Introduction to Embryology PDF

INTRODUCTION TO EMBRYOLOGY Andrea Streit, Centre for Craniofacial & Regenerative Biology [email protected] Why study development? Understanding normal development is central to Understanding the causes & genetics of birth defects Stem cell research Regenerative medicine and …….….. Movie: Rusty...

INTRODUCTION TO EMBRYOLOGY Andrea Streit, Centre for Craniofacial & Regenerative Biology [email protected] Why study development? Understanding normal development is central to Understanding the causes & genetics of birth defects Stem cell research Regenerative medicine and …….….. Movie: Rusty Lansford, USC Fertilisation Cleavage: Blastocyst All the cells have the same genes but activate them differentially! Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube Blastocyst formation in mouse Differential gene expression Blastocyst formation in mouse Human blastocyst 4-5 days post-fertilisation Trophectoderm 32 cell stage Inner cell mass: Generates the embryo Implantation Begins around 6 days Trophoblast ICM: inner cell mass The entire embryo comes from the inner cell mass Blastocyst 9 days ICM is the source of embryonic stem cells Inner cell mass: source of pluripotent embryonic stem cells Image from Mike Jones Key points: After fertilisation, cell division forms a ball of cells called morula Cells within the morula - begin to transcribe different genes - reorganise to form the outer trophectoderm and the inner cells mass Cavitation generates a fluid-filled cavity (blastocoel) and the inner cell mass locates to one side of the blastocyst The trophectoderm will form large parts of the placenta The inner cell mass will form the embryo and is the source of pluripotent embryonic stem cells Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube Subdivision of the ICM into epiblast and hypoblast Mouse embryo Hypoblast Lining of the yolk sac Epiblast Embryo proper Embryo: 2-layered disc of epiblast and hypoblast Epiblast and hypoblast grow Primitive streak forms: site of gastrulation Primitive streak HypoblastEpiblast 12 days in humans During gastrulation Cells in the epiblast move towards the primitive streak Cells leave the epiblast and ingress into the primitive streak Ingressing cells form mesoderm and endoderm Cells that remain at the surface form ectoderm GASTRULATION: Formation of 3 germ layers Primitive streak ectoderm + mesoderm +endoderm Derivatives of each germ layer Ectoderm Mesoderm Endoderm Skin Nervous system Vertebral column Skeleton Skeletal muscle Circulatory system Blood Kidney Gastrointestinal tract Lung Pancreas Liver Thymus Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube Making the medial-lateral (dorso-ventral) axis Example: mesoderm Axial: notochord and prechordal Centre of vertebral disc, some muscles in the head Head Skeleton, muscles, dermis Intermediate Kidney, part of genital system Lateral plate Trunk Viscera, inner lining of the body wall, dermis The medial-lateral (dorso-ventral) axis is established during gastrulation The position of cells along the primitive streak determines their final position in the medial-lateral (dorso-ventral) axis Cells from tip of the streak move into the notochord = midline = dorsal Cells slightly more along the streak move into somites = paraxial = more ventral than notochord The primitive streak is a passageway for cells to their final position in the future back-belly axis vertebrae muscles heart kidney Dorsal flank/ belly Ventral …but the order in which they come out determines their position along the head - tail axis Cells that leave first are rostral, cells that leave later are caudal Establishing the body axes Back to belly = Dorsal – ventral: Position along the primitive streak Head to tail = Rostral – caudal: Timing of when cells leave the primitive streak Key points: Gastrulation: formation of the three germ layers – ectoderm, mesoderm, endoderm During gastrulation, epiblast cells ingress through the primitive streak to become endoderm and mesoderm; cells that do not ingress are the ectoderm The position along the primitive streak defines the medial-lateral axis of the mesoderm The time when cells leave the streak defines their position along the head-tail axis Different organ systems arise from different germ layers Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube While mesoderm is laid down, neural plate forms in the ectoderm…. Neural plate: origin of the central nervous system Neural plate folds into the neural tube The neural tube closes Transverse section Signals from the axial mesoderm Signals from the axial mesoderm: Separation of the eye field eye field Sonic hedgehog Xenopus zebrafish frog chick mouse (Ybot-Gonzalez et al (2002) Development 129:2007-17) Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube How to form a body from flat germ layers? ? Folding of the body wall VENTRAL DORSAL Folding of the lateral body wall Blastocyst inner cell mass enveloped by outer layer (trophectoderm) Gastrulation generation of 3 germ layers: endoderm, mesoderm, ectoderm Establishment of body axes head-tail, back-belly Neural plate formation & Neurulation ectoderm forms the neural plate, which folds up into a tube Folding of the body walls Neural crest cells migrating cells from the neural tube Neural crest cells: migratory cells from the dorsal neural tube Neural crest cells: migratory cells from the dorsal neural tube Trunk: melanocytes Sensory neurons Sympathetic neurons Enteric neurons Glia Neural crest cells: migratory cells from the dorsal neural tube Head: Melanocytes Cranial ganglia Craniofacial skeleton Odontogenic mesenchyme = ectomesenchyme Key points The neural plate forms in the ectoderm and generates the central nervous system Signals from surrounding tissues pattern the neural tube; example: shh from the notochord is essential for eye field separation Fusion of the body wall: lateral folds fuse in the ventral midline Neural crest cells: derivatives and importance in the head Larsen's Human Embryology, 4th Edition Authors: Gary C. Schoenwolf, Steven B. Bleyl, Philip R. Brauer, & Philippa H. Francis-West https://syllabus.med.unc.edu/courseware/em bryo_images/unitwelcome/welcome_htms/contents.htm