Skin is the largest organ of the body and has two main layers - the epidermis and dermis. The epidermis is made up of stratified squamous epithelium with the stratum corneum being the outermost layer of dead skin cells. Below this is the dermis which contains collagen, fibroblasts and fat. Skin has appendages like hair, nails, sweat and sebaceous glands which help regulate body temperature and protect the skin.

2. SKIN ( INTEGUMENT )
Skin is the largest organ of the
body. It is not uniformly thick. At some
places, it is thick and in some places, it is
thin. The average thickness of the skin is
about 1-2 mm. In the sole, palm and in the
interscapular region, it is considerably
thick, measuring about 5 mm. In other
areas of the body, the skin is thin. It is
thinnest over eyelids and penis measuring
about 0.5 mm only.

3. Skin is made up of two layers :
1. Outer epidermis
2. Inner dermis
and sub - cutaneous loose connective tissue
contain lot of fat

4. EPIDERMIS
The epidermis is the outer layer of the skin.
It is formed by stratified epithelium, which
consists of five layers :
1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum germinativum

5. STRUCTURE OF EPIDERMIS

6. The important features of epidermis is that,
it does not have blood vessels. The nutrition
is provided to epidermis by the capillaries of
dermis.

7. STRATUM CORNEUM
It is the most superficial layer of the skin. It is
also known as horny layer. It is the outer most
layer and consists of dead cells, which are called
corneocytes. These cells lose their nucleus due to
pressure and become dead cells. The cytoplasm is
flattened with fibrous protein known as keratin.
Apart from this, these cells also contain
phospholipids and glycogen.

8. STRATUM LUCIDUM
It is made up of flattended epithelial
cells. It is 3 to 5 cell deep. Many cells have
degenerated nucleus and, in some cells, the
nucleus is absent. As these cells exhibit shiny
character, the layer looks like a homogeneous
translucent zone. So, this layer is called stratum
lucidum (Lucid = clear). The cytoplasm
contains eleidin, which is precursor of
keratin.

9. STRATUM GRANULOSUM
It is a thin layer with 2 - 5 rows of
flattened rhomboid cells. The cytoplasm
contains granules of a protein called
keratohyaline. The keratohyaline is the
precursor of keratin.

10. STRATUM SPINOSUM
This is a broad layer of variable
thickness and is made up of polyhedral cells.
The surface of these cells are apparently
covered with minute spine which interdigited
with similar spines of adjacent cells. It is also
known as prickle cell layer because the cells
of this layer possess same spine like
protoplasmic projections.

11. STRATUM GERMINATIVUM
It is a thick layer made up of polygonal
cells superficial and columnar or cuboidal
epithelial cells in the deeper parts. Here, new cells
are constantly formed by mitotic division. The
newly formed cells move continuously towards the
stratum corneum. The stem cells, which give rise
to new cells, are known as keratinocytes.
Another type of cells called melanocytes are
scattered between the keratinocytes. The
melanocytes produce the pigment called melanin.
The colour of the skin depends upon melanin.

12. From this layer, some
projections called rete ridges extend down up
to dermis. These projections provide
anchoring and nutritional function.

13. DERMIS
Dermis is the inner layer of the skin. It is a
connective tissue layer made up of dense and
stout collagen fibers, fibroblasts and histiocytes. The
collagen fibers exhibit elastic property and are
capable of storing or holding water. The collagen
fibers contain the enzyme collagenase, which is
responsible for wound healing. Dermis is made up
of 2 layers :
1. Superficial papillary layer
2. Deeper reticular layer

14. PAPILLARY LAYER
This layer projects into the epidermis. It
contain blood vessels, lymphatics and nerve fibers.
This layer also has some pigment containing cells
known as chromatophores.
Dermal papillae are finger-like projections
arising from the superficial papillary dermis. Each
papilla contains a plexus of capillaries and lymphatics
which are oriented perpendicular to the skin surface.
The papillae are surrounded by rete ridges extending
from the epidermis.

15. RETICULAR LAYER
Reticular layer is made up of reticular and
elastic fibers. These fibers are found around the hair
bulbs, sweat glands and sebaceous glands. The
reticular layer also contains mast cells, nerve endings,
lymphatics, epidermal appendages and fibroblasts.
Immediately below the dermis, subcutaneous
tissue is present. It is a loose connective tissue, which
connects the skin with the internal surfaces of the
body. It serves as an insulator to protect the body from
excessive heat and cold of the environment. Lot of
smooth muscles called arrector Pilli are also found in
skin around the hair follicles.

16. GLANDS OF THE SKIN
1. Sweat gland 2. Sebaceous gland

17. SWEAT GLAND
Sweat glands are simple, coiled, tubular
glands for the secretion of sweat. These are
found on all the body surface except margin
of lips, glans penis and inner surface of
prepuce.
Sweat glands are of two types :
1) Eccrine or, Merocrine sweat gland
2) Apocrine sweat gland

18. ECCRINE SWEAT GLAND

19. ECCRINE SWEAT GLAND
The eccrine sweat gland is a tubular coiled gland.
It consists of two parts-
A coiled portion lying deeper in dermis, which secretes
the sweat.
A duct portion, which passes through dermis and
epidermis and opens out through the sweat pore,
distributed over the entire skin surface.
The coiled portion is formed by single layer of
columnar or cuboidal epithelial cells, which are secretory
in nature. The epithelial cells are interposed by the
myoepithelial cells.

20. Secretory activity of eccrine glands
Eccrine glands function throughout life
since birth . These glands secrete a clear
watery sweat . The secretion increase during
increase in temperature and emotional
condition .
Eccrine gland play important role in
regulating the body temperature by secreting
sweat. Sweat contains water, sodium chloride,
urea and lactic acid.

21. Control of eccrine gland
Eccrine Glands are under nervous
control and are supplied by
sympathetic postganglionic nerve fiber,
which secret acetylcholine
(sympathetic cholinergic fibers).
Stimulation by these nerves causes
secretion of sweat.

22. APOCRINE SWEAT GLAND

23. APOCRINE SWEAT GLAND
The structure of apocrine glands is
similar to that of eccrine glands. The
coiled portion lies in deep dermis. But
the duct opens into the hair follicle
above the opening of the sebaceous
gland. Apocrine gland are situated only
in certain areas of the body like –Axilla,
Areola, Pubis, Umbilicus.

24. Secretory activity of apocrine gland
Apocrine sweat glands are non functional
till puberty and starts functioning only at
the time of puberty. In old age , the
function of these glands declines .
The secretion of the apocrine is viscid. At
the time of the secretion , it is odorless. It’s
acrid odor is due to bacterial activities. The
apocrine gland do not play any role in
temperature regulation like eccrine glands. .

25. Composition of Sweat
The average composition of sweat is 0.5% solid
and 99.5% water.
In solid –
Sodium chloride – 0.2% to 0.5%
Urea – 0.03%
Potassium – 0.02% to 0.12%
Calcium – 5mg%
Lactic acid – 0.07%
A trace of creatinine, amino acid, phosphorus and iron
are also present.
Sp.gr. – 1.002 to 1.003 pH – 3.8 to 6.5

26. Mechanism of Sweat secretion
Stimulation of hypothalamus by excess heat causes
sweating. The impulses from hypothalamus that cause
sweating are transmitted in the autonomic pathway to
the cord and then through the sympathetic out flow to
the skin everywhere in the body.
The sweat glands are stimulated by sympathetic
cholinergic nerve fibers. The sweat gland of the hands
and feet do have some adrenergic nerve fibers. These
glands can also be stimulated to some extent by
epinephrine or, nor-epinephrine circulating in the blood.
Local sweating in emotional state and sweating during
exercise are adrenergic activities.

27. A polypeptide, Bradykinin is believed to be liberated
during sweat secretion. It causes dilatation of the
cutaneous blood vessels and blood vessels supplying
the sweat gland.
When the rate of sweat secretion is very low, the
sodium and chloride concentration of the sweat are
also very low, because most of these ions reabsorbed
before it reaches the surface of the skin.
On the other hand, when the rate of secretion
becomes progressively greater, the rate of sodium
chloride reabsorption does not increase. So that then
their concentration in the sweat usually rise.
Sweating occurs at an environmental temperature of
29 C or more. Sweating occurs more quickly in hot and
humid weather. Approximately 500ml to 1000ml sweat
is secreted per hour in hot humid day.

28. SEBACEOUS GLAND

29. SEBACEOUS GLAND
The sebaceous glands are compound
alveolar glands, ovoid or spherical in
shape and are situated at the neck of
the hair follicle. Glands have connective
tissue capsule. The alveoli of the glands
are lined by stratified epithelial cells.
Sometimes sebaceous glands are
present but not communicate with hair
follicle in some area like
face, lips, nipples etc.

30. SEBUM
The sebaceous glands secrete an oily
substance called sebum. Sebum is formed
by the liquefaction of the alveolar cells and
poured out through the ducts either via
the hair follicle or directly into the
exterior. Sebum contains free fatty acids,
triglycerides, squalene, wax and paraffin.
It protect the skin from fungal and
bacterial infection. It also give smoothness
to skin and hairs.

31. Control of secretion of sebum
Sebaceous secretion is influenced by
gonad hormones, both testosterone in male
and progesterone in female. These
hormones affect the secretion by
affecting mitotic activities of the
secretory cells. This is the reason why
secretion of sebaceous glands increase
during puberty, second half of
menstruation and also during pregnancy.
Contraction of arrector pilli helps in the
expulsion of sebum.

32. APPENDAGES OF SKIN
NAILS HAIRS

33. APPENDAGES OF SKIN
Skin appendages are associated
with the skin and serve a particular
function. These are derived from
the skin, and are usually adjacent
to it.
In humans some of the more
common skin appendages are hairs
and nails.

34. NAILS
A nail is a horn-like
envelope covering the
dorsal aspect of the
terminal phalanges of
fingers and toes in
humans. Fingernails and
toenails are modified
stratum corneum made
of a tough protein called
keratin. Average rate of
growth in finger nails is
about 1mm per week.

35. STRUCTURE OF NAIL
 The nail plate or body of nail) is the actual nail,
and like hair and skin, made of translucent hard
keratin. In the nail it forms a strong flexible
material made of several layers of dead, flattened
cells. The plate appears pink because of the
underlying capillaries.
 The free margin (margo liber) or distal edge is
the anterior margin of the nail plate
corresponding to the abrasive or cutting edge of
the nail.
 The nail root is the base of the nail embedded
underneath the skin. It originates from the actively
growing tissue below, the matrix.

36. STRUCTURE OF NAIL
 The matrix or, nail bed is the skin beneath the nail
plate upon which the nail rests. The part of the nail
bed that extends beneath the nail root contains
nerves, lymph and blood vessels.
 The lunula (occasionally called simply "the moon")
is the visible part of the matrix, the whitish
crescent-shaped base of the visible nail. The lunula
is largest in the thumb and often absent in the
little finger.
 The furrow between the nail fold and nail bed is
called nail groove.

37. HAIR
 Hair is an elastic, keratinized and thread like structure
developed from the dermis.
The word "hair" often refers to two distinct structures:
• the part beneath the skin, called the hair follicle or
when pulled from the skin, called the bulb. This organ is
located in the dermis and maintains stem cells which re-
grow the hair after it falls out.
• the shaft, which is the hard filamentous part that
extends above the skin surface. A cross section of the
hair shaft may be divided roughly into three zones :

38. STRUCTURE OF HAIR

39.  Hair fibers have a structure consisting of
following layers from outside :
•the cuticle which consists of several layers of
flat, thin cells laid out overlapping one another
•the cortex, which contains the keratin
bundles in cell structures that remain roughly
rod-like
•the medulla is the central portion consisting
of two to three layers of cells. The cells are
cubical in the root and cornified in the shaft.

40. FUNCTIONS OF SKIN
Protective function – Intact skin protect our body
from air, water and bacteria. It gives a protective
covering to the delicate structures underneath it.
Nails and sensory function of the skin also helps in the
defense work. Melanin pigments offer protection
against ultra violet light.
Sensory function – Free nerve endings on the skin
are sensitive to pain, touch, heat and cold, resulting in
either voluntary or reflex activities.
Secretory function – Sweat help in temperature
regulation and sebum makes skin smooth.
Heat regulatory function – Sweating and cutaneous
blood flow help in temperature regulation.

41.  Excretory function – Through the secretion of
glands of the skin – water, salt, fatty substances
and urea are excreted.
 Synthetic function – Sun’s ultra violet rays help in
synthesis of natural vitamin D. skin can also
manufacture melanin pigment.
 Water balance – Skin serve a useful means in
regulating water balance of the body by
perspiration.
 Absorption – Skin surface can absorb water, oil etc.
 Storage function – Skin can store fat, salt, water,
glucose etc.
 Reflect nutritional status – The sign of
dehydration, vitamin deficiency, age and
malnutrition are manifested in the skin.