Anatomy of the Skin Anatomy of the Skin

Dr Mesfin HunegnawDr Mesfin Hunegnaw

Consultant Dermatologist & VenerologistConsultant Dermatologist & Venerologist

AAU, Medical facultyAAU, Medical faculty

Dept. of DermtovenerologyDept. of Dermtovenerology

The EpidermisThe Epidermis

IntroductionIntroduction Skin is the largest Organ System in the BodySkin is the largest Organ System in the Body MorphologicallyMorphologically - Epidermis - Epidermis - Dermis- Dermis - Panniculus Adiposus (Subcutis)- Panniculus Adiposus (Subcutis) FunctionallyFunctionally - Protective barrier- Protective barrier - Temperature and Fluid regulation- Temperature and Fluid regulation - Immune Surveillance- Immune Surveillance - Sensory Perception- Sensory Perception - Endocrine Function- Endocrine Function Type – Glaborous (non hairy) and Hair bearing Type – Glaborous (non hairy) and Hair bearing

Skin Skin

The EpidermisThe Epidermis superficial, continually reviving, stratified squamous superficial, continually reviving, stratified squamous

epithelium with progressive stages of differentiation from epithelium with progressive stages of differentiation from deeper to more superficial layers deeper to more superficial layers

Composed of four distinct named layers including the Composed of four distinct named layers including the stratum germinativum, stratum spinosum, stratum stratum germinativum, stratum spinosum, stratum granulosum, and stratum corneum. granulosum, and stratum corneum.

0.4 – 1.5 mm in thickness out of the total 1.5- 4mm skin 0.4 – 1.5 mm in thickness out of the total 1.5- 4mm skin thickness.thickness.

composed of majority keratinocytes (80 -95%) with other composed of majority keratinocytes (80 -95%) with other cells as the melanocytes, langerhan cells and merkel cells as the melanocytes, langerhan cells and merkel cells and transient cells as lymphocytes.cells and transient cells as lymphocytes.

Epidermis contains no blood vessels and is entirely Epidermis contains no blood vessels and is entirely dependent on the underlying dermis for nutrient delivery dependent on the underlying dermis for nutrient delivery and waste disposal via diffusion through the and waste disposal via diffusion through the

dermoepidermal junction.dermoepidermal junction.

Embryology of the EpidermisEmbryology of the Epidermis Embryonic Epidermis is derived from the surface ectoderm with

the contribution of the neural crest for the melanocytes and dermis derived from the mesoderm.

In about the third week of fetal life, the epidermis consists of no more than a single layer of undifferentiated glycogen-flled cells.

In a 4- to 6-week-old embryo two layers of cells can be distinguished, the periderm or and a stratum germinativum

Embryology...Embryology... Between eighth and eleven weeksBetween eighth and eleven weeks an intermediate layer of an intermediate layer of

stratum intermedium appears with aggregation of mesenchymal stratum intermedium appears with aggregation of mesenchymal cells precursors of the presumptive hair follicle and cells precursors of the presumptive hair follicle and desmosomes and hemidesmosomes appear.desmosomes and hemidesmosomes appear.

Between twelfth and sixteenth weeks more intermediate layers Between twelfth and sixteenth weeks more intermediate layers appear and cells elabotate basal keratins and skin appear and cells elabotate basal keratins and skin differentiation keratins with filaggrin.differentiation keratins with filaggrin.

Embryology...Embryology... Between sixteenth and twenty sixth weeks basal cells will grow Between sixteenth and twenty sixth weeks basal cells will grow

downward and complete the formation of the pilosebaceous downward and complete the formation of the pilosebaceous unit with the mesenchymal cells forming the hair papilla, unit with the mesenchymal cells forming the hair papilla, keratohyalin granules appear by by week 21 and the peridem keratohyalin granules appear by by week 21 and the peridem separates from the embryo at week 24.separates from the embryo at week 24.

Embryology...Embryology...

The KeratinocyteThe Keratinocyte is an ectodermally derivedis an ectodermally derived that constitutes at least 80% of the that constitutes at least 80% of the

total population of cells in the epidermis containing keratin total population of cells in the epidermis containing keratin intermediate filaments and form desmosomes and intermediate filaments and form desmosomes and hemidesmosomes.hemidesmosomes.

Keratin filaments are the hallmark of keratinocytes with over 30 Keratin filaments are the hallmark of keratinocytes with over 30 different types ranging from 40-70 kDa.different types ranging from 40-70 kDa.

Keratin can be broadly classified into Type 1 which is acidic and Keratin can be broadly classified into Type 1 which is acidic and designated from K1-K8 and Type 2 which is basic and designated from K1-K8 and Type 2 which is basic and designated from K9-K19.designated from K9-K19.

Keratins exhibit “Obligate Heteropolymerism”Keratins exhibit “Obligate Heteropolymerism”

eg. K5/K15eg. K5/K15 Keratin pairs differ in each of the epidermal layers reflecting theKeratin pairs differ in each of the epidermal layers reflecting the

status of keratinocyte differentiation.status of keratinocyte differentiation.

Layers of the EpidermisLayers of the Epidermis The characteristics of each epidermal layer is reflected by the The characteristics of each epidermal layer is reflected by the

mitotic and metabolic properties of keratinocytes and their state mitotic and metabolic properties of keratinocytes and their state of differentiation.of differentiation.

Keratinization being the hallmark of these characters is a Keratinization being the hallmark of these characters is a genetically programmed complex metabolic and morphologic genetically programmed complex metabolic and morphologic changes controlled by extrinsic and intrinsic factors involving:changes controlled by extrinsic and intrinsic factors involving:

1. increased cell size and flattening.1. increased cell size and flattening.

2. appearance, reorganization and degradation of organelles.2. appearance, reorganization and degradation of organelles.

3. eventual ‘focused’ metabolism.3. eventual ‘focused’ metabolism.

4. alterations of plasma proteins, antigens and receptors.4. alterations of plasma proteins, antigens and receptors.

5. dehydration.5. dehydration.

Layers...Layers... Stratum GerninativumStratum Gerninativum

- also known as the Stratum Basale- also known as the Stratum Basale

- usually a one cell layer but could be 2-3 cells thick in - usually a one cell layer but could be 2-3 cells thick in

glaborous skin.glaborous skin.

- composed of the basal, mitotic, columnar keratinocytes - composed of the basal, mitotic, columnar keratinocytes

with a large nucleu a a prominent nucleolus.with a large nucleu a a prominent nucleolus.

- cells endowed with the ‘house keeping’ organelles with - cells endowed with the ‘house keeping’ organelles with

membrane bound vacuoles of melanosomes.membrane bound vacuoles of melanosomes.

- cells contain K5/K14 with K19 and microfilaments.- cells contain K5/K14 with K19 and microfilaments.

- is the primary location of mitotically active cells of the - is the primary location of mitotically active cells of the

epidermis: Epidermal Stem Cells(10%), Transient Amplifying epidermis: Epidermal Stem Cells(10%), Transient Amplifying

Cells(50%), Postmitotic cells(40%). Cells(50%), Postmitotic cells(40%).

Layers...Layers... StratumStratum Spinosum Spinosum

- also called the spinous/prickle layer because of the spine like - also called the spinous/prickle layer because of the spine like

appearance of the cells due to the numerous desmosomes.appearance of the cells due to the numerous desmosomes.

- lower cells are polyhedral with the a round nucleus while the - lower cells are polyhedral with the a round nucleus while the upper cells are large and flattened with lamellar granules(new upper cells are large and flattened with lamellar granules(new organelles containing phospholipids, glycolipids, and free organelles containing phospholipids, glycolipids, and free sterols with enzymes which aid in differentiation).sterols with enzymes which aid in differentiation).

- cells contain K5/K14 plus K1/K10.- cells contain K5/K14 plus K1/K10. - ‘Spines’ or Desmosomes are calcium dependent cell surface - ‘Spines’ or Desmosomes are calcium dependent cell surface modifications that promote adhesion and resistance to modifications that promote adhesion and resistance to mechanical stress anchoring keratin and bridging adjacentmechanical stress anchoring keratin and bridging adjacent keratinocytes.keratinocytes.

Layers...Layers... Stratum GranulosumStratum Granulosum - also called the granular layer.- also called the granular layer. - most apparent structures in the cells of this layer are the - most apparent structures in the cells of this layer are the basophilic, Keratohyalin granules.basophilic, Keratohyalin granules. - These granules contain Profilaggrin, Keratin Filaments and - These granules contain Profilaggrin, Keratin Filaments and Loricrin. Loricrin. - cells form the Cornified Cell Envelope from Involucrin and - cells form the Cornified Cell Envelope from Involucrin and Keratolinin by the action transglutaminases.Keratolinin by the action transglutaminases. - Lamellar granules function by discharging their contents to - Lamellar granules function by discharging their contents to the intercellular space and change of the probarrier lipids to the intercellular space and change of the probarrier lipids to barrier lipids under enzymatic action. barrier lipids under enzymatic action. - cells undergo programmed apoptosis as they transit to the - cells undergo programmed apoptosis as they transit to the cornified layer losing nucleus and most cytoplasmic contents.cornified layer losing nucleus and most cytoplasmic contents. - K1/K10 to K2/K11- K1/K10 to K2/K11

Layers...Layers... Stratun CorneumStratun Corneum - also called the cornified or horny layer.- also called the cornified or horny layer. - comprised of flattened , polyhedral cells, largest in the - comprised of flattened , polyhedral cells, largest in the epidermis and with thickness ranging from 15-100 cells.epidermis and with thickness ranging from 15-100 cells. - shape, surface, structure adapted to maintain integrity yet - shape, surface, structure adapted to maintain integrity yet allow desquamation and provide mechanical protection, allow desquamation and provide mechanical protection, barrier to water loss, permeation of soluble substancesbarrier to water loss, permeation of soluble substances - corneocytes contain keratin filaments aligned to disulfide - corneocytes contain keratin filaments aligned to disulfide bonded macrofibers under the influence of filaggrin which bonded macrofibers under the influence of filaggrin which account for 80% of cell mass.account for 80% of cell mass. - Also posses an insoluble cornified envelope formed by - Also posses an insoluble cornified envelope formed by crosslinkinking soluble protien precursors as Involucrin.crosslinkinking soluble protien precursors as Involucrin. - Stratum Compactum – thicker, parallel filaments, fragile CCE- Stratum Compactum – thicker, parallel filaments, fragile CCE - Stratum dysjunctum- Stratum dysjunctum - ?Dead - ?Dead