INTEGUMENT (Skin) - includes skin covering surface of body and specialized
derivatives of skin (nails & claws, hair, sebaceous & sweat glands, teeth; in
nonmammals - feathers and scales)

FUNCTIONS:

1) Protection of underlying tissues
2) Prevents desiccation
3) Provides insulation (hair and feathers)
4) Increases heat loss by changes in hair or feather position or by increased evaporative
water loss (sweating)
5) Reception of tactile, thermal or pain stimuli
6) Removal of excess water and salts
7) Production of vitamin D
8) Respiration in amphibians

COMPOSITION: 2 layers

1) Epidermis = upper layer, ectodermal origin; specialized epithelium - in mammals
composed of stratified squamous, can be thick or thin depending on stresses on that
area of skin, can also be keratinized or nonkeratinized
2) Dermis = below epidermis, mesodermal origin, dense irregular CT, thicker than
epidermis; sensory endings (tactile, thermal, pain) found throughout
- Tela subcutanea (fascia) = underlies dermis, areolar to adipose CT, connects skin to
structures underneath

EPIDERMAL STRUCTURE

1) Epidermis conforms to irregularities in dermis (e.g., fingerprints) or fills in valleys
and is thin over dermal ridges (smooth skin)
2) Epidermis of thick skin (palms and soles) has 5 distinct layers. Epidermal cells are
continuously replaced as upper cells die and become keratinized. The 5 layers
are (bottom to top):

a) Stratum germinativum = single layer of low columnar cells, gives rise to
keratinocytes (epidermal cells that transform as they move up in the epidermis
eventually forming keratin "scales"); underlain by basal lamina to which it is attached
by hemidesmosomes; functions as the main mitotically active (germinative) layer
b) Stratum spinosum = several cells thick, polyhedral cells with interconnecting
cytoplasmic processes ("prickle cells") - contact between cells occurs via desmosomes
at process ends so no cytoplasmic continuity; tonofilaments (keratin) present in
cytoskeleton enable cells to withstand abrasion; mitotically active
c) Stratum granulosum = 3-5 cells thick, flattened polyhedral cells; nuclei lack
nucleolus (evidence for decreased cellular activity); cytoplasm contains keratohyalin
granules - involved in keratin formation and associated intimately wit
h tonofilaments
d) Stratum lucidum = 3-5 cells thick, pale-staining translucent layer; cells are dead
and tightly packed; nuclei indistinct or absent; keratohyalin present (secreted from
granules)
e) Stratum corneum = composed of clear, dead, scale-like cells without a nucleus;
keratohyalin abundant; cytoplasm replaced with keratin from tonofilaments mainly;
dead cells are continually sloughed off (desquamation)
SEE HANDOUT FOR REVIEW OF THE 5 LAYERS
- Keratohyalin is very basophilic, so basophilia increases away from basal lamina
- Nuclei become pycnotic (more compact, densely stained) away from basal lamina
- Stratum granulosum produces vitamin D (cholecalciferol) - requires UV irradiation of precursor molecule (7-dehydrocholesterol); Vitamin D increases intestinal Ca2+
absorption and is important in proper ossification/remodeling of bone

3) Thin Skin = all layers are present but reduced in thickness, stratum lucidum may be
absent; keratinization is less marked; hair is present in thin skin of mammals

- Thin skin pigmentation (or lack of it) is due to presence of melanin (pigment) in
epidermis, melanin is synthesized by melanocytes present in the upper layers of the
dermis or between cells of the stratum germinativum; precise details of secretion
not certain, but thought that melanin found in vesicles in cell processes of
melanocytes that become pinched off and taken into cells of stratum germinativum
by phagocytosis; Melanin is synthesized and released in response to UV irradiation -
leads to suntan and acts to protect cells from harmful UV irradiation

DERMAL STRUCTURE: 2 layers
1) Papillary Layer = next to epidermis, contains high vascularity
2) Reticular Layer = below #1
- Epidermis being an epithelium is avascular so dermis brings blood vessels into close
approximation in papillary layer
- Dermis is folded into papillae which extend into epidermis (papillary layer)
- Composed of dense irregular CT with collagen and elastic fibers, less dense in
papillary layer
Capillary
Dermal papilla
Stratum corneum
Epidermis
Str. germinativum
Dermis

HAIR FOLLICLES (SEE HANDOUT FOR REVIEW)

= develop as invagination of epidermis into dermis (same process
for other skin derivatives)
- Invagination continues passing through dermis and into subcutaneous tissue below
- Follicle surrounded by CT sheath from dermis and is closely associated with
sebaceous glands and arrector pili muscles (smooth muscle strands, act to elevate hair
and squeeze sebum into follicle)
- Structural Features:

1) External Root Sheath = simple tubular invagination of epidermis
2) Internal Root Sheath = lining of soft keratin, similar to that at surface of epidermis,
surrounds hair
3) Hair Matrix = proliferating mass of cells at base of follicle
4) Dermal Papilla = extends into hair matrix providing nutrients
5) Hair:

a) central core of keratin (medulla)
b) surrounding medulla is a layer of hard keratin (stronger form of keratin similar to
that in nails, claws, etc.) = cortex
c) Cuticle = outermost layer of hair; composed of thin, hard keratin; cells overlap like
shingles on a roof

SKIN GLANDS

1) Sebaceous Glands = simple alveolar gland; opens to hair follicle; secretes oily substance (sebum) that functions as a lubricant and moisturizer for hair and skin
- Secretion occurs by holocrine mechanism (cells die and become part of the secretion)

- Cells in the basal layer undergo mitosis, synthesize and accumulate lipids, become
displaced toward interior of gland and therefore are removed from capillaries in
dermis (source of nutrition) and die
- Sebaceous Gland secretion under hormonal control (sex hormones), become more
active at puberty. Hyperactivity leads to excessive sebum production, may become
impacted and rupture into dermis, leads to inflammatory response which results
in acne

2) Eccrine Sweat Glands = simple tubular glands with coiled secretory unit, generally
located beneath dermis in subcutaneous layer

- Merocrine secretion (exocytosis)
- Abundant in thick skin (sweaty palms)
- Myoepithelial cells surround secretory portion of gland
- Sweat composition = water, ions (Na+, K+, Cl-), waste products (urea, ammonia, lactic
acid)

3) Apocrine Sweat Glands = simple tubular glands that open into hair follicle, secrete
by merocrine mechanism (exocytosis)

- limited to armpits, breasts, pubic regions
- secrete only after puberty, secretion is a cloudy substance that becomes odiferous upon
bacterial degeneration

- thought that these glands served as distinctive recognition signals and sex attractants
in human ancestors
4) Ceruminous Glands = produce ear wax, these glands are considered modified sweat
glands
5) Meibomian Glands = secretion lubricates surface of eye, cornea

- located in upper eyelid
- merocrine secretion
- compound alveolar gland

6) Mammary Glands = will treat in detail with female reproductive system although
they are modified skin glands

NONMAMMAL VERTEBRATE SKIN GLANDS

1) Uropygial Gland = preen gland of birds (this is the only integumentary gland in
birds)

- located at base of tail on dorsal side, compound gland
- secretion contains oils and wax (lipids), functions to waterproof feathers and in
feather and bill maintenance

2) Femoral Gland (Reptiles) = located on undersurface of thigh in male lizards
- opens through short duct passing through a spike-like projection
- presumed to function in sexual attraction
3) Mucous and Poison Glands = found in amphibian skin

- Mucous glands serve as an aid in keeping skin moist, important for respiration
- Poison glands produce noxious substances involved as a defense mechanism against
predators

CUTANEOUS SENSORY RECEPTORS (SEE HANDOUT FOR REVIEW)

1) Free Nerve Endings = sensory endings in deep layers of epidermis and papillary
layer of dermis; responsive to pain, temperature, movement (associated with hair
follicles).
- Merkel Cell Complexes = Merkel cells are found in stratum germinativum,
particularly abundant in fingertips. Nucleus is oblong and usually parallel to basal
lamina. These cells contain granules of unknown function around the nucleus
and in cell processes. Merkel cells interact with free nerve endings to form a complex
that is thought to function in mechanoreception.
2) Encapsulated Nerve Endings: several types
a) Pacinian Corpuscle = distributed throughout dermis and subcutaneous layer;
responsive to pressure; structure is like an onion bulb, the nerve ending is surrounded
by concentric layers of flat cells (modified Schwann cells)
b) Meissner's Corpuscle = areas of high tactile receptivity (e.g., fingertips); responsive
to touch; corpuscle is ovoid and contains a stack of flattened cells (modified
Schwann cells) intertwined with nerve endings and surrounded by a CT capsule
c) Ruffini Corpuscle = present in deep dermis and subcutaneous layer; spindle-shaped
corpuscles; serve as mechanoreceptors (tension)
d) Krause End Bulbs = present in dermis; thick capsule with branched, coiled nerve
endings; may be responsive to tension or cold

To Lecture 14