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6.4: Week 4 Neurulation

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    6.4.1 Embryonic Folding

    During week 4 the embryonic disc forms a cylindrical tube, with simultaneous folding occurring in both lateral and cephalocaudal directions. The somites curve down and under the caudal end, the heart prominence moves from near the end down to the middle of the embryo, and the neural groove curves up and over the cranial end of the embryo.

    Figure 6.12 Embryonic Folding
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    6.4.1.1 Folding Directions

    Folding directions: Cephalocaudal folding is over the ends and bows to the inside, bending like a banana, whereas lateral folding is around to the center along the embryo’s length like a tube.

    Figure 6.13 Folding Directions
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    6.4.1.2 Lateral Folding

    Lateral folding starts between day 19-21. First, the neural folds fuse to form the neural tube and the neural crest develops. Then the coelom grows and divides the lateral mesoderm into somatic mesoderm (the body wall) and splanchnic mesoderm (the gut). On day 22, the rapidly growing spinal cord and somites trigger lateral folding. The Somatic mesoderm abuts the ectoderm to form the body, the splanchnic mesoderm abuts the endoderm to form the gut. Initially, there is a wide connection between the gut and umbilical vesicle. On day 26 the lateral folding continues as the amniotic cavity expands around the embryo and a coelomic cavity forms around the gut as the umbilical vesicle extends out of the embryo. Finally on day 28, the lateral folding is complete. The embryo is within the amniotic cavity and the gut tube is surrounded by a coelom.

    Figure 6.14 Lateral Folding
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    6.4.1.3 Cephalocaudal Folding Animation

    Video 6.4

    Watch CEPHALOCAUDAL FOLDING Animation - From the Clinical Anatomy MENU, select Week 4 - 5.4 "Cephalocaudal folding animation"

    6.4.1.4 Basic Body Plan

    By the end of week 4, the embryo has completed both gastrulation and folding. The embryo has differentiated into three distinct layers of tissue: ectoderm, mesoderm and endoderm. It also has folded in two major directions: lateral folding and cephalocaudal folding. This leaves us with a basic body plan to start developing more complex organ structures as we start week 5. The embryo is shown in a transverse and longitudinal section.

    Figure 6.15 Basic Body Plan
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    6.4.1.5 Embryo Layers

    The embryo's three layers can further be divided into different types of tissues.

    • Ectoderm

    The ectoderm and organs that originate from ectoderm are shown labeled. The surface ectoderm is around the outside of all the tissues, the neuroectoderm comprise of the neural crest and neural tube.

    Figure 6.16 Ectoderm
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    • Mesoderm

    The mesoderm and organs that originate are categorized as paraxial mesoderm (including somites), intermediate mesoderm, and lateral mesoderm (including somatic mesoderm and splanchnic mesoderm.

    Figure 6.17 Mesoderm
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    • Endoderm

    The endoderm and organs that originate from the endoderm are present after lateral folding and form the gut tube and umbilical vesicle.

    Figure 6.18 Endoderm
    Image Source: University of British Columbia. Clinical Anatomy. (ND) This work is licensed under a Creative Commons Attribution NonCommercial-ShareAlike 4.0 International     License. Available from University of British Columbia

    6.4: Week 4 Neurulation is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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