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Questions and Answers
What condition describes the complete absence of a limb?
What condition describes the complete absence of a limb?
- Amelia (correct)
- Ectrodactyly
- Phocomelia
- Meromelia
Which limb defect is characterized by the partial absence of a limb?
Which limb defect is characterized by the partial absence of a limb?
- Polydactyly
- Meromelia (correct)
- Amelia
- Brachydactyly
What is the primary cause of syndactyly?
What is the primary cause of syndactyly?
- Insufficient apoptotic cell death between digital rays (correct)
- Excessive proliferation of mesoderm
- Additional sites of apoptotic cell death
- Development of abnormal AER
Ectrodactyly is often referred to as which of the following?
Ectrodactyly is often referred to as which of the following?
Which condition is characterized by the presence of extra digits?
Which condition is characterized by the presence of extra digits?
What is the role of the zone of polarizing activity (ZPA) in limb development?
What is the role of the zone of polarizing activity (ZPA) in limb development?
In which week do the first digital rays appear in the developing hand and foot?
In which week do the first digital rays appear in the developing hand and foot?
Which statement accurately describes limb bone differentiation?
Which statement accurately describes limb bone differentiation?
How does limb rotation differ between the upper and lower limbs?
How does limb rotation differ between the upper and lower limbs?
What is the primary result of the rotation of the lower limb during development?
What is the primary result of the rotation of the lower limb during development?
Which of the following describes the mechanism of digit formation from digital rays?
Which of the following describes the mechanism of digit formation from digital rays?
What is the primary cause of limb defects noted in clinical correlations?
What is the primary cause of limb defects noted in clinical correlations?
What is the primary signal produced by the zone of polarizing activity (ZPA)?
What is the primary signal produced by the zone of polarizing activity (ZPA)?
What is the primary origin of the posterior skull and vertebral column?
What is the primary origin of the posterior skull and vertebral column?
Which structure differentiates into muscle and contributes to the dermis of the back?
Which structure differentiates into muscle and contributes to the dermis of the back?
From which cells does the craniofacial skeleton develop?
From which cells does the craniofacial skeleton develop?
What is the segmental pattern of innervation in the adult derived from?
What is the segmental pattern of innervation in the adult derived from?
What is formed by the ventro-medial cells of the somite?
What is formed by the ventro-medial cells of the somite?
Which part of the somite is associated with intrinsic bones and muscles of the back?
Which part of the somite is associated with intrinsic bones and muscles of the back?
Which mesodermal structure gives rise to the sternum and appendicular skeleton?
Which mesodermal structure gives rise to the sternum and appendicular skeleton?
During somite differentiation, what signals the ventro-medial cells to become mesenchymal?
During somite differentiation, what signals the ventro-medial cells to become mesenchymal?
Which of the following is the correct order of development from somites?
Which of the following is the correct order of development from somites?
What does the hypomere develop into?
What does the hypomere develop into?
What is the process by which sclerotome cells organize around the neural tube?
What is the process by which sclerotome cells organize around the neural tube?
Which part of the sclerotome fuses with the cranial half of the adjacent sclerotome?
Which part of the sclerotome fuses with the cranial half of the adjacent sclerotome?
What happens to the notochord during the formation of vertebrae?
What happens to the notochord during the formation of vertebrae?
What is the cause of congenital scoliosis?
What is the cause of congenital scoliosis?
Which condition results from incomplete resegmentation of sclerotomes?
Which condition results from incomplete resegmentation of sclerotomes?
Which embryonic structure gives rise to all skeletal muscle?
Which embryonic structure gives rise to all skeletal muscle?
What are the muscles derived from epimere called?
What are the muscles derived from epimere called?
What is the primary event occurring during the limb bud development at week 4?
What is the primary event occurring during the limb bud development at week 4?
What signals the overlying ectoderm to grow during limb bud development?
What signals the overlying ectoderm to grow during limb bud development?
Which domain contains only somite-derived cells?
Which domain contains only somite-derived cells?
What condition is characterized by a depressed sternum?
What condition is characterized by a depressed sternum?
Which of the following correctly describes the structure of ribs?
Which of the following correctly describes the structure of ribs?
What is the effect of Lmx-1 expression in limb development?
What is the effect of Lmx-1 expression in limb development?
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Study Notes
Introduction to Musculoskeletal Development
- Objectives: Understand cell types from somites, development of axial and appendicular skeletons, and limb development.
- Timeline: Development occurs around Week 5 (35-38 days into gestation).
Origin of Skeletal Tissue
- Paraxial Mesoderm: Forms posterior skull, vertebral column, and ribs.
- Lateral Plate Mesoderm: Gives rise to sternum and appendicular skeleton.
- Neural Crest Cells: Contribute to craniofacial skeleton.
Somite Structure and Function
- Somites: Segmental structures lateral to the neural tube that differentiate into various tissues.
- Differentiation:
- Ventro-medial cells become mesenchymal, forming sclerotome (tendons, ligaments, bones).
- Dorso-lateral cells form dermomyotome (muscle, dermis).
Innervation of Somites
- Each somite correlates with one spinal nerve, leading to segmental innervation patterns for muscles and skin.
- Divisions:
- Epimere: Associated with dorsal rami, forming intrinsic back structures.
- Hypomere: Associated with ventral rami, forming muscles/connective tissues of the trunk and limbs.
Axial Skeleton Development
- Vertebrae Resegmentation:
- Sclerotome cells reorganize around the neural tube before they split and fuse to form vertebrae.
- Original notochord transforms into nucleus pulposus.
- Clinical Correlations:
- Congenital Scoliosis: Results from hemivertebra formation due to partial vertebra development.
- Klippel-Feil Syndrome: Incomplete resegmentation causes fused vertebrae, leading to a short neck, low hairline, and limited neck motion.
Formation of Ribs and Sternum
- Ribs: Develop from sclerotomes that form thoracic vertebrae with three embryonic structures (centrum, neural arch, costal process).
- Sternum: Unique as it originates from parietal lateral plate mesoderm.
Appendicular Skeleton Development
- Limbs derive from parietal lateral plate mesoderm.
- Skeletal Muscle Development:
- Somites are the source of all skeletal muscle; epaxial (intrinsic back) and hypaxial (other skeletal muscle) classifications based on somite origin.
Limb Bud Formation
- Limb Buds: Upper limbs appear in Week 4, lower limbs in Week 5, consisting of ectodermal caps and mesodermal cores.
- Apical Ectoderm Ridge (AER): Crucial for limb growth, directing mesoderm proliferation via signaling molecules (FGF-10 and FGF-8).
Limb Axis and Differentiation
- Proximal/Distal Axis: AER signals distal limb formation; dorsal/ventral axis differentiation involves homeobox genes like Lmx-1.
- Cranial/Caudal Axis: Zone of Polarizing Activity (ZPA) directs digit positioning via Sonic hedgehog (SHH) signaling, contributing to varying digit lengths.
Limb Bone Development
- Differentiation: Limb bones arise from proximal to distal: humerus/femur (stylopod), radius/ulna/tibia/fibula (zeugopod), and carpals/metacarpals/phalanges/tarsals/metatarsals (autopod).
- Ossification: Bones form through endochondral ossification, starting from a cartilage template.
Digital Formation and Limb Rotation
- Digital Rays: Form by apoptosis in the AER; complete separation of digits typically occurs by Week 8.
- Rotation:
- Upper limbs rotate laterally (extensor muscles lateral).
- Lower limbs rotate medially (extensor muscles anterior).
Limb Innervation
- Dermatomes: Skin regions supplied by specific spinal nerves; limb rotation modifies original innervation patterns.
- Motor Innervation: Reflects segmental development; includes specific mappings for upper limb muscle innervation.
Clinical Correlations in Limb Development
- Limb Defects:
- Amelia: Complete limb absence.
- Meromelia: Partial limb absence.
- Phocomelia: Absence of long limb bones.
- Brachydactyly: Shortened digits from inadequate proliferation.
- Syndactyly: Fused digits from insufficient apoptosis.
- Polydactyly: Extra digits resulting from additional apoptotic site development.
- Ectrodactyly: Missing digits leading to cleft conditions.
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