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Musculoskeletal Development
Objectives
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Review of the subdivisions of mesoderm development.
Differentiation of somites Development of the axial skeleton – skull and vertebral column. Resegmentation of the sclerotome
Development of the vertebrae and their specializations. Development of the skull. Cartilage formation
Bone formation
Development of skeletal muscle Citation: The Developing Human: clinically oriented embryology 9th ed. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. Philadelphia, PA: Saunders, 2011. Chapter 14 ‐ Skeletal System
Chapter 15 ‐ Muscular System
Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009. Chapter 8 ‐ Development of the Musculoskeletal System
Mesoderm has a different fate depending on where it is on the A/P axis and the mediolateral axis The head paraxial mesoderm is unsegmented. Neural crest cells invade into the head mesoderm and both contribute to head formation The trunk paraxial mesoderm segments into somites that are thought to form by a clock‐wavefront model. Starting from the cranial end of the embryo, the pairs of somites start to form at regular time intervals either side of the midline. From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.
The formation of muscle and bone in the trunk region
the products of the paraxial>somitic mesoderm
Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
Somitic segmentation occurs at the boundaries of gene expression
Transition from somitomere to somite. (A) Expression pattern of receptor tyrosine kinase EphA4 (blue) and its ligand, ephrinB2 (red) as somites develop. The somite boundary forms at the junction between the region of ephrin expression on the posterior of the last formed somite and the region of Eph expression on the anterior of the next somite to form. In the presomitic mesoderm, the pattern is created anew as each somite buds off. The posteriormost region of the next somite to form does not express ephrin until that somite is ready to separate.
Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
Differentiation of the somite (chick)
Under inductive signals emanating from the notochord and neural tube floorplate (SHH) and from the
overlying ectoderm (WNT/BMP4) the ventromedial somite undergoes an epithelial>mesenchymal
transition creating the sclerotome. The dorsolateral part retains an epithelial character and becomes
the dermamyotome.
Somite
Dermamyotome
Neural
tube
SHH
SHH
Notochord
Somite
Sclerotome – axial
cartilage then bone
(A) The somites divide into sclerotome cells and dermamyotome cells. (B) The sclerotome cells lose their adhesion to one another and migrate toward the neural tube. (C) The remaining dermamyotome cells divide. The medial cells form an epaxial myotome beneath the dermamyotome, while the lateral cells form a hypaxial myotome. (D) A layer of muscle cell precursors (the myotome) forms beneath the epithelial dermamyotome.
Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
The cranial‐caudal expression boundary created at initial segmentation is maintained in the sclerotome and correlates with the boundary between a loosely organized cranial region and a cell‐dense caudal region. The division between these domains is called Von Ebner’s fissure. Later the sclerotomes split along this line and they fuse to adjacent sclerotomal segments to create the vertebral bodies. This allows the spinal nerves to pass between the vertebral bodies to contact the muscles developing in the myotomal blocks
The sclerotome undergoes a process of resegmentation that involves the fusion of the caudal and cranial regions of successive sclerotomal masses. This gives rise to the vertebral body and neural arch.
The making of the somite: molecular events in vertebrate segmentation
Yumiko Saga & Hiroyuki Takeda
Nature Reviews Genetics 2, 835‐845 (November 2001)
The first 5 somites contribute to the occipital bone of the skull. The atlas is formed from the caudal region of the 5th somite and the cranial region of the 6th. This mechanism creates the situation in which there are 8 cervical spinal nerves but only 7 cervical vertebrae. In the thoracic, lumbar and sacral regions, the number of spinal nerves matches the number of vertebrae. When the sclerotome splits, cells in the plane of the division coalesce to form the annulus fibrosus of the IV disc. The notochord cells enclosed by this form the nucleus pulposus. Elsewhere the notochord degenerates. The intersegmental arteries fall in the middle region of the vertebral body. Schoenwolf: Larsen's Human Embryology, 4th ed.
Copyright © 2008
From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.
Sclerotomal mesenchyme surrounding the neural tube forms the neural arch of the vertebrae. From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.
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Schoenwolf: Larsen's Human Embryology, 4th ed.
Copyright © 2008
Vertebrae specialize along the A/P axis
33 in total – 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 5 coccygeal
The bodies are represented in yellow, the vertebral arches in red, and the costal processes in blue
Mesenchyme of the costal processes in the thoracic region forms the ribs
Seven pairs of true ribs attach directly to the sternum through their own cartilage
Five pairs of false ribs attach via the cartilage of another rib or ribs
Last 2 are floating ribs
The sternum develops from sternal bars that emerge in the ventrolateral body wall
Chondrification occurs as they move medially
At 10 weeks they fuse in the median plane
Form cartilaginous models of manubrium, sternebrae and the xiphoid process. Centers of ossification appear before birth
Cranium development
Can be subdivided into 4 main centres and 2 origins
1. Neurocranium – bones enclosing the brain
2. Viscerocranium – bones of the face derived from pharyngeal arches
Cartilaginous neurocranium
Forms the base of the brain box from temporary cartilage models
a.
Parachordal cartilage forms at the cranial end of the notochord and fuses with the occipital sclerotomes
b. Hypophysial cartilage forms around the pituitary and forms the sphenoid
c.
Trabeculae cranii fuse to form the body of the ethmoid
d. Ala orbitalis forms the lesser wing of the sphenoid
e. Otic capsules form around the otic vesicles and contribute to the temporal bone
f.
Nasal capsules form around the nasal sacs and contribute to the ethmoid
Membranous neurocranium
Forms bone directly from the mesenchyme at the sides and top of the brain to form the calvaria (skullcap). Parietal and frontal with 6 fibrous fontanelles > sutures
Cartilaginous Viscerocranium
Derived from neural crest in the 1st two pharyngeal arches and develops via the intermediate formation of a cartilaginous model
a. 1st pharyngeal arch – malleus and incus b. 2nd pharyngeal arch – stapes and styloid process of temporal bone
c. 3rd pharyngeal arch – contribute to the hyoid
d. 4th pharyngeal arch – laryngeal cartilage Membranous Viscerocranium
Membranous ossification in the maxillary prominence of the1st arch –
forms the squamous, temporal, maxillary and zygomatic bones
The mandible is formed by membranous ossification around a cartilaginous model Formation of cartilage
Cartilage development begins during the 5th week at sites of mesenchymal condensation called chondrification centres.
Mesenchymal cells differentiate into prechondrocytes then chondoblasts, which secrete an extracellular matrix of ground substance (carbohydrates ‐hyaluronan, chondroitin sulfates and keratan sulfate) and the protein tropocollagen (type II) which polymerizes extracellularly to form collagen fibres.
Types of cartilage Hyaline – the basic form Elastic cartilage – has elastic fibres mixed in. forms the epiglottic cartilage, the larynx, external ear and auditory tube
Fibrocartilage – contains type I collagen as well as type II collagen. Flexible and tough. Forms the annulus fibrosus of the intervertebral discs and the pubic symphysis.
Articular cartilage – lines the joint surfaces
Bone formation ‐ Endochondrial Ossification
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Occurs in preexisting cartilaginous models
Majority of skeleton formed by this process (vertebra, limb long bones) Osteoblasts replace cartilage matrix with a matrix rich in type I collagen
Chondrocytes undergo hypertrophy then apoptosis
Blood vessels enter bringing in osteoblasts which deposit bone matrix (osteoid)
Ossification centres (primary and secondary) create an epiphyseal cartilaginous growth plate near the ends of long bones
Some invading cells differentiate into haematopoietic cells – bone marrow
Perichondrium converts into periosteum
Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
Intramembranous Ossification
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Forms in mesenchyme that has formed a membranous sheath Mesenchyme condenses and becomes highly vascular
Precursor cells differentiate directly into osteoblasts and start to deposit bone matrix (osteoid).
Calcium phosphate is deposited in the osteoid and the osteoblasts become trapped within the matrix to form osteocytes. Initially has no pattern – just spicules of bone which then organize into lamellae (layers). Bone van be continuously remodeled though resorption via osteoclasts and new deposition via osteoblasts Developmental Biology. 6th edition.
Gilbert SF.
Sunderland (MA): Sinauer Associates; 2000.
Bone formation is dependent on the action of the Runx2 (Cbfa1) transcription factor
Gene targeting of Cbfa1 in mice causes lack of bone formation. Newborn mice (wild‐
type and homozygotes for Cbfa1) were stained with alcian blue (for cartilage) and alizarin red (for bone). Cartilage development in both mice was normal. (A) Wild‐type littermate. (B) Homozygous mutant showing cartilage, but an absence of ossification throughout the entire body. (Otto et al. 1997)
Muscle – Myogenesis
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Smooth muscle – some cells originate from undifferentiated splanchnic mesoderm mesenchymal cells. Smooth muscle surrounding blood vessels originates from somatic mesoderm. Others (iris of the eye, myoepithelial cells in mammary and sweat glands) originate from ectoderm. Differentiate to express smooth muscle actin and myosin for contraction but remain mononuclear
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Skeletal muscle ‐ cells originate from the paraxial mesoderm. Myoblasts undergo frequent divisions and then fuse to form multinucleated, syncytial myotubes that mature to form adult muscle fibres.
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Cardiac muscle ‐ cells originate from the prechordal splanchnic mesoderm. Discussed later
Skeletal Muscle has a
syncitial cellular structure
Muscles, muscle fibres and
myofibrils
Properties of Muscle
Fiber Types
Characteristic
V
(speed of shortening)
Fast fibers
IIb
IIa
Type I
Intermediate
Low
High/moderate
High
Predominant energy system Anaerobic
Combination
Aerobic
Myoglobin
Low
Medium
High
Capillary density
Low
Medium
High
max
Resistance to fatigue
Highest
IIx
Slow fibers
Low
Motor control of muscle fibres
Motor unit – the -motor neuron and all the fibres under
its control
Motor units
may control <5 muscle fibres in the eye or
small hand muscles or >2000 fibres in the
gastrocnemius
The origin of embryonic myoblasts in the chick
Epaxial
Wnt
Somite
Neural
tube
Notochord
Shh
Dermamyotome
Hypaxial
Sclerotome
Pax3-expressing
myoblasts migrate
into limb-bud
Myogenesis in the mouse
Formation of the myotome
Muscle progenitors delaminate from the edges
of the dermamyotome to form the myotome.
Some cells migrate into the limb buds. At E10.5
the dermamyotome disintegrates centrally and
the main myotome is formed
Expression of the myogenic
regulatory factor (MRF) gene MyoD
Epaxial and hypaxial components of the
myotome E11.5 mouse embryos.
Eloy-Trinquet S , Nicolas J Development 2002;129:111-122
Myogenesis
Proliferative phase
Myogenic
progenitors
specification
Myoblasts
determination
differentiation
Myotube
maturation
growth
hypertrophy
Maturation hypertrophy
to increase size and
expression of adult
myofilament genes =
mature muscle fiber
Differentiation of primary myotubes in the mouse hind-limb (12-14
dpc) and the beginning of fibre type patterning
Fusion of myoblasts is ordered and
synchronous. Nerve is not required for fusion or
Myosin Heavy Chain Slow expression
Tendon formation from
sclerotome-derived cells
– marked by expression
of Scleraxis (Scx).
Induced by the
myotome.
MyHC expression
1. Embryonic
2. Neonatal
3. Slow
Secondary myotube formation – mouse hindlimb 14dpc - birth and
continuing fibre type patterning
14-16 dpc - Pioneer motor
axons contact primary
myotubes. Necessary for
survival of myotube and
secondary myotube cluster
formation
Secondary myotubes form in
Clusters around primaries.
MyHC gene expression
1. Embryonic
2. Neonatal
Late fetal stage- clusters disperse.
MyHC gene expression
Primaries – slow MyHC
Secondaries - neonatal MyHC
EM sections of developing iliofibularis muscle in chick embryos
Secondary myogenesis
Primary myogenesis
In situ hybridisation analysis of Troponin I isoforms in mouse crural sections
G = Gastrocnemius
S = Soleus
E = EDL
T = Anterior tibialis
Tnni1 is the gene that
encodes the inhibitory
subunit of the Troponin
complex that is found in
slow-twitch fibres.
Postnatal fibre CONVERSION:
slow fiber number declines and neonatal MyHC is replaced by the adult fast
fibre MyHCs
A
Tibia
Tibialis anterior muscle
EDL muscle
Fibula
Soleus muscle
Medial Gastrocnemius
muscle
Lateral Gastrocnemius
muscle
Transverse sections of hindlimbs from postnatal mice
2days and 6 weeks after
birth – stained for Myosin
heavy chain slow and
Myosin heavy chain 2A
Plasticity and Regeneration of
Adult Muscle
Muscle Adaptation to Exercise Training
Adaptations to exercise training, particularly elevation in oxidative capacity of exercised
muscle but also some myosin isoform changes mainly in fast subtypes.
Cross‐Reinnervation
Buller et al. (1960) – Motor nerves supplying the (slow) soleus and (fast) FDL muscles swapped around. Contraction speed of soleus got faster, FDL slower.
Chronic Low‐Frequency Stimulation (CLFS)
Artificial electrical stimulation of a nerve supplying a fast muscle with a tonic pattern
mimics the impulse pattern of a slow nerve and induces fast to slow transformation Pette et al. (1973).
Regeneration
Injured muscle can regenerate itself using a population of stem cells that are laid down during embryogenesis – called satellite cells. Satellite cells lie between the sarcolemma and the basal lamina of each muscle fibre and activated by injury.
MATURATION HYPERTROPHY that occurs in mouse SOLEUS
muscle fibres between birth and adulthood
Images taken at same magnification – HIGH power
BIRTH
ADULT
Muscle hypertrophy and hyperplasia – exercise induced hypertrophy
and genetic control of hyperplasia via Myostatin. Mutation of this gene causes double muscling in the Belgian Blue breed.