Lecture 9 Flashcards
List the subdivisions of the vertebrate skeletal system.
Vertebrates are characterized by an internal skeleton made of cartilage and/or bone. This internal framework can be divided into subdivisions as follows:
• Axial skeleton:
• Chondrocranium:
- Braincase and capsules for organs of special sense
• Viscerocranium:
- Gill arches and jaws
• Vertebral column
• Appendicular skeleton:
• Limb girdles (pectoral and pelvic)
• Limbs (forelimbs and hindlimbs, including fins
Describe vertebral column
The vertebral column of vertebrates is an obvious example of segmentation in vertebrates.
• The vertebral column consists of a series of segments called vertebrae as well as additional related structures.
• The vertebrae and related structures may be composed entirely of cartilage or bone or both.
Describe centrum
- The centrum is typically a cylindrical structure that is connected to similar cylindrical centra both cranially and caudally.
- This chain of centra replaces the notochord of the protochordates, such as amphioxus, as the principal axial support structure.
- This chain of centra is flexible, but may not be as flexible as a notochord.
- A flexible chain of centra would be useful for swimming, but would not be as useful for supporting a terrestrial life-style.
- In many vertebrates (including humans) parts of the notochord may persist within and/or between adjacent vertebrae.
Describe neural arch
- Sits on top of the centrum (dorsal surface) and provides a protective passageway for the spinal cord.
- A neural spine often projects from the dorsum of the neural arch.
- Provides lever arm for muscle attachment.
- In Latimeria the neural spines are hollow and filled with cartilage – “coelacanth” means hollow spine.
- A corresponding hemal arch is present on the ventral surface of the centrum of each of the caudal (tail) vertebrae.
- A caudal artery and vein pass through the hemal arch.
Components of a shark vertebral column
A Intercalary plate (fill in gaps between adjacent neural arches and protects spinal cord) B Neural spine C Neural arch D Spinal cord E Centrum of vertebra F Caudal artery G Caudal vein H Hemal arch I Hemal spine
Regional variations within vertebral column in fishes
• Precaudal vertebrae:
- Include all the more cranial vertebrae that lack a hemal arch
• Caudal vertebrae:
- Include all the vertebrae caudal to the precaudal
vertebrae
- Possess hemal arches
Vertebral column fishes
Centra tend to be more ossified.
• Each neural arch is topped by a long, slender neural spine.
• Prominent notochord instead of ossified centra are still retained in more primitive bony fishes such as the sturgeon.
• Pelvic and pectoral girdles are not tied into fish vertebral column.
Compartmentalization of vertebrate body
- The vertebrate body plan is tied together by sheets of connective tissue referred to as myosepta.
- The myosepta tie into the vertebral column.
- Collectively, the myosepta form compartments in which are enclosed masses of skeletal muscle.
Vertebral Column: Early Tetrapods
Early tetrapod centrum developed from two parts:
• Intercentrum.
• Pleurocentrum.
• One or the other or both contributed to the formation of the centrum in early tetrapods.
• Vertebral column replaced notochord as the main axial support.
• Precaudal vertebrae became specialized (correlated with terrestriality):
• Cervical: articulated with skull
• Trunk: processes for rib attachment
• Sacral: articulated with pelvic girdle
• Pelvic girdle tied into vertebral column via ilia.
Describe vertebral column of Crossopterygians
Each centrum consisted of:
• Single, larger intercentrum:
- “U”-shaped and cranial
• Paired, smaller pleurocentra:
- Caudal and dorsal to intercentrumDescribe vertebral column of Stereospondylous labyrinthodonts
Centrum entirely derived from
intercentrum
Describe vertebral column of Embolomerous labyrinthodonts
Intercentrum and pleurocentrum
equal in size.
Describe vertebral column of modern amphibians
Tendency in later tetrapods was to reduce:
- The intercentrum to a small,
cranial, ventral element to which attached the capitulum (head) of the rib.
- Considerably increase the size of
the pleurocentrum.
• In later tetrapods, notochord was
either completely eliminated in the adult or persisted as a slender rod passing through the pleurocentrum
or confined to spaces between intercentra.
• Uncertain in modern amphibians whether centrum is derived from pleurocentrum or intercentrum.
Vertebral column of salamanders
Regional variations within the vertebral column seem to be correlated with terrestriality.
• Specializations of precaudal vertebrae (First seen in primitive tetrapods):
• First precaudal vertebra = cervical vertebra
- Only one in amphibians
• Last precaudal vertebra = sacral vertebra
- Only one in amphibians
• Vertebrae between cervical and sacral are trunk vertebrae
- Numerous in salamanders
- Reduced to about six or seven in anurans
- Transverse processes have attachment sites for ribs.
- Neural arches with short, blunt neural spines are located on the dorsal surfaces of the centra.
• Caudal vertebrae in salamanders also have hemal arches
Describe ribs in Necturus
Ribs are bicipital:
• The tuberculum attaches to the dorsal part of the transverse process (diapophysis).
• The capitulum attaches to the ventral part of the transverse process (parapophysis).
Vertebral column in Anurans
Regional variation in anuran vertebral column:
• Cervical vertebra* (1)
• Trunk vertebrae* (variable)
• Sacral vertebra* (1)
• Caudal vertebrae (1)
- Urostyle = single caudal vertebra in
anurans.
- Modified adaptation for saltatory
locomotion:
- Attachment site for muscles involved in
jumping.
• Ilia tie pelvic girdle into vertebral column
through sacral vertebra.
• Collectively comprise the precaudal vertebrae
Two pairs of zygapophyses in salamanders
The cranial (anterior) zygapophyses have articular facets that face dorsally. • The caudal (posterior) zygapophyses have articular facets that face ventrally
Describe amniote vertebra
Parts of an amniote vertebra are similar to those of the
Necturus vertebra.
- They consist of:
• Centrum
• Neural and hemal arches (with spines)
• Transverse processes
• zygapophyses
Describe amniote centrum
The centrum functionally replaces the
notochord.
• Classification of centrum based on shape of
cranial and caudal ends:
• Amphicoelous: both ends are concave
(Some reptiles)
• Procoelous: only cranial end is concave
(Most reptiles)
• Opisthocoelous: only caudal end is concave
• Acoelous: both ends are flat (Mammals)
What can be found in tails
In the tails of reptiles and some mammals
small pieces of bone may be found between
the ventral edges of centra:
• Intercentra (or hypocentra)
• Form hemal arches (chevron bones)
Describe pleura/intercentrum in amniotes
- In primitive reptiles the bulk of the centrum
is formed from the pleurocentrum. - In modern reptiles and mammals the intercentrum has disappeared and the centrum is formed entirely from the pleurocentrum
- Intercentrum persists in Sphenodon
Describe neural arch in amniotes
Neural arch consists of a neural spine and
diapophyses.
• Diapophyses are for rib attachment.
Neural arch is composed of lateral and dorsal plates which form the neural canal.
• Dorsal plates are the laminae and meet in the midline to fuse with the neural spine.
• Lateral plates are the pediceles which fuse to the centrum ventrally and to the laminae dorsally to form the transverse processes.
• The posterior edge of each pedicele is typically notched so that when vertebrae are articulated, holes are formed between them to allow the passage of spinal nerves.
Describe development of vertebral column in amniotes
The vertebral column develops from the
sclerotome portion of the embryonic somites.
• Somites are paired blocks of mesodermal tissue that form along the notochord during embryonic
development.
• Each somite consists of three major
components:
• Dermatome:
• Forms dermis of integument.
• Myotome:
• Forms axial muscles.
• Sclerotome:
• Forms parts of vertebrae.Describe sclerotomal breakup
• Sclerotomal portion of each
somite breaks up into an anterior and a posterior
portion.
• Posterior portion of one
somite joins the anterior portion of an adjacent
somite, etc.
• This creates gaps that allow
nerves to grow out from the neural tube into the epaxial segmental musculature derived from the myotomes.
