The origins of the circumventricular organs

Abstract

The circumventricular organs (CVOs) are specialised neuroepithelial structures found in the midline of the brain, grouped around the third and fourth ventricles. They mediate the communication between the brain and the periphery by performing sensory and secretory roles, facilitated by increased vascularisation and the absence of a blood-brain barrier. Surprisingly little is known about the origins of the CVOs (both developmental and evolutionary), but their functional and organisational similarities raise the question of the extent of their relationship. Here, I review our current knowledge of the embryonic development of the seven major CVOs (area postrema, median eminence, neurohypophysis, organum vasculosum of the lamina terminalis, pineal organ, subcommissural organ, subfornical organ) in embryos of different vertebrate species. Although there are conspicuous similarities between subsets of CVOs, no unifying feature characteristic of their development has been identified. Cross-species comparisons suggest that CVOs also display a high degree of evolutionary flexibility. Thus, the term 'CVO' is merely a functional definition, and features shared by multiple CVOs may be the result of homoplasy rather than ontogenetic or phylogenetic relationships.

Impact of P-Glycoprotein Function on the Brain Kinetics of the Weak Substrate 11C-Metoclopramide Assessed with PET Imaging in Humans

“…An increase in V T after ABCB1 inhibition was not observed in the pituitary gland, consistent with a lack of ABCB1-mediated efflux in this brain region (16,17). The small chemoreceptor trigger zone of the area postrema, which is the known pharmacologic target tissue of metoclopramide in the brain, is also not protected by the BBB (26). ABCB1 function may thus preferentially control the CNS side effects of metoclopramide (akathisia and dystonia), which are associated with the blockage of D 2 R in the striatum (24), rather than its therapeutic (antiemetic) effect.…”

Endogenous amylin contributes to birth of microglial cells in arcuate nucleus of hypothalamus and area postrema during fetal development

“…A high number of BrdUϩ cells were not identified as neurons, astrocytes, or microglia. Since the AP and the mediobasal hypothalamus are highly vascularised brain nuclei (26), a number of these nonidentified cells born embryonically may become vascular endothelial cells.…”

Pineal progenitors originate from a non-neural territory limited by FGF signalling

“…Whereas classical fate-mapping studies postulated a neural origin of pineal progenitors ( Oksche, 1965 ; ), we have found here that at least some pineal progenitors originate in the PPR, an area of non-neural ectoderm that flanks the anterior neural plate and gives rise to the cranial placodes: epithelial specialisations that form the sensory organs of the vertebrate head ( Graham and Shimeld, 2013 ; Schlosser, 2014 ). Thus, the pineal organ appears to be similar to the pituitary gland, another neuroendocrine gland that develops on the opposite (ventral) side of the diencephalon with a non-neural placodal (Rathke's pouch, adenohypophysis, anterior pituitary) and a neural (neurohypophysis, posterior pituitary) contribution ( Sánchez-Arrones et al, 2015 ; Kiecker, 2018 ). A dual origin of the pineal organ would also be reminiscent of the vertebrate eye that forms from neural (optic vesicle) and placodal (lens) tissue ( Graw, 2010 ).…”

“…Comparably little is known about the mechanisms that regulate pineal development in vertebrate embryos ( Joly et al, 2007 ; Rath et al, 2013 ; Sapède and Cau, 2013 ; Kiecker, 2018 ). The homeodomain transcription factors Not1/Noto (also known as floating head, Flh, in zebrafish), Pax6, Otx2 and Bsx are required for pinealogenesis in rodents, zebrafish and frogs, and individuals with mutations in PAX6 frequently lack the pineal organ ( Masai et al, 1997 ; Estivill-Torrús et al, 2001 ; Cau and Wilson, 2003 ; Mitchell et al, 2003 ; Nishida et al, 2003 ; Foucher et al, 2006 ; Abouzeid et al, 2009 ; D'Autilia et al, 2010 ; Chatterjee et al, 2014 ; Khuansuwan et al, 2016 ; Schredelseker and Driever, 2018 ).…”