Anatomy and Physiology of the Skin ISRAEL G.M INTRODUCTION

• Interface between humans and their environment • Largest and heaviest organ in the body. • Weighs an abt 4 kg • Covers an area of 22 feet square. • A barrier; • Protecting from harsh external conditions • Preventing the loss of water. Embryology • The skin has a dual origin; 1. The epidermis develops from the surface ectoderm 2. Dermis develops from the underlying mesenchyme. Development of the Epidermis; • - Single layer of ectodermal cell covering the embryo divides during the 2nd mth forming PERIDERM. • - A 3rd intermediate zone is formed with a further proliferation of cells in the basal layer at 7th week. • - About the end of 4th mth, definitive epidermis formed. Embryology

Development of the dermis • Derived from the lateral plate mesoderm and the dermatomes from somites • Around the 3rd &4th mth, the corium forms many irregular papillary structures-dermal papillae which project upwards into the epidermis. • Dermal papillae contains capillary & sensory nerve end organ.

• The Sub corium gives rise to the hypodermis which contains fatty tissues

INTRODUCTION

The skin consist of three layers; The epidermis – outermost layer The dermis- Consist of connective tissues The subcutis/hypodermis – the fatty tissue layer, blood vessels, nerve ends EPIDERMIS- Contains no blood vessels. - Varies in thickness - < 0.1 mm on the eyelids • CROSS SECTION OF - Nearly 1 mm on the palms and THICKNESS SKIN soles

- As dead surface squames are shed, the thickness is kept constant by cellsdividing in the deepest layer.

- The journey from the basal layer to the surface takes about 60 days.

Layers of the epidermis. (left) Light microscopy and (right) electron micrograph. THE BASAL LAYER

• The basal layer – - Rests on a BM. - Single layer of columnar cells - basal surfaces fine processes and hemidesmosomesa nchoring them to the lamina densa of BM THE BASAL LAYER • In normal skin; • - some 30% of basal cells are preparing for division (growth fraction). • Following mitosis, a cell enters the G1 phase – synthesizes RNA and protein, – Grows in size. • DNA is synthesized (S phase) and chromosomal DNA is replicated. • G2 phase -- further growth occurs before mitosis (M). • DNA synthesis continues through the S and G2 phases – not during mitosis. THE BASAL LAYER

-G1 phase is then repeated

- one of the daughter cells moves into the suprabasal Layer having lost the capacity to divide, and synthesizes keratins.

- Some basal cells remain inactive in a so-called G0 phase

- May re-enter the cycle and resume proliferationcell cycle time - G1 phase takes 50–200 hour The spinous or prickle cell layer

Composed of keratinocytes. -Differentiating cells - Synthesising keratins -Larger than basal cells. -Attached to each other by; ◦ Small interlocking cytoplasmic processes ◦ Desmosomes ◦ Intercellular cement of glycoproteins ◦ lipoproteins.  Under the light microscope, the desmosomes look like ‘prickles’. Spinous or prickle cell layer CT

They contain desmoplakins, desmogleins and desmocollins.

- Autoantibodies to these proteins are found in pemphigus

- Responsible for the detachment of keratinocytes from one another leading to intraepidermal blister formation.

• Cytoplasmic continuity between keratinocytes occurs at gap junctions, specialized areas on opposing cell walls Granular layer • Differentiating cell • Consists of 2 or 3 three layers – flatter than the spinous layer cell – have more tonofibrils. • Contain large irregular basophilic granules of keratohyalin • keratohyalin granules proteins, include; – Involucrin – loricrin – profilaggrin.

• keratohyalin granules lost with upward migration

Other cells in the epidermis

• Other cells in the epidermis • Keratinocytes - 85% of cells in the Epidermis three other types of cell are also found there: melanocytes Langerhans cells Merkel cells.

Melanocytes

 Melanocytes are the only cells that can synthesize melanin.

 They migrate from the neural crest into the basal layer of the ectoderm where, in human embryos as early as 8th wk of gestation.

 Found in hair bulbs, the retina and pia arachnoid. Melanocytes

• Each dendritic melanocyte associates with a number of keratinocytes, forming an ‘epidermal melanin unit’ • The dendritic processes of melanocytes wind between the epidermal cells and end as discs in contact with them. • Their cytoplasm contains discrete organelles, the melanosomes, containing varying amounts of the pigment melanin The dermo-epidermal junction

• The interface between epidermis and dermis

• EM shows that the lamina densa (rich in type IV collagen) separated from the basal cells by an electron-lucent area, the lamina lucida.

• The plasma membrane of basal cells has hemidesmosomes • Hemidesmosome contains bullous pemphigoid antigens, collagen XVII and α6 β4 integrin.

• The lamina lucida contains the adhesive macromolecules, – laminin-1, laminin-5 and entactin. Anchoring fibrils (of type VII collagen), dermal microfibril bundles and single small collagen fibres (types I and III) The dermo-epidermal junction • The structures within the dermo-epidermal junction

– Provide mechanical support

– Encouraging the adhesion,

– Growth, differentiation and migration of the overlying basal cells, and

also act as a semipermeable filter

– Regulates the transfer of nutrients and cells from dermis to epidermis.

Dermis

-The dermis lies between the epidermis and the subcut -It supports the epidermis structurally and nutritionally. - Its thickness varies; greatest in the palms and soles and least in the eyelids and penis. -In old age, the dermis thins and loses its elasticity. - The dermis interdigitates with the epidermis DERMIS

- This interdigitation is responsible for the ridges seen most readily on the fingertips (as fingerprints). - Important in the adhesion between epidermis and dermis as it increases the area of contact between them. • Dermis has three components: – Cells – Fibres – Amorphous ground substance Cells of the dermis

• The main cells of the dermis are fibroblasts, numbers of resident and transitory mononuclear phagocytes, lymphocytes, Langerhans cells and mast cells. Other blood cells, e.g. polymorphs, are seen during inflammation

Hair

• Distinguishing features of mammals

• Distribution, function, density, and texture varies across

mammalian species.

• Humans are relatively hairless, with only the scalp, face,

pubis, and axillae being densely haired. HAIR

Each hair consists of a diagonally positioned shaft, root, and bulb

-The shaft ; visible, dead portion above the surface of the skin.

-The bulb; enlarged base of the root within the hair follicle.

- Hair develops from stratum basale cells within the bulb

- As the cells divide,away from nutrient,cellular death and keratinization occur.

- Hair grows at the rate of approximately 1 mm every 3 days. HAIR

- Men and women have about the same density of hair on their bodies

-Generally more obvious on men as a result of male hormones.

-Hairless areas: soles, lips, nipples, penis, and parts of the female genitalia HAIR

• Three layers can be observed in hair that is cut in cross section. • The inner medulla - composed of loosely surrounding the medulla consists of hardened, tightly packed cells. • A cuticle covers the cortex and forms the toughened outer layer of the hair. • Cells of the cuticle have serrated edges that give a hair a scaly appearance when observed under a dissecting scope HAIR HAIR

. The texture of hair is determined by the cross-sectional shape: straight hair is round in cross section wavy hair is oval kinky hair is flat. • Sebaceous glands and arrectores pilorum muscle are attached to the hair follicle • The arrectores pilorum muscles are involuntary, responding to thermal or psychological stimuli. • When they contract, the hair is pulled into a more vertical position, causing goose bumps. Types of Hair • Humans have three distinct kinds of hair:

• 1. Lanugo- is a fine, silky fetal hair that appears during the last trimester of development.

It is usually seen only on premature infants.

• 2. Vellus. Vellus is a short, fine hair that replaces lanugo.

– abundant in children and women just barely extending from the hair follicules.

• 3. Terminal hair. Terminal hair is coarse, pigmented (except in most elderly people), and sometimes curly. HAIR

• Angora hair is terminal hair that grows continuously. Found on the scalp and on the faces of mature males.

• Definitive hair is terminal hair that grows to a certain length and then stops

It is the most common type of hair.e.g Eyelashes, eyebrows, pubic, and axillary hair. NAIL

• The sides of the nail body are protected by a nail Fold

• The furrow between the sides and body is the nail groove.

• The free border of the nail extends over a thickened region of the stratum corneum called the hyponychium

• The root of the nail is attached at the base.

• An eponychium (cuticle) covers the hidden border of the nail.

• The eponychium frequently splits, causing a hangnail.

Glands

Although they originate in the epidermal layer, all are located in the dermis, where they are physically supported and receive nutrients.

• Glands of the skin are referred to as exocrine, because they are externally secreting glands that either release their secretions directly or through ducts. • The glands of the skin are of three basic types: • sebaceous • sudoriferous • ceruminous GLANDS Sebaceous Glands

• Commonly called oil glands

• Found every where except palm and sole.

• Numerous on head, chest, neck and back””seborheic areas””

• Forms part of pilo-sebaceous unit

• Sebaceous glands are prominent at birth, under the influence of maternal hormones, but atrophy soon after, and do not enlarge again until puberty.

• Enlargement of the glands and sebum production at puberty are stimulated by androgens.

• Growth hormone ,thyroid hormones also affect sebum production Sudoriferous Glands

• Commonly called sweat glands • Excrete sweat onto the surface of the skin. • Sweat is composed of water, salts, urea, and uric acid. • Sweat glands are most numerous on the palms, soles, axillary and pubic regions, and on the forehead. They are coiled and tubular and are of two types: eccrine and apocrine • Eccrine sweat glands are innervated by the sympathetic nervous system, but the neurotransmitter is acetylcholine • Apocrine sweat glands are found principally in the axillae and anogenital region • AG -active at puberty, and secretion is controlled by adrenergic nerve fibres. Sudoriferous Glands

• 1. Eccrine sweat glands – widely distributed over the body, especially on the forehead, back, palms, and soles. – formed before birth and function in evaporative cooling • 2. Apocrine sweat glands – much larger than the eccrine glands. – They are found in the axillary and pubic regions, where they secrete into hair follicles. – Apocrine glands are not functional until puberty, and their odoriferous secretion is thought to act as a sexual attractant. Sudoriferous Glands

– secrete milk during lactation – The breasts of the female reach their greatest development during childbearing years, under the stimulus of pituitary and ovarian hormones. – Good routine hygiene is very important for health and social reasons. – Washing away the dried residue of perspiration an sebum eliminates dirt. – Excessive bathing, however, can wash off the natural sebum and dry the skin, causing it to itch or crack. Ceruminous Glands

These specialized glands are found only in the external auditory canal – where they secrete cerumen Cerumen is a water and insect repellent, and also keeps the tympanic membrane pliable. Excessive amounts of cerumen may interfere with hearing. Blood vessels

• Although the skin consumes little oxygen, its abundant blood supply regulates body temperature. The blood vessels lie in two main horizontal layers • The deep plexus is just above the subcutaneous fat Blood vessels Blood vessels  its arterioles supply the sweat glands and hair  papillae.  The superficial plexus; ◦ papillary dermis ◦ arterioles from it become capillary loops in the dermal papillae.  An arteriole arising in the deep dermis supplies an inverted cone of tissue, with its base at the epidermis. ◦ The blood vessels in the skin are important in thermoregulation. ◦ Under sympathetic nervous control, arteriovenous anastamoses at the level of the deep plexus can shunt blood to the venous plexus at the expense of the capillary loops, thereby reducing surface heat loss by convection Cutaneous lymphatics

• Afferent lymphatics begin as blind-ended capillaries in the dermal papilla and pass to a superficial lymphatic plexus in the papillary dermis.

• There are also two deeper horizontal plexuses, and collecting lymphatics from the deeper one run with the veins in the superficial fascia. Nerves The skin is liberally supplied with an estimated one million nerve fibres.

Most are found in the face and extremities.

Their cell bodies lie in the dorsal root ganglia.

Both myelinated and non-myelinated fibres exist

Most free sensory nerves end in the dermis; however, a few non-myelinated nerve endings penetrate into the epidermis.

Some of these are associated with Merkel cells

Free nerve endings detect the potentially damaging stimuli of heat FUNCTION OF THE SKIN