Laboratory 1, Bio 325, 2011
Skeletons and some materials and tissues
Preamble
Vertebrate bones can be re-assembled, glued and wired into their relative positions as they were
during life. Such a „specimen‟ (see rat skeleton of today‟s lab) shows all of the struts and none of the ties.
[Struts are designed to resist compression: pushes or squeezes in other words, forces (weights) acting to
crush; ties are designed to resist tension: pulls in other words, forces acting to rend.] Tendons, ligaments
are ties, they join to bone: on such a prepared „skeleton specimen‟ there are no ties present: so it is very
incomplete. [Also missing are muscles, the „power units‟, the structures that do work in a certain time in
order to make things move.] The point is that skeletons need to be understood together with their ties in
order to understand their function. Looking at a rat skeleton doesn‟t tell you a whole lot about how it
works.
“…the form of an object “-- animal body part –“is a diagram of forces” (D‟Arcy Thompson).
This first lab illustrates three major skeletal types: hydrostatic skeleton (earthworm), endoskeleton
(vertebrate) and exoskeleton (insect); it also presents some of the materials (tissues) that make up animal
bodies. [Tissues are similar cells specialized and grouped together to serve a common function.
Examples range from muscle fibres to epithelium, from nervous tissue to connective tissues such as
bone, cartilage, chitin. Many materials are composite: gaining functionality, better strength or better
resilience, from being more than one material in combination; the materials retaining their separate
identity and properties within the combination.] In the general body plan of an animal, skeleton is a
critical component and both skeletons and body plans are best understood in the context of locomotion.
There are three phyla involved in the lab today: Arthropoda, Chordata [subphylum Vertebrata] and
Annelida. Major features of each phylum are given below. Lab directions are underlined.
1. Phylum Chordata: Subphylum Vertebrata
The animals placed in this taxon share the following structural features (diagnostic: meaning these
features are exclusively chordate). Some of these structures are more obvious in the embryo. Bilateral
symmetry is also a feature of vertebrates, but it is not diagnostic.
1.
2.
3.
4.
They have a notochord (or its evolutionary descendant the vertebral axial skeleton)
Hollow nerve cord situated dorsally in the body
Pharyngeal pouches/gill slits, i.e., perforated pharynx
Blood circulation proceeding forward in a main ventral vessel and posteriorly in a main dorsal
vessel.
5. Tail: that is, the digestive tract‟s ending is not terminal.
Chordates are divided into four subphyla: Urochordata (tunicates), Cephalochordata (amphioxus
Branchiostoma), Hemichordata (acorn worms), Vertebrata.
You already know a lot about animals: to demonstrate this to yourself, list the common names of ten
species of fish, ten amphibians, ten reptiles, ten species of bird and ten species of mammal.
2. Phylum Arthropoda
This phylum is very large: it contains more than three times the number of all other animal
species combined and the largest taxon within the phylum is of course the insects (>750000 spp.)The
animals placed in this phylum share the following structural features (not necessarily all diagnostic:
Triploblastic coelomate bilateria; metamerism (coelom forming by schizocoely of mesoderm); metamerism
with extensive tagmatization, chitinous exoskeleton molting under hormonal control, dorsal anterior brain
followed by a ventrally located nerve cord with segmental ganglia; holoblastic determinate cleavage
during development; specialization of serial appendages.
Chitinous exoskeleton is diagnostic. To understand the above „recipe‟ of the phylum you need to
understand several important concepts such as tagmatization (grouping of serially repeated body
segments so as to serve a common function: e.g, abdomen, or thorax, or head of an insect.
You already know a lot about arthropods. Using just common names you know beetle, butterfly,
grasshopper etc. Each of these common names corresponds to a order within the class insecta. Thus
order coleoptera contains all the beetles, order lepidoptera all the butterflies and moths and order
orthoptera grasshoppers, crickets, katydids „and their allies‟. We do a great deal of work with the
migratory locust, genus locusta migratoria in this course. Explain how the insect tagma „abdomen‟
because of its form might usefully serve diverse functions.
3. Phylum Annelida
The animals placed in this group are not common in our daily north temperate terrestrial
environments: mostly we see only leeches and earthworms. Most of the ~8800 species of annelid are
marine and include burrowing sediment feeders such as Arenicola, the lugworm and predatory swimmers
such as Nereis, the clamworm. Here is a recipe for Annelida, to compare with the one above for
Arthropoda.
Triploblastic coelomate bilateria; body cavity a schizocoel metamerically segmented; limited
tagmatization; longitudinal and circular muscles surround a hydraulic skeleton (coelom); paired setae on
each segment; extracellular digestion in a straight digestive tract running from anterior mouth to posterior
anus; gut supported in coelom by longitudinal mesenteries and septa; circulatory system closed (within
vessels so higher pressure; exretion by nephridia (specialized tubules); ventral nerve cord with segmental
ganglia and anterior brain.
You already know something about annelids: leeches have suckers and eat blood, that
earthworms crawl underground and are hunted by robins. These two taxa (hirudinea and oligochaeta
respectively) are more specialized than their common marine relatives (polychaeta). What structural
features of leeches and earthworms might one expect as a result of their more specialized existence?
Skeletons and translocation of forces
There are three major sorts of skeleton: hydrostatic, exoskeleton and endoskeleton. All three
types translocate forces. For example, a limb muscle may originate on one bone, cross a joint, and
inserts on the next more distal bone. (Muscle „origin‟ is taken as the end of the muscle moving least; while
the insertion is the end of the muscle most displaced.) Contraction of the limb muscle causes one bone to
pivot on the other. The force generated by the contracting muscle is 'translocated' (its point of application
is shifted) along the bone from origin to insertion. [Vertebrates use cartilage to cover the surfaces where
the bones slide or pivot upon each other to reduce friction; and there is also synovial fluid enclosed in a
sac as a lubricant at such a joint.]
Arthropods also translocate forces, but with exoskeleton made of chitin and stiffened to some
degree by sclerotization (see darker parts of the exoskeleton). A large inflection of the exoskeleton
(cuticle) called an apodeme can project inward into the body from a mandible or between leg segments.
(If you have eaten crab or lobster you have stripped muscle from one of these smooth rubbery blades
inside the claw.) Muscle inserts on this apodeme, running to it from some other exoskeletal location;
when the muscle contracts the force imparted to the apodeme is translocated to the insertion.
And many animals or parts of animals also move and change shape without these relatively rigid
skeletal elements: instead they use fluid as a way of translocating forces: these are hydrostatic skeletons.
[„Static‟ refers to the fact that the fluid involved doesn‟t circulate, though it clearly moves when the animal
changes body shape.]
Materials: struts and ties: tensile, pliant, rigid
Mentioned above was the idea of a strut (a „member‟ that resists compression: the two by four
stud in the wall of a frame house). Vertebrate bones can act as effective struts resisting compression;
tendons, ligaments are the ties used by vertebrates; they join muscle to bone (tendons) and they can also
function as a kind of cordage („rope‟) (ligaments).Tendons are stiffer (less elastic) than ligaments. A
muscle works only by shortening, by pulling on a bone and stretch in a ligament would reduce the
effectiveness of force translocation: instead of pulling the bone where it is supposed to go, some of the
force would be wasted in stretching the tendon if it were too elastic. Elasticity can have a useful
cushioning effect by absorbing unwanted forces. Thus for example, a prominent elastic nuchal ligament
associated with the neck of many mammals carries the head with reduced forces in running.
Solid materials can be classified by how they behave mechanically: tensile, pliant, rigid. They can
be classified by whether they are simple or composite: which means comprised of one „chemical species‟
vs having a combination of two or more components. In composite materials the resulting material retains
its properties and so the composite material reflects different contributions from its constituents (see
below).
TENSILE MATERIALS are effective in resisting being pulled: a kind of „biological cordage‟ or „animal
ropes‟. Four kinds are common among organisms (Vogel 1988, 2003):
1. Silk: cocoons of Lepidoptera, webs of spiders, feeding nets of caddisfly larvae (Hydropsychidae):
„pure‟ in the case of spider silk but a composite material as with „glue‟ in a cocoon.
2. Collagens: proteins; collagen is a composite material used in skin, artery walls, muscle; makes
muscle (meat) tough
3. Cellulose: sugar polymer [polysaccharide] is the basic tensile material of plants; usually
functions as a composite in plant cell walls, pure in „drag-increasing‟ fibres of wind-dispersed
seeds; ropes; fabrics such as linen, cotton; rayon is cellulose extracted from wood; cellulose is
rare in animals but occurs in tunicates (sea squirts).
4. Chitin: after collagen the most widely used tensile material in animals. It connects muscles to
exoskeleton in Arthropoda (insects, crustaceans, spiders etc.) as the material of apodemes.
PLIANT MATERIALS. The usefulness of some materials arises from their deformation under stress (or
strain). “…no material is perfectly rigid” (Vogel 1988).
1. Springs/elastes [„elastes‟ is a word for an organ that assists the act of leaping, see „elater‟].
These work by strain energy storage. They act like rubber. Three „rubbers‟ exist in nature and
may form part of animal elastes: resilin, abdunctin, elastin. They function by storing energy. A
muscle does work by exerting a force through a distance and this „work of extension‟ stretches
the elastes i.e., deforms it. This energy can then be delivered back at a later time and faster.
Being faster is often adaptive. The faster this stored work is returned the greater the power.
(Power is the rate of doing work.) a) resilin: localized small pads near insect wing bases that store
and return energy from wing upstroke to downstroke and vice versa; b) abductin: located at the
hinge of a bivalve as a ligament that enables shell opening in opposition to the adductor muscles;
c) elastin: mammals have this in skin and artery walls: the “nearest thing to pure elastin is the
nuchal ligament of large grazing mammals” (Vogel 1988). All three of these rubbers are proteins
but the protein structure is quite different. “…pulling on them amounts to pulling on molecular
entanglements rather than on chemical bonds, which is why they are stretchier than the tensile
materials” (Vogel 1988).
2. Pliant composites: composite materials are common in animal bodies. Usually two materials are
together to make up a third material but the components retain their separate identity. Fibreglass
is an example of a composite material: oriented glass fibres and a „glue‟ (resin); plywood is
another (wood and glue with varying grain. Bone is a composite material in animal skeletons: the
protein collagen combined with bone salts such as calcium carbonate. To see the usefulness of
composite materials consider one that is not: glass. A crack starts running through the glass
because perhaps the framing is stressing it. There are compressive/tensile forces producing
stresses and strains within the glass. As a crack moves through the glass the broken cracked part
lengthening behind is no longer under stress/strain: the forces concentrate at the farthest point
reached by the crack; this is why it tends to continue. But in a composite material like bone, the
inclusion of collagen fibres which are more elastic than the salts, will allow the tip of a crack to
withstand the stress/strain concentration. In fibreglas cracks that start in glass fibres tend to stop
at the resin which has greater give (elasticity).
RIGID MATERIALS: These are materials that resist stresses without undergoing much deformation. All
are composites – rather than a simple chemical compound.
1. Arthropod cuticle/exoskeleton: a composite of chitin fibres within a proteinaceous matrix. [Note
that this can also be a tensile material – see apodemes above.] In Crustacea (crabs and allies)
Calcium Carbonate salt gets added to the cuticle. The chitin fibres are arranged in sheets with
variable orientation analogous to plywood.
2. Bone: This is a composite of collagen fibres (about 50% by volume) with some other protein and
deposited Calcium phosphate salt. Mammalian bone starts out as cartilage in the embryo and
then is gradually replaced by bone in some but not all, of the developing skeleton. (In some
animals such as sharks cartilage is the definitive adult skeleton.) Cartilage tends to be retained in
the adult skeleton wherever flexible support is required: the ends of the ribs where they meet at
the sternum, the external ear (pinna), the nose. (Why do these work better if they are flexible
instead of brittle?) Cartilage also covers the surfaces where bones move together upon each
other at joints: along with synovial fluid it lubricates and reduces friction.
3. Keratin: a composite material of microfibrils in a matrix this is the constituent of hair, horns, the
feathers of birds, baleen of whales.
4. Wood: cell walls of plants, cellulose microfibrils specifically oritented in an unstructured
matrix mainly of a substance called lignin
5. Stony materials or ‘biological ceramics’
Tissues
Histology is the study of tissue. Tissue is an aggregation of similarly differentiated cells,
morphologically similar cells associated to perform a common function: e.g. the tissue of the cortex of the
adrenal gland secretes hormone. The tissue of the brain encourages thinking. There are four major types
of tissue: muscle, nerve, epithelial (tissues that line surfaces) and connective (cartilage, bone etc.).
Muscular tissue made up of cells also called fibres that are contractile and can shorten. Nervous tissue
with thin far-reaching cell extensions whose depolarization conveys information. Epithelium is tissue that
invests (covers) body surfaces – not meaning the integument only but also the surface of organs, the
inner surface of the gut tube, the lining of the lungs, the „inside‟ of urogenital ducts. Connective tissues
are made up of fibres and matrix (extracellular cell-secreted material that accumulates into a continuous
multibranching „web‟ that shoves the secreting cells farther apart during its production); the matrix may be
the functional part of the tissue as in cartilage or bone.
All tissues can be considered to consist of cells and matrix but in epithlial tissue very little
intercellular matrix is evident: this is characteristic of epithelia.Epithelia have many possible functions:
protection (skin), secretion (glands), sensory reception (eye) etc. Two basic kinds of epithelium: simple
and stratified. The first is a single carpet of cells; the second is characterized by several cell layers. These
rest upon a basement membrane (matrix). Epithlial cells sometimes described per their shape as cuboidal
or columnar. In the latter the cell height is greater than its width and the nuclei are not central but basal.
Simple columnar epithelia are found in vertebrate respiratory systems, lining digestive tracts where they
function in digestion, lining many glands and ducts. The free surface of the epithelial cells, at the end
distad of the basement membrane may show specializations such as a striate border (short protoplasmic
filaments, e.g., intestinal epithelium. Secretory epithelium by its nature goes through cycles involving
elaboration, storage, discharge, resting: this will mean differences in structural appearance.
Stratified epitelium such as in the skin of a vertebrate involves a layer of cells resting on an
innermost basement membrane. Daughter cells are continually produce by cell division of this layer and
are pushed more peripherally becoming more flattened and ultimately „squamous‟ epithelium that is then
sloughed off. So the skin of a tetrapod renews itself from below.
Connective tissue contrasts with epithelia in that the cell is no longer functionally important but
rather the matrix produced by the cell. In the embryo all connective tissue is the same: mesenchyme:
stellate (star-shaped) cells that are mobile and move about in a relatively fluid environment. With
maturation different forms of connective tissue are produced. White fibrous connective tissue is
characterized by collagen (a protein) fibres. This is the most abundant protein in the human body: one
third of total protein and 6% of body weight. Collagen fibres have a high tensile strength. They are „wavy‟
and do not branch. Collagen fibres are the principal component of tendons and ligaments.
There are also reticular fibres: these are relatively thin and form a delicate network or reticulum.
Reticular fibres form the basement membrane of many epithelia, the framework of lymph organs [tonsils,
lymph nodes], they are part of the connective tissue binding the liver and pancreas. And there are elastic
fibres
In adult connective tissue there are white, elastic and reticular fibres -- and there are also various
living cells: fibroblasts, large oval pale staining nuclei, spindle-shaped with several branching processes.
These are the cells that „remake‟ the connective tissue. (Fat tissue, adipose tissue, is sometimes included
under connective tissue: see fat-body of insects.)
Cartilage consists of cells known as chondrocytes, embedded in a gel-like matrix along with e.g.,
collagen fibres. These cells lie within lacunae (spaces) within the extracellular matrix. Sometimes two or
three cells are in the same lacuna. Cartilage occurs of different types: hyaline, elastic, fibrous.
The most rigid of all connective tissues is bone: cells, fibres, matrix and calcium salts. It is laid
down as layers (lamellae) by cells which in mature bone are dead, but once occupied the lacunae of what
is termed a Haversian system. There are two kinds of adult bone: compact and spongy. The former is a
continuous mass containing only the Haversian systems with their microscopic cavities. The latter is
made up of tiny bars called trabeculae. (The word is derived from trabs L. meaning „beam‟ as in beam of
a house roof; trabeculae are „little beams‟.) In spongy bone the spaces between the trabeculae are filled
with marrow. There is an interesting picture in D‟Arcy Thompson showing the trabeculae following lines of
stress in the sectioned head of a human femur.
Observe microscope slides of sectioned frog duodenum, compact bone and of both teased (and
not teased) white collagen fibres. Searching Google images is worthwhile and will turn up many useful
pictures of the various tissues discussed above.
Earthworm: genus Lumbricus and its hydrostatic skeleton
Using a compound microscope and a prepared slide of a transverse section of lumbricus, identify:
cuticle, hypodermis, circular muscles, longitudinal muscles, coelom [where coelomic fluid is located in
life], peritoneum, dorsal blood vessel (blood flows forward [anteriorly] in this muscular-walled tube by
peristalsis; contrast with blood circulation in a mammal), ventral blood vessel (blood flows posteriorly in
this vessel), nerve cord (located low on the ventral side), gut, secretory epithelium, gut cavity, typhlosole,
chloragogue cells, chaeta . What are possible functions of the secretory epithelium? Of the typhlosole?
Muscle contraction creates forces. Skeletons function in part to move forces from one location to
another. The hydraulic skeleton of annelids is simple enough to describe: fluid in a body cavity slightly
under pressure is surrounded by highly flexible layers of muscle: one circular, one longitudinal. When one
layer of muscle contracts the coelom causes it to stretch the other back to a precontracted length. The
two sets of muscle are antagonists. Which layer is proximad to the body axis? Which layer is distad? Is
there any adaptive basis for this arrangement, i.e., [imagine it otherwise] what if the ordering of the
muscle layers was reversed; would this have any effect on their function in earthworm locomotion?
Endoskeleton of rat
Identify the limb bones of the rat skeleton: see provided labelled diagram of a cat. [Axial
skeleton includes vertebrae divided into 5 groups/regions: cervical, thoracic, lumbar, sacral,caudal; the
appendicular (limbs) skeleton involves humerus, radius, ulna of fore limb and the femur, fibula, tibia of the
hind limb; the bones are of course homologous (structural correspondence) .]
A prominent hind limb muscle is the gastrocnemius. This calf muscle contracts to lift your body
weight up onto tip- toes. It inserts via the Achilles tendon. Its muscle fibres are arranged pinnately rather
than parallel. This pinnate arrangement (it means fibres are angled so ressembling a feather) involves
shorter fibres, which thus move through a shorter distance when they contract -- but there can be more of
them packed into the same volume. So pinnate muscle is designed „to move a heavy load through a short
distance‟ as in fact you do when going up onto your toes.
Determine on which bones of the hind limb of the rat the gastrocnemius muscle originates and
where its Achilles tendon inserts. Understand how a rat‟s bones and its gastrocnemius muscle would
contribute to a jump.
The nuchal ligament is prominent in large grazing mammals; it runs elaborately between the skull
and the spinous processes of some of the thoracic vertebrae. It is „stretchy‟ rather than stiff (has a low
stiffness; tendon has a higher stiffness) and smooths out stresses that result from the movement of the
cantilevered head, as when the animal runs; that is it tends to act like a shock absorber. Its material is
mostly the pliant protein elastin.
Though of course ties are absent from the rat skeleton specimen, the vertebrae can show
structural features that relate to ligament insertion. What evidence do the vertebrae of the rat give of any
ligament support for the head?
Exoskeleton of cicada (female)
Obtain a female cicada and examine it under 70% alcohol in a dissecting dish using a dissection
microscope. Identify all visible sensory structures associated with the head. On the basis of structure how
do you think this animal feeds? Is there anything unusual (e.g., compare to a grasshopper) about the face
region above the mouthparts? Examine the wings and identify the largest vein – the radius. Observe the
base of the wing and the tiny chitinous parts (axillary sclerites) that serve in wing deployment (extension
and flexion) and flight. Observe terminalia and suggest their function. Being an insect this animal has
three tagma: head, thorax, abdomen. Find and state major differences between the thorax and abdomen.
What are the functions of the abdomen?
With a razor blade (single edge) make a longitudinal section through the body of the cicada as
near as possible to the midline. Examine the face of the cut under alcohol in a dissecting dish under a
dissection microscope. Identify apodemes and the internal manifestations of external pits and sutures.
How does the exoskeleton vary in thickness and colour? This has to do with sclerotization. Identify:
cuticle, gut, haemocoel, Malpighian tubules, muscle, mouthparts, compound eyes, ventral nerve cord,
tracheae, There are three tagma in Insecta: head, thorax, abdomen. What are the functions of the insect
abdomen?
Human pinnae
Examine and draw the external ear of a fellow student. Make the drawing large, about a third of a
8 ½ x 11 sheet of paper. Make it in pencil and include a scale. Invent names for appropriate parts if you
can‟t discover the real names. Cartilage makes up a large portion of the „material‟ of the pinna. Would
cartilage be less effective or more effective if it were stiffer and less flexible? 1) How similar are human
pinnae in structure? Is there much variation in the cartilages? Compare drawings. 2) One hypothesis of
pinna cartilage function has to do with phase and sound frequencies: see if you can understand this
hypothesis. 3) Which animals show minimal or no presence of pinnae and why?
Sources
Alexander, R. McNeill 1992. Exploring Biomechanics: Animals in Motion. Scientific American
Library.
Gordon, J.E. 1978. Structures or Why Things Don‟t Fall Down. Plenum Press, New York.
Gordon, J.E. 1976. The New Science of Strong Materials. Penguin, Harmondsworth. [Reprinted
Princeton Univ. Press 1984.]
Thompson, D‟Arcy 1966. On Growth and Form. Bonner, J.T. (Ed.) Abridged Edition. Cambridge
University Press, Cambridge.
Vogel, S. 1988. Life‟s devices: the Physical World of Animals and Plants. Princeton Univ. Press,
Princeton, N.J.
Vogel, S. 2003. Comparative Biomechanics: Life‟s Physical World. Princeton Univ. Press,
Princeton, N.J.