DEVELOPMENT OF THE CIRCULATORY SYSTEM

I. Develops relative to embryo’s needs A. During early development diffusion of oxygen, wastes, nutrients, etc. suffices B. As embryo grows larger, metabolic needs increase, diffusion no longer sufficient. C. Circulatory system begins to develop II. First evidence of circulatory structures seen in yolk sac, extraembryonic splanchnic mesoderm - area opaca vasculosa - blood islands

(primitive blood cells and blood stem cells)

III. Blood stem cells - where do they come from? A. Sites of production from stem cells change as the human embryo develops. 1. At 4 weeks after fertilization - stem cells are located in the extraembryonic splanchnic mesoderm of the yolk sac. 2. At 5 weeks - in body mesenchyme of embryo. 3. At 6 weeks - in developing liver. 4. At 8-16 weeks - in developing spleen, thymus and lymph nodes. 5. At 16 weeks and beyond - in bone marrow

B. Evidence suggests there are two different sources for blood stem cells. 1. Initially derived from extraembryonic splanchnic mesoderm of the yolk sac. a. “Primitive stem cells” b. Produce mature erythrocytes that have a nucleus. 2. Later in development a new population of stem cells arises. Recent research suggests that these stem cells arise from endothelial cells that line the aorta. a. By 16 weeks of development these stem cells populate the bone marrow b. These cells are the blood stem cells that give rise to enucleate erythrocytes (red blood cells).

IV. Circulatory system in mammals A. Basic circulatory loop arterioles capillaries arteries venules ventricle atrium veins

B. Basic adult mammalian circulation

C. Early embryonic circulation

D. How is the adult circulatory pattern established?

If we look at adult vertebrate species from primitive fish, to reptiles, to birds, to mammals, there are gross structural differences in the pattern of circulation. These differences are there to accommodate the specific adult needs (e.g. fish have gills, mammals don’t). These differences are so great, that if only the adult circulatory system was used to establish taxonomic classification, in some cases we would probably say that 2 or more species of vertebrates were not related. If we look at the embryonic circulatory systems of all vertebrates, we find that they are basically the same. The adult systems are derived from the basic embryonic system.

Conversion of the early embryonic circulatory system to the adult configuration. Involves: A. Degeneration of parts of some embryonic vessels or their parts. B. Hypertrophy of parts of some vessels. C. Anastomosis (fusion) of some vessels. D. Separation of one embryonic vessel into two. E. Loss of connection between some vessels. F. Formation of new vessels.

Views from the ventral side of the animal Right Left Views from the ventral side of the animal Right Left From Carlson, B.M. 1996. Patten’s Foundations of Embryology. McGraw-Hill, Inc. New York. 6th edition. p. 618

Right Left

In the embryo Right Left

Fate of the Trunkus Arteriosus 4 trunkus pulmonary trunk 4th systemic trunk Right Left Fate of the Trunkus Arteriosus Contributes to the systemic trunk and a portion of the pulmonary trunk. Divided into the bases of the pulmonary and systemic trunks

RIGHT LEFT green - dorsal aortic roots purple - 3rd aortic arches (Seventh intersegmental) green - dorsal aortic roots Modified from Carlson, B.M. 1996. Patten’s Foundations of Embryology. McGraw-Hill, Inc. New York. 6th edition. p. 618 purple - 3rd aortic arches red - 4th aortic arches black - ventral aortic roots and trunkus arteriosus orange - 6th aortic arches yellow - bulbus arteriosus blue - intersegmental arteries These figures present a ventral view

Adult vessels Right Left Right Left Color indicates embryonic derivation green - dorsal aortic roots purple - 3rd aortic arches red - 4th aortic arches black - ventral aortic roots and trunkus orange - 6th aortic arches yellow - bulbus arteriosus blue - interseg-mental arteries Vertebral artery Subclavian artery These figures present a ventral view Modified from Carlson, B.M. 1996. Patten’s Foundations of Embryology. McGraw-Hill, Inc. New York. 6th edition. p. 618

Venous Circulation Four Major Systems 1. Systemic (other than hepatic) 3. Pulmonary 4. Placental (Umbilical)

Conversion of the early embryonic circulatory system to the adult configuration. Involves: A. Degeneration of some embryonic vessels or their parts.* B. Hypertrophy of parts of some vessels.* C. Anastomosis (fusion) of some vessels.* D. Loss of connection between some vessels.* E. Formation of new vessels.*

Formation of the inferior vena cava Posterior Systemic circulation Formation of the inferior vena cava posterior anterior anterior posterior Adapted from Hopper, A.F. and N.H. Hart, 1985. Foundations of animal development. Oxford University press. New York, p. 434

These figures present a ventral view Green - anterior cardinal veins Yellow - vitelline veins Orange - supracardinal veins Purple - common cardinal veins Blue, blue - posterior cardinal veins Olive - sinus venosus Red, red - subcardinal veins Hepatic segment Mesenteric segment Renal segment Sub-/Supra-cardinal anastomosis Adapted from Hopper, A.F. and N.H. Hart, 1985. Foundations of animal development. Oxford University press. New York, p. 434

These figures present a ventral view Green - anterior cardinal veins Yellow - vitelline veins Orange - supracardinal veins Purple - common cardinal veins Blue, blue - posterior cardinal veins Olive - sinus venosus Red, red - subcardinal veins (Mesenteric segment) Internal iliac Adapted from Hopper, A.F. and N.H. Hart, 1985. Foundations of animal development. Oxford University press. New York, p. 434

These figures present a ventral view Green - anterior cardinal veins Yellow - vitelline veins Orange - supracardinal veins Purple - common cardinal veins Blue, blue - posterior cardinal veins Olive - sinus venosus Red, red - subcardinal veins Adapted from Hopper, A.F. and N.H. Hart, 1985. Foundations of animal development. Oxford University press. New York, p. 434

portion between the right subclavian and the left brachiocephalic vein forms the right brachiocephalic (innominate) vein.

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(brachiocephalic) anterior cardinal veins Common cardinal vein (brachiocephalic) http://education.yahoo.com/reference/gray/subjects/subject?id=135

anterior cardinal veins Common cardinal vein (brachiocephalic)

These figures present a ventral view Right Left Right Left Right Left Right vitelline vein These figures present a ventral view Green - anterior cardinal vein Purple - common cardinal veins Future Inferior vena cava Blue - posterior cardinal veins Brown - sinus venosus Orange - umbilical veins Red - right vitelline vein Yellow - left vitelline vein Right Left Red/yellow speckles - anastomoses between right and left vitelline veins Adapted from Hopper, A.F. and N.H. Hart, 1985. Foundations of animal development. Oxford University press. New York, p. 431

Ductus venosus: Present at birth, but looses functionality within minutes. Structurally closed 3-7 days after birth Leaves a fibrous remnant in the liver called the ligamentum venosum

Pulmonary venous system The pulmonary veins are not derived from pre-existing embryonic veins. They form de novo as the lungs develop and drain the capillary beds of the lung tissue into the left atrium. Initially this drainage is via a single trunk; however, as the embryo develops, this trunk is incorporated into the wall of the left atrium. By 8-9 weeks, this results in the 4 pulmonary veins that originally connected to the common trunk, emptying separately into the left atrium.

Umbilical Arteries Develop as branches off the posterior dorsal aorta that extend along the allantoic stalk out to the placenta. Umbilical Veins As the placental circulation develops, two umbilical veins initially return blood from the placenta to the sinus venosus. As development continues, the right umbilical vein degenerates and the placental blood ends up being returned to the heart by the left umbilical vein via the ductus venosus. This blood flow ceases at birth when the umbilical cord is cut. Subsequently, the lumen within the left umbilical vein is obliterated by cell growth from the walls and the remnant of this vessel becomes the round ligament of the liver.