ANA 212 LECTURE WEEK ONE SUMMER SEMESTER 2019/2020 EMBRYOLOGY &HISTOLOGY

Lecture Objectives, Outcomes and Learning Milestones. Lectures Aims: To facilitate the pedagogic and didactic process for a lucid comprehension of these themes by the student of Clinical anatomy. Lecture Objectives, Outcomes and Learning Milestones. The objective of this Lecture is to delineate the salient OF features of the Embryology and Histology and its clinical application to facilitate its lucid comprehension by the student of Clinical Anatomy. At the end of this Lecture Notes study, The Student of Applied Anatomy: The Student will understand the relevance and import of studying the Human Embryology and Histology. The Student will be able to accurately describe the salient features and thematic aspects of Human Embryology and Histology.

Human Embryology (Essense) It is the study of the development and growth process in a human being from conception until birth. It is a component of Anatomy. Ontogeny is the study of the human being from conception until all his life span. It is synonymous with developmental Anatomy. Clinical Embryology is very much related to Fetal Pathology.

The Bilaminar Germ Disc Layer. It corresponds with the second week of developmental period. The cells of the inner cell mass or embryoblast differentiates into two layers the hypoblast and the epiblast. The hypoblast layer is a layer of small, cuboidal cells adjacent to the blastocyst cavity which has a sustentacular and subicular function. The epiblast is a layer of high columnar cells adjacent to the amniotic cavity which differentiated further into the endoderm, mesoderm and the ectoderm. The cells of each of the Bilaminar germ layers form a flat disc and together they are known as the Bilaminar disc.

The Trilaminar Germ Disc Layer (Essence) The most characteristic event that occurs at the third week is gastrulation, the process which establishes all the three germ layers in the embryo. Gastrulation begins with the formation of the primitive streak on the surface of the epiblast. The cephalic end of the streak, known as the primitive node. The Primitive node consists of a slightly elevated area surrounding the small primitive pit. In this period a transverse section through the region of the primitive groove demonstrates that a new cell layer has developed between the epiblast and the hypoblast.

The Trilaminar Germ Disc Layer (Development) During this period cells of the epiblast migrate in the direction of the primitive streak to form the mesoderm and the intraembryonic endoderm. Invagination in embryonic terms is the process by which on arrival in the region of the primitive streak, the daughter epiblast cells becomes flask shaped detach from their parent epiblast and slips beneath it. Once the daughter epiblast cells have invaginated, some displace the hypoblast, thereby creating the embryonic endoderm, while some come to lie between the epiblast and the newly created endoderm to form the mesoderm. The ectoderm is formed from cells remaining in the epiblast. [E]-The epiblast through the process of gastrulation is the source of all the germ; layers of the embryo. (i.e. the ectoderm, mesoderm and endoderm)

The Trilaminar Germ Disc Layer (Development) Cells invaginating in the primitive pit move straight forward until they reach the prochordal plate. The above cells form a tube like process known as the notochordal or head process. With further development and differentiation, the lumen of the notochordal process disappears and a solid cord the notochord is formed. The notochord forms a midline axis, which will serve as the basis of the axial skeleton. By the end of the third week, the three germ layers consisting of the ectoderm, the mesoderm and the endoderm are laid down and further tissue and organ differentiation could commence.

The Embryonic Period (Derivatives of The Ectodermal Germ Layer). In general terms it may be stated that the ectodermal germ layer gives rise to those organs and structures that maintain contact with the outside world. The Central Nervous System is ectodermal in origin. The Peripheral Nervous System is ectodermal in origin. The Sensory Epithelium of the ear, nose, and eye are ectodermal in origin. The epidermis including the hair and nails, in addition to the subcutaneous glands, the mammary glands, the pituitary gland and the enamel of the teeth are of ectodermal origin.

The Embryonic Period (Derivatives of the Mesodermal Layer). By about the 17 th day, the mesodermal cells close to the midline proliferate and form a thickened plate of tissue, known as the paraxial mesoderm The more laterally disposed mesodermal layer remains thin and is referred to as the lateral plate mesoderm. With the appearance and coalescence of intercellular cavities in the lateral plate, this tissue is divided into two layers. The layer of the lateral plate mesoderm which is continuous with the mesoderm covering the amnion is known as the somatic or parietal mesoderm. The layer of the lateral plate mesoderm which is continuous with the mesoderm covering the yolk sac is the splanchnic or visceral mesoderm.

The Embryonic Period (Derivatives of the Mesodermal Layer). The tissue connecting the paraxial mesoderm and the lateral plate mesoderm is known as the intermediate mesoderm. By the beginning of the 3rd week paraxial mesoderm becomes organized into segments known as somitomeres. Somitomeres first appear in the cephalic region of the embryo and their formation proceeds in a cephalocaudal direction. Each Somitomere consists of mesodermal cells arranged in concentric whorls around the centre of the unit. In the head region, such structures form in association with segmentation of the neural plate into neuromeres and contribute to the majority of the head mesenchyme

The Embryonic Period (Derivatives of the Mesodermal Layer). From the occipital region caudally, somitomeres becomes further organized into somites. The First pair of somites arises in the cervical region of the embryo at approximately the 20th day of development. From the 20th day of development, new somites appear in craniocaudal sequence approximately three per day, until at the end of the 5th week when about 42 to 44 pairs would have been formed. There are 4 occipital, 8 cervical, 12 thoracic, 5 Lumbar, 5 Sacral and 8 to 10 Coccygeal pairs. The first occipital and the last 5 to 7 coccygeal somites later disappear, while the remainder forms the axial skeleton.

The Embryonic Period (Derivatives of the Mesodermal Layer). During this period of development the age of the embryo is expressed in the number of somites. By the beginning of the 4th week, cells forming the ventral and medial walls of the somite lose their compact organization, become polymorphic and shift their position to surround the notochord forming the sclerotome. The mesenchyme a loosely woven tissue is formed by the Sclerotomes. Dermomyotomes are derivatives of the remaining part of the dorsal somite wall after the Sclerotomes and mesenchymes have been formed. The myotomes are derivates of the dermomyotomes which are characterized by pale nuclei and darkly stained nucleoli which provides the musculature for its own segment.

The Embryonic Period (Derivatives of the Mesodermal Layer). After the cells of the dermomyotome have formed the myotome, they lose their epithelial characteristics and spread out under the overlying ectoderm where they form the dermis and the subcutaneous tissue of the skin Each somite forms its own scleretome (the cartilage and bone component). Each somite forms its own myotome (proving the segmental muscle component) Each somite forms its own dermatome (the segmental skin component) Each myotome and dermatome has its own segmental nerve component

The Embryonic Period (Derivatives of the Mesodermal Layer. The Following tissues and organs are considered to be part of mesodermal origin. The supporting tissues such as connective tissue, cartilage and bone. Striated and Smooth musculature. Blood and lymph cells and the walls of the heart, blood, and lymph vessels. The kidneys, gonads and their corresponding ducts. The Cortical portions of the adrenal gland and the spleen.

Derivatives of the Endodermal Germ Layer. The gastrointestinal tract is the main organ derived from the endodermal germ layer. The formation of the gastrointestinal tract is greatly dependent on cephalocaudal and lateral foldings of the embryo. The cephalocaudal folding is caused mainly by the rapid, longitudinal growth of the central nervous system. The transverse or lateral folding is produced by the formation of the rapidly growing somites. The formation of the tube-like gut is a passive event and consists of the inversion and incorporation of part of the endoderm-lined york sac into the body cavity.

Derivatives of the Endodermal Germ Layer. At first the endodermal germ layer has the shape of a flat disc, forming the roof of the yolk sac and closely apposed to the ectoderm. With the development and the growth of the brain vesicles, however, the embryonic disc begins to bulge into the amniotic cavity and to fold in a cephalocaudal direction. The cephalocaudal folding of the embryonic disc is most pronounced in the regions of the head and tail, where the so called head-fold and tail fold are formed. As a result of the cephalocaudal folding, a continuously larger portion of the endoderm-lined cavity is incorporated into the body of the embryo proper. In the anterior part, the endoderm forms the foregut, in the tail region the embryo forms the hindgut.

Derivatives of the Endodermal Germ Layer. The midgut remains temporarily in open connection with the yolk sac by way of a broad stalk, the omphalomesenteric or vitelline duct. The omphalomesenteric or vitelline duct is initially wide, but with further growth of the embryo it becomes narrow and much longer. At its cephalic end the foregut is temporarily bounded by the prochordal plate, an ectodermal-endodermal membrane, which is now called the buccopharyngeal membrane. At the end of the third week, the buccopharyngeal membrane ruptures, thus establishing an open-connection between the amniotic cavity and the primitive gut. The hindgut also terminates temporarily at a membrane known as the cloacal membrane.

Derivatives of the Endodermal Germ Layer. As a result of the rapid growth of the somites the initial flat embryonic disc begins to fold in lateral direction and the embryo obtains a round appearance. Contemporaneously as the above process is taking place, the ventral body wall of the embryo is established, with the exception of a small part in the ventral abdominal region where the yolk sac stalk is attached. While the foregut and hind gut are established mainly as a result of the formation of the head fold and tail fold, respectively, the midgut remains in communication with the yolk sac. Initially, this connection is `wide, but as a result of the lateral folding it gradually becomes long and narrow, the vitelline duct. Only much later, when the vitelline duct is obliterated, does the midgut lose its connection with the original endoderm lined cavity and obtain its free position in the abdominal cavity. Another important result of the cephalocaudal and lateral folding is the partial incorporation of the allantois into the body of the embryo, where it forms the cloaca.The distal portion of the allantois remains in the connecting stalk.

Derivatives of the Endodermal Germ Layer. By the fifth week, the yolk sac stalk and the connecting stalk fuse to form the umbilical cord. In humans, the yolk sac is vestigial and in all probability has a nutritive role only in the early stages of development. In the second month of development the yolk sac is found in the chorionic cavity. The endodermal germ layer initially forms the epithelial lining of the primitive gut. The endodermal germ layer initially forms the epithelial lining the intraembryonic portions of the allantois and the vitelline duct.

Derivatives of the Endodermal Germ Layer. (During further development the endodermal germ layer ) Derivatives of the Endodermal Germ Layer. (During further development the endodermal germ layer gives rise to the: The epithelial lining of the respiratory tract. The parenchyma of the thyroid, parathyroids, liver, and pancreas. The reticular stroma of the tonsils and thymus. The epithelial lining of the urinary bladder and urethra. The epithelial lining of the tympanic cavity and Eustachian tube.

The Embryonic Period from The (Third to The Eight Week) All major organs and organ systems are formed during the period from the 4th to the 8th week. This period is therefore also called the period of organogenesis. It is the time that the embryo is therefore most susceptible to factors interfering with development. Most congenital malformations seen at birth find their origin during this critical period. Being familiar with the main events of organogenesis will be of great help in identifying the time that a particular abnormality arose.

The Embryological Aspects of the Skeletal System. The skeletal system develops from the paraxial and lateral plate (somatic layer) mesoderm and from the migratory neural crests cells. The Paraxial mesoderm forms a segmented series of tissue blocks on each side known as somitomeres in the head region and somites from the occipital region caudally. Somites differentiate into a ventromedial part, the scleretome, and a dorsal lateral part, the dermomyotome. At the end of the 4th week, the scleretome cells become polymorphous and form a loosely woven tissue known as the mesenchyme or embryonic connective tissue. It is characteristic for the mesenchymal cells to migrate and to differentiate (especially along with the ectodermal neural crest cells) in many different ways, becoming fibroblasts,chondroblasts or osteoblasts(the bone forming cells)

The Embryological Aspects of the Skeletal System. The bone forming capacity of the mesenchyme is not restricted to the cells of the scleretome, but occurs also in the somatic mesoderm layer of the body wall, which contributes mesoderm cells for the formation of the pelvic and shoulder girdles and the long bones of the limbs. It has been demonstrated that the neural crest cells in the head region differentiate into mesenchyme and participate in the formation of bones of the face. It has been proven that occipital somites `and somitomeres form most of the cranial vault and base of the skull. In some bones such as the flat bones of the skull, the mesenchyme differentiates directly into bone in a process known as membranous ossification. In most bones however, the mesenchymal cells first give rise to hyaline cartilage models, which in turn become ossified by intramembranous ossification

The Embryological Aspects of the Skeletal System (The Skull The Embryological Aspects of the Skeletal System (The Skull) The skull could be divided into two parts. The neurocranium which is divided into two parts is derived from occipital somites and somitomeres and forms a protective case around the brain. The viscerocranium is derived from neural crest and forms the skeleton of the face. The membranous part of the neurocranium consists of flat bones surrounds the brain as a vault. The cartilagenous part the neurocranium or chondrocranium forms the bones of the base of the skull.

The Embryological Aspects of the Skeletal System (The Skull ) The sides and roof of the skull develop from mesenchyme investing the brain and undergo membranous ossification. During membranous ossification of the skull bones, membranous bones are formed which are characterized by the presence of needle-like bone spicules. The bone spicules progressively radiate from the primary ossification centres toward the periphery. With further growth during fetal life and post natal life, the membranous bones enlarge by apposition of new layers on the outer surface. With further growth during fetal life and post natal life, the membranous bones enlarge by simultaneous osteoclastic resoprtion from the inside.

The development of the diaphragm. The elements contributing to the diaphragm : The development of the diaphragm. The elements contributing to the diaphragm include: The septum transversum, The dorsal mesentery of the oesophagus, The body wall mesodermal myoblasts and The pleuroperitoneal membrane.

The development of the diaphragm. The diaphragm receives its entire motor supply from the phrenic nerve (C3, 4, 5) whose long course from the neck follows the embryological migration of the muscle of the diaphragm from the cervical region to definitive position at the anterior part of the diaphragm. During this migration, the cervical myotomes and nerves contribute muscle and nerve supply respectively, thus accounting for the long course of the phrenic nerve (C3, 4 and 5) from the neck to the diaphragm. However, with such a complex embryological story, one may be surprised to know that congenital abnormalities of the diaphragm are unusual. However, a number of defects may occur, giving rise to a variety of congenital herniae through the diaphragm. These may be one through the foramen of Morgagni; anteriorly between the xiphoid and costal origins; And another through the foramen of Bochdalekthe pleuroperitoneal canallying posteriorly; Through a deficiency of the whole central tendon (occasionally such a Hernia may be traumatic in origin); There may also be congenital hernias through a congenitally large oesophageal hiatus.

Development Of The Diaphragm Far more common are the acquired hiatus herniae (subdivided into Sliding and rolling herniae). These are found in patients usually of middle age where weakening and widening of the oesophageal hiatus has occurred. In the sliding hernia the upper stomach and lower oesophagus slide upwards into the chest through the lax hiatus when the patient lies down or bends over; the competence of the cardia is often disturbed and peptic juice can therefore regurgitate into the gullet in lying down or bending over. This may be followed by oesophagitis with consequent heartburn, bleeding and, eventually, stricture formation. In the rolling hernia (which is far less common) the cardia remains in its normal position and the cardio-oesophageal junction is intact, but the fundus of the stomach rolls up through the hiatus in front of the oesophagus, hence the alternative term of para-oesophageal hernia. In such a case there may be epigastric discomfort, flatulence and even dysphagia, but no regurgitation because the cardiac mechanism is undisturbed.

The Embryology and The Histological Structure OF The Trachea. The patency of the trachea is maintained by a series of 15–20 U-shaped cartilages. Posteriorly, where the cartilage is deficient, the trachea is flattened and its wall completed by fibrous tissue and a sheet of smooth muscle (the trachealis). Within, it is lined by a ciliated columnar epithelium with many goblet cells. Embryologically the trachea is a derivative of the foregut. Recognized congenital abnormalities of the trachea include atresia, stenosis and fistulation with the oesophagus.

The embryological development of the heart (Essense and Process) The primitive heart is a single tube which soon shows grooves demarcating the sinus venosus, atrium, ventricle and bulbus cordis from behind forwards. As the heart tube enlarges it kinks so that its caudal end, receiving venous blood, comes to lie behind its cephalic end with its emerging arteries. The sinus venosus later absorbs into the atrium and the bulbus becomes incorporated into the ventricle so that, in the fully developed heart, the atria and great veins come to lie posterior to the ventricles and the roots of the great arteries. The boundary tissue between the primitive single atrial cavity and single ventricle grows out as a dorsal and a ventral endocardial cushion which meet in the midline, thus dividing the common atrio-ventricular orifice into a right (tricuspid) and left (mitral) orifice. The division of the primitive atrium into two is a complicated process but an important one in the understanding of congenital septal defects

The embryological development of the heart (Essense and Process) A partition, the septum primum, grows downwards from the posterior and superior walls of the primitive common atrium to fuse with endocardial cushions. Before fusion is complete, however, a hole appears in the upper part of this septum which is termed the foramen secundum in the septum primum. A second membrane, the septum secundum, then develops to the right of the primum but this is never complete. The septum primum and septum secundum which form the interatrial septum, leaving the foramen ovale as a valve-like opening passing between them. The two overlapping defects in the septa form the valve-like foramen ovale which shunts blood from the right to left heart in the fetus. After birth, this foramen ovale usually becomes completely fused leaving only the fossa ovalis on the septal wall of the right atrium as its memorial.

The embryological development of the heart (Essense and Process) In about 10% of adult subjects, however, a probe can still be insinuated through an anatomically patent, although functionally sealed foramen. Division of the ventricle is commenced by the upgrowth of a fleshy septum from the apex of the heart towards the endocardial cushions. However, this stops short of dividing the ventricle completely and thus it has an upper free border, forming a temporary interventricular foramen. Contemperoneously, the single truncus arteriosus is divided into aorta and pulmonary trunk by a spiral septum (hence the spiral relations of these two vessels), which grows downwards to the ventricle and fuses accurately with the upper free border of the ventricular septum. The spiral septum contributes the small pars membranacea septi, which completes the separation of the ventricle in such a way that blood on the left of the septum flows into the aorta and on the right into the pulmonary trunk. The primitive sinus venosus absorbs into the right atrium so that the venae cavae draining into the sinus come to open separately into this atrium. The smooth-walled part of the adult atrium represents the contribution of the sinus venosus; the pectinate part represents the portion derived from the primitive atrium.

The embryological development of the heart (Essense and Process) When the truncus arteriosus splits longitudinally to form the ascending aorta and pulmonary trunk, the 6th arch, unlike the others, remains linked with the latter and forms the right and left pulmonary arteries. On the left side the 6th arch retains its connection with the dorsal aorta to form the ductus arteriosus (the ligamentum arteriosum of adult anatomy). The asymmetrical development of the aortic arches accounts for the different course taken by the recurrent laryngeal nerve on each side. In the early fetus the vagus nerve lies lateral to the primitive pharynx, separated from it by the aortic arches. What are to become the recurrent laryngeal nerves pass medially, caudal to the aortic arches, to supply the developing larynx. With elongation of the neck and caudal migration of the heart, the recurrent nerves are caught up and dragged down by the descending aortic arches. On the right side the 5th and distal part of the 6th arch absorb, leaving the nerve to hook round the 4th arch (i.e. the right subclavian artery). On the left side, the nerve remains looped around the persisting distal part the 6th arch (the ligamentum arteriosum) which is overlapped and dwarfed by the arch of the aorta.

The fetal circulation (Essense and Transition at Birth) The circulation of the blood in the embryo is a remarkable example of economy in nature and results in the shunting of well-oxygenated blood from the placenta to the brain and the heart, leaving relatively desaturated blood for less essential structures. Blood is returned from the placenta by the umbilical vein to the inferior vena cava and thence the right atrium, most of it by-passing the liver in the ductus venosus. Relatively little mixing of oxygenated and deoxygenated blood occurs in the right atrium since the valve overlying the orifice of the inferior vena cava serves to direct the flow of oxygenated blood from that vessel through the foramen ovale into the left atrium, while the deoxygenated stream from the superior vena cava is directed through the tricuspid valve into the right ventricle. From the left atrium the oxygenated blood (together with a small amount of deoxygenated blood from the lungs) passes into the left ventricle and hence into the ascending aorta for the supply of the brain and heart via the vertebral, carotid and coronary arteries. As the lungs of the fetus are inactive, most of the deoxygenated blood from the right ventricle is short-circuited by way of the ductus arteriosus from the pulmonary trunk into the descending aorta. This blood supplies the abdominal viscera and the lower limbs and is shunted to the placenta, for oxygenation, along the umbilical arteries arising from the internal iliac arteries.

The fetal circulation (Essense and Transition at Birth)-Applied Anatomy. At birth, the septum primum and septum secundum are forced together, closing the flap valve of the foramen ovale. Fusion usually takes place at about 3 months after birth. In about 10% of subjects, this fusion may be incomplete. However, the two septa overlap and this patency of the foramen ovale is of no functional significance. If the septum secundum is too short to cover the foramen secundum in the septum primum, an atrial septal defect persists after the septum primum and septum secundum are pressed together at birth. This results in an ostium secundum defect, which allows shunting of blood from the left to the right atrium. This defect lies high up in the atrial wall and is relatively easy to close surgically. An atrial septal defect results if the septum primum fails to fuse with the endocardial cushions. This ostium primum defect lies immediately above the atrioventricular boundary and may be associated with a defect of the pars membranacea septi of the ventricular septum. In such a case, the child is born with both an atrial and ventricular septal defect. Occasionally the ventricular septal defect is so huge that the ventricles form a single cavity, giving a trilocular heart. Congenital pulmonary stenosis may affect the trunk of the pulmonary artery, its valve or the infundibulum of the right ventricle. If stenosis occurs in the conjunction with a septal defect, the compensatory hypertrophy of the right ventricle (developed to force blood through the pulmonary obstruction) develops a sufficiently high pressure to shunt blood through the defect into the left heart; this mixing of the deoxygenated right heart blood with the oxygenated left- sided blood results in the child being cyanosed at birth.

The fetal circulation (Essense and Transition at Birth)-Applied Anatomy. The commonest combination of congenital abnormalities causing cyanosis is Fallots tetralogy. This results from unequal division of the truncus arteriosus by the spinal septum, resulting in a stenosed pulmonary trunk and a wide aorta which overrides the orifices of both the ventricles. The displaced septum is unable to close the interventricular septum, which results in a ventricular septal defect. Right ventricular hypertrophy develops as a consequence of the pulmonary stenosis. Cyanosis results from the shunting of large amounts of unsaturated blood from the right ventricle through the ventricular septal defect into the left ventricle and also directly into the aorta.

The fetal circulation (Essense and Transition at Birth)and Applied Anatomy. At birth, expansion of the lungs leads to an increased blood flow in the pulmonary arteries; the resulting pressure changes in the two atria bring the overlapping septum primum and septum secundum into apposition which effectively closes off the foramen ovale. At the same time active contraction of the muscular wall of the ductus arteriosus results in a functional closure of this arterial shunt and, in the course of the next 2–3 months, its complete obliteration. Similarly, ligature of the umbilical cord is followed by thrombosis and obliteration of the umbilical vessels. The complex development of the heart and major arteries accounts for the multitude of congenital abnormalities which may affect these structures, either alone or in combination. Dextro-rotation of the heart means that this organ and its emerging vessels lie as a mirror-image to the normal anatomy. It may be associated with reversal of all the intra- abdominal organs A correct diagnosis of an acute appendicitis as the cause of a patients severe left iliac fossa pain has been achieved, because the apex beat of the heart was heard on the right side.

The fetal circulation (Essense and Transition at Birth)-Applied Anatomy. A persistent ductus arteriosus is a relatively common congenital defect. If left uncorrected, it causes progressive work hypertrophy of the left heart and pulmonary hypertension. Aortic coarctation is thought to be due to an abnormality of the obliterative process which normally occludes the ductus arteriosus. There may be an extensive obstruction of the aorta from the left subclavian artery to the ductus, which is widely patent and maintains the circulation to the The persistent ductus arteriosus has a close relationship to the left recurrent laryngeal nerve. With coarctation of the aorta; more often than not there are multiple other defects and frequently infants so afflicted die at an early age. In some cases of co-coarctation of the aorta, circulation to the lower limbs is maintained via collateral arteries around the scapula anastomosing with the intercostal arteries, and via the link-up between the internal thoracic and inferior epigastric arteries.

The fetal circulation (Essense and Transition at Birth)-Applied Anatomy The fetal circulation (Essense and Transition at Birth)-Applied Anatomy. Clinically, this circulation may be manifest by enlarged vessels being palpable around the scapular margins; radiologically, dilatation of the engorged intercostal arteries results in notching of the inferior borders of the ribs. Abnormal development of the primitive aortic arches may result in the aortic arch being on the right or actually being double. An abnormal right subclavian artery may arise from the dorsal aorta and pass behind the oesophagusa rare cause of difficulty in swallowing (dysphagia lusoria). Rarely, the division of the truncus into aorta and pulmonary artery is incomplete, leaving an aorta–pulmonary window, the most unusual congenital fistula between the two sides of the heart. Persistent ductus arteriosus showing its close relationship to the left recurrent laryngeal nerve. (b) Coarctation of the aorta.

Development of the oesophagus. The oesophagus develops from the distal part of the primitive fore-gut. From the floor of the fore-gut also differentiate the larynx and trachea, Initially as a groove (the laryngotracheal groove) which then converts into a tube, a bud on each side of which develops and ramifies into the lung. The close relationship between the origins of the oesophagus and trachea accounts for the relatively common malformation in which the upper part of the oesophagus ends blindly while the lower part opens into the lower trachea at the level of T4 (oesophageal atresia with tracheoesophageal fistula). Less commonly, the upper part of the oesophagus opens into the trachea, or oesophageal atresia occurs without concomitant fistula into the trachea. Rarely, there is a tracheo-oesophageal fistula without atresia. Oesophageal-tracheal fistula could present with feeding difficulties at birth.

The development of the gastro-intestinal system. The primitive endodermal tube of the gut is divided into: the fore-gut (supplied by the coeliac axis) extending as far as the entry of the bile duct into the duodenum; The mid-gut (supplied by the superior mesenteric artery) continuing as far as the distal transverse colon; The hind-gut (supplied by the inferior mesenteric artery) extending thence to the ectodermal part of the anal canal. In stages in rotation of the bowel; the herniation of the mid-gut loop takes place at six weeks, and it returns to the abdomen before twelve weeks during return the caecum descends to its definitive position and there is completion of stomach-rotation with the formation of the lesser sac (omental bursa). At an early stage rapid proliferation of the gut wall obliterates its lumen and this is followed by subsequent recanalization.

The development of the gastro-intestinal system. In the development of the gastro-intestinal system. The fore-gut becomes rotated with the development of the lesser sac so that the original right wall of the stomach comes to form its posterior surface and the left wall its anterior surface. The vagi rotate with the stomach and therefore lie anteriorly and posteriorly to it at the oesophageal hiatus. The rotation swings the duodenum to the right and the mesentery of this organ then blends with the peritoneum of the posterior abdominal wall and this blending process is termed zygosis. The mid-gut enlarges rapidly in the 5-week fetus, becomes too large to be contained within the abdomen and herniates into the umbilical cord. The apex of this herniated bowel is continuous with the vitello-intestinal duct and the yolk sac, but this connection, even at this early stage of fetal life, is already reduced to a fibrous strand.

In the development of the gastro-intestinal system. In the development of the gastro-intestinal system. The axis of this herniated loop of gut is formed by the superior mesenteric artery, which demarcates a cephalic and a caudal limb. The cephalic element develops into the proximal small intestine; the caudal segment differentiates into the terminal 2 feet (62 cm) of ileum, the caecum and the colon as far as the junction of the middle and left thirds of the transverse colon. A bud which develops on the caudal segment indicates the site of subsequent formation of the caecum; it may well be that this bud delays the return of the caudal limb in favour of the cephalic gut during the subsequent reduction of the herniated bowel. At 10 weeks this return of the bowel into the abdominal cavity commences. The mid-gut loop first rotates anti-clockwise through 90° so that the cephalic limb now lies to the right and the caudal limb to the left.

The development of the gastro-intestinal system. In the development of the gastro-intestinal system. The cephalic limb of the gastrointestinal tract returns first, passing upwards and to the left into the space left available by the bulky liver. During its return, mid-gut passes behind the superior mesenteric artery (which thus comes to cross the third part of the duodenum) and also pushes the hind- gutthe definitive distal colonover to the left. When the caudal limb of the gastrointestinal tract returns returns, it lies in the only space remaining to it, superficial to, and above, the small intestine with the caecum lying immediately below the liver. During the return of the primitive bowel loops, the caecum then descends into its definitive position in the right iliac fossa, dragging the colon with it. The transverse colon will come to lie in front of the superior mesenteric vessels and the small intestine during the return.

The development of the gastro-intestinal system. In the development of the gastro-intestinal system. Finally, the mesenteries of the ascending and descending parts of the colon blend with the posterior abdominal wall peritoneum by zygosis. The embryological fusion of peritoneal surfaces is of major surgical importance. In mobilising the right or left colon, an incision made along this avascular line of zygosis lateral to the bowel, allows it to be mobilized with its mesocolon and blood supply because of embryological peritoneal fusion. Because of peritoneal The duodenum, head of pancreas and termination of the common bile duct can be mobilized bloodlessly by incising the peritoneum along the right border of the duodenum because of embryological peritoneal fusion. Numerous anomalies may occur in the highly complex developmental process of the gastrointestinal system.

The development of the gastro-intestinal system. In the development of the gastro-intestinal system. -Atresia or stenosis of the bowel may result from failure of recanalization of the lumen or due damage to the blood supply to the bowel within the fetal umbilical hernia with consequent ischaemic changes. Meckels diverticulum represents the remains of the embryonic vitellointestinal duct (communication between the primitive mid-gut and yolksac) and is, therefore, always on the anti-mesenteric border of the bowel. As an approximation to the truth it can be said to occur in 2% of subjects, twice as often in males as females, to be situated at 2 feet (62 cm) from the ileocaecal junction and to be 2 in (5 cm) long. In fact, it may occur anywhere from 6 in (15 cm) to 12 feet (3.5 m) from the terminal ileum and vary from a tiny stump to a 6 in (15 cm) long sac. Occasionally the diverticulum ends in a whip-like solid strand. As well as a diverticulum, the commonest form, this duct may persist as a fistula or band connecting the intestine to the umbilicus, as a cyst hanging from the anti-mesenteric border of the ileum or as a raspberry tumour at the umbilicus, formed by the red mucosa of a persistent umbilical extremity of the diverticulum pouting at the navel. The mucosa lining the diverticulum may contain islands of peptic epithelium with oxyntic (acid-secreting) cells. Peptic ulceration of adjacent intestinal epithelium may then occur with haemorrhage or perforation. In the development of the gastro-intestinal system. The caecum may fail to descend; the peritoneal fold which normally seals it in the right iliac fossa passes, instead, across the duodenum and causes a neonatal intestinal obstruction. The mesentery of the small intestine -to umbilicus in such a case above is left as a narrow pedicle, which allows volvulus of the whole small intestine to occur (volvulus neonatorum). Occasionally, reversed rotation occurs, in which the transverse colon comes to lie behind the superior mesenteric vessels with the duodenum in front of them; this may again be accompanied by extrinsic duodenal obstruction due to a peritoneal fold. Exomphalos is persistence of the mid-gut herniation at the umbilicus after birth. Exomphalus could be classified into Major and Minor on the basis of the size.

The gastrointestinal adnexae, liver, gall-bladder and its ducts, pancreas and spleen. The ligamentum teres is the obliterated remains of the left umbilical vein which, in utero, brings blood from the placenta back into the fetus. The ligamentum venosum is the fibrous remnant of the fetal ductus venosus which shunts oxygenated blood from this left umbilical vein to the inferior vena cava, short-circuiting the liver. The grooves for the ligamentum teres, ligamentum venosum and inferior vena cava, representing as they do the pathway of a fetal venous trunk, are continuous in the adult. Lying in the porta hepatis (which is 2 in (5 cm) long) are: the common hepatic ductanteriorly; the hepatic arteryin the middle; the portal veinposteriorly. Embryologically the Spleen is related to the gastrointestinal system through its blood supply.

Development ofThe Pancreas The pancreas develops from a larger dorsal diverticulum from the duodenum and a smaller ventral outpouching from the side of the common bile duct. The ventral pouch swings round posteriorly to fuse with the lower aspect of the dorsal diverticulum, trapping the superior mesenteric vessels between the two parts. The ducts of the two formative segments of the pancreas communicates The accessory pancreatic duct takes over the main pancreatic flow to form the main duct, leaving the original duct of the larger portion of the gland as the accessory duct. The Ducts of Santorini and Wirsurng are eponymous names for pancreatic ducts.

The embryology and congenital abnormalities of the kidney and ureter The kidney and ureter are mesodermal in origin and develop in an unusual manner of considerable interest to the comparative anatomist. The pronephros, of importance in the lower vertebrates, is transient in humans, but the distal part of its duct receives the tubules of the next renal organ to develop, the mesonephros, and now becomes the mesonephric or Wolffian duct. The mesonephros itself then disappears except for some of its ducts which form the efferent tubules of the testis. A diverticulum then appears at the lower end of the mesonephric duct which develops into the metanephric duct; on top of the latter a cap of tissue differentiates to form the definitive kidney or metanephros. The metanephric duct develops into the ureter, pelvis, calyces and collecting tubules, the metanephros into the glomeruli and the proximal part of the renal duct system

The embryology and congenital abnormalities of the kidney and ureter The mesonephric duct now loses its renal connection, atrophies in the female (remaining only as the epoöphoron) but persists in the male, to become the epididymis and vas deferens. The kidney first develops in the pelvis and then migrates upwards. Its blood supply is first obtained from the common iliac artery but, during migration, a series of vessels form to supply it, only to involute again when the renal artery takes over this duty. It is common for one or more distally placed arteries to persist (aberrant renal arteries) and one may even run to the kidney from the common iliac artery. Occasionally the kidney will fail to migrate cranially, resulting in a persistent pelvic kidney. The two metanephric masses may fuse in development, forming a horseshoe kidney linked across the midline which may have ascent difficulties and remains in the pelvic cavity.

Embryology of the Fallopian tubes, uterus and vagina. The paramesonephric (or Mullerian) ducts develop, one on each side, adjacent to the mesonephric (Wolffian) ducts in the posterior abdominal wallthey are mesodermal in origin. All the above four tubes lie close together caudally, projecting into the anterior (urogenital) compartment of the cloaca. One of the above systems disappears in the male, the other in the female, each leaving behind congenital remnants of some interest to the clinician. In the male, the paramesonephric duct disappears, apart from the appendix testis and the prostatic utricle. In the female, the mesonephric system (which in the male develops into the vas deferens and epididymal ducts) persist as remnants in the broad ligament termed the epöophoron,paröophoron and ducts of Gärtner.

Embryology of the Fallopian tubes, uterus and vagina. The paramesonephric ducts in the female form the Fallopian tubes cranially. More caudally, the paramesonephric ducts come together and fuse in the midline (dragging, as they do so, a peritoneal fold from the side wall of the pelvis which becomes the broad ligament). The median structure so formed, by the fusion of the paramesonephric ducts differentiates into the epithelium of the uterine body (endometrium), cervical canal and upper one-third of the vagina, which are first solid and later become canalized. The rest of the vaginal epithelium develops by canalization of the solid sinuvaginal node at the back of the urogenital sinus. The different embryological origin of the vagina accounts for the differences in lymphatic drainage of the upper and lower vagina

The embryology and congenital abnormalities of the kidney and ureter In 1 in 2400 births there is complete failure of development of one kidney (congenital absence of the kidney). Congenital polycystic kidneys (which are nearly always bilateral) are believed to result from failure of metanephric tissue to link up with some of the metanephric duct collecting tubules; blind ducts therefore form which subsequently become distended with fluid, although this theory of origin does not explain their occasional association with multiple cysts of the liver, pancreas, lung and ovary. The mesonephric duct may give off a double metanephric bud so that two ureters may develop on one or both sides. These ureters may fuse into a single duct anywhere along their course or open separately into the bladder There may be an aberrant renal artery Congenital Polycystic kidneys could be autosomal dominant or autosomal recessive.