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2/2/2011ZOO 401- Embryology-Dr. Salah A. MartinDEVELOPMENT OF THE DIGESTIVESYSTEM Development of Endodermal OrgansWednesday, February 02, 2011Development of Ectodermal Organs1Wednesday, February 02, 2011ZOO 401- Embryology-Dr. Salah A. Martin Primitive Gut Tube2Proctodeum and StomodeumThe primitive gut tube is derived from the dorsal partof the yolk sac, which is incorporated into the body ofthe embryo during folding of the embryo during thefourth week.The primitive gut tube is divided into three sections.The epithelium of and the parenchyma ofglands associated with the digestive tract (e.g., liver andpancreas) are derived from endoderm.The muscular walls of the digestive tract (laminapropria, muscularis mucosae, submucosa, muscularisexterna, adventitia and/or serosa) are derivedfrom splanchnic mesoderm.During the solid stage of development the endodermof the gut tube proliferates until the gut is a solid tube.A process of recanalization restores the lumen.Wednesday, February 02, 2011DEVELOPMENT OF THE DIGESTIVE SYSTEMZOO 401- Embryology-Dr. Salah A. MartinPrimitive Gut Tube Primitive Gut TubeProctodeum and StomodeumStomachDuodenumPancreasLiver and Biliary ApparatusSpleenMidgut3The proctodeum (anal pit) is the primordialanus, and the stomodeum is the primordialmouth. In both of these areas ectoderm is in directcontact with endoderm without interveningmesoderm, eventually leading to degenerationof both tissue layers. Foregut, Esophagus. The tracheoesophageal septum divides theforegut into the esophagus andtrachea. information. Wednesday, February 02, 2011Proctodeum and Stomodeum4ZOO 401- Embryology-Dr. Salah A. MartinDuodenumStomach The primordium of the primitive stomach isvisible about the end of the fourth week.It is initially oriented in the median plane andsuspended from the dorsal wall of the abdominalcavity by the dorsalmesentery or mesogastrium.During development the stomach rotates 90degrees in a clockwise direction along itslongitudinal axis, placing theleft vagus nerve alongits anterior side and the right vagus nerve alongits posterior side.Rotation of the stomach creates the omentalbursa or lesser peritoneal sac.Wednesday, February 02, 2011Stomach5 The duodenum acquiresits C-shaped loop as thestomach rotates.Because of its location atthe junction of theforegut and the midgut,branches of boththe celiac trunkandthe superiormesentericartery supply theduodenum.Wednesday, February 02, 2011Duodenum61

2/2/2011ZOO 401- Embryology-Dr. Salah A. MartinZOO 401- Embryology-Dr. Salah A. MartinPancreas Liver and Biliary ApparatusThe pancreas develops from two outgrowths of theendodermal epithelium, the dorsal pancreaticbud and the ventral pancreatic bud.During rotation of the gut these primordial cometogether to form a single pancreas.The ventral pancreatic bud forms the uncinateprocess and part of the head, while the dorsalpancreatic bud forms the remainder of the head,body, and tail of the pancreas.The ducts of the pancreatic buds join together toform the main pancreatic duct, but the proximalpart of the duct of the dorsal pancreatic bud maypersist as an accessory pancreatic duct.Wednesday, February 02, 2011Pancreas 7The liver develops from endodermal cells that form the hepaticdiverticulum.The liver grows in close association with the septumtransversum, which later forms part of the diaphragm.As it grows the hepatic diverticulum divides into a cranial part,which forms the parenchyma of the liver, and the caudalpart, which gives rise to thegallbladder and cystic duct.The hemopoietic cells, Kupffer cells, and connectivetissue of the liver are derived from mesenchyme in theseptum transversum.The embryonic liver is large and fills much of the abdominalcavity during the seventh through ninth weeks of development.Blood formation (hemopoiesis) begins in the liver during thesixth week of development, and bile formation begins in thetwelfth week.Wednesday, February 02, 2011Liver and Biliary Apparatus8ZOO 401- Embryology-Dr. Salah A. MartinSpleen MidgutThe midgut communicates with the yolk sac viathe yolk stalk. As the midgut forms, it elongates into a Ushaped loop (midgut loop) that temporarilyprojects into the umbilical cord (physiologicalumbilical herniation). The cranial limb of the midgut elongatesrapidly during development and formsthe jejunum and cranial portion of theileum. The caudal limb forms the cecum, appendix, The spleendevelopsfrommesenchymal cellslocatedbetweenlayers ofthe dorsalmesogastrium.Wednesday, February 02, 2011Spleen9Wednesday, February 02, 2011ZOO 401- Embryology-Dr. Salah A. MartinLower Respiratory System Lower Respiratory System Larynx Trachea Bronchi RESPIRATORY SYSTEM10ZOO 401- Embryology-Dr. Salah A. MartinRESPIRATORY SYSTEMWednesday, February 02, 2011Midgut11The primordium of the lower respiratory systemdevelops in about the fourth week.The laryngotracheal diverticulum arises fromendoderm on the ventral wall of the foregut.Tracheoesophageal folds develop on either side andjoin to form a tracheoesophageal septum thatseparates it from the rest of the foregut.This divides the foregut intothe laryngotracheal tube (ventral) andthe esophagus (dorsal).The caudal end of the laryngotracheal diverticulumenlarges to form the lung bud, which is surroundedby splanchnic mesenchyme.Wednesday, February 02, 2011Lower Respiratory System122

2/2/2011ZOO 401- Embryology-Dr. Salah A. MartinZOO 401- Embryology-Dr. Salah A. MartinLarynxTracheaThe opening of the laryngotracheal tubebecomes the inlet of the larynx. The laryngeal cartilages are derivedfrom the fourth and sixth pharyngealarches. Wednesday, February 02, 2011Larynx13The epithelium and glands of thetrachea develop from the endoderm of thelaryngotracheal tube. The cartilage, connective tissue, andsmooth muscle are derived from thesurrounding splanchnic mesenchyme. Wednesday, February 02, 2011ZOO 401- Embryology-Dr. Salah A. Martin At the end of the fourth week the lung bud divides intotwo bronchial buds, which enlarge to form the primarybronchi.The right bronchus is larger and moreverticallyoriented than the left one, and this relationship persiststhroughout life.In the fifth week, each bronchial bud divides into secondarybronchi.In the eighth week the secondary bronchi divide to formthe segmental bronchi (tertiary bronchi), ten in the right lungand eight in the left.Each segmental bronchus becomes a bronchopulmonarysegment (segment in a lung).The smooth muscle, connective tissue, and cartilaginousplates in the bronchi are derived from splanchnicmesenchyme.Wednesday, February 02, 2011Bronchi15BODY CAVITIES Intraembryonic Coelom DiaphragmWednesday, February 02, 2011ZOO 401- Embryology-Dr. Salah A. Martin Intraembryonic Coelom16DiaphragmThe primitive intraembryonic coelom forms inthe lateral and cardiogenic mesoderm about the fourthweek of development.The embryo undergoes two foldings and this cavity is eventuallydivided into the pericardial, pleural,and peritoneal embryonic body cavities.During the fourth week the septum transversum grows toseparate the pericardial cavity from the pleural cavities.During the sixth week the pleuroperitonealmembranes grow to separate the pleural cavities from theperitoneal cavity.During the seventh week the pleuropericardialmembranes separate the pericardial cavity from the pleuralcavities. In the adult the pleuropericardial membranes formthe fibrous pericardium of the heart.Wednesday, February 02, 2011BODY CAVITIESZOO 401- Embryology-Dr. Salah A. MartinIntraembryonic Coelom 14ZOO 401- Embryology-Dr. Salah A. MartinBronchi Trachea17 The diaphragm separates the thoracic and abdominalcavities. It arises from tissue from four sources: The septum transversum, which forms the centraltendon of the diaphragm. The pleuroperitoneal membranes, which contribute onlya small amount to the adult diaphragm The dorsal mesentery of the esophagus, which formsthe crura and median portion of the diaphragm The body wall, which forms the periphery of the diaphragm The diaphragm develops initially at the level of cervicalsomites 3-5 and it “descends” to the level of L1 as theembryo grows.As it moves it takes along its innervation, which explainswhy the phrenic nerve arises from cervical rootsthree, four, and five (“C3-4-5 keeps a man alive.”)Wednesday, February 02, 2011Diaphragm183

2/2/2011ZOO 401- Embryology-Dr. Salah A. MartinSENSE ORGANSORGANS-EYE About the fourth week, optic sulci (optic grooves) develop inthe diencephalon. The optic sulci evaginate to form optic vesicles. The optic vesicles enlarge and form hollow optic stalks. The optic vesicles induce the surface ectoderm of the head to form lensplacodes. The optic vesicles then invaginate to form double-walled optic cups, and theventral surfaces of the optic stalks invaginate to form optic fissures. Mesenchyme within each optic cup forms the hyaloid artery and hyaloidvein. In the meantime, the lens placodes have sunk in to form lens pits. The pits detach from the surface ectoderm to form lens vesicles. The retina is derived from the walls of the optic cups. The proximal parts of the hyaloid vessels form the central artery and vein ofthe retina. The distal parts of the hyaloid vessels disappear before birth.Wednesday, February 02, 2011SENSE ORGANS-EYEGrowth and Diffentiation19Wednesday, February 02, 2011Development of Ectodermal Organs20ZOO 401- Embryology-Dr. Salah A. MartinGrowth Growth is the enlargement of a tissue or organism.Growth continues after the embryonal stage, and occurs through cellproliferation, enlargement of cells or accumulation of extracellularmaterial.In plants, growth results in an adult organism that is strikingly differentfrom the embryo.The proliferating cells tend to be distinct from differentiated cells (stemcell and progenitor cell).In some tissues proliferating cells are restricted to specialised areas, suchas the growth plates of bones.But some stem cells migrate to where they are needed, suchas mesenchymal stem cells which can migrate from the bonemarrow to form e.g. muscle, bone or adipose tissue.The size of an organ frequently determines its growth, as in the case ofthe liver which grows back to its previous size if a part is removed.Growth factors, such as fibroblast growth factors in the animalembryo and growth hormone in juvenile mammals, also control theextent of growth.Wednesday, February 02, 2011Growth21Wednesday, February 02, 201122ZOO 401- Embryology-Dr. Salah A. MartinMetamorphosisMost animals have a larval stage, with a bodyplan different from that of the adult organism. The larva abrubtly develops into an adult in aprocess called metamorphosis. For example, caterpillars (butterfly larvae)are specialized for feeding whereas adultbutterflies (imagos) are specialised for flightand reproduction. When the caterpillar has grown enough, itturns into an immobile pupa. Here, the imago develops from imaginaldiscs found inside the larva. Wednesday, February 02, 2011Metamorphosis23ButterflyFrogWednesday, February 02, 2011244

2/2/2011ZOO 401- Embryology-Dr. Salah A. MartinRegeneration Regeneration is the reactivation of developmentso that a missing body part grows back.This phenomenon has been studied particularlyin salamanders, where the adults can reconstructa whole limb after it has been amputated.Researchers hope to one day be able to induceregeneration in humans (regenerative medicine).There is little spontaneous regeneration in adulthumans, although the liver is a notable exception.Like for salamanders, the regeneration of the liverinvolves dedifferentiation of some cells to a moreembryonal state.Wednesday, February 02, 2011Regeneration255

Wednesday, February 02, 2011 RESPIRATORY SYSTEM 11 ZOO 401-Embryology-Dr. SalahA. Martin Lower Respiratory System The primordiumof the lower respiratory system develops in about the fourth week. . Wednesday, February 02, 2011 Lower Respiratory System 12. 2/2/2011 3 ZOO 401-Embryology-Dr. SalahA. Martin Larynx The opening of the .