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The structure of some vessels of the foetal circulation e.g. umbilical veins and arteries, the ductus arteriosus and ductus.
Typology: Exercises
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(B.V.Sc., Khartoum University, 2005)
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CONTENT Dedication .................................................................................................................. i ACKNOWLEDGEMENTS .................................................................................... ii
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REFERENCES.................................................................................................... 107 LEGANT OF FIGURES ..................................................................................... 121
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Sudan has the second largest camel population in the world, second only to Somalia and it is estimated to be about three million camels (Salih, 1988). Tribal groups in Sudan breed distinctive types of camels (Mason and Maule, 1960). Well-known among these are the Anafi and Bishareen, prized for their racing and riding capacities, the Rashaidi, sturdy transport camels with superior drought resistance, and the large whitish Lahaween camels which give high meat yield. Over the past few decades, camels have begun to regain recognition for their food-producing potential in arid and semi-arid areas of Sudan and other countries. After having been dismissed as uneconomical domestic animals, their vital role in supporting human populations in some of the poorest and frequently drought-stricken areas of the world has now been widely acknowledged (Hjort af Ornäs, 1988). The Arab breed of camels is well suited for meat production and transportation and camel milk is important at the subsistence level but is rarely marketed. The export of camels for slaughter, mostly to Egypt and Libyan Arab Jamahiriya and other countries, is an important source of foreign currency which should not be overlooked. Monetary returns of camel husbandry are low but, on the other hand, it is ecologically sound in long term to sustainable strategy for arid land exploitation. Scientists have presented evidence that camel grazing is beneficial for range vegetation (Gauthier-Pilters, 1984) and it appears likely that camel husbandry might discard some of the ecologically destructive effects of monocropping in sensitive environment throughout Sudan (Gauthier-Pilters, 1984). Research on the different aspects of camels is really appreciated during recent years (Majid, 2006). Topics dealing with embryology of
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1.1- The mammalian foetal circulation The embryonic circulatory system can be analyzed in term of three major circulatory arcs with the heart as the common center and pumping station. One arc is entirely intraembryonic in its distribution. Its vessels distribute food material and oxygen to all parts of the growing body and return waste materials and carbon dioxide. The other two circulatory arcs have both intra-and extraembryonic components (Carlson, 1981). The vitelline arc carries blood to and from yolk sac. The other arc carries blood to and from the allantois for gaseous interchange. As the blood from the three arcs is returned to the heart for recirculation, it is constantly being mixed so that its food material, oxygen and waste products are maintained at a tolerable level (Carlson, 1981). In placental mammals, radical changes in the source of food supply and basic living conditions occur. In mammalian embryos, the allantoic arc takes over all functions to keep the embryo alive. Maternal blood and foetal blood are brought so close together that the foetal blood can absorb food and oxygen from the maternal blood and pass its own waste materials back to the maternal circulatory system (Carlson, 1981). Blood in the mammalian foetus passes from the placenta and enters the foetus via one large umbilical vein which is found in the umbilical cord. From the umbilical vein blood goes to the liver. Most of blood continues via the ductus venosus into the posterior vena cava and finally into the right atrium (Kent and Carr,
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2001), while a small amount of blood enters the liver sinusoids and then into the posterior vena cava. From the right atrium, the greater volume of blood passes through an interatrial foramen (foramen ovale) into the left atrium bypassing a route to the lungs. From the left atrium blood is pumped to the left ventricle and then to the systemic circulation. Unoxygenated blood returned to the left atrium from the lung is a very small quantity and mixed with blood which comes in through the foramen ovale. Most of the blood returning to the right atrium from the major venous channels enters the right ventricle and pumped into the pulmonary trunk and all but a small amount of this blood is shunted away from the lungs via the ductus arteriosus into the dorsal aorta (Kent and Carr, 2001). Blood is returned to the placenta through two large umbilical arteries which are branches of the internal iliac arteries (Getty, 1975).
1.2- Embryogenesis of the foetal circulation 1.2.1- Umbilical cord The umbilical cord is the means of connection between the foetus and the placenta. Its length at full term, as a rule, is about equal to the length of the human foetus (Gray, 1918).
It is formed by the fifth week of development and it functions throughout pregnancy to protect the vessels that connect the foetus to the placenta (Kliman, 1998).
The line of reflection between the amnion and embryonic ectoderm is oval and is known as the primitive umbilical ring. At the 5 th^ week of development in human foetuses, the following structures pass through the ring: (a) the connecting stalk, containing the allantois
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stalk comes to lie on the ventral surface of the embryo. The cord is covered by a layer of ectoderm which is continuous with that of the amnion and its various constituents are enveloped by embryonic gelatinous tissue (jelly of Wharton). The vitelline vessels and duct, together with the right umbilical vein, undergo atrophy and disappear; and the cord, at birth, contains a pair of umbilical arteries and the left umbilical vein (Gray, 1918). In mammals, the first vessels are laid down in the area vasculosa on the surface of the yolk sac where they develop from the mesenchymal cells.
The mesoderm remains as a permanent connection between the embryo and the wall of the chorionic vesicle, and is called the body stalk (Saunders, 1982). An endodermal diverticulum from the posterior end of the future embryo pushes into the body stalk to form the allantois. The allantois and body stalk are the forerunners of the umbilical cord (Saunders, 1982).The allantois arises as a tubular diverticulum of the hindgut which grows into the extra-embryonic coelom (McGeady, 2006) and when the hind gut is developed the allantois is carried backward with it and then opens into the cloaca or terminal part of the hind gut: it grows out into the body stalk, a mass of mesoderm which lies below and around the tail end of the embryo. The diverticulum is lined by entoderm and covered by mesoderm, and in the latter are carried the allantoic or umbilical vessels (Gray, 1918). In reptiles, birds, and many mammals, the allantois becomes expanded into a vesicle which projects into the extraembryonic coelom. During further development in birds, it is seen to project to the right side of the embryo, and gradually expands and spreads over its dorsal surface as a flattened sac between the amnion and serosa, and extends in all
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directions and ultimately surrounds the yolk (Gray, 1918). By the end of the third week of development, the human embryo is attached to the placenta via a connecting stalk (Kliman, 1998). At approximately 25 days, the yolk sac forms and by 28 days, at the level of the anterior wall of the embryo, the yolk sac is pinched down to a vitelline duct which is surrounded by a primitive umbilical ring (Kliman, 1998).
The early formed yolk sac is observed in the embryo of camelus bacterianus at the stage of 7 pairs of somites (at the age of 22-24 days) (Shagaev and Baptidanova, 1976). The yolk placenta is formed at the stage of 17-22 pairs of somites (at the age of 27-28 days). The yolk placenta of the camelus bacterianus is functional for a long time: during the embryonic and prefoetal period (Shagaev and Baptidanova, 1976). By the end of the 5 th^ week, the primitive umbilical ring contains a connecting stalk within which passes the allantois (primitive excretory duct), two umbilical arteries and one vein, the vitelline duct (yolk sac stalk), and a canal which connects the intra- and extra-embryonic coelomic cavities (Kliman, 1998). Occasionally residual portion of the vitelline and allantoic duct, and their associated vessels, can still be seen even in full term umbilical cords, especially if the foetal end of the cord is examined (Kliman, 1998). Skidmore, Wooding and Allen (1996) reported that the foetus of camelus dromedarius was situated in the middle of the left uterine horn between 35 and 56 days of age, but the chorion and the large allantoic sac extend early into the other uterine horn (Ghazi, Oryan, and Pourmirzaei, 1994; Skidmore et al ., 1996; Tibary, 1997; Sumar, 1999).
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from the belly stalk to the sinus venous (Carlson, 1981). These veins have their origin in the capillary vessels of the allantois and their relations are similar whether the allantois remains saccular, as in the pig, or forms only a rudimentary lumen as in man (Carlson, 1981). As the liver grows in bulk, it fuses with the lateral body wall, and where this fusion occurs, vessels develop, connecting the umbilical veins with the plexus of the vessels in the liver. Once these connections are established, the umbilical stem tends more and more to pass by way of the connections to the liver; the old channels to the sinus venosus gradually degenerate (Carlson, 1981). Meanwhile, the umbilical veins, distal to their entrance into the body, become fused with each other so that only a single vein is established in the umbilical cord. Following this fusion in the cord, the intra-embryonic part of the umbilical channel also loses its original paired condition. The right umbilical vein is abandoned as a route to the liver, and all the placental blood is returned via the left umbilical vein. Part of the right umbilical vein persists, draining the body wall (Carlson, 1981). The remnant of left umbilical vein which persists and is present in the adult as the round ligament of the liver is contained within the falciform ligament (McGeady et al., 2006).
1.2.2 -The Ductus venosus When first diverted into the liver, the umbilical blood stream passes via a meshwork of small anastomosing sinusoids. As the blood volume increases, it excavates a main channel through the substance of the liver known as the ductus venosus. The ductus venosus becomes confluent with the hepatic vein which drains small sinusoids in the liver, and then it joints the posterior vena cava
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(Carlson, 1981). The ductus venosus is closed, in human babies, within 3 to 7 days after birth and the remnant is known as ligamentum venosum. In the guinea-pig, the ductus venosus is an intrahepatic branch of the vena umbilicalis. The ductus venosus persists up to the time of delivary in carnivores and ruminants but atrophies during gestation in horses and pigs (McGeady et al., 2006).
1.2.3-The Ductus arteriosus The sixth aortic arch changes its original relationship somewhat earlier than the other aortic arches. At the early stage of development, these arches extend branches from their right and left limbs toward the lungs. After these pulmonary vessels have been established, the right side of the sixth aortic arch loses communication with the dorsal aortic root and disappears. On the left, the six aortic arch retains its communication with the dorsal aortic root. The portion of this aortic arch, between the dorsal aorta and the point where the pulmonary artery is given off, is called the ductus arteriosus (Carlson, 1981). After birth, the ductus arteriosus becomes the ligamentum arteriosum.
1.2.4- The Carotid body It is not clear to what extent the migration of cells from the neural crest, dorsal mesoderm, endoderm and epibranchial placodes contribute to the primary cell aggregate which ultimately differentiates into the carotid body (Rogers, 1965). It has previously been suggested that the cell matrix of the third aortic arch is not an ordinary mesenchyme but contains cells derived from both the epibranchial placode of the glossopharyngeal nerve (de Winiwarter, 1939; Halley, 1955; Batten, 1960) and from the endoderm (de Winiwarter, 1939). It has also been suggested that, in chordates, neural crest cells play a part
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Possible contributions from the glossopharyngeal nerve have been described by Kohn (1900), Rabl (1922) and Benoit (1928). It is not improbable that cells of presumptive neural origin after migration through the glossopharyngeal nerve undergo differentiation rendering them visible as small, dark cells at the site on in the vicinity of, the peripheral contact of the sinus nerve with the anlage of the carotid body. In the rat, these cells contribute to the formation of most of the cortical region of the developing carotid body (Rogers, 1965).
The small, dark cells derived from the glossopharyngeal nerve and a few cells from the sympathetic system differentiate into cells which cannot be described as neural (Rogers, 1965).
The presence of a condensation of large, pale-staining cells about the third aortic arch has prompted Rabl (1922) and Boyd (1937) to identify this with a mesenchymatous origin for the glomus cells of the carotid body. Other investigators including Smith (1924), Ochoterena (1936), Watzka (1943), Schwarz-Karsten (1944), Ito (1950) and Celestino da Costa (1955) also believe that this primary condensation is mesenchyme, but that the glomus cells are derived from invading neuroblasts. It is suggested that the small dark cells which migrate down the sinus nerve to the carotid body differentiate into cells comparable to type П cells (de Kock, 1954) and sustentacular cells (Ross, 1959).
1.3- Anatomy 1.3.1- Umbilical cord It contains two arteries and one vein in addition to a widely patent, thin walled allantoic duct in the horse (Whitwell, 1975; Hong, Donahue, Giles, Petrites- Murphy, Poonacha, Tramontin, Tuttle, and
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Swercze, 1993). McGeady et al. (2006) reported that the body of the cord in the mare and sow consists of foetal mucoid connective tissue, surrounding two umbilical arteries, two umbilical veins fused in the cord, the urachus, and the vestige of the yolk sac. The African lion has two arteries and two veins in the umbilical cord (Benirschke and Miller, 1982). The umbilical cord of the horse is long and the proximal three-fifths of the cord are surrounded by the amnion and the distal two-fifths by the allantois ((Noden and de Lahunta, 1985). In horses, dogs and cats, the umbilical cord is divided into an amniotic and allantoic portions due to the arrangement of the foetal membranes in these species (McGeady et al., 2006). In cattle, sheep and pigs, the amnion is reflected onto the surface of the umbilical cord. In these animals, the cord is short and is cut of at birth. The umbilical cord of horses has somewhat a long portion within the amnionic cavity and a short segment in the allantoic sac (Whitwell, 1975). The umbilical cord of ruminants is short, consisting only of the allantoic stalk and blood vessels (two arteries and two veins) and is surrounded by the amnion. At 3 months of gestation, amniotic plaques develop on the amniotic and the umbilical stalk ectoderm (Noden and de Lahunta, 1985).
Morton (1961) stated that the amnion covers most of the umbilical cord of the dromedary camel foetus whereas Mohammed, (2008) stated that the amnion starts from the navel and completely surrounds the umbilical cord. Also Morton (1961) stated that the umbilical cord of the dromedary camel at full term had small amniotic pustules, measuring up to 1 cm in diameter and fine bristle-like horns up to 15 mm. in length were present close to the umbilical cord.
