THE SKIN PHYSIOLOGY
Lesson 1 (Slides 1-25)
Jesmin Akter
Lecturer
Department of Pharmacy
Bangladesh University
Skin is not a simple protective wrap for the body
but also a busy frontier between the organism
and the environment.
Total area ranges from 2500 cm2 at birth to 18000
cm2 in the adult and weight varies from 4.8 kg in
men to 3.2 kg in women.
Functions:
1. Controls loss of valuable fluid
2. Prevents the penetration of foreign materials
3. Cushions against mechanical shock
4. Regulates heat loss
5. Transduces incoming stimuli
6. Regulates blood pressure.
7. Limits the passage of chemicals into and out of the
body.
8. It expresses emotion.
9. It identifies individuals through the characteristics
particular to humans, e.g. color, hair, odor and
texture.
10. Protects against radiation.
11. Synthesis and metabolism
12. Contains body fluids and tissues.
Physiology of skin
Human skin comprises three tissue layers:
1. The stratified cellular epidermis
2. Dermis
3. The subcutaneous fat tissue.
Epidermis
The epidermis is the external layer of the skin,
varying in thickness from 0.16 mm on the
eyelids to 0.8 mm on the palms and soles. It is
stratified and keratinized.
It functions as a protective barrier against
bacteria, chemical irritant, allergens etc. It is
avascular. Free nerve endings are found in it.
Layers of Epidermis:
Stratum corneum (Horny layer)
Stratum lucidum
Stratum granulosum
Stratum spinosum (Prickle layer)
Stratum germinativum (Basal layer)
Continued……..
Stratum corneum composed of keratinized cells called
corneocytes. The cells are almost filled with keratins.
They have lost their nuclei and almost all of their
cytoplasmic organelles. The horny cells are continuously
shed from the skin surface.
Stratum lucidum is considered as a subdivision of the
horny layer and can be recognized only in palmar and
plantar skin.
Stratum granulosum contain numerous granules of a
material called keratohyalin.
Stratum spinosum or prickle cell layer is termed so
because of the numerous desmosomes or
Continued…………….
attachment plaques at their surfaces which give the
cells a shiny appearance.
Stratum germinativum or basal cell layer gives
rise to all keratinocytes. It is represented by a single
layer of cells which are mitotically active. The cells
are small and cuboidal or columnar. The cells have
large nuclei, ribosomes, mitochondria,
tonofilaments and also melanins.
Pigmentary system
Major determinant of skin color is a dark pigment
called melanin. It is the product of a special cell
called melanocyte.
Melanocytes are derived from the neural crest in the
embryo and migrate to many tissues of the body
including the basal layers of the epidermis. They
differ from other cells of the stratum basale by the
possession of dendritic processes by which they
transfer pigment to a group of keratinocytes and
thus the forming an ‘epidermal melanin unit’. They
have no desmosomes.
The characteristic feature of melanocytes is a special
cytoplasmic organelle known as ‘melanosome’ on
which the melanin is produced by the action of the
enzyme tyrosinase. Melanosomes are spherical
membrane-bound vesicle and arise in the zone of
Golgi apparatus.
Melanins are of two kinds –
Phaeomelanins :
Yellow or red , soluble in dilute alkali.
Eumelanins :
Black and insoluble in dilute alkali.
Formation of eumelanins
Upon exposure to UV radiation, DNA damage
triggers cytokines, growth factors and other
inflammatory factors to stimulate melanin
production.
Both are formed by the same initial steps which
involve oxidation of tyrosine to 3,4dihydroxyphenylalanine(DOPA) and its
dehydrogenation to DOPA quinone. After several
further steps indole-5,6- quinone is produced which
is polymerized to produce eumelanine.
Components of skin color
Skin colour has two components
-constitutive i.e. genetic and
- facultative i.e. environmental.
These factors impart various degrees of pigmentation in
different ethnic groups. The differences are in the amount
of melanin produced and not in the numbers of
melanocytes present. Pigmentation can be enhanced by
exposure to sun or by endocrine factors, e.g. in pregnancy
and also by certain hormones
Major function of melanin is to protect against solar
radiation . It may also be activated to a free radical state and
eliminate genetically damaged cells by phototoxic
mechanism.
Langerhans cells
This cells are similar in form to melanocytes but free
from pigment. They are capable of limited phagocytosis
and may be concerned with immune functions.
Dermis :
Dermis is a tough and resilient tissue and cushions the
body against mechanical injury and provides nutrients
to the epidermis and cutaneous appendages.
It consists of protein fibres and mucopolysaccharide. It
contains fibroblasts, mast cells, macrophage,
lymphocyte, leucocyte, melanocytes etc. It has blood
vessels, lymphatic and nervous system, hair follicles,
with its associated glands and the eccrine sweat glands.
Continued……………..
Collagen
It is the major fibrous constituent of dermis accounting
for 75% of the dry weight.
Elastin and reticulin
Elastic fibres make up about 4% of the dry weight. They
can be stretched by 100% or mpre but return to their
original length when stretch is removed.
Reticulin constitutes only 0.4% of the dermis and made up
with fine branching fibres.
Ground substances
This amorphous substances contain carbohydrates,
proteins and lipids mostly mucopolysaccharides.
Continued……………
Fibroblasts
Fibroblast is an actively secreting cell and secret
collagen.
Mast cells
The cells are characterized by a cytoplasm filled with
granules. They contain and release heparin and
histamine. Rupture of cells due to skin damage results in
release of granules. This histamine is responsible for
inflammation, irritation and other skin disorders.
Subcutaneous tissue:
It is a specialized fat tissue which acts as a cushion and
heat insulator.
Skin Appendages
 Sweat gland
 Sebaceous gland
 Hair follicle
 Nail
Sweat gland
Two types:
 Eccrine sweat gland
 Apocrine sweat gland
Continued………
Eccrine sweat gland:
 Most numerous
 Occurs over the majority of body surface chiefly on
palms, soles, forehead etc.
 Cylindrical spiral duct that extends from their visible
opening in the epidermis down into the deep dermis
where the duct becomes coiled and convoluted.
 Manufactures odorless sweat that rises up the duct.
 The glands respond to environmental temperature and
other stimuli. On palms and soles they increase friction.
 Secretion is watery.
 The secretion composed of electrolytes, urea, amino
acids, lipid etc.
Apocrine gland
 Tubular glands attached to hair follicle
 Most of them are found in the axillary, anogenital
areas and in the areola of the nipple.
 Become functional at puberty.
 Secretion of the gland is milky, viscous
 Secretion is composed of lipid, protein and
pheromones.
Sebaceous glands
 Secret sebum
 Occur throughout the body and normally
associated with hair follicles.
 Greatest concentrations are found on scalp, face,
upper chest and shoulders.
 None on palms and soles
 Holocrine glands
 Activity is under hormonal control
 Output increases during puberty.
 Composed of glycerides, free fatty acids, wax
esters, cholesterol etc.
Hair Follicle
 Tubular inpushings of the epidermis
 Produced by the keratinization of cells formed by division
in the matrix at the base of the follicle.
 There are about 120000 follicles on human scalp
 About 100 hairs are normally lost from the scalp each day.
Hair shaft
Sebaceous gland
Hair root
Hair bulb
Common disorders of the skin
 Hyperpigmentary disorders
Ephelides Lentigens
Moles
 Hypopigmentary disorder
Vitiligo
Continued……..
 Disorders of the sebaceous gland
Acne
 Disorders of the sweat gland
Miliaria
 Skin scaling disorders
Psoriasis
Dandruff
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