Questions and Answers
What significant process begins around day 14 of human development?
Which part of the embryo is primarily associated with the formation of the primitive groove?
What anatomical reference does the primitive groove define in the embryo?
What forms at the cranial end of the primitive groove during gastrulation?
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How does the shape of the bilaminar disc change during gastrulation?
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What happens to the epiblast cells that migrate through the primitive groove?
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What is the term for the normal left-right symmetry that begins to form during the development of the groove?
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What structure forms alongside the primitive node to create the primitive streak?
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What is the role of epiblast cells in forming the embryonic endoderm layer?
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What defines multipotent cells in the germ layers?
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Which structures do the cranial and caudal bilaminar regions develop into?
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What is the fate of the notochord in adult life?
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What occurs around day 15 of development concerning the ectoderm layer?
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Which of the following describes the composition of the trilaminar disc?
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What key feature distinguishes the extraembryonic mesoderm?
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What is the significance of the notochord during early development?
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Study Notes
Human Development Week 3
- Blastocyst becomes fully embedded in endometrial tissue, known as decidua, around week 3.
- Gastrulation starts approximately on day 14, transforming the blastocyst into a gastrula through significant cellular reorganization.
Gastrulation Process
- Formation of the primitive groove, a narrow depression in the epiblast, marks the beginning of gastrulation.
- The groove establishes the cranial-caudal axis, showcasing initial bilateral symmetry: left and right sides of the embryo.
- Medial refers to the area closer to the midline (groove), while lateral refers to the edges.
- The primitive groove is located on the dorsal side of the embryo, making the dorsal-ventral axis apparent.
Morphological Changes
- The bilaminar disc elongates, resembling a guitar pick; narrow at the caudal end and wide at the cranial end.
- The primitive node, a mound of tissue at the cranial end of the groove, contains the primitive pit.
Cell Migration and Differentiation
- Epiblast cells migrate towards the primitive groove, descend into it, and differentiate into new cell layers.
- Cells that dive deeply form the embryonic endoderm, replacing the ventral hypoblast layer.
- More shallowly migrating cells become the embryonic mesoderm, which integrates with pre-existing extraembryonic mesoderm, dividing it into parietal and visceral layers.
- Remaining epiblast cells generate the embryonic ectoderm layer.
Formation of Germ Layers
- Results in a trilaminar disc consisting of ectoderm, mesoderm, and endoderm—collectively known as germ layers, which are multipotent and can differentiate into various tissues and organs.
Bilaminar Regions Formation
- By day 15, two ectoderm areas (cranial and caudal) push ventrally and fuse with the endoderm, creating bilaminar regions.
- Cranial bilaminar region develops into the oropharyngeal membrane, which disintegrates in week 4, forming the mouth opening.
- Caudal bilaminar region forms the cloacal membrane, disintegrating in week 7 to create the anus and genitourinary tracts.
Notochord Development
- Around day 17, mesoderm cells form the notochord, a solid rod along the craniocaudal axis.
- The notochord is transient and contributes to the nucleus pulposus of intervertebral discs in adults.
- Critical for early development, serving significant signaling roles and structural support.
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Description
This quiz covers key concepts of human development during week 3, focusing on the process of gastrulation and the transformation from blastocyst to gastrula. Understand the formation of the primitive groove and the significance of these early developmental stages.
