### Date : 2025-02-15 16:08 ---- ### **1.1 Structure of the Skin** Understanding the **skin’s structure** is essential in **hair transplantation**, as it influences **donor site selection, graft survival, and wound healing**. The skin consists of three primary layers: **epidermis, dermis, and hypodermis**, each with distinct roles. --- ## **1️⃣ Epidermis (표피) – The Protective Barrier** The **epidermis** is the **outermost layer** of the skin and serves as a **protective barrier**. It is **avascular** (contains no blood vessels) and consists mainly of **keratinocytes**.  ### **Key Features:** - Thickness: **0.05 mm (eyelids) – 1.5 mm (palms, soles)** - Composed of **stratified squamous epithelium** - Undergoes **constant renewal** (every ~28 days) ### **Layers of the Epidermis (Deep to Superficial)** 💡 **Mnemonic: "Come, Let’s Get SunBurned"** (Corneum, Lucidum, Granulosum, Spinosum, Basale) 1. **Stratum basale (Basal layer)** - Contains **stem cells** → new keratinocytes form here - **Melanocytes** (pigment production) & **Merkel cells** (touch receptors) are present - Attached to the basement membrane 2. **Stratum spinosum (Prickle cell layer)** - Desmosome connections between keratinocytes (**spiny appearance**) - Provides **mechanical strength** 3. **Stratum granulosum (Granular layer)** - Contains **keratohyalin granules** (precursors of keratin) - Forms a **waterproof barrier** 4. **Stratum lucidum (Clear layer) – Only in thick skin** - Found in **palms & soles** - Consists of **dead, flattened keratinocytes** 5. **Stratum corneum (Horny layer)** - Fully **keratinized, dead cells** - Protective **barrier against microbes & dehydration** ### **Clinical Relevance to Hair Transplantation** - The **epidermis is removed** along with the graft in FUE/FUT. - Poor graft handling can lead to **epidermal desiccation (drying out)** → reduced survival. - Scarring occurs when **epidermal basal cells fail to regenerate properly**. --- ## **2️⃣ Dermis (진피) – The Structural Layer** The **dermis** lies beneath the **epidermis** and provides **strength, elasticity, and vascular support**.  ### **Key Features:** dr - **Thickness**: 1-4 mm (thicker in scalp) - **Rich in blood vessels, nerves, and connective tissue** - Contains **collagen & elastin fibers** (provides strength and flexibility) ### **Layers of the Dermis** 6. **Papillary Dermis (Upper layer)** - Loose **connective tissue**, rich in **capillaries** - Contains **Meissner corpuscles** (touch receptors) - **Nourishes the epidermis** via diffusion 7. **Reticular Dermis (Lower layer)** - Dense **collagen & elastin fibers** (provides tensile strength) - Contains **deep blood vessels, lymphatics, and sensory nerves** - Houses **hair follicles, sebaceous glands, and sweat glands** ### **Clinical Relevance to Hair Transplantation** - The **dermis is where the follicular unit is anchored**. - Proper **punch depth control** is essential to avoid **deep dermal transection**. - **Dermal fibrosis** (due to poor healing) can make **subsequent hair transplants more difficult**. --- ## **3️⃣ Hypodermis (피하조직) – The Fatty Layer** The **hypodermis** (also called the **subcutaneous tissue**) is the **deepest** skin layer. ### **Key Features:** - Composed mainly of **fat lobules** and **connective tissue**. - Contains **large blood vessels** and **lymphatics**. - Acts as a **cushion** to protect underlying muscles and bones. ### **Clinical Relevance to Hair Transplantation** - Overharvesting in FUE can **penetrate the hypodermis**, leading to **visible scarring and skin depressions**. - Adequate **hypodermal fat preservation** in grafts improves **follicular survival**. - Excessive **hypodermal damage** can cause **prolonged post-op swelling (edema)**. --- ## **4️⃣ Hair Follicles and Sebaceous Glands** The **hair follicle** is a **complex mini-organ** embedded in the **dermis** and extending into the **hypodermis**. ### **Structure of the Hair Follicle** - **Hair Bulb (deepest part)** - Contains **dermal papilla** → critical for hair growth. - **Outer Root Sheath (ORS)** - Contains **stem cells** responsible for **regeneration**. - **Inner Root Sheath (IRS)** - Provides **structural support** during hair growth. - **Sebaceous Glands** - Secrete **sebum** (oil) → lubricates the hair. - Overactivity = **seborrheic dermatitis** (common in hair loss patients). ### **Clinical Relevance to Hair Transplantation** - FUE must **extract the entire follicle (including dermal papilla)** to ensure regrowth. - **Sebaceous gland preservation** helps prevent **graft desiccation**. - **Poor follicle anchoring** (due to dermal fibrosis) can lead to **poor growth rates** in secondary transplants. --- # **Summary Chart for Quick Review** |**Layer**|**Key Features**|**Hair Transplantation Relevance**| |---|---|---| |**Epidermis**|Protective outer layer, keratinocytes, stratified squamous epithelium|Avoid epidermal desiccation; epidermis regenerates after extraction| |**Dermis**|Strength, elasticity, vascular supply, houses hair follicles|Proper depth control prevents graft trauma| |**Hypodermis**|Fat layer, cushions deeper structures|Overharvesting in FUE can cause depressions & fibrosis| |**Hair Follicle**|Contains dermal papilla, stem cells, and sebaceous glands|Must extract complete follicle for survival| |**Sebaceous Glands**|Produces oil (sebum) to lubricate hair|Helps maintain graft hydration| --- # **Practical Tips for Mastering This Section** ✅ **Watch dissection videos**: Observe the epidermis, dermis, and follicular structures in cadaver studies. ✅ **Use a dermatoscope**: Examine different scalp conditions (seborrheic dermatitis, folliculitis, etc.). ✅ **Train in micro-dissection**: Practice **separating follicles under a microscope** (improves FUE skills). ✅ **Study donor site physiology**: Knowing scalp thickness variations helps in **reducing transection rates**. ---