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Questions and Answers
At what day does the cranial neuropore close?
At what day does the cranial neuropore close?
Failure of neural tube closure can result in spina bifida.
Failure of neural tube closure can result in spina bifida.
True
What do neural crest cells contribute to in the peripheral nervous system?
What do neural crest cells contribute to in the peripheral nervous system?
Sensory and autonomic ganglia, Schwann cells, satellite cells
The __________ is a developmental disorder resulting from incomplete closure of the cranial neuropore.
The __________ is a developmental disorder resulting from incomplete closure of the cranial neuropore.
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Match the following structures with their contributions from neural crest cells:
Match the following structures with their contributions from neural crest cells:
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Which of the following is NOT a consequence of abnormal neural crest cell migration?
Which of the following is NOT a consequence of abnormal neural crest cell migration?
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Neural crest cells are multipotent and arise from the ectoderm during neurulation.
Neural crest cells are multipotent and arise from the ectoderm during neurulation.
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Name one structure contributed by neural crest cells in the craniofacial region.
Name one structure contributed by neural crest cells in the craniofacial region.
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What is the primary function of sclerotome cells in vertebral formation?
What is the primary function of sclerotome cells in vertebral formation?
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Ribs are developed from costal processes extended from the sclerotome in the lumbar region.
Ribs are developed from costal processes extended from the sclerotome in the lumbar region.
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What process involves the division of each sclerotome into cranial and caudal halves?
What process involves the division of each sclerotome into cranial and caudal halves?
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Abnormal sclerotome development can lead to __________ defects such as spina bifida.
Abnormal sclerotome development can lead to __________ defects such as spina bifida.
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The dermatome contributes to which part of the body?
The dermatome contributes to which part of the body?
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Match the following muscle types with their locations and innervation:
Match the following muscle types with their locations and innervation:
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Disruptions in myotome development can lead to Poland syndrome.
Disruptions in myotome development can lead to Poland syndrome.
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Myotomal cells differentiate into __________ muscles and __________ muscles in the limbs.
Myotomal cells differentiate into __________ muscles and __________ muscles in the limbs.
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What is the axial skeleton composed of?
What is the axial skeleton composed of?
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Which of the following is NOT one of the primary brain vesicles?
Which of the following is NOT one of the primary brain vesicles?
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The midbrain divides into two secondary vesicles.
The midbrain divides into two secondary vesicles.
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What structures are formed by the metencephalon?
What structures are formed by the metencephalon?
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The __________ is responsible for processing visual and auditory information.
The __________ is responsible for processing visual and auditory information.
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Match the secondary brain vesicle with its corresponding structure:
Match the secondary brain vesicle with its corresponding structure:
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Which structure does the diencephalon NOT form?
Which structure does the diencephalon NOT form?
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Holoprosencephaly is a disorder characterized by the division of the prosencephalon into hemispheres.
Holoprosencephaly is a disorder characterized by the division of the prosencephalon into hemispheres.
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What is microcephaly commonly linked to?
What is microcephaly commonly linked to?
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The __________ forms the cortex responsible for higher cognitive functions.
The __________ forms the cortex responsible for higher cognitive functions.
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Which part of the brainstem is responsible for vital reflexes and autonomic functions?
Which part of the brainstem is responsible for vital reflexes and autonomic functions?
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What does the notochord persist as in the adult human body?
What does the notochord persist as in the adult human body?
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The heart begins to beat around day 30 of embryonic development.
The heart begins to beat around day 30 of embryonic development.
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What are the three components formed from somites?
What are the three components formed from somites?
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The _____ mesoderm gives rise to the two endocardial tubes during heart development.
The _____ mesoderm gives rise to the two endocardial tubes during heart development.
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Match the following somite components with the structures they form:
Match the following somite components with the structures they form:
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Which of the following is NOT a role of the mesenchyme in limb development?
Which of the following is NOT a role of the mesenchyme in limb development?
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The cardiovascular system is the first functional organ system to form during embryonic development.
The cardiovascular system is the first functional organ system to form during embryonic development.
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What happens around day 21 in heart tube formation?
What happens around day 21 in heart tube formation?
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The _____ region is where the heart forms initially as a straight tubular structure.
The _____ region is where the heart forms initially as a straight tubular structure.
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Which part of the heart tube is responsible for receiving blood?
Which part of the heart tube is responsible for receiving blood?
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Which mesoderm contributes mesenchymal cells for skeletal and connective tissue components of the limbs?
Which mesoderm contributes mesenchymal cells for skeletal and connective tissue components of the limbs?
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The ectoderm contributes only to the outer covering of the limb buds.
The ectoderm contributes only to the outer covering of the limb buds.
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What role does the AER play in limb development?
What role does the AER play in limb development?
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During limb development, disruptions can lead to conditions such as _______ and _______.
During limb development, disruptions can lead to conditions such as _______ and _______.
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What regulates the Anterior-Posterior Axis during limb development?
What regulates the Anterior-Posterior Axis during limb development?
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Match the following conditions to their descriptions:
Match the following conditions to their descriptions:
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At what days do the upper and lower limb buds appear?
At what days do the upper and lower limb buds appear?
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Study Notes
Week 4 of Embryonic Development
- Embryonic development in the fourth week is crucial for establishing the groundwork for organ systems.
- Morphological changes, particularly folding, are pivotal.
Morphological Changes: Folding of the Embryo
Cranial Folding
- Embryo's cranial region bends ventrally due to rapid neural tube and somite growth.
- Forebrain develops prominently, placed above the primitive heart.
- Oropharyngeal membrane and stomodeum (future mouth) shift to defined positions.
Caudal Folding
- Caudal region folds ventrally due to neural tube lengthening.
- Connecting stalk (future umbilical cord) shifts ventrally to align with gut tube and other embryonic structures.
Lateral Folding
- Lateral edges of embryonic disc fold inward.
- Intraembryonic coelom closes, forming primitive thoracic and abdominal cavities.
- Gut tube is enclosed, and will later develop into the gastrointestinal tract.
Nervous System Development
- Neurulation is the process forming the neural tube, the foundation of the central nervous system (brain and spinal cord).
- The process begins in the third week and is completed by the fourth week.
- First steps involve the formation of the neural plate, the neural plate shaping and folding, and the neural tube closure.
Neurulation Steps
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Formation of the Neural Plate: Ectoderm thickens above the notochord, influenced by signaling molecules (like Sonic Hedgehog). This marks the start of CNS development.
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Shaping and Folding: Lateral edges of the neural plate elevate to create neural folds and a neural groove, which eventually fuse.
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Closure of the Neural Tube: Starts in the cervical region and progresses bidirectionally (cranial and caudal). Neuropores (temporary openings) close around days 25 (cranial) and 27 (caudal).
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Neural tube closure failure can lead to spina bifida (incomplete caudal neuropore closure) or anencephaly (incomplete cranial neuropore closure, resulting in a lack of brain development).
Neural Crest Cells
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Neural crest cells are multipotent and arise at the neural plate/ectoderm junction.
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They undergo EMT (epithelial-to-mesenchymal transition) and migrate extensively through the embryo.
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Peripheral Nervous System: Contributing to sensory and autonomic ganglia (e.g. dorsal root ganglia), Schwann cells, and satellite cells (support cells for peripheral nerves).
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Craniofacial Structures: Contributing to cartilage, bones, and connective tissues of the face and skull.
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Endocrine and Cardiac Structures: Contributing to adrenal medulla, and parts of the heart.
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Pigment Cells: Contributing to melanocytes, responsible for skin pigmentation.
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Other Structures: Contributing to some meninges, and some blood vessel smooth muscle.
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Abnormalities in neural crest cell migration or differentiation can lead to conditions such as Hirschsprung disease, cleft palate, and neurofibromatosis.
Brain Development
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The cranial end of the neural tube expands and differentiates into the brain.
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This happens in stages, starting with primary brain vesicles (forebrain, midbrain, hindbrain) which further divide into secondary vesicles.
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Forebrain forms the cerebral hemispheres and lateral ventricles, as well as the diencephalon (thalamus, hypothalamus, epithalamus, third ventricle).
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Midbrain remains as a single vesicle, forming parts of the brainstem (tectum and tegmentum), pathways for visual and auditory processing.
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Hindbrain divides into metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata).
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Brain structures like the cerebral hemispheres expand and develop, becoming responsible for higher cognitive functions like memory, reasoning, and voluntary movements.
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Brainstem structures have vital reflexes and autonomic functions
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Spinal cord develops from caudal neural tube, elongating as the body develops.
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Disorders of brain development include holoprosencephaly and microcephaly.
Musculoskeletal System Development
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Somites, paired blocks of mesoderm, contribute to vertebrae, ribs, skeletal muscles, and dermis.
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Somites originate from paraxial mesoderm, adjacent to the neural tube and notochord.
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Somites segment into paired structures along the cranial-to-caudal axis.
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Somites develop at a rate of about 3-4 pairs per day until the end of the fifth week, resulting in 42-44 pairs of somites. Some undergo resorption. Leaving a human embryo with 37 pairs.
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Sclerotome: Forms vertebrae and ribs, and migrates medially toward the notochord and neural tube.
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Dermatome: Forms connective tissue (dermis) of the skin.
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Myotome: Forms skeletal muscles of the trunk and limbs.
Cardiovascular System Development
- The heart forms as a straight tubular structure from the mesoderm in the cardiogenic region.
- Key steps in heart tube formation include the migration of cardiogenic mesoderm, the formation of endocardial tubes, and the merging of these tubes into a single heart tube.
- The tube elongates, segments into regions (sinus venosus, primitive atrium, primitive ventricle, bulbus cordis, and truncus arteriosus).
- The heart begins to beat around day 22.
- Primitive blood vessels form through vasculogenesis and angiogenesis and connect to the heart tube to form initial circulatory circuits.
- Circulation is established via the vitelline and umbilical circulations which allow for nutrient, oxygen, and waste exchange with the yolk sac and placenta, respectively.
Limb Bud Formation
- Limbs develop from lateral plate mesoderm, with mesenchyme surrounded by ectoderm.
- The mesenchyme condenses and differentiates into cartilage, which eventually ossifies.
- Limb bud formation involves timing and location, as well as cellular composition (mesenchymal core from lateral plate mesoderm, apical ectodermal ridge, or AER formed from ectoderm).
- Signaling pathways, like FGFs and SHH, plays significant role in limb development.
Pharyngeal Arches
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Pharyngeal arches appear in the developing embryo during the fourth week.
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Each arch has a core of mesoderm and neural crest cells, an outer layer of ectoderm, an inner lining of endoderm, and its own artery, muscles, nerves, and cartilage components.
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The first pharyngeal arch (mandibular arch) forms parts of the upper jaw and lower jaw, muscles of mastication, and the mandibular division of the trigeminal nerve (CN V3).
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The second pharyngeal arch (hyoid arch) forms parts of the hyoid apparatus, muscles of facial expression, and facial nerve (CN VII).
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Description
Explore the critical morphological changes during the fourth week of embryonic development, including cranial, caudal, and lateral folding. Understand how these processes lay the foundation for organ systems and the development of the nervous system.
