Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2022 Nov 1.
Published in final edited form as: J Neuroendocrinol. 2021 Sep 22;33(11):e13039. doi: 10.1111/jne.13039

Hidden ‘pit’falls in deciphering the gonadotropin releasing hormone neuroendocrine cell lineage

Y Shan 1, S Wray 1
PMCID: PMC8616834  NIHMSID: NIHMS1738687  PMID: 34553448

Abstract

To this day, the identity of GnRH progenitors remains unclear. However, the visualization of different developmental markers in subsets of GnRH neurons during early embryonic stages raised the possibility of at least two GnRH subpopulations. This observation led directly to a second question. Does visualization of different developmental markers in subsets of GnRH neurons reflect functional heterogeneity? This question remains unanswered, but as we learn more about the GnRH system, functional GnRH subpopulations becomes critically important to understanding GnRH function. This review addresses the development of the neuroendocrine GnRH system, specifically the heterogeneity of the GnRH neuroendocrine population.

Introduction

In vertebrates, the gonadotropin releasing hormone (GnRH) neuroendocrine cells reside postnatally within the brain and control reproductive function. However, these cells (expressing the GnRH1 gene in mammals and the GnRH3 gene in zebrafish1) arise outside the central nervous system (CNS) and migrate into the forebrain during embryonic development2,3. Reviews have highlighted this event in multiple species and as well as the cues that play a role in this process4–7. In mammals, when early stages of development have been examined, the site where GnRH1 neuroendocrine cells are first visualized using either antibodies or in situ hybridization histochemistry is in the nose, particularly, in the developing olfactory pit (Figure 1). The olfactory pit develops from the olfactory placode, a structure which gives rise to both the main olfactory epithelium as well as the vomeronasal organ (VNO). GnRH1 cells are found within the boundaries of the developing VNO. Based on location, it was first proposed that, in mammals, GnRH1 cells originate in the olfactory placode8, a structure that starts out as a thickening of ectoderm cells at the tip of the nose. Subsequent mouse genetic studies indicated that disrupting genes involved in olfactory placode development or olfactory axon targeting to the olfactory bulb often disrupted the GnRH1 system and consequently function9,10, reinforcing the association of GnRH1 neurons with the olfactory placode. Although the VNO does not persist postnatally in humans, it has been identified during prenatal development11 and GnRH1-positive neuroendocrine cells are detected within this structure12. Certainly, the identification of the prenatal journey of GnRH1 cells led to an explosion in research and understanding of Kallmann syndrome, patients with reproductive dysfunction and anosmia7. Together the data on the development of the GnRH neuroendocrine cells have conclusively shown that the development of the GnRH1 system and the olfactory system are entwined.

Figure 1.

Figure 1.

Open in a new tab

Developmental stages of mouse nasal region where GnRH neurons are first detected. (A and B) Schematics showing structure of E9.5 and E10.5 mouse embryos. Grey areas represent the olfactory placode (in E9.5) and olfactory pit (OP, in E10.5). T=telencephalic vesicle. (C) E10.5 mouse section stained with TUJ1, an antibody against neuron specific β-tubulin III. TUJ1 positive cells are detected in the epithelium of the olfactory pit (OP, arrows), developing forebrain (fb) and a migratory mass of cells (mm, arrowheads) leaving the caudal region of the OP towards the developing forebrain. (D) Image showing whole mount E11.5 mouse transgenic embryo with OP, eye (E) and telencephalon (T). (E) Schematic of section through E11.5 embryo (D) showing regions of interest. The olfactory pit has invaginated and the respiratory epithelium (RE), olfactory epithelium (OE) and presumptive vomeronasal organ (pVNO) are now visible. (F) Staining of an E11.5 section for GnRH detects immunopositive cells in the developing VNO (brown, arrowheads, boxed region in E). (G) Schematic of E12.5 mouse embryonic section through nasal region, olfactory epithelium (OE) and VNO are well formed. (H and I) GnRH neurons (dark purple, arrowheads) are seen migrating out of the VNO at E12.5 along olfactory sensory axons positive for peripherin4 (brown, open arrowheads), targeting the nasal forebrain junction (NFJ), just beneath the developing olfactory bulbs (OB). Grey area in all schematics = structures developed from the olfactory placode. Scale: F: 100 μm, I: 50 μm. Panels D,F, H and I adapted45,7,29 respectively.

Brief History:

A placodal origin for the GnRH1 cell population was surprising, as no other sensory placodes have been found to give rise to neuroendocrine populations. However, the developing olfactory placode is flanked by cells of the adenohypophyseal placode (forms the anterior pituitary) and neural crest cells, both known to give rise to neuroendocrine cells13. In fact, studies in zebrafish addressing the lineage of GnRH3 (equivalent to GnRH1 in mammals) cells raised questions about the origin of GnRH neuroendocrine cells, suggesting a dual origin from the adenohypophysial placode as well as the olfactory placode-indicating that the later GnRH cells have a neural crest origin while the others derive from preplacodal ectoderm13. Subsequent experiments in mice indicated that GnRH1 cells do not arise from the adenohypophysial placode14. However, lineage tracing techniques in mice produced evidence that at least two GnRH1 subpopulations existed in the olfactory placode (Figure 2), one (the majority, 70%) originating from preplacodal ectoderm and the other (30%) formed from neural crest15. Yet in chick, ablation of the region giving rise to olfactory sensory neurons did not result in loss of forebrain GnRH1 cells, while ablation of the respiratory epithelium did16. Notably, the ablated region which perturbed the respiratory epithelium and GnRH1 cells also gives rise to precursors of the anterior pituitary placode. In amphibians, removal of the olfactory placodes results in loss of the olfactory epithelium, nerve and bulb, as well as loss of GnRH neurons17. It is important to note that many ablation studies were conducted in developmental stages after olfactory placode formation. By the time that the placode has formed, neural crest cells may have already migrated in and become intimately associated with the olfactory placode. Any manipulations conducted on the olfactory placode at this stage will also affect the underlying neural crest cells. Thus, using a variety of techniques, each with their own pros and cons, the identity of GnRH progenitors still remains unclear.

Figure 2.

Figure 2.

Open in a new tab

Wnt expression delineates two GnRH populations in the developing olfactory pit. (A) Fluorescent photograph showing Wnt1Cre/RYFP expression (green) in a whole mount embryo head at E10.525; a positive signal is detected in the facial mesenchyme, highlighting the OP (white arrow). (B) At E11.5, Wnt1Cre/RLacZ staining shows β-gal positive cells (blue) in the respiratory epithelium (RE), vomeronasal organ (VNO, black arrowhead), parts of the development olfactory epithelium (OE) and surrounding nasal mesenchyme (NM). (C) High magnification photomicrograph from the VNO region of a E11.5 Wnt1Cre/RYFP mouse section. Three GnRH cells can be seen. Two of these GnRH cells are negative for GFP (white arrows) while one cell is positive for both GnRH and GFP (asterisk) - revealing two subpopulations of GnRH neurons. Scale: B: 50 μm, C: 10 μm. Panels A, B, adapted25, and C adapted29.

Certainly, unique attributes of species can always be used to explain developmental differences, including those noted for GnRH cells. However, GnRH is an old molecule, pre-dating the appearance of jawed vertebrates18. Thus, is it likely that each group of animals in which GnRH is involved in reproductive function, evolved different mechanisms for incorporating GnRH cells into the CNS system? Migration cues and movement dynamics have been shown to be fairly applicable across species7,9,10,19–21. Studies from chicken, mouse and zebrafish concur that GnRH+ cells are detected leaving the olfactory placode, migrating on an olfactory axonal pathway, and pausing at the nasal forebrain junction, prior to entrance into the CNS3,22,23. As such, one must wonder why the progenitors giving rise to the GnRH neuroendocrine might be different? Alternatively, technical issues may cloud data interpretation and conserved mechanisms regarding the progenitors of the GnRH neuroendocrine cells in different species may exist. Perhaps most relevant to assaying the source of GnRH cells is the lack of data at equivalent developmental stages across species and the lack of appropriate markers/techniques. Development is a continuum of events and even embryos within the same rodent litter can show developmental differences depending on their location within the uterus due to differences in blood supply across the placenta, hormones from neighboring embryos, to name just two of the known variables. In addition, although a straight line might be drawn between expression of a marker at two different stages, in fact an ‘on-off-on’ sequence may occur in which the ‘on’ events are regulated differently and represent two separated unrelated developmental events. Finally, one needs to think about how many progenitors one might be ‘looking’ for. In an early paper examining mouse, it was found that GnRH cells were born between E10.0 and E11.0. During this embryonic age, the mitotic cycle is −8.5 hr2. Based on these data, the authors posit that ~100 progenitor GnRH cells (two bilateral groups of 50 cells each), committed to becoming GnRH-expressing cells, underwent three divisions in 24 hr and thereby produced the total postnatal GnRH population of ~800 cells. Making several broad stroke assumptions, based on Forni et al. subpopulation data, one is potentially looking for ~15 cells/side from neural crest lineage. Although chickens contain more GnRH cells (~4000), zebrafish contain fewer GnRH3 cells that migrate in the brain (~10023). As such, the total number of GnRH cells could confound our ability to identify progenitors of a small GnRH subpopulation. With these caveats in mind, we will consolidate recent data on where GnRH cells come from, whether they originate from multiple sources, and could the origin of the GnRH cells be indicative of functional subpopulations. But remember, we can always fall back on species differences.

Where we are:

Mouse:

The olfactory placode becomes apparent at ~E9.5 (Figure 1) and subsequently gives rise to both olfactory epithelium as well as the VNO and a variety of cell types including olfactory sensory neurons, pheromone receptor neurons, olfactory ensheathing cells, sustentacular support cells and the neuroendocrine GnRH cells5. GnRH1 cells are found within the boundaries of the developing VNO at E11.5. From E11.5-E12.5, GnRH1 cells leave the developing VNO, migrating along olfactory axons (sensory axons, from both olfactory sensory neurons and pheromone receptor neurons, as well as the nervus terminalis) to the base of the developing forebrain3,24 where they pause4 (also shown in chick22and zebrafish13). During migration, the cells and axons are surrounded by olfactory ensheathing cells. After reaching the nasal forebrain junction (NFJ, at the base of the cribriform plate), the main and accessory olfactory axons and olfactory ensheathing cells target the olfactory bulb, while GnRH neurons migrate along a subset of axons (nervus terminalis) which turn caudally and enter the developing forebrain5,25. Cre-recombinase genetic lineage tracing revealed that the olfactory placodes are comprised of both placodal ectoderm cells and neural crest cells, with the neural crest cells migrating into the existing structure by E9.525–27. These studies further identified that the olfactory ensheathing cells25–27 and a subpopulation of GnRH1 cells25 were negative for the placodal ectodermal lineage marker and positive for neural crest lineage markers (Figure 2). The remaining GnRH1 cells were positive for the placodal ectoderm lineage marker but negative for the neural crest lineage marker, consistent with two GnRH1 subpopulations at early stages of development25. Sox10 is a key neural crest specifier gene. Studies using Sox10 mutants showed olfactory ensheathing cells were reduced but present and that the olfactory, VNO, and terminal nerves had formed in the homozygous mutant embryos, and that GnRH1 cells were present, but migration into the forebrain disrupted. One study showed that the total number of GnRH1 cells was not altered in Sox10 mutants28. The presence of both olfactory ensheathing cells and GnRH1 cells in these mutant mice remains unclear, though is consistent with a non-neural crest origin or rescue by alternative factors.

Two independent studies have been performed examining Islet-1/2 expression in GnRH1 cells during development29,30. Islet-1 and Islet-2 are LIM-homeodomain transcription factors. Islet-1 is expressed in many embryonic cells, being one of the earliest markers of differentiation31. Shan et al29 found that labeling for Islet-1/2 distinguishes two subpopulations of GnRH1 neurons in E11.5 and E12.5 embryonic mice (Figure 3). The majority of GnRH1 neurons were Islet-1/2(+) at both ages. However, At E11.5, ~16% of GnRH1 neurons in the developing olfactory pit were Islet-1/2(−), at E12.5, ~23% of GnRH1 neurons on the migratory track were negative for Islet-1/2(−). Notably, the Wnt promoter-driven CRE recombination of Rosa-YFP also highlighted the GnRH1 neuronal subpopulation lacking Islet-1/2, at both ages. These data are consistent with two separate origins of GnRH1 cells in mouse and indicate that these subpopulations can be distinguished by Islet-1/2 expression and that the expression of Islet-1/2 is a suitable marker for tracing the ectodermal lineage.

Figure 3.

Figure 3.

Open in a new tab

A subpopulation of mouse GnRH neurons do not express Islet1/2. (A-D) While the majority of GnRH neurons express Islet1/2 (asterisks), GnRH neurons negative for Islet1/2 (white arrows) can be seen in both E11.5 and E12.5 mouse embryonic sections. Blue dashed boxes in schematics A and C show locations of high magnification photographs B and D. Red solid box in schematic A shows location of high magnification in E-G. (E-G) Triple immunostaining for GnRH (white), GFP (green) and Islet-1/2 (red) indicates GnRH/GFP positive cells are islet-1/2 negative (E-G, dashed box and G1). In contrast, GnRH cells that are GFP negative are islet-1/2 positive (E-G, solid box and arrowheads, and G2). (H and I) Preliminary data from an adult NIH swiss mouse (>PN45) section immunostained for GnRH (blue) and Islet-1/2 (brown) using standard ABC peroxidase procedures45. H = low magnification at the level of organum vasculosum lamina terminalis (schematic inset). I = higher magnification of area boxed in H. GnRH neurons positive for Islet-1/2 (black arrows), and GnRH cells negative for Islet-1/2 (white arrows) are shown. (J) Fluorescent image from a brain section from an adult Wnt1Cre/RYFP mouse shows that GnRH/GFP positive neurons (J1) and GnRH/GFP negative neurons (J2) are still detected. Panels B,D and E-G adapted29. Panels H-J unpublished data from Shan and Wray.

In a second study by Taroc et al30, approximately 11% of GnRH1 cells at E11.5 were negative for Islet-1/2 immunoreactivity, similar to that found by Shan et al29. However, histochemistry at E13.5 and E15.5 showed strong Islet-1/2 immunoreactivity in virtually all GnRH1 neurons, suggesting Islet-1/2 expression is not limited to specific subpopulations of GnRH1 cells and/or that Islet-1/2 expression is upregulated later in the initial Islet-1/2 negative population. Using two different strategies for knocking out Islet-1 from GnRH1 neurons, Taroc et al30 found no changes in GnRH1 neuronal formation or migration, suggesting compensatory mechanisms, but leaving the role of this transcription factor in GnRH1 neurons during early placodal neurogenesis unresolved. Notably in one line, GnRH1Cre;Isl1flox/flox conditional mutants, they reported only loss of Islet-1 immunoreactvity in ~40% of GnRH1 neurons. It would be of interest to know whether expansion of the Islet-1/2 negative GnRH cells detected at E11.5 may have occurred in these mutants. Preliminary data in our lab indicates that in adult mice, a subpopulation of GnRH neurons was Islet-1/2 negative (Figure 3). Anatomically, the GnRH(+)/ Islet-1/2(−) cells were found throughout the forebrain, being ~25% of the total GnRH cells counted, similar to that found at E11.5-E12.5 by Shan et al29. Notably, analysis by region showed a difference in the distribution of these cells, with 20% of the anterior GnRH cells were Islet-1/2(−), 22% of the GnRH cells located close to the level of the organum vasculosum lamina terminalis were Islet-1/2(−), while 52% of the GnRH cells caudally were Islet-1/2(−). Whether these are the same cells identified prenatally in our earlier study29 remains to be determined, but certainly a correlation might be found by examining GnRH positive cells that are Wnt1Cre;RosaYFP positive for Islet-1/2.

Taken together, the work in mice can be interpreted two ways. One is that Wnt1Cre tracing and Islet-1/2 immunoreactivity defines two distinct embryonic lineages for GnRH1 neurons. The other is that the Wnt1Cre positive subpopulation of GnRH1 neurons differ in its timing and/or expression levels of Islet-1/2 from the majority of GnRH1, not necessary confirming two different lineages but certainly, distinguishing two subpopulations of GnRH neurons.

Zebrafish:

The olfactory placode first becomes apparent around 18 hour post-fertilization (hpf)32,33 and GnRH3 positive cells can be identified in this structure shortly after, with time of detection dependent on the technique used. Aguillon et al.28, concluded that all zebrafish GnRH3 neurons have a homogenous origin from preplacodal ectodermal progenitors, based on co-labeling with Islet-1/2, use of a mutant line devoid of Sox10 as well as lines for lineage tracing. At 48hpf (~E13.5 in mouse) high resolution imaging techniques revealed all GnRH3 cells expressed Islet-1/2. In addition, no change in Islet-1/2(+) cell number was found in Sox10 mutant embryos. Finally, using lineage reconstruction based on backtracking in time-lapse confocal datasets, and confirmed by photoconversion experiments, this group reported that GnRH3 neurons lack neural crest markers. Similarly, they found that all of the microvillous sensory neurons (sensing pheromones in fish, equivalent to vomerosensory neurons in mammals) also arise from preplacodal progenitors and did not require Sox10 for specification. The latter results directly contradicted data in which the cranial neural crest was found to be the primary source of microvillous sensory neurons in zebrafish34,35. This discrepancy highlights unresolved problems when using present day tools in lineage analysis. Although experiments presented by Aguillon et al. are elegant studies, one must always be concerned about visualizing small populations, redundant factors and progenitor expansion. Would one be able to find GnRH3 positive cells/Islet-1/2 negative cells if only a small percentage of cells were counted (indicated 6–10 cells/epithelium28)? Would the Sox10 KO lines show a statistical difference if the loss was a small percent or did redundant factors step in to insure GnRH neuronal survival? Could the loss of a small population induce the preplacodal population to expand in zebrafish? These are questions yet to be answered.

Chicken:

The olfactory placode first becomes apparent around Hamburger Hamilton (HH) stage 17–1922. GnRH cells are detected in the olfactory epithelium by stage 19 and within the brain by stage 29, with a total population varying between 4000–6000 neurons22. The large number of GnRH cells is consistent with data showing that overall, birds have twice the number of neurons as mammals of the same weight36. In contrast to the data above in zebrafish and mice, Palaniappan et al37, provided evidence in chicken that olfactory placode-derived Islet-1+, and GnRH1+ cells were distinct, ie. little overlap in expression. In addition, they detected co-expression of Sox10 in olfactory ensheathing cells, but only by stages HH24 and HH26, suggesting that these cells never invaded the developing olfactory placode as seen in mice. The authors concluded that since GnRH1 expression was largely nonoverlapping with Islet-1 (or a second LIM-homeodomain transcription factors, Lhx2), that at least three cell types were differentially specified in the olfactory placode prior to or during cell migration and Islet-1 was not a marker of GnRH cell lineage.

Humans:

The nasal placodes are present at week 4 (~E10.5 in mouse). The placodes then invaginate to form the nasal pits (~5th week of gestation38). At 6–7 weeks, the VNOs are present39, which thereafter degenerates by week 11 (Comparison Carnegie charts put stage 12 as E10.5 mouse and ~28 days in human; CS14 as E11.5 in mouse and ~33 days in human). Lund et al40 modeled the development of human GnRH neurons by generating a stable GNRH1-TdTomato reporter cell line in human pluripotent stem cells (hPSCs) using CRISPR Cas9 genome editing. RNA-sequencing of the reporter clone, differentiated by dual SMAD inhibition and FGF8 treatment, revealed Islet-1, as one of the top 50 most upregulated genes in GnRH neurons compared to progenitors. Examination of Islet-1 was confirmed in GnRH neurons in 10.5 gestational week-old human fetus (~E16 mouse). It was reported that all GnRH-immunoreactive neurons expressed Islet-1, throughout the migratory pathway, both in the nasal compartment and brain area sampled. The authors conclude that Islet-1 is expressed in fetal GnRH neurons after their differentiation, and expression of Islet-1 persists during the migration to the hypothalamus. These findings are consistent with the results of Taroc et al30, who examined GnRH cells in mice at later stages and found that the majority of GnRH cells were Islet-l1/2 positive. Notably, in human, newly formed GnRH neurons can only be detected in these later stages, after they delaminate from the neuroepithelium into the nasal mesenchyme12. As such the neuronal progenitors that give rise to GnRH neurons and their gene expression profile are awaiting new markers.

GnRH neuronal subpopulations:

In postnatal mammals, extensive heterogeneity has been described within the GnRH1 neuroendocrine population, including behavior associated c-fos responses, and expression of receptors for neuromodulators as well as neurotransmitters7. Heterogeneity in GnRH1 cell responses in vivo have most often been attributed to afferent inputs. However, numerous experiments have shown that the GnRH1 cells mature during prenatal development41. Notably when GnRH1 cells are assayed in explants generated from embryonic tissue (e.g. cells that have not entered the brain and are without the presence of numerous afferents), they show characteristics similar to that recorded in adult slices or whole animals42, including pulsatile-like secretion as well as subpopulation responses to neuromodulators or neurotransmitters7,42. Thus, the existence of GnRH subpopulations is not new, being identified in primary GnRH neuroendocrine cells in vivo and in vitro using a number of different assays. However, whether these are the same subpopulations identified during early development in some species but not in others, remains to be determined - and remember, we can always fall back on species differences.

Conclusion

The existence of GnRH subpopulations during development needs to be further investigated, to fully understanding GnRH function postnatally. GnRH cells have been documented migrating towards pallial and subpallial telencephalic regions in both human and mice19, and non-neuroendocrine GnRH functions have been proposed for decades based on GnRH projections to areas other than the median eminence in a variety of species42. Thus, both neuroendocrine as well as non-neuroendocrine GnRH cells arise in nasal regions in mammals. The lineage of these two cell types has yet to be examined. In addition, in mouse, distinct subpopulations of neuroendocrine GnRH cells that are responsive to different neuromodulators of reproductive function are present within the developing nasal region and later within the brain. The presence of these subpopulations prenatally suggests initiation of different genetic programs. This leads to three questions:1) Why would one GnRH neuroendocrine cell express a specific receptor and another not, when both reside within the same microenvironment? 2) Is it the resolution of our assays or are there innate differences in the neuroendocrine GnRH cell population? And finally 3) are there species differences in the origin of the neuroendocrine GnRH population? These questions will be difficult to answer but certainly intriguing to solve.

Acknowkedgments:

S. Wray thanks all past and present members of the laboratory that have contributed to work on the development and regulation of the GnRH neuron. This work was supported by the Intramural Research Program of the National Institutes of Health, National Institute of Neurological Disorders and Stroke (ZIA NS002824).

Footnotes

Disclosure summary: “Authors report no conflict of interest”

References

  • 1.Okubo K, Nagahama Y. Structural and functional evolution of gonadotropin-releasing hormone in vertebrates. Acta Physiol. 2008;193:3–15. [DOI] [PubMed] [Google Scholar]
  • 2.Wray S, Grant P, Gainer H. Evidence that cells expressing luteinizing hormone-releasing hormone mRNA in the mouse are derived from progenitor cells in the olfactory placode. Proc Natl Acad Sci U S A. 1989;86:8132–8136. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Wray S, Nieburgs A, Elkabes S. Spatiotemporal cell expression of luteinizing hormone-releasing hormone in the prenatal mouse: evidence for an embryonic origin in the olfactory placode. Dev Brain Res. 1989;46:309–318. [DOI] [PubMed] [Google Scholar]
  • 4.Wray S Development of gonadotropin-releasing hormone-1 neurons. Front Neuroendocrinol. 2002;23:292–316. [DOI] [PubMed] [Google Scholar]
  • 5.Wray S From nose to brain: Development of gonadotrophin-releasing hormone −1 neurones. Journal of Neuroendocrinology;22. Epub ahead of print 2010. DOI: 10.1111/j.1365-2826.2010.02034.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Whitlock KE. Origin and development of GnRH neurons. Trends in Endocrinology and Metabolism;16. Epub ahead of print 2005. DOI: 10.1016/j.tem.2005.03.005. [DOI] [PubMed] [Google Scholar]
  • 7.Cho H-J, Shan Y, Whittington NC, et al. Nasal Placode Development, GnRH Neuronal Migration and Kallmann Syndrome. Front Cell Dev Biol;7. Epub ahead of print 11 July 2019. DOI: 10.3389/fcell.2019.00121. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Wray S, Gähwiler BH, Gainer H. Slice cultures of LHRH neurons in the presence and absence of brainstem and pituitary. Peptides. Epub ahead of print 1988. DOI: 10.1016/0196-9781(88)90103-9. [DOI] [PubMed] [Google Scholar]
  • 9.Kramer PR, Wray S. Novel gene expressed in nasal region influences outgrowth of olfactory axons and migration of luteinizing hormone-releasing hormone (LHRH) neurons. Genes Dev;14. Epub ahead of print 2000. DOI: 10.1101/gad.14.14.1824. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Toba Y, Tiong JD, Ma Q, et al. CXCR4/SDF-1 system modulates development of GnRH-1 neurons and the olfactory system. Dev Neurobiol. 2008;68:487–503. [DOI] [PubMed] [Google Scholar]
  • 11.Lund C, Pulli K, Yellapragada V, et al. Development of Gonadotropin-Releasing Hormone-Secreting Neurons from Human Pluripotent Stem Cells. Stem Cell Reports;7. Epub ahead of print 2016. DOI: 10.1016/j.stemcr.2016.06.007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Casoni F, Malone SA, Belle M, et al. Development of the neurons controlling fertility in humans: New insights from 3D imaging and transparent fetal brains. Dev;143. Epub ahead of print 2016. DOI: 10.1242/dev.139444. [DOI] [PubMed] [Google Scholar]
  • 13.Whitlock KE, Wolf CD, Boyce ML. Gonadotropin-releasing hormone (GnRH) cells arise from cranial neural crest and adenohypophyseal regions of the neural plate in the zebrafish, Danio rerio. Dev Biol;257. Epub ahead of print 2003. DOI: 10.1016/S0012-1606(03)00039-3. [DOI] [PubMed] [Google Scholar]
  • 14.Metz H, Wray S. Use of mutant mouse lines to investigate origin of gonadotropin-releasing hormone-1 neurons: Lineage independent of the adenohypophysis. Endocrinology;151. Epub ahead of print 2010. DOI: 10.1210/en.2009-0875. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Forni PE, Wray S. Neural crest and olfactory system: New prospective. Mol Neurobiol. 2012;46:349–360. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.El Amraoui A, Dubois PM. Experimental evidence for an early commitment of gonadotropin-releasing hormone neurons, with special regard to their origin from the ectoderm of nasal cavity presumptive territory. Neuroendocrinology;57. Epub ahead of print 1993. DOI: 10.1159/000126490. [DOI] [PubMed] [Google Scholar]
  • 17.Murakami S, Kikuyama S, Arai Y. The origin of the luteinizing hormone-releasing hormone (LHRH) neurons in newts (Cynops pyrrhogaster): the effect of olfactory placode ablation. Cell Tissue Res. 1992;269:21–27. [DOI] [PubMed] [Google Scholar]
  • 18.Kavanaugh SI, Nozaki M, Sower SA. Origins of gonadotropin-releasing hormone (GnRH) in vertebrates: Identification of a novel GnRH in a basal vertebrate, the sea lamprey. Endocrinology;149. Epub ahead of print 2008. DOI: 10.1210/en.2008-0184. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Casoni F, Hutchins II, Donohue D, et al. SDF and GABA interact to regulate axophilic migration of GnRH neurons. J Cell Sci. 2012;125:5015–5025. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Song Y, Tao B, Chen J, et al. GABAergic neurons and their modulatory effects on GnRH3 in zebrafish. Endocrinology. 2017;158:874–886. [DOI] [PubMed] [Google Scholar]
  • 21.Palevitch O, Abraham E, Borodovsky N, et al. Nasal embryonic LHRH factor plays a role in the developmental migration and projection of gonadotropin-releasing hormone 3 neurons in zebrafish. Dev Dyn. 2009;238:66–75. [DOI] [PubMed] [Google Scholar]
  • 22.Mulrenin EM, Witkin JW, Silverman AJ. Embryonic development of the gonadotropin-releasing hormone (GnRH) system in the chick: A spatio-temporal analysis of gnrh neuronal generation, site of origin, and migration. Endocrinology;140. Epub ahead of print 1999. DOI: 10.1210/endo.140.1.6425. [DOI] [PubMed] [Google Scholar]
  • 23.Golan M, Boulanger-Weill J, Pinot A, et al. Synaptic communication mediates the assembly of a self-organizing circuit that controls reproduction. Sci Adv;7. Epub ahead of print 2021. DOI: 10.1126/sciadv.abc8475. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Schwanzel-Fukuda M, Pfaff DW. Origin of luteinizing hormone-releasing hormone neurons. Nature. 1989;338:161–164. [DOI] [PubMed] [Google Scholar]
  • 25.Forni PE, Taylor-Burds C, Melvin VS, et al. Neural crest and ectodermal cells intermix in the nasal placode to give rise to GnRH-1 neurons, sensory neurons, and olfactory ensheathing cells. J Neurosci. 2011;31:6915–6927. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Barraud P, Seferiadis AA, Tyson LD, et al. Neural crest origin of olfactory ensheathing glia. Proc Natl Acad Sci U S A. 2010;107:21040–21045. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Katoh H, Shibata S, Fukuda K, et al. The dual origin of the peripheral olfactory system: Placode and neural crest. Mol Brain;4. Epub ahead of print 2011. DOI: 10.1186/1756-6606-4-34. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Aguillon R, Batut J, Subramanian A, et al. Cell-type heterogeneity in the early zebrafish olfactory epithelium is generated from progenitors within preplacodal ectoderm. Elife;7. Epub ahead of print 2018. DOI: 10.7554/eLife.32041. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Shan Y, Saadi H, Wray S. Heterogeneous Origin of Gonadotropin Releasing Hormone-1 Neurons in Mouse Embryos Detected by Islet-1/2 Expression. Front Cell Dev Biol;8. Epub ahead of print 2020. DOI: 10.3389/fcell.2020.00035. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Taroc EZM, Katreddi RR, Forni PE. Identifying Isl1 Genetic Lineage in the Developing Olfactory System and in GnRH-1 Neurons. Front Physiol;11. Epub ahead of print 2020. DOI: 10.3389/fphys.2020.601923. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Ericson J, Thor S, Edlund T, et al. Early stages of motor neuron differentiation revealed by expression of homeobox gene Islet-1. Science (80- ). 1992;256:1555–1560. [DOI] [PubMed] [Google Scholar]
  • 32.Hansen A, Zeiske E. Development of the olfactory organ in the zebrafish, Brachydanio rerio. J Comp Neurol. 1993;333:289–300. [DOI] [PubMed] [Google Scholar]
  • 33.Miyasaka N, Knaut H, Yoshihara Y. Cxcl12/Cxcr4 chemokine signaling is required for placode assembly and sensory axon pathfinding in the zebrafish olfactory system. Development. 2007;134:2459–2468. [DOI] [PubMed] [Google Scholar]
  • 34.Saxena A, Peng BN, Bronner ME. Sox10-dependent neural crest origin of olfactory microvillous neurons in zebrafish. Elife;2013. Epub ahead of print 19 March 2013. DOI: 10.7554/eLife.00336. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35.Torres-Paz J, Whitlock KE. Olfactory sensory system develops from coordinated movements within the neural plate. Dev Dyn. 2014;243:1619–1631. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Kuenzel WJ Mapping the brain of the chicken (Gallus gallus), with emphasis on the septal-hypothalamic region Gen Comp Endocrinol. 2018. January 15;256:4–15. [DOI] [PubMed] [Google Scholar]
  • 37.Palaniappan TK, Slekiene L, Gunhaga L, et al. Extensive apoptosis during the formation of the terminal nerve ganglion by olfactory placode-derived cells with distinct molecular markers. Differentiation;110. Epub ahead of print 2019. DOI: 10.1016/j.diff.2019.09.003. [DOI] [PubMed] [Google Scholar]
  • 38.Tiwana PS, Madsen MJ. Cleft Lip and Palate: Primary Cleft Palate Repair. In: Current Therapy in Oral and Maxillofacial Surgery. 2012. Epub ahead of print 2012. DOI: 10.1016/B978-1-4160-2527-6.00086-4. [DOI] [Google Scholar]
  • 39.Carlson BM. Human Embryology and Developmental Biology: Fifth Edition. 2013.
  • 40.Lund C, Yellapragada V, Vuoristo S, et al. Characterization of the human GnRH neuron developmental transcriptome using a GNRH1-TdTomato reporter line in human pluripotent stem cells. DMM Dis Model Mech;13. Epub ahead of print 2020. DOI: 10.1242/dmm.040105. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Livne I, Gibson MJ, Silverman AJ Biochemical differentiation and intercellular interactions of migratory gonadotropin-releasing hormone (GnRH) cells in the mouse. Dev Biol 1993. October;159(2):643–56.doi: 10.1006/dbio.1993.1271. [DOI] [PubMed] [Google Scholar]
  • 42.Constantin S, Caraty A, Wray S, et al. Development of gonadotropin-releasing hormone-1 secretion in mouse nasal explants. Endocrinology. 2009;150:3221–3227. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Barry J, Hoffman GE and Wray S. LHRH containing systems. In Handbook of Chemical Neuroanatomy. Vol. 4. GABA and Neuropeptides in the CNS, pg. 166–215, Björklund A and Hökfelt T (Eds). Elsevier Pub. Co., Inc., New York, 1985. [Google Scholar]
  • 44.Witkin JW, Dao D, Livne I, Dunn IC, Zhou XL, Pula K, Silverman AJ. Early expression of chicken gonadotropin-releasing hormone-1 in the developing chick. J Neuroendocrinol. 2003. September;15(9):865–70. doi: 10.1046/j.1365-2826.2003.01073.x. [DOI] [PubMed] [Google Scholar]
  • 45.Forni PE, Bharti K, Flannery EM, Shimogori T, Wray S The indirect role of fibroblast growth factor-8 in defining neurogenic niches of the olfactory/GnRH systems. J Neurosci. 2013. December 11;33(50):19620–34. doi: 10.1523/JNEUROSCI.3238-13.2013. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES