Overview
This lecture introduces the integumentary system (skin), covering its embryological development, major functions, and structural organization, focusing especially on the multiple layers and key cell types of the epidermis.
Embryological Development of the Integumentary System
- The integumentary system is derived from both ectoderm (epidermis) and mesoderm (dermis and hypodermis).
- The epidermis develops from ectoderm.
- The dermis forms from mesoderm, specifically the dermatome of somites.
- The hypodermis originates from mesenchyme, another mesodermal tissue type.
Functions of the Integumentary System
- Provides protection against ultraviolet (UV) light, microorganisms, and water loss (desiccation).
- Aids in thermoregulation via sweating, shivering, vasoconstriction, and vasodilation.
- Contributes to immune defense through both a physical barrier and phagocytic skin cells.
- Produces inactive vitamin D from cholesterol when exposed to UV, which is activated in the kidneys.
- Excretes small amounts of water and waste ions (e.g., sodium) via sweat.
- Facilitates communication and sensory reception through touch and environmental sampling.
Structure and Layers of the Skin
- The skin has three main layers: epidermis (outermost), dermis (middle), and hypodermis (deepest).
- The epidermis is an epithelial tissue forming the cutaneous membrane and containing glands and multiple cell types.
- Major epidermal cells: keratinocytes (produce keratin, ~90% of epidermal cells), melanocytes (produce melanin), macrophages (phagocytic white blood cells), and tactile cells (sensory role).
- The epidermis is organized into distinct layers; focus is on the four primary layers (excluding a fifth seen only in thick skin).
- The deepest layer, stratum basale, contains simple cuboidal/columnar cells, divides to regenerate the epidermis, and secretes the basement membrane.
- Keratinocytes in the stratum basale are the only dividing cells, pushing upward as they multiply.
- Melanocytes in the stratum basale derive from neural crest cells and produce varying amounts of melanin, determining skin color.
- Melanin is injected into surrounding keratinocytes, not just localized in melanocytes.
Key Terms & Definitions
- Integumentary System — The organ system comprising the skin and its derivatives.
- Epidermis — Outermost skin layer, derived from ectoderm, consists mainly of keratinocytes.
- Dermis — Middle skin layer, derived from the dermatome (mesoderm).
- Hypodermis — Deepest skin layer, derived from mesenchyme (mesoderm).
- Keratinocyte — Main epidermal cell producing the protein keratin.
- Melanocyte — Pigment-producing cell in the epidermis that synthesizes melanin.
- Desiccation — Loss of moisture; the skin prevents this.
- Stratum Basale — The deepest layer of the epidermis where cell division occurs.
- Phagocytosis — Process where cells engulf and digest particles, important for immunity.
Action Items / Next Steps
- Review the four primary layers of the epidermis for upcoming lectures.
- Read textbook section on skin structure and function.
- Prepare questions about vitamin D synthesis for discussion.