The embryonic structure which establishes body axes and patterns surrounding tissues is called the notochord. The notochord is a midline column of cells running in a rostrocaudal direction (head-tail) within the mesoderm layer. It exists as a transient developmental patterning structure with a role in molecular signaling (patterning) and controlling the direction of embryonic disc folding (mechanical). |
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Carnegie Stages 7Features: embryonic disc, primitive node, primative streak, primitive groove, yolk sac Facts: Week 3, 15 - 17 days, 0.4 mm View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side. Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm. Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract. |
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Axes: embryonic disc is shown rostral (head) to top and caudal (tail) to bottom. Left and right are the lateral margins of the disc as shown.
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Folding: all edges of the embryonic disc will fold ventrally, forming a rostro-caudal "C" shaped tube. |
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Gastrulation: Through the primitive streak cells migrate continuously through week 3 into week 4. Initial cells replace hypoblast as an epithelial layer the endoderm. Later migrating cells spread between the two epithelial layers to form mesoderm. |
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Axial Process: From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process, then notochordal process, and finally the notochord. The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract. Next: Carnegie Stage 8 Unlabelled | Stage 8 Labelled  |
Diagrams showing formation of the axial process which then forms the notochord (also called axial mesoderm). This mesodermal column of cells determines the direction of embryonic disc folding and signals the differentiation of nearby mesoderm (somites) and overlying ectoderm (neural plate then tube).
The notochordal process begins as a fold of ectoderm extending cranially toward the prechordal plate region. The sequence of differentiation: notochordal process -> notochordal plate -> notochord.
(a) Elongation of the notochordal process cranially from the primitive pit as a hollow tube (notochordal canal) in the midline of the embryonic disc underlying the ectoderm.
(b) The notochordal canal may appear to break down on the endodermal side forming a notochordal plate continuous with the endodermal layer.
(c) Notochordal plate folds to form notochord. The notochord (also called axial mesoderm) is an embryonic structure that regulates differentiation of surrounding structures including the overlying ectoderm (neural plate) and mesoderm (somites).
(figures- Moore and Persaud, 1998)
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A simplified description of terms more detailed definitions are available from the Glossary link above.
conceptus - all cells produced from the zygote, including the embryo and extraembryonic tissues (fetal membranes and placenta) all cells will also have the same genetic origin/makeup.
cytotrophoblast "cellular" trophoblast layer surrounding (forming a "shell") the early implanting conceptus. Beginning at uterine adplantation, proliferation and fusion of these cells is thought to form a second outer trophoblast layer, thesyncytiotrophoblast. The cytotrophoblast layer contributes to formation of the placental villi, the functional component of the fetal placenta. (More? Week 2 Notes | Placenta Notes)
dysraphism (Greek, rhaphe = suture) clinical term used to describe defective fusion of neural folds. The position and degree of failure of fusion will result in either embryonic death or a range of different neural defects. The way (mode) in which the human neural tube fuses has been a source of contention. In humans, fusion appears to initiate at multiple sites but the mode is different from that found in many animal species used in developmental studies. (More? Neural Abnormalities | Neural Notes | Neural Tube Defects)
gastrulation - the process of forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm/mesoderm/ectoderm) and establish the initial body axis. These 3 germ layers will form all the future tissues of the entire embryo. (More? Gastrulation)
hypoblast - epithelium that forms during week 2 of human development that lines the blastoceol, and forms part of the bilaminar embryo (epiblast/hypoblast) from the inner cell mass. During trilaminar embryo development, the process of gastrulation replaces the hyoblast layer within the embryo with endoderm germ layer. (More? Week 2 Notes)
neural crest - cell region at edge of neural plate, then atop the neural folds, that remains outside and initially dorsal to the neural tube when it forms. These paired dorsal lateral streaks of cells migrate throughout the embryo and can differentiate into many different cell types (= pluripotential). Those that remain on the dorsal neural tube form the sensory spinal ganglia (DRG). Neural crest cells also migrate into the somites. (More? Neural Crest Notes)
neural groove - the second stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neural plate - the first stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neural tube - The third stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neuroenteric canal - canal or opening existing early in trilaminar embryo development that provides transient communication between the amnion and the yolk sac. Canal forms as part of axial process development, which is the precursor to the notochord (axial mesoderm). (More? Neural Notes)
neurogenesis - the process of formation of the neural system. This begins with segregation of the neural plate from the ectoderm of the trilaminar embryo by folding to form initially the neural groove, which then fuses to form the neural tube (the central nervous system progenitor, brain and spinal cord) and associated neural crest. (More? Neural Notes)
neuropore - the initial two openings at either end of neural tube are the neuropores: cranial=rostral=anterior, caudal=posterior. The cranial neuropore closes (day 25) approx. 2 days (human) before caudal. The developmental abnormality classed as Neural Tube Defects (NTDs) are mainly dure to failure of these two neuropores to developmentally close. (More? Neural Notes | Neural Abnormalities)
nidation - means implantation within the uterus. (More? Week 2 Notes)
notochord - (= axial mesoderm) rod of cells lying in the midline of the trilaminar embryo mesoderm layer ventral to the neural tube. Two roles: 1. Mechanical, influencing the folding of the early embryo; 2. Morphogenic, secreting sonic hedgehog a protein which regulates the development of surrounding tissues (neural plate, somites, endoderm and other organs). (More? Mesoderm Notes | Neural Notes)
syncytiotrophoblast a multinucleated cell currently thought to form by the fusion of cytotrophoblasts (trophoblasts) within the trophoblast layer (shell) of the implanting conceptus. In early development, these cells mediate implantation of the conceptus into the uterine wall and secrete the hormone (human Chorionic Gonadotrophin, hCG) responsible for feedback maintainance of the corpus luteum (in maternal ovary) and therefore maintaining early pregnancy. (More? Week 2 Notes | Placenta Notes | human Chorionic Gonadotrophin)
trophoblast - (Greek "trophe" = nutrition) the trophoblast cells have an important contribution to extraembryonic tissues of the fetal placenta and membranes. Week 1 blastocyst formation the outer layer of cells (adjacent to the zona pellucida) form a flat squamous epithelial layer of cells, the trophoblast layer. Week 2 following blastocyst hatching the trophoblast layer is involved with initial adhesion to the uterine wall and subsequent implantation within the wall. During this period the trophoblast layer differentiates into two distinct layers (syncitiotrophoblast, cytotrophoblast). (More? Week 2 Notes | Week 3 Notes | Placenta Notes)
uterus - the space within the mother where the embryo grows. It also grows when the embryo does and has lots of blood to feed the embryo and strong muscles in its walls for childbirth.
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