Skin (Integumentary System)

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Integumentary System:-


1. protection
mechanical
chemical
bacterial
UV
desiccation. 

2. temperature homeostasis sweat glands, flushing
arrector pili, pale. 

3.cretion
affects fluid & electrolyte balance
sweat glands release: water, salts, ammonia
oil glands release: lipids, acids. 

4. sensation
touch (light touch, wind, etc)
pressure
heat
cold
pain. 

5. synthesis
vitamin D precursor passes through capillaries
in skin and light converts it to vitamin D. 

6. nonverbal communication
eg. humans and other primates have much
more expressive faces than other animals.


Layers of Skin:

1) Epidermis
2) Dermis
3) Hypodermis

Epidermis

Stratified squamous epithelium
upper layers dead, filled with keratin (waxy protein)
lower layers living cells
replaced every 35-45 days
subdivided into 5 identifiable layers:

1) Stratum Basale:

Lowermost layer of epidermis
single cell layer thick
only cells that get adequate nutrition and oxygen by diffusion
from tissues below
actively dividing cells
bordered below by basement membrane

2)Stratum Spinosum:

Several layers thick
less mitosis
flattened, irregular, spine like projections. 

3)Stratum granulosum:

Very thin; 2-3 cell layers thick
as cells move up from s. basale they die & get flatter
and thinner
keratinization begins here. 

4)Stratum lucidum:

Thin translucent band
only found in thick areas of epidermis:
soles of feet
palms of hand. 

5)Stratum corneum:

Thickest of all layers; 3/4th ‘s the thickness of epidermis
20-30 cell layers thick
dead cells completely filled with keratin
water resistant
main protection against biological and chemical assault
takes keratinocytes 30-40 days from their formation in
s basale until they flake off of the s corneum. 

Cell of epidermis:-

1.Stem cells undifferentiated cells found only in deepest layer
2. Keratinocytes  most cells in epidermis synthesize keratin
3. Melanocytes also in deepest layer synthesize pigment
melanin
4. Merkel cells  touch receptors, attached to nerve cell
5. Dendrite cells(Langerhans cells)found in s spinosum
and s granulosum
are macrophages that stand guard against toxins,
microbes and other pathogens that penetrate the
skin‡ if detected they alert immune system. 

°°Dermis :-

Strong, flexible, connective tissue
gives skin its strength and resilience
gel-like matrix
contains collagen, elastic and reticular fibers
rich in nerves, receptors, blood vessels, lymph vessels
hair follicles and sweat glands extend into it
two layers:

a. Papillary layer:-

Mainly areolar connective tissue
lots of blood vessels
dermal papillae
capillary loops
sensory cells
produce finger prints. 

b. Reticular layer:-

Mainly dense (irregular connective tissue)
lots of collagen fibers
lines of cleavage between collagen bundles
tension lines
longitudinal in limbs
circular around trunk
incisions parallel to lines heal quicker. 

°°Subcutaneous Layer:-


Hypodermis or superficial fascia
below skin
mainly adipose tissue (ie subcutaneous fat)
insulation
infants and elderly have less of this than adults and are therefore more
sensitive to cold. 

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