Anatomy of Muscles and Skin - ANA 211

Introduction to Muscles, Bursae and Synovial Sheaths

• The topic is Muscles, Bursae and Synovial Sheaths from the Department of Anatomy, PUMS Introductory Lecture Series ANA 211.
• The lecturer is M. A. Amadi.

Learning Objectives

• Introduce the basic structure of the skin as an important organ.
• Provide a suitable definition for skin.
• Know the principal components of skin.
• Describe the recognized layers of skin.
• Give a brief account of the microscopic anatomy of skin.
• Understand the anatomical basis of surgical incisions.

Skin as an Organ

• The skin is part of the integumentary system of organs.
• It is the largest organ of the human body, weighing 6-9 kg and measuring about 2 square meters.
• It acts as a protective container for organs.
• It acts as a barrier between internal organs, vital body fluids, and the external environment.
• Structures like the tongue and vagina have specialized or modified skin.
• Skin can display clinical evidence of internal disease, such as peau de range in breast cancer.

Layers of Skin

• Epidermis:
  • The most superficial of the three skin layers.
  • Its thickness averages 0.6–1 mm.
  • It is multicellular, composed of up to 50 rows of epithelial cells.
  • It is normally avascular.
  • Contains keratinocytes, Merkel cells, and dendritic cells (melanocytes and Langerhans' cells). Keratinocytes make up 80%.
  • Particularly thick in the palms and soles.
• Dermis:
  • The layer beneath the epidermis.
  • Thicker than the epidermis.
  • Responsible for skin's tensile strength, elasticity, and appearance.
  • Contains arteriovenous anastomosis for thermoregulation.
  • Anatomically divided into two compartments.
  • Composed of collagen fibers (including reticulin fibers) and elastic fibers, held together by ground substance (mucopolysaccharide gel). Types 1 and 111 collagen form the fibers.
  • The dermis contains cellular elements like mast cells, fibroblasts, endothelial cells, vascular smooth muscle cells, specialized myoepithelial cells, nerve, and hematopoietic cells (macrophages, lymphocytes, and plasma).
• Hypodermis (Subcutaneous Layer):
  • Flexible areolar connective tissue bridge between skin and underlying bones and muscles.
  • Comprises the largest volume of fatty tissue in the body.
  • Fat cells vary in thickness depending on location and use.
  • Contains fibrous septae that separate fat cells into lobules.
  • Contains blood vessels, lymphatics, and nerves that nourish and keep skin healthy, passing from underlying tissues to the dermis.

Appendages

• Nails, hair, and teeth are appendages of the skin.
• Exocrine glands (e.g., sweat glands, apocrine glands, sebaceous glands) open onto the skin surface.
• Clinical example: Boil – an infected sebaceous gland.

Langer's Lines (Skin Tension Lines)

• Invisible cleavage lines along which surgical cuts or incisions should be made for best healing.
• Based on the direction of orientation of underlying collagen fibers.

Fasciae

• General: A band of connective tissue (primarily collagen) beneath the skin that attaches, stabilizes, encloses, and separates muscles and internal organs. It's classified by layer (superficial, deep, visceral/parietal), function, and location. Like ligaments and tendons, fascia is oriented in a wavy pattern parallel to the pull direction.
• Superficial fascia:
  • The lowest layer of skin, blending with the reticular dermis.
  • A mixture of loose areolar and adipose connective tissue, influencing body shape.
  • Surrounds organs, glands, neurovascular bundles, and fills unoccupied spaces.
  • Present but lacking fat in eyelids, ear, scrotum, penis, and clitoris.
  • Facilitates skin movement, acts as a passage for cutaneous vessels and nerves, protects from heat loss.
  • Dense in some areas (e.g., scalp, palm, sole), containing collagen bundles.
  • Thin in others (e.g., eyelids, auricle), devoid of adipose tissue.
• Deep fascia:
  • A layer of dense fibrous connective tissue that surrounds individual muscles and divides groups into compartments.
  • High density of elastin fibers determines extensibility and resilience.
  • Examples include fascia lata, fascia cruris, brachial fascia, plantar fascia, thoracolumbar fascia, and Buck's fascia.
• Visceral or Subserous fascia:
  • Suspends organs within cavities, enveloped in connective tissue membranes.
  • Each organ is encased in a double fascia layer, separated by a serous membrane.
  • Outermost layer is parietal, innermost is visceral.
  • Specialized names exist for specific locations (e.g., meninges in brain, pericardium in heart, pleura in lungs, peritoneum in abdomen).
  • Visceral fascia is less extensible than superficial fascia due to its suspensory role. Laxity can lead to organ prolapse, while hypertonicity hinders organ mobility.