Effects of the Brachyury (T) mutation on morphogenetic movement in the mouse embryo

$\text{T}\text{T}$ embryos have been first distinguishable at 8 days post coitum by their gross morphological abnormalities. By quantitative morphometry of histological sections, anomalies in the homozygotes were expressed numerically. At 8 days p.c., morphologically identifiable T-homozygotes had an increased number of ectodermal and a reduced number of mesodermal cells compared to the wild type. At 7 days p.c., embryos with a low mesoderm/ectoderm ratio were found only in litters of $\text{T}\text{+}\text{×}\text{T}\text{+}$ matings at the expected frequency. At 6 days p.c., one-fourth of the embryos in $\text{T}\text{+}\text{×}\text{T}\text{+}$ litters showed a delay in the transition from cuboidal to squamous endoderm. No such embryos were found in the +/+ × +/+ matings. In 6-, 7-, and 8-day mutant embryos, cells proliferated at statistically normal rates. Therefore, it may be said that advanced morphological irregularities of 8-day homozygotes cannot be accounted for by anomalies in cell proliferation. When the total cell number was 5 × 10⁴/embryo (8 days), a sudden change was observed in the regional distribution of mesodermal and ectodermal cells along the anteroposterior axis of $\text{T}\text{T}$ embryos. Since no regional difference in the cell cycle time was observed, these abnormalities may best be explained by anomalies in cell migration. These results strongly suggest abnormal morphology of $\text{T}\text{T}$ mutants resulting from defects in morphogenetic movement.     

A Review of the Eyeless Mutant in the Mexican Axolotl

SYNOPSIS. As a homozygous recessive, gene e in the Mexican axolotlprevents optic vesicles from forming, thus producing an eyelessanimal. Previous experimental evidence has indicated that thegene acts by affecting the ability of anterior medullary plateectoderm in the eye field to respond to inductive mesodermalsignals. Other possible mechanisms of gene action are described.The hypothalamus is also affected and "eyeless" animals aresterile. The absence of eyes results in increased levels ofcirculating MSH and thus the animal is also highly pigmented.Eyes may be grafted into the heads of "eyeless" axolotls. Theseeyes become functional and lead to normal pigmentation. Whenpresent as a homozygous recessive, gene r acts to allow thepenetration of heterozygous e (E/e r/r). This results in abnormaleye development.     

Autonomy for a specific gene product in oocytes: Experimental evidence in the polychaetous annelid,Platynereis dumerilii

Oocytes of Platynereis dumerilii in early vitellogenesis were injected into female worms with oocytes of similar diameter. The donor oocytes were labeled by the or gene controlling eye pigmentation and, after some weeks of growth, were spawned together with the host oocytes. In most cases, a few donor progeny could be found among the offspring produced by the hosts. Donor progeny were examined with respect to an or gene-dependent maternal effect which normally causes wild-type eye color in homozygous ($\text{or}\text{or}$) larvae originating from the crossings of heterozygous ($\text{or}{}^{\mathrm{+}}\text{or}$) females and homozygous ($\text{or}\text{or}$) males. This maternal effect was absent from homozygous ($\text{or}\text{or}$) larvae derived from homozygous ($\text{or}\text{or}$) donor oocytes which had developed in heterozygous females. Conversely, this maternal effect was observed in homozygous ($\text{or}\text{or}$) larvae derived from heterozygous ($\text{or}{}^{\mathrm{+}}\text{or}$) donor oocytes which had developed in homozygous ($\text{or}\text{or}$) host females. It is concluded that the oocyte genome is active at the or⁺ locus during oogenesis and that the oocyte is autonomous with respect to the product of synthesis of the or⁺ locus. In the present case, the “maternal effect” is therefore caused by synthetic activity in the growing oocyte. The results are discussed with respect to current information on gene products from animal genomes.     

Inductive and axial properties of prospective wing-bud mesoderm in the chick embryo

Prospective wing-bud mesoderm, stripped of ectoderm mechanically through the use of glass needles, or chemically by means of EDTA or trypsin, was obtained from donor embryos of stages 11 through 21. Grafts were made in both homopleural (aadd and apdv) and heteropleural (aadv and apdd) combinations to the right flank of host embryos of the same range of stages. Flank ectoderm from the host healed over the graft in a few hours and, in combinations between donors and hosts in the range of stages 12 through 17, the composite formed, with high frequency, a limb bud capped by an apical ectodermal ridge, and then developed into a supernumerary wing in which all proximodistal levels were represented. When either member of the combination was older than stage 17, only incomplete limbs, if any, were formed. Regardless of their orientation on the host, the supernumerary limbs always showed the axial characteristics appropriate to their side of origin.Supernumerary wings failed to form if the grafts were inserted into a space tunneled between flank ectoderm and its underlying mesoderm. If the covering ectoderm were deliberately torn and forced to heal over the graft, however, an ectodermal ridge formed and a supernumerary limb developed.It is concluded, therefore, that: (1) the wing-bud mesoderm, appropriately combined with flank ectoderm, has the property of morphological and axial self-differentiation by stage 12; (2) the apical ectodermal ridge is induced in flank ectoderm by prospective wing-bud mesoderm; (3) this inductive power is restricted to prospective wing-bud mesoderm from donors of stages 12 through 17; (4) the response competence is limited to flank ectoderm that has healed over the mesoderm; and (5) this competence is lost by the end of stage 17.     

Inductive interactions in the spatial and temporal restriction of lens-forming potential in embryonic ectoderm of Xenopus laevis

The process of lens cell determination in amphibians is currently viewed as one involving a series of inductive interactions. On the basis of previous investigations, these interactions are thought to begin during gastrulation when the presumptive foregut endoderm and then the heart mesoderm come into contact with the presumptive lens ectoderm. This earlier period of induction is followed by the later interaction of the optic vesicle with the lens-forming ectoderm. Transplantation experiments were performed to determine the relative significance of the early and later periods of induction in the process of lens cell determination in the anuran Xenopus laevis. Various ectodermal tissues were transplanted either into the lens-forming region of open neural plate stage host embryos or over the newly formed optic vesicle of later neurula stage embryos. All transplanted tissues were labeled with the intracellular marker horseradish peroxidase to assess the exact origins of any induced lens structures. The results indicate that all nonneural ectodermal tissues have some lens-forming potential early during gastrulation; however, this potential is restricted to the lens-forming region, and perhaps nearby regions, later in development during the time of neurulation. Furthermore, the results show that the optic vesicle is not a substantial inductor of the lens in tissues that have not been previously exposed to the earlier series of inductive interactions that take place during gastrulation and neurulation. Since the optic vesicle does not appear to be a sufficient inductor of the lens, these earlier inductive interactions are, therefore, essential in the process of lens cell determination in Xenopus. These earlier inductive interactions lead to a steady increase in what may be called a lens-forming bias in the presumptive lens ectoderm during this period of development. The eventual loss in the ability of nonlens ventral ectoderm to respond to these lens inductors is presumably the result of other determinative processes that occur in this tissue.     

A re-examination of lens induction in chicken embryos: in vitro studies of early tissue interactions

Early studies on lens induction suggested that the optic vesicle, the precursor of the retina, was the primary inducer of the lens; however, more recent experiments with amphibians establish an important role for earlier inductive interactions between anterior neural plate and adjacent presumptive lens ectoderm in lens formation. We report here experiments assessing key inductive interactions in chicken embryos to see if features of amphibian systems are conserved in birds. We first examined the issue of specification of head ectoderm for a lens fate. A large region of head ectoderm, in addition to the presumptive lens ectoderm, is specified for a lens fate before the time of neural tube closure, well before the optic vesicle first contacts the presumptive lens ectoderm. This positive lens response was observed in cultures grown in a wide range of culture media. We also tested whether the optic vesicle can induce lenses in recombinant cultures with ectoderm and find that, at least with the ectodermal tissues we examined, it generally cannot induce a lens response. Finally, we addressed how lens potential is suppressed in non-lens head ectoderm and show an inhibitory role for head mesenchyme. This mesenchyme is infiltrated by neural crest cells in most regions of the head. Taken together, these results suggest that, as in amphibians, the optic vesicle cannot be solely responsible for lens induction in chicken embryos; other tissue interactions must send early signals required for lens specification, while inhibitory interactions from mesenchyme suppress lens-forming ability outside of the lens area.     

Identification and characterization of Xenopus kctd15, an ectodermal gene repressed by the FGF pathway

The FGF pathway regulates a variety of developmental processes in animals through activation and/or repression of numerous target genes. Here we have identified a Xenopus homolog of potassium channel tetramerization domain containing 15 (KCTD15) as an FGF-repressed gene. Kctd15 expression is first detected at the gastrula stage and gradually increases until the tadpole stage. Whole-mount in situ hybridization reveals that the spatial expression of kctd15 is tightly regulated during early embryogenesis. While kctd15 is uniformly expressed throughout the presumptive ectoderm at the early gastrula stage, its expression becomes restricted to the non-neural ectoderm and is excluded from the neural plate at the early neurula stage. At the mid-neurula stage, kctd15 shows a more restricted distribution pattern in regions that are located at the anterior, lateral or medial edge of the neural fold, including the preplacodal ectoderm, the craniofacial neural crest and the prospective roof plate. At the tailbud stage, kctd15 expression is mainly detected in neural crest- or placode-derived tissues that are located around the eye, including the mandibular arch, trigeminal ganglia and the olfactory placode. FGF represses kctd15 expression in ectodermal explants, and the inhibition of FGF receptor with a chemical compound dramatically expands the region expressing kctd15 in whole embryos. Dorsal depletion of kctd15 in Xenopus embryos leads to bent axes with reduced head structures, defective eyes and abnormal somites, while ventral depletion causes defects in ventral and caudal morphologies. These results suggest that kctd15 is an FGF-repressed ectodermal gene required for both dorsal and ventral development.     

Inhibition of DNA synthesis in embryonic mouse retina as a result of gene interaction.

It has been shown by autoradiography using ³H-thymidine that 11-day mouse embryos doubly homozygous for the autosomal recessive genes fidget (gene symbol fi) and ocular retardation (or), have three to five times fewer labelled nuclei in their retina anlages as do normal (genotype +/+ +/+) embryos singly homozygous for fidget (+/+ $\text{fi}\text{fi}$) or ocular retardation (+/+ $\text{or}\text{or}$). In 11-day embryos of +/+ +/+, +/+ $\text{fi}\text{fi}$ and +/+ $\text{or}\text{or}$ genotypes the labelled nuclei are localized mainly in the inner zone of the retina anlage. However, in double homozygotes the indices of labelled nuclei were not significantly different in the inner and outer zones of the retina anlage. The retina anlage of 12-day double homozygote, $\text{fi}\text{fi}\text{or}\text{or}$, has practically no nuclei synthesizing DNA. Consequently, the mutant genes fi and or which prolong the G₁ period of the cell cycle in single homozygotes, act synergetically to stop DNA synthesis in the retina anlage cells of 12-day $\text{fi}\text{fi}\text{or}\text{or}$ embryos.     

The cell cycle and retinal histogenesis fidget mutant mice.

The autoradiographic method using [³H]thymidine has shown that the autosomal recessive mutant gene fidget (gene symbol fi) prolonging the presynthetic period of the cell cycle in the retinal anlage in homozygotes retards the transition of retinal cells to the differentiated state. Some retinal cells of normal embryos (+/+) start their transition to the differentiated state on the 11th day of embryogenesis, while in $\text{fi}\text{fi}$ embryos this process starts only on the 12th day. An active transition of retinal cells to the differentiated state especially in the peripheral zone of the mutant retina takes place 2 days later as compared to normal embryos. The number of differentiating cells in the retina of mutants at the stages of development studied is considerably lower as compared to the norm. The analysis of the cell cycle parameters in 15-day embryos has shown that in the mutants the retina is less mature as compared to +/+ embryos. The sequence of transition of various cell types to the differentiated state in the retina of $\text{fi}\text{fi}$ embryos is the same as in the norm. Gene fidget seems to interfere with proliferative rather than critical (quantal) cell cycles in the developing mouse retina.     

Control of pattern duplication in the retinotectal system of Xenopus. Induction of duplication in eye fragments by secondary cuts.

Following surgical ablation of the temporal (posterior) region of the eye-bud in stage 32 Xenopus frog embryos, the surviving nasal (anterior) fragment gradually rounds up to form a functional eye and orderly retinotectal map. Large nasal fragments ($\text{N}\text{-}\text{2}\text{3}$) assemble topographically normal maps, as does the majority of nasal “half-eye” fragments; small nasal fragments ($\text{N}\text{-}\text{1}\text{3}$), and a minority of nasal half-eye fragments, give a characteristic, mirror-symmetrical duplication map, similar (but not identical) to the “double-nasal” maps which develop when two nasal half-eyes are fused to form a frank NN double-eye. Ventral fragments and temporal fragments show similar size-dependent behavior, although their characteristic duplicate maps are topographically different from those of nasal fragments and more similar to the “double-ventral” and “double-temporal” maps of VV and TT recombinant eyes. Here we show that a simple surgical transection, applied either dorsally or ventrally to large nasal ($\text{N}\text{-}\text{2}\text{3}$) fragments so as to isolate a subregion of the tissue at the dorsum or venturm of the fragment, induces full or partial duplication of the nasal type in the majority of cases. The results refute the hypothesis that special properties at the eye-bud center, by their presence or absence in the fragment, control pattern duplication, and point instead toward interactions around the circumference of the eye-bud as a crucial parameter in determining positional information in the retina.     

Ultraviolet effects on presumptive primordial germ cells (pPGCs) in Xenopus laevis after the cleavage stage

The presumptive primordial germ cell (pPGC) number with development after the cleavage stage and the fate of pPGCs damaged by uv irradiation were studied in successive Epon sections (0.5 μm thick) with the light microscope in both uv-irradiated and unirradiated Xenopus embryos. taking survival rate and sterility into consideration. The pPGCs of the uv-irradiated embryos occupy nearly the same location in the embryos as those of the unirradiated embryos at stages 12, 17, 23, and 28 [see Ikenishi, K., and Kotani, M. (1975). Develop. Growth Different. 17, 101–110]. At stage $\text{33}\text{34}$ they are found in the central part of the endoderm cell mass in the uv-irradiated embryos, while they are situated in the lateral or dorsal part of the endoderm cell mass in the unirradiated. In the uv-irradiated embryos, a cavity which was never found in the unirradiated embryos was observed in the endoderm cell mass beneath the archenteron cavity and in the almost-median part of the posterior endoderm cell mass at stages 17 and 23, respectively, and some vacuoles in pPGCs as well as in somatic cells around those pPGCs were noticed at stages $\text{17–}\text{33}\text{34}$. The number of pPGCs of the unirradiated enbryos increases about three- or fourfold during stages 12–46, while the pPGCs of the uv-irradiated embryos slowly increase in number from stage 17 to stage 28, indicating that the division occurs in pPGCs, then decrease with development and finally disappear from the tadpole.     

Ontogeny of the retina and optic nerve of Xenopus laevis: IV. Ultrastructural evidence of early ganglion cell differentiation

Ultrastructural evidence indicates that Xenopus retinal ganglion cell axons differentiate early, between stages 28 and 32. Light microscope studies indicated the presence of argryophilic material in the ventral retina and optic stalk of early embryos. Ultrastructural analysis of this region confirmed the presence of axons in the stalk and interstices of ventral retinal cells. Axons containing aligned microtubules and neurofilaments and elongated mitochondria with a paucity of other cell inclusions are found with increasing frequency in the ventral retina from stages 28 through $\text{33}\text{34}$. Central and dorsal regions of the retinas examined show little or no evidence of axons. A discrete, small bundle of axons is found in the optic stalk of stage 28 embryos and by stage $\text{30}\text{31}$ the number of axons in bundles has increased, suggesting early fasciculation. Between stages 28 and $\text{33}\text{34}$ (± 12 hr) extracellular space surrounding early axons diminishes and processes from neuroretinal cells in contact with axons surround developing axon bundles. The evidence presented suggests that axon initiation occurs in stages much earlier than previously reported. Other investigators have failed to detect ganglion cell differentiation prior to stage 32 possibly because they examined regions of the retina with few axons. Thus, experiments which rotate the retina in the orbit may have to be reevaluated since regenerating axons may use previously established pathways to organize and “home in” on tectal target cells.     

Studies on an anophthalmic strain of mice. VI. Lens and cup interaction

In the embryology of the eye region in the anophthalmic strain of mice ($\text{ZRDCT}\text{Ch}$), development proceeds normally until Day 10 (26 somites). At this time a lens is induced, but it is smaller in size and may be improperly centered in the optic cup. Where the lens is centered in relation to the optic cup determines whether microphthalmia or anophthalmia will occur. Also, we observed that optic cup formation is different in normal control strains.     

The partial purification of growth-inhibiting factor of the brachypodism-H mouse embryos

The extract of 13-day-old $\text{bp}{}^{\mathrm{H}}\text{bp}{}^{\mathrm{H}}$ embryos contains the factor causing inhibition of the growth of the long bone rudiments of normal embryos in vitro. A 400-fold purification of the factor has been achieved. The molecular weight of the main protein component of the preparation obtained is 76,000. The synthesis of this protein appears to be controlled by bp^H gene.     

Evidence for dosage compensation of an X-linked gene in the 6-day embryo of the mouse.

The time at which dosage compensation of an X-linked gene in the mouse is established has been estimated by measuring the activity levels of two glycolytic enzymes, phosphoglycerate kinase (EC 2.7.2.3) and triosephosphate isomerase (EC 5.3.1.1), during early development of embryos from $\text{X}\text{X}$ and $\text{X}\text{O}$ mice. During preimplantation development the level of phosphoglycerate kinase in embryos from $\text{X}\text{X}$ mice was constant for the first 48 hr of development (2.55–2.71 nmoles/hr/embryo) and then dropped to one-half the earlier level (1.44 nmoles/hr/embryo) by 72 hr of development. The general developmental profile of phosphoglycerate kinase was similar in embryos from $\text{X}\text{O}$ mice; however, the absolute level of enzyme activity was reduced to approximately 1.4 nmoles/hr/embryo during the first 48 hr of development and to 0.9 nmoles/hr/embryo by 72 hr of development. These differences in phosphoglycerate kinase levels between embryos from $\text{X}\text{X}$ and $\text{X}\text{O}$ mice disappeared between the blastocyst and egg cylinder stages (150 hr postplug) during which time the activity levels had increased to 183 and 193 nmoles/hr/egg cylinder for embryos from $\text{X}\text{O}$ and $\text{X}\text{X}$ mice, respectively.The activity level for triosephosphate isomerase in embryos from $\text{X}\text{X}$ and $\text{X}\text{O}$ mothers did not differ at any stage of development; this suggests that the gene coding for triosephosphate isomerase is autosomal. In addition the level of activity remained constant during preimplantation development (approximately 3 nmoles/hr/embryo) and then, like phosphoglycerate kinase, increased 100-fold between the blastocyst and egg cylinder stages.The frequency distribution of the activity ratio (triosephosphate isomerase to phosphoglycerate kinase) in the egg cylinder of individual embryos from both $\text{X}\text{X}$ and $\text{X}\text{O}$ mothers was determined in order to detect differences among $\text{X}\text{X}\text{,}\text{X}\text{O}$ and $\text{X}\text{Y}$ embryos. These frequency distributions were unimodal, and hence these results coupled with the gene dosage differences observed during preimplantation development indicate that dosage compensation for an X-linked gene has been established in the 6-day mouse embryo.