Transcript INTEGUMENT - SKIN I. Skin - General considerations: A. Skin is an organ that fulfills and/or mediates many different functions.

INTEGUMENT - SKIN
I. Skin - General considerations:
A. Skin is an organ that fulfills and/or mediates
many different functions. (i.e. excretion,
temperature control, communication with external
environment, insulation, protection from abrasion
and some dissolved substances, retards
dehydration.)
Some people also find other uses for it.
B. It is the largest/heaviest single organ in
body (16% body wt.).
C. Consists of two major tissue layers
1. epidermis (ectoderm) - stratified squamous
epithelium
2. dermis (mesoderm) - connective tissue
http://www.vh.org/Providers/Textbooks/MicroscopicAnatomy/Section07/Plate07135.html
D. Thickness and some aspects of the
structure of these layers varies according to
whether we are dealing with thick or thin
skin.
1. Most of the body is covered with thin
skin.
http://www.leeds.ac.uk/chb/pcd1855/img0020.jpg
2. Parts of the body that are subject to
frequent abrasion such as the palms of the
hands and soles of the feet are covered
with thick skin.
http://www.udel.edu/Biology/Wags/
histopage/colorpage/cin/cin.htm
E. Where epidermis and dermis meet,
both are thrown into a series of folds that
interdigitate.
1. The dermal folds are called dermal papillae.
2. Epidermal folds are called epidermal
ridges.
F. We can also consider a third layer that is
not actually part of the skin, but that binds
dermis to underlying or subjacent tissues.
This is the subcutaneous layer or
hypodermis.
http://www.lab.anhb.uwa.edu.au/mb140/
http://www.medinfo.ufl.edu/year1/histo/images/h9h.jpg
STRUCTURE AND FUNCTION OF
SKIN LAYERS
I. Epidermis:
A. There are 4 cell types
1. Keratinocytes
2. Melanocytes
3. Langerhans cells
4. Merkel cells
http://medic.med.uth.tmc.edu/slnk/slnkimg/00000057.gif
KERATINOCYTES
B. First and most abundant cell type - cells
of stratified squamous keratinized
epithelium.
1. The cells of this epithelium become
keratinized and are called keratinocytes.
2. They form a protective outer layer that is
continuously shed and replaced from
below.
http://www.leeds.ac.uk/chb/pcd1855/img0018.jpg
3. The epidermis (keratinized stratified
squamous epithelium) can be divided into
a series of sub-layers.
Sub-layers of the epidermis. Starting at the
surface.
a. Stratum corneum
b. Stratum lucidum
c. Stratum granulosum
d. Stratum spinosum
e. Stratum basale = stratum germinativum
f. In thick skin such as palms of hands and
soles of feet, all layers can be easily identified.
In thin skin that covers rest of body the stratum
lucidum is not always apparent.
http://www.lab.anhb.uwa.edu.au/mb140/
4. Stratum basale = stratum germinativum
a. A basophilic, low columnar layer of cells (one
cell thick) which lies on basement membrane that
attaches it to the underlying dermis.
b. Desmosomes bind the lateral and upper
surfaces of these cells so they adhere strongly to
each other.
c. Hemi-desmosomes are found in the basal cell
membranes and are involved in binding cells to
the basal lamina.
e. There is lots of mitosis occurring in this layer
since it is responsible for providing cells to replace
those shed as the upper layer of keratinocytes is
worn away.
f. These cells, and particularly they’re progeny,
synthesize keratin that is stored as intermediate
filaments in their cytoplasm.
http://www.lab.anhb.uwa.edu.au/mb140/
5. Stratum spinosum
a. Comprises the largest portion of living cells
in the epidermis.
b. Cell shape varies - consists of cuboidal,
polygonal, or slightly flattened cells which
have a plasmalemma that is thrown into a
series of many short processes.
c. These processes are filled with
intermediate filaments and terminate with
desmosomes formed between processes of
adjacent cells.
d. The processes of these cells give them a
spiny appearance when viewed with the light
microscope. Thus the name "spinosum". Also
sometimes called the "prickly" layer.
http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/skinlsn
/stspin.htm
http://www.vh.org/Providers/Textbooks/MicroscopicAnatomy/
Section07/Plate07137.html
http://www.lab.anhb.uwa.edu.au/mb140/
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab14/EXAMPLES/Exspinos.htm
Psoriasis= skin disorder
where excessive cell
division leads to
increased thickening of
stratum basale and
spinosum
http://www.med.uiuc.edu/histo/large/atlas/image/tem40/2750a1.htm
d. These tightly bound processes are thought to impart resistance to abrasion to
the epidermis. This is supported by the fact that the stratum spinosum is thicker
in portions of skin subject to frequent abrasion such as palms and soles.
e. The basal portion of the spinosum, next to the stratum basale (germinativum),
contains additional mitosing cells. This germinative area is called the
MALPIGHIAN LAYER.
6. Stratum granulosum
a. This layer may be difficult to discern in thin skin
b. It consists of 3-5 layers of flattened
polygonal cells that are characterized by two
types of granules.
* keratohyalin granules - contain histidine
rich protein. No membrane around these
granules.
http://www.lab.anhb.uwa.edu.au/mb140/
* Membrane-coating lamellar granules (lamellar
granules)
** These granules are formed by the golgi bodies
and are membrane bound.
**They move to the plasmalemma closest to the
external environment and are exocytosed into the
intercellular space.
**The contents of these granules are
glycosaminoglycans and phospholipids.
**These substances help to cement cells together
and also act as a barrier to penetration of the skin
by foreign materials. (i.e. they seal the skin and
decrease its permeability - helps prevent
dehydration).
** First appeared in reptiles, not found in
amphibians. The evolutionary development of the
stratum granulosum was a critical event that
allowed the evolution of fully terrestrial life forms.
http://www.lab.anhb.uwa.edu.au/mb140/
7. Stratum lucidum
a. This layer is most apparent in thick skin. May not
be apparent in thin skin.
b. Transition region between living and dead cells.
It consists of a thin layer of extremely flattened
cells.
c. Organelles and nuclei are no longer evident in
the cytoplasm.
d. The cytoplasm is rich in densely packed keratin
filaments in an electron-dense matrix called eleidin.
http://www.lab.anhb.uwa.edu.au/mb140/
8. Stratum corneum
a. This is the outermost layer of the skin.
b. Consists of flattened, dead, non-nucleated, keratinized
cells.
c. The cytoplasm of these cells is filled with keratin
filaments embedded in matrix.
d. These keratinized cells are called horny cells.
e. The stratum corneum is very thick in areas subject to
frequent abrasion such as the palms and soles (e.g. 3mm
on soles). In thin skin, this layer is relatively thin and
delicate.
http://www.lab.anhb.uwa.edu.au/mb140/
9. Since the upper layer of skin is heavily
keratinized and the lower layer (stratum
germinativum) is not, it is clear that as cells of
the epidermis progress toward the external
surface, one of the things that is happening is
that higher and higher concentrations of keratin
are being synthesized and deposited in their
cytoplasm.
10. Also, since cells of the stratum corneum lack
organelles, something is occurring to rid the cells
of these structures.
a. So, what we find is that as keratinization proceeds,
more and more autophagosomes and other
lysosomes are present in the cytoplasm of epidermal
cells.
b. The autophagosomes are responsible for ridding
the cytoplasm of organelles. Other lysosomes provide
the enzymes necessary to digest these organelles.
http://www.vh.org/Providers/Textbooks/MicroscopicAnatomy/
Section07/Plate07136.html
C. Second skin cell type - Melanocytes pigment producing cells
1. Melanocytes are derived from neural crest
cells (which you'll recall are also involved in
the formation of the adrenyl medulla and tooth
dentin).
2. Cell bodies of melanocytes are found
between or beneath those of the stratum
basale and stratum spinosum.
3. They extend cytoplasmic processes
between cells of these two layers. These
processes end in invaginations of the cells that
comprise the stratum spinosum and stratum
basale.
4. Melanin is synthesized by melanocytes via
the action of an enzyme called tyrosinase on
the amino acid tyrosine.
http://www.leeds.ac.uk/chb/pcd1855/img0020.jpg
http://www.leeds.ac.uk/chb/pcd1855/img0023.jpg
5. Melanin synthesis occurs in vesicles formed by the Golgi body and consists of 4
stages.
a. Tyrosinase precursors are synthesized on ribosomes of the endoplasmic reticulum
and accumulate in vesicles formed by the Golgi body.
* Stage I vesicle - slight tyrosinase activity, so either the golgi packaged precursor is
undergoing further processing, or tyrosinase in being added to Golgi vesicles as these
vesicles fuse with each other.
* Stage II vesicle - vesicle contains intermediate filaments which melanin is deposited
on.
* Stage III vesicle - concentration of melanin increases obscuring the filaments.
* Stage IV vesicle - mature melanin granule, no filaments visible.
6. The melanin vesicles are transported into the processes of the melanocytes and are
transferred to epidermal cells in the stratum basale (germinativum) and stratum spinosum.
7. Pigmentation of the skin is due to accumulations of melanin in epidermal cells
(keratinocytes) rather the number of melanocytes present in the epidermis.
8. Thus, differences in skin color in people of the same race and of different races
are due mainly to differences in the concentration of melanin granules in epidermal
keratinocytes, not to differences in the number of melanocytes present.
9. As skin cells mature and become keratinized, melanin is enzymatically broken
down such that dead cells of the stratum corneum have lost their pigmentation by
time they are shed.
D. Third skin cell type - Langerhans cells
1. These cells are a specialized type of macrophage
found in the stratum spinosum
2. Also called histiocytes
3. Recent evidence indicates that these cells "entrap"
foreign antigens that enter the skin. Acts as an antigen
presenting cell.
4. Defining characteristic - Rod (racket) shaped
granules called Birbeck granules are present in
cytoplasm.
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=cell0509
http://sprojects.mmi.mcgill.ca/dermatology/immunology.htm#Langerhans
http://zappa.ultrakohl.com/Wsit97/Aug97/aug97p4.htm
E. Fourth type of skin cell - Merkel cells
1. Present in hair follicles, thick skin of palms and soles, and oral mucosa.
2. Large, oval, poorly stained
3. Unmyelinated axons penetrate the
stratum basale and terminate as expanded
disks on the base of Merkel cells - may
function in sensing both touch and
pressure.
4. Membrane bound granules resembling
neurosecretory vesicles suggest a possible
hormonal function for these cells.
http://cs.swau.edu/~durkin/biol101/lecture27/notes.html
II. Dermis
A. This is the connective tissue that supports epidermis and binds it to subjacent tissue
B. The dermis is composed of two layers:
1. Papillary layer - thin
2. Reticular layer - thick
C. Papillary layer
1. Composed of loose connective tissue, mostly fibroblasts, but also mast cells,
macrophages, and extravasated leukocytes.
2. This portion of the dermis interdigitates with the epidermis forming folds of the
dermis that are called dermal papillae.
a. Most papillae contain a capillary bed that provides
nutrients to overlying and surrounding cells.
b. Some papillae are "tactile papillae" and house sensory
structures for touch called Meissner's corpuscles.
* stacks of fibroblasts or modified Schwann cells with
unmyelinated axons zig-zagging between them (the axons
lose their myelin sheaths as they enter the corpuscle)
http://www.udel.edu/Biology/Wags/
histopage/colorpage/cin/cintsmc2.gif
http://medocs.ucdavis.edu/CHA/402/studyset/lab10/slide15.htm
3. From the papillary layer, collagen fibrils penetrate
the dermal papillae and insert into the basal lamina of
the epidermis, just beneath the stratum
basale/germinativum.
a. This binds the two layers together.
b. The collagen fibrils are called anchoring fibrils.
D. Reticular layer
1. This layer is thicker than the papillary layer and is
composed of dense irregular connective tissue that
contains collagen and elastic fibers.
a. Fibers are the major component.
b. There is a lower cell density in the reticular layer than
the papillary layer.
http://www.lab.anhb.uwa.edu.au/mb140/
2. Elastic fibers emerge from this layer and
penetrate the papillary layer.
a. As is the case with the anchoring fibers
mentioned above, the ends of the elastic
fibers penetrate the papillary layer and insert
in the basal lamina below the stratum basale
(germinativum).
b. This elastic network is responsible for the
elasticity of the skin.
http://www.lab.anhb.uwa.edu.au/mb140/
3. The functional cells of hair follicles, sweat glands, and oil glands are found
in the reticular layer of the dermis.
4. Pacinian corpuscles may be found in the
reticular layer.
a. Sensory structures for pressure and possibly
vibration.
b. Consist of concentric layers (lamellae)
composed of collagenous fibers with fibroblasts in
between.
c. An axon extends centrally into this onion like
arrangement.
d.The axon is unmyelinated within the corpuscle.
e. Thick dense connective tissue sheath encloses
the corpuscle.
E. While the epidermis contains no blood or lymph vessels, the dermis contains many.
IV. Underlying the skin (epidermis +
dermis) is a fatty, loose connective
tissue layer called the hypodermis.
This layer is not considered part of
the skin!
http://www.meddean.luc.edu/lumen/MedEd/Histo/HistoImages/hl6-11.jpg
V. Appendages of the skin
A. There are a number of structures that can be defined as
appendages of the skin:
1. Hair/hair follicle
2. Finger and toe nails
3. Sebaceous glands
4. Sweat glands
B. Hair follicles
1. The hair follicle, in a sense, is a deep
invagination of the epidermis that involves
the stratum basale and stratum spinosum
(sort of like an extra long epidermal ridge
specialized for hair production). It penetrates
into the reticular layer of the dermis and
“pushes” a portion of this layer into the
hypodermis.
2. The hair itself is essentially a specialized
form of the keratinized layers of the
epidermis.
http://www.lab.anhb.uwa.edu.au/mb140/
3. The hair follicle extends deep into the
reticular layer of the dermis and terminates
at a vascularized "dermal papilla".
* Note that this dermal papilla is close to the
base of the reticular layer and, in that sense,
is different from the "dermal papilla" that
forms as an interdigitation with the
epidermis in the papillary layer of the
dermis.
* The basal region of the follicle, including
the dermal papilla, is called the bulb.
* The epidermal cells covering the dermal
papilla in the lower half of the bulb are called
matrix cells and are responsible for
producing the shaft of the hair.
http://www.lab.anhb.uwa.edu.au/mb140/
2. The hair shaft itself, consists of 3
regions,
a.The Medulla
* 2-3 layers of vacuolated cuboidal cells
* present near base of hair, doe not
extend through entire shaft
b. The Cortex
* several layers of elongated, heavily
keratinized cells.
* Melanin is sequestered between or
within these cells
* These cells become more flattened as
you move toward the tip of the hair.
c. The Hair Cuticle
* a layer of clear, flat,
anucleate (dead) cells
* these cells overlap one and
other in a scale like fashion
3. The root of the follicle is surrounded by an inner sheath
consisting of 3 layers. These are,
a. The innermost layer (innermost cuticle)
* Flat overlapping cells adjacent to the cuticle of the hair
shaft
* The edges of the cells of the innermost layer
interdigitate with those of the hair cuticle forming a
robust bond between the hair and the follicle wall.
b. Huxley's layer - the middle layer
* Several layers of elongated cells
* Contain trichohyalin granules (a type of kerotohyalin
granule)
c. Henle's layer - the outermost layer
* Single layer of flat, clear cells that are highly
keratinized.
4. Henle's layer is surrounded by the
outer (or external) root sheath.
* This is continuous with the stratum
spinosum and stratum basale
* A thin, hyalin, modified basement
membrane called the glassy membrane,
surrounds the outer root sheath
* Outside the glassy membrane is a
poorly defined, connective tissue
investment.
http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cinhsl1.gif
http://www.udel.edu/Biology/Wags/histopage/colorpage/cin/cinhft3.GIF
READ ABOUT HAIR GROWTH, SWEAT GLANDS, AND SEBACEOUS GLANDS IN
YOUR TEXT.