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.