Skin and Body Membranes: Structure and Function (Module 4 Study Notes)

Skin and Body Membranes

Overview

Body membranes cover surfaces, line body cavities, and form protective (and often lubricating) sheets around organs. They play essential roles in protection, secretion, and sensation throughout the body.

Major Groups of Body Membranes

• Epithelial membranes:

  • Cutaneous membrane: Commonly known as the skin or integumentary system, this is the outer covering that provides protection.

  • Mucous membrane: Lines all body cavities that open to the exterior, such as those of the respiratory, digestive, urinary, and reproductive tracts. These membranes secrete mucus for lubrication and protection.

  • Serous membrane: Lines body cavities that are closed to the exterior (except for the dorsal body cavity and joint cavities). They secrete serous fluid to reduce friction between organs.

• Connective tissue membranes:

  • Synovial membranes: Line the fibrous capsules surrounding joints, producing synovial fluid for lubrication.

Functions of the Integumentary System

• Protection against:

  • Mechanical damage (e.g., bumps)

  • Chemical damage (e.g., acids and bases)

  • Microbial damage

  • Ultraviolet (UV) radiation

  • Thermal damage (heat or cold)

  • Desiccation (drying out)

• Water retention: Prevents loss of water and other precious molecules from the body.

The Integumentary System (Skin)

The skin is the largest organ of the body and serves as a barrier to protect internal organs and tissues from external threats.

Structure of the Skin

• Epidermis: The outermost layer, composed mainly of keratinocytes that produce keratin, a tough, protective protein. The epidermis is avascular (lacks blood supply).

• Dermis: The deeper, thicker layer beneath the epidermis, providing strength and elasticity.

Layers of the Epidermis (Strata)

• Stratum Basale: Deepest layer, attached to the dermis; site of rapid cell division.

• Stratum Spinosum: Several layers of keratinocytes; cells begin to flatten.

• Stratum Granulosum: Cells become more flattened and keratinized.

• Stratum Lucidum: Present only in thick skin (palms, soles); provides extra protection.

• Stratum Corneum: Outermost layer; consists of 20–30 layers of dead, keratinized cells; accounts for most of the epidermal thickness.

Specialized Cells and Structures in the Epidermis

• Keratinocytes: Main cell type; produce keratin.

• Desmosomes: Junctions that connect keratinocytes, providing structural integrity.

• Melanocytes: Produce melanin, a pigment that protects against UV radiation. Melanin granules are stored in the cytoplasm of these cells.

• Merkel cells: Associated with sensory nerve endings; function as touch receptors (Merkel discs).

• Nerve endings: Detect pain, touch, pressure, and temperature.

• Epidermal dendritic cells: Immune cells that help detect and fight pathogens.

The Dermis

The dermis is a strong, flexible connective tissue layer that binds the body together and supports the epidermis.

Regions of the Dermis

• Papillary Layer: Superficial layer; contains dermal papillae that form ridges (fingerprints) and increase grip. Contains capillaries and sensory receptors.

• Reticular Layer: Deepest skin layer; contains dense irregular connective tissue, blood vessels, sweat and oil glands, and deep pressure receptors (lamellar corpuscles).

Cutaneous Sensory Receptors

• Meissner's corpuscles: Detect light touch.

• Pacinian (lamellar) corpuscles: Detect deep pressure and vibration.

• Free nerve endings: Detect pain and temperature.

• Root hair plexus: Detect hair movement.

Summary Table: Types of Body Membranes

Membrane Type	Location	Main Function
Cutaneous	Skin (external body surface)	Protection, prevents water loss
Mucous	Lines body cavities open to exterior (digestive, respiratory, urinary, reproductive tracts)	Secretion, absorption, protection
Serous	Lines closed body cavities (thoracic, abdominal)	Reduces friction between organs
Synovial	Lines joint cavities	Secretes synovial fluid for lubrication

Key Terms and Definitions

• Keratinization: The process by which keratinocytes produce keratin and become tough, protective cells.

• Melanin: Pigment produced by melanocytes; protects against UV radiation.

• Desmosome: Specialized cell junction that holds adjacent cells together.

• Lamellar corpuscle: Deep pressure receptor found in the dermis.

• Merkel disc: Sensory receptor for touch.

Additional info:

• The skin also plays a role in thermoregulation, vitamin D synthesis, and excretion of wastes through sweat.

• Disorders of the skin can include burns, infections, and cancers, which are covered in later modules.