Skeletons Flashcards
What is a skeleton
Any structure that:
Maintains body shape
Supports and protects a body
Transmits contractile forces
3 types of skeletons
Hydrostatic skeletons
Exoskeleton
Endoskeleton
Hydrostatic skeleton
Associated with the presence of a fluid-filled body cavity
Body cavity
Distinguished by presence of coelom (body cavity)
Coelom is filled with coelomic fluid - separates intestines and organs
- fluid absorbs shock and acts as hydrostatic Skelton
Coelom is lined with peritoneum
Pseudocoelomate
Coelom lost and replaced with pseudocoelom (persistent blastocoel).
Unlined cavity.
What type of skeleton is the ancestral condition for most coelomate bilaterians
Hydrostatic skeleton
Characteristics of hydrostatic skeletons
Fluid (water-filled) skeleton
Supported by fluid pressure
Hydrostatic have constant volume—> transmits muscle contractile forces
Cylindrical bodies
Polyps and vermiform (worm-like) animals
Support structure of hydrostatic skeleton
Body walls reinforced with a mesh of inelastic fibres
-orthogonal pattern = doesn’t allow for changes in length, bends until failure from kinking , allows for torsion
-cross-helical pattern = allows for changes in length, bends in a curve and resists torsion
Most common pattern of support structures in hydrostatic skeleton mesh
Cross-helical pattern
Muscle structure and movement in hydrostatic skeleton
Longitudinal muscles
Circular muscles
Muscles can only contract (not push)
Localised muscle contraction displaces fluid to another part of the body (where muscles are relaxed)
Movement using a hydrostatic skeleton
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 hydrostatic skeleton
In annelids (e.g., earthworm), 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
Form of a calcareous shell protecting a soft-bodied animal with a hydrostatic skeleton
What secretes the calcareous shell in molluscs
Mantle epithelium
Secreted into extrapallial space
Protected by periostracum
Calcareous shell- 3 layers
Periostracum (P)
-Outermost ‘leathery’ organic layer
-Made of the protein conchiolin
Prismatic layer (PL)
-CaCO3
Nacreous layer (NL)
-CaCO3
-Pearly
Prismatic layer of molluscs shell
The middle & 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 of molluscs shell
Inner layer of the shell
In some species looks like mother of pearl
Forms from thin sheets of calcium carbonate alternating with organic matter (eg keratin, collagen, chitin)
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 mollusc shells
When conditions are harsh the mantle may stop secreting the shell.
When conditions improve, the mantle starts again
These can be seen as growth rings
Biomineralisation pathway
Calcium carbonate (CaCO3) is formed from calcium and bicarbonate ion (HCO3-), the derivative of carbonic acid, contained in sea water. The Calcium ions in sea water are funnelled through into the extrapallial space through intercellular diffusion (1), active transport via calcium ATPase ion pumps (2), and calcium channels (3). The extrapallial space is then the calcification site.
Arthropod exoskeleton
Composed of a thick hard cuticle
Protects internal tissues from dehydration & infection and offers support for the internal organs.
It also provides sites for muscle attachment allowing movement.
Structure of arthropod exoskeleton
The cuticle is secreted from the epidermis of the body wall
The cuticle is essentially layers of protein + a waterproof polysaccharide called chitin
In crustaceans (crabs, lobsters etc.) the exoskeleton contains calcium carbonate crystals – making it very inflexible
Layers of arthropod exoskeleton
The epicuticle is the hardened outer layer – made of waxy lipoprotein - it is waterproof and acts as a barrier to microorganisms.
The procuticle (the combined exocuticle and endocuticle) is largely chitin & proteins.
The procuticle hardens through a process of sclerotization (tanning – protein layers are cross-bonded to one another).
Sclerotization
Protein layers are cross-bonded to one another
Arthropod joints
Invaginations of exoskeleton result in ridges for muscle attachment
At the joints the procuticle is thinner and less hardened
This is called the articular membrane
Growth and exoskeletons
Moulting or ecdysis
Ecdysis
Epidermal glands secrete enzymes that begin to digest the procuticle
This separates the epidermis and the procuticle.
New procuticle and epicuticle are secreted
Old exoskeleton splits along ecdysal lines when the animal expands by air or water intake
Pores in the procuticle secrete additional epicuticle
Calcium carbonate (crustaceans) or sclerotization hardens exoskeleton
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.
Echinoderm body wall
Consists of:
Thin cuticle
Monolayered epidermis
Thick connective-tissue dermis (houses skeletal ossicles)
Coelomic epithelium of myoepithelial cells
Peritoneum
Components of echinoderm endoskeleton
the calcareous ossicles and the collagenous connective tissues.
Skeletal ossicles provide rigidity and muscle attachment sites.
Echinoderm connective tissue
The connective tissues surrounding the skeletal ossicles also play a key skeletal role.
Collagenous ligaments suture ossicles together to create the skeletal framework.
Echinoderms can reversibly vary the rigidity of their dermis and general connective tissue.
Skeletal ossicles
Skeletal ossicles form intracellularly in a syncytium of fused 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.
Types of skeletal supporting structures
Cartilage
Bone
Cartilage
Hard and pliant material
Chondroitin suplhates (polysaccharides) which bind with ground substance proteins to form proteoglycans
Chondrocytes produce a large amount of extracellular chondroitin sulphate matrix interspersed with collagen out or elastic protein fibres
3 types of cartilage
Hyaline cartilage
Fibrocartilage
Elastic cartilage
What is appearance and functional role of cartilage dependent on
Number and type of protein fibres
Where is hyaline cartilage formed
Embryonic bones
Nose
Tips of ribs
Tracheal rings
Articular ends of long bones
Hyaline cartilage
Most common type of cartilage in the body.
Consists of short and dispersed collagen fibres and contains large amounts of proteoglycans.
No fibres are visible when viewed under light microscopy.
A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood.
Where is fibrocartilage found
Intervertebral disks
Pubic symphysis
Fibrocartilage
Collagen fibres are abundant giving mechanical resistance to tensile forces
Found under conditions where tensile or warping loads applied
Where is elastic cartilage found
Internal support for ear
Epiglottis
Elastic cartilage
Predominate protein fibre is elastin, which makes the cartilage springy and flexible.
Elastic cartilage gives rigid support as well as elasticity.
Bone structure
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 bone strong but flexible enough to withstand stress.
Types of bone cells
Osteoblasts – osteogenesis (producing new bone)
Osteoclasts – remove existing bone
Osteocytes – maintain equilibrium in fully formed bone
Osteoblasts
the bone cells responsible for producing new bone, or osteogenesis. Osteoblasts synthesise and secrete the collagen matrix and calcium salts
Osteoclasts
responsible for removing existing bone. Osteoclasts are multinucleated and originate from white blood cells instead of from osteogenic stem cells. Osteoclasts are continually breaking down old bone while osteoblasts are constantly forming new bone.
Osteocytes
formed from osteoblasts that get trapped within the calcified matrix that they secreted.
Osteocytes are the primary cell of mature bone and the most common type of bone cell.
Osteocytes maintain the mineral concentration of the matrix via secretion of enzymes.
Osteon
Series of concentric rings of bone cells and layers of bone matrix around a central canal that houses blood vessels, lymph vessels and nerves
2 types of bone
Compact bone
Spongy/cancellous bone
Compact bone
Dense
Formed of osteons
Spongy/cancellous bone
Osteocytes housed in lacunae inside trabeculae - maximise strength at areas of stress, with each trabecula forming along lines of stress
The spaces of the trabeculae network provide balance to the dense and heavy compact bone by making bone lighter. Marrow occupies the cavities and is lined with endosteum. Marrow also contains connective tissue fibres, blood vessels, nerve fibres and adipose tissue. Hemopoietic tissue (red marrow) produces red blood cells and some white blood cells.
2 parts of a long bone
Diaphysis
Epiphysis
Diaphysis
Tubular shaft
Medullary cavity- hollow region filled with yellow marrow
Composed of compact bone matrix around
Epiphysis
Wider section at each end of the bone
Filled with spongy bone
Metaphysis
Where the epiphysis and diaphysis meet
Contains the epiphyseal plate - a layer of hyaline cartilage in growing bone
2 types of bone development
Endochondral
Intramembranous
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 and erode calcified cartilage to form initial spaces of marrow
Osteoblasts appear in the core of the bone and primary centre of ossification appears. Old cartilage replaced by bone. Trabeculae form. Cartilage replacement moves to the metaphysis.
Epiphyseal plate is last region of cartilage proliferation.
Fishes, amphibians and reptiles have indeterminate growth – they can continue to grow through life.
Birds & 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 epiphyses 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
Mesenchymal cells group into clusters and ossification centres form.
Secreted osteoid traps osteoblasts, which then become osteocytes.
Trabecular matrix and periosteum form.
Compact bone develops superficial to the trabecular bone and crowded blood vessels condense into red marrow.
Types of vertebrate skeletons
Cartilaginous
Bony
Osteichthyes
Have bony endoskeletons
Formed through Endochondral ossification
Bone replaces cartilage
Basal vertebrates
Cartilaginous endoskeletons
Could show some level of mineralisation
Cartilage calcification
-Accumulation of calcium salt in cartilage
Perichondral ossification
-Bone forms on cartilage surface
Placoderms
have cartilaginous endoskeletons with ossification perichondral ossification
Head and torso covered in extensive bony 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
Basic Bauplan of vertebrate skeletons
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, patella
Developmental origins- endoderm
Splanchnocranium
Developmental origins- mesoderm
Splanchnocranium
Neurocranium
Axial skeleton
Appendicular skeleton
Developmental origins - ectoderm
Splanchnocranium
Dermal skeleton
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, patella
Evolution of the exoskeleton
Bony exoskeletons are widespread across both cartilaginous and bony vertebrates, first appearing in Galeaspida.
Exoskeletons form through cartilage calcification, intramembranous ossification or perichondral ossification
Evolution of ossification
In stem vertebrates, endoskeleton composed entirely of cartilage.
Osteostracans and non-osteichthyes jawed vertebrates evolved ossified endoskeletons. Endo- and exoskeletons developed on the surface of cartilage (perichondral ossification).
Osteichthyes acquired endochondral ossification
Bony tissues are produced within (as well as on top of) cartilage.
Bony tissues eventually replace cartilage.
Tetrapod Bauplan
Cranial skeleton
-Skull and mandible
Axial skeleton
-Cervical skeleton
-Thoracic skeleton
-Caudal skeleton
Appendicular skeleton
-Limbs and girdles
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 structures
An attached cartilaginous skull and gill arches
Fin rays
Basal vertebrates
only possessed cartilage
Lack mineralised skeletons
Calcified cartilage in galeaspids but no bones
Living examples:
Cyclostomes (Hagfish & lampreys)
Extinct examples:
Conodonts
Galeaspids
Perichondral ossification
Bone formation on outside of cartilage
