Early Development of Heart - DUHS

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					                          Early Development

   Describe Vasculogenesis and Angiogenesis
   Discuss the formation of Endothelial tube
   Discuss the Carcinogenic Area
   Describe the formation of heart tube
   Discuss the further development of heart
   Discuss the formation of different layers of heart
   Discuss the division of heart tube
   Describe the Rotation of Heart tube
    Development of Cardiovascular System

   The cardiovascular system is first system to function in the embryo.
   The primordial of heart and blood vascular system appear in the middle of third
    week of embryonic development.
   The heart start to function at the beginning of fourth


                         External Features of Heart

                         Internal Features of Heart
                     Vasculogenesis and Angiogenesis
   Vasculogenesis means formation of blood vessels and Angiogenesis means
    formation of blood cells. Both the vasculogenesis and angiogenesis both occur
    parallel and both processes are controlled by one regulating gene factor. VEGF
    is a major regulator of both cell types and plays a critical role, in coordination with
    other signaling pathways and transcriptional regulators, in controlling the
    differentiation and behavior of endothelial and blood cells during early embryonic
   The formation of embryonic vascular system involves two processes:
    vasculogenesis and angiogenesis. Blood vessel formation (vasculogenesis) in
    the embryo and extraembryonic membrane during the third week: the different
    step of vasculogenesis are:
   Mesenchymal cells differentiate into endothelial cell precursor- Angioblast
    (vessel forming cells) which aggregate to form isolated angiogenic cell clusters,
    the blood islands
   Small cavities appear within the blood island by confluence of intercellular clefts.
   Angioblasts flatten to form the endothelial cells that arrange themselves around
    the cavities in the blood island to form endothelium
   These endothelium lined cavities soon fused to form the endothelial channels

                                    BLOOD ISLAND

    From Noden and La Hunta p 211
    Vessels sprout into adjacent areas by endothelia budding and fuse other vessels
   The first blood cells arise in the splanchnic mesoderm on the endodermal wall of
    the yolk sac.
   Cells in the outer zone of these blood islands become vascular endothelium and
    enclose haematopoietic stem cells.
   The function of haematopoiesis is transferred during foetal development from
    yolk sac to liver to bone marrow.

                                    Formation of Endothelial Tubes
   Day 17 - Blood islands form first in the extra-embryonic mesoderm
   Day 18 - Blood islands form next in the intra-embryonic mesoderm
   Day 19 - Blood islands form in the cardiogenic mesoderm and coalesce to form
    a pair of endothelial heart tubes

                                                   Cardiogenic area
   The carcinogenic area first forms between cranial edge of the trilaminar germ
    disc and the neural plate and just lateral to the cranial end of the neural plate, on
    about day 19. Splanchnopleuric mesodermal cells in, adjacent to the endodermal
    layer will form angioblasts that form vascular cords which will begin to coalesce
    into the paired lateral endocardial tubes. Later these lateral endocardial tubes will
    fold under the embryo, fuse, and form the primitive heart tube
                             Formation of Heart Tube
   The endothelial heart tubes fuse to form a single primitive heart tube with a
    cranial (arterial) end and a caudal (venous) end.
   The heart tubes are derived from the cardiogenic mesoderm situated next to
    the pericardial cavity, the cranial-most end of the intra-embryonic coelom. After
    the formation of the head fold (at 20 days) the carcinogenic mesoderm is shifted
    ventrally and comes to lie ventral to the primitive pharynx.

                                Folding of Embryo

                        Location of Cardiogenic Area
   At first the horseshoe-shaped cardiogenic area lies lateral and cranial to the
    neural tube
   At later stage of development, due in folding of embryo in carnio-cadual and
    lateral directions, the cardiogenic area changes its position and lies in front of
    foregut (Primitive Pharynx)
      Different steps of in-folding of embryo and shifting of
                 developing heart and pericardium

                              Development of Heart
   The primordium of the heart first evident at 18 days. In the cardiogenic area,
    splanchnic mesenchymal cells ventral to the pericardial coelom aggregates and
    arrange themselves side by side to form two cardiac primodium, the Angioblastic
    Cords. These cords canalize to form two thin walled endocardial heart tubes. As
    the lateral folding occurs, the endocardial tubes approach each other and fuse to
    form single heart tube. Fusion of heart tubes begins at the cranial end of
    developing heart and extends caudally.

                  Development of Three Layers of Heart
   Primodium of myocardium develops from the Splanchnic mesoderm surrounding
    the pericardial coelom. Twenty-three days following conception, the single,
    simple epithelial heart tube lies within the embryo's pericardial cavity. At this time
    there are three cell layers present within the heart tube.

                                   Layers of Heart
   Inner thin layer is the thin endothelial tube, that is separated from a thick
    muscular tube (primodium of myocardium) by gelatinous connective tissue called
    as cardiac jelly. The cardiac jelly is a structure less mass of cells which contain
    very few nuclei.

                          Layers of Developing Heart
   The endothelial tube becomes the internal endothelial lining of heart, the
    Endocardium, and the primodium of myocardium becomes the muscular wall of
    heart, the Myocardium. The visceral pericardium, or Epicardium is derived from
    mesothelial cells that arise from the eternal surface of sinus venosus
   Division of Heart Tube
   As folding of head region of embryo occurs, the heart and pericardial cavity come
    to lie ventral to the foregut and caudal to the oropharyngeal membrane. At the
    same time the tubular heart elongates and develops alternate constrictions and
    dilatations, the trunsus arteriosus, bulbus cordis, ventricle, atrium and sinus
                              Division of Heart Tube
   The tubular Truncus Arteriosus is continuous cranially with the aortic sac from
    which the aortic arches are arising. The Sinus Venosus receives the umbilical,
    vitelline and common cardinal veins from the chorion, yolk sac and embryo
    respectively. The arterial and venous ends of the heart are fixed by the
    pharyngeal arches and septum transverum respectively.

                     Rotation of different Parts of Heart
   The bulbus cordis and ventricle grow faster than other regions, so that heart
    bends on itself forming U-shaped Bulbo-ventricular loop. As the primordial heart
    bends the atrium and sinus venosus come to lie dorsal to the truncus arteriosus ,
    blubus cordis and ventricle. At this stage the sinus venosus has developed lateral
    expansion, the right and left horns of sinus venosus. As the heart develops it
    gradually invaginates the pericardial cavity. The heart initially suspended from
    the dorsal wall by mesentery, the dorsal mesocardium.

                    Formation & Rotation of Heart Tube

                         Transverse Pericardial Sinus
   The central part of dorsal mesocardium soon degenerates, forming a
    communication called as Transverse Pericardial sinus between right and left
    sides of the pericardial cavity. At this stage the heart is attached only at its cranial
    and caudal ends.
              Circulation Through the Primordial heart
   The initial contractions of heart originate in muscle, that is they are of myogenic
    origin. Blood enters the sinus venosus from a number of sites, including the:
   Embryo through common cardinal veins
   Placenta through the umbilical veins
   Yolk sac through the vitelline veins

        Development of Veins associated with the Heart
   Three paired Veins drain into tubular heart of four week embryo;
   Vitelline veins return poorly oxygenated blood from the yolk sac
   Umbilical veins carry well-oxygenated blood from chorion, the primordial
    placenta, only the left umbilical vein persists.
   Common Cardinal Veins return poorly oxygenated blood from the body of