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Questions and Answers
What are the two layers of the lateral plate mesoderm called?
What are the two layers of the lateral plate mesoderm called?
How does somitomere formation progress in the embryo?
How does somitomere formation progress in the embryo?
Which of the following correctly represents the number of segments in somites from the occipital region caudally?
Which of the following correctly represents the number of segments in somites from the occipital region caudally?
What signifies the development stage of an embryo in terms of somite count?
What signifies the development stage of an embryo in terms of somite count?
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Which statement about the fate of somites is true?
Which statement about the fate of somites is true?
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What is the significance of the apical ectodermal ridge (AER) in limb development?
What is the significance of the apical ectodermal ridge (AER) in limb development?
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During the formation of digits, what process leads to the separation of the apical ectodermal ridge into five parts?
During the formation of digits, what process leads to the separation of the apical ectodermal ridge into five parts?
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What do the rotations of the upper and lower limbs during development signify?
What do the rotations of the upper and lower limbs during development signify?
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At what stage of development do the first hyaline cartilage models of the extremities begin to form?
At what stage of development do the first hyaline cartilage models of the extremities begin to form?
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What process leads to the formation of joints during limb development?
What process leads to the formation of joints during limb development?
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Study Notes
Organization of the Mesoderm
- Mesoderm is classified into three regions: paraxial, intermediate, and lateral plate mesoderm.
- Lateral plate mesoderm divides into somatic (parietal) and splanchnic (visceral) layers, lining the intraembryonic cavity.
- Intermediate mesoderm links paraxial and lateral mesoderm and plays a vital role in development.
Somitomeres and Somites
- In the third week, paraxial mesoderm organizes into somitomeres, appearing first in the cephalic region.
- Somitomeres consist of concentric mesodermal cell whorls and contribute to head mesenchyme.
- Somitomeres caudally organize into somites: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal pairs.
- Somites form the axial skeleton and indicate the age of the embryo.
Development of the Skull
- The skull develops from mesodermal and neural crest cells, forming the cranial vault and base.
- Newly formed skull in newborns shows fontanelles and sutures, crucial for brain growth and passage through birth canal.
Development of the Limbs
- Apical ectodermal ridge (AER) forms, influencing mesenchymal cells in limb buds to remain undifferentiated.
- Limb development occurs proximodistally; fingers and toes form via programmed cell death in AER.
- Upper limbs develop faster than lower limbs, rotating 90° laterally; lower limbs rotate medially.
Formation of Joints
- Joints arise from interzones in cartilaginous condensations where chondrogenesis stops, leading to joint cavity formation through cell death.
- Joint capsules differentiate from surrounding cells to create functional joints.
Clinical Correlates: Bone Age
- Radiologists assess ossification centers via X-rays to determine children's bone maturation.
- Ultrasonography can provide insights on fetal growth and gestational age.
Limb Abnormalities
- Abnormalities include meromelia (partial absence), amelia (complete absence), phocomelia (rudimentary limbs), and micromelia (short limbs).
- Additional anomalies: polydactyly (extra digits), ectrodactyly (absence of digits), syndactyly (fused digits).
Vertebral Column, Ribs, and Sternum
- Sclerotome cells envelop spinal cord and notochord, forming vertebral column structures.
- Notochord degenerates, forming nucleus pulposus; annulus fibrosus arises from surrounding tissue.
- Ribs originate from thoracic vertebrae's costal processes, while the sternum develops from somatic mesoderm and forms through fusion of sternal bands.
Clinical Correlates: Vertebral Defects
- Asymmetrical fusion or absence of vertebrae can lead to scoliosis.
- Spina bifida results from imperfect vertebral arch fusion; outcomes range from spina bifida occulta (no neurological deficits) to spina bifida cystica (exposed neural tissue).
Molecular Regulations of Bone Development
- Hox genes determine limb positioning along the craniocaudal axis and influence bone shape.
- Fibroblast growth factors (FGFs) are critical for regulating growth along three body axes.
- Bone morphogenetic proteins (BMPs) induce AER formation, guiding limb development.
Muscular System Development
- Muscular system (skeletal, smooth, cardiac muscle) primarily derives from mesodermal layer.
- Skeletal muscle originates from paraxial mesoderm; myogenic precursors arise from somatic and splanchnic mesoderm.
- Myogenesis is marked by the elongation of myoblasts and their fusion into myotubes, leading to skeletal muscle formation.
Molecular Regulations of Muscle Development
- BMP4 and FGFs from lateral plate mesoderm signal gene expression for muscle formation.
- MYO-D and MYF5, key muscle-specific genes, activate pathways for myoblast development into myofibers, defining skeletal muscle structure.
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Description
This quiz covers the intricate development of the skeletal system, focusing on the organization of mesoderm, development of the skull and limbs, as well as clinical correlates. It includes important details on somitomeres, somites, and molecular regulations of bone development, making it essential for understanding human anatomy.
