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#notochord

13 public questions tagged with this topic.

Specification Determination-ll

When the 4 blastomere pairs of an 8-cell stage tunicate embryo are dissociated, each forms most of the structures it wou

Tunicate embryos at the eight-cell stage exhibit predominantly autonomous development, as dissociated blastomere pairs still differentiate into most tissues normally derived from them, reflecting segregation of determinants like macho-1 for muscle. However, notochord and nervous system lineages deviate from this rule, failing to form in isolation and requiring cell-cell interaction between animal and vegetal partners. Inductive signals involving FGF and BMP antagonism from presumptive notochord precursors influence neighboring ectoderm. Therefore most lineages demonstrate autonomous specification, while notochord and neural fate exemplifies conditional specification dependent on extrinsic instructive communication for proper induction.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Tunicates and Conditional Specification of Notochord.

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Animal Classification

Cephalochordates retain notochord

Cephalochordates exemplified by Branchiostoma lanceolatum or amphioxus retain notochord as persistent axial skeleton extending from anterior tip beyond cerebral vesicle to posterior tip of tail throughout entire life, without replacement by cartilage or bone. Notochord consists of stacked vacuolated myofibril-containing lamellar cells capable of slow contraction. Dorsal nerve cord lies above, segmental myotomes alongside. Lifelong retention illustrates primitive chordate condition before vertebral evolution and contrasts sharply with tunicates where notochord exists only in larval tail and vertebrates where it is transient embryonic structure replaced during skeletogenesis, making it model for chordate ancestry.

Ref: Campbell Biology, 12th ed., Chapter 34: Cephalochordata; NCBI Bookshelf, Amphioxus Notochord Persistence

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Animal Classification

Notochord in vertebrates is replaced by

During vertebrate embryogenesis, notochord serves as axial scaffold and midline signaling organizer secreting Sonic hedgehog and chordin, patterning neural tube and somites. Paraxial mesoderm sclerotome migrates around notochord and neural tube, forming cartilaginous vertebrae that later ossify into vertebral column. Notochord regresses largely, vestiges remaining as nucleus pulposus inside intervertebral discs. Thus vertebral bodies replace notochordal support providing rigid yet flexible axis for muscle attachment, weight bearing and protection of spinal cord. Cartilage alone, exoskeleton or skull do not fully substitute notochord, making vertebral column its adult replacement.

Ref: Campbell Biology, 12th ed., Chapter 34: Vertebrate Skeleton; NCBI Bookshelf, Notochord to Vertebral Column

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