Origin and early development of the chicken adenohypophysis

In vitro modeling of cranial placode differentiation: Recent advances, challenges, and perspectives

Cranial placodes are transient ectodermal thickenings that contribute to a diverse array of organs in the vertebrate head. They develop from a common territory, the pre-placodal region that over time segregates along the antero-posterior axis into individual placodal domains: the adenohypophyseal, olfactory, lens, trigeminal, otic, and epibranchial placodes. These placodes terminally differentiate into the anterior pituitary, the lens, and contribute to sensory organs including the olfactory epithelium, and inner ear, as well as several cranial ganglia. To study cranial placodes and their derivatives and generate cells for therapeutic purposes, several groups have turned to in vitro derivation of placodal cells from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs). In this review, we summarize the signaling cues and mechanisms involved in cranial placode induction, specification, and differentiation in vivo, and discuss how this knowledge has informed protocols to derive cranial placodes in vitro. We also discuss the benefits and limitations of these protocols, and the potential of in vitro cranial placode modeling in regenerative medicine to treat cranial placode-related pathologies.

Missing pieces of the pituitary puzzle: participation of extra-adenohypophyseal placode-lineage cells in the adult pituitary gland

The pituitary gland is a major endocrine tissue composing of two distinct entities, the adenohypophysis (anterior pituitary, cranial placode origin) and the neurohypophysis (posterior pituitary, neural ectoderm origin), and plays important roles in maintaining vital homeostasis. This tissue is maintained by a slow, consistent cell-renewal system of adult stem/progenitor cells. Recent accumulating evidence shows that neural crest-, head mesenchyme-, and endoderm lineage cells invade during pituitary development and contribute to the maintenance of the adult pituitary gland. Based on these novel observations, this article discusses whether these lineage cells are involved in pituitary organogenesis, maintenance, regeneration, dysplasia, or tumors.

The Neurod1/4-Ntrk3-Src pathway regulates gonadotrope cell adhesion and motility

Pituitary gonadotrope cells are essential for the endocrine regulation of reproduction in vertebrates. These cells emerge early during embryogenesis, colonize the pituitary glands and organize in tridimensional networks, which are believed to be crucial to ensure proper regulation of fertility. However, the molecular mechanisms regulating the organization of gonadotrope cell population during embryogenesis remain poorly understood. In this work, we characterized the target genes of NEUROD1 and NEUROD4 transcription factors in the immature gonadotrope αT3-1 cell model by in silico functional genomic analyses. We demonstrated that NEUROD1/4 regulate genes belonging to the focal adhesion pathway. Using CRISPR/Cas9 knock-out approaches, we established a double NEUROD1/4 knock-out αT3-1 cell model and demonstrated that NEUROD1/4 regulate cell adhesion and cell motility. We then characterized, by immuno-fluorescence, focal adhesion number and signaling in the context of NEUROD1/4 insufficiency. We demonstrated that NEUROD1/4 knock-out leads to an increase in the number of focal adhesions associated with signaling abnormalities implicating the c-Src kinase. We further showed that the neurotrophin tyrosine kinase receptor 3 NTRK3, a target of NEUROD1/4, interacts physically with c-Src. Furthermore, using motility rescue experiments and time-lapse video microscopy, we demonstrated that NTRK3 is a major regulator of gonadotrope cell motility. Finally, using a Ntrk3 knock-out mouse model, we showed that NTRK3 regulates gonadotrope cells positioning in the developing pituitary, in vivo. Altogether our study demonstrates that the Neurod1/4-Ntrk3-cSrc pathway is a major actor of gonadotrope cell mobility, and thus provides new insights in the regulation of gonadotrope cell organization within the pituitary gland.

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

Is There a Prechordal Region and an Acroterminal Domain in Amphioxus?

This essay re-examines the singular case of the supposedly unique rostrally elongated notochord described classically in amphioxus. We start from our previous observations in hpf 21 larvae [Albuixech-Crespo et al.: PLoS Biol. 2017;15(4):e2001573] indicating that the brain vesicle has rostrally a rather standard hypothalamic molecular configuration. This correlates with the notochord across a possible rostromedian acroterminal hypothalamic domain. The notochord shows some molecular differences that specifically characterize its pre-acroterminal extension beyond its normal rostral end under the mamillary region. We explored an alternative interpretation that the putative extension of this notochord actually represents a variant form of the prechordal plate in amphioxus, some of whose cells would adopt the notochordal typology, but would lack notochordal patterning properties, and might have some (but not all) prechordal ones instead. We survey in detail the classic and recent literature on gastrulation, prechordal plate, and notochord formation in amphioxus, compare the observed patterns with those of some other vertebrates of interest, and re-examine the literature on differential gene expression patterns in this rostralmost area of the head. We noted that previous literature failed to identify the amphioxus prechordal primordia at appropriate stages. Under this interpretation, a consistent picture can be drawn for cephalochordates, tunicates, and vertebrates. Moreover, there is little evidence for an intrinsic capacity of the early notochord to grow rostralwards (it normally elongates caudalwards). Altogether, we conclude that the hypothesis of a prechordal nature of the elongated amphioxus notochord is consistent with the evidence presented.

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

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

The embryonic development of the pineal organ, a neuroendocrine gland on top of the diencephalon, remains enigmatic. Classic fate-mapping studies suggested that pineal progenitors originate from the lateral border of the anterior neural plate. We show here, using gene expression and fate mapping/lineage tracing in zebrafish, that pineal progenitors originate, at least in part, from the non-neural ectoderm. Gene expression in chick indicates that this non-neural origin of pineal progenitors is conserved in amniotes. Genetic repression of placodal, but not neural crest, cell fate results in pineal hypoplasia in zebrafish, while mis-expression of transcription factors known to specify placodal identity during gastrulation promotes the formation of ectopic pineal progenitors. We also demonstrate that fibroblast growth factors (FGFs) position the pineal progenitor domain within the non-neural border by repressing pineal fate and that the Otx transcription factors promote pinealogenesis by inhibiting this FGF activity. The non-neural origin of the pineal organ reveals an underlying similarity in the formation of the pineal and pituitary glands, and suggests that all CNS neuroendocrine organs may require a non-neural contribution to form neurosecretory cells.

Highlighted Article: Gene expression and fate mapping/lineage tracing in zebrafish reveals that the pineal organ develops from the non-neural pre-placodal ectoderm under the control of FGF signalling.

The origins of the circumventricular organs

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.

Adenohypophysis placodal precursors exhibit distinctive features within the rostral preplacodal ectoderm

Placodes are discrete thickenings of the vertebrate cranial ectoderm that generate morpho-functionally distinct structures, such as the adenohypophysis, olfactory epithelium and lens. All placodes arise from a horseshoe-shaped preplacodal ectoderm in which the precursors of individual placodes are intermingled. However, fate-map studies indicated that cells positioned at the preplacodal midline give rise to only the adenohypophyseal placode, suggesting a unique organization of these precursors within the preplacode. To test this possibility, we combined embryological and molecular approaches in chick embryos to show that, at gastrula stage, adenohypophyseal precursors are clustered in the median preplacodal ectoderm, largely segregated from those of the adjacent olfactory placode. Median precursors are elongated, densely packed and, at neurula stage, express a molecular signature that distinguishes them from the remaining preplacodal cells. Olfactory placode precursors and midline neural cells can replace ablated adenohypophyseal precursors up to head-fold stage, although with a more plastic organization. We thus propose that adenohypophyseal placode precursors are unique within the preplacodal ectoderm possibly because they originate the only single placode and the only one with an endocrine character.

Fgf10+ progenitors give rise to the chick hypothalamus by rostral and caudal growth and differentiation

Classical descriptions of the hypothalamus divide it into three rostro-caudal domains but little is known about their embryonic origins. To investigate this, we performed targeted fate-mapping, molecular characterisation and cell cycle analyses in the embryonic chick. Presumptive hypothalamic cells derive from the rostral diencephalic ventral midline, lie above the prechordal mesendoderm and express Fgf10Fgf10⁺ progenitors undergo anisotropic growth: those displaced rostrally differentiate into anterior cells, then those displaced caudally differentiate into mammillary cells. A stable population of Fgf10⁺ progenitors is retained within the tuberal domain; a subset of these gives rise to the tuberal infundibulum - the precursor of the posterior pituitary. Pharmacological approaches reveal that Shh signalling promotes the growth and differentiation of anterior progenitors, and also orchestrates the development of the infundibulum and Rathke's pouch - the precursor of the anterior pituitary. Together, our studies identify a hypothalamic progenitor population defined by Fgf10 and highlight a role for Shh signalling in the integrated development of the hypothalamus and pituitary.

Regulation of pituitary stem cells by epithelial to mesenchymal transition events and signaling pathways

The anterior pituitary gland is comprised of specialized cell-types that produce and secrete polypeptide hormones in response to hypothalamic input and feedback from target organs. These specialized cells arise from stem cells that express SOX2 and the pituitary transcription factor PROP1, which is necessary to establish the stem cell pool and promote an epithelial to mesenchymal-like transition, releasing progenitors from the niche. The adult anterior pituitary responds to physiological challenge by mobilizing the SOX2-expressing progenitor pool and producing additional hormone-producing cells. Knowledge of the role of signaling pathways and extracellular matrix components in these processes may lead to improvements in the efficiency of differentiation of embryonic stem cells or induced pluripotent stem cells into hormone producing cells in vitro. Advances in our basic understanding of pituitary stem cell regulation and differentiation may lead to improved diagnosis and treatment for patients with hypopituitarism.

β-catenin is required in the neural crest and mesencephalon for pituitary gland organogenesis

Background

The pituitary gland is a highly vascularized tissue that requires coordinated interactions between the neural ectoderm, oral ectoderm, and head mesenchyme during development for proper physiological function. The interactions between the neural ectoderm and oral ectoderm, especially the role of the pituitary organizer in shaping the pituitary precursor, Rathke’s pouch, are well described. However, less is known about the role of head mesenchyme in pituitary organogenesis. The head mesenchyme is derived from definitive mesoderm and neural crest, but the relative contributions of these tissues to the mesenchyme adjacent to the pituitary are not known.

Results

We carried out lineage tracing experiments using two neural crest-specific mouse cre lines, Wnt1-cre and P0-cre, and determined that the head mesenchyme rostral to the pituitary gland is neural crest derived. To assess the role of the neural crest in pituitary development we ablated it, using Wnt1-cre to delete Ctnnb1 (β-catenin), which is required for neural crest development. The Wnt1-cre is active in the neural ectoderm, principally in the mesencephalon, but also in the posterior diencephalon. Loss of β-catenin in this domain causes a rostral shift in the ventral diencephalon, including the pituitary organizer, resulting in pituitary dysmorphology. The neural crest deficient embryos have abnormally dilated pituitary vasculature due to a loss of neural crest derived pericytes.

Conclusions

β-catenin in the Wnt1 expression domain, including the neural crest, plays a critical role in regulation of pituitary gland growth, development, and vascularization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0118-9) contains supplementary material, which is available to authorized users.