Chapter 21 – Development of Skin
Introduction
• Skin is the largest organ in the body, covering the entire external surface and performing
crucial roles:
o Protection against physical, microbial, and chemical insults.
o Regulation of body temperature and water balance.
o Sensory reception.
o Immune defense.
o Excretion and synthesis of Vitamin D.
• Skin has a dual embryological origin:
1. Epidermis: Derived from surface ectoderm.
2. Dermis: Derived from underlying mesenchyme (from mesoderm and neural crest).
Development of the Epidermis
1. Early Development
• Initially, at Week 5, the embryo is covered by a single layer of ectodermal cells.
• Week 7:
o Proliferation of ectoderm forms:
§ Basal Layer (Stratum Germinativum).
§ A superficial periderm layer (temporary epithelial covering).
• The periderm is shed during the second half of intrauterine life into the amniotic fluid.
2. Formation of the Definitive Epidermis
• By the 4th month, the definitive multilayered epidermis is established, containing:
1. Basal Layer (Stratum Basale):
§ Composed of mitotically active progenitor cells.
§ Produces new keratinocytes.
§ Forms epidermal ridges, responsible for fingerprints.
2. Spinous Layer (Stratum Spinosum):
§ Polyhedral cells linked by desmosomes, creating a spiny appearance.
§ Contains tonofibrils (bundles of keratin filaments), providing mechanical
strength.
3. Granular Layer (Stratum Granulosum):
§ Contains keratohyalin granules, which aggregate keratin filaments.
4. Horny Layer (Stratum Corneum):
§ Composed of keratinized, dead cells (squames).
, § Provides a tough, protective outer barrier.
3. Epidermal Cellular Components
1. Keratinocytes:
o The primary cell type, forming all epidermal layers.
o Undergo keratinization, producing keratin proteins for strength and waterproofing.
2. Melanocytes:
o Derived from neural crest cells.
o Migrate into the basal layer and hair follicles during the 3rd month.
o Synthesize melanin within melanosomes.
§ Melanin shields keratinocyte DNA from UV radiation by forming protective
caps above nuclei.
o Melanosomes are transferred to keratinocytes via dendritic processes.
3. Langerhans Cells:
o Originate from bone marrow-derived mesodermal precursors.
o Migrate into the epidermis and function as antigen-presenting cells in immune
defense.
4. Merkel Cells:
o Found in the basal layer, especially in areas of tactile sensitivity.
o Act as mechanoreceptors, transmitting touch signals through synaptic contacts
with sensory nerves.
Development of the Dermis
1. Origin of the Dermis
• The dermis develops from mesenchyme, which has different embryological origins:
1. Neural Crest Cells: Contribute to dermis in the head and neck region.
2. Paraxial Mesoderm: Forms dermis in the trunk and limbs.
3. Lateral Plate Mesoderm: Contributes to dermis in limbs and body wall.
2. Layers of the Dermis
• By Week 12, the dermis differentiates into three layers:
1. Papillary Layer:
§ Contains loose connective tissue.
§ Forms dermal ridges, which correspond to epidermal ridges (fingerprints).
§ Rich in capillaries and sensory nerve endings.
2. Reticular Layer:
§ Composed of dense irregular connective tissue.
Introduction
• Skin is the largest organ in the body, covering the entire external surface and performing
crucial roles:
o Protection against physical, microbial, and chemical insults.
o Regulation of body temperature and water balance.
o Sensory reception.
o Immune defense.
o Excretion and synthesis of Vitamin D.
• Skin has a dual embryological origin:
1. Epidermis: Derived from surface ectoderm.
2. Dermis: Derived from underlying mesenchyme (from mesoderm and neural crest).
Development of the Epidermis
1. Early Development
• Initially, at Week 5, the embryo is covered by a single layer of ectodermal cells.
• Week 7:
o Proliferation of ectoderm forms:
§ Basal Layer (Stratum Germinativum).
§ A superficial periderm layer (temporary epithelial covering).
• The periderm is shed during the second half of intrauterine life into the amniotic fluid.
2. Formation of the Definitive Epidermis
• By the 4th month, the definitive multilayered epidermis is established, containing:
1. Basal Layer (Stratum Basale):
§ Composed of mitotically active progenitor cells.
§ Produces new keratinocytes.
§ Forms epidermal ridges, responsible for fingerprints.
2. Spinous Layer (Stratum Spinosum):
§ Polyhedral cells linked by desmosomes, creating a spiny appearance.
§ Contains tonofibrils (bundles of keratin filaments), providing mechanical
strength.
3. Granular Layer (Stratum Granulosum):
§ Contains keratohyalin granules, which aggregate keratin filaments.
4. Horny Layer (Stratum Corneum):
§ Composed of keratinized, dead cells (squames).
, § Provides a tough, protective outer barrier.
3. Epidermal Cellular Components
1. Keratinocytes:
o The primary cell type, forming all epidermal layers.
o Undergo keratinization, producing keratin proteins for strength and waterproofing.
2. Melanocytes:
o Derived from neural crest cells.
o Migrate into the basal layer and hair follicles during the 3rd month.
o Synthesize melanin within melanosomes.
§ Melanin shields keratinocyte DNA from UV radiation by forming protective
caps above nuclei.
o Melanosomes are transferred to keratinocytes via dendritic processes.
3. Langerhans Cells:
o Originate from bone marrow-derived mesodermal precursors.
o Migrate into the epidermis and function as antigen-presenting cells in immune
defense.
4. Merkel Cells:
o Found in the basal layer, especially in areas of tactile sensitivity.
o Act as mechanoreceptors, transmitting touch signals through synaptic contacts
with sensory nerves.
Development of the Dermis
1. Origin of the Dermis
• The dermis develops from mesenchyme, which has different embryological origins:
1. Neural Crest Cells: Contribute to dermis in the head and neck region.
2. Paraxial Mesoderm: Forms dermis in the trunk and limbs.
3. Lateral Plate Mesoderm: Contributes to dermis in limbs and body wall.
2. Layers of the Dermis
• By Week 12, the dermis differentiates into three layers:
1. Papillary Layer:
§ Contains loose connective tissue.
§ Forms dermal ridges, which correspond to epidermal ridges (fingerprints).
§ Rich in capillaries and sensory nerve endings.
2. Reticular Layer:
§ Composed of dense irregular connective tissue.
