Bruce Heyer 12/12/07 Page 1 of 11 Vertebrate Structure & Adaptation Biology 6A / Sanhita Datta Winter 2008 As we have examined in a previous lab, one of the major keys to the success of vertebrate animals is the power, speed and agility resulting from having an articulated, bony endoskeleton. We can learn much about the correlation of body form and function by studying the basic structure of the vertebrate skeleton and some of the many derivations adapted to specific life histories. Bone provides support and movement via attachments for soft tissue and muscle. It protects vital organs, and plays a role in the metabolism of minerals such as calcium and phosphorus. The inorganic components of bone (largely calcium-phosphate hydroxylapatite crystals) comprise 60% of its dry weight and provide the compressive strength of bone. The organic component is primarily collagen, which gives bone great tensile strength. We will examine some details of the mammalian skeleton using our own large-brained, bipedal human structure, along with the more typically quadrupedal cat. We will then look at some functional variations in form displayed by a variety of mammalian skeletal parts. Thirdly, we will study the specializations of the tetrapod skeleton for flight as demonstrated by an assortment of birds. Development & Structure of Bone In mammals, there are two basic structural types of bone. Compact bone forms the outer shell of all bones and also the shafts in long bones. Spongy bone is found at the expanded heads of long bones and fills most irregular bones. Spongy bone also may contain marrow — yellow marrow for fat storage, or red marrow for the production of blood cells. Bone formation, or ossification, begins with an aggregation of embryonic mesenchymal cells (nonspecialized connective tissue in the mesoderm). These cells then develop into either fibroblasts that will lay down the collagen matrix, or osteoblasts to form bone cells. Specific bones are typically classified by their type of ossification: With intramembranous bone formation, also called dermal bone, ossification occurs within the developing dermis of the mesoderm. This process gives rise to the bones of the lower jaw, the paired bone plates forming the roof of the skull, and the clavicle of the pectoral girdle. It also produces dentin within teeth and other bone that develops in the skin, such as the scutes of turtles and crocodiles. Endochondral [“within cartilage”] bone formation, also called replacement bone, is the process by which bone is deposited within pre-existing cartilage skeletal elements. This is the mechanism that forms the majority of bones in the tetrapod skeleton.  Examine the prepared slides of both compact and spongy bone. Recognize the similarities and differences between the two types. Recall that bone is a type of living tissue. Identify the lacuna spaces for the osteocytes, and the central canals allowing penetration for blood vessels and nerves.  Observe the longitudinal sectioned human long bone with a dissecting scope. Identify the location of both compact and spongy bone. How does their form & location relate to their respective functions? The tetrapod skeleton can be subdivided into two major regions: I. The axial skeleton includes the skull and vertebral column along with the attached rib cage. Also included is the visceral skeleton with structures derived from the pharyngeal arches, but located in the throat region rather than in the skull. These are the bony and cartilaginousBruce Heyer 12/12/07 Page 2 of 11 components of the hyoid apparatus and larynx that provide attachment for the muscles of the tongue, pharynx, and voicebox, and the tracheal rings that keep the airway open. II. The appendicular skeleton includes the pectoral girdle with its associated forelimbs, and the pelvic girdle with the hind limbs. Some terms used to describe bone structure: • A process is a projection or spine extending from the surface of a bone to increase the surface area for muscle attachment. Larger processes imply larger, more powerful muscles. A ridge-like process may be called a crest. Two processes can fuse to form an arch. • A foramen is a hole in a bone, usually to allow the penetration of nerves and blood vessels. The most conspicuous is the foramen magnum, the hole in the back of the skull allowing the spinal cord to extend from the brain down the vertebral column. • A joint is a connection between bones. A suture is a fixed joint, such as between the plates forming the cranium. A symphysis is a joint with limited flexibility, such as the connection between the two halves of the jaw or pubis. The connection between bones at a suture or symphysis is made by connective tissue for membrane bone and by cartilage for replacement bone. A diathrosis is a freely movable joint. It is generally characterized by a condyle (swelling or protrusion) on one bone that fits smoothly into the fossa (pocket or groove) of the second bone. Each articulating surface is lubricated by an extremely smooth layer of hyaline cartilage, and the two units are held together by dense connective tissue ligaments. Diathroses come in many variations, but they can be classified into three functional groups. o A hinge joint, with a cylindrical condyle turning in a grooved fossa, limits motion primarily to a single plane. Example: mandible to skull. o A ball-and-socket joint, with a hemispherical condyle and nearly congruous fossa, allows movement and rotation in many directions. Examples: shoulder and hip joints. o A pivot joint, with a disk-shaped condyle in a notch-like fossa, restricts motion to rotation around the long axis of the bone. Example: radius pivots on the ulna as the wrist is rotated. See Campbell, Figure 49.26.  Examine a human skeleton. Observe the overall layout of the axial, visceral, and appendicular skeleton. Locate examples of a suture joint, a symphysis, and the three types of diathroses. For each diathrosis, identify its fossa and condyle. Demonstrate the range of motion for each of these joints in your own body.  Examine the cat skeleton. Locate the corresponding skeletal components described above for the human skeleton in the cat. Ia. Axial Skeleton — skull The skull is constructed of the cranium (braincase), facial bones, and mandible (lower jaw). Cranium. The base of the cranium is the occipital bone that forms around the foramen magnum. Here the skull