The skin is composed of two main layers - the epidermis and dermis. The epidermis is made up of stratified squamous epithelium with the basal layer containing keratinocytes that divide and push outward. As they move upward through the granular and stratum corneum layers, the keratinocytes produce more keratin and eventually shed their nuclei. The dermis lies below and contains collagen, fibers and vasculature that provide strength and nutrients to the epidermis. The basement membrane separates the two layers and is important for structural integrity of the skin.

1. SKIN BASIC STRUCTURE
AND FUNCTION
I. INTRODUCTION
II. EPIDERMIS AND ITS LAYERS
III. EPIDERMAL CELLS
IV. BASEMENT MEMBRANE ZONE
V. DERMIS
VI. VESCULAR SUPPLY OF DERMIS
VII. ULTRA STRUCTURE OF DEJ
VIII. CUTANEOUS GLANDS, HAIR & NAIL
IX. FUNCTION OF SKIN

2. • Skin is the largest organ in the body.
• A 70 kg individual the Skin weighs over 5 kg
& surface area 2 meter square.
• The skin is composed of two basic layers
1. outermost layer- Epidermis
2. underlying connective tissue-Dermis
BELOW THE DERMIS Subcutaneous fatty layer is
Hypodermis

4. Layers of epidermis
• Stratum basale (the deepest layer)
• Stratum spinosum
• Stratum granulosum
• Stratum lucidum (only in thick skin)
• Stratum corneum (most superficial layer of
epidermis).

5. Layers of epidermis
Lowest BASAL LAYER
SPINOUS LAYER
GRANULAR LAYER
Upper most STRATUM CORNEUM
Epidermis formed by the division of cells in the basal layer.
which give rise to the spinous layer. This layer contains cells
that move outwards and progressively differentiate, forming
the granular layer and the stratum corneum.

6. EPIDERMIS :-
• Epidermis derived from ectoderm
• The epidermis is mainly composed of
keratinocytes ,
• Thickness is 0.05–0.1 mm .
• Thickest on plams and soles (approx-1.5mm)
• Thinnest on eyelids & scrotum (approx-
0.1mm)
• The cellular progression from the basal layer
to the skin surface takes about 30 days but
is accelerated in diseases such as psoriasis.

7. DESMOSOMES
HEMI DESMOSOMES

8. Stratum Basale
• The stratum germinativum (basal layer) -
single layer of basophilic columnar or cuboidal
cells
• All cells contain intermediate keratin
filaments.
• Number of keratin filaments increases as cells
progress upward.

9. Stratum Spinosum
• Also contain the dividing cells as in basale.
Cells contain bundles of intermediate filament
(tonofilaments) projecting into the processses of
cells which give attachment to the desmosomes, so
giving spined appearance hence called Prickle Layer
• Tonofilaments provide resistant to the abrasion so
this layer is thicker in the areas prone to abrasion
(thick skin) .
• Keratinization begins in the stratum spinosum

10. stratum granulosum
• Consists of polygonal cells , cytoplasm of which
is filled with the basophilic granule ,keratohyaline
granules.
• It is rich in phosphorylated histidine and cystine.
Cells contain, lamellated bodies, made up of
lipid.
• It fuses with the cell membrane and it come out
of cells and function as a intercellular cement or
sealing agent.

11. Stratum Lucidum
• More prominent in thick skin .
• Cellular organells and nuclei are not
prominent.
• It is composed of clear non-nucleated cells.
• In the palms and soles, the stratum
lucidum is present.
• The tan colored protein blocks the underlying
melanocytes from view

12. Stratum corneum
• The main difference between thick skin and thin skin
relates to the thickness of the Stratum corneum.
• These are the dead cells, flaking off. The cells lose their
nucleus and fuse to form squamous sheets, which are
eventually shed from the surface (desquamation).
• The mean turnover or renewal time of epidermis is
28-30 Days i.e. time for a cell to move from the
stratum basale to the distal edge of the stratum
corneum and shed.

13. CELLS OF EPIDERMIS
• A. Keratinocyte
• B. Melanocyte
• C. Langerhans Cells
• D. Merkel Cells ( Haascheiben cells)

14. A. Keratinocyte
• The cells are named after its intermediate
filament called keratin which provides mechanical
strength to this cells ( strength is required for
resisting daily trauma and mechanical pressure
on the epidermis ).
• Keratinocytes devide and generate the upper
layers. Transit time / turnover time is the time it
takes for basal cells to reach the skin surface it is
28 days.
• More the need for strength, more the need for
keratin

15. SB
ss
SG
SC
14 DAYS
14 DAYS
28
DAYS

16. • Since SC require the maximum strength
maximum keratin is in SC and least amount of
keratin is in SB.
• This process of increasing the keartin in the
cells from SB upward to SC is called
keratinization of epidermis
• As function of SC is protection and it does not
divide, it sheds off its nucleus .

17. • Hence as the basal cells go up, gradually in 4 weeks the
nucleus is lost and hence, stratum corneum is non-
nucleated.
• 95% of all epidermal cells are keratinocytes.
• Orignal resident in epidermis incontrast, melanocyte,
langerhans cells and merkel’s cells are immigrants.
• There are changes in the keratin filaments (K) from
basal to upper layers –
K5 and K14 - basal layer
K1 and K10 – suprabasal layers
Mutation in the genes for keratins 5 and 14 are
associated with epidermolysis bullosa simplex – [ EBS ]

18. Keratin filaments join together in the upper
epidermis by Filaggrin.
Since upper layers require more strength than lower
layer, Filaggrin synthesis is in upper layer

19. Disease with abnormal keratinization
• A. Parakeratosis:- in psoriasis the basal layer
divides too fast

20. • In psoriasis the basal layer divides too fast. Hence
the basal layer reach the SC faster and donot
shed the nucleus
• Hence as nuclei are retained in psoriasis,
nucleated stratum corneum is seen on
histopathology this is called parakeratosis
• In the granular layer the keratohyaline granules
contain these filaggrin molecules (Hence granular
layer is called granular)

21. • B. Dyskeratosis :-
keratinization in the lower epidermis
normally keratinisation occurs in upper epidermis.
In Darier’s dis These abnormally overkeratinized cell
in lower layers are called Corps ronds .
Dyskeratosis is often associated with acantholysis
(sepration of cells in the epidermis with formation of
circular keratinocytes)

22. B. Melanocytes
• Present in basal layer
• Derived from the neural crest
• Produce Melanin
• Package it in organells (Melanosomes) and
Transfer it via finger-like processes called
Dendrites to Keratinocyes
• Each melanocyte transfer melanin to 36
keratinocytes
• Forming the epidermal-melanin unit [1:36]

23. Through these
Dendrites melanin
transfer to
keratinocytes
Keratinocytes
Dendritic
process
Melanocyte in basal layer

24. • Melanin protects cells form Uv light.
• They differ from keratinocytes by possessing
no desmosomes.
• Colour of skin depends on :-
1. Amount of melanin inside melanocytes
2. Number and size of melanosomes
3. Degree of transfer into keratinocytes

25. • It does not depend on the number of
melanocyte (all humans have the same
number of melanocytes)
Types of melanin :-
1. Eumelanin:- brown or black pigment
found in dark colored races.
2. Pheomelanin:- yellow-red in caucasian
skin

26. C. Langerhans Cells
• Derived from bone marrow
• Dendritic cells expressing
:-CD45, HLA-DR, CD 1C
• Role in adaptive immune responses in the skin
since they are antigen-presenting cells (APC)
Processing antigens and transporting them to
local lymph nodes
• Like melanocytes they are not connected to
adjacent keratinocyte by the desmosomes.
• Contain tennis racket shaped “Birbeck granules”

27. D. Merkel Cells ( Haascheiben cells)
• Seen amongst the basal keratinocytes. contain
desmosomes
• Derivation of this cell is controversial-evidence
supports both differentiation from epidermal
keratinocytes as well as migration from the
neural crest.
• Slow adapting touch receptors.

28. Dermis
• It is connective tissue that support the epidermis and
attaches the epidermis to the hypodermis.
• Dermis is 15-40 times thicker than the epidermis
• Its surface consists of many ridges (dermal papillae) which
interdigitate with epidermal ridges.
• The dermis is also the area where all the glands of the
body are located.
• Has 2 layers/compartments
1. A thin zone immediately beneath the epidermis the papillary
dermis
2. A thick zone of Reticular dermis that extends from the base of
the papillary dermis to the surface of the subcutaneous fat

29. Papillary dermis
• Papillary layer –The papillary dermis is the uppermost
layer of the dermis,composed of thin haphazardly
arranged collagen bundles,delicate branching elastic
fibers,numerous fibrocytes,abundant ground
substance.
• A highly developed microcirculation composed
of arterioles,capillaries and venules Its superior surface
is uneven (fingerlike projections) which forms the
characteristic fingerprint of the finger.
• This layer provides the epidermis with nutrients. Pain
and touch receptors are found here

30. Reticular dermis
• Dense irregular Connective Tissue Has thick bundles of
Collagen and coarse Elastic fibers.
• Proportionally, there are fewer fibrocytes and
blood vessels and less ground substance compared to
papillary dermis
• Arrangement of bundle in the direction of mechanical
force give rise to the cleavage lines of Langer.
• Strongest layer of the Dermis. Gives the area
strength.Contains sweat,sebaceous glands and
pressure receptors
• Leather is made of this layer.

31. HYPODERMIS
• Consists of loose connective tissue which
helps in sliding the skin over the deep
structure.
• Consists of layer of fat according to the
nutritional status of the person.
• Also called as superficial fascia or panniculus
adiposus

32. VESSELS IN SKIN
• Arteries form the 2 plexuses One at the
junction of papillary and reticular layer( sub-
papillary plexus) and another at junction of
dermis and hypodermis (cutaneous plexus).
• Veins form the 3 plexuses –
2 in same position as for arterial and another
in the middle of the dermis

33. BASEMENT MEMBRANE ZONE
• THE JUNCTION BETWEEN EPIDERMIS AND
DERMIS IS CALLED THE BASEMENT
MEMBRANE ZONE
• ULTRASTRUCTURALLY THIS ZONE IS
COMPOSED OF THREE COMPONENTS FROM
TOP TO BOTTOM

34. 1. BASAL KERATINOCYTE
2. DEJ
3. DERMAL CONNECTIVE TISSUE

35. ULTRA STRUCTURE OF DEJ
HEMIDESMOSOMES
LAMINA LUCIDA
LAMINA DENSA
DEJ
TYPE 4 COLLAGEN FIBER
ANCHORING FIBER
DERMAL
CONNETIVE TISSUE

36. • PLASMA MEMBRANE OF BASAL KERATINOCYTE WITH THE
SPECIALIZED ATTACHMENT PLATES = HEMIDESMOSOMES
• DE JUNCTION:- IT IS DEVIDED INTO THE
UPPER – LAMINA LUCIDA
LOWER –LAMINA DENSA
TYPE –IV COLLAGEN IS THE MAJOR COMPONENT OF LAMINA
DENSA
• ANCHORING FIBERS:-ATTACHES THE LAMINA DENSA ABOVE TO
THE UNDERLYING CONNECTIVE TISSUES
TYPE VII COLLAGEN IS THE MAJOR PROTEIN IN THE ANCHORING
FIBER

37. BASAL KERATINOCYTE
TYPE -4 COLL.FIBER
BP-1
BP-2LAMININ
DERMAL
CONNECTIVE TISSUE
ANCHORING FIBER
BP (anti body against BP-1)
BP,CP,LAD (anti-b against BP-2)
EBA (antibody against collagen 7)
EBD (absent collagen 7 since birth)
EBJ (Absent
laminin
since birth)

38. Cutaneous Glands
• 1. Sebaceous (oil) glands- Sebaceous glands
are microscopic glands in the skin which
secrete an oily matter, called sebum, In the
hair follicles to lubricate the hair.
• In humans, they are found in greatest
abundance on the face and scalp, though they
are distributed throughout all skin sites except
the palms and soles.
• infection causes acne

39. 2. Sweat glands –
• Sweat glands are exocrine glands, found in the skin , that are used
for body temperature regulation.
a) Eccrine glands -Eccrine glands (or merocrine glands) are
found at virtually all sites on the human body. They produce
clear liquid (perspiration), consisting of water, salts, and urea.
b) Apocrine glands- Apocrine glands are found in axillary and
genital areas, secrete a milky protein and fat substance. This
mixture is an excellent source of nutrients for bacteria which
produce body odour.

40. HAIR
• Follicle- A hair follicle is a part of the skin that
grows hair by packing old cells together

41. • Arrector pili -Arrectores pilorum (singular
Arrector pili) are tiny smooth muscle fibers
attached to each hair follicle, which contract
to make the hairs stand on end, causing goose
bumps.

42. Nails
• Fingernails and toenails are made of a tough
protein called keratin. Along with hair and
teeth they are an appendage of the skin.

43. • Free edge- The part of the nail that extends past
the finger, beyond the nail plate. There should
always be a free edge present to prevent
infections.
• Nail folds (cuticle)- A fold of hard skin
overlapping the base and sides of a fingernail or
toenail.
• Nail Matrix- This is the only living part of the nail.
It is situated behind and underneath the Nail Fold
and produces protein keratin which makes up the
Nail Plate

44. Embryology of skin

45. • The skin of the embryo begins to form during the first
20 to 30 days of embryonic life, the period of active
organogenesis in human development
• The skin arises by the juxtaposition of two major
embryological elements
1. The prospective epidermis, originates from a
surface area of the early gastrula; ectoderm.
2. The prospective mesoderm, which is brought into
contact with the inner surface of the epidermis.
• The neural crest also makes contribution to the skin

46. Development of epidermis
EMBRYONIC GESTATIONAL AGE EVENTS
3 Weeks Single Layer Of Flattened
Epithelial Cells
4 Weeks Basal Germinative Layer &
Periderm
3 Months Intermediate Cells
Tonofilaments-desmosomes
5 Months Keratohyaline Granules, signs
Of Cornification Starts
6 Months Cornification Completed
Term Increase In Thickness Of Cornified Layers

47. Physiological function of skin
Epidermis is a barrier to water loss and also barrier to
prevent entry of harmful substance like microbes and
UV light into the body .
Loss of this barrier will lead to increase of infection
and more loss of fluids from skin (in burn patient)
This barrier is created by polygonal shape of
keratinocytes and better sticking together by glue like
lipid secreted by keratinocytes.
It is compared to a brick and cement model where
bricks are synonymous with keratinocytes and cement
is the glue.

48. BRICK=KERATINOCYTE
CEMENT = INTER CELLULAR LIPID LAMELLAR BODY IN KERATINOCYTE
SYNTHESIZES LIPID AND EXPELS IT INTO
EXTRACELLULAR SPACE
LIPID INTO INTERCELLULAR
SPACE

49. BARRIER LOSS
• Barrier loss can either be due to problem in the
keratinocytes (brick damage) or problem in the glue
(cement collapse).
A. Problem in the keratinocytes (brick damage) :-
1.Damage to intercellular junction desmosomes
2. Primary destruction of the keratinocyte itself (cytolysis)
3. Problem in calcium pump

50. • B. Problem in the Glue (cement damage) :-
– Intercellular cement contain ceramide, Squalene, fatty
acids.
– Loss of intercellular glue cause eczema. The small gap
creates a barrier function loss. This Gap then fill up with
fluid this is called intercellular edema or spongiosis.
– Fluid can ooze out of skin clinically. These microscopic gap
also allows entry of organism in to skin [ Pt. with atopic
eczema have more infectious especially staphylococcal ].
– In eczema to replace this loss of ceramide moisturising
containing ceramide are often given to replace this
intercellular cement.

51. • Langerhans cells which serve as sentinel cells whose prime
function is to survey the epidermal environment and to initiate an
immune response against microbial threats.
• Melanin, which is mostly found in basal keratinocytes, also provides
some protection against DNA damage from ultraviolet radiation.
• Thermoregulation:-Vasodilatation or vasoconstriction of the blood
vessels in the deep or superficial plexuses helps regulate heat loss.
• Skin lubrication and waterproofing is provided by sebum secreted
from sebaceous glands.
• Subcutaneous fat has important roles in cushioning trauma aswell
as providing insulation and a calorie reserve.
• Endocrine function:- releasing the hormone leptin, which acts on
the hypothalamus to regulate hunger and energy metabolism
• Vitamin D synthesis from UV-B