Lab 3 Answers

Lab 3: Sample Solutions – Post-cranial Skeleton Please note that some of the sample solutions provided below include additional explanatory information to help support the answer. Some of the details included in these answers are found in the display materials in the lab. Vertebral Column Spinal Cord Notochord Dorsal Hollow Nerve Cord Structure & composition: - strong, supportive structure - made up of vertebrae, which are composed of cartilage or endochondral bone Function(s): - protects delicate spinal cord; muscle attachment sites, axial support, attaches to pelvic girdle (in tetrapods) and ribs Embryonic origin/Development: -Sclerotome epimere mesoderm Structure & composition: - Delicate structure comprised of neurons and support cells Function(s): - Part of the central nervous system - Used to rapidly send messages (action potentials) Embryonic origin/Development: - formed during neurulation by invagination of neural plate ectoderm Structure & composition: - flexible rod Function(s): - allows lateral flexion but prevents anterior/posterior collapse of the body Embryonic origin/Development: - arises from mesoderm Structure & composition: -hollow, fluid-filled structure Function(s): - forms the central nervous system (the brain and spinal cord) Embryonic origin/Development: - formed during neurulation by invagination of neural ectoderm 1. Observe the microscope slides of the three different types of cartilage. What is the main difference in their structure? (Hint: look at the protein fibres of the cartilage matrix.) Predict how these structural differences affect the functions of these three different cartilage types . The main structural difference is the type (and relative amount and arrangement) of protein fibres within the polysaccharide matrix. Fibrous cartilage contains many protein fibres, and they are mostly collagen (type I and type II). Fibrous cartilage is very tough and can handle both compression and tension (the many collagen fibres deal well with tensile forces). Elastic cartilage also contains many protein fibres – it has both elastin and collagen fibres with a higher proportion of elastin fibres, which allows this type of cartilage to be very flexible and able to handle repeated bending (it is best at handling tensile forces but not compressive forces). Hyaline cartilage has relatively few protein fibres, and they are mostly collagen (type II collagen), so it is less stretchy and flexible than fibrous or elastic cartilage but can handle compressive forces well. 2. Examine the display material (microscope slides, pictures, and models) on the structures of cartilage and bone . a) Describe two similarities in the structure of bone and cartilage. • Both bone and cartilage are composed of a dense, supportive matrix. • Both bone and cartilage contain living cells that are located within lacunae (tiny spaces within the dense matrix). b) Describe two differences in the structure of bone and cartilage. 1

• In bone, the matrix is composed of calcium phosphate (and other mineral salts) and protein fibres, while in cartilage the matrix is composed of polysaccharides and protein fibres (such as elastin and collagen). • In bone, blood vessels and nerves pass through canals in the bone matrix, while the cartilage matrix contains neither nerves nor blood vessels. • Compact bone has a network of canals like Haversian canals, Volkmann’s canals, and tiny canaliculi that pass through the bone matrix to connect the lacunae, while cartilage has no canals and the lacunae are not connected. Consider: how do these structural differences affect the way that the living cells in bone and in cartilage are able to receive nutrients, gases, and other substances, and get rid of wastes? Cartilages are thinner and substances must diffuse through the cartilage matrix to/from the cells. 3. Two types of forces that can act on bone are compression (pressing or pushing) and tension (pulling or stretching). Which one of these two forces do you think bone is best able to withstand? Explain. Bone is strongest when withstanding compressive forces. Bone is a hard, stiff tissue, so can withstand a great deal of compressive pressure. However, bone is not elastic, so it does not withstand stretching as well as soft tissues such as tendon or muscle, which are more elastic. 4. Name the endochondral bones you have seen in this week’s lab. Name the dermal bones you have seen in this week’s lab. Answer this question after you have viewed all of the displays for this lab. Endochondral bones: Vertebrae, ribs, scapula, suprascapula, coracoid, ilium, ischium, pubis, humerus, radius, ulna, carpals, metacarpals, phalanges, femur, tibia, fibula, tarsals, metatarsals Dermal bones: Clavicle (and cleithrum, supracleithrum, post-temporal ) 5. Consider the structure of a long bone such as the femur of a mammal. Look at the cross-sections and longitudinal sections on display to see the internal structure. a) Make a simple drawing of a longitudinal section of the femur and label the following: periosteum, trabecular bone, and compact bone. b) What is the function of the trabeculae? Trabeculae are supportive struts that reinforce bones along stress lines. Trabeculae increase the strength of the bone while still allowing it to have some spaces inside – these spaces help to reduce the overall weight of the bone. The spaces in between the trabeculae often contain marrow, which produces blood cells and stores fat. c) The femur is an endochondral bone. Imagine finding a femur while walking in the forest. How could you determine whether this femur was from a young mammal or an adult mammal? 2

Amphicoelous Chondrichthyes, Osteichthyes, some Amphibia Concave on both ends Procoelous Some Amphibia, some Sauropsida Anterior is concave, posterior is convex Opisthocoelous Some Sauropsida Posterior is concave, anterior is convex Acoelous Mammalia Relatively flat on both ends (no concavities on either end) Heterocoelous Aves (cervical region) (and cervical region of Testudinata) Saddle-shaped b) Amphicoelous centra have a narrow, hollow tunnel that passes through the middle of the centrum from anterior to posterior. What structure is located within this tunnel? The notochord passes through the centrum of each vertebra. 8. In mammalian quadrupeds (animals that use four limbs for locomotion) such as cats, the limbs act like upright pillars (columns) that support the body weight. The vertebral column acts like a bridge that transfers the weight of the internal organs, the head, and the tail, to the forelimbs and hindlimbs. a) Examine a cat skeleton. Describe the general shape of the vertebral column in the region between the forelimbs and the hindlimbs. The vertebral column is shaped like an arch, with the highest (most dorsal) point of the arch midway between the forelimbs and the hindlimbs. Anterior to this high point, the vertebral column curves ventrally towards the forelimbs, and posterior to this high point, it curves ventrally towards the hindlimbs. b) Examine the neural spines of the vertebrae in the cat skeleton. Do the neural spines in the lumbar region point towards the same direction as the neural spines in the thoracic region? Describe your observations. No; generally the neural spines in the lumbar region point towards the anterior of the body while the neural spines in the thoracic region point posteriorly. 4

9. In birds (Aves), the sacral, lumbar, and some caudal and thoracic vertebrae all fuse together to form the synsacrum , making this large section of the vertebral column completely inflexible. What are the functional advantages of the synsacrum in birds? The rigid synsacrum stabilizes the trunk of the body during flight. Fusing these vertebrae together also means that fewer muscles are required to stabilize the vertebral column, decreasing body weight for flight: when there are mobile joints between vertebrae, each vertebra requires several pairs of muscles to move it and stabilize it, whereas if many vertebrae are fused together, each vertebra does not requires its own separate muscles, so the overall number of muscles can be decreased. Fusing vertebrae together also strengthens this region of the vertebral column, such that individual vertebrae can be smaller while still providing the same support, and this also decreases weight. 10. a) Examine the vertebrae on display. Identify the various structures or structural features that allow you to differentiate among cervical, thoracic, lumbar, sacral, and caudal vertebrae in mammals. Region Cervical Thoracic Lumbar Sacral Caudal Which structures allow you to identify vertebrae from this region? transverse foramina (holes in the transverse processes protect blood vessels to the head) sites (on the transverse process and on the centrum) for articulation with ribs (rib facets) no transverse foramina, no rib facets; (sometimes) large transverse processes Sites for attachment to the ilium of the pelvic girdle. A sacrum may be composed of an individual vertebra (e.g. non-amniote s such as frog or salamander), but often several vertebrae fuse together (e.g. many amniotes) Often elongated, with reduced or absent processes. (Often it is the centrum with very small processes.) b) In Amniota, the two most anterior cervical vertebrae are specialized: these are the atlas and the axis. How can you distinguish the atlas? How can you distinguish the axis? Both the atlas and axis are cervical vertebrae, so they have transverse foramina. The atlas has no centrum nor neural spine. The axis has an odontoid process that protrudes on the anterior side of the centrum. 5

explain the locations of the bones. Also consider whether there are any useful ‘Landmarks’ or functional regions in the pelvis that are useful for your identifications.) The ilium attaches to the vertebral column via the sacral region. The pubis is located more ventrally/anteriorly, while the ischium is located more dorsally/posteriorly. The Ilium, ischium and pubis bones all unite and fuse together at the location of the joint for the proximal end of the femur (the acetabulum). 13. Examine the tetrapod limbs on display in the lab, and compare the different types of foot posture. a) Which bones contact the ground in plantigrade tetrapods? In digitigrades? In unguligrades? Plantigrades: carpals/tarsals, metacarpals/metatarsals, phalanges Digitigrades: phalanges Unguligrades: only the most distal tips of the phalanges b) Are the limbs longer (relative to the rest of the body) in plantigrades or in digitigrades and unguligrades? Explain why – consider both the relative length of the metacarpals/metatarsals and the location of these bones relative to the foot. Digitigrades and unguligrades generally have relatively longer limbs than plantigrades. The metacarpals and metatarsals are relatively longer in digitigrades and unguligrades compared to in plantigrades. Also, in plantigrades, the metacarpals and metatarsals are part of the foot, while in digitigrades and unguligrades, the metacarpals and metatarsals are incorporated into the leg rather than the foot. Therefore, the metacarpals/metatarsals increase the overall limb length in digitigrades and unguligrades compared to in plantigrades. c) What is a functional advantage of unguligrade versus plantigrade foot posture (e.g., for running)? What is one functional disadvantage of unguligrade versus plantigrade foot posture? Explain. Unguligrades have relatively longer limbs than plantigrades, so they have a greater stride length, meaning that they can cover more distance with each step – this makes running faster and more efficient. Tetrapods with plantigrade foot posture are more stable since a larger surface area of the foot contacts the ground than in unguligrades (for example, plantigrades are better able to balance on only two feet). In humans, plantigrade foot posture can also be more efficient for walking long distances (perhaps this is also true for other bipedal vertebrates, but it has only been studied in humans). 14. How many digits are present in a human foot? How many digits are present in a horse foot? What is a functional advantage for the horse of having a reduced number of digits? Humans have five digits per foot while horses have only one. Decreasing the number of digits decreases the relative mass of the foot. By decreasing the mass on the distal end of the limb, it decreases the inertia that must be overcome to swing the limbs during locomotion, decreasing the energy required for each step and therefore increasing the efficiency of locomotion. This also allows the limbs to swing forward more quickly, which increases the stride rate. 7