Skeletal System

• Skeletons are supportive systems that provide protection, support, and a place for muscle attachment.
• In the hydrostatic skeleton of an earthworm, muscles in the body wall develop force by contracting against incompressible coelomic fluids.
• Alternate contractions of circular and longitudinal muscles of the body wall enable a worm to move forward.
• Muscular hydrostats work because they are composed of incompressible tissues.
• Examples of Muscular hydrostats: Elephant’s trunk, mammal & reptile tongues, cephalopod tentacles.
• Functions of Rigid skeletons are:

  • Provide anchor points for pairs of opposing muscles.
  • Provides protection & support.
• Two types of Rigid Skeleton: Exoskeleton and Endoskeleton.
• Exoskeleton – found in molluscs & arthropods and some other invertebrates.
• Endoskeleton – found in echinoderms, chordates, and some cnidarians.
• Vertebrate Endoskeleton is composed of bone and cartilage (types of connective tissue).
• Vertebrate Skeleton

  Bone provides support, protection, and serves as a reservoir for calcium and phosphorous.
• The notochord is a supportive rod found in protochordates and developing vertebrates.
• Notochord

  • Derived from mesoderm.
  • Except in jawless vertebrates, the notochord is replaced by the backbone.
• Notochord> Fibrous Sheath > Elastic Sheath
• Cartilaginous Skeletons:

  • Found in jawless fishes and elasmobranchs
  • Derived feature since their ancestors had bony skeletons.
• Bony Skeletons

  • Most vertebrates have this with some cartilaginous parts.
• Cartilage is a soft, pliable tissue that resists compression and is variable in form.
• Hyaline cartilage

  • has a clear, glassy appearance with chondrocytes surrounded by a matrix.
  • No blood vessels.
• Cartilage is often found at articulating surfaces of many bone joints, and as supporting rings of the passageways in the respiratory system.
• Cartilage similar to hyaline cartilage is found in many invertebrates.

  • Radula of gastropods
  • Lophophore of brachiopods
• Bone is highly vascular living tissue that contains significant deposits of inorganic calcium salts.
• Endochondral (replacement) bone

  • develops from another form of connective tissue – usually cartilage.
• Intramembranous bone

  • develops directly from sheets of embryonic cells.
  • Face, cranium, clavicle, dermal bone.
• Bone can vary in density.
• Spongy bone consists of open, interlacing framework of bony tissue, oriented to give strength.
• Compact bone

  • is dense
  • the open framework of spongy bone has been filled in by additional calcium salts.
• Compact bone is composed of a calcified bone matrix arranged in sets of concentric rings - osteons.
• Bones consist of bundles of osteons interconnected with blood vessels and nerves.
• Between the rings (osteons) are lacunae (cavities)
• Lacunae (cavities) is filled with osteocytes (bone cells) connected by tiny passageways that distribute nutrients.
• Bone is a dynamic tissue.
• Osteoblasts are bone building cells.
• Osteoclasts are bone resorbing cells.
• Hormones

  • are responsible for maintaining a constant calcium level in the blood.
  • Parathyroid Hormone for resorption
  • Calcitonin for deposition
• Axial skeleton includes the skull, vertebral column, ribs, and sternum.
• Appendicular skeleton includes the limbs and pectoral and pelvic girdles.
• Over time, the number of skull bones has been reduced from as many as 180 in some early fishes to 35 or fewer in mammals.
• The vertebral column serves as the main stiffening axis.
• Vertebral Column

  • In fishes it provides points for muscle attachment,
  • provides stiffness, and preserves body shape during muscle contraction – much like the notochord from which it is derived.
• Most vertebrates have paired appendages.