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Skin- Embryology and Anatomy

By Edited Nov 13, 2013 0 0

During the third month of foetal life, three layers of cells are recognizable, the periderm,
the intermediate and the basal layer which is close to derma. The basal
cells multiply rapidly and keep pushing the older cells towards the
periderm and thus by the fifth month a stratum of these cells (prickle
cells) superficial to basal cells forms a definite stratum Malpighii.

End of second month of intrauterine life, the derma consists of closely packed,
spindle shaped mesenchymal cells and by the third month of intra-uterine
life, fine reticulum fibres are demonstrable, which later increase in
number and thickness and form the collagenous fibers. The elastic fibres
appear during the sixth month the foetal life.
The subcutaneous fat is apparent by the end of third month of intrauterine
life, but becomes abundant only during the later months of foetal
life. The nail starts as an epidermal specialization on the dorsum of the
tips if the digits by the third month of foetal life. Most of the sebaceous
glands in the body develop in connection with hair follicles during the
fifth month of foetal life.
The skin is composed of a superficial epithelial layer- The Epidermis
and underlying connective tissue layer, the Dermis (or) Corium. Beneath
the Corium is another connective tissue layer, rather loose in texture – the
‘Hypodermis’ (or) subcutaneous layer.
Epidermis:
The epidermis is formed of non-vascular stratified epithelium. Its
usual thickness is between 0.07mm and 0.12mm. The epidermis is mainly
two divisible they are keratinising (or) Malpighian system
(Keratinocytes) which produces the pigment.
There are seven layers in the epidermis.
1. Stratum Germinatum or Stratum Basale:
This is the deepest portion of the epidermis and is composed of
columnar cells placed perpendicular to the skin surface. The whole of the
epidemis germinates from this stratum, hence the name stratum
Germinatium.
2. Stratum Malpighii (on) the Prickle cell layer:
It is superficial to the basel cell layer and is composed of several
layers of polyhedral cells connected to each other by intercellular brides.
3. Stratum Granulosum:
It is composed of flat, fusiform cells which are one to three layers
thick. It is superficial to the Stratum Malpighii. These cells contain
irregular granules of Keratohyalin.
4. Stratum Lucidum:
It is pale and wavy looking layer, present superficial to the stratum
granulosum. This layer contains refractile droplets of eleidin.
5. Stratum Corneum:
This is the most superficial layer, the outer surface that is exposed
to the atmosphere. It consists of many layers of non-nucleated, flattened,
cornfield cells.
6. Dentritic cells of Epidermis:
These are melanocytes, Langerhan’s cells and indeterminate cells.
The cells of langerhan’s are found in the middle of epidermis.
7. Basal Lamina (Basement Membrane):
Dermal side of the basal lamina contains of few scattered collagen
fibres.
Dermis
The dermis is bounded distally by its junction with the epidermis
and proximally by the subcutaneous fat. The base of the dermis is a
supporting matrix (or) ground substance in which polysaccharides. The
matrix contains two kinds of proteins. They are,

  • Collagen - When has great tenscle strength
  • Elastin - Which has considerable eleasticity

Hair follicles, various types of sebaceous and sweat glands, plain
muscel fibres, and sensory end organs like pacinian and adipose tissue are
seen in the microscope section of the dermis.
The dermis contains few cells, which are fibroblast mast cells,
histocytes (or) macrophages, lymphocytes (or) other lencocytes and
melanocytes. In the deeper layer of dermis then in arterio venous
anastonomosis surrounded by sphincter group of smooth muscles under
autonomic control.
Sebaceous glands
They are situated in the upper half of the Corium. The Sebaceous
glands are derived from the epithelial cells of the hair follicle and present
everywhere in the skin except on the palms and soles.
They are multi lobulated and covered by a connective tissue
capsule within which is a layer of small epithelial cells. As these cells
mature towards the center of the lobules and they elarge, their cytoplasm
becoming arranged in a delicate network surrounding globular of fat
(Sebum).
Sweat glands
These are found in all areas of the skin. The sweat glands originate
as down growths from the epidermis. They consist of a single unbranced
tube which terminates in the form of a coil in the mid-corium.
The coil is the secretary segment and is lined by a single layer of
epithelial cells. The duct runs straight upwards from it to the epidermis,
which it transverses in a corkscrew meshwork of horn cells.
Apocrine glands
They occur in the axillae, areola and nipples of breasts, umbilicus,
around the anus and the genitalia. The myo-epithelial cells are highly
developed and more abundant in these glands. They are specified sweat
glands and their secretion is odoriferous with a secondary sexual
significance.
Hair
Hair is found on almost every part of the body surface except on
the palms and the soles, the dorsal surface of the terminal phalanges, the
inner surface of the labia, the inner surface of the prepuce and glanspenis.
Hair growth development is under endocrine control.
Hair is made of hard keratin and is analogous to nail. It is formed
by the hair matrix, a layer of specified epidermal cells. Capping the
papilla, the two structures making up the hair bulb. Melanocytes are
present in the matrix and form the pigment of hair. The portion of a hair
below the surface of the scalp is known as the hair root. Above the
surface of the scalp the hair is composed of the medulla, cortex and
cuticle.
The medulla consists of seven rows of soft keratin, but is
discontinuous (or) even absent in most human hairs. The cortex is the
main structural component and is made up of tightly packed fusiform
keratinised cells.
Nails
These are semitransparent plates like structures, covering the dorsal
surfaces of the distal phalanges of the fingers and toes. The nails are
composed of many layers of flattened keratinised cells fused into a
homogenous mass. The arise from epidermis lining and invaginating of
skin at the base of the nail, this specialized epidermis known as the nail
matrix. The invagination of skin at the base of the nail is called the nail
fold. The anterior borer encourches upon the nail plate as a flattened
keratinous rim, the cuticle and forms a protective barrier against irritants
and infections.
Blood Vessels
The blood supply of the skin originates from the large number of
arteries forming anastamosis in the deepest part of cortex. From here
single vessels run upwards and form a second network in the upper cortex
finally termina; arteioles ascend into the papillae ending in capillary
loops, which drain into connecting venules. The blood is returned to the
large veins in the subcutaneous tissues.
Lymphatics
The skin contains a rich network on lymphatics which drain into a
few larger vessels in the hypodermis.
Nerve Supply
The nerve supply of the skin consists of a motor sympathetic
portion derived from the sympathetic ganglia. The sympathetic fibers
innervate the blood vessel, erector pilorum muscels and apocrine duct,
where the fibers are adnergic and cause contraction.


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