Editing Hemichordate (section)

== Development ==
Together with the [[echinoderms]], the hemichordates form the [[Ambulacraria]], which are the closest extant phylogenetic relatives of [[chordates]]. Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly.<ref>{{cite book | pmid = 15575607 | volume = 74 | year = 2004 | pages = [https://archive.org/details/isbn_9780124802780/page/171 171–194] | last1 = Lowe | first1 = CJ | last2 = Tagawa | first2 = K | last3 = Humphreys | first3 = T | last4 = Kirschner | first4 = M | last5 = Gerhart | first5 = J | title = Development of Sea Urchins, Ascidians, and Other Invertebrate Deuterostomes: Experimental Approaches | chapter = Hemichordate Embryos: Procurement, Culture, and Basic Methods | doi = 10.1016/S0091-679X(04)74008-X | series = Methods in Cell Biology | isbn = 9780124802780 | url = https://archive.org/details/isbn_9780124802780/page/171 | editor1= Charles A. Ettensohn| editor2= Gregory A. Wray| editor3= Gary M. Wessel }}</ref> Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm.

The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm.<ref>{{cite journal | last1 = Tagawa | first1 = K. | last2 = Nishino | first2 = A | last3 = Humphreys | first3 = T | last4 = Satoh | first4 = N. | date = 1 January 1998 | title = The Spawning and Early Development of the Hawaiian Acorn worm (Hemichordate), Ptycodhera flava | journal = Zoological Science | volume = 15 | issue = 1| pages = 85–91 | doi = 10.2108/zsj.15.85 | pmid = 18429670 | hdl = 2433/57230 | s2cid = 36332878 | hdl-access = free }}</ref> The Pterobranch genus most extensively studied is ''Rhabdopleura'' from Plymouth, England and from Bermuda.<ref>{{cite journal|last=Stebbing|first=ARD|title=Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)|journal=Marine Biology|year=1970|volume=5|pages=205–212|doi=10.1007/BF00346908|issue=3|bibcode=1970MarBi...5..205S |s2cid=84014156}}</ref><ref>{{cite journal|last=Dilly|first=PN|title=The larva of Rhabdopleura compacta (Hemichordata)|journal=Marine Biology|date=January 1973|volume=18|issue=1 |pages=69–86|doi=10.1007/BF00347923|bibcode=1973MarBi..18...69D |s2cid=86563917}}</ref><ref>{{cite journal|last=Lester|first=SM|title=Settlement and metamorphosis of Rhabdopleura normani (Hemichordata: Pterobranchia)|journal=Acta Zoologica|date=June 1988|volume=69|pages=111–120|doi=10.1111/j.1463-6395.1988.tb00907.x|issue=2}}</ref><ref>{{cite journal|last=Lester|first=SM|title=Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleura normani|journal=Acta Zoologica|year=1986|volume=69|pages=95–109|doi=10.1111/j.1463-6395.1988.tb00906.x|issue=2}}</ref>

The following details the development of two popularly studied species of the hemichordata phylum ''Saccoglossus kowalevskii'' and ''Ptychodera flava''. ''Saccoglossus kowalevskii'' is a direct developer and ''Ptychodera flava'' is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.

[[File:Hemichordate development.jpg|thumb|320px|right|Schematic of embryonic cleavage and development in ''P. flava'' and ''S. kowalevskii'']]

=== ''Ptychodera flava'' ===
''P. flava’s'' early cleavage pattern is similar to that of ''S. kowalevskii''. The first and second cleavages from the single cell zygote of ''P. flava'' are equal cleavages, are [[orthogonal]] to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four [[blastomere]]s both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. ''P. flava'' has a 16 cell embryo with four vegetal micromeres, eight animal [[mesomere]]s and four larger macromeres. Further divisions occur until ''P. flava'' finishes the [[blastula]] stage and goes on to [[gastrulation]]. The animal mesomeres of ''P. flava'' go on to give rise to the larva’s [[ectoderm]], animal [[blastomere]]s also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues.<ref>{{cite journal | pmid = 11806633 | volume=3 | issue=6 | title=Deuterostome evolution: early development in the enteropneust hemichordate, ''Ptychodera flava'' | date=November–December 2001| pages=375–90 | doi=10.1046/j.1525-142x.2001.01051.x | journal=Evolution & Development | last1 = Henry | first1 = JQ | last2 = Tagawa | first2 = K | last3 = Martindale | first3 = MQ| s2cid=24071389 }}</ref> Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development ''P. flava'' blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage.<ref>{{cite journal | last1 = Colwin | first1 = A | last2 = Colwin | first2 = L | year = 1950 | title = The developmental capacities of separated early blastomeres of an enteropneust, ''Saccoglossus kowalevskii'' | journal = Journal of Experimental Zoology | volume = 155 | issue = 2| pages = 263–296| doi = 10.1002/jez.1401150204  | bibcode = 1950JEZ...115..263C }}</ref>

=== ''Saccoglossus kowalevskii'' ===
Eggs of ''S. kowalevskii'' are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm.<ref>{{cite journal | last1 = Colwin | first1 = A | last2 = Colwin | first2 = L | year = 1951 | title = Relationships between the egg and larva of Saccoglossus kowalevskii (Enteropneusta): axes and planes; general prospective significance of the early blastomeres | journal = Journal of Experimental Zoology | volume = 117 | issue = 1 | pages = 111–138 | doi=10.1002/jez.1401170107| bibcode = 1951JEZ...117..111C }}</ref><ref>{{cite journal |first1=Arthur L. |last1=Colwin |first2=Laura Hunter |last2=Colwin |date=May 1953 |title=The normal embryology of saccoglossus kowalevskii (enteropneusta) | journal = Journal of Morphology | volume = 92 |issue=3 | pages = 401–453 | doi = 10.1002/jmor.1050920302 |s2cid=85420179 }}</ref>

=== Genetic control of dorsal-ventral hemichordate patterning ===
Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on ''S. kowalevskii'', and like chordates ''S. kowalevskii'' has dorsalizing bmp-like factors such as ''bmp 2/4'', which is homologous to ''Drosophila''’s decapentaplegic dpp. The expression of ''bmp2/4'' begins at the onset of gastrulation on the ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post-anal tail. The bmp antagonist chordin is also expressed in the [[endoderm]] of gastrulating ''S. kowalevskii''. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in ''S. kowalevskii'', such as a netrin that groups with netrin gene class 1 and 2.<ref name=Lowe2006 /> Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but ''S. kowalevskii'' was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.