4. SKIN
It is the largest single organ of the body
accounting for about 16% of total body weight
in adults and 1.2 – 2.3 m2
of surface
It is composed of:
• epidermis – an epithelial layer of ectodermal
origin
• dermis – a layer of connective tissue of
mesenchymal origin
Beneath the dermis lies the hypodermis (hypo=under, derma=skin)
= subcutaneus tissue, a loose connective tissue which may contain
adipose cells (panniculus adiposus)
The hypodermis is not part of the skin, but binds skin loosely to the subjacent tissues
5. SKIN
• Thick (glabrous, smooth or
nonhairy) found on the palms
and soles
• Thin (hairy) skin found in other
areas of the body
The designation „thick” or „thin” refers to the thickness of the epidermal layer
Total skin thickness also varies according to the site of the body
> 5 mm 1 – 2 mm
6. SKIN functions:
• The external layer of skin is relatively impermeable to water, it
prevents water loss by evaporation
• Skin is a receptor organ in continuous comunication with the
environment
• Skin protects the organism from impact and friction injuries
• Melanin – a pigment produced and stored in the cells of epidermis
protects against the UV rays
7. SKIN functions:
• Glands of the skin, blood vessels, adipose tissue participate in:
thermoregulation,
body metabolism,
excretion of various substances
• Vitamin D3 is formed in the skin (from precursors) under the solar
irradiation
• Skin can expand and cover large areas in edema and pregnancy
8. Epidermis
Consists mainly of stratified squamous keratinized epithelium, but it
also contains three less abundant cell types:
• Melanocytes
• Langerhans cells
• Merkel`s cells
• The keratinizing epidermal cells are called keratinocytes
9. Epidermis - Stratum basale
Consists of five layers of keratin-producing cells (keratinocytes):
1. Stratum basale (stratum germinativum) consists of the single layer
of basophilic columnar cells resting on the basal lamina at the
dermal-epidermal junction
• Desmosomes bind the cells of this layer together in their lateral and
upper surfaces
• Hemidesmosomes bind these cells to the basal lamina
10. Epidermis - Stratum basale
• All cells in the stratum basale
contain intermediate keratin
filaments of about 10 nm in
diameter
• The filaments play an important
role in maitaining cohesion among
the cells and in resisting the
effects of abrasion
• As the cells progress upward, the
number of filaments increases
until they represent half the total
protein in the stratum corneum
11. Epidermis
Keratin contains at least six different polypeptides with molecular mass
ranging from 40 to 70 kDa
Three polypeptide chains coil around one another to form subunits of
the tonofilament
Nine of three-chain subunits coil around each other, forming a filament
of about 10 nm in diameter
End-to-end aggregation of three-chain subunits increases the length of
the tonofilament
The composition of tonofilaments changes as epidermal cells
differentiate
Basal cells contain polypeptides of lower molecular weight, whereas
more differentiated cells synthesize the higher-molecular-weight
polypeptides
12. Epidermis - Stratum basale
• This stratum is characterized by intense mitotic activity and is
responsible (in conjunction with the initial portion of the next layer)
for constant renewal of epidermal cells
• These two layers are called malpighian layer
• The human epidermis is renewed about every 15-30 days
13. Epidermis
2. Stratum spinosum consists of cuboidal or slightly flattened cells
with a central nucleus and a cytoplasm filled with bundles of keratin
filaments (tonofibrils)
• These filaments end at and insert into the cytoplasmic densities of
the desmosomes
• The cells of this layer are bound together by desmosomes that
punctuate the cell surface, giving a spine-studded appearance
14. Epidermis – Stratum spinosum
• The filaments play an important role in maitaining cohesion among the
cells and in resisting the effects of abrasion
• The epidermis of areas subjected to continuous friction and pressure
(such as the soles of the feet) has a thicker stratum spinosum with more
abundant tonofibrils and desmosomes
• The cytoplasm displays membrane-coating granules – lamellar bodies
containing lamellar disks which are formed by lipid bilayers
15. Epidermis
3. Stratum granulosum consists of 3-5 layers of flattened polygonal
cells whose cytoplasm is filled with coarse basophilic granules –
keratohyalin granules which are not surrounded by a membrane
These granules contain:
Profilaggrin
Involucrin
Small proline-rich protein
Loricrin
16. Epidermis – Stratum granulosum
• Lamellar bodies increase in number
• These granules fuse with the cell membrane and discharge their
contents (glycolipid – acylglucosylceramide) into the intercellular
spaces, where they are deposited in the form of sheets containing
lipid
• The function of this extruded material is similar to that
of an intercellular cement, it acts as a water barrier
and as a barrier to penetration by foreign materials
17. Epidermis
4. Stratum lucidum: present in thick skin, the stratum is a translucent,
thin layer of extremely flattened eosinophilic cells
The organelles and nucleus are no longer evident
The cytoplasm consists primarily of densely packed keratin filaments
embedded in an electron-dense matrix
Desmosomes are still evident between adjacent cells
18. Epidermis
5. Stratum corneum consists of 15-20 layers of flattened
nonnuceated keratinized cells whose cytoplasm is filled
with a filamentous (fibrillar) scleroprotein - keratin and
amorfous proteins, and have thickened plasma membranes
- they are called horny cells
Tonofilaments are packed together in a matrix contributed by the
keratohyalin granules
These cells are continuously shed at the surface of the stratum
corneum
19.
20. In psoriasis, a common skin disease, there is an increase in the number of
proliferating cells in the stratum basale and spinosum as well as decrease
in the cycle time of these cells. This results in greater epidermal thickness
and more rapid renewal of epidermis – 4-7 days instead of 15-30 days
21. Melanocytes
Melanocytes - neural crest derived cells
They have rounded cell bodies from which long irregular extensions branch
into the epidermis running between the cells of the strata basale and
spinosum
Melanocytes contain numerous, small mitochondria, a well-developed Golgi
complex, short cisternae of rough endoplasmic reticulum, intermediate
filaments
Melanocytes are not attached to the adjacent keratinocytes by
desmosomes, they are bound to the basal lamina by hemidesmosomes
22. Melanocytes
The color of the skin is the result of several factors,
the most important of which are:
• content of melanin and carotene
• the number of blood vessels in the dermis
• and the color of the blood flowing in them
Melanin is synthesized in the melanocytes, with tyrosinase playing an
important role in the process
The extensions terminate in
invaginations of keratinocytes
25. Langerhans cells
• They are bone marrow–derived macrophages (Dendritic cells) they
can bind, process and present antigens to T lymphocytes
• They play a significant role in immunological skin reactions
• Star-shaped cells present mainly in the stratum spinosum of the
epidermis, associating with keratinocytes through E-cadherin
27. Immunologic activity in the skin
SALT (Skin Associated Lymphoid Tissue)
• Keratinocytes of the stratum basale produce:
tymopoietin – promotes T cell`s terminal maturation in the skin;
interleukin-1
• T lymphocytes
• Langerhans cells
28. Merkel`s cells (neural crest derived cells)
• Merkel`s cells are generally present in the thick skin of palms and
soles
• Free nerve endings are present at the base of Merkel`s cells
• Merkel`s cells may act as sensory mechanoreceptors or may have
functions related to the diffuse neuroendocrine system
29. Dermis
• A basal lamina is found between the stratum germinativum and the
papillary layer of the dermis
• Abnormalities of the dermal-epidermal junction can lead to one type
of blistering disorder (pemphigoid)
30. • Dermis is the connective tissue that supports the epidermis and
binds it to the subcutaneous tissue (hypodermis)
• The thickness of the dermis varies according to region of the body
and reaches its max of 4 mm on the back
• The surface of the dermis is very irregular and has many projections
(dermal papillae) that interdigitate with projections of the epidermis
(epidermal ridges)
Dermis
31. Dermis
• Dermis contains two layers:
• Papillary layer and reticular layer
• Papillary layer is composed of loose connective tissue (fibroblasts,
fibrocytes, mast cells, macrophages, leukocytes)
• It constitutes the dermal papillae
• From this layer special collagen fibrils (type VII) insert into the basal
lamina and extend into the dermis. They bind the dermis to the
epidermis and are called anchoring fibrils
32. Dermis
• Reticular layer is thicker, composed of irregular dense connective
tissue, it has more fibers (mainly type I and III collagen) and fewer
cells than in the papillary layer
• The dermis contains a network of the elastic system fibers
(oxytalan, elaunin, elastic fibers)
• This elastic network is responsible for the elasticity of the skin
• The principal gycosaminoglycan is dermatan sulfate
34. • Hairs are keratinized structures
present on all surface of the body,
except on the thick skin of the palms
and soles
• Each hair consists of two parts: hair
follicle and hair shaft
• The hair follicle is a tubular
invagination of the epidermis and is
responsible for the growth of hair
• The hair bulb is the end portion of the
hair follicle
• Follicular bulb contains stem cells –
clonogenic keratinocytes –
that can migrate and regenerate the
hair shaft
• Dermal papilla is very well
vascularized connective tissue
• The hair follicle consists of external
root sheath and internal root sheath
36. Sebaceus glands
• Sebaceus gland is a holocrine saccular gland
present in the thin skin
• The secretory portion of the sebaceus gland
lies in the dermis, excretory duct opens into
the hair follicle (or directly on the skin surface
of the lips, the corner of the mouth, the glans
penis, the labia minora and mammary nipple)
• The secretory portion of the sebaceus gland
consists of groups of alveoli connected to the
excretory duct by a short ductule. Each
alveolus is lined by cells resembling
adipocytes with numerous small lipid droplets
• The excretory duct is lined by stratified
squamous epithelium continuous with the
external root sheath of the hair
• The oily secretion of the gland (sebum) is
released on the surface of the hair and the
epidermis
37.
38. Sweat glands
There are two types of sweat glands:
• Eccrine sweat glands
• „Apocrine” sweat glands
Both types of sweat glands are tubular glands and both release
their secretion by a merocrine process
Eccrine sweat glands:
• are present in the all regions of the skin
• are functional through a whole life
• participate in control of body temperature
The secretory portion of eccrine sweat glands is
convoluted tube composed of three cell types:
• clear, dark, myoepithelial cells
39. Clear cells are devoid of secretory granules
Their basal plasmalemma has numerous invaginations
characteristic of cells involved in transepithelial salt transport
Mucoid cells are pyramidal cells
Line most of the luminal surface of secretory portion
The fluid secreted by eccrine sweat glands is not viscous and contains little protein
Its main components are: water, sodium chloride, urea, ammonia, uric acid
After its release on the skin surface, sweat evaporates, cooling the surface
40. Sweat glands
„Apocrine” sweat glands:
• occur in the axilla, mons pubis, anal area,
ceruminous glands in the external auditory meatus
and the glands of Moll of the margin of the eyelids
• are functional after puberty
• are supplied by adrenergic nerves
The secretory portion of „apocrine”
sweat gland is located in the dermis
or in hypodermis
The excretory duct opens into the hair follicle
The glands produce a viscous secretion which is odorless
but may acquire odor as a result of bacterial decompositon
41. Mammary gland
• consist of 15-25 lobes of the compound
tubuloalveolar type whose function
is to secrete milk to nourish newborns
• The histologic structure of the mammary gland varies according to
sex, age and physiologic status
• Each lobe is separated from the others by dense connective tissue
and adipose tissue, and is embedded in intralobular loose
connective tissue
• Each lobe has its own excretory lactiferous duct which opens in the
nipple
42. Mammary gland
• During puberty breasts increase in size
and develop a prominent nipple
• Breast enlargement is a result of the accumulation of adipose tissue
and connective tissue with increased growth and branching of
lactiferous ducts (due to increase of ovarian estrogens)
• In adult woman each lobe is embeded in loose connective tissue;
a denser, less cellular connective tissue separates the lobes
• Near the opening of the nipple the lactiferous ducts dilate to form the
lactiferous sinuses – lined with stratified squamous epithelium at the
external openings
43. Mammary gland
Simple cuboidal epithelium covered by myoepithelial cells
Stratified columnar or cuboidal epithelium
stratified squamous epithelium
44. Mammary gland
• In the connective tissue surrounding the alveoli there are lymphocytes and
plasma cells, which population increases during pregnancy - toward the end
of pregnancy; it is responsible for the secretion of immunoglobulins (Ig A)
that confer passive immunity on the newborn
• The mammary glands undergo intense growth during pregnancy as a result
of the proliferation of alveoli at the ends of the terminal interlobular ducts
• Alveoli are spherical collections of epithelial cells which secrete milk
• A fat droplets and membrane-limited secretory vacuoles containing dense
aggregates of milk proteins are visible in apical part of cells
Caseins, α-lactalbumin, Ig A,
lactose
46. Epidermis
• After keratinization, the cells consist of only fibrillar and amorfous
proteins and thickened plasma membranes, they are called horny
cells
• These cells are continuously shed at the surface of the stratum
corneum
• The human epidermis is renewed about every 15-30 days
47. Melanocytes
• Eumelanin is dark brown pigment produced by the melanocytes, a
specialized cells of the epidermis found beneath or between the
cells of the stratum basale and in the hair follicles
• The pigment found in red hair is called pheomelanin and contains
cysteine as part of its structure
