mendoza,hannah-isabela h._cardiovascular system

8/2/2019 Mendoza,Hannah-Isabela h._cardiovascular System

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+ESTABLISHMENT OF THE CARDIOGENIC FIELD

+FORMATION AND POSITION OF THE HEART TUBE

+FORMATION OF THE CARDIAC LOOP

+ABNORMALITIES OF CARDIAC LOOPING

+MOLECULAR REGULATION OF CARDIACDEVELOPMENT

+DEVELOPMENT OF THE SINUS VENOSUS

+FORMATION OF THE CARDIAC SEPTA

+ENDOCARDIAL CUSHIONS AND HEART DEFECTS

+SEPTUM FORMATION IN THE COMMON ATRIUM

+FURTHER DIFFERENTIATION OF THE ATRIA

CARDIOVASCULAR SYSTEM


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ESTABLISHMENT OF THECARDIOGENIC FIELD

Dorsal view of a late presomiteembryo (approximately 18 days)after removal of the amnion.Prospective myoblasts andhemangioblasts reside in thesplanchnic mesoderm in front ofthe neural plate and on each sideof the embryo.B.Transverse section throughasimilar-staged embryo to showthe position of the bloodislands inthe splanchnic mesoderm layer.C.Cephalocaudal section through

a similar-staged embryo showingthe position of the pericardialcavity and cardiogenic field.D.Scanning electron micrograph ofa mouse embryo equivalent to 19days in the human, showingcoalescence of the blood islands

by vasculogenesis into ahorseshoe-shaped heart tube


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The vascular system appears in the middle of the thirdweek, when the embryo is no longer able to satisfy itsnutritional requirements by diffu-sion alone.

Cardiac progenitor cells lie in theepiblast,immediatelylateraltotheprimitivestreak.Fromtherethey migrate through the streak. Cells destined toformcranial segments of the heart, the outflow tract,migratefirst, and cells forming more caudal portions,rightventricle, left ventricle, and sinus venosus,respectively,migrate in sequential order

The cells proceed toward the cranium and position

themselves rostral to the buccopharyngeal membrane andneural folds (Fig. 11.1). Here they reside in the splanchniclayer of the lateral plate mesoderm. At this time, late in thepresomite stage of development, they are induced by theunderlying pharyngeal endoderm to form cardiacmyoblasts. Blood islands also appear in this mesoderm,

where they will fom blood cells and vessels by the processof vasculogenesis


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With time, the islands unite and form ahorseshoe-shaped endothelial-lined tubesurrounded by myoblasts.This region is known asthe cardiogenic field; the intraembryoniccavity

over it later develops into the pericardial cavity


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Formation and Position of the HeartTube

Initially, the central portion of the cardiogenic areais anterior to the buccopharyngeal membrane andthe neural plate (Fig. 11.2 A). With closure of theneural tube and formation of the brain vesicles,

however, the central nervous system growscephalad so rapidly that it extends over thecentracardiogenic area andthe future pericardialcavity (Fig. 11.2). As a result of growth of the

brain andcephalic folding of the embryo, thebuccopharyngeal membrane is pulled for-ward,while the heart and pericardial cavity move first tothe cervical region and finally to the thorax


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Formation of the Cardiac Loop The heart tube continues to elongate and bend

on day 23. The cephalic portion of the tube bendsventrally, caudally, and to the right (Fig. 11.6, Band C ), and the atrial (caudal) portion shifts

dorso cranially and to the left (Figs. 11.6and11.7 A). This bending, which may be due tocell shape changes, creates the cardiac loop. It iscomplete by day 28


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Figure 11.6Formation of the cardiac loop.

A. 22 days. B. 23 days.C.24days. Brokenline, pericardium. Dand E. Scanning electronmicrographs of mouse embryosshowing frontal views of theprocess shown in the diagrams.Initially the cardiac tube is short

and relatively straight (D),but asit lengthens, it bends (loops),bringing the atrial region cranialand dorsal to the ventricularregion(E). The tube is organizedin segments, illustrated by thedifferent colors, from the outflowregion to the right ventricle to theleft ventricle to the atrial region.These segments represent acraniocaudal axis that appears tobe regulated by homeobox geneexpression.A, primitive

atrium;arrow,septumtransversum;S , sinus


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ABNORMALITIES OF CARDIACLOOPING

Dextrocardia, in which the heart lies on the rightside of the thorax instead of the left, is causedbecause the heart loops to the left instead of theright. Dextrocardia may coincide with situs

inversus,a complete reversal of asym-metry in allorgans. Situs inversus, which occurs in 1/7000individuals, usually is associated with normalphysiology, although there is a slight risk of

heartdefects. In other cases sidedness is random,such that some organs are re-versed and othersare not; this is heterotaxy.


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Molecular Regulation of CardiacDevelopment

Heart induction. BMPs secreted in the posteriorportion of the primitivestreak and periphery of theembryo, in combination with inhibitionof WNT expression by crescent in the anterior

half of the embryo, induce expressionof NKX2.5 in theheart forming region of thelateral plate mesoderm (splanchniclayer).NKX2.5 is thenresponsible for heart

induction.


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WNTinhibitors(crescent) NKX-2.5

BMP 2,4


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Development of the Sinus

Venosus In the middle of the fourth week, the sinus venosus receives

venous bloodfrom the right and left sinus horns (Fig. 11.10 A).Each horn receives blood rom three important veins: (a) thevitelline or omphalomesenteric vein,(b) the umbilical vein,and (c)the common cardinal vein. At first communi-cation between the

sinus and the atrium is wide. Soon, however, the entrance of thesinus shifts to the right (Fig. 11.10B). This shift is causedprimarily by left to right shunts of blood,which occur in thevenous system during the fourth and fifth weeks of development


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Ventral view of coronalsections through the heart at

the level of theatrioventricular canal to showdevelopment of the venousvalves.A. 5 weeks.B. Scanningelectron micrograph of a

similar-staged mouse heartshowing initial formatio of theseptum primum; septumspurium is not visible. Notethe atrioventricular canal(arrow ).C.Fetal stage. The

sinus venarum (blue) issmooth walled; it derivesfromthe rightsinushorn.Arrows,blood flow.D.High magnification of theinteratrial sep-tum (arrows )

of a mouse embryo at asta e similar toC.


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Endocardial Cushions and HeartDefects

Because of their key location,abnormalities inendocardial cushion formation contribute to manycardiac malformations, including atrial andventricularseptal defects and defects involving the

great vessels (i.e., transposition of the greatvessels and tetralogy of Fallot).Since cellspopulating thec onotruncal cushions includeneural crest cells and since crest cells also

contribute extensively to development of the headand neck,abnormalities in these cells,producedby teratogenic agents or genetic causes, oftenproduce both heartand craniofacial defects in the

same individual.


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SEPTUM FORMATION IN THECOMMON ATRIUM

Septum Formation in the Atrium.The septum primum,asickle-shaped crest descending from the roof of theatrium, begins to divide the atrium in two but leaves alumen,the ostiumprimum,for communication between

the two sides(Fig. 11.14). Later, when the ostiumprimum is obliterated by fusion of the septum primumwith the endocardial cushions, the ostium secundumis formed by cell death that creates an opening in theseptum primum. Finally, as eptum secundum forms,but an interatrial opening, the oval foramen,persists.Only at birth, when pressure in the left atriumincreases, do the two septa press against each otherand close the communication between the two.

Abnormal-ities in the atrial septum may vary from total


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Further Differentiation of the Atria While the primitive right atrium enlarges by

incorporation of the right sinus horn,the primitiveleft atrium is likewise expanding. Initially, a singleembryonic pulmonary vein develops as an

outgrowth of the posterior left atrial wall,just to theleft of the septum primum (Fig. 11.15A). This veingains connection with eins of the developing lungbuds. During further development, thepulmonaryvein and its branches are incorporated

into the left atrium, forming the large smooth-walled part of the adult atrium. Although initiallyone vein enter the left atrium, ultimately fourpulmonary veins enter(Fig.11.15 B)as the

branches are incorporated into the expandingatrial wall


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in the fully developed heart, the originalembryonic left atrium is rep-resented by littlemore than the trabeculated atrialappendage,while the smooth-walled part

originates from the pulmonary veins (Fig. 11.15).On the right side the original embryonic rightatrium becomes the trabeculated right atrialappendage containing the pectinate muscles, and

the smooth-walled sinus venarum originates fromthe right horn of the sinus venosus.


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Septumprimum

Interseptovalvularspace

Septumspurium

Rightvenousvalve

Sinuatrialorifice

Leftvenousvalv

e

Pulmonary veins

Superior vena

cava

Sinusvenarum

Cristaterminalis

Septumsecundum Septumprimum


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Prepared by:

HANNAH-ISABELA

H.MENDOZA

mendoza,hannah-isabela h._cardiovascular system

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