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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




  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]