Anatomy & Function of the Skin pdf - D. Hadf

Anatomy & Histology of the Skin

D. Hadf

Learning objectives

By the end of this lecture; the student should be able to:

1.  List the components of the integumentary system, including their physical relationships.
2.  Specify the functions of the integumentary system.
3.  Describe the main features and functions of the epidermis and dermis.
4.  Explain the structure and function of the various skin appendages.

Largest organ in the body

Surface

 About 2 sq. meters

Thickness varies according to area:

 0.2-0.5 mm on eyelid & prepuce
 3-5 mm on palm & sole

Weight

 4-5 kg
 20 kg with hypodermis

Functions of the skin

 The most important function is protection:

o Serving as a barrier against infection, UV light & disease

  Helping to regulate body temperature
  Removing waste products from the body
  Vitamin D

synthesis

  Sensory organ
  Calorie reserve & heat insulation
  Beauty organ

Layers of skin

 Epidermis

 Dermis

 Subcutaneous fatty layer:



not part of skin

The Epidermis

o The epidermis is the outer most layer of the skin

o composed of layers keratinocytes, some undergo rapid mitosis

• Thin skin = four layers (strata) as in hairy skin

• Thick skin = five layers as in glabrous skin (palm & sole)

Histology of epidermis

Avascular stratified squamous epithelium

1- Keratinocytes: arranged in 5 layers

o  Stratum germinativum (basal cell layer)
o  Stratum spinosum (prickle cell layer)
o  Stratum granulosum (granular cell layer)

o Stratum lucidum: only in palm & sole
o stratum corneum(horny layer)- non-viable epidermis ----------

2- Dendritic cells: melanocytes, Langerhans's cells, merkel’s cells

The ultimate function of epidermis is to produce keratin

o As new cells are produced, they push older cells to the surface of the skin where they

become flattened, lose their cellular content & start making keratin

o It is a tough fibrous protein which forms the basic structure of hair, nail & skin
o Eventually the keratin producing cells(keratinocytes) die & form a tough, flexible,

waterproof covering of the surface of the body

o This is shed or washed away once every 14-28 days

Viable epidermis

Non-viable epidermis

1-Basal cell layer

o  Single layer, tall columnar cells, have nuclei & all organelles
o  Site of DNA synthesis & mitosis
o  Connected to each other by desmosomes & to basement membrane by

hemidesmosome

2-Prickle cell layer

5-20  layers,  polygonal,  nucleated,  cytoplasm
become full of keratin bundles that are attached
to  desmosomes:  which  are  small  interlocking
cytoplasmic processes  which are thickenings  on
the cell membrane of two opposing cell surfaces,
allowing  the  sliding  of  adjacent  cells  on  each
other  without  separation  upon  trauma,  links  are
so  strong  that  dead  cells  are  shed  in  sheets  not
individually.

The upper part of this layer contain lamellar granules (Odland’s bodies, keratinosomes) which
contain lipids & polysaccharides & their contents are discharged into the intercellular space at
the interface with granular layer

Forming the hydrophobic barrier

Granular cell layer

 3-10 layers, flattened cells, cytoplasm

full of basophilic keratohyaline granules

 Dissolution of nucleus & other cell

organelles

 Keratin filaments in large bundles

 Keratinosomes migrate to the periphery

of cells & discharge their lipid content

Horny cell layer

 Flattened cells arranged in vertical stacks

that have lost nuclei & cellular organelles

 Keratin filaments arranged into macro

fibers under influence of fillagrin

 Highly insoluble cornified envelope within

plasma membrane

 Desmosomes are lost

Epidermal cell cycle

 After reaching the surface, corneocytes are shed continuously being replaced by

newer cells from beneath

 The whole cell cycle takes around 4 weeks normally from basal layer to be shed at

the surface as a scale.

 This rate is accelerated in certain disease conditions such as psoriasis to be less than

1 week

Dendritic cells: 1- melanocytes

• Dendritic cells of neuronal origin localized

between basal cells at a rate of one in ten, fixed
in all races

• Contain melanosomes: specialized organelles

that  synthetize  melanin  from  tyrosine  under
action  of  tyrosinase  enzyme  then  transfer  it  to
surrounding  keratinocytes,  forming  epidermal-
melanin unit

 Melanosomes are responsible for the difference in normal skin color between races;

being more in no.

 Larger & more dispersed in darker skin

Question
what would happen to the skin if tyrosinase enzyme was deficient?

2- Langerhans's cell

• Dendritic cell of mesenchymal origin, localized in suprabasal layer

• By electron microscope show Birbeck granules in cytoplasm

• Antigen presenting cell in the skin: process antigens encountered on skin & present it

to local lymph nodes, thus have a key role in adaptive immune response.

3- Merkel’s cells:

• Dendritic cells localized between basal cells directly above basement membrane

• Associated with unmyelinated nerve endings & act as mechano-sensory receptors in

response to touch

4- Indeterminate cells:

They have the same ultrastructure of Langerhans’s cells but without Birbeck granules

Dermo-epidermal junction

In light microscope is one layer, actually it is 3
layers:

The upper part is formed by the basement
membrane of basal layer with its attached
hemidesmosomes

  Lamina Lucida
  Lamina densa
  Sub laminal fibrous band

The dermis

Dermis organization

Papillary layer

 Contains blood vessels,
 lymphatics, sensory nerves of epidermis

Reticular layer

 Contains network of collagen and elastic fibers to resist tension

Dermis

The dermis forms the main bulk of the skin, lies under the epidermis & supports it both
structurally and nutritionally, they interdigitate so that upward projections of the dermis (the
dermal papillae) interlock with downward ridges of the epidermis (the rete-pegs), this
increases the force of adhesion & the contact area.

Composition of dermis

Components of dermis:

 Cells: fibroblasts, mast cells, macrophages & all the cells in the blood

 Fibers: 80-85% is collagen mainly type I & III, the remainder is composed of elastic &

reticular fibers

 Ground substance: composed of glycosaminoglycan/ proteoglycan macromolecules,

they constitute 0.1-0.3% of the weight of dry dermis but are responsible for the

hydration of the dermis due to the high water binding capacity of hyaluronic acid.

60% of the weight of the dermis is water

Skin appendages: Hair

Hair types

1. Lanugo hair: intrauterine life, fine long hair

2. Vellus hair: peach fuzz; all over body, fine

short hair

3. Terminal hair: coarse long hair on scalp

The Anatomy of a Single Hair

Composition of hair

  Originate in hair follicle
  Composed of root and shaft
  Root base (hair papilla) surrounded by hair bulb and root hair plexus
  Hairs have soft medulla and hard cortex
  Cuticle = superficial dead protective layer

Hair

A bundle of smooth muscle, the arrector pili, extends at an angle between the surface of the
dermis and a point in the follicle wall. Supplied by adrenergic fibers causing hair erection during
fear, anger, & cold.

Cycles of hair growth

  Anagen: growth phase lasts 2-3 years
  Catagen: transition phase 2-3 weeks
  Telogen: resting phase 2-3months, after which a club hair is shed

Sebaceous glands

 Present all over body but mostly in seborrheic areas: scalp, face, upper part of: chest,

shoulders & back

 Attached to hair follicles

 Secrete sebum: a complex lipid which is bactericidal & fungistatic

 Holocrine type of secretion: degeneration of the whole gland after it is filled &

release of sebum

Pilosebaceous follicles

Exocrine sweat glands

  2-4 millions
  All over body, mostly palms, soles & axillae
  Two parts :

1- the secretory coil deep in the dermis
2- the duct: extends from the gland & opens directly onto skin surface independent of

hair follicles

 Sweat glands are innervated by cholinergic fibers of the sympathetic nervous system
 Important in thermoregulation

Apocrine sweat glands

 modified sweat glands limited to the axillae, nipples, periumbilical area, perineum &

genitalia

  Opens directly into hair follicle
  Secretion by decapitation
  Responsible for the odor of the body
  Under action of androgen hormone

The nail

1- Nail plate

2- Nail matrix

3- Nail bed

4- Nail folds

5- The cuticle

Blood supply of skin

The dermis is the source of nutrition of the skin, the blood vessels lie in 2 horizontal layers:

1- The deep plexus: just above the subcutaneous fat

2- A superficial plexus: in the papillary dermis with interconnecting channels between the

two.

Summary

 Now you should be familiar with:

 The components of the integumentary system, including their physical relationships.

 The functions of the integumentary system.

 The main features and functions of the epidermis and dermis.

 The structure and function of the various accessory organs of the skin.

Mubark A. Wilkins