General Considerations of Endoskeleton

• First endoskeleton of the chordates and the principal endoskeleton of lower chordates.


  Notochord
• Internal skeleton of the body

  Endoskeleton
• In the vertebrates, the notochord is always more or less replaced by a skull and vertebral column.
• Parts of the endoskeleton:

  • Skull
  • Visceral skeleton - composed of gill arches
  • Vertebral column
  • Ribs
  • Sternum
  • Pectoral girdle
  • Pelvic girdle
  • Skeleton of the appendages
• Axial skeleton

  • Skull
  • Visceral skeleton
  • Vertebral column
  • Ribs
  • Sternum
• Appendicular skeleton

  • Pectoral girdle
  • Pelvic girdle
  • Skeleton of the appendages
• The endoskeleton develops from the mesenchyme. The mesenchyme accumulates in certain regions known as the skeletogenous regions where the skeleton is to be formed.
• The mesenchyme in the process of forming endoskeleton first produces cartilage.
• All of the endoskeleton proper is first composed of cartilage, and in the lower vertebrates the endoskeleton may remain wholly or partly cartilaginous.
• The endoskeleton of elasmobranchs, for example, is composed entirely of cartilage. Such cartilage may be and often is stiffened by the deposition within it of calcium salts. In such cases the cartilage is said to be calcified.
• Cartilage replaced during development by bone, which is secreted by osteoblasts, is said to be ossified.
• [t/f] TRUE

  Some bones develop directly from mesenchyme without passing through a cartilage stage. Such bones are called membrane bones.

  They are really derived from the dermis of the skin. So, they are consequently parts of the exoskeleton. They have sunk inward from their original position in the skin and have attached themselves to the endoskeleton with which they are now so closely associated that they must be considered a part of it.
• The vertebrae arise from the sclerotome. A sclerotome is a mass of mesenchyme originating from the medial wall of each epimere.
• There are 4 pairs of arcualia to each vertebrae:

  1. Posterior half of the two sclerotomes that form the anterior half of a vertebra

  • Basidorsals
  • Basiventrals

  2. Anterior half of the two sclerotomes that form the posterior half of a vertebra

  • Interdorsals
  • Interventrals
• The basidorsals extend dorsally around the neural tube to form the neural arch enclosing the neural tube.
• The interdorsals form an intercalary arc.
• The basiventrals form the hemal arch.
• The interventrals form the interhemal arch.
• Centrum - body or central mass of vertebra that is produced in either of 2 ways:

  1. By developing within the sheath of the notochord and becoming secondarily fused to the arcualia.
  2. From the arcualia, either by fusion of the bases of the arches, or from entire arcualia
• Centrum encloses the notochord
• Neural arch encloses the neural tube
• Hemal arch encloses blood vessels
• Apophyses – projecting processes of the centrum serving for articulations with adjoining vertebrae or with ribs or for the attachment of muscles.
• Dorsal or true ribs

  • A.K.A. intermuscular ribs
  • formed at the intersection of each myoseptum with the horizontal skeletogenous septum.
  • characteristic of majority of vertebrates
• Ventral or false ribs

  • formed at the intersection of the myosepta with the ventral skeletogenous septum.
  • A.K.A. subperitoneal ribs
  • lie outside the coelomic lining between the coelomic wall and the muscle layer
• Ventral ribs are a characteristic of bony fishes.
• The ribs are extensions of the basiventral arcualia.
• Girdle - crescent-shaped or arch-shaped portions of the endoskeleton that function for the support of paired appendages.
• Pectoral girdle supports the anterior appendages.
• Pelvic girdle supports the posterior appendages.
• Sternum or breastbone is an elongated structure lying in the median ventral line of the anterior part of the trunk region. It is commonly composed of a chain or cartilages or bones, or both.
• The ribs and the pectoral girdle usually articulate with the sternum. This allows the strengthening of the anterior part of the trunk and facilitates respiration.
• Paired appendages consist of fins in fishes and limbs in tetrapods.
• The pelvic girdle is commonly stronger and more massive than the pectoral girdle because support of posterior appendages is generally more important.
• 3 main parts of pelvic girdle

  • Ilium - commonly articulated to sacral vertebra
  • Pubis
  • Ischium
• 3 regions of hind limb:

  • Proximal segment - consists of thigh
  • Middle segment - consists of shank
  • Distal segment - consists of the foot or pes
• 3 main parts of pectoral girdle

  • scapula
  • procoracoid or precoracoid
  • coracoid
• 3 regions of forelimb:

  • Proximal segment - consists of upper arm or brachium
  • Middle segment - consists of forearm or antebrachium
  • Distal segment - consists of hand or manus
• Skull or cranium is the part of the endoskeleton which is found within the head that protects the brain.
• Skull is the most complex of all the parts of the endoskeleton because it is derived from several different sources.