6.2. DEVELOPMENT OF ENDODERMAL ORGANS Pavel Krejci Endoderm forms the epithelial lining of the gut together with its outgrowths - liver, pancreas and respiratory systém. The outer coats of these organs - smooth muscle, connective tissue and blood vessels are formed from splanchnic mesoderm. Splanchnic mesoderm + endoderm = splanchnopleure. GUT DEVELOPMENT In chick, the endoderm is formed as lower layer of definitivě endoderm that emerges from the primitive streak. The gut lumen originates from the space beneath the blastoderm. The gut tube itself is formed by the folding of the body away from the blastoderm. Anterior Head fold Stomatodeal piate Posterior Ectoderm Mesoderm Endoderm Amniotic cavity Amnion (a) Anterior intestinal portal Stomatodeal plate Stage 9 Cloaca] membrane Tai! gut Stage 20 Pouch 1 Arch 2 ORGANIZATION OF THE GUT TUBE: In the region of the Arch , pharynx are formed the pharyngeal pouches, there are 4 major pouches and two or more rudimentary. Buds from the 1st pouch form the cavities of the Pojch 2 middle ear and the Eustachian Arch 3 ----- tubes. The ventral midline region Pouch 3 — forms the thyroid gland. The 2nd Arch 4------ pair forms the tonsils, 3rd pair the Pouch 4 — thymus, 4th pair the parathyroid. ____ In the ventral midline opposite to rr Pouch 5 the pouches the trachea forms and produces the paired buds that later Arch 6 generate bronchi and lung tissue. Rudimentary The floor of pharynx becomes the pouc tongue. St o m a to de u m Thyroid — Slit 2 Parathyroids Thymus Ultimobranchia! body Lungs Esophagus Esophagus and stomach follow the pharingeal region. At the exit of the stomach is the pyloric sfíncter leading to small intestine. The liver is ventral outgrowth of the endodermal epithelium that expands to the adjacent ventral mesentery. The pancreas arises from a large dorsal bud and a small ventral bud, that later fuse to form a single organ. The junction between small and large intestine is often marked by presentee of outgrowth called caeca. At the posterior end the large intestine becomes rectum in and mammals joins the exterior through the anal sfíncter. ta) 2 days Esophagus Stomach Liver Small intestine 4 days DETERMINATION OF THE ENDODERM: The initial formation of the endoderm has been mainly characterized in Xenopus and zebrafish. In Xenopus the endoderm forms from the tissue corresponding to the eight most vegetal blastomeres at the 32-cell stage. The endoderm is formed by the action of transcription factors VegT, Soxl7, Mixl, Mixer and GATA1-4. Some of these factors require intercellular signaling out of which the critical in nodal signaling. Animals with nodal loss-of-function can not form the definitive endoderm. The transcription factors mentioned above specify endoderm as a germ layer rather than any specific type of epithelium or organ. Endoderm REGIONAL SPECIFICATION 5 M i N i OF ENDODERM: Many | i transcription factors are expressed locally in endoderm and are important for regional specification. These include pax9 that is critical for development of thymus and parathyroid, Nkx2.1 is essential for for thyroid and lungs, Pdxl that is a critical regulator of H* ! pancreas development, and FoxA2 which loss-of function leads to lack of development of fore- and midgut. 3 PANCREATIC DEVELOPMENT INDUCTION OF PANCREATIC BUDS: Both dorsal and ventral buds give rise to the same range of pancreatic cell they arise in different ways. The dorsal bud appears the region where notochord contacts the gut roof. It is the region of suppression of the otherwise ubiquitous expression of Shh and Indian hedgehog (Ihh). The effect of notochord can be mimicked by administration of Activin or FGF. The ventral pancreas is formed from the adjacent region of the foregut floor to the liver only in the absence of FGF, that functions in maintaining the Shh. It thus appears that dorsal bud is induced by FGF whereas the ventral bud develops because of an absence of FGF, although the common factor is suppression of Shh expression in the endoderm. Once both buds are formed, their continued outgrowth and differentiation depends on close proximity of the pancreatic mesenchyme, that executes a permissive effect on bud outgrowth. A signal that carries this function appears to be FGF10. Liver Ventral pancreatic bud 30 days Stomach Dorsal pancreatic bud Cystic diverticulum Ventral pancreatic buc 32 days Dorsal pancreatic bud Accessory pancreatic duct Minor papilla Major papilla 42 days PANCREATIC TRANSCRIPTION FACTORS: Pdxl is essential for pancreas formation and is expressed in a region of the duodenum including the prospective territories of the pancreatic buds. Later in the development, the Pdx expression is downregulated but persists in ß-cells where it serves as transcription factor for insulin. Hb9 precedes Pdxl expression and the K.O. animals do not develop dorsal pancreas. PTF1 is essential for expression of various exocrine genes, its K.O. do not develop ventral bud. Pax4, Pax6 and neurogenin 3 (Ngn3) are essential for endocrine cell development. Sox 7 7 3 Endoderm Pdxl, HixbB 3 Pre-pancreatic PPcell Pdxl, PTF1 Ngn3 Endocrine precursor Exocrine precursor PTF1 Exocrine cell Pax4f Nkx2.2, NkxS. 1 Pax4 P-cel 5-cell PaxE rt-cietl