BIO 337 Lecture Part 2
Skeletal System
THE SKELETAL SYSTEM
Function: Support and Movement
Most functionally active tissues are epithelial or “soft” tissues, such as muscle, nervous
…but if organism was composed entirely of this, it would be a “blob”
Therefore, the need for supporting tissue.
Three basic types:
1. Notochord
2. Connective Tissue
3. Skeletal (or Sclerotized) Tissue
1. Notochord
Vacuolated cells surrounded by a fibrous sheath, forming a firm cylindrical support structure.
2. Connective Tissue
a. Loose or Areolar (adipose, mucous,
embryonal)
Relatively open arrangement of cells and fibers
Three fiber types may be present:
i. Collagenous (collagen)
ii. Elastic (elastin)
iii. Reticular (collagen)
b. Compact
Fewer cells, more fibers (Elastic or
Collagenous)
The mix of fibers depends on the need for flexibility
i. Irregular or Interwoven
Withstands stress from multiple directions
Fascia, organ capsules, periosteum, perichondrium, perimysium, dura mater
ii. Regular or parallel
Withstands stress from one direction
Tendons, ligaments, aponeuroses
3. Skeletal (= Sclerotized) Tissue
Requirements of sclerotized tissue:
A .Scleroblasts (most from mesodermal
mesenchyme
Chondroblasts à Chondrocytes
Osteoblasts à Osteocytes
Ameloblasts à Amelocytes
Odontoblasts* à Odontocytes
*from Neural Crest
b. Organic matrix (Fibroproteins, Protein-
polysaccharides,Phospholipids )
c. Inorganic Matrix (Mineral salts)
d. Vascular supply
A. Cartilage
Basic structure: Chondroblasts within lacunae lay down matrix of chondromucoprotein and various fibers. Chondroblasts mature into chondrocytes.
Types:
1. Hyaline (“glassy”) – Embryos, trachea, diarthroses,
costal cartilages, elasmobranch skeleton.
2. Elastic (more flexible, elastin fibers) – Pinnae,
epiglottis, nasal cartilages.
3. Fibrocartilage (abundance of collagen fibers)
–Intervertebral discs, pubic symphysis.
Basic structure: Similar to cartilage. Chondroblast in lacunae lay down matrix of minerals and fibers. Primary matrix constituent is hydroxyapatite.
Departure from cartilage: vascular supply with lacunar connections via canaliculi.
Histologically, two types of bone:
Primary Vascular Bone -------------- Primary Vascular Bone
(embryo) (adult)
:
:
:
Secondary Osteonal Bone
(Haversian Bone)
The transition from Primary Vascular Bone to Secondary Osteonal (=Haversian) Bone:
In developing bone: Ridges and grooves on the bone’s surface contain blood vessels and osteoblasts build up around these vessels laying down matrix as this occurs. Eventually, the vessels become completely encircled and the concentric deposition of matrix converts the tunnel into a typical Haversian canal.
In bone remodeling: Osteoclasts erode a groove in outer region of bone. As groove deepens, blood vessels from periosteum invade area. Bone is converted to tunnel as osteoblasts lay down matrix. Successive layers (lamellae) form the osteon.
Bone Development
Three modes of development:
1. Membrane, or Dermal Bone (intramembranous).
Abundant in fishes
Restricted to the skull in tetrapods
2. Endochondral or Replacement Bone
Bone begins as cartilaginous template or model.
3. Heterotopic or Heteroplastic bone
Bone forms in fully differentiated tissue
Sesamoid bones (including patella)
Baculum (os penis) of mice, squirrels, mink, bats
Os Cordis of many ruminants
Pessulus within the avian syrinx
Bone density varies
Spongy or cancellous bone
Dense or compact bone
Flat bones of the skull:
Inner table, Outer table, Diploe
The Bone-Bone Interface
Joints and Articulations
Diarthrosis (=moveable)
Articular cartilage surrounded by synovial capsule
Amphiarthrosis (=partially moveable)
Intervertebral joints, interosseus membrane
Synarthrosis (=immoveable)
Sutures (skull bones), Symphyses (pubic, mandibular)
Ankylosis (=fusion)
Complete fusion of adult bones: maxillary,
premaxillary, sacrum, innominate
VERTEBRATE SKELETAL ELEMENTS
I. Dermal Skeleton
Bony plates and scales, developed in skin
Never preformed in cartilage
From mesoderm
II. Endoskeleton
Elements preformed in cartilage
May remain cartilaginous
Two Divisions:
A. Visceral Skeleton
Lies between and operate gill openings and structures derived from these (jaws, ear ossicles)
From Neural Crest
B. Somatic Skeleton
All remaining internal skeletal elements
From mesoderm
1.Axial Skeleton
Most of skull
Most of vertebrae
Ribs and sternum
2. Appendicular skeleton
Pectoral girdle (most)
Pelvic girdle
Limbs, fins (incl. Median)
Some adult elements are derived from 2 or more categories:
Shoulder = dermal + somatic endoskeleton
Lower jaw = dermal + visceral endoskeleton
Skull = dermal + visceral & somatic endoskeleton
THE AXIAL SKELETON - THE VERTEBRATE SKULL
That part of the axial skeleton that
1) encloses the brain
2) houses the organs of special sense
3) forms structures intimately involved with feeding and gas exchange.
The Cranium is a fusion of 3 major parts:
1) NEUROCRANIUM (=CHONDROCRANIUM)
“Primary Braincase”…..from mesoderm
2) DERMATOCRANIUM
“Palatal Complex”
“Dermal Shield”
……..from mesoderm
3) SPLANCHNOCRANIUM
“Visceral Skeleton”….from Neural Crest
THE NEUROCRANIUM
Forms the majority of endochondral, replacement bones of the skull.
Development proceeds as cartilaginous elements associated with the notochord are stimulated by epidermal placodes adjacent to the organs of special sense.
The neurocranium grows to surround the sensory capsules, and in most vertebrates, ossification follows.
Neurocranium elements:
Occiptial Condyles
One in Fish, Reptiles, & Birds (from basioccipital)
Two in Amphibia, Mammals (from exoccipital)
DERMATOCRANIUM - PALATAL
COMPLEX
Dermal bones that form the roof of the mouth, surrounding the internal nares.
Not present in cartilaginous fishes.
Dermatocranium – Palatal Complex elements:
Evolution of the Secondary palate
DERMATOCRANIUM - DERMAL SHIELD
A shield of membrane (=dermal) bone over the top and sides of the head, broken only by external nares, orbits and parietal foramen.
Dermatocranium – Dermal Shield elements:
Cranial Summary:
Dermatocranium is absent in the cartilaginous fishes
Tetrapods show a progressive reduction in skull
Elements
The Dermal Shield becomes perforated by temporal
fenestrae in many tetrapods…Adaptive
significance:
1. Reduction in mass
2. Muscular expansion
3. Increased surface area for muscle origin
THE SPLANCHNOCRANIUM - VISCERAL
SKELETON
Derived from Neural Crest
Contributes to the jaws
In gnathostome fishes, gill support becomes more complex compared to the agnathans, primarily to enhance ventilatory efficiency.
The anterior-most branchial elements contribute to the phylogeny of the vertebrate jaws. Two theories:
1. Serial Theory
2. Composite Theory
Vertebrate Jaw suspension and the fate of the Visceral
Arches:
Cranial Kinesis
Movement of one part of the skull relative to another
Weberian Ossicles
Sound transmission in fishes…Not homologous with the middle ear ossicles
NonCranial derivatives of the Visceral Skeleton
Hyoid Apparatus (Arch II, with parts from others)
Laryngeal skeleton (Arches III+) thyroid, Cricoid,
Arytenoid cartilages
Tracheal rings, Epiglottis (Arches III+)
THE AXIAL SKELETON – VERTEBRAE
Typical structure of Vertebrae
Primitive Condition
General Features of fish vertebrae
General Features of tetrapod vertebrae
Development of the Vertebrae
Derived from the sclerotome and chordamesoderm
Three developmental courses for centrum:
1. Acentrous: No centrum forms
Lamprey, Hagfish, Chondrostei, Sturgeon
2. Chordal and Perichordal Centers: Notochordal sheath ossifies…or
chondrifies.
Many fishes, bony and cartilaginous, amphibian
3. Autocenters: Perichordal mesenchyme units to ossify around chordal center
Amniotes
Here, the contribution of various developmental components (primarily the Pleurocentrum; Intercentrum) varies across taxa
Several vertebral types based on this, ranging from the primitive rhachitomous condition to the holospondylous condition.
Vertebrae and associated musculature are
Intersegmental
Two hypotheses as to how this comes about
Sclerotomal Resegmentation
Cranial Sclerotomal Degeneration
Vertebral Articulation:
Amphicoelous, Opisthocoelous, Procoelous, Platyan,
or Heterocoelous
Autotomous Vertebrae
THE AXIAL SKELETON – THE RIBS
FISHES
In the fishes, very little musculature is attached to the vertebrae…..
Force is directed against the myocommata (= septa)
Ribs have developed as an adaptation to strengthen this membrane….i.e. they develop between muscle masses.
Three rib types in fishes:
Dorsal Ribs
Ventral Ribs
Intermuscular bones
These ribs are all within the myosepta, and act to increase muscular efficiency.
Sharks have very short dorsal ribs
No ribs in cyclostomes or in the seahorses.
TETRAPODS
Tetrapods exhibit a much reduced body wall musculature.
Ribs become more important for support.
The Bicipital Rib is an adaptation for support against gravitational pull.
Rib Modifications:
Uncinate processes in tuatara, crocodilians,
birds
Turtle Carapace (ribs flatten and fuse)
THE APPENDUCULAR SKELETON
It is generally accepted that the tetrapod limbs originated from fish fins (pelvic and pectoral)
But the origin of paired fins is more perplexing.
Gill Arch Hypothesis (Gegenbauer)
This hypothetical fin is very similar to modern lungfish fins.This accounts for pectoral fins but not pelvic fins.
Fin-Fold Theory (Balfour & Thacher; Jarvik)
Fins may have arisen from ventrolateral folds that also possessed spines.
Primitive extant fishes possess an Archipterygial fin, where radial elements project from the axis.
Most bony fishes possess a Metapterygial fin, where fin rays extend (posteriorly) from one plane.
The Tetrapod limb configuration is more complex.
VERTEBRATE GIRDLES
Pectoral Girdle
The pectoral girdle is a “duplex” structure containing both dermal and somatic endoskeletal elements
In fishes, the pectoral girdle is connected to the skull
No dermal elements are found in elasmobranches
There is a ventral scapulocoracoid connection in
Elasmobranchs.
In Tetrapods, the pectoral girdle is not connected to
the skull.
The clavicle + interclavicle is ankylosed in birds.
In all vertebrates with fins/limbs, the endoskeletal
elements carry the limb articulation.
Pelvic Girdle
All endoskeletal….no dermal elements
In Fishes, no connection to the vertebrae
Located in muscle
May make ventral contact
In tetrapods, connected to the vertebral column
Ventral contact via pubis (pubic symphysis)