In a model vertebrate of your choice, explain how the neural tube becomes patterned along the anterior-posterior axis.
Patterning is essential in early embryonic development. It is the result of the uneven concentration gradient of morphogens spread throughout the embryo space. Anterior-Posterior patterning is one of the keys to inducing different cellular fates to form distinct types of cells correctly. Frog (Xenopus Laevis) is an example vertebrate to explore this patterning.
When the stage of mesoderm formation, Nieuwkoop Centre signals the Spemann Organizer to produce Bone Morphogenetic Protein (BMP) inhibitors, such as Noggin, Chordin, Follistatin and Cerberus to return cells into Neural Stem Cells (NSC) fate. AP patterning in the neural tube starts once this signalling is removed. NSC formed previously, keep proliferating the neural plate to span the dorsal surface anteriorly until it eventually wraps as the neural tube to be patterned. Spemann Organizer stops producing BMP inhibitors and starts to secrete Fibroblast Growth Factor (FGF). As result, Spemann Organizer, located in the posterior part, establishes an FGF gradient with a higher concentration in the posterior.
In the anterior part, the prechordal plate and mesoderm take the function of producing BMP inhibitors, and numerous inhibitors for other morphogens. These include WNT inhibitors like Dickkopf (DKK) and Retinoic Acids (RA) inhibitors like CYP26. Together, BMP, RA and BMP blockers morphogen establish the gradient where the highest concentration is in the anterior part.
In contrast, the posterior side is dominated by FGF from Spemann Organizer, and WNT8, RA released by the paraxial mesoderm which is located at the sides of the notochord in the posterior. Here, RA morphogen is a key component to elaborate the Homeobox (HOX) gene afterwards, especially in the first trimester (Moore, Persaud and Torchia 2013).
Remember that Xenopus’ embryonic stem cells are by default falling into Neural Stem Cells (NSC) fate, this is because of master transcription factors SRY-box 2 (SOX2) and Octamer-binding Transcription Factor 4 (OCT4). As it is not allowed to have all stem cells as NSC, BMP and other morphogens were produced because they can suppress their SOX2 and OCT4 expression, to form Non-NSC (NNSC). Later, Spemann Organizer was used to producing BMP blockers to reset the default behaviour to NSC. Similarly, now, the activation of SOX2-OCT4 in anterior due to BMP blockers leads to the expression of Master Anterior Identity Gene: Orthodenticle homeobox 2 (OTX2). This means in some way that the whole NSC are destined by default to form anterior. Nevertheless, when Spemann Organizer stops from secreting BMP blockers and turns to FGF, it also brings back SOX2 and OCT4 suppressing family, impeding anterior formation in the posterior part. In opposition to OTX2 domination in the anterior, the morphogens in the posterior part express Gastrulation Brain Homeobox 2 (GBX2).
This is the first main confrontation to extend gradients in Anterior-Posterior patterning, WNT8, RA and FGF compete with their inhibitors to express GBX2 and OTX2, respectively. During this battle, the intermedial zone they create because of low concentration, forms the Isthmic Organizer. This boundary has relatively naïve NSC, so this zone will be patterned in the future by another gene. Also, this boundary is important in the sense that it shapes the boundary of the midbrain and hindbrain.
At this stage, three different sections are drawn in the Anterior-Posterior patterning of the neural tube. The first section is the anterior part. It results from OTX2 and has been developing so far by default fate of the NSC. OTX2 is critical for the formation of the anterior, as lack of leads to not-formation of anterior and facia skull (Sanes, Reh, Harris and Landgraf 2019). Mutants without it lose completely the forebrain and midbrain, parts of the hindbrain are lost too because the anterior is important to the formation of the Isthmic Organizer.
The second and third sections both came from the same sets of morphogens, but they resulted in different activated genes due to the difference in concentration. Lower concentrations of WNT8, RA and FGF exists in the hindbrain of the neural tube, and therefore, can only attach to high-affinity genes. Whereas the closer it is toward the posterior, in this case, spinal cord, the concentration is higher, so it can activate both low-affinity and high-affinity genes. Note that normally low-affinity genes repress the expression of high-affinity ones, causing a different gradient of cell types. Accordingly, the second main pattern section is in the hindbrain and is dominated by GBX2. Not only has this zone GBX2, but also has other 8 HOX genes getting involved in building the hindbrain. From which, one is used to having the closest connection to anterior to structure the cerebellum, and the other seven genes paint different HOX gradients. Lack of GBX2 will fail correct hindbrain formation, and the anterior part will connect to the spinal cord directly.
Subsequently, Caudal Homeobox (CDX) gene patterns the third and the most posterior area of the neural tube. Two other HOX genes pattern together with CDX to fully discriminate cells. Mutants absent of CDX will miss the spinal cord.
Once the anterior patterning stabilises and confirms forebrain and hindbrain fate, it stops secreting inhibitors as they are no longer needed. Therefore, many inactivated genes begin to express again. From them, FGF8 raises as the fundamental gene to form Isthmic Organizer, together with OTX2 and GBX2 (Joyner, Liu and Millet 2000). This new organising centre, apart from helping the transition of OTX2 and GBX2, also patterns its surrounding. It is a building block of future mesencephalon and metencephalon, which includes colliculi of the tectum and cerebellum. If a Xenopus mutant lack FGF8, it will have a deficit formation of the midbrain and hindbrain.
The Anterior-Posterior axis patterning is indispensable to the wide range of cell type formation along the neural tube of vertebrates. Anterior-Posterior patterning plus Dorsal-Ventral patterning and different homeobox genes together give neuroblasts unique identities to form relevant cell types and perform future tasks.
References:
- Joyner, A.L., Liu, A., and Millet S. (2000) “Otx2, Gbx2 and Fgf8 interact to position and maintain a mid–hindbrain organizer”, Current opinion in cell biology, 12(6), pp. 736-741.
- Moore, K.L., Persaud, T.V.N., and Torchia, M.G. (2013) The Developing Human: Clinically Oriented Embryology. 9th. Saunders, pp. 506-509.
- Sanes, D.H., Reh, T.A., Harris, W.A., and Landgraf, M. (2019) Development of the Nervous System. 4th. Academic Press, pp. 27-52