The skin has two basic layers - the epidermis and dermis. The epidermis is made up of stratified squamous epithelium and contains melanocytes, Merkel cells, and Langerhans cells. It has five layers - stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The dermis lies below the epidermis and contains collagen, elastic fibers, and cutaneous structures. It has two layers - a thin papillary dermis and a thick reticular dermis. The skin develops from the ectoderm and mesoderm germ layers during embryogenesis.

1. SKIN - basic STRUCTURE
DR.P.NIKHILESH REDDY

2. introduction
 Skin is the largest organ of the human body
 Accounts for 16-20% of body weight…it weighs twice
as much as your brain
 For the average adult human, the skin has a surface
area of between 1.5-2.0 sq.mtrs
 The skin is composed of two basic layers (regions)..
◦ Epidermis – outermost layer
◦ Dermis –underlying connective tissue
 Subcutaneous fat (Hypodermis),inspite of its close
anatomic relationship and tendency to respond
jointly to pathologic processes,is not a part of skin
basic structure

3. EPIDERMIS
 Primarily made up of keratinized stratified squamous
epithelium(keratinocytes)
 Gives strength to the skin.
 Varies in thickness from thick skin to thin skin
 Eyelids- 0.04 mm,Palms- 1.6 mm,average 0.1 mm
 It does not have any vascularization, so it relies on
the connective tissues deep to it.
 Also contain melanocytes, merkel’s cells and
Langerhans cell

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)

6. Stratum Basale
 The stratum germinativum (or basal layer,
stratum basale) Consists of single layer of
basophilic columnar or cuboidal cells.
 Along with S. spinosum, it is a component of
Malpighian layer
 Cells are bound to each other by desmosomes
and to basal lamina by hemidesmosomes.
 All cells contain intermediate keratin filaments,
number of which increases as cells progress
upward.

7. 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.
 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.

8. 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.
 This sealing effect is first evolutionary
adaptation to terrestrial life

9. 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

10. 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
39 days(13+12+14) i.e.,time for a cell to move
from the stratum basale to the distal edge of the
stratum corneum and shed
 13 days for proliferative compartment( lower two
rows),12 days for differentiated compartment,14
days for cornified layer

11. 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) and around adnexa ( the periadnexal
dermis).The combination of papillary and periadnexal
dermis is called Adventitial dermis
2. A thick zone of Reticular dermis that extends from the
base of the papillary dermis to the surface of the
subcutaneous fat

12. 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
 Together,the papillary dermis and epidermis form a
morphologic and functional unit whose intimacy is
reflected in their alteration jointly in various
inflammatory processes
 A similar interrelationship exists b/w periadnexal layer
and its adjacent epithelium

13. 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.

14. 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
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 three plexuses – 2 in same position as for
arterial and another in the middle of the dermis

16. 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 skin and 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. An infection causes acne
2. Sweat (sudoriferous) 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.

17. hair
 Follicle- A hair follicle is a part of the skin that grows hair by
packing old cells together.
 Root
 Shaft
 Hair bulb
 Arrector pili -Arrectores pilorum (singular Arrector pili)
are tiny muscle fibers attached to each hair follicle, which
contract to make the hairs stand on end, causing goose
bumps. Arrectores pilorum are smooth muscle, not skeletal
muscle, which explains why humans cannot voluntarily give
themselves goose bumps.

18. nails
 Fingernails and toenails are made of a tough protein
called keratin. Along with hair and teeth they are an
appendage of the skin.
 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.

19. Embryology of skin
DR.P.NIKHILESH REDDY

20. 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:
 The prospective epidermis, originates from a
surface area of the early gastrula; ectoderm.
 The prospective mesoderm, which is brought into
contact with the inner surface of the epidermis.
The neural crest also makes contribution to the skin

21. Derivates of germinal layers
Ectoderm:
Epithelial structures like
 Epidermis
 Folliculo-sebaceous-apocrine units
 Eccrine units
 Nail units
& Merkel Cells ( From Primitive Ectodermal Cells From
Embryonic Epidermis)
Neuroectoderm:
 Melanocytes
 Nerves &
 Specialised sensory receptors

22. Mesoderm:
 Langerhans cells
 Macrophages
 Mast cells
 Fibrocytes
 Blood vessels
 Lymph vessels
 Muscles
 adipocytes

24. Development of epidermis
 In about the third week of fetal life, the
epidermis consists of a single layer of
undifferentiated, glycogen-filled, a single layer of
cells.
 Present only in prekeratinized, developing skin
sloughed to amniotic fluid.
The periderm:
In a 4- to 6-week-old fetus, two layers of cells can
be distinguished, the periderm or epitrichial
layer and a stratum germinativum ( basal
germinative epithelium)

26. Development of epidermis
EGA 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

28. Development of cells in epidermis
Melanocytes:
Derived from Neural Crest cells
8 weeks - Reach epidermis
4-6 Months - Become dendritic, synthesize &
transfer melanosomes
Langerhans cells :
Derived from Bone marrow ( Mesoderm)
In fetal life : yolk sac and/or Liver
6-7 weeks – Appear
12-14 wks - mature
Merkel cells :
Derived from neural crest ??
Evidence points towards origin from primitive
ectodermal cells within embryonic epidermis
12 wks – appear in plantar skin
16 wks – palmar skin
( as early as 9th week in Hair )

29. Development of dej
EGA EVENTS
Early Flat Interface
1st Trimester Basal Lamina
12 Wks Interface Undulated
End Of 12 Wks Mature DEJ ( As Viewed
Through An Electron
Microscope)
6 Months Dermal Papillae

30. development of dermis
 Intially, embryonic dermis comsists of stellate
mesenchymal cells suspended in acid muco substance
EGA EVENTS
12 weeks fibrocyte produce delicate collagen
bundles
16 weeks i. Mature collagen bundles
ii. Dermis with papillary (thin
collagen bundles) and Reticular
(thicker collagen bundles)
becomes recognizable
24 weeks Fibrocyte derived elastic fibres
appear interspread among collagen
bundles

31. Devp. Of blood vessels,cells of dermis and
sub cutaneous fat
EGA EVENTS
1st Trimester Dermal network of blood & lymph vessels
1st appears
2nd Trimester Mast cells and macrophages appear in
the dermis
Late 2nd
Trimester
Beneath the dermis, mesenchymal cells
surrounding BV begin to differentiate into
lipid filled primitive adipocytes,as a
consequence subcutaneous fat comes
into being
3rd Trimester Arborizing arterial and Venous plexuses

32. Develeopment of neural network
 Origin-ectoderm of neural crest
 5th week – detectable in the embryonic dermis
 In succeeding weeks, elaborate neural network develop
consisting of autonomic motor nerves that innervate
i. Blood vessels
ii. Hair erector muscles
iii. Eccrine & apocrine glands
iv. Somatic sensory nerves
v. Specialised sensory end organs ( pacini corpuscle,
meissner corpuscle,mucocutaneous end organs)

33. Development of adnexa
Folliculo-sebaceous-apocrine unit
 Hair follicle
 The earliest development of the hair rudiments occurs
at about 9 weeks in the regions of the eyebrow, upper
lip and chin.
 The bulk of the remaining follicles begin to develop at
approximately 4 to 5 months gestation in a cephalad-to-
caudad direction.
 By 17th week-first fine wisps of hair emerge from
ostia on the eyebrows and forehead and cover the
entire scalp by 18 weeks
 By 20 weeks,these lanugo hairs cover the whole
cutaneous surface,except for the palms,soles,terminal
phalanges of the digits,glans penis and labia minora

34. Follicular germ stage
condensation of mesenchymal cells just beneath the slight downgrowth or
“bud” of fetal basal keratinocytes.

35. Follicular peg stage, organization of keratinocytes
in the follicle and the mesenchyme of the follicular
sheath and follicular papilla located at the tip of
the follicle.

36. Bulbous hair peg stage,(near 16th week). Two prominent bulge
outgrowths
the uppermost becoms the sebaceous gland and the lowermost is
the insertion site of the arrector pili muscle as well as the
presumptive site of the hair follicle stem cells.
In many follicles, a third bud later appears above the sebaceous
gland; this is the rudiment of the apocrine gland.

38. Sebaceous glands
 The sebaceous glands become differentiated at 13-15
weeks, and are then large and functional.
 These are, at first, solid, hemispherical protuberances on
the posterior surfaces of the hair pegs.
 The cells contain moderate amounts of glycogen, but
soon the cells in the centre lose this, and become larger
and foamy as they accumulate droplets of lipid.
Apocrine glands
• Anlagen of apocrine glands probably develop in all hair
follicles, but after the fifth month,most begin to
regress,so that by term they persist in only a few sites
namely the axillae,areola and the periumbilical and
anogenital skin
• At 24 weeks,cord of cells which becomes coiled at its
base
• Although the apocrine secretory segment secretes a
milky fluid beginning at 7 months,apocrine glands are
dormant postnatally until they resume secretory function
around puberty

39. Eccrine glands
 In embryos of 12 weeks, the rudiments of eccrine
sweat glands are first identifiable as regularly spaced
undulations of the stratum germ.
 These start to develop on the palms and soles at
about 3 months, but not over the rest of the body
until the fifth month.
 Cells forming them lie palisading and closely together,
but otherwise they do not differ from the rest of the
stratum germinativum.
 By 14-15 weeks, the tips of the eccrine sweat-gland
rudiments have penetrated deeply into the dermis,
and have begun to form the coils

41. nails
 The nail apparatus develops during the 9th embryonic
week from the epidermis of the dorsal tip of the digit
as a rectangular area, the nail field.
 The proximal border of the nail field extends
downward and proximally into the dermis to form the
nail matrix primordium.
 at 13 weeks, four morphologic components are
recognizable in the epithelium of a developing nail
unit. They are the basal zone,the spinous zone,the
granular zone and the cornified zone.This region now
termed e[ithelium of nail bed, loses its granular zone
by the twentieth week
 At 14 weeks,cornified cells mature at the proximal
end of nail bed to form nail plate
 By 16 weeks,nail plate advances to cover proximal
half of the nail bed
 By 20th week,covers its completely at which time the
fetal nail resembles that of the adult

43. Mechanism that govern embryonal
development of skin
I. Mesenchyme Epithelial interaction
Can occur via direct cell to cell contact or diffusible macromolecules
 This interdependence is exemplified by embryogenesis of follicular
unit
 Epithelial unit will not develop from epidermis in absence of
mesenchymal papilla and conversely a follicular papilla will not
form in the absence of a covering epithelium
II. Stratification of epidermal cells is dependant on the
intactness of basal lamina
 Seen in re epithelisation of healing wounds
 The reconstitution of epidermis from keratinocytes of all
ectodermally derived epithelial structures of adnexa demonstrates
the pluripotentiality of adnexal keratinocytes

44. In conclusion
 The development & maintenance of skin depend on
interactions between epithelium and mesenchyme,between
generative epithelium cells & components of their basal
lamina,and of epithelial cells with one another.
 These interactions collectively result in a heterogenous but
unified structure i.e., skin with marked regional
differentiation in form,color,consistency
References
i. Samuel L. Moschella and Harry J. Hurley Dermatology 3/e
ii. Jean L Bolognia MD ,Joseph L Jorizzo MD ,Ronald P Rapini
MD Dermatology 3/e