Editing Rotifer (section)

=== Retrocerebral organ ===
Despite over 100 years of research, rotifer anatomy still has many poorly understood components. One of the more mysterious organs in rotifers is the "retrocerebral organ" (RCO), which still remains very enigmatic in its morphology, function, development, and evolution. Lying close to the brain, this organ usually consists of one or more glands and a sac or reservoir. The sac drains into a duct before opening through pores on the uppermost part of the head. Current data shows a wide diversity in structure and potential function. <ref>Fontaneto, D., & De Smet, W. H. (2015). Rotifera, chapter 4 Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifera, Volume 3, Gastrotricha and Gnathifera Schmidt-Rhaesa, Andreas.</ref>  In some species it is reduced or may even be absent completely. [[Benthic zone|Benthic]] species have larger RCO's than [[Plankton|planktonic]] species. Despite this diversity, positional correspondence of RCOs strongly suggests [[Homology (biology)|homology]].<ref name=IZ/><ref name="wallace">
Robert Lee Wallace.
[https://academic.oup.com/icb/article/42/3/660/724027/Rotifers-Exquisite-Metazoans1 "Rotifers: Exquisite Metazoans"].
2002.
{{doi | 10.1093/icb/42.3.660 }}
quote:
"What is the function of the retrocerebral organ?"
</ref><ref name=Hochberg/>

A 2023 study using [[transmission electron microscopy]] and [[Confocal microscopy|confocal laser scanning microscopy]] has illuminated the fine structure of this organ further. The study, the first of its kind, investigated the RCO in one species, ''Trichocerca similis''. It was determined to be a [[Syncytium|syncytial organ]], composed of a [[Anatomical terms of location|posterior]] glandular region, an expansive reservoir, and an anterior [[Duct (anatomy)|duct]]. The [[Gland|glandular]] portion has an active cytoplasm with paired [[Cell nucleus|nuclei]], abundant [[Endoplasmic reticulum|rough ER]], [[Ribosome|ribosomes]], [[Golgi apparatus|Golgi]], and [[Mitochondrion|mitochondria]]. Secretion granules accumulate at the [[Anatomical terms of location|anterior]] end of the gland where they undergo [[Cell fusion|homotypic fusion]] to create larger granules with numerous "mesh-like" contents.  These contents gradually fuse into tubular secretions that accumulate in the reservoir, awaiting secretion. [[Striated muscle tissue|Cross-striated longitudinal muscles]] form a partial sleeve around the reservoir and may function to squeeze the secretions through the gland's duct that often penetrates through the cerebral ganglion.<ref name=Hochberg> Hochberg, R., Araújo, T. Q., Walsh, E. J., Mohl, J. E., & Wallace, R. L. (2023). Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta). Invertebrate Biology, 142(1), e12396. https://doi.org/10.1111/ivb.12396.</ref>
====Retrocerebral organ secretions====
Much like the organ itself, the precise function and biochemical makeup of the secretions is still unknown. The small size of rotifers and small volume of the secretions makes isolation immensely difficult. The secretions have some similarities to the hydrogel secretions that form gelatinous housings in some rotifer species.
[[Ultrastructure]] analysis of ''T. similis'' secretions showed them to be a series of tube-like secretions with a highly filamentous framework. This is highly suggestive of a [[glycosaminoglycan]] structure- proteins with negatively charged [[polysaccharide]] chains forming [[proteoglycan]] molecules. These molecules are standard in vertebrate and invertebrate [[Gelatin|gelatins]] such as [[mucus]]. <ref name=Hochberg/> 

Despite recent advancements in understanding RCO organ and secretion ultrastructure, the exact function of the organ is still ultimately unclear. The leading hypotheses are that the RCO secretes a mucus-like substance that aids in [[Benthic boundary layer|benthic]] locomotion, adhesion, and/or reproduction (i.e., attachment of eggs to a substrate), although more research is needed to explore function and evaluate the homology between species.<ref name=Hochberg/>