Vertebrate Structure and Development
The body skeleton which includes everything but the head has
a number of functions: 1) protect internal organs; 2) help in
respiration; 3) store minerals - especially calcium and
phosphorus; 4) provide a rigid internal frame; and 5) provide
muscle attachment and with movable segments allow movement.
We can divide the skeleton in another way. The axial
skeleton includes the head and backbone. The appendicular
skeleton includes the pectoral girdle and front limbs and the
pelvic girdle and hind limbs.
We've started our discussion of the axial skeleton so we'll
finish that first. The main structures left are the ribs,
vertebrae and sternum.
The vertebrae come from mesenchyme that aggregates on each
side of the notochord and neural tube. The somites are divided
into various parts: sclerotome, myotome, and dermotome. Dermatome
forms skin, myotome forms muscle, and sclerotome forms the
vertebrae. Sclerotome breaks up into mesenchyme and surrounds
the spinal cord, and notochord, and becomes cartilage. The sclerotomes break up into
mesenchyme and surround the spinal cord and notochord and also become cartilage.
Initially, the vertebrae are layed down as cartilage and later
replacement bone comes in.
The myotomes and the precursors to the vertebrae develop
between each other in an alternating fashion. In the middle of
each myotome (muscle segment) a spinal ganglion is formed. The
spinal ganglion innervates its muscle segment. The vertebrae
form between the spinal ganglia and therefore sit underneath 2
The vertebrae can be subdivided into various parts. The
main part or body is called the centrum. This sits below the
dorsal nerve cord (or spinal cord). The centrum may be
associated with the notochord or actually replace it. In some
vertebrate groups the centrum has 2 parts. The more anterior
part is the intercentrum and the hind part is the pleurocentrum.
Dorsal to the centrum is an arch - the neural arch which
surrounds the spinal cord. In some species a sharp projection -
the neural spine sticks up dorsally. Less frequently there is a
hemal arch and hemal spine ventral to the centrum.
When 2 vertebrae meet (articulate) it is by the centra or in
tetrapods by the centra and projections on the neural arch called
zygapophyses. The prezygapophysis sits anteriorly and the
postzygopophysis sits posteriorly. Some projections stick
straight out at the sides of the vertebrae. These are the
Anatomists can use the articular surfaces of a vertebra to
tell about functions, evolution and what group it belong to.
Amphicoelous means both ends of the centrum are concave (fish).
This allows limited movement. Procoelous means concave in front
and convex in back (anurans and reptiles). Opisthocoelous means
convex in front and concave in back (terrestrial salamanders).
These allow more movement. Acoelous refers to vertebrae than are
flat on both ends (mammals).
The notochord is present in the 3 classes of fishes
throughout life. In the advanced osteichthyes the notochord is
replaced by the centrum. In the fishes the vertebral column is
uniform along its entire length. This takes a drastic change
when vertebrates moved onto land. The spinal column became
involved in limb support and we started having differentiation.
In tetrapods the cervical vertebrae are in the neck. The
trunk vertebrae have the ribs (except anurans). The exception to
this is in amniotes (except snakes and turtles) where there are 2
types of trunk vertebrae: the thoracic vertebrae have ribs and
the lumbar vertebrae don't. Sacral vertebrae are associated with
the pelvic girdle and caudal vertebrae with the tail.
Some tetrapod exceptions or additions are the urostyle of
anurans. The synsacrum of birds which is made of fused trunk,
sacral and caudal vertebrae. The pygostyle of birds which
supports tail feathers. The sacrum of mammals which is fused
Ribs articulate with the vertebrae. Only the Agnatha lack
them. Fishes have both dorsal and ventral ribs. Tetrapods have
retained the dorsal set, but they have moved ventrally.
Ribs start as mesenchyme derived from sclerotomes and
lateral mesoderm. Initially cartilage is layed down and then it
may be replaced by bone.
The sternum is a ventral bone unique to tetrapods, but a
number of groups such as some amphibians, snakes and turtles lack
it. Birds have a ridge on the sternum called a keel (carina) to
attach flight muscles. We aren't really sure where the sternum
The appendicular skeleton includes the fins of fishes which
are used for movement and staying upright during swimming. In
fishes the pectoral girdle is larger and more complex than the
pelvic girdle. The pectoral girdle articulates with the skull.
In amphibians this connection is lost. The bones of the pectoral
girdle are the clavicle, scapula, cleithrum, supracleithrum,
suprascapula, and coracoid. The furcula of birds is the
wishbone. Tetrapods have larger pelvic girdles than fishes. The
3 bones are the ilium, ischium and pubis.
All vertebrates have paired appendages except Agnatha. Fish
used their fins for swimming and probably evolved fins to walk
along the bottom of the water or get on the edge to catch bugs.
The limbs of tetrapods are very similar in all 4 classes. There
is 1 proximal limb bone, then 2 bones in the next distal segment,
followed by numerous small bones - tarsals or carpals. Hand or
foot bones - metacarpals or metatarsals and phalanges. Some
mammalian distinct features include the calcaneus or heel bone
and the olecranon process. We also have tiny - usually nameless
bones embedded in tendons near joints called sesamoid bones.
These can show great individual variation. The largest, most
consistent is the patella. Some mammals have a bone within the
penis called the baculum.
Last updated on 1 Mar 2007
Provide comments to Lynnette Sievert at firstname.lastname@example.org
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