Lecture - Week 3 Development
2010 Lecture | placentation | gastrulation | Notochord
Objectives
- Understand broadly the events of week 3 of human development
- Understand the process early placentation, villi formation
- Understand the process of gastrulation
- Understand the process of axis formation
- Brief understanding of folding
References
The Developing Human: Clinically oriented embryology
Citation: The Developing Human: clinically oriented embryology 9th ed. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. Philadelphia, PA: Saunders, 2011. |
Larsen's human embryology
Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009. |
UNSW Embryology
Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW. |
Movies
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Template:Week 3 notochord movie |
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Early Placentation
The trophoblast layer has now differentiated into two morphologically distinct cellular layers.
- Syncitiotrophoblasts - form a multinucleated cytoplasmic mass by cytotrophoblast cell fusion and both invade the decidua and secrete hCG
- Cytotrophoblasts - form a cellular layer around the blastocyst, proliferates and extends behind syncitiotrophoblasts
Early Utero-Placental exchange
- transfer of nutrition from maternal lacunae filled with secretions from uterine glands and maternal blood from blood vessels.
- development of trophoblast villi extending into the uterine decidua.
- ongoing process with placental development (covered in future lecture)
There are three stages of villi development:
- Primary Villi - cytotrophoblast
- Secondary Villi - cytotrophoblast + extraembryonic mesoderm
- Tertiary Villi - cytotrophoblast + extraembryonic mesoderm+ blood vessels
There are two main types of early villi:
- Anchoring villi - attached to decidua
- Floating villi - not attached to decidua, floating in maternal lacunae.
Gastrulation
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Embryonic Disc showing primitive streak |
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Primitive StreakThe primitive streak is the visible feature which represents the site of cell migration to form the additional layers.
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Epithelial to Mesenchymal Transition
- Epithelial cells (organised cellular layer) which loose their organisation and migrate/proliferate as a mesenchymal cells (disorganised cellular layers) are said to have undergone an Epithelial Mesenchymal Transition (EMT).
- Mesenchymal cells have an embryonic connective tissue-like cellular arrangement
- cells that have undergone this process may at a later time and under specific signaling conditions undergo the opposite process, mesenchyme to epithelia.
- In development, this process can be repeated several times during tissue differentiation.
This process occurs at the primitive streak where epiblast cells undergo an epithelial to mesenchymal transition in order to delaminate and migrate.
MH - there are a number of common cellular changes that occur during embryonic development at different times and in different tissues, which we can classify into "developmental mechanisms".
Notochord
- The notochord is a structure which has an early mechanical role in embryonic disc folding and a major signaling role in patterning surrounding embryonic tissue development.
- A developmental feature not present in the adult anatomy.
- This signaling role patterns many different tissues (neural plate, neural tube, somites, endodermal organs).
- Has own sequence of development from a primitive axial process
- axial process an initial epiblast hollow epithelial tube which extends in the midline from the primitive pit, cranially in the embryonic disc (toward the oral membrane).
- neuroenteric canal is a transient communication between the amnionic cavity and the yolk sac cavity formed by the axial process.
- notochordal plate forms from the axial process merging with the endoderm layer.
- notochord forms from the notochordal plate which then separates back into the mesoderm layer as a solid column of cells lying in the midline of the embryonic disc and running rostro-caudally (head to tail).
- An alternate name for the notochord is "axial mesoderm".
MH - Much of our knowledge of this structure comes from the study of animal models of development.
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Template:Week 3 notochord movie |
Embryo Folding
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Embryonic Disc Folding - all edges of the embryonic disc will fold ventrally, forming a rostro-caudal "C" shaped tube. |
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Timeline
Human Development |
Page | Play |
Next: Lab 2 - Fertilization/Week 1 and 2
Glossary Links
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Cite this page: Hill, M.A. (2024, June 14) Embryology Lecture - Week 3 Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Week_3_Development
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G