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Small cavities appear in the extraembryonic mesoderm and coalesce to form the extraembryonic coelom (chorionic cavity) women's health clinic unionville purchase aygestin american express, except for a mesodermal stalk that connects the amnion to the trophoblast womens health partners summerville sc purchase aygestin without a prescription. This remaining mesodermal stalk later forms the connecting stalk (future umbilical cord) womens health 334 tamu purchase aygestin master card. With the formation of the extraembryonic coelom women's health center teaneck buy 5 mg aygestin with amex, the extraembryonic mesoderm becomes separated into two thin layers, completing formation of the three extraembryonic membranes. The trophoblast, with its inner coating of extraembryonic mesoderm, is now called the chorion. The primitive amnion with its outer coating of extraembryonic mesoderm is the amnion proper; and the primitive umbilical vesicle (yolk sac) becomes the umbilical vesicle proper with its outer coating of mesoderm. The primitive streak and node give rise to a third germ layer, the intraembryonic mesoderm, situated between the ectoderm and endoderm. This creation of intraembryonic mesoderm is through the process of gastrulation, occurring during the third week. The mesodermal cells migrate laterally and cranially until ectoderm and endoderm are separated from each other by intraembryonic mesoderm, except at the cephalic oropharyngeal membrane and the caudal cloacal membrane, where ectoderm remains in contact with endoderm. The primitive streak mesoderm cranial to oropharyngeal membrane in the midline is the cardiogenic mesoderm. Laterally, all along the margin of the embryonic disc, the intraembryonic mesoderm is continuous with the extraembryonic mesoderm. The embryo, now trilaminar, becomes more elongated and pear shaped when viewed from its dorsal (ectodermal) or ventral (endodermal) side. The embryo is attached at its narrow caudal end to the chorion by the connecting stalk. The ovulation age of the embryo at this stage of development is about 20 days, and its length is almost 1. The time is now rapidly approaching when simple diffusion of oxygen and nutrients cannot adequately provide for the greatly increasing metabolic needs of the embryo, and a functional circulatory system becomes necessary. The vascular system grows from a simple, bilaterally symmetric plexus into an asymmetric, complex system of arteries, veins, and capillaries-a necessarily dynamic process involving the formation of new vessels and temporary detours, rerouting of the bloodstream, and the disappearance of previously dominant channels or even of entire vascular subsystems. The vascular system needs to enlarge as the embryo grows, adapting to marked changes in embryonic shape and developmental changes in other organ systems. While hard at work, the heart also must grow and differentiate from a simple tube into a complex, fourchambered organ with sets of valves. Finally, because the very young embryo is tiny compared to the mass of extraembryonic (placental) tissue, which the young heart also supplies with blood, this heart is relatively enormous compared with its relative size in the adult. Describing the development of the cardiovascular system first requires review of the intraembryonic coelom ("body cavity") formed by the confluence of small, initially isolated spaces that appear in the lateral mesoderm and cardiogenic mesoderm. The spaces fuse together and form the single, horseshoe-shaped intraembryonic coelom that extends the length of the embryo in the lateral mesoderm on each side, communicating across the midline cranially in the cardiogenic mesoderm. Later in development, a communication develops on each side between the caudal ends of the intraembryonic coelom and the extraembryonic coelom. The formation of the coelom separates the lateral mesoderm into two layers: the parietal layer in contact with the ectoderm and the visceral layer in contact with the endoderm. The ectoderm with its parietal layer of lateral plate mesoderm is called somatopleure; endoderm with its visceral mesodermal layer is called splanchnopleure. In the late presomite embryo, scattered masses of angiogenic cells differentiate in the cardiogenic mesoderm and the splanchnopleure mesoderm ventral to the entire extent of the horseshoe coelom. The angiogenic cell clusters, called blood islands, rapidly increase in number and size, acquire a lumen surrounded by a simple squamous endothelium, and unite to form a plexus of vessels. In the cardiogenic mesoderm, paired endothelial heart tubes develop from paired angioblastic cords. The tubes are ventral to the central part of the U-shaped coelom that will form the pericardial cavity. The paired heart tubes begin to beat by day 21 or 22 and fuse into a single heart tube a few days later. Another pair of angioblastic cords appear bilaterally, parallel and rather close to the midline of the embryo. These cords also acquire a lumen and form a pair of longitudinal vessels, the dorsal aortae (aortas). These vessels connect with the dorsocranial aspect of the endothelial (endocardial) heart tubes to establish the arterial pole of the developing heart. The caudal ends of the endothelial heart tubes make contact with vessels arising in the yolk sac mesoderm (vitelline veins) and later with the developing umbilical veins and common cardinal veins. Thus the first pair of aortic arches, or pharyngeal (branchial) arch arteries, appear. With the folding of the trilaminar embryonic disc, the endoderm is shaped into a tube within the embryo. The midgut is continuous with the umbilical vesicle (yolk sac) that extends ventrally from the embryo. The foregut is an extension into the head region of the embryo, and the hindgut is a caudal extension. The trilaminar embryonic disc folds into a cylinder, and the amnion tucks around the embryo on each side. The amnion also envelops the head end of the embryo as the ectodermal tube of the forebrain rapidly increases in size in a cranial and ventral direction. The result is a 180-degree sagittal plane rotation of the cardiogenic mesoderm and oropharyngeal membrane, which were originally cranial to the neural plate and the developing neural tube.
Thus the right sinus horn becomes larger menopause excessive bleeding generic aygestin 5 mg without prescription, more vertical women's health center abington aygestin 5mg mastercard, and incorporated into the part of the primitive atrium that will become the right atrium menstruation gastrointestinal problems purchase aygestin 5 mg visa. The right horn will form the smooth posterior wall of the right atrium pregnancy blood test cheap aygestin 5mg mastercard, the sinus venarum, named after its sinus venosus origin. The communication between the sinus venosus and the developing right atrium is now limited to the right sinus horn. The sinoatrial orifice is tall and narrow, and the folds on either side constitute the valve of the sinus venosus, with the right fold larger than the left fold. The left fold of the valve fuses with the septum secundum to become part of the interatrial septum. The cranial part of the right valve fold becomes a thick, vertical ridge of muscle, the crista terminalis, that marks the boundary between the two primordia (sinus venosus and primitive atrium) that contribute to the right atrial wall. Posterior to the crista terminalis is the smooth-walled sinus venarum; anterior to the crista terminalis is the wall of the right atrium lined with pectinate muscle, including the right atrial appendage (auricle). The inferior part of the right fold of the valve of the sinus venosus becomes the valve of the inferior vena cava and the smaller valve of the coronary sinus. Plate 4-11 summarizes the primitive heart tube chambers and their adult derivatives. The left and right cusps diminish in size and are usually identifiable in the adult valve as very small left and right commissural cusps. The right tricuspid valve develops similarly, except three cusps develop instead of the original four (becoming two) in the mitral valve. Initially thick and fleshy, the chordae tendineae become thin and fibrous as their muscular component disappears. Development of the basic structure of the mitral valve is completed by the end of the sixth week; the tricuspid valve is completed soon after (see Plates 4-12 and 4-13). The primordia of the aortic and pulmonary semilunar valves appear near the end of partitioning of the truncus arteriosus by the truncal component of the spiral septum. Four swellings of mesenchyme surround the lumen of the truncus arteriosus (see Plate 4-9). Left and right swellings are divided by the aorticopulmonary septum to form left and right valve cusps in both the ascending aorta and pulmonary trunk. The anterior swelling forms the anterior cusp in the pulmonary valve, and the posterior swelling in the truncus arteriosus forms the posterior cusp of the aortic valve. Excavation of the superior surfaces of the swellings and later thinning result in the semilunar shape of each cusp. Dilatation of the proximal origins of the ascending aorta and pulmonary trunk gives rise to the pulmonary and aortic sinuses, the expanded space between each cusp and the walls of the arteries. The left and right coronary arteries arise from the left and right aortic sinuses (of Valsalva), respectively. Preferential flow related to the development of organ systems, however, leads to enlargement of certain channels in the plexus. This expansion is brought about in part by the fusion and confluence of adjacent smaller vessels and by the enlargement of individual capillaries. The various vascular systems are also continuously modified to satisfy changing needs. Initially, the arteries and veins consist simply of endothelial tubes and cannot be distinguished from each other histologically. In later development, typical vessel walls are differentiated from the surrounding mesenchyme. The final pattern of the vascular system is genetically determined and varies with the animal species. Variations are, however, extremely common in both arterial and venous patterns, and local modifications occur in cases of abnormal development of organs. Distal portion of ascending aorta, brachiocephalic trunk, and aortic arch up to origin of left common carotid artery. Left: Aortic arch segment between left common carotid and left subclavian arteries. Right: Proximal part becomes proximal segment of right pulmonary artery; distal part disappears early. Left: Proximal part becomes proximal segment of left pulmonary artery; distal part persists, until birth, as ductus arteriosus. Two days later (4-mm embryo), the first arch has largely disappeared, but part of it persists as a portion of the maxillary artery. The second arch also regresses; all that remains of it is the tiny stapedial artery in the middle ear. The fourth and sixth arches form as ventral and dorsal sprouts from the aortic sac and dorsal aortae, respectively, fuse with each other. The ventral portion of the sixth arches already have their major branches, the proximal portions of the left and right primitive Derivatives of the aortic arch arteries and related vessels 1. Sixth arches: the major arteries in an early embryo are represented by a pair of vessels, the dorsal aortae, which run with the long axis of the embryo and form the continuation of the endocardial heart tubes. Because of the changing position of the cardiogenic mesoderm containing the heart tubes, the cranial portion of each dorsal aorta comes to describe an arc on both sides of the foregut, thus establishing the first pair of aortic arch arteries, termed aortic arches (see Plate 4-14). In primitive vertebrates, six pairs of aortic arches appear in conjunction with the development of the corresponding pharyngeal ("branchial") arches, which are transverse swellings of mesenchyme flanking the foregut ventrally and laterally. The pharyngeal arches and their blood supply initially evolved in part to form the gills (branchiae) of aquatic vertebrates, thus their original designation as "branchial arches. As part of this process, certain aortic arches (pharyngeal arch arteries) are retained and modified to form the large arteries of the neck and thorax. Left seventh intersegmental artery: Near the end of the third week (3-mm embryo), the first pair of arches is large; the second pair is just forming.
Of the right atrial branches of the right coronary artery women's health center gretna buy 5mg aygestin with mastercard, one is of great importance menstrual cramps 8dpo generic aygestin 5 mg on line. Apical part of anterior (sternocostal) surface supplied by branches from posterior interventricular (posterior descending) branch of right coronary artery curving around apex women's health magazine tips cheapest generic aygestin uk. Posterior interventricular (posterior descending) branch is derived from circumflex branch of left coronary artery instead of from right coronary artery women's health zambia buy aygestin australia. Area supplied chiefly by small branches from circumflex branch of left coronary artery and from right coronary artery. Area supplied chiefly by elongated anterior interventricular (left anterior descending) branch curving around apex. The three largest veins are the great cardiac vein, middle cardiac vein, and posterior left ventricular vein. The ostia of these veins may be guarded by fairly welldeveloped unicuspid or bicuspid valves. The oblique vein of the left atrium (of Marshall) enters the sinus near the orifice of the great cardiac vein, and its ostium never has a valve. The small cardiac vein may enter the right atrium independently, and the anterior cardiac veins always do. Small venous systems in the atrial septum (and probably in ventricular walls and septum) enter the cardiac chambers directly, called the thebesian veins. The existence of so-called arterioluminal and arteriosinusoidal vessels is debatable and the evidence inconclusive. The cervical and upper thoracic sympathetic trunk ganglia contribute cardiac branches, all of which pass through the cardiac plexus, usually without forming synapses. These ganglia are ultimately distributed to the various layers of the heart wall through the coronary plexuses. Three pairs of sympathetic cardiac nerves are derived from the cervical ganglia of the sympathetic trunks, and others arise from the upper thoracic ganglia. The superior cervical sympathetic cardiac nerve originates by several rootlets from the corresponding ganglion. It often unites with the superior vagal cardiac nerve(s), and this conjoined nerve then descends behind the carotid sheath, communicating en route through slender rami with the pharyngeal, laryngeal, carotid, and thyroid nerves. On the right side, the conjoined nerve passes posterolateral to the subclavian and brachiocephalic arteries and aortic arch; on the left it curves downward across the left side of the aortic arch. The middle cervical sympathetic cardiac nerve is often the largest of the cervical cardiac nerves. It is formed by filaments from the middle and vertebral ganglia of the sympathetic trunk. This cardiac nerve usually runs independent of the cardiac plexus but may unite with other cardiac nerves, and it is interconnected with tracheal, esophageal, and thyroid branches of the sympathetic trunks. The inferior cervical sympathetic cardiac nerves consist of filaments arising from the stellate (cervicothoracic) ganglion and ansa subclavia. These cardiac nerves often combine with each other or with other cardiac nerves before reaching the cardiac plexus, and inconstant communications exist between these nerves and the phrenic nerves. The thoracic sympathetic cardiac nerves are four or five slender branches on each side that arise from the corresponding upper thoracic sympathetic trunk ganglia. Some enter the plexus directly, whereas others are united for variable distances with filaments destined for the lungs, aorta, trachea, and esophagus. The vagal (parasympathetic) cardiac branches vary in size, number, and arrangement but can be grouped as superior and inferior cervical and thoracic vagal cardiac nerves. The superior cervical vagal cardiac nerve forms from two or three filaments that leave the vagus in the upper part of the neck and usually unites with the corresponding sympathetic cardiac nerve. The inferior cervical vagal cardiac nerve(s), one to three in number, arise in the lower third of the neck and often join or communicate with the cardiac branches from the middle cervical sympathetic ganglia and the vertebral and/or stellate sympathetic ganglia. If they remain separate, these cardiac nerves lie posterolateral to the brachiocephalic artery and aortic arch on the right side and lateral to the left common carotid artery and aortic arch on the left side. Inferior cervical (sympathetic) cardiac nerves Thoracic cardiac branch of vagus nerve 4th thoracic sympathetic ganglion Thoracic (sympathetic) cardiac branches Cardiac plexus Phrenic nerve (cut) 3rd thoracic sympathetic ganglion Thoracic (sympathetic) cardiac branches Thoracic cardiac branch of vagus nerve Left recurrent laryngeal nerve the thoracic vagal cardiac nerves are a series of filaments arising from the vagus nerve of each side, at or below the level of the thoracic inlet, and also from both recurrent laryngeal nerves, with the left contributing more filaments than the right. These often unite with other cardiac nerves in their passage to the cardiac plexus. The cardiac plexus lies between the concavity of the aortic arch and the tracheal bifurcation and is sometimes described as consisting of superficial and deep parts, although their depths vary minimally, and they are intimately interconnected. A proportion of the vagal fibers relay in several ganglia present in the cardiac plexus. Relatively few cardiac ganglia are found over the ventricles, but enough exist to question the view that the ventricular innervation is entirely or predominantly sympathetic. The cardiac sympathetic and parasympathetic nerves carry both afferent and efferent fibers. The efferents carry impulses that are modified reflexively by afferent impulses from the heart and great vessels. Efferent fibers are under the overall control of the higher centers in the brain, the hypothalamus, and the brainstem. Afferents from the heart and the great vessels are shown traveling to the cord via the sympathetic cardiac nerves, whereas others are carried upward to nuclei in the medulla oblongata by the vagus nerves. The cell bodies of the afferent neurons are situated in the dorsal root ganglia of the upper four or five thoracic nerves and in the inferior vagal ganglia. The preganglionic parasympathetic fibers are the axons of cells in the dorsal vagal nuclei, and these fibers relay in cardiac plexus or intrinsic cardiac ganglia. The preganglionic sympathetic fibers are the axons of cells located in the lateral gray columns of the upper four or five thoracic segments. These fibers enter the corresponding spinal nerves and leave them in white rami communicantes.
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Diseases
Dandy Walker malformation with mental retardation, macrocephaly, myopia, and brachytelephalangy
