SKIN STRUCTURE OF THE EMBRYO
In the earliest days of fetal life, the epidermis is a single sheet of
cells, which is doubled between the fifth and sixth week .The outer layer,
is the periderm and the inner layer is the stratum germanitivum. The
stratum germinatum is responsible for the development of most epithelial
structures of the skin such as the basal cell layer, and eccrine sweat
glands. The primary epithelial germ cells give rise to the sebaceous
glands, apocrine glands and hair follicles.
Stratum Malpighi appears in the four months of fetal life.
STRUCTURE OF INFANT SKIN
The skin of babies has a smooth vulnerable appearance and the histologic
features are mainly like adult skin with some variations. The epidermis
has the same structure but the stratum corneum is less tough in infants.
The main differences include immaturity of collagen, hair follicles, and
sebaceous glands where these become more modified with increase of age.
The dermo-epidermal adhesion in babies is less than in adults. This
explains the stronger skin reactions in infants and young children in
response to certain stimuli such as in bullous papular urticaria due to
insect bites or other stimuli.
The heat regulation mechanism in infants differs than in adult skin. The
differences are mainly due to variations in area-to-body volume ratio of
skin ,vascular reactivity and the tendency of loosing heat by sweating
where, all of these can lead to more loss of heat in infants after minor
OF NEONATAL SKIN AND ITS APPENDAGES
A variety of skin lesions commonly seen in the newborn are regarded as
At birth the skin is covered with a whitish greasy film, which is the vernix
caseosa. The vernix may cover the entire skin surface or it may be
present only in body folds such as the groins. It normally dries rapidly
and starts to flake off within a few hours after birth.
Different color changes may
appear in the newborn. Golden yellow staining occurs in hemolytic diseases
of the newborn and post maturity. Bile pigments staining of the vernix and
muconium occurs in fetal distress.
Vivid color difference may appear along the midline at some time
during the first week of life. This phenomenon occurs when the baby is
lying on its side, the upper half of the body becomes pale while the lower
half has a deep red color with a sharp midline demarcation between the
(or acrocyanosis) is a feature particularly seen
in the full term newborn.
Generalized hyperemia is
usually known as erythema neonatorum, which fades spontaneously within few
Scaling of the skin of the
newborn occurs in most normal neonates. Superficial cutaneous desquamation
is often physiological.
One or two solitary blisters or erosions
may be present
at birth on the fingers, lips or forearms. These are due to vigorous
sucking in utero; hence the term sucking blister is often applied.
Sebaceous glands hyperplasia is
usually considered a physiological event in the newborn, reflecting the
influence of maternal androgens.
Multiple, uniform, pinpoint yellowish papules appear mainly on the nose,
cheeks, upper lip forehead, genitalia and the limbs. The lesion usually
clears spontaneously without treatment
Sweat ducts - papules may appear on the face and genitalia due to
temporary obstruction of the sweat ducts, which usually disappear during
the first weeks of life.
These changes are related to maternal and placental hormones.
Hair - lanugo hair covers the
skin of the newborn that may be pigmented. This usually sheds within few
The Female Genitalia - a mucoid
vaginal discharge is common in the female newborn. Vaginal epithelium
desquamates to leave a more normal infantile mucosa , this desquamation may
be accompanied by a creamy white discharge. Frank withdrawal bleeding may
occur from the uterus on the third or fourth day, usually lasting 2 - 3
The male genitalia appear large and well developed at birth.
Both sex show hypertrophy of the mammary glands at birth. After a few days
, the breasts may become engorged and show lactation ‘witch‘s milk‘.
The swelling subsides during the second week and usually becomes
undetectable by the end of the fourth week. In some girls, however, it may
be more persistent predisposing to infection due to stagnant milk, leading
to mastitis and abscess formation.
STRUCTURE OF ADULT HUMAN SKIN
The epidermis is purely a cellular structure without blood vessels or
The different layers
of the epidermis include : the stratum corneum, stratum granulosum,
stratum malpighii (prickle cell layer), stratum spinosum, (ret mucosum)
and the basal layer (stratum germinativum). In palmoplantar skin
there is an additional zone, the stratum lucidum between the stratum
granulosum and the stratum corneum.
The Stratum Corneum-
consists of several rows of flattened cornfield horny cells
containing a fatty or waxy material, which are non-nucleated cells.
The surface of the cells appears as thin, dry scales. The thickness
of this layer varies according to age and site. It is thickest on
palms and soles and thinnest on the eyelids.
Fig.1. Structure of Skin
Stratum Lucidium -
lies on the lower portion of the stratum corneum and consists of a
thin clear strip of glistening, translucent, flattened cells without
granules or nuclei. Stratum lucidum is present only in the skin of
palms and soles.
Fig. 2. Layers of the Epidermis
Stratum Granulosum (granular
layer) - lies on the top of the prickle cells and consists of rows of
flattened nucleated cells, which contain keratohyaline granules.
Fig. 3. The Epidermis
Stratum Malpighi (prickle cell
layer) - lies on the top of the basal layer and following its direction.
This is a very important layer and is involved in most of the pathological
conditions of the skin.
Stratum Germinativum (Basal
Layer) - is the deeper layer of the epidermis and is composed of a single
row of columnar cells arranged vertically to the basement membrane. The
cells of the basal layer become modified to form all other cells of the
The epidermis is maintained by the division of the germinate cells present
in the basal layer and differentiate later to keratinocytes in about seven
Epidermal cells are
composed of keratinocytes, melanocytes,mast , Langerhans cells and
As the keratinocytes move upwards reaching the granular layer of the
epidermis, loose their nuclei. Keratinocytes become flattened and heaped
on the skin surface as horny dead cells. The thickness of this layer
varies according to age and different body sites.
Vitamin A is found to have an important role in the development of
Fetal epidermis, with incomplete keratinization, has abundant glycogen,
which is present in the outer root sheath of active hair follicles.
Glycogen is found to increase after epidermal injury, which has a direct
relation to cellular metabolism.
Keratinocytes possess A and B blood group antigens and share with the red
blood cells the same antibodies that are absorbed selectively in some
Functions of keratinocytes
Synthesis of the fibrillar proteins giving the stratum corneum its
Secretion of a large number of cytokines, which have an important
effect on lymphocyte and granulocyte function.
Synthesis of a wide range of growth factors that have an important role
in wound healing. The factors controlling synthesis and secretion of these
factors may be important in the pathogenesis of skin disease.
Langerhans cells - have immunological functions, which can provide
traps for contact antigens and present them to T-cells.
Melanocytes - are dendritic cells derived from the neural crest and lie
on the basement membrane. Melanocytes synthesize melanin from
phenylalanine via tyrosine by series of reactions catalyzed initially by
tyrosinase giving the skin its color. Pigmentation is related more to
melanin synthesis than to the number of skin melanocytes.
The dermis is a dense fibrous layer beneath the epidermis. It is composed
mainly of fibrous tissue, elastic fibres and muscle fibres. The upper
surface of the dermis is the papillary layer that is a finger like
projection into the epidermis and contains the terminal capillaries and
sensory nerve endings. Dermoepidermal junction is undulating where ridges
of the epidermis, known as ret ridges, project into the dermis. The
junction provides mechanical support for the epidermis.
The entire surface of the
dermis is beset with numerous papillae which are more numerous in some
locations such as areola of the nipple and terminal phalanges. Most
inflammatory diseases of the skin involve the papillary dermis.
The deeper part of the
dermis is the reticular layer, where injury below that level leads to
scarring after healing.
The dermis contains and
supports blood vessels, lymph vessels, nerves, hair follicles, glands and
The cells of the dermis
are derived from the reticulum cell, the primitive mesenchymal cell. The
cells of the dermis include :
form the fibrous tissue and the matrix (ground substance).
Functions of fibroblasts:
Production of collagen
and elastic tissue.
Synthesize of mucopolysaccharides .
Metabolize cholesterol and steroids.
are part of the reticulo-endothelial system. Histiocytes are
large phagocytic cells either fixed to the interstitial tissues
or wandering .
are specialised cells that synthesize histamine and heparin.
Mast cells are numerous in the papillary layer of the epidermis,
in the beds of capillaries, in the reticular layer and in the
areolar tissue that surrounds the cutaneous appendages .
have an important role in the immune reactions.
cells: form Immunoglobulins
inhibit the action of histamine.
The dermis is composed
Collagen fibers: The
dermis is composed mainly of collagen fibers, which are synthesized by
fibroblast. The thickness of collagen depends on different factors such
as age, sex and body sites. The collagen layer is organized into a
smooth superficial layer under the epidermis at the level of papillae
and is coarse in the deeper layers. The collage fibers give the skin its
toughness and elasticity.
Elastic fibers: are
associated with the collagen fibers and both are surrounded by
The base of the dermis is a supporting matrix or ground substance which
is composed of polysaccharides and protein which are linked to produce
macromolecules with a remarkable capacity for holding water in their
domain. Within and associated with this matrix are two kinds of protein
fibers: collagen, which has a great tensile strength and forms the major
constituent of the dermis, and elastin, which forms only a small
proportion of the bulk.
The dermis contains the
specialized sensory organs and the skin appendages. Below the dermis is a
fatty layer known as the subcutaneous tissue.
BLOOD SUPPLY OF
The dermis has a very
rich blood supply, though no vessels pass the dermoepidermal junction.
depends upon dermal nerve network, which has special sensory nerve
The motor innervation of
the skin is autonomic. This includes a cholinergic component to the
eccrine sweat glands and adrenergic components to both the eccrine,
apocrine glands, to the smooth muscle, the blood vessels and to the
erector pilorum muscles.
The sensory nerve endings
are of several kinds: some are free, some terminate in the hair follicles
and others have expanded tips.
The special sensory nerve
endings in the dermis are of several different types mainly, the
Vater-Pacini corpuscles and the Meissner corpuscles.
Merkel‘s corpuscles are
found on the sides of the tongue. Krause‘s corpuscles are found on the
conjuctiva while Ruffins corpuscles are present on the soles.
Hair is present on all parts
of the human skin except the palms, soles, red portions of the lips,
ungual phalanges and glans penis.
Types of Hair
This is unpigmented and unmedullated hair appears on all
parts of the skin surface except areas which are usually
devoid of hair. These are vestigial hairs with large sebaceous
glands and lack the erector pilorum muscles.
Terminal hair. This type of
hair covers certain part of the body such as the scalp, eyebrows,
eyelashes, axilla, pubic area, the beard and moustache areas. The
hair that is covering these areas is pigmented and medullated.
Axillary, pubic, beard and body hair growth is under the control
of sex hormones.
Fig. 5. Pilosebaceous Structure
Structures of The
Hair papillae - this is
the lower end of the hair. It appears as a bulbous swelling known
as the hair bulb.
Hair root - this is the
intrafollicular portion of the hair is known as the hair matrix or
the hair root. Keratinocytes form the hair matrix.
Hair shaft - this is made
up of keratinized cells. It is composed of the hair cuticle or
sheath, the cortex beneath it and the medulla in the center.
The hair cuticle is the
outer layer. The inner root sheath is made up of the cuticle,
Huxley and Henle layer.
Fig. 6. Hair Structure
The outer root sheath
extends from the epidermis to the hair bulb.
The nails are convex,
translucent horny plates that are composed of:
The body - the
exposed part of the nail.
The free edge - the
anterior extension from the body.
Nail root - the
portion of the nail extending under the skin.
Nail lanula - the
whitish crescent near the base of the nail.
Nail bed - part of
the epidermis on which the nail lies.
Nail matrix - the
part of the nail below the nail root.
Nail folds - the folds
that bound the nail posteriorly and laterally.
Eponychium - the
crescent shaped thin membrane that stretches for a short distance over the
Hyponychium - the
horny layer under the free edge of the distal nail plate.
Fig. 7. Nail