes of deeper melanophore

(i)      Overlain by xanthophores and erythrophores

1.      Different colors result from different combinations of pigment cells

a.       Melanin dispersed in melanophore processes results in dark appearance

b.      Melanin withdrawn from melanophore processes causes light to reach iridophores

i.         Short wavelengths (blue) are reflected through the skin (filtered by yellow xanothophores to produce green)

ii.       Long wavelengths absorbed by melanophores

vi)     Structural coloration – minute ridges on surface cells refract light

(1)   Causes color change with changes in angle of observation

b)      Pigmentary color changes

i)        Synthesis of pigment can be adapted to different backgrounds, seasons, or solar radiation (suntan)

(1)   Morphological color changes – slow changes in pigment levels

(2)   Physiological changes – rapid changes in pigment levels

(a)    Migration of pigment within chromatophores

(i)      Sympathetic nerves in fishes

1.      Aggregation of melanosomes (norepinephrine)

(ii)    Pituitary gland – melanophore-stimulating hormone

1.      Dispersal of melanosomes

c)      Role of skin color

i)        Cryptic – concealing skin color to benefit animal

(1)   Hide from predators

(2)   Stalk prey undetected

ii)       Aposematic – color that signals the presence of danger

(1)   Venomous and distasteful species reinforce unpleasant experience of predators

iii)     Species recognition – courtship and communication

iv)     Territory establishment

v)      Thermoregulation (in amphibians)

(1)   Darker pigment helps collect solar radiation to warm the body

vi)     Protection of deeper tissues against UV radiation

d)      Adaptive camouflage in flatfishes (flounders) – Focus 6-1

i)        Disguise functions as a malleable mirror that reflects immediate local surroundings

(1)   Designs for visibility

(a)    Color resemblance

(b)   Obliterative shading

(i)      Graded lightening and darkening on the surface

(c)    Disruptive coloration

(i)      Pattern of colors and tones that divide the continuous body surface into sections

(d)   Shadow elimination

ii)       Disruptive coloration in flatfishes

(1)   Adaptive changes in skin reflectance and contrast between skin areas

(a)    Distribution of chromatophores and reflecting cells (iridophores)

(i)      Sympathetic nerve stimulation, melanophore-stimulating hormone, and melanophore-concentrating hormone

(ii)    Control system is superimposed by visual system

1.      Blind flatfishes darken (cannot camouflage)

(b)   Limited range of preset patterns of stimulation of chromatophore units

(c)    Eliminate sharp boundaries of body using transparent fins along body perimeter

(d)   Produces average reflectance of background

4)      Skin of fishes

a)      Structure

i)        Epidermis is relatively thin

(1)   Most cells are living

(2)   Keratin deposited in limited areas (horny teeth of lampreys)

ii)       Unicellular glands – produce mucus (common)

(1)   Secretions form a mucous cuticle (slimy surface)

(a)    Reduces water exchanges between fish and environment (maintain stable internal environment)

(2)   Protects body from bacterial invasion

(3)   Reduces friction and drag

(4)   May contain an alarm substance that acts as a chemical messenger when injured

(a)    Triggers a fright reaction in nearby members of the species

iii)     Multicellular glands – grow into the dermis (uncommon)

(1)   Poison glands that produce toxic materials (fin spines)

(2)   Photophores – light-generating glands (deep-sea species)

(a)    Secretions may nourish a bacterial colony that generates light

(b)   Chemical reactions may produce light

(c)    Used for recognition, luring, and feeding

(i)      Flashlight fishes that produce light with a symbiotic bacteria possess a shutter mechanism to cover and expose the organ (Fig 6-3)

iv)     Dermis

(1)   Helical cross fibering - collagen fibers are arranged regularly in layers that spiral around the body

(a)    Adjacent fibers are perpendicular

(i)      Arrangement strengthens the body to maintain shape during swimming

b)      Bony scales

i)        Bone, dentine, and enamel contribute to scales

(1)   Bone consists of extracellular matrix of collagen fibers embedded in a ground substance of protein polysaccharides and calcium phosphate

(a)    Osteoblasts (from mesenchymal cells of the dermis) form bone

(i)      Hydroxyapatite – calcium phosphate crystals synthesized by osteoblasts

(2)   Cellular bone structure (Fig 6-4)

(a)    Osteocytes (differentiated osteoblasts) – bone-forming cells trapped in the matrix in lacunae (small cavities)

(i)      Interconnected by canaliculi (small canals)

(3)   Teleost fish possess acellular bone

(a)    Bone-forming cells are found in periphery during development

(i)      Cells move away from the center of the scale as bone is formed

ii)       Dentine and enamel may be deposited on top of bone to form surface

(1)   Scales develop embryonically (similar to teeth)

(a)    Mesenchymal cells aggregate in small papillae below basal lamina of epidermis

(b)   Basal cells of overlying epidermis differentiate into ameloblasts that form an enamel organ (enamel)

(i)      Induction – underlying dermal cells differentiate into odontoblasts that lay down dentine below enamel organ

1.      Dentine

a.       Produced by cells of neural crest origin

b.      Resembles bone (differs in arrangement)

c.       Cosmine dentine – dentine tubules (form from processes of retreating odontoblasts) grouped into radiating tufts

(c)    Enamel – produced by enamel organ on top of dentine

(i)      Epidermal origin

(ii)    Hardest substance in body (96% hydroxyapatite by weight)

(iii)   Matrix includes proteins called amelogenins

(iv)  Ganoine enamel – enamel produced in waves resulting in a lamellar appearance (difference in pattern not in content from teeth)

iii)     Cosmoid plates – primary dermal armor of ancestral jawless vertebrates (Fig 6-6A)

(1)   Deepest part of plates made up of layers of cellular bone

(2)   Spongy bone is superficial to lamellar bone

(3)   Spaces in scale accommodate blood vessels and electroreceptive systems

(4)   Surface is composed of denticles that contain cosmine dentine called enameloid (developed from dermis)

(5)   Armor helped keep bottom feeders on the bottom and protected

(6)   Bone served as calcium/phosphate reservoir to allow movement to freshwater

(7)   Stopped osmosis in freshwater

(a)    Ancestral armor was lost in lampreys (more active and mobile)

iv)     Bony material disappeared in cartilaginous fishes

(1)   Spiny dermal denticles or placoid scales of sharks and other cartilaginous fishes are composed of dentine surrounded by a vascular pulp cavity capped by a hard material (Fig 6-6)

(a)    Contains enamel amelogenin proteins and fibrous dermal material

v)      Cosmoid scales – early bony fishes retained heavy, bony scales

(1)   No denticles

(2)   Present in lungfishes

vi)     Ganoid scales – composedof lamellar bone overlain by layers of enamel (ganoine)

(1)   Some ontained small amounts of vascular spongy bone and dentine

vii)   Imbricating scales – develop in overlapping skinfolds in some actinopterygians

(1)   Composed primarily of acellular bone underlain by a dense, fibrous material (Fig 6-6D)

(a)    Flexible

viii)  Cycloid scales – sculptured into a pattern of growth rings (Fig 6-6E)

ix)     Ctenoid scales – series of comb-like projections (ctenii) on posterior skin surface (Fig 6-6F)

x)      Fins

(1)   Lepidotrichia (scale hair) – scales associated with fins lined up in branching columns to form flexible fin rays (in osteichthyans) – Fig 6-7A)

(2)   Ceratotrichia – horny supports of fins (chondrichthyan)

(3)   Spines – lie on leading edge of fin to stiffen and act as cutwater (Fig 6-7B)

(a)    Evolved by the enlargement of a single scale

(b)   Often associated with poison glands

5)      Skin of amphibians

a)      Larvae possess skin similar to fishes without scales

b)      After metamorphosis, epidermis remains thin and synthesizes keratin

i)        Stratum corneum – accumulated keratin in dead cells forms a horny layer (Fig 6-8)

(1)   Thin to allow for cutaneous respiration

(a)    Provides adequate protection

(2)   Desquamation – outer layer sloughs off periodically in large sheets

(a)    Hormonally controlled

(3)   Nuptial pads – keratinized pads that appear during mating season to aid male in holding on to female

c)      Unicellular glands are absent

d)      Multicellular glands – alveolar-shaped glands that grow into dermis from stratum germinativum

i)        Mucus glands – secretions protect skin surface, reduce water loss, keep surface moist

e)      Dermal vascularization – aids in gas and ionic exchange across the skin

f)        Granular glands – secrete toxic substances

i)        Parotid (or parotoid) gland – aggregation of granular glands behind ear

(1)   Secretions discourage predators

ii)       Toxic skin glands – some tropical frogs use aposematic (warning) coloration in combination with these glands

g)      Cutaneous glands – secrete substances that protect against pathogenic bacteria in damp areas

h)      Coloration

i)        Chromatophores (including melanophores) in epidermis and dermis

ii)       Xanthophores and iridophores in dermis

i)        Bony scales are absent

i)        Some anurans retain homologues of dermal scales called osteoderms

(1)   Interclavicle – one large osteoderm incorporated into the pectoral girdle

6)      Skin of reptiles

a)      Thick stratum corneum

i)        Composed of many layers of dead, keratin-filled cells

(1)   Organized as horny plates in turtles

(2)   Organized as horny scales in lizards and snakes (Fig 6-9A)

(3)   Loose little water through skin due to phospholipids bound to keratin

b)      Functional scale – complex epidermis

i)        Outer epidermal generation (Fig 6-9B)

(1)   Outer layers heavily keratinized

(2)   Deepest layers are living

ii)       Inner epidermal generation – produced by stratum germinativum

(1)   Lies beneath outer epidermis

(2)   Keratinized and living layers begin to differentiate before skin is shed

iii)     Ecdysis – shedding or molting

(1)   Deep, unkeratinized layer of cells of the outer epidermal generation breaks down

(2)   Fission zone develops

(3)   Outer epidermal generation is lost as a unit

iv)     Plates on turtle shell wear away (not shed)

c)      Terminal phalanx of each digit encased in protective claw

i)        Aid in grip during locomotion

ii)       Derivative of stratum germinativum (like horny scales)

iii)     Calcium salts provide hardness

d)      Epidermal scale modification

i)        Setae – scales over tips of gecko digits become small hair-like structures

(1)   Form adhesive force with surface areas

(a)    Enable movement upside down

ii)       Apical pits

(1)   One to seven in a row near the posterior margin of epidermal scales

(2)   Tactile function (sensation)

(3)   No other sense organs are present in skin

iii)     Scent glands

(1)   Produce secretions (musk) used during mating

(2)   No mucus glands

iv)     Osteoderms

(1)   Retained in some reptiles

(2)   Form gastralia – rib-like structures in abdominal wall

(a)    Stiffen pleuroperitoneal cavity as a mechanism for lung ventilation

e)      Color

i)        Kaleidoscope color change – sympathetic nerves (adrenaline) induce pigment expansion in chromatophores

(1)   Erythrophores and xanthophores produce red and yellow

(2)   Melanocytes produce brown/black pigment

7)      Skin of birds

a)      Epidermis is relatively thin

i)        Not heavily cornified

ii)       Horny scales develop on legs and feet

iii)     Claws are present

iv)     Toothless jaws covered by horny beak

b)      Feathers – derivatives of epidermis

i)        Form flying surface of wing

ii)       Entrap air (evolved before flight?)

(1)   Reduces evaporation

(2)   Forms insulating layer (high constant body temperature)

iii)     Contour feathers – cover body surface (Fig 6-10A and B)

(1)   Flight feathers – larger and stiffer feathers from flying surface

(2)   Structure

(a)    Central axis

(i)      Calamus (quill) – base of axis lodged in feather follicle in dermis

(ii)    Rachis (shaft) – distal region supports a vane

1.      Dermal tissue and blood vessels enter proximal end of quill during development

2.      Bipinnate structure – vane is composed of barbs that branch obliquely

a.       Barbs bear barbules with minute hooklets that interlock with adjacent barbule hooklets (like Velcro)

(iii)   After-feather – small, second vane arises from superior umbilicus (distal calamus)

(b)   Produces a strong, flexible structure

iv)     Down feather (Fig 6-10C)

(1)   Found beneath contour feathers

(2)   Consists of fluffy barbs at distal end of quill

(3)   Provides good insulation

v)      Filoplume feathers (Fig 6-10C)

(1)   Short, stiff bristles

(2)   Barbs reduced or lost

(3)   Protect against foreign substances

vi)     Colors

(1)   Physical structure – causes light scattering

(a)    White – air in cells and polygonal shape of barbule cells

(b)   Blue – scattering action of cellular particles beneath feather barbs

(c)    Iridescent colors

(2)   Chemical pigments

(a)    Red, orange, and pink – carotenoids from keratinized cells of feather

(b)   Brown and black – melanin from germinal region of follicle

vii)   Development

(1)   Epithelial-mesenchymal interaction between epidermis and dermis

(a)    Formation of crown-shaped dermal papilla (Fig 6-11)

(b)   Mitotic divisions in stratum germinativum form crown of barbs

(i)      Covered by a horny feather sheath

(ii)    Cells on one side divide more rapidly to extend shaft upwards

(c)    Base of feather recedes into skin (with epithelium)

(2)   Pterylae – distinct feather tracts from which feathers form (not uniform)

(3)   Feathers are replaced during molting periods

(a)    A few feathers are lost at a time (period of weeks)

c)      Dermis

i)        Determines whether epidermis remains flat

ii)       Determines whether epidermis has feathers, scales

(1)   Type of feathers

iii)     Characteristics

(1)   Lacks ossifications

(2)   Thin, well-vascularized

(3)   Interlacing collagen fibers

(4)   Sensory receptors

iv)     Feather position – flight, behavior, and thermoregulation (Fig 6-12)

(1)   Erectors – lift feathers

(2)   Depressors – lower feathers

(3)   Retractors – pull feathers inward

(4)   Rotators – turn feathers

d)      Evolution of feathers – Focus 6-2

i)        Feathers and horny scales may be homologous structures since they develop by similar mechanisms (dermal papilla pushes into overlying epidermal cells)

(1)   Stage I – first feather-like structure was a filament with a dermal core that protruded from a follicle

(a)    similar to early stage in embryonic development

(2)   Stage II – tuft of unbranched barbs on cone (resembles down feather)

(a)    Communication, thermal insulation, water repellency

(3)   Stages III-V – bipinnate feather with coherent vane

ii)       Theropod dinosaurs contained integumentary structures consisting of (un)branched filaments with an array of collagen fibers within the skin (similar to stage I and II of feather evolution)

(1)   Other theropods possessed bipinnate feathers (too small for flight) on the arm for bipedal balance

iii)     Evolution of skin and feathers is closely correlated with endothermy

(1)   Trapped air serves as insulation

(a)    Thickness of air layer controlled by dermal smooth muscles

(2)   Skin is vascular

(a)    Blood flow regulation through skin determines heat conservation

iv)     Integumentary glands

(1)   Uropygial gland – single branched, alveolar gland located above the base of the tail

(a)    Produces fatty/waxy secretions to be spread over feathers (waterproof)

8)      Skin of mammals

a)      Epidermis and derivatives

i)        Thick with many-layered stratum corneum

(1)   Dead, flattened cells filled with keratin and phospholipids (Fig 6-13)

(2)   Reduces water loss

(3)   Layers between germinal layer consist of cells differentiating and accumulating keratin

(4)   Forms footpads on feet and toes of many mammals

ii)       Hair and keratin derivatives

(1)   Functions

(a)    Affords mechanical protection (in furred species)

(b)   Entraps layer of air as insulation (in furred species)

(c)    Tactile (in humans)

(2)   Structure

(a)    Shaft – dead keratinized cells

(i)      Medulla – consists of shrunken, dead cells

(ii)    Cortex – thickened region that imparts strength

(iii)   Cuticular scales – outer cuticle (Fig 6-14)

(b)   Hair follicle – base of shaft is embedded in dermis (Fig 6-13)

(i)      Wall composed of epidermal cells

1.      Surrounded by fibrous dermal tissue with nerves

(c)    Root – enlarged base of follicle

(i)      Papilla – conical region in root that contains nerve endings and capillaries to nourish developing hair

1.      New cells formed by mitosis above papilla – added to base of hair shaft

(d)   Regional specialization

(i)      Life span ranges from a few months (armpits) to ~four years (scalp)

1.      Following a period of quiescence, mitosis resumes

a.       New shaft pushes out old hair

(3)   Color – results from pigment cells of cortex

(a)    White – absence of pigment

(4)   Movement

(a)    Arrector pili muscle – originates from upper layer of dermis (except in marine mammals)

(i)      Attached to one side of follicle

(ii)    Contraction pulls hair upright

(iii)   Controlled by sympathetic nervous system

1.      Stress response

2.      Temperature control – raised upon exposure to cold temperatures to increase thermal air insulation

(5)   Species specificity

(a)    Guard hairs – protect a denser coat of soft underhair (in furred species)

(i)      Pelage – fur formed by hairs

(b)   Vibrissae – long, tactile whiskers on snouts (cats, dogs, rats, etc.)

(c)    Hair is reduced in some hot-climate species (elephants, hippos, etc.)

(6)   Development

(a)    Unique derivative of epidermis

(b)   Develops from ingrowth of epidermis into dermis (Fig 6-15)

(i)      Hair did not evolve from scales (present with scales on tails of some rodents)

1.      Hairs may have evolved as tactile outgrowth between scales

(7)   Claws – laterally compressed, compacted, cornified projections at the end of each digit (Fig 6-16)

(a)    Present in most mammals, birds, reptiles

(8)   Nails – modified claws in primates

(a)    Tightly compacted, cornified epithelial cells on surface of fingers and toes

(i)      Nail matrix – forms new tissue at nail base

1.      Pushes old material to worn free edge

2.      Friction free zone

3.      Firm grip

(9)   Horns – used in defense and courtship (sheep and cattle)

(a)    Core of dermal bone covered by layer of keratinized cells

(i)      Develop from stratum germinativum (Fig 6-17)

(ii)    Not generally shed

(iii)   Grow and wear continually

(10)                       Antlers – used in defense and courtship (deer – don’t confuse with horns)

(a)    Usually found in males

(b)   Bony outgrowths of skull

(i)      Velvet covering during growth

(ii)    Reabsorbed at base of antlers – fall off after mating season

(11)                       Baleen plates – palate plates in large, toothless whales (Fig 6-18)

(a)    Composed of hairlike keratin fibers

iii)     Glands – multicellular cutaneous secretory structures

(1)   Sebaceous glands – branched alveolar glands (Figs 6-13 and 6-15)

(a)    Produce oily and waxy secretions

(b)   Holocrine – secretion by breakdown of producing cells

(c)    Develop as buds of hair follicles

(i)      Lubricate and waterproof hairs

(d)   Tarsal glands – open on edge of eyelids

(i)      Coat rim of eyelid so tears do not overflow

(2)   Sweat glands – coiled, tubular glands

(a)    Widely distributed in humans

(b)   Merocrine secretion – secretion across cell surface

(c)    Apocrine secretion – tip of cell ruptures during secretion

(i)      Develop as outgrowths from hair follicles

(ii)    Discharge secretions into follicles

(iii)   Located in armpits and genital areas of humans

(iv)  Secretions of organic molecules (body odor)

1.      Scent glands – modified apocrine glands that produce pheromones

a.       Used in mating and marking of territory

(v)    Wax glands – modified apocrine glands in ear

(d)   Eccrine sweat glands – invaginations from skin surface

(i)      Secrete watery solution onto sensory and frictional surfaces (footpad)

1.      Enhance tactile perception and frictional adhesion

(ii)    Secretions of water with some urea and salts

(e)    Involved in evaporative cooling in certain primates

(3)   Mammary glands – mixture of characteristics of sebaceous and apocrine sweat glands

(a)    Restricted to females in most mammals (except monotremes)

(b)   Mammary gland sebaceous characters

(i)      Complex alveolar structure

(ii)    Synthesize complex organic molecules

(iii)   Mature at puberty under the control of steroid hormones

(iv)  Lack motor innervation

(c)    Mammary gland apocrine characters

(i)      Myoepithelial cells

(ii)    Copious secretion

(d)   Develop from pair of continuous milk ridges that grow into dermis (Fig 6-19A-C)

(i)      Definitive glands may develop along all or part of ridge (from armpit to groin)

(e)    Contain adipose tissue and secretory cells (Fig 6-19D)

(i)      Secrete milk under hormonal control during lactation

1.      Contains water, carbohydrates, fat, protein, minerals, and antibodies

(f)     Ducts

(i)      Store milk

(ii)    Discharge into small depression in belly (monotremes)

1.      Young lap milk from hair (due to bills)

(iii)   Open into nipples (teats) – Fig 6-19E and F

1.      Teat – formed by elevation of a skin collar around large cistern

(g)    Necessitate a close relationship between young and mother

(h)    May have evolved as a neomorphic tissue (not from either sebaceous or apocrine sweat glands)

iv)     Vitamin D synthesis – the “sunshine” vitamin

(1)   Provitamin (7-dehydrocholesterol) in epidermal cells is converted to vitamin D under the influence of UV rays

(a)    Absorption of calcium from intesting

(b)   Deposition of calcium in bone

(2)   Rate of synthesis depends on skin pigmentation

(a)    Heavy pigmentation limits the amount of UV radiation penetrating skin

(i)      Tanning may be a mechanism to maintain a constant rate of vitamin D synthesis

b)      Dermis and subcutaneous tissue

i)        Dermal bone is uncommon in mammalian skin

(1)   Dermal bony elements of skull and pectoral girdle develop in deeper parts of skin

ii)       Extensive vascularization (Fig 6-20)

(1)   Important in thermoregulation and blood pressure control

(a)    Regulation of flow through dermal capillaries

(i)      Precapillary sphincters – present in arterioles

1.      Open to allow heat radiation from body to cause cooling

2.      Close to reduce blood flow and conserve heat

(ii)    Postcapillary sphincters – present in venules

(b)   Shunts and arteriovenous anastomoses regulate blood volume in other parts of the body (blood reservoir)

(i)      As blood pressure rises, arteriovenous shunts close and precapillary sphincters open

1.      Greater blood volume in extensive capillary beds

a.       Lowers blood pressure

(ii)    As blood pressure drops, blood is shunted away from skin

1.      Increases blood pressure

iii)     Subcutaneous tissue

(1)   Composed of bundles of connective tissue interspersed with fat (between dermis and muscles)

(a)    Provides insulation (in cold water feeding grounds)

(i)      Heat is lost in thermal windows in whales

(b)   Blubber in some marine animals

(i)      ~40% of body weight in large whales

(ii)    Streamlines body shape

(iii)   Provides buoyancy

(iv)  Fuel reserve (whales feed little during migration)

9)      Evolution of vertebrate skin (Fig 6-21)

a)      Primary dermal armor of bony plates

i)        Transformed into denticulated cosmoid scales (in extinct placoderms)

(1)   Evolution of teeth from scales in the mouth

b)      Dermal denticles of denticulate cosmoid scales transformed into placoid scales (Chondrichthyes)

i)        Adenticulate – lost tooth-like projections on scales (bony fishes)

(1)   Transformed into ganoid scales and on to cycloid and ctenoid scales (actinopterygian fishes)

c)      Dermal pectoral girdle – transformed original protective function of armor into locomotory function

d)      Feathers and hair – key evolutionary innovations

i)        Permitted radically new ways of life

ii)       Essential to endothermy

(1)   Homeothermy – high body temperature maintained at constant level

(a)    High metabolic activity necessary for flight and nocturnal domination of mammals

(i)      Nervous system functions more rapidly

(b)   Great stamina and aerobic capacity (muscle contractions)

iii)     Preadaptation – organ that originated in a context different from its eventual function

(1)   Feathers derived from reptilian scales

(a)    Originally functioned in thermoregulation (preadaptation for flight)

(2)   Hair originally functioned as tactile structure

(a)    Converted to devices for endothermy (preadaptation for thermoregulation)