Anatomical Structure of the Skin

1. Epidermis: The epidermis is the outermost layer of the skin. It is a layer of keratinized, polystratified epithelium that covers the entire surface of the dermis and adheres to it, providing protection from the environment. It is the layer of skin with the largest number of cells and a high turnover rate.

The epidermis is composed of five layers:

1.1 Stratum Corneum: Made up of layers of keratin-filled corneocytes. The most superficial layers are continuously shed.

1.2 Stratum Lucidum: Only found in areas where the skin is thickest, it is a thin transition layer between the stratum granulosum and the stratum corneum.

1.3 Stratum Granulosum: Consists of 4 or 5 layers of granule cells, rich in keratohyalin granules. 1.4 Stratum Spinosum: Consists of spiny cells, so named for their spine-like extensions. The cells form several integrated rows, tightly joined by desmosomes. Desmosomes: These are intercellular junctions that connect the cell membranes and intermediate plasma filaments of adjacent cells. 1.5 Stratum Basalum or Germinative: Contains the skin's stem cells. Mitosis occurs continuously here, and cells slough off and differentiate into the stratum spinosum.

Epidermal Cells: There are four cellular blocks:

Keratinocytes: These form the protective covering of the epidermis. They are so named because they produce a protein called keratin, which is impermeable to water and protects the skin and tissues from external aggressions and abrasions. It regenerates approximately every 28 days.

Melanocytes: These are cells of nervous origin that synthesize melanin. They have dendritic processes located in the deepest layer of the epidermis. Their function, in addition to providing pigment, is to protect against UV radiation. Langerhans cells: These are cells originating from the bone marrow that migrate to the epidermis. They have a phagocytic function and are also said to present antigens to lymphocytes. They constitute an important defense of the body against foreign agents. They are usually located in the spinous, granular, and basal layers. Merkel cells: These are mechanoreceptors associated with nerve endings. These cells act as touch receptors and are located in the basal layers of the epidermis.

2. Dermis: It is the intermediate layer located below the epidermis, it is structured in 2 layers, an upper one called papillary dermis and a thicker lower layer called reticular dermis.

2.1 Papillary Dermis: This is the most superficial layer, made up of loose connective tissue and dermal papillae that raise the deeper layers of the epidermis. It contains nerve endings and extensive capillary vascularization. 2.2 Reticular Dermis: This is the deepest layer, made up of dense connective tissue containing collagen and elastin fibers arranged in a lattice or reticular structure. It also contains numerous blood vessels, lymphatic vessels, and nerve fibers. The cutaneous plexus forms here.

Extracellular Matrix (ECM): This is a fluid compartment that surrounds tissue cells. It is composed of protein fibers, proteoglycans, glycoproteins, and ground substance. It functions as a kind of network that binds cells together, provides support and mechanical resistance to tissues, provides nutrition to cells, and allows for interaction between them.

Components of the Extracellular Matrix:

Protein Fibers (collagen, elastin, and fibronectin).

Collagen: This protein is found in the ECM and is essential for tissue structure, so it's found in all body structures. To date, there are 29 known types of collagen, but our skin contains types I, III, and V. Collagen levels begin to decline after age 30.

Type I collagen provides firmness, elasticity, and hydration to the skin. It is the most common type of collagen in the body, representing 90% of the total. Type III collagen provides support to the skin and helps tissues expand. It is also important for the development of the skin and the cardiovascular system. Type V collagen improves skin radiance and forms the structural pillars of cells. Elastin: This is a protein found in skin and connective tissue, giving it elasticity and stretchability. It is an essential component of the skin, allowing it to regain its shape after stretching or contracting. Fibronectin: This protein is found in the skin. It participates in cell adhesion, helps accelerate the healing process, can improve the appearance of the skin, and protects it.

Sundamental Substance: it is a component of the extracellular matrix. It has a gelatinous consistency and contains water, proteoglycans, glycoproteins, and glycosaminoglycans. These give it its viscous nature and the ability to bind large amounts of water for hydration, nutrient diffusion, and tissue nourishment.

Composition of the Dermis

The dermis is rich in structural and functional components that allow it to fulfill its multiple roles. It is made up of fibroelastic connective tissue, blood vessels, lymphatic vessels, nerve fibers, and skin appendages (sebaceous and sweat glands, hair, and nails). All of these structures provide support, strength, and elasticity to the skin.

Dermal Cells

• Fibroblasts: These are the most numerous and important cells in the dermis. They are found in connective tissue and produce collagen and elastin fibers, the fundamental substance that holds the dermis and other components of the extracellular matrix together. They are responsible for the synthesis and degradation of proteins (collagen, elastin, glycosaminoglycans), making them essential for maintaining the skin's firmness and density.

  
  

• Mast Cells: These are cells that are part of the immune system and are responsible for defending the body against parasitic infections and allergens.
  

• Histiocytes: These are a type of white blood cell that is part of the immune system and helps protect the body from infections caused by viruses, bacteria, fungi, and parasites.

Fibroelastic Tissue

Collagen Fibers: These constitute the main structural component, providing strength and support to the skin. Elastin Fibers: responsible for the elasticity and flexibility of the skin.

Capillary Blood Vessels: These are responsible for transporting oxygen and allowing the exchange of nutrients in skin cells. Lymphatic Vessels: These are thin tubes that branch and connect to the lymph nodes. They transport lymph (a clear, watery fluid). Their function is to remove metabolic waste and regulate local immunity. Nerve Fibers: The skin is densely populated with nerve endings. The nerves in the skin are classified as sensory and autonomic.

Sensory nerves receive sensations from the skin such as temperature, pain, touch, and itching. Autonomic nerves control functions such as sweating.

Skin Appendages: Are the appendages or parts attached to the skin:

• Sebaceous Glands: These are cells found in the dermis. They produce sebum, a natural oil that moisturizes and lubricates the skin, prevents dryness, eliminates old skin cells, and protects the skin from germs.

• Sweat Glands: These are tubular structures found in the dermis. They produce sweat. Sweat is a watery secretion that helps regulate body temperature and protect the skin from microorganisms. 

• Hair: These are highly keratinized structures contained in an invagination called a hair follicle and originating in the hair matrix, which is located at the base of the hair follicle. Hair Follicle: A tubular invagination of the epidermis that penetrates the dermis and contains the hair root.
  

• Nails: Nails are epidermal formations in the form of keratin sheets that protect the tips of the fingers and toes. They are hard, flexible, convex, translucent, and smooth. 

3. Hypodermis (Subcutaneous Tissue): This is the deepest layer, made up primarily of adipose tissue. It acts as a thermal insulator and energy reserve. This layer has several important functions, including: Insulation: It helps regulate body temperature by acting as a thermal insulator. Cushioning: It provides a layer of protection against shock and injury by absorbing impact. Storage: It serves as an energy reserve by storing fat. Connection: It anchors the skin to underlying tissues, such as muscles and bones.