Skin and its Appendages

Skin is the largest organ of the body, accounting for approximately 16% of body weight. It is composed of three layers from the outside in:

• Epidermis: The outermost layer, formed from the ectoderm. The thickness of the epidermis ranges from 0.07-2.5mm.

• Dermis: Located beneath the epidermis, formed from the mesoderm.

• Hypodermis: The deepest layer of skin, connecting the dermis to underlying organs.

Epidermis

The epidermis is a stratified squamous keratinized epithelium, consisting of five layers from inside to outside:

• Stratum basale: Consists of a single row of cuboidal or columnar cells, capable of reproduction. The cytoplasm of the basal layer is slightly basophilic, the nucleus is oval or spherical, and contains many chromatin located on the basement membrane.

• Stratum spinosum: Consists of 5-20 rows of polyhedral cells, containing dense granules of phospholipids. This layer is also capable of reproduction.

• Stratum granulosum: The cytoplasm has keratohyalin granules, which stain intensely with bases, signaling the beginning of keratinization.

• Stratum lucidum: The cells are flatter and longer, fewer organelles, the nucleus shrinks and disappears. This layer is often seen in thick skin.

• Stratum corneum: The cells turn into thin keratin flakes, no nucleus, containing a lot of keratin. This layer ensures water impermeability and prevents water vapor from evaporating through the skin.

Dermis

The dermis is a strong fibrous connective tissue, containing many blood vessels and nerves. It is composed of two layers:

• Papillary layer: Has protrusions called dermal papillae, forming fingerprints and footprints that are unique to each individual.

• Reticular layer: A loose connective tissue, consisting of collagen fibers that are parallel to the skin surface.

Hypodermis

The hypodermis is a loose connective tissue, containing subcutaneous fat lobules, blood vessels, lymphatic vessels, and nerve endings.

Skin Appendages

• Sebaceous glands: Located in the dermis, secrete sebum (oil) to make the skin and hair soft.

• Sweat glands: Found everywhere on the skin, helping to secrete sweat to regulate body temperature.

Hair

Hair grows from a funnel-shaped depression of the epidermis called the hair follicle. The structure of the hair follicle consists of three parts:

• Inner epithelial sheath: Originates from epidermal cells at the bottom of the hair shaft.

• Outer epithelial sheath: Lies outside the inner epithelial sheath.

• Fibrous sheath: The outermost sheath of the hair follicle.

Hair is divided into three parts from inside out:

• Medulla: The core of the hair.

• Cortex: Surrounds the medulla.

• Cuticle: The outermost layer of the hair.

Nails

Nails are flat, horny plates that cover the dorsal surface of the fingertips and toes.

Functions of Skin

• Protect against water loss and evaporation.

• Protect against radiation due to melanin.

• Protect against friction, prevent water and acid solutions from penetrating.

• Prevent bacteria and parasites from entering the body.

• Receive stimuli from the external environment.

• Participate in the excretion of some substances.

• Participate in regulating body temperature.