Study Notes on the Integumentary System

Introduction to the Integumentary System

• Presented by Genevieve, this module provides an in-depth exploration of the integumentary system.

• Focus of Chapter 6 in the textbook, which delves into the microscopic and macroscopic anatomy, as well as the physiological functions.

• Lab manual exercises mentioned: Chapter 5, offering practical application and identification of structures.

• Importance of supplementary reading from the textbook and lab manual for a comprehensive understanding of complex topics and clinical correlations.

Overview

• The integumentary system is one of the largest and most extensive organ systems in the body, primarily composed of the skin and its associated structures.

• Composed of all four primary tissue types (epithelial, connective, muscular, and nervous tissue), it forms a dynamic and vital interface between the body's interior and the external environment.

• Essential functions of the integumentary system include:

  • Covering and protecting underlying structures: Provides a physical barrier against physical trauma, pathogens, and harmful UV radiation.

  • Preventing desiccation (drying out): The keratinized epidermis and lipid-rich lamellar granules create an impermeable barrier, crucial for maintaining fluid balance.

  • Assisting in the maintenance of normal body temperature (thermoregulation): Achieved through processes like vasodilation/vasoconstriction of dermal blood vessels and sweat production.

  • Excreting water, salt, and organic waste: Via eccrine glands producing sweat and sebaceous glands producing sebum, contributing to homeostatic balance.

  • Receiving and conveying sensory information: Through specialized nerve endings and receptors, enabling sensation of touch, pressure, pain, and temperature.

  • Synthesizing vitamin D via sun exposure: When exposed to ultraviolet B (UVB) radiation, a precursor molecule (7-dehydrocholesterol) in the skin is converted into cholecalciferol (D_3), which is then further processed in the liver and kidneys to its active form, calcitriol, essential for calcium absorption.

  • Storing nutrients: Primarily adipose tissue in the hypodermis stores triglycerides, serving as an energy reserve.

Structural Components of the Integumentary System

• Skin: The primary organ, composed of two main layers: the more superficial epidermis and the deeper dermis.

  • Related structures include specialized epidermal derivatives such as horns, hooves, claws, feathers, scales, skin glands (e.g., sebaceous, sudoriferous), and hair.

• Keratinization: A vital cellular process where epithelial cells (keratinocytes) in the epidermis undergo a terminal differentiation, replacing their organelles and nuclei with a tough, fibrous protein called keratin. This process makes the outermost structures resilient, durable, and highly waterproof, forming a protective barrier.

Epidermis

• Definition: The outermost, avascular layer of the skin, primarily consisting of stratified squamous epithelium composed predominantly of keratinized cells.

• Layers of the Epidermis: The epidermis is typically divided into five distinct layers in thick skin (e.g., foot pads, nose planum) and four layers in thin skin.

  1. Stratum Corneum (horny layer):

     • Outermost, most superficial layer, composed of 20-30 rows of flattened, anucleated, dead keratinocytes, often referred to as “corneocytes.”

     • Constitutes up to 75% of total epidermal thickness, providing the primary physical barrier.

     • Composed of highly keratinized, dead cells with nuclei that are faint or absent and no longer functional, forming a protective, durable, and desiccation-resistant surface.

  2. Stratum Lucidum (clear layer):

     • Present only in thick skin (e.g., paw pads) and visible as a narrow, clear, homogenous band under the microscope, consisting of 2-3 layers of flattened, densely packed dead cells.

     • These cells contain eleidin, a clear protein intermediate in the formation of keratin, and lack nuclei and organelles.

     • Does not appear in most areas of thin skin in mammals, except in specific regions where skin is subjected to high friction and abrasion.

  3. Stratum Granulosum (granular layer):

     • Middle layer, typically composed of 2-4 layers of flattened, diamond-shaped keratinocytes that are undergoing apoptosis.

     • Contains prominent keratohyaline granules (involved in keratin aggregation) and lamellated granules (release a lipid-rich secretion for waterproofing), leading to the degeneration of nuclei and other organelles; this marks the beginning of programmed cell death.

     • The contents of lamellated granules are extruded into the intracellular space, contributing significantly to the waterproofing and barrier function of the skin by forming a lipid cement.

  4. Stratum Spinosum (spiny layer):

     • Known as the spiny layer due to the presence of desmosomes, which appear as