15

Musculo-Skeletal System(Trunk, Limbs, and Head)

General Statements: Bilaterally, paraxial mesoderm become somites and somitomeres. (Somitomeres develop ros-tral to the notochord in the head. They are like somites, but smaller and less distinctly organized.) The mesoderm comprising each somite differentiates into three regions: — dermatome (lateral) which migrates to form dermis of the skin — sclerotome (medial) forms most of the axial skeleton (vertebrae, ribs, and base of the skull). — myotome (middle) forms skeletal mus-culature. Individual adult muscles are produced by merger of adjacent myotomes.

Note: Nerves make early connections with adjacent myotomes and dermatomes, establishing a segmental innervation pattern. As myotome/dermatome cells migrate to assume adult positions, the segmental nerve supply must follow along to maintain its connection to the innervation target. (Recurrent laryngeal & phrenic nerves travel long distances because their targets migrated far away.)

Skin. Consists of dermis and epidermis. Epidermis, including hair follicles & glands, is derived from ectoderm. Neural crest cells migrate into epidermis and become melanocytes. (Other neural crest cells become tactile disc receptors.) Dermis arises from mesoderm (dermatomes of somites). Each dermatome forms a continu-ous area of skin innervated by one spinal nerve. Because adjacent dermatomes overlap, a locus of adult skin is formed by 2 or 3 dermatomes, and innervated by 2 or 3 spinal nerves.

Muscle. Muscles develop from mesoderm, except for muscles of the iris which arise from optic cup ectoderm. Cardiac and smooth muscles originate from splanchnic mesoderm. All skeletal muscle is derived from paraxial meso-derm that forms somites and, in the rostral region of the head, somitomeres. Mesodermal cells of the myotome region of each so-mite/somitomere differentiate into myoblasts which fuse to form multinucleate muscle cells that synthesize myosin & actin and appear striated. Developing muscles and tendons must be under tension (stretched by growing bone) in order to grow to proper lengths. Muscle development requires innervation. Muscles release trophic molecules that determine muscle cell type (I, IIa, IIb). Also, muscles release trophic molecules that affect nerve growth.

Note: Each anatomical muscle is genetically allocated a specific number of myoblasts that is deter-mined by the time of birth. Thereafter, muscle cell growth is due solely to cellular hypertro-phy. Regeneration (hyperplasia) of adult muscle cells does not occur.

Mesoderm Regions

ectoderm

endoderm

neural tube

neural crest

aorta

coelom

somite:dermatome

sclerotomemyotome

intermediatemesoderm

somaticmesoderm

splanchnic mesoderm

notochord

head

heart limbbudeye

somitomeresin

pharyngeal arches

somites

Somites & Somitomeres

The INNNERVATION PROBLEM: How to establish connections between nerves and their targets during embryonic development? Would you . . .

A] First, have targets migrate to their locations, then

have nerves find them. B] First, connect nerves to targets, then have nerves

follow targets as they migrate.

15

Musculo-Skeletal System(Trunk, Limbs, and Head)

General Statements: Bilaterally, paraxial mesoderm become somites and somitomeres. (Somitomeres develop ros-tral to the notochord in the head. They are like somites, but smaller and less distinctly organized.) The mesoderm comprising each somite differentiates into three regions: — dermatome (lateral) which migrates to form dermis of the skin — sclerotome (medial) forms most of the axial skeleton (vertebrae, ribs, and base of the skull). — myotome (middle) forms skeletal mus-culature. Individual adult muscles are produced by merger of adjacent myotomes.

Note: Nerves make early connections with adjacent myotomes and dermatomes, establishing a segmental innervation pattern. As myotome/dermatome cells migrate to assume adult positions, the segmental nerve supply must follow along to maintain its connection to the innervation target. (Recurrent laryngeal & phrenic nerves travel long distances because their targets migrated far away.)

Skin. Consists of dermis and epidermis. Epidermis, including hair follicles & glands, is derived from ectoderm. Neural crest cells migrate into epidermis and become melanocytes. (Other neural crest cells become tactile disc receptors.) Dermis arises from mesoderm (dermatomes of somites). Each dermatome forms a continu-ous area of skin innervated by one spinal nerve. Because adjacent dermatomes overlap, a locus of adult skin is formed by 2 or 3 dermatomes, and innervated by 2 or 3 spinal nerves.

Muscle. Muscles develop from mesoderm, except for muscles of the iris which arise from optic cup ectoderm. Cardiac and smooth muscles originate from splanchnic mesoderm. All skeletal muscle is derived from paraxial meso-derm that forms somites and, in the rostral region of the head, somitomeres. Mesodermal cells of the myotome region of each so-mite/somitomere differentiate into myoblasts which fuse to form multinucleate muscle cells that synthesize myosin & actin and appear striated. Developing muscles and tendons must be under tension (stretched by growing bone) in order to grow to proper lengths. Muscle development requires innervation. Muscles release trophic molecules that determine muscle cell type (I, IIa, IIb). Also, muscles release trophic molecules that affect nerve growth.

Note: Each anatomical muscle is genetically allocated a specific number of myoblasts that is deter-mined by the time of birth. Thereafter, muscle cell growth is due solely to cellular hypertro-phy. Regeneration (hyperplasia) of adult muscle cells does not occur.

Mesoderm Regions

ectoderm

endoderm

neural tube

neural crest

aorta

coelom

somite:dermatome

sclerotomemyotome

intermediatemesoderm

somaticmesoderm

splanchnic mesoderm

notochord

head

heart limbbudeye

somitomeresin

pharyngeal arches

somites

Somites & Somitomeres

15

Musculo-Skeletal System(Trunk, Limbs, and Head)

General Statements: Bilaterally, paraxial mesoderm become somites and somitomeres. (Somitomeres develop ros-tral to the notochord in the head. They are like somites, but smaller and less distinctly organized.) The mesoderm comprising each somite differentiates into three regions: — dermatome (lateral) which migrates to form dermis of the skin — sclerotome (medial) forms most of the axial skeleton (vertebrae, ribs, and base of the skull). — myotome (middle) forms skeletal mus-culature. Individual adult muscles are produced by merger of adjacent myotomes.

Note: Nerves make early connections with adjacent myotomes and dermatomes, establishing a segmental innervation pattern. As myotome/dermatome cells migrate to assume adult positions, the segmental nerve supply must follow along to maintain its connection to the innervation target. (Recurrent laryngeal & phrenic nerves travel long distances because their targets migrated far away.)

Skin. Consists of dermis and epidermis. Epidermis, including hair follicles & glands, is derived from ectoderm. Neural crest cells migrate into epidermis and become melanocytes. (Other neural crest cells become tactile disc receptors.) Dermis arises from mesoderm (dermatomes of somites). Each dermatome forms a continu-ous area of skin innervated by one spinal nerve. Because adjacent dermatomes overlap, a locus of adult skin is formed by 2 or 3 dermatomes, and innervated by 2 or 3 spinal nerves.

Muscle. Muscles develop from mesoderm, except for muscles of the iris which arise from optic cup ectoderm. Cardiac and smooth muscles originate from splanchnic mesoderm. All skeletal muscle is derived from paraxial meso-derm that forms somites and, in the rostral region of the head, somitomeres. Mesodermal cells of the myotome region of each so-mite/somitomere differentiate into myoblasts which fuse to form multinucleate muscle cells that synthesize myosin & actin and appear striated. Developing muscles and tendons must be under tension (stretched by growing bone) in order to grow to proper lengths. Muscle development requires innervation. Muscles release trophic molecules that determine muscle cell type (I, IIa, IIb). Also, muscles release trophic molecules that affect nerve growth.

Note: Each anatomical muscle is genetically allocated a specific number of myoblasts that is deter-mined by the time of birth. Thereafter, muscle cell growth is due solely to cellular hypertro-phy. Regeneration (hyperplasia) of adult muscle cells does not occur.

Mesoderm Regions

ectoderm

endoderm

neural tube

neural crest

aorta

coelom

somite:dermatome

sclerotomemyotome

intermediatemesoderm

somaticmesoderm

splanchnic mesoderm

notochord

head

heart limbbudeye

somitomeresin

pharyngeal arches

somites

Somites & Somitomeres

16

Bone.Bonesoriginatefromparaxial mesoderm(endochondralaxialskeletonfromsclero-tomes),somatic mesoderm (endochondralappendicularskeleton),orectomesenchyme (intramem-branous bonesofthecalvariaandfacefromneuralcrest).Ligaments,tendons,andmuscle-relatedconnectivetissueoriginatefromlocalmesenchymeorectomesenchyme. Thus, most bones are formed endochondrally (ossification of a cartilage model), but bones ofthecalvaria(topoftheskull)andthefaceareformedintramembranously(ectomesenchymecellsbecomeosteoblastsdirectlyratherthanbecomingchondroblasts).

Endochondral bone formation: —localmesenchymeundergoescondensation;somecellsdifferentiateintochondroblasts —chondroblastssecretematrixtoproduceacartilagemodelofthefuturebone;themodelissurrounded by perichondral fibrous tissue — the diaphysis of the cartilage model undergoes ossification first; epiphyseal ossification occurs later; physis ossification is postponed until bones stop growing in length. —overallboneshapeisgeneticallydetermined;surfaceirregularitiesofboneareacquiredduetolocalizedtension(stress)producedbyligamentsandtendons.

Joints.Condensationofmesenchymeproducesaninterzoneregionwithinperichondraltis-sueconnectingadjacentcartilagemodelsofbones.Accordingtothenatureofthefuturejoint,theinterzone becomes fibrous connective tissue, or fibrocartilage, or a synovial cavity.

Synovial jointsformasfollows: —mesenchymeatthecenteroftheinterzoneundergoescavitationandthetissueborderingthecavitybecomesyno-vial membrane;unevenexpansionofthesynovialcavitycreatessynovialfolds(theinterzonemesenchymealsoformsintra-articularligamentsandtendonswherethesearepresent); —perichondraltissuesurroundingtheinterzonebecomesjoint capsuleandlocalizedthick-eningsofthejointcapsuleformsligaments.

Note: Muscleactivityisessentialforpropersynovialjointdevelopmentafterthejointcavitydevelops.Jointsmustmoveduringin uteroandpostnataldevelopment to prevent ankylosis (fixed/frozen joint).

Regional Specifics

Trunk Region: Skeletalmusculatureisformedbysomitemyotomeswhichfusetoformbroadmusclesthataresegmentallyinnervated(eachmyotomebringsitsowninnervationasitmergeswithadjacentmyotomes).Myotomeaccumulationssegregateintoadorsalmass(epimere)innervatedbydorsalbranchesofspinalnervesandaventralmass(hypomere)innervatedbyventralbranchesofspinalnerves.Theepimerebecomesepaxial musclesandthehypomerebecomeshypaxial muscles. Sclerotomesgiverisetovertebraeandribs. The sternum develops differently, from chondrification/os-sification of somatic mesenchyme of the ventral thorax.

Synovial Joint Development

perichondral layer

cartilage (bone)

interzone ligament

cavitation

fibrouscapsule

synovialmembrane

synovialcavity

16

Bone.Bonesoriginatefromparaxial mesoderm(endochondralaxialskeletonfromsclero-tomes),somatic mesoderm (endochondralappendicularskeleton),orectomesenchyme (intramem-branous bonesofthecalvariaandfacefromneuralcrest).Ligaments,tendons,andmuscle-relatedconnectivetissueoriginatefromlocalmesenchymeorectomesenchyme. Thus, most bones are formed endochondrally (ossification of a cartilage model), but bones ofthecalvaria(topoftheskull)andthefaceareformedintramembranously(ectomesenchymecellsbecomeosteoblastsdirectlyratherthanbecomingchondroblasts).

Endochondral bone formation: —localmesenchymeundergoescondensation;somecellsdifferentiateintochondroblasts —chondroblastssecretematrixtoproduceacartilagemodelofthefuturebone;themodelissurrounded by perichondral fibrous tissue — the diaphysis of the cartilage model undergoes ossification first; epiphyseal ossification occurs later; physis ossification is postponed until bones stop growing in length. —overallboneshapeisgeneticallydetermined;surfaceirregularitiesofboneareacquiredduetolocalizedtension(stress)producedbyligamentsandtendons.

Joints.Condensationofmesenchymeproducesaninterzoneregionwithinperichondraltis-sueconnectingadjacentcartilagemodelsofbones.Accordingtothenatureofthefuturejoint,theinterzone becomes fibrous connective tissue, or fibrocartilage, or a synovial cavity.

Synovial jointsformasfollows: —mesenchymeatthecenteroftheinterzoneundergoescavitationandthetissueborderingthecavitybecomesyno-vial membrane;unevenexpansionofthesynovialcavitycreatessynovialfolds(theinterzonemesenchymealsoformsintra-articularligamentsandtendonswherethesearepresent); —perichondraltissuesurroundingtheinterzonebecomesjoint capsuleandlocalizedthick-eningsofthejointcapsuleformsligaments.

Note: Muscleactivityisessentialforpropersynovialjointdevelopmentafterthejointcavitydevelops.Jointsmustmoveduringin uteroandpostnataldevelopment to prevent ankylosis (fixed/frozen joint).

Regional Specifics

Trunk Region: Skeletalmusculatureisformedbysomitemyotomeswhichfusetoformbroadmusclesthataresegmentallyinnervated(eachmyotomebringsitsowninnervationasitmergeswithadjacentmyotomes).Myotomeaccumulationssegregateintoadorsalmass(epimere)innervatedbydorsalbranchesofspinalnervesandaventralmass(hypomere)innervatedbyventralbranchesofspinalnerves.Theepimerebecomesepaxial musclesandthehypomerebecomeshypaxial muscles. Sclerotomesgiverisetovertebraeandribs. The sternum develops differently, from chondrification/os-sification of somatic mesenchyme of the ventral thorax.

Synovial Joint Development

perichondral layer

cartilage (bone)

interzone ligament

cavitation

fibrouscapsule

synovialmembrane

synovialcavity

17

Formation of Vertebrae and Ribs: —sclerotomeregionsofsomitesmigrate&becomeacon-tinuousmasssurroundingthenotochordandneuraltube.Thustheoriginalsomitesegmentationislost —thecontinuousmassdifferentiatesintodiffuse&denseregionsperoriginalsclerotome.Thediffuseregionfromonesomitecombineswiththedenseregionofanadjacentsomitetoproduceacartilagemodelofonevertebra. —betweennewlyformedvertebrae(intervertebraldiscregions)sclerotomemesenchymeformsannulus fibrousandnotochordformsnucleus pulposus(notochorddegeneratesintheregionofthevertebralbody) —ribsdevelopasprocessesofthoracicvertebrae.

Note: Asaresultoftheabovere-segmentation,vertebraeareshiftedrelativetoothersegmentalstructures(seenextpage).Consequently,musclesspanadjacentvertebrae;spinalnervestraverseintervertebralforamina(locateddorsaltointervertebraldiscs);andembryonicintersegmentalarteriesbecomespinalarteriesthatrunalongsidevertebralbodies.

Vertebralanomaliesinclude:stenosisofthevertebralcanal;mal-articu-lation; hemivertebra; and spinal bifida (absent vertebral arch). The notochord, neuraltube,andneuralcrestallplayaroledirectingsomitedifferentiationandvertebralsegmentation(formation).

Note:ThedensoriginatesasthebodyofvertebraC1(atlas),butitfuseswithvertebraC2(axis).

Limbs:

Limbsgrowoutwardfrombodywallsomatopleureaslimbbuds.Bone,cartilage,andconnectivetissueofthelimbarisefromsomaticmesodermofthelimbbud.Dermisandskeletalmusclecomefromdermatomeandmyotomemigrationsintothelimb.

Limb Morphogenesis: — a limb begins as a limb field (an area of somatopleure committedtoformingalimb) —next,alimb budisproducedbylocalizedproliferation&condensationofmesenchyme,coveredbyectoderm —regionsofthelimbdevelopinproximodistalorderasthelimb bud elongates (the shoulder/hip appears first, the manus/pes is thelasttobeadded) — the distal end of the limb bud (footplate) is flattened like apaddleandectodermalongitsoutermarginthickenstoformanapicalridge(theridgeisinducedtoformbyunderlyingmesodermanditinducesthemesodermtocontinuegrowinganddifferentiatingintoalimb)

dorsalbranch

ventralbranch

epaxialmuscles

hypaxialmuscles

coelom

spinalnerve

gut

MyotomeSegregation

L-2

L-3

Canine Dermatomes

14

intervertebralforamen

Features of Vertebrae

nucleus pulposusanulus fibrosus

Intervertebral Disc:

lamina

pedicle

Vertebral pcesses:

spinous

transverse

articular

Vertebral body

Vertebral arch:

Transverse Section Through A Vertebra

vertebralforamen

Lateral View of Vertebrae

caudalarticular process

spinousprocess

cranialarticularprocess

transverse process

intervertebral discsVertebra

vertebralforamen

transverseprocess

cranial articularprocess

costal fovea(rib articulation)

spinaous process

Craniolateral Viewof a Thoracic

Vertebra

A Laminectomy Exposes Spinal Cord Within Vertebral Canal

vertebral canal

intervertebralforamen

four primarybranches of a spinal nerve

spinal nerve

Transverse Sectionthrough an Intervertebral Disc

cut vertebral arch (pedicle) spinal cordwing ofatlas

spinal ganglion (T2)

vertebra T5

processes:

18

caudal

sclerotome

spinal n.

neural segmentstube

cranial

myotome

continuous mass

dense diffuse

vertebra

intervertebral discmuscle

neural tube dorsal rootectoderm

notochord

somite

sclerotome myotome

dermatome

ossificationspinous process

ribtubercle

ribhead

vertebralcanal

transverse process

vertebralbody notochord

Sclerotomes to Vertebrae

LIMB GROWTH: Limbs arise from the lateral wall of the body. Thus limbs are products mainly of . . .

A] paraxial mesoderm B] intermediate mesoderm C] somatic lateral mesoderm D] splanchnic lateral mesoderm

LIMB GROWTH

How do limbs grow? A] proximal to distal order B] distal to proximal order

How do digits grow? A] digits grow outward from manus/pes B] digits appear by selective degeneration of manus/

17

Formation of Vertebrae and Ribs: —sclerotomeregionsofsomitesmigrate&becomeacon-tinuousmasssurroundingthenotochordandneuraltube.Thustheoriginalsomitesegmentationislost —thecontinuousmassdifferentiatesintodiffuse&denseregionsperoriginalsclerotome.Thediffuseregionfromonesomitecombineswiththedenseregionofanadjacentsomitetoproduceacartilagemodelofonevertebra. —betweennewlyformedvertebrae(intervertebraldiscregions)sclerotomemesenchymeformsannulus fibrousandnotochordformsnucleus pulposus(notochorddegeneratesintheregionofthevertebralbody) —ribsdevelopasprocessesofthoracicvertebrae.

Note: Asaresultoftheabovere-segmentation,vertebraeareshiftedrelativetoothersegmentalstructures(seenextpage).Consequently,musclesspanadjacentvertebrae;spinalnervestraverseintervertebralforamina(locateddorsaltointervertebraldiscs);andembryonicintersegmentalarteriesbecomespinalarteriesthatrunalongsidevertebralbodies.

Vertebralanomaliesinclude:stenosisofthevertebralcanal;mal-articu-lation; hemivertebra; and spinal bifida (absent vertebral arch). The notochord, neuraltube,andneuralcrestallplayaroledirectingsomitedifferentiationandvertebralsegmentation(formation).

Note:ThedensoriginatesasthebodyofvertebraC1(atlas),butitfuseswithvertebraC2(axis).

Limbs:

Limbsgrowoutwardfrombodywallsomatopleureaslimbbuds.Bone,cartilage,andconnectivetissueofthelimbarisefromsomaticmesodermofthelimbbud.Dermisandskeletalmusclecomefromdermatomeandmyotomemigrationsintothelimb.

Limb Morphogenesis: — a limb begins as a limb field (an area of somatopleure committedtoformingalimb) —next,alimb budisproducedbylocalizedproliferation&condensationofmesenchyme,coveredbyectoderm —regionsofthelimbdevelopinproximodistalorderasthelimb bud elongates (the shoulder/hip appears first, the manus/pes is thelasttobeadded) — the distal end of the limb bud (footplate) is flattened like apaddleandectodermalongitsoutermarginthickenstoformanapicalridge(theridgeisinducedtoformbyunderlyingmesodermanditinducesthemesodermtocontinuegrowinganddifferentiatingintoalimb)

dorsalbranch

ventralbranch

epaxialmuscles

hypaxialmuscles

coelom

spinalnerve

gut

MyotomeSegregation

L-2

L-3

Canine Dermatomes

19

—mechanically,limbgrowthconsistsof: - elongation of a dorsoventrally flattened limb bud - ventroflexion of the distal half of the limb (ventral now faces medially) -pronationofthedistalhalf(previousmedialsurfacenowbecomescaudal) —separatedigitsareproducedbyinterdigitalnecroticzones(species withfewerdigitsundergofurtherdegenerationand/orfusionofdigits); —localmesenchymecondensestoformcartilagemodelsoflimbbones —myotomecellsmigrateintothebaseofthelimbformingextensor& flexor muscle masses that subsequently segregate into the individualmusclesofthelimb; —vesselsandnervesgrowintothelimb.

Clinical considerations: Achondroplasia (dwarfism; Dachshund) — inherited, systemic prema-ture ossification of physes of extremities.

Arthrogryposis[Gr.gryposis=crooked]canresultfrommalformedjoints,denervation,abnormalmuscletension,orimpairedmobilityin utero. Polydactyly(extradigits);syndactyly(fuseddigits);brachydactyly(stumpydigitsGr.dactylos=digit] Amelia(nolimb);meromelia(absenceofpartoflimb);micromelia(smalllimbGr.melos=limb] Note:phocomelia(seallimb)=absenceofproximalsegment(s)oflimbwasaconsequenceofpregnantwomentakingthalidomideinthelate1950s.

Head Region: Theheadconsistsofacranium,whichcontainsthebrainwithinacranialcavity,andaface.

The face develops separately from the frontonasal process and first pharyngeal arch. Since the face and cranium have different embryonic origins they can be independently influenced genetically (e.g., inthecaseofbrachycephalicbreeds)orbyteratogens.

Skull. Bones of the base of the cranium develop endochondrally; the relatively flat bones that comprisethecalvaria(roofofthecranium)andthefacedevelopintramembranously.(Themandiblehasacomplexorigininvolvingbothendochondralandintramembranousdevelopment.)

Theendochondralbonesareformedfromsclerotomesofsomitomersandsclerotomesofthefirst four somites (occipitalsomites).

Theintramembranousbonesarisefromectomesenchyme(mesenchymederivedfromneuralcrest),whichgivesrisetocartilage,bone,andconnectivetissueofthefaceanddorsalhead.Intramem-branous bones articulate by means of fibrous joints called sutures. Widened suture areas, at the corners of growing bones,arecalledfontanels.Suturesandfontanelsallowbonyplatestooverlaponeanotherduringparturition.

Manus/PesDevelopment

foot plate

cell death

digit

face(intramembranous)

base of cranium (endochondral )

Regions of the Skullcalvaria of cranium(intramembranous )

19

—mechanically,limbgrowthconsistsof: - elongation of a dorsoventrally flattened limb bud - ventroflexion of the distal half of the limb (ventral now faces medially) -pronationofthedistalhalf(previousmedialsurfacenowbecomescaudal) —separatedigitsareproducedbyinterdigitalnecroticzones(species withfewerdigitsundergofurtherdegenerationand/orfusionofdigits); —localmesenchymecondensestoformcartilagemodelsoflimbbones —myotomecellsmigrateintothebaseofthelimbformingextensor& flexor muscle masses that subsequently segregate into the individualmusclesofthelimb; —vesselsandnervesgrowintothelimb.

Clinical considerations: Achondroplasia (dwarfism; Dachshund) — inherited, systemic prema-ture ossification of physes of extremities.

Arthrogryposis[Gr.gryposis=crooked]canresultfrommalformedjoints,denervation,abnormalmuscletension,orimpairedmobilityin utero. Polydactyly(extradigits);syndactyly(fuseddigits);brachydactyly(stumpydigitsGr.dactylos=digit] Amelia(nolimb);meromelia(absenceofpartoflimb);micromelia(smalllimbGr.melos=limb] Note:phocomelia(seallimb)=absenceofproximalsegment(s)oflimbwasaconsequenceofpregnantwomentakingthalidomideinthelate1950s.

Head Region: Theheadconsistsofacranium,whichcontainsthebrainwithinacranialcavity,andaface.

The face develops separately from the frontonasal process and first pharyngeal arch. Since the face and cranium have different embryonic origins they can be independently influenced genetically (e.g., inthecaseofbrachycephalicbreeds)orbyteratogens.

Skull. Bones of the base of the cranium develop endochondrally; the relatively flat bones that comprisethecalvaria(roofofthecranium)andthefacedevelopintramembranously.(Themandiblehasacomplexorigininvolvingbothendochondralandintramembranousdevelopment.)

Theendochondralbonesareformedfromsclerotomesofsomitomersandsclerotomesofthefirst four somites (occipitalsomites).

Theintramembranousbonesarisefromectomesenchyme(mesenchymederivedfromneuralcrest),whichgivesrisetocartilage,bone,andconnectivetissueofthefaceanddorsalhead.Intramem-branous bones articulate by means of fibrous joints called sutures. Widened suture areas, at the corners of growing bones,arecalledfontanels.Suturesandfontanelsallowbonyplatestooverlaponeanotherduringparturition.

Manus/PesDevelopment

foot plate

cell death

digit

face(intramembranous)

base of cranium (endochondral )

Regions of the Skullcalvaria of cranium(intramembranous )

20

Note:AuditoryossiclesariseendochondrallyfrompharyngealarchesI(malleus&incus)andII(stapes).

Muscles.Musclesoftheheadarisefrommyotomesderivedfromsomitomeres(seven)orsomites(fouroccipitalsomites: Somitomeremyotomesmigratetotheorbit(twogivingriseeyemuscles)ortheymigratetopharyngealarches(becomingmusclesofmastication,facialexpressionmuscles). Somitemyotomesbecometongueandneckmusclesandtheymigratetopharyngealarches(IV-VI),becomingpharyngeal,laryngeal&esophagealmuscles. Cranialnervesestablishearlyconnectionswithadjacentsomitomeres&somitesandac-company them to definitive muscle sites. Pharyngeal arches are each innervated by specific cranial nerves(I=trigeminal;II=facial;III=glossopharyngeal;IV-VI=vagus).

Pharyngeal (Branchial) Arch Summary: Ectomesenchymemigratestopharyngealarchestoformconnectivetissue,cartilageandbone.Somitomere/somitemyotomesmigrateintothearchesandgiverisetoskeletalmuscle.Eacharchisinnervatedbyaparticularcranialnerve.

Firstarch.(innervatedbycranialnerveV) —jawbones(mandible&maxilla);also,ossiclesofthemiddleear(incus&stapes) —musclesofmastication,plusrostraldigastricus,mylohyoid,&tensortympanimm.

Secondarch:(innervatedbycranialnerveVII) —hyoidbones&stapes(ossicleofthemiddleear) —musclesoffacialexpression,includingcaudaldigastricus&stapediusmm.

Thirdarch:(innervatedbycranialnerveIX) —hyoidbones —onepharyngealmuscle(stylopharyngeusmm.)

ArchesIVthroughVI:(innervatedbycranialnerveX) —laryngealcartilages —pharyngealmm&cricothyroidm—innervatedbycranialbranchofX —intrinsiclaryngealmm—innervatedbyrecurrentlaryngealn.ofX