Lateral line placodes of aquatic vertebrates are evolutionarily conserved in mammals

Patterns of senescence and apoptosis during development of branchial arches, epibranchial placodes, and pharyngeal pouches

BACKGROUND

Many developmental processes are coregulated by apoptosis and senescence. However, there is a lack of data on the development of branchial arches, epibranchial placodes, and pharyngeal pouches, which harbor epibranchial signaling centers.

RESULTS

Using immunohistochemical, histochemical, and 3D reconstruction methods, we show that in mice, senescence and apoptosis together may contribute to the invagination of the branchial clefts and the deepening of the cervical sinus floor, in antagonism to the proliferation acting in the evaginating branchial arches. The concomitant apoptotic elimination of lateral line rudiments occurs in the absence of senescence. In the epibranchial placodes, senescence and apoptosis appear to (1) support invagination or at least indentation by immobilizing the margins of the centrally proliferating pit, (2) coregulate the number and fate of Pax8+ precursors, (3) progressively narrow neuroblast delamination sites, and (4) contribute to placode regression. Putative epibranchial signaling centers in the pharyngeal pouches are likely deactivated by rostral senescence and caudal apoptosis.

CONCLUSIONS

Our results reveal a plethora of novel patterns of apoptosis and senescence, some overlapping, some complementary, whose functional contributions to the development of the branchial region, including the epibranchial placodes and their signaling centers, can now be tested experimentally.

Programmed Cell Death Not as Sledgehammer but as Chisel: Apoptosis in Normal and Abnormal Craniofacial Patterning and Development

Coordination of craniofacial development involves an complex, intricate, genetically controlled and tightly regulated spatiotemporal series of reciprocal inductive and responsive interactions among the embryonic cephalic epithelia (both endodermal and ectodermal) and the cephalic mesenchyme — particularly the cranial neural crest (CNC). The coordinated regulation of these interactions is critical both ontogenetically and evolutionarily, and the clinical importance and mechanistic sensitivity to perturbation of this developmental system is reflected by the fact that one-third of all human congenital malformations affect the head and face. Here, we focus on one element of this elaborate process, apoptotic cell death, and its role in normal and abnormal craniofacial development. We highlight four themes in the temporospatial elaboration of craniofacial apoptosis during development, namely its occurrence at (1) positions of epithelial-epithelial apposition, (2) within intra-epithelial morphogenesis, (3) during epithelial compartmentalization, and (4) with CNC metameric organization. Using the genetic perturbation of Satb2, Pbx1/2, Fgf8, and Foxg1 as exemplars, we examine the role of apoptosis in the elaboration of jaw modules, the evolution and elaboration of the lambdoidal junction, the developmental integration at the mandibular arch hinge, and the control of upper jaw identity, patterning and development. Lastly, we posit that apoptosis uniquely acts during craniofacial development to control patterning cues emanating from core organizing centres.

Responses of Epibranchial Placodes to Disruptions of the FGF and BMP Signaling Pathways in Embryonic Mice

Placodes are ectodermal thickenings of the embryonic vertebrate head. Their descendants contribute to sensory organ development, but also give rise to sensory neurons of the cranial nerves. In mammals, the signaling pathways which regulate the morphogenesis and neurogenesis of epibranchial placodes, localized dorsocaudally to the pharyngeal clefts, are poorly understood. Therefore, we performed mouse whole embryo culture experiments to assess the impact of pan-fibroblast growth factor receptor (FGFR) inhibitors, anti-FGFR3 neutralizing antibodies or the pan-bone morphogenetic protein receptor (BMPR) inhibitor LDN193189 on epibranchial development. We demonstrate that each of the three paired epibranchial placodes is regulated by a unique combination of FGF and/or bone morphogenetic protein (BMP) signaling. Thus, neurogenesis depends on fibroblast growth factor (FGF) signals, albeit to different degrees, in all epibranchial placodes (EP), whereas only EP1 and EP3 significantly rely on neurogenic BMP signals. Furthermore, individual epibranchial placodes vary in the extent to which FGF and/or BMP signals (1) have access to certain receptor subtypes, (2) affect the production of Neurogenin (Ngn)2⁺ and/or Ngn1⁺ neuroblasts, and (3) regulate either neurogenesis alone or together with structural maintenance. In EP2 and EP3, all FGF-dependent production of Ngn2⁺ neuroblasts is mediated via FGFR3 whereas, in EP1, it depends on FGFR1 and FGFR3. Differently, production of FGF-dependent Ngn1⁺ neuroblasts almost completely depends on FGFR3 in EP1 and EP2, but not in EP3. Finally, FGF signals turned out to be responsible for the maintenance of both placodal thickening and neurogenesis in all epibranchial placodes, whereas administration of the pan-BMPR inhibitor, apart from its negative neurogenic effects in EP1 and EP3, causes only decreases in the thickness of EP3. Experimentally applied inhibitors most probably not only blocked receptors in the epibranchial placodes, but also endodermal receptors in the pharyngeal pouches, which act as epibranchial signaling centers. While high doses of pan-FGFR inhibitors impaired the development of all pharyngeal pouches, high doses of the pan-BMPR inhibitor negatively affected only the pharyngeal pouches 3 and 4. In combination with partly concordant, partly divergent findings in other vertebrate classes our observations open up new approaches for research into the complex regulation of neurogenic placode development.

Senescence and Apoptosis: Architects of Mammalian Development

Mammalian development involves an exquisite choreography of cell division, differentiation, locomotion, programmed cell death, and senescence that directs the transformation of a single cell zygote to a mature organism containing on the order of 40 trillion cells in humans. How a single totipotent zygote undergoes the rapid stages of embryonic development to form over 200 different cell types is complex in the extreme and remains the focus of active research. Processes such as programmed cell death or apoptosis has long been known to occur during development to help sculpt organs and tissue systems. Other processes such as cellular senescence, long thought to only occur in pathologic states such as aging and tumorigenesis have been recently reported to play a vital role in development. In this review, we focus on apoptosis and senescence; the former as an integral mechanism that plays a critical role not only in mature organisms, but that is also essential in shaping mammalian development. The latter as a well-defined feature of aging for which some reports indicate a function in development. We will dissect the dual roles of major gene families, pathways such as Hox, Rb, p53, and epigenetic regulators such as the ING proteins in both early and the late stages and how they play antagonistic roles by increasing fitness and decreasing mortality early in life but contribute to deleterious effects and pathologies later in life.

Evolutionary and Developmental Associations of Neural Crest and Placodes in the Vertebrate Head: Insights From Jawless Vertebrates

Neural crest and placodes are key innovations of the vertebrate clade. These cells arise within the dorsal ectoderm of all vertebrate embryos and have the developmental potential to form many of the morphological novelties within the vertebrate head. Each cell population has its own distinct developmental features and generates unique cell types. However, it is essential that neural crest and placodes associate together throughout embryonic development to coordinate the emergence of several features in the head, including almost all of the cranial peripheral sensory nervous system and organs of special sense. Despite the significance of this developmental feat, its evolutionary origins have remained unclear, owing largely to the fact that there has been little comparative (evolutionary) work done on this topic between the jawed vertebrates and cyclostomes—the jawless lampreys and hagfishes. In this review, we briefly summarize the developmental mechanisms and genetics of neural crest and placodes in both jawed and jawless vertebrates. We then discuss recent studies on the role of neural crest and placodes—and their developmental association—in the head of lamprey embryos, and how comparisons with jawed vertebrates can provide insights into the causes and consequences of this event in early vertebrate evolution.

Notch signalling regulates epibranchial placode patterning and segregation

Epibranchial placodes are the geniculate, petrosal and nodose placodes that generate parts of cranial nerves VII, IX and X, respectively. How the three spatially separated placodes are derived from the common posterior placodal area is poorly understood. Here, we reveal that the broad posterior placode area is first patterned into a Vgll2⁺/Irx5⁺ rostral domain and a Sox2⁺/Fgf3⁺/Etv5⁺ caudal domain relative to the first pharyngeal cleft. This initial rostral and caudal patterning is then sequentially repeated along each pharyngeal cleft for each epibranchial placode. The caudal domains give rise to the neuronal and non-neuronal cells in the placode, whereas the rostral domains are previously unrecognized structures, serving as spacers between the final placodes. Notch signalling regulates the balance between the rostral and caudal domains: high levels of Notch signalling expand the caudal domain at the expense of the rostral domain, whereas loss of Notch signalling produces the converse phenotype. Collectively, these data unravel a new patterning principle for the early phases of epibranchial placode development and a role for Notch signalling in orchestrating epibranchial placode segregation and differentiation.

Chicken embryos share mammalian patterns of apoptosis in the posterior placodal area

In the posterior placodal area (PPA) of C57BL/6N mice and primate-related Tupaia belangeri (Scandentia), apoptosis helps to establish morphologically separated otic and epibranchial placodes. Here, we demonstrate that basically identical patterns of apoptosis pass rostrocaudally through the Pax2⁺ PPA of chicken embryos. Interplacodal apoptosis eliminates unneeded cells either between the otic anlage and the epibranchial placodes 1, 2 and/or 3, respectively (type A), or between neighbouring epibranchial placodes (type B). These observations support the idea that in chicken embryos, as in mammals, interplacodal apoptosis serves to remove vestigial lateral line placodes (Washausen & Knabe, 2018, Biol Open 7, bio031815). A special case represents the recently discovered Pax2^- /Sox2⁺ paratympanic organ (PTO) placode that has been postulated to be molecularly distinct from and developmentally independent of the ventrally adjacent first epibranchial (or 'geniculate') placode (O'Neill et al. 2012, Nat Commun 3, 1041). We show that Sox2⁺ (PTO placodal) cells seem to segregate from the Pax2⁺ geniculate placode, and that absence of Pax2 in the mature PTO placode is due to secondary loss. We further report that, between Hamburger-Hamilton (HH) stages HH14 and HH26, apoptosis in the combined anlage of the first epibranchial and PTO placodes is almost exclusively found within and/or immediately adjacent to the dorsally located PTO placode. Hence, apoptosis appears to support decision-making processes among precursor cells of the early developing PTO placode and, later, regression of the epibranchial placodes 2 and 3.