Evolution of the notochord

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

	Amphioxus larvae and adults	Tunicate larvae	Vertebrate embryos^a
Cell types	I. Discoidal cells (stacked like coins^b; each cell containing transverse myofilaments) II. Müller cells: sparsely distributed; no known function	Early larva: discoidal cells (stacked like coins^b; no myofilaments) Late larva: above cells change to squamous epithelium around fluid-filled lumen^c	I. Inner core cells^d: each with a large vacuole; no myofilaments II. Surrounding epithelial cell layer
Extracellular sheath	Inside to out: external lamina^e, circular collagen layer, and longitudinal collagen layer^f	External lamina^e	Inside to out: external lamina^e, circular collagen layer, and longitudinal collagen layer^f
Organizer genes involved in notochord formation^g	Comparable to those of vertebrates	Highly divergent^h	Comparable to those of amphioxus
Hedgehog from notochord involved in patterning central nervous system	Yesⁱ	No^j	Yes

^aAlso in adults of more basal vertebrates (e.g., cyclostomes, sturgeons)
^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]
^cTransitional stages described for ascidians in [18]; there is a similar arrangement in adult appendicularian tunicates [20]
^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]
^eAlso sometimes termed the basal lamina or the elastica interna
^fAlso sometimes termed the elastica externa
^gDiscussed in [22]
^hDiscussed in [113]
ⁱDiscussed in [23]
^jDiscussed in [24]

ISSN: 2041-9139