DOCSLIB.ORG

Gastrulation & Neurulation

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Class 17 Thursday, 2 April 2020 Gastrulation & Neurulation Exams will be graded and returned on Tuesday We will go over questions and answers Xenopus: simpler, fertilized egg produces embryo only – no maternal interactions, no extra-embryonic tissues – useful for looking at major morphogenic processes but “unique” (timing / size / speed - non-placental) Mouse: human (mammalian) relevance but distinctive differences As an example, we will return to nine-banded armadillo, which always produces monozygotic quadruplets (good for embryonic noise studies) videos (time-lapse) Xenopus Richard Harland (UC Berkeley) 2: The Cellular Basis of Gastrulation watch through ~ 12:30 What happens if Dvl does not function? Two receptor complexes are formed on opposite sides of a cell: • Frizzled forms a complex with Flamingo, disheveled (Dvl), and Diego • on the other side the complex consists of Flamingo, Van Gogh, and Prickle. • The extracellular parts of these complexes interact controlling cellular polarization Absence of Wnt: GSK3 binds Axin & APC to form the β-catenin destruction complex. • Some Wnts stimulate Frizzled & LRP5/6, inhibit GSK3 & stabilize β-catenin • GSK3 inhibition also leads to target gene derepression by promoting TCF3 phosphorylation by homeodomain-interacting protein kinase 2 (HIPK2)- mechanism involves β-catenin • Other Wnts use distinct receptors in addition to Frizzled - control the cytoskeletal organization through core planar cell polarity (PCP) proteins, small GTPases (Rho/ Rac/ Cdc42), and c-Jun amino- terminal kinase (JNK). Double-gradient model of embryonic axis formation in Xenopus: perpendicular activity gradients of Wnts and BMPs regulate head- to-tail and dorsal–ventral patterning. Patterning begins at gastrula stages, but for clarity, it is depicted in an early neurula. The formation of head, trunk and tail requires increasing Wnt activity. mouse / human (placental mammal) pre-implantation mouse embryos from the 2- cell stage to over 100-cells as the expanded blastocysts hatch from their zona pellucidas compaction: time lapse video Implantation of the blastocyst ` In toto live imaging of mouse development from gastrulation to early organogenesis schematic gastrulation video schematic “embryo folding" video Gastrulation - establishment of outer inner between cells outer cell layer - ectoderm inner layer - endoderm between ectoderm / endoderm layers - mesoderm • Prior to implantation, both human and mouse embryos similarly undergo cell divisions culminating in the development of a blastocyst comprising a discernible ICM and TE. • Mouse embryonic genome activation begins at the 2-cell stage. • The mouse blastocyst forms between days 3 and 4. • Pre-implantation blastocysts comprise an outer layer of trophectoderm (TE) cells, which form the trophoblast lineage of the placenta, and an inner cell mass (ICM) that segregates into epiblast (Epi) and primitive endoderm (PE) layers. Epiblast cells give rise to future fetus, whereas the PE gives rise to extra-embryonic endoderm (ExEn) cells that will form the yolk sac. • In the mouse, the TE gives rise to a proliferative stem cell pool of extra-embryonic ectoderm (ExEc) cells that bud off differentiated polyploid trophoblast giant (TG) cells. • Prior to implantation, both human and mouse embryos similarly undergo cell divisions culminating in the development of a blastocyst comprising a discernible ICM and TE. Mouse • Embryonic genome activation in human begins at ~4- to 8-cell stage on day 3. • Compaction and blastocyst formation are delayed in human embryos (compared to mouse); human blastocysts form between days 5 and 6. • Pre-implantation blastocysts comprise an outer layer of trophectoderm (TE) cells, which form the trophoblast lineage of the placenta, and an inner cell mass (ICM) that segregates into epiblast and primitive endoderm layers. • In human TE gives rise to villous cytotrophoblast (VCT) cells, a multinucleated syncytium (Syn) and extravillous trophoblast cells anti-bmp READ Submit one question / call for clarification via beSocratic.
Recommended publications
  • Works Neuroembryology

    Works Neuroembryology

    Swarthmore College Works Biology Faculty Works Biology 1-1-2017 Neuroembryology D. Darnell Scott F. Gilbert Swarthmore College, [email protected] Follow this and additional works at: https://works.swarthmore.edu/fac-biology Part of the Biology Commons Let us know how access to these works benefits ouy Recommended Citation D. Darnell and Scott F. Gilbert. (2017). "Neuroembryology". Wiley Interdisciplinary Reviews: Developmental Biology. Volume 6, Issue 1. DOI: 10.1002/wdev.215 https://works.swarthmore.edu/fac-biology/493 This work is brought to you for free by Swarthmore College Libraries' Works. It has been accepted for inclusion in Biology Faculty Works by an authorized administrator of Works. For more information, please contact [email protected]. HHS Public Access Author manuscript Author ManuscriptAuthor Manuscript Author Wiley Interdiscip Manuscript Author Rev Dev Manuscript Author Biol. Author manuscript; available in PMC 2018 January 01. Published in final edited form as: Wiley Interdiscip Rev Dev Biol. 2017 January ; 6(1): . doi:10.1002/wdev.215. Neuroembryology Diana Darnell1 and Scott F. Gilbert2 1University of Arizona College of Medicine 2Swarthmore College and University of Helsinki Abstract How is it that some cells become neurons? And how is it that neurons become organized in the spinal cord and brain to allow us to walk and talk, to see, recall events in our lives, feel pain, keep our balance, and think? The cells that are specified to form the brain and spinal cord are originally located on the outside surface of the embryo. They loop inward to form the neural tube in a process called neurulation.
  • Gastrulation

    Gastrulation

    Embryology of the spine and spinal cord Andrea Rossi, MD Neuroradiology Unit Istituto Giannina Gaslini Hospital Genoa, Italy [email protected] LEARNING OBJECTIVES: LEARNING OBJECTIVES: 1) To understand the basics of spinal 1) To understand the basics of spinal cord development cord development 2) To understand the general rules of the 2) To understand the general rules of the development of the spine development of the spine 3) To understand the peculiar variations 3) To understand the peculiar variations to the normal spine plan that occur at to the normal spine plan that occur at the CVJ the CVJ Summary of week 1 Week 2-3 GASTRULATION "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986) Gastrulation Conversion of the embryonic disk from a bilaminar to a trilaminar arrangement and establishment of the notochord The three primary germ layers are established The basic body plan is established, including the physical construction of the rudimentary primary body axes As a result of the movements of gastrulation, cells are brought into new positions, allowing them to interact with cells that were initially not near them. This paves the way for inductive interactions, which are the hallmark of neurulation and organogenesis Day 16 H E Day 15 Dorsal view of a 0.4 mm embryo BILAMINAR DISK CRANIAL Epiblast faces the amniotic sac node Hypoblast Primitive pit (primitive endoderm) faces the yolk sac Primitive streak CAUDAL Prospective notochordal cells Dias Dias During
  • Human Blastocyst Morphological Quality Is Significantly Improved In

    Human Blastocyst Morphological Quality Is Significantly Improved In

    Human blastocyst morphological quality is significantly improved in embryos classified as fast on day 3 (R10 cells), bringing into question current embryological dogma Martha Luna, M.D.,a,b Alan B. Copperman, M.D.,a,b Marlena Duke, M.Sc.,a,b Diego Ezcurra, D.V.M.,c Benjamin Sandler, M.D.,a,b and Jason Barritt, Ph.D.a,b a Mount Sinai School of Medicine, Department of Obstetrics and Gynecology, Department of Reproductive Endocrinology and Infertility, and b Reproductive Medicine Associates of New York, New York, New York; and c EMD Serono, Rockland, Massachusetts Objective: To evaluate developmental potential of fast cleaving day 3 embryos. Design: Retrospective analysis. Setting: Academic reproductive center. Patient(s): Three thousand five hundred twenty-nine embryos. Intervention(s): Day 3 embryos were classified according to cell number: slow cleaving: %6 cells, intermediate cleaving: 7–9 cells, and fast cleaving: R10 cells, and further evaluated on day 5. The preimplantation genetic diagnosis (PGD) results of 43 fast cleaving embryos were correlated to blastocyst formation. Clinical outcomes of transfers involving only fast cleaving embryos (n ¼ 4) were evaluated. Main Outcome Measure(s): Blastocyst morphology correlated to day 3 blastomere number. Relationship between euploidy and blastocyst formation of fast cleaving embryos. Implantation, pregnancy (PR), and birth rates resulting from fast embryo transfers. Result(s): Blastocyst formation rate was significantly greater in the intermediate cleaving (72.7%) and fast cleav- ing (54.2%) groups when compared to the slow cleaving group (38%). Highest quality blastocysts were formed significantly more often in the fast cleaving group. Twenty fast cleaving embryos that underwent PGD, formed blastocysts, of which 45% (9/20) were diagnosed as euploid.
  • The Genetic Basis of Mammalian Neurulation

    The Genetic Basis of Mammalian Neurulation

    REVIEWS THE GENETIC BASIS OF MAMMALIAN NEURULATION Andrew J. Copp*, Nicholas D. E. Greene* and Jennifer N. Murdoch‡ More than 80 mutant mouse genes disrupt neurulation and allow an in-depth analysis of the underlying developmental mechanisms. Although many of the genetic mutants have been studied in only rudimentary detail, several molecular pathways can already be identified as crucial for normal neurulation. These include the planar cell-polarity pathway, which is required for the initiation of neural tube closure, and the sonic hedgehog signalling pathway that regulates neural plate bending. Mutant mice also offer an opportunity to unravel the mechanisms by which folic acid prevents neural tube defects, and to develop new therapies for folate-resistant defects. 6 ECTODERM Neurulation is a fundamental event of embryogenesis distinct locations in the brain and spinal cord .By The outer of the three that culminates in the formation of the neural tube, contrast, the mechanisms that underlie the forma- embryonic (germ) layers that which is the precursor of the brain and spinal cord. A tion, elevation and fusion of the neural folds have gives rise to the entire central region of specialized dorsal ECTODERM, the neural plate, remained elusive. nervous system, plus other organs and embryonic develops bilateral neural folds at its junction with sur- An opportunity has now arisen for an incisive analy- structures. face (non-neural) ectoderm. These folds elevate, come sis of neurulation mechanisms using the growing battery into contact (appose) in the midline and fuse to create of genetically targeted and other mutant mouse strains NEURAL CREST the neural tube, which, thereafter, becomes covered by in which NTDs form part of the mutant phenotype7.At A migratory cell population that future epidermal ectoderm.
  • Terminologia Embryologica

    Terminologia Embryologica

    Terminologia Embryologica МЕЖДУНАРОДНЫЕ ТЕРМИНЫ ПО ЭМБРИОЛОГИИ ЧЕЛОВЕКА С ОФИЦИАЛЬНЫМ СПИСКОМ РУССКИХ ЭКВИВАЛЕНТОВ FEDERATIVE INTERNATIONAL PROGRAMME ON ANATOMICAL TERMINOLOGIES (FIPAT) РОССИЙСКАЯ ЭМБРИОЛОГИЧЕСКАЯ НОМЕНКЛАТУРНАЯ КОМИССИЯ Под редакцией акад. РАН Л.Л. Колесникова, проф. Н.Н. Шевлюка, проф. Л.М. Ерофеевой 2014 VI СОДЕРЖАНИЕ 44 Facies Лицо Face 46 Systema digestorium Пищеварительная система Alimentary system 47 Cavitas oris Ротовая полость Oral cavity 51 Pharynx Глотка Pharynx 52 Canalis digestorius; Canalis Пищеварительный канал Alimentary canal oesophagogastrointestinalis 52 Oesophagus Пищевод Oesophagus▲ 53 Gaster Желудок Stomach 54 Duodenum Двенадцатиперстная кишка Duodenum 55 Ansa umbilicalis intestini Пупочная кишечная петля Midgut loop; Umbilical intestinal loop 56 Jejunum et Ileum Тощая и подвздошная кишка Jejunum and Ileum 56 Intestinum crassum Толстая кишка Large intestine 58 Canalis analis Анальный канал Anal canal 58 Urenteron; Pars postcloacalis Постклоакальная часть кишки Postcloacal gut; intestini Tailgut; Endgut 59 Hepar Печень Liver 60 Ductus choledochus; Ductus Жёлчный проток Bile duct biliaris 61 Vesica biliaris et ductus Жёлчный пузырь и пузырный про- Gallbladder and cystic cysticus ток duct 61 Pancreas Поджелудочная железа Pancreas 63 Systema respiratorium Дыхательная система Respiratory system 63 Nasus Нос Nose 64 Pharynx Глотка, зев Pharynx 64 Formatio arboris respiratoriae Формирование дыхательной Formation of системы (бронхиального дерева) respiratory tree 67 Systema urinarium Мочевая система Urinary
  • Mechanisms of Programmed Cell Death in the Developing Brain

    Mechanisms of Programmed Cell Death in the Developing Brain

    _TINS July 2000 [final corr.] 12/6/00 10:39 am Page 291 R EVIEW Mechanisms of programmed cell death in the developing brain Chia-Yi Kuan, Kevin A. Roth, Richard A. Flavell and Pasko Rakic Programmed cell death (apoptosis) is an important mechanism that determines the size and shape of the vertebrate nervous system. Recent gene-targeting studies have indicated that homologs of the cell-death pathway in the nematode Caenorhabditis elegans have analogous functions in apoptosis in the developing mammalian brain.However,epistatic genetic analysis has revealed that the apoptosis of progenitor cells during early embryonic development and apoptosis of postmitotic neurons at later stage of brain development have distinct roles and mechanisms.These results provide new insight on the significance and mechanism of neural cell death in mammalian brain development. Trends Neurosci. (2000) 23, 291–297 ELL DEATH has long been recognized to occur in homologs of ced-3 comprise a family of cysteine- Cmost neuronal populations during normal devel- containing, aspartate-specific proteases called caspases5. opment of the vertebrate nervous system (reviewed in The ced-4 homolog is identified as one of the apoptosis Ref. 1). Traditionally, the investigation of neural death protease-activating factors (APAFs)6. The mammalian in development focused on the role of target-derived homologs of ced-9 belong to a growing family of Bcl2 survival factors such as NGF and related neurotrophins proteins, which share the Bcl2-homology (BH) domain (see Ref. 2 for a review). However, in the past few years, and are either pro- or anti-apoptotic7. The cloning of the genetic analysis of programmed cell death in the egl-1 indicates that it is similar to the BH3-domain- nematode Caenorhabditis elegans has inspired new ap- containing, pro-apoptotic subfamily of Bcl2 proteins4.
  • Programmed Cell Death During Xenopus Development

    Programmed Cell Death During Xenopus Development

    DEVELOPMENTAL BIOLOGY 203, 36–48 (1998) ARTICLE NO. DB989028 View metadata, citation and similar papers at core.ac.uk brought to you by CORE Programmed Cell Death during Xenopus provided by Elsevier - Publisher Connector Development: A Spatio-temporal Analysis Carmel Hensey and Jean Gautier1 Department of Genetics and Development and Department of Dermatology, College of Physicians and Surgeons of Columbia University, 630 West 168th Street, New York, New York 10032 Programmed cell death (PCD) is an integral part of many developmental processes. In vertebrates little is yet known on the patterns of PCD and its role during the early phases of development, when embryonic tissue layers migrate and pattern formation takes place. We describe the spatio-temporal patterns of cell death during early Xenopus development, from fertilization to the tadpole stage (stage 35/36). Cell death was analyzed by a whole-mount in situ DNA end-labeling technique (the TUNEL protocol), as well as by serial sections of paraffin-embedded TUNEL-stained embryos. The first cell death was detected during gastrulation, and as development progressed followed highly dynamic and reproducible patterns, strongly suggesting it is an important component of development at these stages. The detection of PCD during neural induction, neural plate patterning, and later during the development of the nervous system highlights the role of PCD throughout neurogenesis. Additionally, high levels of cell death were detected in the developing tail and sensory organs. This is the first detailed description of PCD throughout early development of a vertebrate, and provides the basis for further studies on its role in the patterning and morphogenesis of the embryo.
  • Embryo Makes Contact with the Endometrial Lining of the Uterus

    Embryo Makes Contact with the Endometrial Lining of the Uterus

    Week 1 • Week 1 - Early zygote • Stage 1 starts at the beginning of • Week 1 Carnegie stage – 1,2,3,4, fertilization • Fertilization • Stage 2 begins with the division • of the zygote into two cells and Zygote ends with the appearance of the • Morula blastocystic cavity • Blastocyst • Stage 3 begins when the blastocystic cavity first appears in the morula and ends when the zona (capsula) pellucida is shed as the embryo makes contact with the endometrial lining of the uterus. • Stage 4 is reserved for the attaching blastocyst to the endometrial lining Week 2 • Week 2 Implantation • Stage 5 Two distinct layers • Week 2 Carnegie stage -5,6 are evident in the • Trophoblast - outer cell trophoblast; 1) a thicker layer outer layer without cell boundaries, called the • Embryoblast - inner cell syncytiotrophoblast and 2) mass a thinner inner layer with • Implantation cell boundaries called the • Bilaminar embryo cytotrophoblast. • Stage 6 the first appearance of chorionic villi. Week 3 • • Stage 7 the presomite • Week 3 - Embryonic disc period and well defined • Week 3 - Carnegie stage – embryonic disc appearance 7,8, &9 of the notochordal process and the gastrulation • Gastrulation (primitive) node. • Notochord formation • Trilaminar embryo • Mesoderm • Somitogenesis • Neurogenesis Week 4 • The heart begins • Week 4 - Carnegie stage -10,11,12 &13 • Heart • Placodes • Pharyngeal arches • Week 5 • Week 7 - Head and limb • Carnegie stages stage 14 development stage 15 • Carnegie stages stage 18 • stage 19 • Week 6 - Early face • Week 8 deevelopment • Last embryonic stage • Carnegie stages Carnegie stage – 20 21 22 • Week 6 - Carnegie stage 16 &23 & 17 • Last week of embryonic development.
  • The Involvement of Cell Death and Survival in Neural Tube Defects: a Distinct Role for Apoptosis and Autophagy?

    The Involvement of Cell Death and Survival in Neural Tube Defects: a Distinct Role for Apoptosis and Autophagy?

    Cell Death and Differentiation (2008) 15, 1170–1177 & 2008 Nature Publishing Group All rights reserved 1350-9047/08 $30.00 www.nature.com/cdd Review The involvement of cell death and survival in neural tube defects: a distinct role for apoptosis and autophagy? F Cecconi*,1,2, M Piacentini3,4 and GM Fimia3 Neural tube defects (NTDs), such as spina bifida (SB) or exencephaly, are common congenital malformations leading to infant mortality or severe disability. The etiology of NTDs is multifactorial with a strong genetic component. More than 70 NTD mouse models have been reported, suggesting the involvement of distinct pathogenetic mechanisms, including faulty cell death regulation. In this review, we focus on the contribution of functional genomics in elucidating the role of apoptosis and autophagy genes in neurodevelopment. On the basis of compared phenotypical analysis, here we discuss the relative importance of a tuned control of both apoptosome-mediated cell death and basal autophagy for regulating the correct morphogenesis and cell number in developing central nervous system (CNS). The pharmacological modulation of genes involved in these processes may thus represent a novel strategy for interfering with the occurrence of NTDs Cell Death and Differentiation (2008) 15, 1170–1177; doi:10.1038/cdd.2008.64; published online 2 May 2008 Neural tube defects (NTDs) are common (1 in 1000 by participating in folding, pinching off and fusion of neural pregnancies) congenital malformations in humans leading to walls, in neural precursor selection and in postmitotic infant mortality or severe disability. NTD results from failure of competition of neurons for their cellular targets (reviewed in complete neurulation during the fourth week of embryo- De Zio et al.,18Hidalgo and ffrench-Constant,19 Kuan et al.,20 genesis.
  • Embryology and Teratology in the Curricula of Healthcare Courses

    Embryology and Teratology in the Curricula of Healthcare Courses

    ANATOMICAL EDUCATION Eur. J. Anat. 21 (1): 77-91 (2017) Embryology and Teratology in the Curricula of Healthcare Courses Bernard J. Moxham 1, Hana Brichova 2, Elpida Emmanouil-Nikoloussi 3, Andy R.M. Chirculescu 4 1Cardiff School of Biosciences, Cardiff University, Museum Avenue, Cardiff CF10 3AX, Wales, United Kingdom and Department of Anatomy, St. George’s University, St George, Grenada, 2First Faculty of Medicine, Institute of Histology and Embryology, Charles University Prague, Albertov 4, 128 01 Prague 2, Czech Republic and Second Medical Facul- ty, Institute of Histology and Embryology, Charles University Prague, V Úvalu 84, 150 00 Prague 5 , Czech Republic, 3The School of Medicine, European University Cyprus, 6 Diogenous str, 2404 Engomi, P.O.Box 22006, 1516 Nicosia, Cyprus , 4Department of Morphological Sciences, Division of Anatomy, Faculty of Medicine, C. Davila University, Bucharest, Romania SUMMARY Key words: Anatomy – Embryology – Education – Syllabus – Medical – Dental – Healthcare Significant changes are occurring worldwide in courses for healthcare studies, including medicine INTRODUCTION and dentistry. Critical evaluation of the place, tim- ing, and content of components that can be collec- Embryology is a sub-discipline of developmental tively grouped as the anatomical sciences has biology that relates to life before birth. Teratology however yet to be adequately undertaken. Surveys (τέρατος (teratos) meaning ‘monster’ or ‘marvel’) of teaching hours for embryology in US and UK relates to abnormal development and congenital medical courses clearly demonstrate that a dra- abnormalities (i.e. morphofunctional impairments). matic decline in the importance of the subject is in Embryological studies are concerned essentially progress, in terms of both a decrease in the num- with the laws and mechanisms associated with ber of hours allocated within the medical course normal development (ontogenesis) from the stage and in relation to changes in pedagogic methodol- of the ovum until parturition and the end of intra- ogies.
  • Early Embryonic Development Till Gastrulation (Humans)

    Early Embryonic Development Till Gastrulation (Humans)

    Gargi College Subject: Comparative Anatomy and Developmental Biology Class: Life Sciences 2 SEM Teacher: Dr Swati Bajaj Date: 17/3/2020 Time: 2:00 pm to 3:00 pm EARLY EMBRYONIC DEVELOPMENT TILL GASTRULATION (HUMANS) CLEAVAGE: Cleavage in mammalian eggs are among the slowest in the animal kingdom, taking place some 12-24 hours apart. The first cleavage occurs along the journey of the embryo from oviduct toward the uterus. Several features distinguish mammalian cleavage: 1. Rotational cleavage: the first cleavage is normal meridional division; however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially. 2. Mammalian blastomeres do not all divide at the same time. Thus the embryo frequently contains odd numbers of cells. 3. The mammalian genome is activated during early cleavage and zygotically transcribed proteins are necessary for cleavage and development. (In humans, the zygotic genes are activated around 8 cell stage) 4. Compaction: Until the eight-cell stage, they form a loosely arranged clump. Following the third cleavage, cell adhesion proteins such as E-cadherin become expressed, and the blastomeres huddle together and form a compact ball of cells. Blatocyst: The descendents of the large group of external cells of Morula become trophoblast (trophoblast produce no embryonic structure but rather form tissues of chorion, extraembryonic membrane and portion of placenta) whereas the small group internal cells give rise to Inner Cell mass (ICM), (which will give rise to embryo proper). During the process of cavitation, the trophoblast cells secrete fluid into the Morula to create blastocoel. As the blastocoel expands, the inner cell mass become positioned on one side of the ring of trophoblast cells, resulting in the distinctive mammalian blastocyst.
  • Junctional Neurulation: a Unique Developmental Program Shaping a Discrete Region of the Spinal Cord Highly Susceptible to Neural Tube Defects

    Junctional Neurulation: a Unique Developmental Program Shaping a Discrete Region of the Spinal Cord Highly Susceptible to Neural Tube Defects

    13208 • The Journal of Neuroscience, September 24, 2014 • 34(39):13208–13221 Development/Plasticity/Repair Junctional Neurulation: A Unique Developmental Program Shaping a Discrete Region of the Spinal Cord Highly Susceptible to Neural Tube Defects Alwyn Dady,1,2 X Emmanuelle Havis,1,2 Virginie Escriou,3 Martin Catala,1,2,4 and Jean-Loup Duband1,2 1Universite´ Pierre et Marie Curie-Paris 6, Laboratoire de Biologie du De´veloppement, 75005 Paris, France, 2Centre National de la Recherche Scientifique, Laboratoire de Biologie du De´veloppement, 75005 Paris, France, 3Unite´ de Technologies Chimiques et Biologiques pour la Sante´, Centre National de la Recherche Scientifique Unite´ Mixte de Recherche 8258, Institut National de la Sante´ et de la Recherche Me´dicale U1022, Universite´ Paris Descartes, Faculte´ de Pharmacie, 75006 Paris, France, and 4Assistance Publique-Hopitaux de Paris, Federation of Neurology, Groupe Hospitalier Pitie´-Salpeˆtrie`re, 75013 Paris, France In higher vertebrates, the primordium of the nervous system, the neural tube, is shaped along the rostrocaudal axis through two consecutive, radically different processes referred to as primary and secondary neurulation. Failures in neurulation lead to severe anomalies of the nervous system, called neural tube defects (NTDs), which are among the most common congenital malformations in humans. Mechanisms causing NTDs in humans remain ill-defined. Of particular interest, the thoracolumbar region, which encompasses many NTD cases in the spine, corresponds to the junction between primary and secondary neurulations. Elucidating which developmen- tal processes operate during neurulation in this region is therefore pivotal to unraveling the etiology of NTDs. Here, using the chick embryo as a model, we show that, at the junction, the neural tube is elaborated by a unique developmental program involving concerted movements of elevation and folding combined with local cell ingression and accretion.