Identification of New Alleles of harpy
The mutant harpy was initially identified as an early arrest phenotype in the Tübingen zebrafish screen for early morphogenetic mutants, represented by a single recessive allele ti245 (Kane et al., 1996). The earliest visible phenotype seen using a dissecting scope was an abnormally bumpy head (Fig. 1A,B) accompanied by a shortened body axis, easily seen by comparing the distance between the nose and tail bud around the ventral aspect of the embryo. As development proceeds, mutant embryos were slightly smaller than normal at 24 hr and show little or no growth thereafter (Fig. 1C). In contrast to the other early arrest mutants, cell death and necrosis was not apparent before 24 hr (Kane et al., 1996), although afterward low numbers of cells appeared to be lysing throughout the embryo, especially in the central nervous system (CNS). (Evidence of low level cellular lysis based on pyknotic nuclei can be seen in Fig. 2H and throughout Fig. 7.)
Figure 1. General phenotype of harpy mutants. A,B: Live wild-type (WT) and mutant embryos at the 10 somite stage. Double sided arrows indicate the distinctive nose–tail bud separation in the mutants. C: Side views of live wild-type and mutant embryos at 24 hr. D,E: Anti-HuC (HU) stained neurons at 16 hr in wild-type and mutant embryos. The trigeminal ganglia (tg) are indicated, and double sided arrows indicate the width of the neural keel. Small arrows indicate single cells; note the larger size of the mutant cells. F,G: The 4′,6-diamidine-2-phenylidole-dihydrochloride (DAPI) staining at 20 hr in wild-type and mutant embryos. F′ and G′ are whole embryo dissociations. Note the cell in mitosis in the WT field; no cells are found in mitosis in the mutants. H: Rca1/emi1 coding region showing individual protein domains and the locations of point mutations (asterisks) found in hrpti245 and hrpx1 mutants. Scale bar = 50 μm in D,E, 5 μm in F′,G′.
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In a screen for neural patterning mutants, one of us (B.B.R.) recovered a recessive lethal mutation, termed x1, which causes a phenotype in which neurons were reduced in number but increased in size (Fig. 1D,E). Further investigation of 4′,6-diamidine-2-phenylidole-dihydrochloride (DAPI) -stained embryos showed similar changes in cell number and size throughout the embryo (Fig. 1F,G), and DAPI staining of dissociated cells revealed that cell nuclei were greatly enlarged in cells from mutant embryos (Fig. 1F′,G′). The early morphological defects of x1 mutants resembled those of hrpti245 and complementation testing confirmed the mutations were allelic. The new allele is henceforth designated as hrpx1.
harpy Is a Mutation in Rca1/emi1
The altered number and size of cells in harpy mutants suggested a defect in the cell division cycle. The nuclei shown in Figure 1G′ are more than twice the diameter of wild-type cells, suggesting that they might be at 8N or greater ploidy. In support, staining hrpx1 mutants for phospho-histone H3 revealed a gradual cessation of mitosis just after the onset of gastrulation. Few if any mitotic cells were detected at any stage after 8 hr (Fig. 2A–F). Identical results were seen for the hrpti245 allele (not shown).
Figure 2. Cycle characterization of cells in harpy mutants. A–F: Anti-phospho-histone H3 (pH3) staining in wild-type and harpy mutant embryos from 6 through 24 hr showed that typically by 8 hr no cells are in mitosis. Only occasional cells were pH3 positive at 24 hr (F). G: Mean number of cells per embryo, based on cell counts. Cells were counted after dissociating individual embryos with collagenase. Each time point shows the mean and standard deviation from 10 embryos. H–N: Bromodeoxyuridine (BrdU) incorporation in wild-type and harpy mutant embryos. H shows an harpy mutant injected at 14 hr and fixed 10 hr later. I–N shows embryos injected and fixed 45 min later. Note that at 30 and 42 hr many cells were positive in mutants; however, at 48 hr, mutants were mostly negative except for a small focus of BrdU incorporation in the tail. Scale bar = 100 μm in A–F, 85 μm in H,J,K,M,N, 200 μm in I,L.
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The mutation tiy121 of Zhang et al. (2008) that disrupts the mitotic regulator Rca1/emi1 produces a phenotype that closely resembles that of hrp. We confirmed that harpy maps within 1 cM of the map position of Rca1/emi1 (data not shown). On sequencing the alleles (Fig. 1H), both contained nonsense mutations that truncate the Rca1/emi1 protein midway through the transcript: hrpti245 has a stop codon within a putative nuclear localization signal and hrpx1 has a stop codon within the F-box domain, as well as two other changes in the coding region that introduce amino acid substitutions in the N-terminal half of the protein. (The tiy121 allele contains a premature stop codon that truncates the protein before the F-box domain. Hereafter, we refer to the tiy121 allele as hrptiy121). Truncation of the C-terminus, which includes the Zn-binding region and other domains that are indispensable for Rca1/emi1 activity (Reimann et al., 2001a; Miller et al., 2006), argues that all three harpy alleles are likely to be amorphic for Rca1/emi1 function. There is a fourth allele of hrphi2648 that has been identified in the insertional screen in Nancy Hopkin's Laboratory at MIT (Amsterdam et al., 2004; Wiellette et al., 2004); this allele is caused by an insertion in the first intron and behaves as a hypomorph (Zhang et al., 2008; Rhodes et al., 2009).
The Cell Cycle in harpy Mutants Is Blocked During Gastrulation at Approximately Division 14
We performed several experiments to better characterize the timing and division characteristics of the cell cycle defect in harpy mutants. To confirm whether cell division is blocked, embryos were dissociated with collagenase at various stages of development and the number of cells was counted directly using a hemocytometer (Fig. 2G). Wild-type embryos showed a steady increase from 4,600 ± 1,100 cells at 6 hr to 82,600 ± 11,800 cells at 24 hr. In contrast, hrpx1 mutants showed little change in cell number during this period, still comprising only 6,400 ± 2,000 cells at 24 hr. These data are consistent with the phospho-histone H3 staining and confirm that cell division ceases soon after 6 hr in harpy mutants.
It was previously reported that hrptiy121 mutants continue to synthesize DNA in endoreduplication cycles through at least 14 hr (Zhang et al., 2008). We examined BrdU incorporation in hrpx1 mutants at later stages to determine the persistence of these cycles. Despite the lack of cell division, many cells continued to synthesize DNA throughout the segmentation period, shown by BrdU incorporation over a 10-hr period (Fig. 2H). To explore the duration of endoreduplication, we used shorter pulses (45 min) at times up until 48 hr. We found that the intensity of label in mutants was slightly lower than wild-type embryos at 30 hr (Fig. 2I,L). Afterward, at 42 hr, the apparent number of cells labeled decreased (Fig. 2J,M), and at 48 hr, the only region of substantial labeling was in the tail (Fig. 2K,N). Thus, endoreduplication continued well into day 2 of development, but eventually tended to stop in all but a few cells of the embryo. It is interesting to note that for all embryos, both wild-type and mutant, individual cells are either labeled intensely or not at all, suggesting that the cell cycle dynamics in the mutant might be similar to that in wild-type embryos, i.e., cells in the harpy mutants may contain a distinctive S-phase. These results are consistent with the results by Rhodes et al. (2009) using flow cytometry to document the appearance of 4 N and greater nuclei during the segmentation stages.
To precisely assess the cycle that is blocked in harpy mutants, we labeled individual cells in mutant and wild-type embryos in divisions immediately after the Midblastula Transition, noting the cell cycle when the cell was labeled. We later counted the number of labeled cells at the beginning of gastrulation (6 hr) and the next day (at approximately 30 hr). In these experiments, we labeled a single exterior blastomere with lineage tracer and followed the progeny over the next day of development. Cells at the surface of the blastula typically alternate between anticlinal and periclinal divisions; hence, after two divisions there are typically two enveloping layer cells and two deep cells. Because of the fate differences in enveloping layer cells and deep cells, we counted each cell type separately, and from those counts, using the starting clone cell cycle and the log to the base 2, we derived the average cycle number that was attained by each enveloping layer or deep cell clone. These experiments are summarized in Figure 3 and Table 1. In normal embryos, deep cells advance to cycle 14 by the beginning of gastrulation and division 14 (into cycle 15) tends to occur during early gastrulation (Kane et al., 1992; Kane and Kimmel, 1993; Kimmel et al., 1994). On the other hand, enveloping layer cells typically arrest or slow their divisions at the beginning of epiboly: this temporary cycle arrest occurs in cycle 13 or, somewhat less frequently, in cycle 14. Within experimental error, these results were recapitulated in our studies. In both mutant and wild-type embryos, deep cells reached mid cycle 14 and enveloping layer cells only just reached cycle 14 at the beginning of gastrulation (Table 1; 6-hr time point).
Figure 3. harpy mutant cells stop dividing at cell cycle 14. A–D: Dorsally derived fates (A,B), and ventrally derived fates (C,D). In each embryo, an individual blastomere was labeled at cycle 11 or 12 and followed by time-lapse video microscopy through early segmentation. Each is restricted to a deep cell lineage (d) or an EVL (e) lineage at the cycle after labeling. The reconstructed cell lineage is displayed by cell cycle. Twenty-four hour histotypes and cell counts for the clones are shown at the bottom of the lineage tree, and left side views of the 24 hr embryos are shown below. E–I: Comparison of different cell types at 24 hr. Whereas notochord cells (E) were normally sized in the mutants, neurons (F) and muscle cells (G) were larger, and blood cells (H) and periderm cells (I) were supersized. In our lineage studies we never saw axons sprouted from presumptive neurons in mutants. Wild-type cells, green; harpy mutant cells, red. edc, endothelial cell; hgl, hatching gland; mus, muscle; nc, notochord; pdm, periderm; rbc, red blood cell. Scale bar = 100 μm in E–I.
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Table 1. Average Number of Cycles Completed in EVL and Deep Clonesa
| ||6 Hours||30 Hours|
| ||Cycle inj.||n||no. of cells||Calc. cycles||No. of cells||Calc. cycles||No. of cells||Calc. cycles||No. of cells||Calc. Cycles|
Cell counts the next day revealed nearly a ×10 difference between the numbers of cells in clones in mutant and those in wild-type embryos. Clones in wild-type embryos underwent 2 to 3 rounds of division between the beginning of gastrulation and 30 hr, reaching mid cycle 16 for EVL clones and mid cycle 17 for deep clones. Our results at 30 hr are consistent with a normal value of 50 to 100 thousand cells in the 1-day embryo. In the same time period, EVL clones in mutant embryos did not divide at all and deep cell clones either did not divide or underwent 1 additional round of division. These results predict that there are approximately 10 thousand cells in the mutant embryo at the time of arrest.
For many of the above experiments, we also time-lapse recorded several embryos (N = 12) from the blastula stage until after the end of gastrulation tail bud stage and reconstructed partial lineages (Fig. 3). Two pairs of wild-type and mutant lineages are shown, matched as ventral and dorsal examples. The wild-type examples were typical of this type of analysis: in the deep lineages there are many early divisions with major tissue restrictions occurring in the gastrula stage; in the enveloping layer lineages, the division rates tended to slow or arrest during epiboly. After gastrulation—and after recording—both lineages went through several divisions; we show the 24–30 hr fates and their numbers present on the next day. In the mutants, cell divisions occurred normally before gastrulation and then arrested just after the onset of gastrulation; when we checked the embryos the next day, no more divisions were apparent.
Global Patterning Was Normal in harpy Mutants but Local Patterning Was Not
In terms of the cell types produced by the above lineages, the lineage relationships in harpy mutant embryos are simple versions of development in wild-type embryos. Notochord lineages are normally related to muscle lineages and blood lineages are normally related to endothelial lineages, and, respectively, such examples are shown in Figure 3A,B and Figure 3C,D.
At 30 hr, the labeled cells in the clones gave an agnostic glimpse into the morphology of individual cells in the mutant embryos. Cells that normally have their terminal mitosis in the gastrula tended to appear normally sized in the mutants. For example, notochord cells (Fig. 3E) seemed to be completely normal (although the sheath cells surrounding them were larger than normal, data not shown). Neurons (Fig. 3F) and muscles (Fig. 3G) were slightly larger than normal; however, neurons tended not to sprout axons, and muscle cells were often dumpy and did not always span an entire somite. On the other hand, cells that normally divide many times in the embryo, such as blood (Fig. 3H) and periderm (Fig. 3I), became supersized versions of their wild-type counterparts. In the case of the blood cells, these cells were actually bigger than the blastomeres that produced them, suggesting that some process—we suggest endoreduplication—might be occurring in these particular cells that is causing abnormal cell sizing.
Many aspects of early development are relatively normal in harpy mutants: defects do not appear during cleavage, early epiboly, nor the beginning of gastrulation. harpy mutants can first be identified morphologically at 14 hr when they exhibit shortened AP axes (Fig. 1A,B). This appears to reflect slowing of convergent extension (Fig. 1D,E) as cells begin to increase in size. Neurogenin staining revealed that many neural progenitors are already enlarged at 10 hr and the neural plate is roughly 30% wider than normal (data not shown); anti-HuC staining showed that it still appeared wider than normal at 16 hr (Fig. 1D,E). The neural tube approaches normal width by 24 hr, but in the head region the neural tube becomes thinner than the wild-type especially in the anterior brain regions.
To provide a global view of specification in the mutant, we examined the mRNA expression patterns of pax2a, islet1, and foxa2, genes which are expressed in many different tissues of the embryo (Fig. 4).
Figure 4. Neural patterning defects in harpy mutants. Panels show wild-type reference embryos and harpy mutants. A–D: Expression of pax2a at 24 hr. Note that the diminution of expression in each domain is the result not of lowered expression in individual cells but of lower total numbers of positive cells. A,C: Whole-mount side views; insets show dorsal views of head-trunk region. B,D: High resolution dorsal views of hindbrain. E–K: Expression of islet1 at 24 hr. E,H: Whole-mount side views; insets show dorsal views of head region. The midline of the central nervous system (CNS; black line) is indicated near the epiphysis. F,G,I–K: Transverse sections at levels indicated in whole-mounts. Note lacking primary motor neurons, normally a bilaterally paired structure. L–O: Expression of foxa2 (axial) at 24 hr. L,N: Show a whole-mount side view; insets show dorsal views of head region. M,O: Show high resolution side views of trunk. Note the gaps between the large cells of the mutant floor plate and hypochord. bmn, branchio-motor nuclei; dc, diencephalon; ep, epiphysis; ed, endoderm; fp, floor plate; hbn, hindbrain neurons; hc, hypochord; mhb, midbrain–hindbrain border; nc, notochord; os, optic stalk; ov, otic vesicle; pcr, pancreas; pp, pharyngeal pouch; pmn, primary motor neurons; prn pronephric neck; prt pronephric tubule; prd, pronephric duct; px, pharynx; rb, Rohon-Beard sensory neuron; scn, spinal cord neurons; tc, telencephalon; vb, ventral brain. Scale bar = 100 μm in A,C,E,H,L,N, 40 μm in B,D,F,G,I,J,K,M,O.
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pax2a mRNA is expressed in cells in the midbrain–hindbrain border, reticulospinal and spinal interneurons, otic placode, and pronephros (Krauss et al., 1991a). In general, all of these cell types were produced at the correct times and locations in harpy mutants (Fig. 4A–D). As expected, based on the division arrest phenotype, there were fewer cells present and they were larger than normal; however, there was a distinct impression that there was less tissue expressing pax2a. This was best seen in the interneurons of the spinal cord, where many regions in the mutant were devoid of pax2a-expressing cells, and also in the hindbrain, which normally has almost a continuous layer of expression in the reticulospinal layers (Fig. 4B,D). There were numerous other defects, notably, the otic vesicle was smaller (discussed below) and expression in the proximal portion of the pronephric tubule was absent.
islet1 is expressed in cells of the neurectoderm, in the epiphysis, in nuclei of the telencephalon and the diencephalon, and in branchiomotor neurons, primary motor neurons and Rohan-Beard sensory cells (Korzh et al., 1993; Inoue et al., 1994). In harpy mutants (Fig. 4E–K) the general pattern of expression was normal, and, with the caveat mentioned above due to the lack of cell division, tissue expression was not as diminished as in the case of pax2a expression. A nice example of normal expression was that of the branchio-motor nuclei overlying the pharynx, which appeared almost normal in the mutant (Fig. 4H inset). The major exception to this general observation was the complete absence of islet1 positive nuclei in the telencephalon and diencephalon of the forebrain, and the almost complete loss of the epiphysis (Fig. 4E,H). Patterning at the cellular level was also abnormal in many regions. Typically, Rohon-Beard cells and primary motor neurons are reiterated paired structures of the spinal cord (Fig. 4F,G); in the mutant there were many missing cells (Fig. 4I–K) often represented by one very large cell on only one side of the spinal cord in tissue sections.
islet1 is also expressed in the endodermally derived portions of the pharynx and the pancreas. These tissues are dramatically reduced. The endodermal portion of the pharynx is the light expression underneath and caudalward of the branchio-motor nuclei, seen in the inset in Figure 4E; this was reduced in the mutant. islet1 is expressed in the endocrine portion of the pancreas; this expression was also reduced in the mutant (Fig. 4F,I). Similar to expression in the pancreas, both of these tissues are among the earliest tissues to proliferate in the developing embryo; in the absence of growth, only founder cells would be present and one would expect the region of expression to appear smaller but not necessarily weaker, and that is what we observed.
foxa2 (also called axial) is expressed in the ventral CNS of the forebrain, the floor plate neurons of the spinal cord, in the mesodermally derived hypochord, and in the endoderm (Strähle et al., 1993; Odenthal and Nüsslein-Volhard, 1998). Normally the floor plate and hypochord are arranged as rows of single cells, as seen in Figure 4M. In the mutant, the two tissues were patterned normally but cellular organization was constrained by the reduction in cell number: both tissues formed a single row of oversized cells which exhibited large gaps (Fig. 4O). On the other hand, the ventral forebrain (Fig. 4L,N) and the mutant pharynx (Fig. 4N inset) showed defects in line with the lack of growth in both of these tissues.
We focused on perturbations in the nervous system using the mRNA expression patterns of deltaA, notch1B, and fgf8, and protein expression patterns of Pax7, 3A10, Grasp, and acetylated tubulin (Fig. 5).
Figure 5. Neural patterning defects in harpy mutants. Panels show wild-type control embryos and harpy mutants. A,B: Expression of deltaA (dlA) at 24 hr. Dorsal view of hindbrain. Note the normal intensity of expression in mutant cells. C,D: Expression of notch1b at 24 hr. Dorsal view of head; insets show lateral view. Note expression in mutants is almost absent. E,F: Expression of fgf8 at 24 hr. Dorsal view of hindbrain. Note ectopic staining in the fourth rhombomere of the mutant. G,H: Anti-Pax7 staining at 24 hr. Arrows indicate neural crest cells migrating over the eye and pharyngeal arches; while these cells are present in the mutant, they are not seen on the migration pathway. I,J: Dorsal view of 3A10-stained Mauthner neurons at 30 hr, displaying abnormal pathfinding and absence of axons in the mutants. K,L: Anti-Grasp staining at 30 hr; dorsal view of the hindbrain. Arrows indicate commissural neurons that in the mutant are chaotic. M,N: Anti-acetylated tubulin (AT) staining at 24 hr; dorsal view of the trunk. Although axons are present in mutants, they do not express acetylated tubulin except in the tail region. mhb, midbrain–hindbrain border; mth, Mauthner neurons; ov, otic vesicle; r4, fourth rhombomere. Scale bar = 50 μm in A–D, 20 μm in F,G, 100 μm in E–N.
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Notch and Delta control neuronal cell fate and proliferation in the vertebrate nervous system much like in Drosophila. Notch expression is generally associated with proliferative zones of undifferentiated cells; these cells are thought to consist primarily of neural stem cells (Bierkamp and Campos-Ortega, 1993). Delta expression is found in scattered cells within proliferative zones; these cells have become post mitotic and are differentiating into neurons (Appel and Eisen, 1998; Haddon et al., 1998).
We found that the expression pattern of deltaA was mostly normal in the CNS of harpy mutants (Fig. 5A,B), although the number of positive cells was reduced. The differences that were observed seemed caused by the large cell size and the reduced number of cells in the mutant CNS. However, notch1b expression was severely reduced throughout the CNS of the mutants (Fig. 5C,D), suggesting that neural stem cell populations diminished in the mutant.
fgf8 is expressed normally in the neural plate during gastrulation (not shown) and continues to be expressed in the midbrain–hindbrain border through 24 hr (Reifers et al., 1998), as seen in Figure 5E. However, harpy mutants also showed an ectopic stripe of fgf8 expression in rhombomere 4 of the hindbrain (Fig. 5F). This represents an expression domain that is normally lost in wild-type embryos by 14 hr (Kwak et al., 2002), but which was aberrantly retained in harpy mutants.
anti-Pax7 marks a population of neural crest cells that migrate extensively and are often seen migrating over the surface of the eye and into lateral regions where cranial placodes and pharyngeal arches form. These cells were produced in harpy mutants, as shown by the presence of migratory Pax7-expressing cells in the head and anterior trunk (Fig. 5G,H); however, they were never observed beyond the lateral edges of the brain, suggesting that migration is impaired.
Axon pathfinding relies on stereotyped molecular cues provided by other cells. We asked if axonal pathfinding was perturbed in harpy mutants, because cues are likely to be missing due to early cessation of cell division.
3A10 antibody marks Mauthner cells (Metcalfe et al., 1990); these large neurons normally extend axons across the midline to join the contralateral medial longitudinal fascicle. Mauthner cells always formed in harpy mutants. However, in approximately a third of the mutants, Mauthner cells failed to extend axons or showed highly aberrant axon trajectories (Fig. 5I,J), in line with the almost complete lack of axons in lesser neurons.
Grasp-positive commissural neurons in the hindbrain normally decussate near rhombomere boundaries to form a regular array of commissures (Trevarrow et al., 1990), but in harpy mutants commissural axons formed highly chaotic patterns (Fig. 5K,L).
Acetylated tubulin antibody labels mature axons, and is strongly expressed in axons of the 24 hr embryo (Fig. 5M). In harpy mutants, although neurons with axons are present, they only expressed acetylated tubulin in the tail (Fig 5N) or not at all.
In Figure 6, we focused on placodal structures and other structures that interact with the epidermis, using the mRNA expression patterns of dlx3b, pax2a, fgf8, and ncad. Most of these structures are induced and their morphogenesis requires many cellular interactions, which could be comprised by the abnormally large size of the mutant cells.
Figure 6. Placodal patterning defects in harpy mutants. Panels show wild-type reference embryos and harpy mutants. A,B: Expression of dlx3b at 24 hr, showing reduced numbers of positive-expressing cells in the mutant. C,D: Optic vesicles in live embryos at 26 hr, showing extruded lens in the mutant. E,F: Otic vesicle in live embryos at 26 hr. Arrows indicate hair cells. Note the absence of layers and miniaturized structures in the mutant. G,H: Expression (arrows) of fgf8 at 24 hr in the otic vesicle. Note that expression is almost absent in the mutant vesicle (demarcated by oval). I–N: Expression of pax2a in the otic tissue. I,J: pax2a mRNA at 24 hr in the otic vesicle. Note that expression reduced in the mutant vesicle (demarcated by oval). K–N: Staining with anti-PAX2 in the otic placodes, showing expansion of the PAX2 territory between 12 and 14 hr in the mutant. O,P: Expression of N-cadherin (ncad) at 24 hr, showing a rosette of the lateral line (arrow head) and the lateral line primordia (arrow). In many mutants, the primordia could not be found. ls, lens; hc, hair cells; np, nose placode; oc, otocyst; op, otic placode; ov, otic vesicle; px, pharynx. Scale bar = 100 μm in A,B, 50 μm in C,D, 10 μm in E,F, 20 μm in G–P.
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dlx3b mRNA is normally expressed in the nasal, lens, and otic placodes (Akimenko et al., 1994), shown in Figure 6A. In the harpy mutant, dlx3b was expressed in far fewer cells (Fig. 6B), indicating that at 24 hr some placodal structures do not contain the correct cell number or that some cells are not yet completely specified. The lens forms in harpy mutants and gives rise to a small but relatively normal lens, but failure of the eye-cup to expand properly leads to extrusion of the lens by 24 hr (Fig. 6C,D). The otic vesicle remains small and usually produces only one or two hair cells with a single otolith (Fig. 6E,F). Reliable support cell markers are not available in zebrafish, but the macula remains essentially a single layer rather than stratifying into apical hair cell- and basal support cell-layers. Additionally, the macula shows little or no expression of fgf8 (Fig. 6G,H), which normally marks both hair cells and support cells of the developing maculae (Leger and Brand, 2002; Millimaki et al., 2007).
pax2a mRNA expression appeared in fewer cells in the mutant otic vesicle at 24 hr (Fig. 6I,J), consistent with its morphology and dlx3b expression at that time. Nevertheless, early development of the otic placode was surprisingly normal in harpy mutants, as shown by Pax2-staining at 12 and 14 hr (Fig. 6K–N). Moreover, despite the absence of cell division, the number of Pax2-positive cells nearly doubled between 12 and 14 hr in mutants (19.2 ± 3.8 cells at 12 hr, n = 6; 37.2 ± 4.4 cells at 14 hr, n = 8), which is similar to the fold-increase seen in wild-type embryos (82.9 ± 7.9 at 12 hr, n = 8; 130.2 ± 7.7 at 14 hr, n = 5) This suggests that early expansion of the otic territory occurs by ongoing otic induction or recruitment, not cell division.
Lastly, ncad mRNA is expressed in many regions of the embryo; here we use it to visualize the lateral line primordia. This group of cells moves down the lateral sides of the embryo and leaves small rosettes of cells that develop in the sensory structures of the lateral line (Fig. 6O). In the mutant, this structure was small and often difficult to find; when we did find it (Fig. 6P), it was hidden in various locations, somewhat randomly, on the sides of the embryo or the yolk sac.
Relationship Between Endoreduplication and Differentiation
During normal development, cell divisions become asynchronous and slower as cells begin to differentiate (Kane et al., 1992; Kane and Kimmel, 1993; Kimmel et al., 1994; Zamir et al., 1997). We hypothesized that the slowing of endoreduplication might be related to the initiation of cytodifferentiation that occurs beginning in the segmentation stages of development. This must occur in only a fraction of the total number of cells because most cells at these stages continue to show S-phase in both mutant and wild-type embryos (Fig. 7A,B). To test this, we examined patterns of BrdU incorporation in embryos stained with markers of various specified and differentiated cell types. We used three antibodies known to label early neuronal and mesodermal cell types: Anti-Islet1/2 antibody stains Rohon-Beard sensory neurons, primary motoneurons and trigeminal ganglion neurons by 10 hr (Inoue et al., 1994). Anti-Engrailed antibody labels the midbrain–hindbrain border and muscle pioneer cells (Hatta et al., 1991). Anti-Pax2 antibody, which primarily binds Pax2a protein (Riley et al., 1999), labels cells in the midbrain–hindbrain border, otic placode, pronephros, reticulospinal neurons in the hindbrain, and CoSA spinal interneurons (Krauss et al., 1991b; Mikkola et al., 1992). 3A10 antibody labels Mauthner neurons (Metcalfe et al., 1990).
Figure 7. Patterns of bromodeoxyuridine (BrdU) incorporation. A–I: Images show lateral views with anterior to the left (A,B,E,F,H) or dorsal views with anterior to the top (C,I) or to the left (D,G). A,B: Head region of a wild-type embryo and harpy mutant incubated with BrdU from 18 to 24 hr. Note that the majority of cells incorporate BrdU in both mutant and wild-type embryos. C–I: harpy mutants injected with BrdU at 12 hr and fixed later at 19 hr. Note that in all the cases shown, cells that express the indicated marker of differentiation (black) never incorporate BrdU (green). C: Anti-Engrailed stained midbrain–hindbrain border. D,E: Anti-Isl1/2 staining showing Rohon-Beard sensory neurons and trigeminal ganglia. F: Anti-Engrailed stained muscle pioneers. G: Anti-Pax2 stained spinal interneurons. H: Anti-Pax2 stained pronephric cells. I: 3A10 antibody-stained Mauthner neurons; these were fixed at 27 hr. mhb, midbrain–hindbrain border; mp, muscle pioneers; Mth, Mauthner neurons; prt, pronephros; sin, spinal interneurons; rb, Rohon-Beard sensory neurons; tg, trigeminal ganglia. Scale bar = 100 μm in A,B, 75 μm in C, 30 μm in D–G,I, 15 μm in H.
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To assess whether DNA synthesis occurred in these cells during the segmentation stages, BrdU was injected at 12 hr and embryos were fixed and co-stained at 19 hr or later for BrdU and the individual cell type markers. Many of the specific cell types expressing the above markers did not show BrdU incorporation (Table 2; Fig. 7D–I), though BrdU was detected broadly in surrounding cells. In particular, BrdU incorporation was not seen in muscle pioneers, Rohon-Beard sensory neurons, primary motoneurons, trigeminal cranial ganglia, Mauthner, reticulospinal, and spinal interneurons, all cell types that are known to have early terminal mitoses (Mendelson, 1986; Myers et al., 1986; Mikkola et al., 1992; Hirsinger et al., 2004; Won et al., 2006; Caron et al., 2008; Rossi et al., 2009).
Table 2. BrdU Incorporation in Cells of harpy Mutants Expressing Markers of Specification and Differentiation
|Marker||Cell type||No. cells BrdU positive||No. cells counted||No. embryos|
|Islet1/2||Rohon-Beard sensory neurons||0||412||29|
| ||Primary motoneurons||0||408||29|
| ||Trigeminal ganglion||0||243||29|
|Pax2||Reticulospinal primary neurons||0||94||26|
| ||Spinal interneurons||0||272||22|
| ||Otic Vesicle||26||451||11|
| ||Midbrain-Hindbrain Border||24||290||9|
In contrast, BrdU was detected in a small fraction of Pax2-staining cells in the developing pronephric duct, within the otic vesicle and in the midbrain–hindbrain border (Table 2; Fig. 7C and data not shown). Each of these domains generate multiple diverse cell types many of which do not differentiate until later. Together, these findings are consistent with the hypothesis that the process of endoreduplication does not continue after cells have begun terminal differentiation, but may still occur during earlier stages of organogenesis.
harpy Acts Both Cell-Autonomously and Cell Non-autonomously
To ask if the division characteristics and the cell morphology of the harpy mutant were cell autonomous, we transplanted mixtures of wild-type and mutant cells into wild-type or mutant host embryos, as described previously (Ho and Kane, 1990). In brief, cells from two donors, one labeled with rhodamine–dextran and one labeled with fluorescein–dextran, were transplanted into one host; donors were genotyped based on their 24 hr phenotypes. We recorded the phenotypes of the individual donor cells as well as the number of cells that were produced by cell division. In general, we found that most of the division and phenotypic characteristics that we identified in our lineage work above were autonomous to the genotype of the transplanted cells. Figure 8 shows examples of transplanted wild-type and mutant cells placed in either wild-type or mutant hosts. Notably, harpy mutant cells that became neurons were larger than their wild-type counterparts, regardless of the genotype of the host. Muscle cells, as noted above, were almost normal in size. However we did note some nonautonomous effects: Normally, harpy mutant neurons tend not to sprout axons. We found that wild-type cells placed in the CNS of harpy mutants also tended not to sprout axons (Fig. 8B,D), whereas harpy cells in wild-type CNS sometimes did (Fig. 8C). We interpret this to mean that some developmental environments in the mutant are not normal. This is an interesting result because Rca1 (in Drosophila) might have some role in axogenesis (Dong et al., 1997). At least in vertebrates, this may be a minor effect compared with nonautonomous effects in the embryonic CNS.
Figure 8. harpy acts both cell autonomously and non-autonomously. A–F: Examples of transplanted wild-type (green) and mutant (red) cells placed in either a wild-type (A,C,E,F) or mutant (B,D) host. A,B: Composite images of a neural and muscle chimera. C: Ultraviolet (UV) image showing a magnified view of neural and muscle chimera in A. Inset shows single harpy interneuron which projected an axon. D: magnified views of neural chimera in B. Note that the wild-type cells did not project axons. E,F: Facial view of telencephalic region (E) and dorsal view of brain (F) neural clones. Note that wild-type cells frequently cross the midline; mutant cells tend to not. G: Comparison of number of cells in clones. For each chimera, two to three cells from two separate individuals were placed into a host embryo at 5 hr, and recorded immediately afterward (txp), then at 6 hr and again at 30 hr. H,I: Mosaic embryos double stained with HRP to detect donor cells (brown/black) and anti-phospho histone H3 to detect mitotic cells (green). H,H′: Labeled mutant cells transferred to an wild-type host; mutant cells are not dividing. I,I′: Labeled wild-type cells transferred to an unlabeled mutant host. A dividing wild-type cell is indicated (arrow). Scale bar = 200 μm in A,B,E,F, 50 μm in C,D, 100 μm in F,I.
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Normally, cells that enter the neural keel have a tendency to divide at the midline with the daughter sibs often moving to opposite sides of the forming neural tube. We noted that when mutant cells were transplanted into the CNS of wild-type or mutant embryos, the mutant cells tended to stay on one side of the neural keel (Fig. 8E,F) even when their wild-type co-transplanted cells moved to both sides.
We also counted the number of donor cells at 30 hr in our host embryos. For each chimera, 2 to 3 cells from two separate individuals were placed into a host embryo in the late blastula. The cell cycle number at this time would be approximately cycle 12 or 13. Immediately after the transplant, each chimeric embryo was recorded. Then at 6 hr, just before gastrulation, the number of cells in the chimera was re-inspected and recorded; similarly at 30 hr. Regardless of host genotype, harpy mutant cells do not tend to proliferate after 6 hr whereas wild-type cells do (Fig. 8G).
To test whether the mitotic block in harpy mutants is cell autonomous, we produced mosaic embryos by transplanting cells from biotin–dextran labeled donor embryos to unlabeled host embryos during blastula stage, fixed the embryos at 24 hr, and double stained them with a horseradish peroxidase reaction product to detect donor cells and anti-phospho histone H3 to detect mitotic cells. When we transplanted cells from a mutant into a wild-type embryo, we found that mutant cells never expressed phospho-histone H3, indicating that they do not enter mitosis (Fig. 8H). Labeled wild-type controls divided normally, even when transplanted into a mutant host (Fig. 8I). These data confirm that disruption of harpy function blocks mitosis cell autonomously.