Dr Jamila EL Medany

OBJECTIVES

Attheendofthelecture,studentsshouldbeableto:qListthedifferentpartsofmesodermandthedifferentdivisionsofsomites.

qDifferentiatebonesaccordingtotheirembryologicaloriginandmodeofossification.

qDescribetheossificationoflongbones.qDescribethemainstepsfordevelopmentoflimbs.qDifferentiatemusclesaccordingtotheirembryologicalorigin.

Amnioticcavity

Yolksac

Embryo

Embryo

Notochord:stimulatesneuraltubeformation

Somaticmesoderm

Splanchnic mesoderm

INTRAEMBRYONIC MESODERMqProliferatesbetweenEctoderm&EndodermEXCEPT inthecentralaxisofembryowhereNOTOCHORD isfound.

qDifferentiatesinto3parts:1. Paraxialmesoderm:oneachsideofnotochord.2. Intermediatemesoderm3. LateralmesodermqParaxialmesodermdividesintounits(Somites).qLateralmesodermdividedbyintraembryonic coelominto:

1. Somaticmesoderm(betweenectoderm&coelom).2. Splanchnic mesoderm(betweenendoderm&coelom).

SOMITE SOMITE

Notochord

SclerotomeSclerotome

Neuraltube

Myotome

qVertebralcolumnqRibs&sternum

Epaxial division:Musclesofback

(ExtensorsofVC&Neck)

Hypaxial division:Musclesofbodywall

Myotome

Myoblasts migrateintolimb:

Limbmuscles

Myoblasts migrateintolimb:

Limbmuscles

Epaxial division:Musclesofback

(ExtensorsofVC&Neck)

DEVELOPMENT OF LIMBS - 1

28DAYS

32DAYS

qThelimbBudappearsasanelevationontheventrolateralbodywallresultingfromproliferationofmesenchymeofthesomaticlayeroflateralmesoderm.qEachlimbbudissurroundedbyanareaofectoderm.qUpperlimbbudsqAppearatday26 oppositethelowercervical segments.qLowerlimbbudsqAppearatday28oppositethelumbar&sacralsegments.

DEVELOPMENT OF LIMBS - 2

qA&G:Apicalectodermal ridge:appearsattheapexoflimbbudandstimulatesproliferationofmesenchyme andelongationoflimbbud.

qB&H:Distalendsofbudsflattenintopaddle-like hand&footplates.

qC&I:Digitalrays:appearasmesenchymal condensationsthatoutlinethepatternsofdigits.

qD&J:Notches:appearbetweendigitalrays.q E&K:Digitsforminsiderays,elongate&appearwebbed.q F&L:Mesenchyme betweendigitsdisappeartoseparate

them.

DEVELOPMENTOFLIMBS - 3

qOriginally,limbbudswereatrightangleofthetrunkwith:-Cranial(preaxial)&Caudal(postaxial) borders:radius andtibia arepreaxial bones.-Ventral&Dorsal surfaces:flexormusclesareventral.

qDuring7th week,adductionoflimbbudsoccurswith90°rotation:-Inupperlimb, rotationoccurslaterally:radiusislateral&flexormusclesareanterior.-Inlowerlimb,rotationoccursmedially: tibiaismedial&flexormusclesareposterior.

Mesenchyme fromlateralmesoderm

Inducesgrowthofmesenchyme &itstransformationintocartilage

Cartilageossifiesby:Endochondral ossification

Myoblasts migratefrommyotomes toform:Musclesoflimbs

Boneincartilaginousstate

Appearanceofprimaryossificcenters:ossificationofdiaphysis

Appearanceofsecondaryossificcenters:ossificationofepiphysis

Ossificationofepiphseal plate:Completeunionofepiphysis&diaphysis

Diaphysis

Epiphysis

Epiphysealplateofcartilage

BIRTH PUBERTY

Diaphysis

Boneincreasesinlengthbyproliferationofepiphyseal plate

Growthofbonestops

OSSIFICATIONOFLONGBONES

Boneageisagoodindexofgeneralmaturation.Boneageisdeterminedby:1. Appearanceofossific centersindiaphysis &epiphysis(specificforeachbone&sex)2. Disappearanceofepiphyseal plate(specificforeachbone&sex)

Epiphysis

DEVELOPMENT OF CRANIUM (SKULL)

qThe skull develops from mesoderm around the developing brain.

qThe skull consists of:1. Neurocranium: protective case for brain2. Viscerocranium: skeleton of faceq Bones of skull ossify either by:

*Endochondral ossification or*Intramembranous ossification

12

FP

ST Z MaxMand

F

F

F

P

P

Bonesofskullthatossifybyintramembranous ossification:1. F=Frontal2. P=Parietal3. Z=Zygomatic4. ST=Squamous temporal5. Mand =Mandible6. Max=Maxilla

JOINTS

Theydevelopfrommesodermbetweenbones:qFibrousjoints:mesodermdifferentiatesintodensefibrousconnectivetissue.

qCartilaginousjoints:mesodermdifferentiatesintocartilage.

qSynovialjoints:asynovialcavityisformedinsidemesoderm;mesodermdifferentiatesintosynovialmembrane,capsule &ligaments.

SUMMARYOFDEVELOPMENTOFBONE

All bones develop from MESODERM.qAXIAL SKELETON:

*Vertebrae, Ribs & Sternum: from Sclerotomes of Somites (Paraxial Mesoderm)*Skull: from Mesoderm surrounding the Brain

qAPPENDICULAR SKELETON: from Somatic part of Lateral Mmesoderm

All bones ossify by Endochondral Ossification EXCEPT:

1. Some bones of Skull2. Clavicle

SUMMARY OF DEVELOPMENT OF MUSCLES

qAllmuscles developfromMESODERM EXCEPT:1. Musclesofiris(eyeball)2. Myoepithelial cellsofECTODERM

mammary&sweatglandsq Allskeletalmusclesdevelopfrommyotomes

ofparaxialmesoderm EXCEPT:q someHead&Neckmusclesfrommesoderm

ofPharyngealArches

SUMMARY OF DEVELOPMENT OF MUSCLES

qCardiac&Smoothmusclesdevelopfromlateralmesoderm:

1. Cardiacmusclesfrom:splanchnic partoflateralmesoderm

2. Smoothmuscles:*Inthewallofviscerafrom:splanchnic partoflateralmesoderm*Inthewallofblood&lymphaticvesselsfrom:somaticpartoflateralmesoderm

QUESTION1

qWhichoneofthefollowinggroupofmusclesarederivativesofepaxial divisionofmyotomes?

1. Musclesofback2. Musclesoflimbs3. Musclesofviscera4. Cardiacmuscles

QUESTION2

q Whichoneofthefollowingbonesossifiesbyintramembranous ossification?

1. Vertebra2. Humerus3. Ribs4. Mandible

QUESTION3

qRegardingtheossificationoflongbones,whichoneofthefollowingstatementiscorrect?

1. Primaryossific centreappearsafterbirth.2. Secondaryossific centreleadsinto

ossificationofdiaphysis.3. Longbonesossifybyintramembranous

ossification.4. Whenepiphysisuniteswithdiaphysis,

growthofbonestops.

THANK YOU