CHAPTER 15-1, LATERAL PLATE MESODERM ZOO3603 The major lineages of the amniote mesodermThe major lineages of lateral plate mesoderm Lateral plate mesoderm Dorsal layer: Somatic mesoderm Ventral layer: Splanchnic mesoderm Space between two: Coelom Somatopleure Splanchnopleure Between two: Extra-embryonic mesoderm Mesodermal development in frog embryos Splanchnic mesoderm together with endoderm forms the splanchnopleure. Somatic mesoderm together with ectoderm forms the somatopleure. Mesodermal development in chick embryos It resembles the amphibian neurula. The development of blood cells Need the stem cell  Everyday, about 1011 red blood cells are killed in the spleen, and need to be replaced.  The replacements come from populations of stem cells. The concept of stem cells A stem cell is a cell that is capable of extensive proliferation, creating more stem cells (self-renewal) as well as more differentiated cellular progeny. Hematopoietic stem cell (HSC) is critical in hematopoiesis. Differences in hematopoiesis  Two Phases: Transient embryonic “primitive” phase Definitive “adult” phase  Different Phases differ in  Sites of blood production  Timing of hematopoiesis  Type of globin genes in red blood cellsSites Of Embryonic Hematopoiesis  Blood islands in the ventral mesoderm near the yolk sac.  Visceral (splanchnic) lateral plate mesoderm near the aorta, the aorta-gonad-mesonephros (AGM) region.  Placenta BMP2 & BMP4 are critical in inducing the blood forming cells.Hematopoiesis in the aorta-gonad-mesonephros (AGM) region Hematopoietic Stem Cell, HSC HSC HSC HSC HSC Aorta 3-Day chick embryo Transition of sources of blood cells to the bone marrow The hematopoietic stem cells colonize the fetal liver, and the stem cells from the liver populate the bone marrow. Chemo-attractant proteins for HSC, which are secreted from the bone and spleen and attract into them. The stem cell niche by which endosteal osteoblasts 1. N-cadherin sticks two cells together. 2. Jagged activates Notch signal. 3. Angiopoietin-1 activates Tie2. 4. Wnt activate Frizzle, and induce the nucleus localization of β-catenin. Wnt pathway is critical for the self-renewal of HSC. A model for the origin of mammalian blood and lymphoid cells Lineage-restricted stem cells Synthesizing hemoglobin No division, RNA synthesis or protein synthesis Expelling nucleus Comparison of blood cells in vertebrate Human American Alligator Amphibians, fish, bird and reptile retain the functionless nucleus in erythrocyte. Hematopoietic inductive microenvironments  The feat of the stem cell is determined by the stromal cells in blood cell productions.  Cytokines are made by several cell types.  Via a paracrine fashion, but the extracellular matrix concentrate and present the factors to the stem cells.  The stem cells become competent to respond to different factors.  The regions of determination are called hematopoietic inductive microenvironments. • Specification of heart tissue • Fusion of the heart rudiments and initial heartbeats • Looping and formation of heart chambers The heart Heart-forming cells of the chick embryo Two symmetrical regions of cardiogenic mesoderm. The cells migrate through the streak and form two group of mesodermal cells lateral to Hensen’s node. Specification of the cardiac precursor cells Outline of the inductive interactions involving Wnt & BMP pathways. Inductions by endoderm through BMP & FGF. endoderm Wnt3a & Wnt8 inhibit heart formation, but promote blood formation. endoderm neural tube notochord Formation of the chick heart from the splanchnic lateral plate mesoderm Cardiogenic mesoderm migration is induced by the gradient of fibronectin in anterior-posterior axis. 25 hours 26 hours Formation of the chick heart from the splanchnic lateral plate mesoderm 28 hours 29 hours The inward folding of the splanchnopleure bring the two cardiac tubes together. Important genes for cardiac precursor migration • GATA5 • Nkx2-5 • Foxp4 Cardia bifida caused by surgery or GATA5 mutation Surgically induced in chick Cardia bifida: Two separate hearts form on on each side of body. Wild type Miles apart mutant In situ hybridization of ventricular myosin in zebrafish Cardia bifida Miles apart expressed in the endoderm is a receptor for a cell surface sphyngolipid molecule Cardia bifida in a Foxp4 deficient mouse embryo 12.5-day embryo Wild type Foxp4 KO Cardia bifida, but developed normally with all four chambers Foxp4 is a transcription factor expressed in the early foregut cells. Establishment of anterior and posterior cardiac domains Double in situ hybridization for the expression of RADH2 and Tbx5 RADH2 (Orange), retinaldehyde dehydrogenase producing retinoic acid; Tbx5 (Blue), a marker for the early heart fields. The posterior region becomes exposed to retinoic acid produced by the posterior mesoderm. 11.25 Anterior-posterior patterning in the chick embryo At embryonic stage-7, mRNA expressions by in situ hybridization Retinaldehyde dehydrogenase-2 Retinoic acid degradation enzymes The posterior fate becomes committed by retinoic acid Initial Cell Differentiation GATA4 is first seen in the precardiac cells. N-cadherin is critical for the fusion of the two heart rudiments into one tube. Formation of the chick heart from the splanchnic lateral plate mesoderm 28 hours 29 hours Joining two tubes together form the pericardial cavity, the sac in which the heart is formed. Small cells of pericardial cavity form the endocardium, the blood vessels. The epithelial cells form the myocardium, rise to the heart muscle. Fusion of the heart rudiments The un-fused posterior portion of the endocardium become the openings of the vitelline veins into the heart. Contraction of the truncus arteriosus speed the blood into the aorta. Initial heartbeats  Pulsations of the chick heart begin while the paired primordia are still fusing.  The sodium-calcium exchange pump in the muscle cell membrane.  Cardiac cell calcium channels in the pacemaker function. Heart Looping: The looping of the chick heart is a complex orchestration of events Cardiac looping and chamber formation in out out in Cascade of heart development Heart looping is depending on:  Left-right pattering protein  Extracellular matrices  Differential cell growth Different gene