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External Appearance – 2 Month

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External Appearance – 2 Month External appearance – 2nd month (week 5 – week 8) 55th ––88th weekweek periodperiod isis thethe timetime ofof organogenesisorganogenesis AtAt thethe endend ofof thethe 44th weekweek thethe mainmain externalexternal featuresfeatures areare thethe somitessomites andand pharyngealpharyngeal archesarches InIn 22 nd monthmonth thethe ageage ofof thethe embryoembryo isis indicatedindicated asas crowncrown --rumprump lengthlength (CRL)(CRL) asas countingcounting somitessomites becomesbecomes difficultdifficult CRLCRL isis thethe distancedistance fromfrom thethe vertexvertex ofof thethe skullskull toto thethe midpointmidpoint betweenbetween thethe apicesapices ofof thethe buttocksbuttocks inin millimetersmillimeters AtAt thethe endend ofof thethe embryonicembryonic period,period, thethe embryoembryo hashas humanhuman appearanceappearance Crown-Rump Length correlated to age External appearance the embryo LateralLateral viewview ofof aa 55 --weekweek --oldold embryoembryo The embryonic period is divided into 2323 CarnegieCarnegie stagesstages based on external and internal morphological criteria CRL WeekWeek 55 -- summarysummary WeeksWeeks 66 --88 -- summarysummary External appearance the fetus weeks SizeSize ofof thethe fetusfetus At the junction of trimesters 1 and 2 (=90 days old) → length of 90 mm At the junction of trimesters 2 and 3 (2→3), the fetus is ~ 250 mm in length and weighs ~ 1,000g 12 weeks 7 months Development of the head & neck Early stages - transformation of the head (cephalic) folds neural walls with the eye primordia otic disc & pharyngeal arches lens & nasal placodes Development of the human face is usually described as a process of merging of 5 swellings (growth centres) surrounding the stomodeum, termed processes (prominences) bilateral maxillary processes bilateral mandibular processes frontonasal prominence PharyngealPharyngeal archesarches CS 15, day 36 Series of bulges on the lateral surface of the es) (groov head and neck clefts Separated by clefts (grooves) 4 pairs are visible by day 30 arches More caudally, no clear-cut arrangement, but a 5th and a 6th arch are distinguished Clefts have internal counterparts, the pharyngeal pouches EarlyEarly 44 thth weekweek Cranial and The developing cervical face is regions make represented by up ~ 50% of the frontonasal up ~ 50% of region, and the the embryo's first pharyngeal length (branchial, visceral) arch LateLate 44 thth –– earlyearly 55 thth weekweek By the time the anterior neuropore closes, the first and second pharyngeal arches are evident The regions between the pharyngeal arches are termed pharyngeal clefts. The indentation just dorsal to the second pharyngeal cleft is the Otic pit developing inner ear, the otic pit. Otic pit EarlyEarly 55 thth weekweek Frontonasal TheThe 11 st pharyngealpharyngeal archarch prominence hashas bothboth aa maxillarymaxillary andand aa mandibularmandibular prominence.prominence. trigeminal maxillary DorsalDorsal toto thethe firstfirst archarch isis anan elevationelevation formedformed byby thethe underlyingunderlying trigeminaltrigeminal ganglionganglion thatthat suppliessupplies tissuestissues derivedderived fromfrom thethe mandibular firstfirst archarch EarlyEarly 55 thth weekweek EpibranchialEpibranchial placodesplacodes areare specializedspecialized regionsregions ofof surfacesurface ectoderm,ectoderm, thethe cellscells ofof whichwhich invaginateinvaginate toto contributecontribute toto thethe formationformation ofof thethe sensorysensory gangliaganglia ofof cranialcranial nervesnerves V,V, VII,VII, IX,IX, andand XX DayDay 2929 buccopharyngeal stomodeum membrane neural neural tube tube 1st arch 1st arch pharynx pouches clefts Remnants of the Clefts separate the buccopharyngeal pharyngeal arches on membrane are seen the external surface between the Pouches separate the stomodeum and the pharyngeal arches on pharynx the internal surface CompositionComposition ofof anan archarch ectoderm endoderm mesenchyme The mesenchyme of the arches is derived in part from neural crest and in part from mesoderm DerivativesDerivatives ofof thethe pharyngealpharyngeal pouchespouches SomeSome ofof thethe neuralneural crestcrest cellscells inin eacheach ofof thethe archesarches becomebecome cartilagecartilage TheThe pharyngealpharyngeal archesarches contributecontribute toto thethe developingdeveloping tonguetongue andand epiglottisepiglottis PharyngealPharyngeal archesarches -- dayday 3535 TheThe 11 st ,, 22 nd ,, 33 rd ,, andand 44 th archesarches areare visiblevisible 1 externally.externally. TheThe sixthsixth 1 archarch doesdoes notnot formform anan 2 externalexternal elevation.elevation. 3 4 TheThe frontonasalfrontonasal prominenceprominence isis composedcomposed ofof thethe tissuetissue thatthat surroundssurrounds thethe forebrainforebrain –– dayday 3535 FollowingFollowing closureclosure ofof thethe anterioranterior neuropore,neuropore, thethe ectectodermoderm thatthat willwill lineline thethe nasalnasal cavitiescavities (( olfactoryolfactory placodesplacodes )) isis locatedlocated onon thethe laterallateral aspectsaspects ofof thethe frontonasalfrontonasal prominenceprominence –– dayday 3535 olfactory placode olfactory placode DuringDuring 55 th weekweek thethe olfactoryolfactory placodesplacodes lineline thethe nasalnasal pitspits olfactory placode medial nasal prominences lateral nasal prominence nasal pit CompositionComposition ofof thethe olfactoryolfactory placodeplacode lateral nasal prominence mesenchyme olfactory placode epithelium medial nasal prominence mesenchyme UpperUpper liplip formationformation –– 66thth weekweek medial Contribution of each of the prominences to the face Union of the medial nasal prominence with the lateral nasal prominence and maxillary prominence is required for normal development of the upper lip At 8 th week the face has human features CraniofacialCraniofacial defectsdefects Complete holoprosencephaly was diagnosed by ultrasound at 22 weeks of gestation, after which pregnancy was terminated at 24 weeks. At autopsy, the female fetus showed agenesis of both eyes and orbits,arhinia and microstomia. LimbLimb developmentdevelopment ByBy thethe beginningbeginning ofof thethe 55th week,week, forelimbsforelimbs andand hindlimbshindlimbs appearappear asas paddlepaddle --shapedshaped budsbuds BBudsuds flattenflatten →→ radialradial groovesgrooves (rays)(rays) appearappear onon thethe distaldistal portionportion ofof thethe budsbuds →→ digitsdigits LimbLimb DevelopmentDevelopment (late(late weekweek 4,4, dayday 28)28) lower TheThe developingdeveloping upperupper limblimb isis evidentevident earlierearlier thanthan thethe lowerlower limblimb upper LimbLimb developmentdevelopment (late(late weekweek 4,4, dayday 28)28) A cut through the limb bud illustrates the mesenchyme which is derived from the lateral plate mesoderm (the source of the skeletal components) and from the myotomes of the paraxial mesoderm (the muscular component) LimbLimb developmentdevelopment (week(week 5,5, dayday 33)33) The thickened ectoderm at the distral rim of the limb bud is termed the apical ectodermal ridge . Integrity of the apical ectodermal ridge is essential for continued limb outgrowth. LimbLimb developmentdevelopment (week(week 5,5, dayday 36)36) As the limb bud grows, indentations become apparent in the hand (or foot) plate LimbLimb developmentdevelopment (Week(Week 7)7) During the 7 th & 8 th weeks of human development the digits of the hand become apparent LimbLimb developmentdevelopment (Week(Week 8)8) At the beginning of the fetal period, touch pads are prominent features of the hands and feet LimbLimb developmentdevelopment (week(week 8)8) As the hand develops, webs that are present between the outgrowing digits must regress Development of the feet is like that of the hands, but in the human it starts approximately 3 -4 days later FormationFormation ofof thethe cardiogeniccardiogenic fieldfield Cardiac progenitor cells lie in the epiblast, A immediately lateral to the primitive streak Y Migrate through the streak → position in front of the neuroectoderm at day 18 the intraembryonic cavity over the cardiogenic field later develops into the pericardial cavity HeartHeart tubetube formationformation –– ~day~day 2020 1 Pericardial cavity 2 Myoepicardium 3 Endocardium 4 Midgut This is a frontal section through the cardiac anlage. The endothelial cells differentiate out of the neighboring mesenchymal cells of the splanchnopleurae and flow together to make up numerous vesicles that form the endocardial plexus . The myocardial mantle arises out of the material of the cardiogenic plate. RapidRapid growthgrowth ofof thethe developingdeveloping brainbrain pushespushes thethe heartheart tubetube ventrallyventrally Rapid growth of the developing brain produces more caudal and ventral positioning of the cardiogenic region and formation of the foregut As the neural cells proliferate and grow forward, the cardiogenic area is folded over and the endoderm -lined foregut is formed EarlyEarly 44 thth week,week, dayday 2222 Neural folds The septum transversum is located just below the developing heart , which at this stage, begins to beat Neural folds BodyBody cavitiescavities (early(early weekweek 4,4, dayday 23)23) The body cavity, which forms between the splanchnic and somatic mesoderm, becomes divided by the septum transversum into the pleuropericardial cavity and the peritoneal cavity ventral view BodyBody cavitiescavities (mid(mid --weekweek 4,4, dayday 26)26) The septum transverum is
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