TYPES OF
SKIN
Skin over entire body is basically similar in
structure, but slight variation is due to thickness, mechanical strength,
degree of keratinization, size and number of hairs, types of glands,
pigmentation,vascularity, innervation and other features.
On the basis of difference in structural and
functional properties there are two major classes of skin.
i) Hairy
skin (Hirsute) – Thin
ii) Hairless
skin (Glabrous) – Thick
i)
Thin hairy skin:
Constitute great majority of body’s covering while
thick hairless skin forming surfaces of palms of hands soles of feet and flexor
surfaces of digit. They also differ in thickness of both epidermal and dermal
component and presence or absence of hairs with glands and arrector pili
muscles.
ii)
Thick hairless skin:
Forms frictional surfaces for manipulation,
locomotion and requires extra strength and numerous sweat glands for cooling
purpose. Thin hairy skin is responsible for general cutaneous functions of body.
Minor areas of skin also have special features which do not fall into either of
major categories. Mucocutaneous junction of lips,outer rim of anal canal and
urethral opening each have a characteristic histology.
e.g. lips have delicate epidermis lacking glands or hairs.
The skin is the first line of defense
against the envioronmental agent and mirror or internal pathology.
v MICROSCOPIC
STRUCTURES OF SKIN
Layers of
Skin:
A)
Epidermis
B) Dermis
A) Epidermis
·
Epidermis is composed of Keratinized stratified
squamous
epithelium.
·
In this tissue there is continuous replacement of
cells, a mitotic
layer
is at the base replacing cells shed at the surface.
Numerous
types of cells are forming epidermis.
q
Cells full of protein keratin – Keratinocyte
q
Pigment forming cells – Melanocyte
q
Cells having phagocytic function – Langerhans cell
q
Neurally associated – Merkel cells
·
Epidermis is divided into number of strata
representing different stages of Keratinocyte maturation from deep to
superficial as follows.
1. Stratum Basale
2. Stratum Spinosum
3. Stratum Granulosum
4. Stratum Lucidum
5. Stratum Corneum
EPIDERMIS
LAYER
A)Epidermis :
First
three of these layers are metabolically active and together known as
‘Germinative Zone’ OR Malpighian layer. Superficial strata of cells achieving
terminal Kertinization constitute, Cornified Zone. Various synonym used for
different layer like,
Stratum
Germinativum for Stratum Basale
Stratum
Malpighii for Stratum Spinosum
Epidermal
appendages or Adnexa include Glands – Sebaceous, Sudorific, Apocrine,
Ceruminous tarsal, Mebomian gland, Glands of Moll, Hairs and Nails.
Epidermal Strata:
1)
Stratum Basale:
It is deepest portion of epidermis.It includes the single layer
of cell which lies in contact with basal lamina.Keratinocytes are formed in
this layer by division of stem cells, then move apically.
Structurally the cells of Stratum Basale
are heterogeneous. Comprises of stem cells,Keratinocytes in various stages of
maturation and non-Keratinocytes. The majorities of basal cells are columnar to
cuboidal and attached to basal Lamina by hemidesomosomes. Ultrastructurally
basal cells have many polyribosomes, many mitrochondria and nuclei basal cell
have many cytoskeletal filament include melanin granules are also present in
many of cells.
Whole
of epidermis germinates from this Stratum hence name ‘Stratum Germinativum’.
Any trauma to this layer results in scarring. While above the level of this
layer wound heal without scarring.
2) Stratum Spinosum
– Stratum
Malpighii /Prickle Cell Layer
It
is superficial to stratum basale It contains more mature Keratinocytes. Cell
packed closely and interdigitation by means of numerous projection and
indentation at their surface they are linked by intercellular bridges called
‘desmosomes’. Internally cell possess
large number of Keratin Filament bundles (Tonofibril or Tonofilament).
This stratum provides much tensile
strength and lucidity to the surface of integument. Cells of this layer tend to
shrink away form each other while preparing for histology so giving spiny
appearance hence name as stratum Spinosum or Prickle cell layer.
In the cytoplasm of cells there are
numerous polyribosomes, vacoules containing melanin (Melanosomes). Melanin is
derived from epidermal melanocyte numerous in deeper part of this strata.
3) Stratum Granulosum:
It is
superficial to Stratum Spinosum. It is composed of flat, fusiform cells which
are one to three layers thick. Cell contains irregular granules of Keratohyalin
and lysosomal enzymes and cystine rich proteins. It is also contains small fine
granular dense homogenous bodies and ellipitical membrane bounded vacuoles
known as membrane,Coating granules, lamellar bodies or odland’s bodies.
Odland’s
bodies take part in barrier function of epidermal permeability. Granular
contents of odland’s bodies are discharged into intercellular space below the
cornified cell layer and form an effective water proof barrier.
4) Stratum Lucidum:
It is
superficial to Stratum Granulosum. This layer is found only in thick, glabrous skin
and it represent poorly understood stage of Keratinization.
It is
pale, wavy-looking layer formed by many layers of flattened and closely packed
cells whose out lines have, become quite indistinct and nuclei have
disappeared.
Characteristically,
it stains more strongly than stratum corneum with acidic dyes it is more
refractile optically and often contains nuclear debris.
5)
Stratum Corneum:
It
is most superficial layer, the surface of which is exposed to the outer
atmosphere. It consists of closely packed layers of non nucleated flattened
cornified cells that are dead Keratinocytes. This layer becomes thicker with
the application of intermittent mechanical pressure due to which almost all the
cellular structure is lost.
Cells are compact and contain high
concentration of Keratin Filament embedded in protein Filaggrin, which is
derived from histidine rich protein. Cells of this layer are known as
‘Corneocytes’ having no organelles, no nuclei but only tonofibrils are seen
embedded in dense structureless matrix. Eventually a cell get disengage from
each other and become detached from epidermal surface process is knonwn as
‘desquamation’.
A) Dermis :
Dermis consists of irregular moderately
dense, soft connective tissue, cells and all dermal appendages.
Connective
tissue is formed by three main components:
1.
Collagen fibres
2.
Elastic fibres
3.
Ground substance
Dermis is profusely supplied by blood
vessels, lymphatic nerve cells and nonstriated myocytes occur in dermis as
‘arrector pili muscle’. Dermis provides
strength and elasticity to skin by virtue of numbers and arrangement of its
collagen fibres and elastic fibres. It also provides a compartment for blood
vessel, lymphatics, nerves and defensive cells. So it is vital for survival of
epidermis.
Dermis can be
divided into two distinct zones.
1.
Superficial
– Papillary layer
2.
Deeper
– Reticular layer
(1) Papillary
Layer:
It
is immediately deep to the epidermis and is specialized to provide mechanical
anchor, metabolic support and trophic preservation. This layer shows rich
network of sensory nerve endings and blood vessels. Superficial surface is marked by ‘numerous
papillae’ which interdigitate with recesses in the base of the epidermis and
form the dermo-epidermal junction at their interface. Papillae have round or
blunt apices which may be divided into several cusps. In thin skin, with little
mechanical stress and minimal sensitivity papillae are few and small, while in
thick skin, papillae are large closely aggregated and arranged in curved
parallel line, forming pattern of ridges and grooves typical of these surface.
2) Reticular Layer:
It merges with deep aspect of Papillary
Layer. Bundles of collagen fibres are thicker than papillary layer. This layer
is made up of Reticular and elastic fibers. These fibres are found around hair
bulbs, sweat gland and sebaceous gland. It also contain mast cell, nerve
ending, lymphatic, epidermal appendages and fibroblast.
Immediately below the dermis
subcutaneous loose connective tissue is present which connect skin with
internal structure of body known to be Hypodermis. Subcutaneous tissue serves
as a insulator to protect body from excessive heat and cold. Lot of smooth
muscle called ‘arrector pili’ are also found in skin around Hair follicles.
SKIN LAYERS
DERMIS
Papillary Layer Reticular Layer
Aa
GLANDS NAIL
HAIR
APPENDAGES OF SKIN
Appendages of skin
v Epidermal
Appendages:
Epidermal appendages are intradermal epithelial
structures lined with epithelial cells with potential for division and
differentiation.
These
include:
1)
Sebaceous Glands
2) Sweat
Glands
3)
Apocrine Glands
4)
Mammary Glands
5) Hair
Follicle
They often are found deep within dermis and in tha
face may even lie in the subcutaneous fat beneath the dermis.
1) Sebaceous
Glands :
§ Found over the entire surface of the body except the
palms, soles and dorsum of the feet.
§ These are simple or branched alvelor glands situated
in dermis of skin.
§ Largest and most concentrated in the face and scalp
where they are sites of origin of acne.
Functions –
To produce and secrete sebum, a group of
complex oils including tryglycerides and Lester fatty acids, breakdown
products, wax esters, squalene, choesterolesters and cholesterol.
Sebum lubricates the skin to protect against
friction and makes it more impervious to moisture.
2) Sweat
Glands: a)
Eccrine Glands
b) Apocrine Glands
a) Eccrine
Glands:
§
These are ordinary, small sized (0.3-0.4mm) sweat
glands which are distributed all over the skin except on nail bed, margin of
lips and glans penis.
§
Most concentrated in the palms and soles of feet
§
Each gland consists of coiled secretary intradermal
portion that connects to epidermis via a relatively straight distal duct.
§
These glands work through out of life.
§
Its secreation is clear and watery and it contains
water, Nacl, urea, lactic acid.
§
Secreation of gland regulates body temp and
emotional condition Secreation of glands are under sympathetic nervous control.
§
Eccrine glands are supply by sympathetic
cholinergic fibre.
b)
Aprocrine Gland:
·
These are specialized large size (3 to 5 mm) sweat
glands, occurs in axillae, areola and nipple of breast, umbilicus, around the
anus and the genitalia.
·
Similar in structure but not identical to eccrine
gland.
·
Apocrine sweat glands are active only in puberty.
Secretion gland is thick, milky and odoriferous with a secondary sexual
significance.
·
Glands are active only in emotional condition and
they are under harmonal control.
·
Apocrine glands are supplied by sympathetic
adrenergic fibres.
·
Mammary gland is considered a modified and highly
specialized type of apocrine gland.
3) Nails
(Ungues):
§ These are semi transparent, plate-like horny
structures, covering the dorsal surface of distal phalanges of fingers of toes.
§ Proximal edge of Nail is known as “Root of Nail”.
§ Visible portion of Nail which looks semi transparent
and red due to abundant vascular supply in the nail bed is called Nail plate.
§ Opaque and slightly whitish semi lunar portion of
Nail plate near its root is known as ‘Lunula’.
§ Surface of skin on which nail-rest is known as Nail
bed.
§ Fold of skin surrounding the lateral and proximal
borders of nail are known as lateral and posterior Nail Fold.
·
Nail develops from
matrix, which is deeper portion of proximal or posterior nail fold and
epithelium of proximal part of Nail bed.
·
Nail plate and proximal
nail fold are joined by thin cuticle called ‘Eponychium’ which make exterior
groove water proof.
4) Hair:
·
Complex structures formed
by Epidermis and Dermis.
·
Hairs are filamentous,
Keratinized structures present over almost the entire body surface except on
palm, soles, dorsal surface of terminal phalanges, inner surface of labia,
inner surface of glans penis and prepuce.
·
Sebaceous glands often
open into hair follicle rather than directly on the skin surface and the entire
complex is known as pilosebaceous unit.
·
The base of the hair
follicle or hair bulb lies deep within the Dermis and thae face may actually
lay down in the subcutaneous fat.This accounts for remarkable ability of the
face to epithelise even the deepest cutaneous wounds.
·
A band of smooth muscle,
the arrector pilli connect the deep portion of the follicle to the superficial
dermis .Contraction of this muscle is under control of sympathetic nervous
system, causes the follicle to assume a more vertical orientation .
Blood
supply of Skin:
Metabolic demands of skin are not to a
large extent so only few capillaries are present in its deeper layer of Dermis.
Whereas epidermis, glands, hair follicles has rich supply of dermal capillary
beds.
Blood enters the skin through small
arteries penetrating the reticular layer from its deep aspect, ramifying and
anastomosing in a sheet like plexus ‘Reticular Plexus’. Reticular plexus is
present at interface between dermis and superficial fascia. Some arterioles
from this plexus supplies to adipose tissue, cutaneous sweat gland and hair
follicle while other arterioles passes superficially and some arterioles
anastomose laterally. Papillary plexus is present at the junction of reticular
and papillary layer of dermis capillaries from this plexus loop into dermal
papillae and supplies to base of epidermis and lastly while to venous plexus.
Venous
capillary bed surrounding the glands and hairfollicle drain into these
plexuses. Vascularization of skin by this superficial and deep plexus help to
maintain skins natural heat conductivity and loss of heat from evaporation of
sweat, convection from cutaneous vessels is vital component of
thermoregulation.
Lymphatic drainage of Skin:
§
Lymphatic capillaries are numerous in the reticular
layer of dermis and drain into plexuses within this stratum and into wider
vessel of superficial fascia.
§
Lymphatic tend to be distended and larges one have
prominent valves; lymph flow through these to deeper lymphatics.
§
Lymphatic of skin are quite profuse and these
appears to be free anastomosis between vessels of all levels, so there is
interchange of lymph between adjacent areas of skin.
