CVA - Cranial skeleton

• Germ layers:

  • Ectoderm: the outermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the epidermis and nerve tissue.
  • Mesoderm: the middle developmental layer between the ectoderm and endoderm, which gives rise to the skeleton, muscle, heart and bones.

  Endoderm: the innermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the lining of the gut and associated structures.
• 2 ways in which cells are organized into tissues

  • Epithelial organization: stationary and reside in 2-dimensional sheets
  • Mesenchymal organization: loosely packed and can move around
• Types of epithelium

  • Simple
  • Pseudostratified
  • Stratified
  • keratinized
• Connective tissues - tissues in which extracellular material predominates over cells
• Cartilage- Type of dense connective tissue that forms when mesenchymal cells develop into chondroblasts.
• 3 types of cartilage

  • Hyaline: most common in mammals, found at end of limb bonds, ventral ends of ribs, and larynx and tracheal rings
  • Elastic: dense network of branching elastic fibers, found in the external ear and epiglottis. 
  • Fibrocartilage: composed of large collagen fibers and found in intervertebral discs and mandibular and pubic symphyses
• Cartilage is the primary skeletal material of vertebrate embryos
• Cartilage is replaced by bone in most adult vertebrates, except hagfish, lamprey, etc.
• Perichondrium: fibrous sheath surrounding cartilaginous elements. Can include blood vessels. Has some passageways where nerves and blood vessels can travel through
• Cartilage of lamprey and hagfish remains highly cellular and lacks collagen
• Several proteins that make up the cartilage of lamprey and hagfish, myxin (hagfish), lamprin (lamprey)
• Calcified cartilage forms in some chondrichthyes when calcium salts infuses with hyaline cartilage
• Bone: a type of connective tissue that develops from osteoblasts that produce a matrix of polysaccharides and collagen bound to calcium phosphate in the form of hydroxyapatite
• Bones are laid down in layers or sheets
• Lamellar bone - slow growing

  Non-lamellar - fast growing
• Haversian bone - instead of being laid down in neat layers, bone is laid down in circular layers, or tubes. Most common in long bones, leg bones, and things that have a lot of force put on them
• Osteon - a tube that’s hollow in the middle for nerves and blood vessels

  Compact bone - looks like one solid bone
• Spongy bone - porous, less dense, laid down in a network of tubes that have pores around them. Makes up the head of long bones
• Hyaline cartilage is laid down and slowly replaced by bone, osteocytes and osteoblasts work outward to form bone
• Osteocytes - maintenance cell

  Bone is continually remodeled and replaced. Around 10% of your skeleton every year is digested and replaced.
• Intramembranous bone - forms directly within connective tissues without a cartilaginous precursor.
• Functions of the skeleton:

  • Protect soft tissues
  • Locomotion
  • Feeding
  • Respiration
  • House blood forming tissues
  • Reservoirs for calcium and phosphate
• Functions of the cranium:

  • Protect soft tissues, especially the brain
  • Food gathering and processing
  • Passage for respiratory flow (air/water)
• The cranial skeleton is composed of the skull and surrounding structures
• Lampreys and hagfish have rudimentary craniums, composed of lots of sheets of cartilage. Roughly articulated bones that protect the sensory organs.
• Chondrocranium – brain case, a cartilaginous element that covers most of the sensory organs. In most vertebrates the the cartilaginous plates are transitory and are soon replaced by bone
• Chondrocranium has 3 parts: ethmoid cartilage, sphenethmoid, basisphenoid. Some of this is homologous to Osteichthyes skulls down the line.
• Splanchnocranium – the skeleton associated with the pharyngeal or gill arches. Has 2 specialized pharyngeal arches, and includes the jaws (palatoquadrate and mandible)

  Cartilaginous in chondrichthyans.
• Supports the gills and offers attachment for the respiratory muscles, contributes to the jaws, hyoid apparatus or gnathostomes, and inner ear bones of mammals.
• Chondrocranium and splanchnocranium usually have bones develop over it, or are replaced
• Dermatocranium is composed of many bones that develop independent from each other, but later fused together into composite sheets.
• In the skull of eusthenopteron (fish), the splanchnocranium came first