Embryonic Segments Research Articles

Disruption of segmentation in a short germ insect embryo

ABSTRACT A heat shock (of 15 min at 48°C) given to early embryos of the locust, Schistocerca gregaria, results in localized abnormalities in the segment pattern subsequently formed. Most defects involve two consecutive segments of the thorax or abdomen, and these are analysed in detail. The abdominal defects fall into three main classes each of which involves the absence of a particular region of the segment pair and, in one class, duplication of the region which remains. The thoracic defects similarly involve absence of parts of the segments and the formation of a single limb base from which one, two, or three limbs develop. Heat shock may result in the absence of parts of segments in two distinct ways. It may interfere with the process of segmentation or it may delete parts of already formed segment primordia. These possibilities are discussed although, at present, neither can be excluded. The duplication observed in some abdominal disruptions and the formation of triple limbs indicates that the absence of parts of embryonic segments is followed by pattern regulation similar to that occurring in regeneration studies on larval segments and appendages of other insects. Two out of the three classes of abnormality can be explained in terms of intercalary regeneration restoring pattern continuity, but it is possible that discontinuities persist in the remaining class.

Development of the eye-antenna imaginal disc and morphogenesis of the adult head in Drosophila melanogaster.

We have studied the organization and development of the eye-antenna imaginal disc of Drosophila melanogaster. We examined the pattern of gynandromorph mosaicism and determined the "sturt distances" between 42 different structures of the head, antenna, and maxillary palpus. A morphogenetic map based on these sturt distances resembles more closely in size and shape that of a single thoracic segment than that of two or more adjacent segments, suggesting that the eye-antenna disc is derived from a single embryonic body segment. We examined the morphology of the eye-antenna discs in situ in late third-instar larvae in serial cross sections. The two discs are connected medially by a thin cellular membrane that probably serves to join the two discs during evagination and morphogenesis of the adult head. A fate map of the imaginal disc was established by cutting the mature disc into fragments and transplanting the fragments into host larvae for metamorphosis. The peripodial layer of the eye-antenna disc is thickened in several regions, and our data suggest that these thickened areas represent primordia of adult head structures. A comparison of the location of precursors in the imaginal disc with those of the differentiated structures of the adult head revealed the nature of the morphogenetic movements that must occur during evagination and differentiation.

Spatially regulated expression of homeotic genes in Drosophila.

The sites of transcript accumulation for six different homeotic loci of the Antennapedia and bithorax gene complexes (ANT-C and BX-C) were identified within embryo tissue sections by in situ hybridization. These six loci belong to the Antennapedia class of the homeo box gene family. Transcripts encoded by each locus are detected primarily in discrete, nonoverlapping regions of the embryonic central nervous system (CNS). The regions of the CNS that contain transcripts encoded by each of these loci correspond to the embryonic segments that are disrupted in mutants for these genes. The maintenance of spatially restricted expression of each ANT-C and BX-C locus could involve hierarchical, cross-regulatory interactions that are mediated by the homeo box protein domains encoded by these genes.

Preferential binding of imaginal disk cells to embryonic segments of Drosophila.

Cell recognition and selective adhesion may be important in pattern formation; such processes in Drosophila melanogaster could be responsible for the maintenance of segment boundaries, the morphogenesis of metamorphosing imaginal disks, and the paths of axon outgrowth during neurogenesis. As cells from different imaginal disks of Drosophila are able to recognize and sort out from one another, we decided to investigate whether these larval cells could recognize and bind to the epidermis of intact embryos. We report here that imaginal disk cells bind preferentially to the epidermis of the embryonic segments from which they are derived: thoracic disk cells to thoracic segments and genital disk cells to abdominal segments. Furthermore, thoracic disk cells recognize and bind, without preference, to all segments of the homoeotic mutant Df(3R)P9 (ref. 6). We conclude, therefore, that cells of the same segmental origin have similar recognition properties at different developmental stages.

The engrailed locus of drosophila: In situ localization of transcripts reveals compartment-specific expression

The engrailed locus plays a unique and critical role in organizing the segmented body plan of Drosophila. Embryos lacking engrailed function die with fused, abnormal segments. Adult mosaics with patches of engrailed cells similarly suffer defects in all of their segments, but only with mutant cells that are in the posterior developmental compartment of each segment. The non-uniform requirement for engrailed function reflects the position-dependent expression of the engrailed locus and we demonstrate it here unambiguously by directly visualizing engrailed transcripts in frozen sections of embryos and larvae and in whole imaginal discs. These results demonstrate that developmental compartments subdivide the embryonic insect segments. In these and in the compartments of the later developmental stages the engrailed locus is expressed in the posterior but not the anterior compartments. With its role in controlling the developmental pathway of the posterior compartment cells, the engrailed locus may be an example of a binary developmental switch.

Further studies onengrailed mutants inDrosophila melanogaster

In 1975, Morata and Lawrence proposed that theengrailed (en) locus was a selector gene that controlled the different pathways of development followed by anterior and posterior compartments. This hypothesis assumed that the phenotype ofen1 flies results from partial inactivity of theen+ product. However, the mutant phenotype ofen1/DF(2R)en− is weaker thanen1/en1. This implies that the partial P→A transformation ofen1 does not result primarily from reduction inen+ activity. Heterozygotes betweenl(2) en alleles andDf(2R)en− deletions express a similar phenotype of fused embryonic segments to that described by Nusslein-Volhard and Wieschaus, and also by Kornberg, forl(2) en homozygoes. By this criterion, the lethal phenotype results from partial or complete lack ofen_ activity. Despite this, the1(2) en alleles give only a weak P→A transformation, whether recovered as embryonic lethals or by failure to complementen1. They appear to define only one locus and, with the exception ofen1, the available genetic date suggest that the complementation pattern at this locus is simple. Thus, it is unlikely thatengrailed is the sole determinant of the A/P compartment separation. It might be one of a number of loci that affect the alternative pathways followed by anterior and posterior compartments.

Implantation of the rabbit blastocyst: Sequential changes in estradiol and progesterone and their receptors

To determine the temporal relationship of the endocrine events involved in rabbit blastocyst implantation, (1) progesterone (P) and estradiol (E) in plasma and uterine flushings, (2) P and E in the cytosol of endometrium and myometrium, and (3) P receptors (PR) and E receptors (ER) in endometrium and myometrium were examined from days 0 through 7 post coitum by radioimmunoassay and radioreceptorassay. Plasma P levels increased significantly from 0.3 +/- 0.06 to 76 +/- 9.4 ng/ml (p = less than 0.005) 2 hours after mating (day 0), declined to 10.0 +/- 1.2 ng/ml by day 6, and increased to 12.0 +/- 1.2 ng/ml by day 7. Cytosol P levels of endometrium and myometrium increased from 3.2 +/- 0.2 to 22.8 +/- 2.2 ng/gm and 2.1 +/- 0.4 to 14.1 +/- 2.1, respectively, from days 1 to 6. On day 7, cytosol P levels of the embryonic segment (8.2 +/- 0.5 ng/gm) were lower than those of the interembryonic segment (12.4 +/- 1.8 ng/gm). P levels in uterine flushings increased significantly from 0.05 +/- 0.01 ng/ml on day 0 to 19.9 +/- 0.7 ng/ml on day 7 (p = less than 0.001). In contrast, E levels in plasma, uterine flushings, and cytosol of uterine tissues showed no significant change from days 1 through 7, but the levels from 6 hours after mating were significantly higher than those before mating (p = less than 0.05). ER in the cytosol and nucleus of all uterine regions increased significantly after mating but decreased after implantation. In contrast, PR in the cytosol and nucleus of all uterine regions showed no significant change on days 0 to 6 post coitum. After implantation, the embryonic segment had significantly higher PR in the nucleus than that in the cytosol (13.0 +/- 1.5 versus 8.0 +/- 0.5 fmol/micrograms), whereas the interembryonic segment had significantly higher PR in the cytosol than that in the nucleus (12.0 +/- 0.5 versus 9.0 +/- 0.5 fmol/micrograms DNA). These findings showed that changes in P in the plasma and uterine flushings preceded those in uterine tissues before implantation. The changes in uterine tissue ER and PR after mating but before and after implantation suggest that the putative nidatory sites are prepared for implantation but undergo endocrine changes to protect the conceptus immediately after implantation.

A molecular analysis of fushi tarazu, a gene in Drosophila melanogaster that encodes a product affecting embryonic segment number and cell fate

Mutations at the fushi tarazu locus in Drosophila melanogaster affect both segment number and the pattern of cuticular structures on alternating segments of embryos. The ftz gene has been cloned and characterized. Two mutations, ftz w20 and ftz Rpl are associated with lesions in a 3.2 kb fragment of DNA cloned in the Antennepedia Complex (ANT-C) chromosome “walk.” The structure of DNA isolated from the ftz w20 and ftz Rpl chromosomes indicates that the mutations are associated with a 4.9 kb insertion of DNA and a chromosomal rearrangement breakpoint, respectively. The 3.2 kb genomic DNA fragment hybridizes to a 1.8 kb polyadenylated transcript which accumulates maximally at 2–4 hr of embryonic development. The ftz w20 and ftz Rpl mutations have different phenotypic consequences for the developing embryo, although both mutations interrupt the 1.8 kb transcription unit. The genetic and molecular data indicate that the 1.8 kb transcript derives from the ftz locus. The gene products are synthesized and utilized several hours prior to the visibly detectable morphogenetic events which the gene apparently regulates.

Chorionic gonadotropin receptors and immunoreactive chorionic gonadotropin in implantation of the rabbit blastocyst

To determine the temporal relationship between immunoreactive chorionic gonadotropin, chorionic gonadotropin receptors, and implantation of the rabbit blastocyst, (1) immunoreactive chorionic gonadotropin in plasma, uterine flushings and blastocysts; (2) chorionic gonadotropin receptors on blastocysts (day 5 and 6) and embryonic and interembryonic segments of the uterus (day 7); and (3) chorionic gonadotropin receptors on the endometrium and myometrium (day 1 through 6) were measured. Binding of I125-labeled β-subunit of human chorionic gonadotropin (hCG) by cell membranes from blastocysts increased significantly from 6.0 ± 1.1 fmol/mg of protein (mean ± SE) on day 5 (N = 6) to 16.1 ± 1.3 fmol/mg of protein on day 6 (n = 6) (P < 0.001). Immunoreactive chorionic gonadotropin levels in blastocyst fluid increased from 0.3 ng/ml of day 5 to 0.65 ng/ml on day 6. Specific binding of I125-labeled hCG was absent in endometrial and myometrial cell membranes before implantation (days 1 to 6) but was found in decidual cells from embryonic segments on day 7. Plasma immunoreactive chorionic gonadotropin or chorionic gonadotropin-like material increased from 6 ng/ml of day 1 to 52 ng/ml on day 7. Uterine flushings had chorionic gonadotropin levels of 0.4 ng/ml of day 2, which increased slowly to 1.0 ng/ml on day 7. Intrauterine instillation of hCG into nonpregnant uterine horns demonstrated transuterine absorption of hCG with plasma hCG levels showing a dose-related response. Our findings demonstrate that (1) immunoreactive chorionic gonadotropin or chorionic gonadotropin-like material is detectable in plasma, uterine flushings, and blastocyst fluid before implantation, (2) chorionic gonadotropin receptors are present on the blastocyst cells before implantation, and (3) the uterus can absorb chorionic gonadotropin from its lumen.

Parameters influencing reversal of segment sequence in posterior egg fragments ofCallosobruchus (Coleoptera).

The "double abdomen" type of embryonic segment pattern can develop in posterior fragments ofCallosobruchus eggs. In this type of pattern, a series of posterior segments is joined in reversed polarity to an equal set from the original pattern persisting in normal polarity. Reversed and non-reversed sets are fused in a plane of mirror symmetry, which shows in the larval cuticle as a "symmetry line". This line may be located anywhere in the posterior thorax or the anterior abdomen. The reversed abdomen may be incomplete caudally due to secondary causes. Polarity reversal and concomitant double abdomen formation occurred only when temporary constriction was terminated before cellularization of the blastoderm, and only when the anterior fragment was degenerating. Maximum reversal frequency was 94% of analyzable posterior partial larvae when the constriction was applied slightly anterior to the middle of the egg when the egg contained 4-32 nuclei. Reversal was often restricted to longitudinal strips of the larval cuticle. The longitudinal borderlines between the reversed and the non-reversed strips ran predominantly along the larval midlines. Such borderlines probably existed in the blastoderm anywhere around its circumference, but borderlines in the future mesoderm and serosa would be internalized during gastrulation and dorsal closure, respectively, and the embryonic midlines would then become secondary borderlines visible in the larval cuticle. If a morphogen is involved in segment pattern formation, its transport in the egg must be polarized longitudinally in order to account for reversals restricted to longitudinal cuticular strips.

ATP Accumulation in Peanut Seeds during Seed-Ripening and During the Dormancy-Breaking Process

Ripening and dormancy-breakage of peanut (Arachis hypogaea L.) seeds are accompanied by an increase, followed by a decrease, in the ability to accumulate ATP (from AMP + phospho enolpyruvate (PEP)) in the embryonic axes as well as in the cotyledonary segment powder. A similar increase and decrease profile in the concentrations of the ATP precursors (AMP + ADP) in the embryonic axes and in the cotyledonary segments is noted during dormancy breakage. The ability of the seeds to germinate during the ripening process (after dormancy-breakage treatment) seems to correlate with the decrease in the ability to accumulate ATP in the embryo, while dormancy breakage seems to be correlated with the same phenomenon in the cotyledon. Imbibition of seeds (as whole seeds or as embryonic axes and cotyledonary segments separately) causes a 10-40-fold increase in the ability to accumulate ATP from AMP and PEP, and a decrease in the concentrations of the precursors of ATP. Ethylene has a stimulatory effect on ATP accumulation in embryonic axes during imbibition.

Genes involved in the activation of the bithorax complex ofDrosophila.

We have studied the genetic properties and developmental effects of several mutations, in eight different loci, which alter the specification of embryonic segments. All of the changes are related to the transformations caused by mutants of the bithorax complex.Several properties: the interactions of these mutants with different mutants of the bithorax complex, the interactions between themselves, the effect of changing dosages of their wildtype alleles and their response to ether induction of phenocopies permit one to distinguish between those mutations which affect activation of the bithorax genes in early embryogenesis and those which affect expression of the bithorax genes during development. This paper deals mainly with the Rg-bx locus and its interactions with other loci which affect segment specification.In particular two loci show genetic and developmental characteristics which seem to conform to those expected for a repressor coding gene (Polycomb) and for an inducer synthesizing gene (Rg-bx) active in a negative control system of the genes in the bithorax complex. A model for the interaction of the wildtype products of these genes to determine the segmental characteristics of both the thorax and abdomen is proposed.

The binding of progesterone in different parts of the rabbit uterus during implantation

Progesterone receptors, both nuclear and cytosolic, were determined in the embryonic and inter-embryonic segments of the rabbit uterus at 6, 7 and 8 daypost-coitum. At day6 postcoitum a higher concentration of nuclear receptor in the embryonic segment was observed compared with that in the inter-embryonic segment. A reverse situation was observed in the case of cytoplasmic receptors. On the 7th daypost-coitum, no significant alteration in the concentration of either kind of the receptors was observed. However, on day 8, a higher concentration of both nuclear and cytosolic receptors at the embryonic site was observed compared to that in inter-embryonic segment. Since receptors are influenced only in the immediate vicinity of the blastocyst, it can be suggested that the blastocyst plays a role in the induction of its own implantation. Further, at day 8 increase in receptor concentration at the embryonic site may be related to the presence of decidual tissue at this site.

Nucleotide sequence of immunoglobulin heavy chain joining segments between translocated VH and mu constant regions genes.

To investigate the mechanism of recombination of immunoglobulin heavy chain variable and constant region genes, we have determined the nucleotide sequence of a large portion of the recombination region between an active C mu gene and its associated VH gene, isolated from an IgM-secreting mouse plasmacytoma, HPC76. By comparison with the sequence of the mu mRNA, we determined the exact boundaries of the intervening sequence between the VH76 and C mu genes. The rearranged VH76 gene encodes up to amino acid 116 without interruption, the 3' 39 nucleotides (the JH76 region) being derived from an embryonic JH segment (JH315) whose sequence was recently determined [Early, P., Huang, H., Davis, M., Calame, K. & Hood, L. (1980) Cell 195, 981-992]. The active JH76 does not use the first two codons of the embryonic JH315 from which it is derived. This indicates that V-J recombination is important in generating diversity within the third hypervariable region of heavy chains. We have identified another JH segment (JHA4), located 336 nucleotides 3' to the rearranged JH76 segment. This JH segment is expressed in the heavy chains of anti-levan myeloma proteins, which have truncated third hypervariable regions. We propose that the nucleotide sequence 5' to JHA4 is important for generating V region genes with shortened third hypervariable regions.

Estrogen Binding by Embryonic and Interembryonic Segments of the Rat Uterus Prior to Implantation

The capacity of embryonic and interembryonic segments of the uterus to bin#{224}� H-estradiol was compared during the early stages of nidation in estrogen-treated “delayed implanting” rats. At the start of nidation, embryonic and interembryonic uterine segments were equally capable of binding H-estradiol. As implantation proceeded, however, the capacity of the embryonic segments (relative to that of interembryonic tissue) to bind newly administered ‘ H-estradiol decreased progressively. This deficiency was observed within 4 h, and by 16 h after the initiation of implantation, embryonic segments bound only one-fourth as much newly administered ‘ H-estradiol as was bound by interembryonic segments. These data are consistent with the hypothesis that estrogen diffusing from the blastocyst partially saturates the hormone binding capacity of the adjacent endometrium. This study also demonstrates that the functional differences between embryonic and interembryonic regions of the rat uterus are associated with and preceded by, differences in estrogen binding. In this system, communication between embryo and uterus appears to commence within 4 h, i.e.,at least20 h before increased vascular permeability at the impending nidation site can be demonstrated.

The development of Drosophila embryos after partial u.v. irradiation

U.v. irradiation of the anterior pole of nuclear multiplication stage Drosophila eggs produces embryos with defective anterior structures. At a low frequency embryos resembling some phenotypes of the bicaudal syndrome of Drosophila were observed. These embryos had no head or thorax and the eight abdominal segments were spread to the anterior of the embryo. Sometimes spiracles, characteristic of the most posterior embryonic segment were observed at the anterior of the embryo. The development of these embryos was followed, and abnormalities occurred as early as blastoderm formation. The extent of the blastoderm defects correlated well with the final abnormality in the embryo.

Ultrastructure of mouse egg-cylinder.

The mouse egg-cylinder prior to and after mesoderm formation was studied by means of electron microscopy. The ultrastructural appearance of the proximal entoderm of both embryonic and extraembryonic segments suggests an intensive absorptive and nutritional activity. Numerous pinocytotic vacuoles, microvilli, primary and secondary lysosomes and fair amounts of rough endoplasmic reticulum and free ribosomes were the most important characteristics of these cells. After mesoderm formation, the extraembryonic entoderm showed the aforementioned characteristics even more prominently, while the cells of embryonic entoderm became flattened and depleted of microvilli and of almost all organelles. The cells of the extraembryonic and embryonic ectoderm prior to and after mesoderm formation had the same ultrastructural appearance as mesodermal cells. The cytoplasm of these cells was replete with free ribosomes, but other organelles such as mitochondria and rough endoplasmic reticulum were few in number. The architecture of all cells of the egg-cylinder except those of the extraembryonic entoderm suggested a very low level of differentiation. The criteria and possibilities for the determination of the degree of differentiation on the ultrastructural level and possible differences in protein synthesis in extraembryonic entoderm as compared with other parts of the embryo are considered.

Morphodifferentiation of teeth in embryonic mandibular segments in tissue culture.

Morphodifferentiation of teeth in embryonic mandibular segments in tissue culture.

Embryonic and larval development and segment formation in Ibla quadrivalivis Cuv. (Cirripedia).

Embryonic development in I. quadrivalvis follows the typical cirripede pattern except for modifications of cleavage and of development of the gut, associated with a large yolky egg. The nauplius is free-swimming but lecithotrophic. It does not grow, and moults irregularly. The development of cypris features begins precociously and the second half of naupliar life is demersal. The cypris stage is unmodified. Settling was not observed. The maxillary segments develop as a delayed continuation of the naupliar segmental sequence, the thoracic segments from a growth zone of seven ectoteloblasts and eight mesoteloblasts. Each row of eight cells budded from the mesoteloblasts develops into the paired somites of a segment. The midgut develops independently of the yolk cells. In cirripedes, increased yolk results in modification of cleavage and gut development but otherwise has little influence on embryonic development. Larval modifications associated with lecithotrophy promote direct development of the cypris but do not include modification of the cypris. Irregularity of moulting is associated with lack of larval growth. Relative brood sizes suggest that increased yolk in cirripedes offers advantages in larval survival. Post-naupliar segment formation in cirripedes resembles in detail that of Malacostraca, indicating a possible phylogenetic affinity between Malacostraca and Maxillopoda which can be tested by further studies on segment formation in non- Malacostraca.

The Development of the Polychaete Haploscoloplos fragilis

ABSTRACT The eggs of Haploscoloplos fragilis, laid in gelatinous cocoons, develop as yolky trochophores which add 4 segments before hatching on the 6th day and two more during a pelagic phase before settling and the onset of feeding on the 10th day. The 6 (embryonic) segments bear a metatroch and 5 amphitrochs. The embryo also has an akrotroch, prototroch, telotroch, and neurotroch. All cilia disappear after settling, when the embryo shows resumption of segment formation. Anterior to the first trunk segment lie the prostomium and mouth region. The latter becomes subdivided by an annulus, but later fuses with the first trunk segment as the peristomium. All other segments become chaetigerous, the 5 embryonic being of thoracic type, the postembryonic abdominal. At the posterior end of the body lies the pygidium, with the segment-forming growth zone immediately in front of it. The development of trunk segments in two series, embryonic and post-embryonic, is characteristic of ariciids, as is the fusion of the first trunk segment with the mouth region to form the peristomium. It is possible that the mouth region incorporates at least one other cephalized segment. Ariciid eggs vary in size among species. No direct relationship exists between egg size and pelagic development. The mode of oviposition also varies. Whether or not a pelagic phase occurs depends on the stage at which the embryo becomes free in the water, which is governed in turn by properties of the cocoon (e.g. jelly viscosity). A pelagic phase is always associated with enhanced ciliation, while in non-pelagic development ciliation is reduced. The basic ciliation (akrotroch, prototroch, metatroch, telotroch, and neurotroch) is usually supplemented by additional segmental bands.