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Table 2 Salient features of notochords compared among amphioxus, tunicates, and vertebrates

From: Evolution of the notochord

  Amphioxus larvae and adults Tunicate larvae Vertebrate embryosa
Cell types I. Discoidal cells (stacked like coinsb; each cell containing transverse myofilaments) II. Müller cells: sparsely distributed; no known function Early larva: discoidal cells (stacked like coinsb; no myofilaments)
Late larva: above cells change to squamous epithelium around fluid-filled lumenc
I. Inner core cellsd: each with a large vacuole; no myofilaments
II. Surrounding epithelial cell layer
Extracellular sheath Inside to out: external laminae, circular collagen layer, and longitudinal collagen layerf External laminae Inside to out: external laminae, circular collagen layer, and longitudinal collagen layerf
Organizer genes involved in notochord formationg Comparable to those of vertebrates Highly divergenth Comparable to those of amphioxus
Hedgehog from notochord involved in patterning central nervous system Yesi Noj Yes
  1. aAlso in adults of more basal vertebrates (e.g., cyclostomes, sturgeons)
  2. bDuring notochord formation, this arrangement is attained rapidly by inconspicuous cell movements [18, 78]; the more marked cell migrations and intercalations establishing the vertebrate notochord have been termed convergent extension [19]
  3. cTransitional stages described for ascidians in [18]; there is a similar arrangement in adult appendicularian tunicates [20]
  4. dOften considered a kind of cartilage [15]; the inner core cells are embedded in an extracellular matrix that is disconcertingly scanty for cartilage, although including some macromolecules characteristic of that tissue [21]
  5. eAlso sometimes termed the basal lamina or the elastica interna
  6. fAlso sometimes termed the elastica externa
  7. gDiscussed in [22]
  8. hDiscussed in [113]
  9. iDiscussed in [23]
  10. jDiscussed in [24]