Consists of the Skin and its derivatives
- hair - nails - glands - receptors (nerve endings)
Anatomy - consists of two principal parts
1. Epidermis - the outer thinner portion composed of stratified squamous epithelium
2. Dermis - the inner thicker portion composed of Dense Irregular Connective Tissue.
Below these two layers we find the subcutaneous region (also called hypodermis or
superficial fascia) which is composed of Areolar Connective Tissue and Adipose
1. Regulation of body temperature (T B) by:
- altering the flow of blood to the surface of the skin.
- perspiration. (evaporative heat loss).
2. Protection - acts as a physical barrier to abrasion, UV, bacteria, water loss.
3. Reception of stimuli - through nerve endings and specialized receptors
4. Excretion of water, salts, and organics
5. Synthesis of vitamin D from a precursor molecule produced in the liver and
modified in the kidney (calcitriol). Vitamin D is a hormone that stimulates the
absorption of calcium and phosphorus in the G.I. tract. The skin may be
considered an endocrine gland because it helps produce this hormone.
6. Immunity. The skin functions as an agent of nonspecific resistance to
pathogens (disease causing organisms).
7. Blood reservoir. 8% to 10% of the blood in an individual may exist in the
dermis at any one time.
- is composed of Stratified squamous epithelium
- is about 0.1mm thick on average - 1.2mm on soles of foot
- has 4 principle types of cells.
1. Keratinocytes make up about 90% of the cells in the epidermis
- they secrete a waterproof protein called keratin
- they are held rigidly together by desmosomes (anchoring junctions)
2. Melanocytes make up about 8% of the cell in the epidermis.
- as the drawing below illustrates they have lots of slender projections that
distribute melanin granules to kerinocytes.
The arms of the melanocyte reach upward between the keratinocytes. The melanin granules are
manufactured in the cytoplasm of the cell body, then are transported up the arms. The end of the arm
pinches off to form a melanin granule, which then migrates to a position above the nucleus of the
keratinocyte cell to protect it from ultraviolet light.
3. Langerhans cells arise in the bone marrow and migrate to skin where they function in immunity
to aid T cells.
4. Merkel cells interact with Merkels' discs in the function of touch.
Keratinocytes - make up five layers in the epidermis which is composed as follows:
1. Stratum Basale (stratum germinativum) - shown below.
2. Stratum Spinosum
3. Stratum Granulosum
4. Stratum Lucidum
5. Stratum Corneum - shown below
Photomicrograph of the epidermis of the skin (palm of hand)
Stratum Basale (Stratum Germinativum)
- single layer of cuboidal to columnar shaped cells.
- bonded to the basal lamina by numerous desmosomes
- cells in this layer exhibit intense mitotic activity
- the epidermis is renewed every 15 to 30 days
- all cells in the Stratum Basale contain filaments (cytokeratins). As the
cells progress upward the numbers of filaments increase until they represent,
in the stratum corneum, half of its total protein.
- contains Merkels' discs - receptors for touch.
Photomicrograph of the Stratum Basale of the epidermis (Palm of hand)
- 25 to 30 rows of flat dead cells filled with keratin.
- is a barrier against light, heat, bacteria and chemicals
- found in its thickest form on the palms of the hands and the soles of the feet.
Photomicrograph of the Stratum Corneum of the epidermis (Palm of hand)
- is composed mainly of dense irregular connective tissue
- is very thick in the palms of the hands and soles of the feet
- very thin in eyelids, penis, and scrotum
- blood vessels, nerves, glands, and hair follicle are found in this layer
Electron micrograph of the dermis of the skin. Note that the collagen fibers
run in all directions giving the dermis equal strength in all directions.
The Hypodermis (subcutaneous, superficial fascia) - consists of loose connective tissue
that binds the skin loosely to the underlying organs, and adipose connective tissue where a lot
of the bodies fat storage occurs. Pacinian corpuscles for sensing deep pressure are also
- elongated keratinized structures derived from an invagination of the epidermal epithelium (in
particular the Stratum Basale)
- found everywhere on the body except:
- the palms of the hands and the soles of the feet
- lips, glands penis, clitoris, labia minora
- face has 600 hairs cm2 the rest of the body about 60 hairs cm2.
- grow discontinuously and have periods of growth followed by periods of rest
- growth occurs in patches
growth of hair
- arises from the hair bulb above a dermal papilla that is highly vascularized, which is vital in
sustaining the hair follicle. The cells that make up the bulb are derived from the stratum basale
of the skin.
- hair color is due to the activity of melanocytes located between the papilla and the epithelial cells of
the hair root.
- dark hair is due to the presence of lots of true melanin
- red and blond hair is due to a melanin variant that contains more iron (red) or
- gray hair is due to loss of pigmentation
- white hair is due to air bubbles in the hair shaft
- there are several types of glands in the skin
1. Sebaceous glands (see image of hair follicle above
- most are connected to hair follicles
- those not connected to hair follicles open directly onto the surface of the skin
in these areas: lips, glands penis, labia minora, tarsal glands of eyes
- the secretion from these glands is sebum which:
- keeps the hair from drying out
- prevents water loss from the skin
- keeps the skin soft
- inhibits the growth of some bacteria
2. Sudoriferous glands
- sweat glands - two types
(1). Apocrine sweat glands
- found in the skin of axilla (armpit), pubic area, and areolae of breasts
- produce a viscous sweat
- begin at puberty
- contains sexual chemicals called pheromones
(2). Eccrine sweat glands
- 3000 inch2 in the palms of the hand
- perspiration is a mixture of water, salt, urea, uric acid, amino acids,
ammonia, sugar, lactic acid, and ascorbic acid
- mammary glands are modified sweat glands
Ceruminous glands – are modified sweat glands found in the External Auditory Meatus. They secrete
a wax called cerumen that traps dust particles.
Homeostasis of Body Temperature
- the skin acts like a radiator to dissipate heat out into the environment. It accomplishes this task
through the control of blood flow. If arteries to the skin are dilated, then blood will flow to the surface and
interact with the environment. If arteries are constricted then blood will not flow to the skin.
- temperature sensitive receptors (thermoreceptors) are found in the skin and deep within the core of the
- if the core thermoreceptors sense a change in the core temperature from 37 oC then the
hypothalamus of the brain is notified. The hypothalamus will then check the thermoreceptors in skin to
determine environmental temperature.
- if the core temperature is too high (above 37 oC) and the environmental temperature is low, then
the hypothalamus will dilate the arteries to the skin, and let blood flow to the surface of the skin, and
dissipate the heat into the environment.
- if the core temperature is too high (above 37 oC) and the environmental temperature is high also,
then the hypothalamus will dilate the arteries to the skin, and let the blood flow to the surface of the skin.
The hypothalamus will also cause the sudoriferous gland to secrete perspiration, which will evaporate on
the surface of the skin and dissipate the heat into the environment.
- if the core temperature is too low (below 37 oC) and the environmental temperature is high, then
the hypothalamus will dilate the arteries and allow the blood to flow to the surface of the skin where heat
can be absorbed from the environment.
- if the core temperature is too low and the environmental temperature is low, then the
hypothalamus will constrict the arteries to the skin, and prevent the flow of blood to the surface. The
hypothalamus will also initiate shivering and increase cellular metabolism.