Composition of the protocerebrum explained

See also: Arthropod head problem.

The protocerebrum is the first segment of the panarthropod brain.

Recent studies suggest that it comprises two regions.

Region associated with the expression of six3

six3 is a transcription factor that marks the anteriormost part of the developing body in a whole host of Metazoa.

In the panarthropod brain, the anteriormost (rostralmost) part of the germband expresses six3. This region is described as medial, and corresponds to the annelid prostomium.^[1]

In arthropods, it contains the pars intercerebralis and pars lateralis.^[1]

six3 is associated with the euarthropod labrum^[2] and the onychophoran frontal appendages (antennae).

Region associated with the expression of orthodenticle

The other region expresses homologues of orthodenticle, Otx or otd. This region is more caudal and lateral, and bears the eyes.^{[1] [3]}

Orthodenticle is associated with the protocerebral bridge, part of the central complex, traditionally a marker of the prosocerebrum.^[4]

In the annelid brain, Otx expression characterises the peristomium, but also creeps forwards into the regions of the prostomium that bear the larval eyes.^[1]

Names of regions

Inconsistent use of the terms archicerebrum and the prosocerebrum makes them confusing.^[1]

The regions were defined by Siewing (1963): the archicerebrum as containing the ocular lobes and the mushroom bodies (= corpora pedunculata), and the prosocerebrum as comprising the central complex.^{[5] [6]}

The archicerebrum has traditionally been equated with the anteriormost, 'non-segmental' part of the protocerebrum, equivalent to the acron in older terminology. The prosocerebrum is then equivalent to the 'segmental' part of the protocerebrum, bordered by segment polarity genes such as engrailed, and (on one interpretation) bearing modified segmental appendages (= camera-type eyes).

But Urbach and Technau (2003) complicate the matter by seeing the prosocerebrum (central complex) + labrum as the anteriormost region

• Strausfeld 2016 identifies the anteriormost part of the brain = labrum = acron with the archicerebrum, followed by an engrailed-bounded 'first segment', bearing appendicular eyes, which he terms the prosocerebrum.^[7]
• Ortega-Hernández et al. (2017)^[8] follow the same model as Strausfeld (2016), but switch the terms prosocerebrum and archicerebrum.
• Archicerebrum = acron, followed by 'labral segment' / 'first head segment': Schmidt-Ott U., González-Gaitán M., Technau G.M. . 1995 . Analysis of neural elements in head-mutant Drosophila embryos suggests segmental origin of the optic lobes . Roux's Archives of Developmental Biology . 205 . 1–2. 31–44 . 10.1007/bf00188841 . 28306063 . 9232602 .
• Urbach & Technau ^[9] say that the archicerebrum = acron = ocular segment = first three protocerebral lobes bears optic lobes and mushroom bodies, and lacks engrailed; the prosocerebrum = fourth protocerebral lobe includes "the central complex and the neurosecretory cells of the pars intercerebralis" and corresponds to the labral segment.
• Weygoldt (1985) ^[10] sees the archicerebrum as anterior to the prosocerebrum, and as being non-segmental (i.e. = acron).
• Hunnekuhl, V.S. and Akam, M.^[11] avoid the terms, instead defining an ocular/preantennal region and an anterior medial region (with pars intercerebralis + neurosecretory cell cluster).

An alternative usage of the terms seems to have been introduced by Steinmetz et al (2010), who seem to flip the terms?

• Steinmetz et al. 2010 ^[1] write "The most anterior part, the

protocerebrum, can be further subdivided into a morelateral region bearing, for example, the optic lobes(archicerebrum) and a median region that includes, forexample, the pars intercerebralis (prosocerebrum). Mostauthors think that the archicerebrum represents the tipof the neuraxis, but this has been disputed."

• Future studies seem to have taken this to denote that the prosocerebrum is the rostralmost part of the body axis,^[12] contradicting a long tradition of usage.

Notes and References

1. Steinmetz P.R.H., Urbach R., Posnien N., Eriksson J., Kostyuchenko R.P., Brena C., Guy K., Akam M., Bucher G., Arendt D. . 2010 . ~Six3~ demarcates the anterior-most developing brain region in bilaterian animals . EvoDevo . 1 . 1. 14 . 10.1186/2041-9139-1-14 . 21190549 . 3025827 . free .
2. Posnien, N., Koniszewski, N.D.B., Hein, H.J. and Bucher, G. 2011: Candidate gene screen in the red flour beetle tribolium reveals six3 as ancient regulator of anterior median head and central complex development. PLoS Genetics 7.
3. Ortega-Hernandez, Javier. Janssen, Ralf. Budd, Graham E.. 10.1016/j.asd.2016.10.011. 1467-8039. Arthropod Structure & Development. 3. 354–379. Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective. 46. 2017. 27989966. free.
4. Scholtz G., Edgecombe G.D. . 2006 . The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence . Development Genes and Evolution . 216 . 7–8. 395–415 . 10.1007/s00427-006-0085-4 . 16816969 . 11300335 .
5. Hunnekuhl V.S., Akam M. . 2014 . An anterior medial cell population with an apical-organ-like transcriptional profile that pioneers the central nervous system in the centipede Strigamia maritima . Developmental Biology . 396 . 1 . 136–149 . 10.1016/j.ydbio.2014.09.020 . 25263198 . free .
6. Scholtz G., Edgecombe G.D. . 2006 . The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence . Development Genes and Evolution . 216 . 7–8. 395–415 . 10.1007/s00427-006-0085-4 . 16816969 . 11300335 .
7. Book: Strausfeld, Nicholas. Arthropod brains.
8. Ortega-Hernández J., Janssen R., Budd G.E. . 2017 . Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective . Arthropod Structure & Development . 46 . 3. 354–379 . 10.1016/j.asd.2016.10.011 . 27989966 . free .
9. Urbach R., Technau G.M. . 2003 . Early steps in building the insect brain: Neuroblast formation and segmental patterning in the developing brain of different insect species . Arthropod Structure and Development . 32 . 1 . 103–123 . 10.1016/s1467-8039(03)00042-2 . 18088998 .
10. Weygoldt, Peter. "Ontogeny of the arachnid central nervous system." Neurobiology of arachnids. Springer, Berlin, Heidelberg, 1985. 20-37.
11. 2014: An anterior medial cell population with an apical-organ-like transcriptional profile that pioneers the central nervous system in the centipede Strigamia maritima. Developmental Biology 396, 136–149.
12. 10.1038/s41467-018-03464-w . 9 . 1 . Brain and eyes of Kerygmachela reveal protocerebral ancestry of the panarthropod head . 2018 . Nature Communications . Tae-Yoon S. Park, Kihm Ji-Hoon, Woo Jusun, Park Changkun, Young Lee Won, Smith M. Paul, Harper David A. T., Young Fletcher, Nielsen Arne T., Vinther Jakob . 29523785 . 5844904 . 1019 . 2018NatCo...9.1019P .