Heart development
| Heart development | |
|---|---|
 Development of the human heart during the first five weeks (top), and the formation of the heart chambers (bottom) at four and eight weeks. In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different pattern from that of an adult.[1][2] | |
| Details | |
| Gives rise to | Heart |
| System | Fetal circulation,circulatory system |
| Anatomical terminology | |
Heart development,also known ascardiogenesis,refers to theprenatal developmentof theheart.This begins with the formation of twoendocardial tubeswhich merge to form thetubular heart,also called the primitive heart tube. The heart is the first functional organ invertebrateembryos.
The tubular heart quickly differentiates into thetruncus arteriosus,bulbus cordis,primitive ventricle,primitive atrium,and thesinus venosus.The truncus arteriosus splits into theascending aortaand thepulmonary trunk.The bulbus cordis forms part of theventricles.The sinus venosus connects to thefetal circulation.
The heart tube elongates on the right side, looping and becoming the first visual sign ofleft-right asymmetryof the body.Septaform within theatriaand ventricles to separate theleftandright sides of the heart.[3]
Early development
[edit]The heart derives from embryonicmesodermalgerm layercells thatdifferentiateaftergastrulationintomesothelium,endothelium,andmyocardium.Heart induction occurs in the anterior mesoderm during gastrulation through interactions with adjacent endoderm (both extra-embryonic and definitive) mediated primarily by endogenous inhibitors of WNT signaling such as DKK1.[4][5]Mesothelialpericardiumforms the outer lining of the heart. The inner lining of the heart – theendocardium,lymphatic and blood vessels, develop from endothelium.[6][2]
Endocardial tubes
[edit]In thesplanchnopleuric mesenchymeon either side of theneural plate,a horseshoe-shaped area develops as the cardiogenic region. This has formed from cardiacmyoblastsandblood islandsas forerunners of blood cells and vessels.[7]By day 19, anendocardial tubebegins to develop in each side of this region. These two tubes grow and by the third week have converged towards each other to merge, usingprogrammed cell deathto form a single tube, thetubular heart.[8]
From splanchnopleuric mesenchyme, the cardiogenic region develops cranially and laterally to theneural plate.In this area, two separateangiogeniccell clusters form on either side and coalesce to form theendocardial tubes.As embryonic folding starts, the two endocardial tubes are pushed into the thoracic cavity, where they begin to fuse together, and this is completed at about 22 days.[9][2]
At around 18 to 19 days after fertilisation, the heart begins to form. The heart begins to develop near the head of the embryo in the cardiogenic area.[1]Followingcell signalling,two strands or cords begin to form in the cardiogenic region[1]As these form, a lumen develops within them, at which point, they are referred to as endocardial tubes.[1]At the same time that the tubes are forming other major heart components are also being formed.[8]The two tubes migrate together and fuse to form a singleprimitive heart tubewhich quickly forms five distinct regions.[1]From head to tail, these are thetruncus arteriosus,bulbus cordis,primitive ventricle,primitive atrium,and thesinus venosus.[1]Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus.[1]The truncus arteriosus will divide to form the aorta and pulmonary artery; the bulbus cordis will develop into the right ventricle; the primitive ventricle will form the left ventricle; the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venosus will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus.[1]
Heart tube position
[edit]The central part of cardiogenic area is in front of the oropharyngeal membrane and the neural plate. The growth of the brain and the cephalic folds push the oropharyngeal membrane forward, while the heart and the pericardial cavity move first to the cervical region and then into the chest. The curved portion of the horseshoe-shaped area expands to form the future ventricularinfundibulumand the ventricular regions, as the heart tube continues to expand. The tube starts receiving venous drainage in its caudal pole and will pump blood out of the firstaortic archand into the dorsal aorta through its polar head. Initially the tube remains attached to the dorsal part of thepericardial cavityby a mesodermal tissue fold called the dorsal mesoderm. This mesoderm disappears to form the twopericardial sinusesthetransverseand theobliquepericardial sinuses, which connect both sides of the pericardial cavity.[7]
Themyocardiumthickens and secretes a thick layer of richextracellular matrixcontaininghyaluronic acidwhich separates theendothelium.Then mesothelial cells form the pericardium and migrate to form most of the epicardium. Then the heart tube is formed by theendocardium,which is the inner endothelial lining of the heart, and the myocardial muscle wall which is the epicardium that covers the outside of the tube.[7]
Heart folding and turning
[edit]The heart tube continues stretching and by day 23, in a process calledmorphogenesis,cardiac looping begins. The cephalic portion curves in a frontal clockwise direction. The atrial portion starts moving in a cephalically and then moves to the left from its original position. This curved shape approaches the heart and finishes its growth on day 28. The conduit forms the atrial and ventricular junctions which connect the common atrium and the common ventricle in the early embryo. The arterial bulb forms the trabecular portion of the right ventricle. A cone will form the infundibula blood of both ventricles. The arterial trunk and the roots will form the proximal portion of the aorta and the pulmonary artery. The junction between the ventricle and the arterial bulb will be called the primary intra-ventricular hole. The tube is divided into cardiac regions along its craniocaudal axis: the primitive ventricle, called primitive left ventricle, and the trabecular proximal arterial bulb, called the primitive right ventricle.[10]This time no septum is present in heart.
A functional explanation for the enigmatic turning of the heart andbowelsis lacking, but one theory does give an explanation for the evolution and development of this phenomenon. According to thisaxial twist theory,this is due to a twist in the body of all vertebrates that occurs in the early embryo. The twist turns the anterior head (with the face and cerebrum) clockwise and the rest of the exterior body anticlockwise, such that the vertebrate body is symmetric on the outside. Since there is no evolutionary pressure on the heart and inner organs for bilateral symmetry, these body parts are excluded from the twisting and remain asymmetric.[11]
Heart chambers
[edit]Sinus venosus
[edit]In the middle of the fourth week, the sinus venosus receives venous blood from the poles of right and left sinus. Each pole receives blood from three major veins: the vitelline vein, the umbilical vein and the common cardinal vein. The sinus opening moves clockwise. This movement is caused mainly by the left to right shunt of blood, which occurs in the venous system during the fourth and fifth week of development.[12]
When the left common cardinal vein disappears in the tenth week only the oblique vein of the left atrium and the coronary sinus remain. The right pole joins the right atrium to form the wall portion of the right atrium. The right and left venous valves fuse and form a peak known as theseptum spurium.At the beginning, these valves are large, but over time the left venous valve and the septum spurium fuse with the developing atrial septum. The upper right venous valve disappears, while the bottom venous valve evolves into the inferior valve of thevena cavaand thecoronary sinus valve.[12]
Heart wall
[edit]The main walls of the heart are formed between day 27 and 37 of the development of the early embryo. The growth consists of two tissue masses actively growing that approach one another until they merge and split light into two separate conduits. Tissue masses calledendocardial cushionsdevelop into atrioventricular andconotruncalregions. In these places, the cushions will help in the formation of auricular septum, ventricular conduits, atrio-ventricular valves and aortic and pulmonary channels.[13]
Atria
[edit]
At the end of the fourth week, a crest grows that leaves the cephalic part. This crest is the first part of theseptum primum.The two ends of the septum extend into the interior of theendocardial cushionsin theatrioventricular canal.The opening between the bottom edge of the septum primum and endocardial cushions is theostium primum(first opening). The extensions of the upper and lower endocardial pads grow along the margin of the septum primum and close the ostium primum. Coalescence of these perforations will form theostium secundum(second opening), which allows blood to flow freely from the right atrium to the left.
When the right of the atrium expands due to the incorporation of the pole of the sinus, a new fold appears, called theseptum secundum.At its right side it is fused with the left venous valve and the septum spurium. A free opening will then appear, called theforamen ovale.The remains of the upper septum primum, will become the valves of the foramen ovale. The passage between the two atrial chambers consists of a long oblique slit through which blood flows from the right atrium to the left.[13]
Ventricles
[edit]Initially, a singlepulmonary veindevelops in the form of a bulge in the back wall of the left atrium. This vein will connect with the veins of the developinglung buds.As development proceeds, the pulmonary vein and its branches are incorporated into the left atrium and they both form the smooth wall of the atrium. The embryonic left atrium remains as the trabecular left atrial appendage, and the embryonic right atrium remains as the right atrial appendage.[14]
Septum formation of the atrioventricular canal
[edit]At the end of the fourth week, two atrioventricular endocardial cushions appear. Initially the atrioventricular canal gives access to the primitive left ventricle, and is separated from arterial bulb by the edge of the ventricular bulb. In the fifth week, the posterior end terminates in the center part of the upper endocardial cushion. Because of this, blood can access both the left primitive ventricle and the right primitive ventricle. As the anterior and posterior pads project inwardly, they merge to form a right and left atrioventricular orifice.[15]
Atrioventricular valves
[edit]When forming intra-atrial septa, atrio-ventricular valves will begin to grow. A muscularinterventricular septumbegins to grow from the common ventricle to the atrio-ventricular endocardial cushions. The division begins in the common ventricle where a furrow in the outer surface of the heart will appear the interventricular foramen eventually disappears. This closure is achieved by further growth of the muscular interventricular septum, a contribution of trunk crest-conal tissue and a membranous component.[16]
Valves and outflow tracts
[edit]Truncus septum formation and arterial cone
[edit]The arterial cone is closed by the infundibular cushions. The trunk cones are closed by the forming of an infundibulotroncal septum, which is made from a straight proximal portion and distal spiral portion. Then, the narrowest portion of the aorta is in the left and dorsal portion. The distal portion of the aorta is pushed forward to the right. The proximal pulmonary artery is right and ventral, and the distal portion of the pulmonary artery is in the left dorsal portion.[13]
Pacemaker and conduction system
[edit]The rhythmic electrical depolarization waves that trigger myocardial contraction is myogenic, which means that they begin in the heart muscle spontaneously and are then responsible for transmitting signals from cell to cell.Myocytesthat were obtained in the primitive heart tube, start beating as they connect together by their walls in asyncytium.Myocytes initiate rhythmic electrical activity, before the fusion of theendocardial tubes.The heartbeat begins in the region of thepacemakerwhich has a spontaneous depolarization time faster than the rest of myocardium.[17]
Theprimitive ventricleacts as initial pacemaker. But this pacemaker activity is actually made by a group of cells that derive from the sinoatrial right venous sinus. These cells form an ovoidsinoatrial node(SAN), on the left venous valve. After the development of the SAN, the superior endocardial cushions begin to form a pacemaker also known as theatrioventricular node.With the development of the SAN, a band of specialized conducting cells start to form creating thebundle of Histhat sends a branch to the right ventricle and one to the left ventricle. Most conduction pathways originate from the cardiogenic mesoderm but the sinus node may be derived from the neural crest.[17]
The humanembryonicheart displays cardiac activity approximately 21 days after fertilization, or five weeks after the last normalmenstrual period(LMP), which is the date normally used to date pregnancy in the medical community. The electrical depolarizations that trigger cardiacmyocytesto contract arise spontaneously within themyocyteitself. The heartbeat is initiated in the pacemaker regions and spreads to the rest of the heart through a conduction pathway. Pacemaker cells develop in the primitive atrium and the sinus venosus to form thesinoatrial nodeand theatrioventricular noderespectively. Conductive cells develop thebundle of Hisand carry thedepolarizationinto the lower heart. Cardiac activity is visible beginning at approximately 5 weeks of pregnancy.
The human heart begins beating at a rate near the mother's, about 75-80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly for the first month of beating, peaking at 165-185 BPM during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, an increase of 100 BPM in the first month.[18]
After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term.
Imaging
[edit]
In thefirst trimester,cardiac activity can be visualized and the fetal heart motion quantified byobstetric ultrasonography.A study of 32 normal pregnancies showed that fetal heart motion was visible at a meanhuman chorionic gonadotropin(hCG) level of 10,000 UI/L (range 8650–12,200).[19]Obstetric ultrasonography can also useDoppler techniqueon key vessels such as theumbilical arteryto detect abnormal flow.
In later stages of pregnancy, a simpleDoppler fetal monitorcan be used to quantify the fetal heart rate.
A fetal heartbeat can be detected at around 17 to 20 weeks of gestation when the chambers of the heart have become sufficiently developed.[20]
Duringchildbirth,the parameter is part ofcardiotocography,which is where the fetal heartbeat anduterine contractionsare continuously recorded.
Additional images
[edit]-
M-mode sonographymeasuring embryonic heart rate. -
Blood flow in aneonate -
Human embryo, 38 mm, 8–9 weeks–anterior view, heart is visible.
References
[edit]- ^abcdefghBetts JG (2013).Anatomy & physiology.pp. 787–846.ISBN978-1938168130.Retrieved11 August2014.
- ^abcHosseini HS, Garcia KE, Taber LA (July 2017)."A new hypothesis for foregut and heart tube formation based on differential growth and actomyosin contraction".Development.144(13): 2381–2391.doi:10.1242/dev.145193.PMC5536863.PMID28526751.
- ^Anderson RH, Webb S, Brown NA, Lamers W, Moorman A (August 2003)."Development of the heart: (2) Septation of the atriums and ventricles".Heart.89(8): 949–958.doi:10.1136/heart.89.8.949.PMC1767797.PMID12860885.
- ^Schneider VA, Mercola M (February 2001)."Wnt antagonism initiates cardiogenesis in Xenopus laevis".Genes & Development.15(3): 304–315.doi:10.1101/gad.855601.PMC312618.PMID11159911.
- ^Marvin MJ, Di Rocco G, Gardiner A, Bush SM, Lassar AB (February 2001)."Inhibition of Wnt activity induces heart formation from posterior mesoderm".Genes & Development.15(3): 316–327.doi:10.1101/gad.855501.PMC312622.PMID11159912.
- ^"Animal Tissues".Users.rcn.com. 2010-08-13. Archived fromthe originalon 2009-05-05.Retrieved2010-10-17.
- ^abcSadler TW (2012).Langman. Embriología Médica.Lippincott Williams & Wilkins. p. 165.ISBN978-84-96921-46-7.
- ^abLarsen W (2001).Human Embryology(3rd ed.). Elsevier Saunders. pp. 159–163.ISBN978-0-443-06583-5.
- ^"Main Frame Heart Development".Meddean.luc.edu.Retrieved2010-10-17.
- ^Rohen J, Lutjen E (2008).Embriología functional: una perspectiva desde la biología del desarrollo.Panamericana. p. 70.ISBN978-84-9835-155-2.
- ^de Lussanet MH, Osse JW (2012). "An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates".Animal Biology.62(2): 193–216.arXiv:1003.1872.doi:10.1163/157075611X617102.S2CID7399128.
- ^abCarlson B (2012).Embriología humana y biología del desarrollo.Mosby. p. 451.ISBN978-84-8174-785-0.
- ^abcFernández PM (2002).Manual de biología del desarrollo.Manual Moderno. p. 243.ISBN978-968-426-976-7.
- ^Eynard A, Valentich M, Rovasio R (2011).Histología y embriología del ser humano: bases celulares y moleculares.Panamericana. p. 283.ISBN978-950-06-0602-8.
- ^Moore KL, Persaud TV (2008).Embriología Clínica.Elsevier Saunders. p. 245.ISBN978-84-8086-337-7.
- ^Tellez de Peralta G (2003).Tratado de cirugía cardiovascular.Díaz de Santos. p. 44.
- ^abLarsen W (2003).Embriología humana.Elsevier Science. p. 177.ISBN978-968-426-976-7.
- ^OBGYN.net "Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies"Archived2006-06-30 at theWayback Machine
- ^Giacomello F, Magliocchetti P, Loyola G, Giovarruscio M (1993). "[Serum beta hCG levels and transvaginal echography in the early phases of pregnancy]".Minerva Ginecologica(in Italian).45(7–8): 333–337.PMID8414139.
- ^"ACOG Guide to Language and Abortion".American College of Obstetricians and Gynecologists.Archived fromthe originalon 16 November 2022.Retrieved17 November2022.
"Heartbeat bill" - It is clinically inaccurate to use the word "heartbeat" to describe the sound that can be heard on ultrasound in very early pregnancy. In fact, there are no chambers of the heart developed at the early stage in pregnancy that this word is used to describe, so there is no recognizable "heartbeat." What pregnant women may hear is the ultrasound machine translating electronic impulses that signify fetal cardiac activity into the sound that we recognize as a heartbeat. "Fetal heartbeat" - Until the chambers of the heart have been developed and can be detected via ultrasound (roughly 17-20 weeks of gestation), it is not accurate to characterize the embryo's or fetus's cardiac development as a heartbeat.
This article incorporates text from theCC BYbook:OpenStax College, Anatomy & Physiology. OpenStax CNX. 30 July 2014.