skeletons and muscles

Skeletons & muscles

Skeleton

any structure that:

maintains body shape

supports and protects a body

transmits contractile forces

types of skeleton:

hydrostatic skeleton

exoskeleton

endoskeleton

Invertebrate skeletons

endoskeleton and exoskeleton are what is thought of when skeleton is mentioned but hydrostatic skeleton is the most wide-spread form of skeleton across Bilatreria ( animals with symmetrical body plans)

The hydrostatic skeleton is associated with the presence of a fluid-filled body cavity

Body cavity

distinguished by presence of coelom ( body cavity)

acoelomate ( no coelom) :

• xenacoelomorpha

• xenoturbellida

• acoelomorpha

worm like creatures that lack features that other bilaterians have ( lacking anus)

coelomate

• other bilaterians

coelom is filled with coelomic fluid - seperates intestines and organs

coelomic fluid absorbs shock and acts as hydrostatic skeleton

coelom is lined with peritoneum

pseudocoelomate

• coelom lost and replaced with pseudocoelom ( persistent blastocoel)

• unlined cavity

Hydrostatic skeletons

is the ancestral condition for most coelomate bilaterians

fluid ( water-filled) skeleton

supported by fluid pressure

hydrostats have constant volume - transmits muscle & contractile forces

cylindrical bodies

polyps and vermiform ( worm like) animals

Support structure:

body walls reinforced with a mesh of inelastic fibres

orthogonal pattern - doesnt allow for changes in length (A-B), bends until failure from kinking (C) allows for torsion(D)

cross-helical pattern - allows for changes in length (E-F) bends in a curve (G) and resists torsion (H)

Muscle structure and movement:

longitudinal muscles

circular muscles

muscles can only contract ( not push)

localised muscled contraction displaced fluid to another part of the body ( where muscles are relaxed)

Movement:

circular muscle at posterior end contracts

forces fluid forwards and extends the front of the animal

longitudinal muscles contract to pull posterior end forwards

segmentation:

in annelids ( e.g earthworms) the coelom is divided into segments by muscular septa

prevents movement of fluid from one segment to another

allows individual segments to operate independently

more complex and variable pattern of movement

protection from injuries

Molluscan exoskeleton

molluscs have an exoskeleton in the form of a calcareous shell protecting a soft-bodied animals

the calcareous shell is secreted from the mantle

shell consists of three layers:

periostracum (P)

• outermost ‘leathery’ organic layer

Prismatic layer (PL)

• CaCO3

Nacreous layer (NL)

• CaCO3

• pearly

shell secretion

shell is secreted from the mantle epithelium

secreted into extrapallial space

protected by periostracum

prismatic layer:

• the middle and thickest layer of the shell is the prismatic layer

• secreted at the mantle edge ( the periostracum acts as a framework on which the calcium carbonate is suspended)

nacreous layer:

• inner layer of the shell

• in some species looks like mother of pearl

• forms from thin sheets of calcium carbonate alternating with organic matter

• cells over the entire epithelial layer of the mantle secrete the nacreous layer - this thickens the shell

• doesn’t limit growth of the animal

growth rings in molluscs shells

when conditions are harsh the mantle may stop secreting the shell

when conditions improve, the mantle starts again

these can be seen as rings

Muscles

major muscles in molluscs include:

foot

pedal retractor muscles

odontophore and radula protractor and retractor muscles

adductor muscles ( bivalves)

Arthropod exoskeleton

arthropods ( jointed foot) have an exoskeleon composed of a thick hard cuticle

protects internal tissues from dehydration and infection and offers support for internal organs

it also provides sites for muscles attachment allowing movement

Structure

the cuticle is secreted from the epidermis of the body wall

the cuticle is essentially layers of proteins and a waterproof polysaccharide called chitin

in crustaceans ( crabs, lobsters etc) the exoskeleton contains calcium carbonate crystals - making it very inflexible

exoskeleton has two layers:

the epicuticle is the hardened outer layer made of waxy lipoprotein - is waterproof and acts as a barrier to microorganisms

the procuticle ( the combined exocuticle and endocuticle) is largely chitin and proteins

the procuticle hardens through a process of sclerotization ( tanning - proteins layers are cross-bonded to one another)

exoskeleton divisions

hardened cuticles are divided into separate plates called sclerites

adjoining sclerites are connected by sections of soft, flexible cuticle called articular membranes

invaginations of exoskeleton result in ridges for muscle attatchement

Muscles

arthropod muscles originate and insert on adjacent sclerites of the body or appendage

muscle blocks bulge when fibres contract

space inside exoskeleton is limited and restricted muscle expansion

arthropod limb muscles are often pennated ( fibres angled about the long axis)

pennated fibres expand along the long axis of the limb

Endoskeletons in invertebrates

echinoderm

endoskeletons:

echinoderms have endoskeletons formed by skeletal ossicles located within the dermis and covered with epidermis

skeletal ossicles provide rigidity and muscle attachment sites

the connective tissues surrounding the skeletal ossicles also play a key skeletal role

body wall:

thin cuticle

monolayered epidermis

thick connective-tissue dermis, which houses skeletal ossicles

coelomic epithelium of myoepithelial cells ( muscles)

peritoneum

endoskeleton:

the echinoderm endoskeleton has two components: the calcareous ossicles and the collagenous connective tissues

skeletal ossicles provide rigidity and muscle attachment sites

connective tissues:

the connective tissues surrounding the skeletal ossicles play a key skeletal role

collagenous ligaments suture ossicles together to create the skeletal framework

skeletal ossicles:

form intracellularly in a syncytium of fussed dermal sclerocytes

ossicles consist of a 3D lattice called a stereom, with the spaces within called the stroma

honeycomb structure reduces weight, increases strength and prevent cracking

spines:

all ossicles, including those that project above the body surface, are endoskeletons and are covered by epidermis

mutable connective tissues: echinoderms can reverisbly vary the rigidity of their dermis and general connective tissue

mutable connective tissue

Vertebrate skeleton

skeleton support the body against external forces and allow forces to be developed by muscles to move specific parts of the body

vertebrate adopt a solid internal skeleton ( endo-skeleton) comprising a solid material with a high-elastic modulus

Types of skeletal supporting structures

supporting structures constructed of two main types of material :

cartilage

bone

Cartilage

is firm but flexible special connective tissue

the matrix primarily consists of chondroitin sulfate ( ground substance) and collagenous or elastic proteins ( fibres)

spaces within the matrix called lacunae house cartilage cells ( chondrocytes)

three types of cartilage:

hyaline cartilage-

the most widespread type of cartliage in the body

means ‘glassy’ referring to the homogenous appearance of the matrix, which resembles pieces of frosted glass

found in : embryonic bones, nose, tips of ribs, tracheal rings and articular ends of long bones

fibrocartilage -

is found where cartilage is subjected to tensile or to warping loads

ground substance is reinforced with collagen fibres

solid ground substance is effective in resisting compressive forces

embedded collagen fibres are effective at addressing tensile forces

found in : intervertebal disks and pubic symphysis

elastic cartilage -

is flexible and springy , owing to the presence of elastic fibres in the matrix

found in : internal support for ear and epoglottis

Bone

bone has a soft framework made of the protein collagen, impregnated with calcium phosphate which adds strength and hardens the framework

this combination of collagen and calcium makes the bone strong but flexible enough to withstand stress

bone cells

bone cells are classified according to their role:

osteoblasts - osteogenesis ( producing new bone)

oeseoclasts - removing existing bone

osteocytes - maintain equilibrium in fully formed bone

types of bone

growth and development of bones

endochondral bone development - develops from cartilage ‘ replacement bone’

intramembranous bone development - direct development from mesenchyme tissue without cartilage precursor

endochondral ossification

mesenchymal cells differentiate to chondrocytes that form the cartilaginous skeletal precursors of bone

hyaline cartilage is surrounded by perichondrium

hyaline cartilage in core of diaphysis ossified by accumulation of inorganic salts. entombed chondrocytes die and blood vessels invade to form initial spaces of marrow

osteoblasts appear in the core of the bones and primary centre of ossification appears, old cartilage replaced by bone. trabeculae form, cartilage replacement moves to the metaphysis.

epophyseal plate is last region of cartilage proliferation - fishes, amphibians and reptiles have indeterminate growth ( they can continue to grow through life)

birds and mammals have determinate growth and stop growing at maturity

mammals, some lizards and birds secondary centres of ossification arise in the epiphysis

at sexual maturity in mammals the epiphysis ossify completely. cartilage remains at joint surface as articular cartilage

Intramembranous ossification

direct development from mesenchyme tissue without cartilage precursor

dermal bone - skull, pectoral girdle and integument

sesamoid bone - associated with tendons

perichondral bone - develops early and retain ability to form bone in the adult

1. mesenchymal cells group into clusters and ossification centres form

2. secreted osteoid traps osteoblasts, which then becomes osteocytes

3. trabecular matrix and periosteum form

4. compact bone develops superficial to the trabecular bone and crowded blood vessels condense into red marrow

vertebrate skeletal Bauplan

Splanchnocranium - primary palate and jaws, branchial elements

neurocranium - braincase

axial skeleton - backbone and ribs

appendicular skeleton - pectoral and pelvic fins or limbs and girdles

dermal skeleton - external portions of the skull, teeth, armour plates, clavicle and patella

developmental origins

endoderm - splanchnocranium

mesoderm - splanchnocranium, neurocranium, axial skeleton, appendicular skeleton

ectoderm - splanchnocranium and dermal skeleton

types of vertebrate skeletons

cartilaginous skeleton

bony skeleton

evolution of the vertebrate skeleton

basal ( non-osteichthyan) vertebrates all have cartilaginous endoskeletons - but could show some level of mineralization :

• cartilage calcification

• accumulation of calcium salt in cartilage

• perichondral ossification - bone forms on cartilage surface

osteichthyes have bony endoskeletons

• formed through endochondral ossification - bone replaces cartilage

evolution of endoskeleton

bony exo-skeleton are widespread across both cartilaginous and bony vertebrates, first appearing in Galeaspida

exoskeletons form through cartilage calcification, intramembranous ossification or perichondral ossification

basal vertebrates only possess cartilage

basal vertebrates only possessed cartilage :

lack mineralised skeletons

calcified cartilage in galeaspids but no bones

living examples - cyclostomes ( hagfish and lampreys)

extinct examples - conodonts and galeaspids

Hagfish skeleton

only have cartilaginous skulls ( chondocranium) and no vertebral column around notochord

but have arcualia ( cartilaginous precursors to vertebrae) in the tail

lamprey skeleton

have an internal skeleton consisting of:

a notochord

vertebra-like structure

an attached cartilaginous skull and gill arches

fin rays

early vertebrate skeletons

ossification in jaw-less fishes

ossification in placorderms

have cartilaginous endoskeletons with ossification perichondral ossification

head and torso covered in extensive body armour plates

first vertebrates to have paired pelvic fins

ossification in chondrichthyans

endoskeleton of prismatic calcified cartilage

cartilaginous skull

bone present in scales ( dermal denticles) and teeth

some sharks show perichondral ossification along vertebrae

cartilage staining blue ( Alcian blue)

dermal denticles staining red for bone ( Alizarin red)

perichondral ossification along outer layer of the vertebrae

Evolution of ossification

1. in stem vertebrates, endoskeleton composed entirely of cartilage

2. osteostracans and non-osteichthyes jawed vertebrates evolved ossified endoskeletons. endo and exo-skeletons developed on the surface of cartilage ( perichondral ossifcation)

c. osteichthyes acquired endochondral ossification -

bony tissues are produced within ( as well as on top of) cartilage)

bony tissues eventually replace cartilage

Muscles

muscles can be classified by their general microscopic appearance

skeletal muscle - voluntary muscle

cardiac muscle - found only in branchial heart and involuntary muscle

smooth muscle - visceral muscle , digestive tract, blood vessels and lungs

skeletal muscle

is the organ that generates forces of motion

comprises muscle fibres or long individual muscle cells

Muscle attachment

muscles attach to bone through various wrappings of connective tissue that extend beyond the ends of the muscle fibres

tendons - cord like attachment

aponeuroses - tendons drawn out into thin, flat sheets of connective tissues

fascia - sheets of connective tissues that wrap and bind parts of the muscle body together

muscle evolution

vertebrates ancestrally have segmental muscle blocks

inherited from chordate ancestors

hypothetical chordate ancestor

vertebrate ancestrally have segmental muscle blocks

inherited from chordate ancestors