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Questions and Answers
What is the future function of the structure formed by the dilation of the terminal part of the hind gut?
What is the future function of the structure formed by the dilation of the terminal part of the hind gut?
What effect does lateral folding primarily have on the embryo?
What effect does lateral folding primarily have on the embryo?
Which part of the mesoderm gives rise to the axial skeleton?
Which part of the mesoderm gives rise to the axial skeleton?
What is the significance of the vitellointestinal duct formed during embryonic development?
What is the significance of the vitellointestinal duct formed during embryonic development?
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Which structure becomes ventral as a result of tail fold formation during embryonic development?
Which structure becomes ventral as a result of tail fold formation during embryonic development?
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What is the primary significance of embryonic folding during the development of the embryo?
What is the primary significance of embryonic folding during the development of the embryo?
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During the cephalocaudal folding, which structure is repositioned to lie ventral to the embryo?
During the cephalocaudal folding, which structure is repositioned to lie ventral to the embryo?
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Which statement accurately describes the tail fold during embryonic folding?
Which statement accurately describes the tail fold during embryonic folding?
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What structures are involved in the cranial part of the embryo after cephalocaudal folding?
What structures are involved in the cranial part of the embryo after cephalocaudal folding?
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When does the process of embryonic folding begin, and what is its primary outcome?
When does the process of embryonic folding begin, and what is its primary outcome?
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Study Notes
Embryonic Period
- The embryonic period, also called organogenesis, happens from week 3 to week 8 of development.
- This is when the three germ layers (ectoderm, mesoderm, and endoderm) give rise to tissues and organs.
- Embryonic folding occurs during this period.
Folding of the Embryo
- Definition: Flat trilaminar embryonic disc transforms into a cylindrical embryo.
- Timing: Begins at the end of the 3rd week and completes by the 4th week.
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Types:
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Cephalocaudal folding (head and tail folds): Driven by rapid longitudinal growth of the central nervous system.
- Head fold: Structures present in the midline of intraembryonic mesoderm (septum transversum, cardiogenic plate, oral membrane) are rearranged ventrally.
- Tail fold: Yolk sac incorporated into the cranial part of the embryo, forming foregut.
- Lateral folding (right and left folds): Due to rapid growth of somites (part of intraembryonic mesoderm). This leads to the formation of the midgut and a narrowing of the communication between the extraembryonic coelom and midgut (yolk stalk).
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Cephalocaudal folding (head and tail folds): Driven by rapid longitudinal growth of the central nervous system.
Results of Folding
- Cylindrical shape: The embryo adopts a cylindrical form.
- Gut formation: The gut forms.
- Craniocaudal arrangement: Structures like the buccopharyngeal membrane, heart, and septum transversum become arranged in a craniocaudal order, becoming ventral in position.
- Ventral position of structures: The cloacal membrane and allantois also move ventral in position.
- Umbilical cord formation: An umbilical cord forms, along with a ventral shifting of the connecting stalk.
- Ventral allantois: The allantois moves from a dorsal to a ventral position.
Differentiation of Germ Layers
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Secondary (intra-embryonic) mesoderm: Divided into three columns on each side of the notochord.
- Paraxial mesoderm: Lies medial to the notochord, divided into somites (crucial for axial skeleton, vertebral muscles, and skin).
- Intermediate cell mass (nephrogenic cord): Gives rise to the urinary and genital systems.
- Lateral plate mesoderm: Creates the intraembryonic coelom. It has two layers: somatic (body wall muscles) and splanchnic (visceral organs and lining of the gut).
Endodermal Derivatives
- Epithelial lining of the gastrointestinal, respiratory, and urinary tracts; parenchyma of the thyroid, parathyroids, liver, and pancreas.
Ectodermal Derivatives
- Central nervous system, peripheral nervous system, sensory epithelium (ear, nose, eye), skin, hair/nails, and glands (mammary, pituitary, sweat).
Neurulation
- Neural plate appears due to notochord influence.
- Neural plate folds forming the neural tube (beginnings of the CNS).
- Neuroectodermal cells form a neural crest between the surface ectoderm and the neural tube.
Clinical Correlations: Birth Defects
- Neural tube defects can arise from abnormal neurulation processes during the third to eighth weeks of development.
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