Evolution of the bilaterian mouth and anus.

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

[External morphological characteristics during the embryonic development of Portunus pelagicus].

The embryonic development of Portunus pelagicus was studied under laboratory conditions at a water temperature of 25-26 Degrees Celsius, salinity of 30, and pH of 7.8-8.4. The embryogenesis of Portunus pelagicus was divided into six stages: cleavage, blastula, gastrula, nauplius, metanauplius, and protozoea. Embryogenesis lasted about 300 h post spawning. Eggs began superficial cleavage about 28 h after spawning when the nucleus appeared at the surface of the egg till the egg divided into 16 cells. The blastula stage was observed about 40 h post spawning and gastrula stage appeared when the presumptive endoderm and other cells near them invaginated. The fourth-stage of embryogenesis, nauplius, was characterized by three pairs of appendages appearing about 90 h post spawning, while metanauplius, the fifth-stage of embryogenesis, was characterized by five pairs of appendages, which appeared about 110 h post spawning. The sixth stage of embryogenesis was protozoea, which was characterized by seven pairs of appendages appearing about 140 h post spawning. The compound eye, heart and pigment cells were also found in the protozoea stage. After the natatory seta formed on the top of maxilliped, the protozoea developed into the zoea at the time of hatching (about 300 h post spawning).

Extracellular matrix assembly and organization during zebrafish gastrulation.

Zebrafish gastrulation entails morphogenetic cell movements that shape the body plan and give rise to an embryo with defined anterior-posterior and dorsal-ventral axes. Regulating these cell movements are diverse signaling pathways and proteins including Wnts, Src-family tyrosine kinases, cadherins, and matrix metalloproteinases. While our knowledge of how these proteins impact cell polarity and migration has advanced considerably in the last decade, almost no data exist regarding the organization of extracellular matrix (ECM) during zebrafish gastrulation. Here, we describe for the first time the assembly of a fibronectin (FN) and laminin containing ECM in the early zebrafish embryo. This matrix was first detected at early gastrulation (65% epiboly) in the form of punctae that localize to tissue boundaries separating germ layers from each other and the underlying yolk cell. Fibrillogenesis increased after mid-gastrulation (80% epiboly) coinciding with the period of planar cell polarity pathway-dependent convergence and extension cell movements. We demonstrate that FN fibrils present beneath deep mesodermal cells are aligned in the direction of membrane protrusion formation. Utilizing antisense morpholino oligonucleotides, we further show that knockdown of FN expression causes a convergence and extension defect. Taken together, our data show that similar to amphibian embryos, the formation of ECM in the zebrafish gastrula is a dynamic process that occurs in parallel to at least a portion of the polarized cell behaviors shaping the embryonic body plan. These results provide a framework for uncovering the interrelationship between ECM structure and cellular processes regulating convergence and extension such as directed migration and mediolateral/radial intercalation.

Frizzled-7-dependent tissue separation in the Xenopus gastrula.

Formation of tissue boundaries can be studied in a simple, inexpensive system, the Xenopus gastrula. Here, the internalized mesoderm and endoderm are separated from the ectodermal blastocoel roof by Brachet's cleft. Non-canonical Wnt signaling mediated by the Wnt receptor, Xfz-7, is essential for this tissue separation event. The function of Wnt pathway components and other factors in tissue separation at Brachet's cleft can be tested in a blastocoel roof assay. Small pieces of mesoderm or endoderm are placed on large blastocoel roof explants, and it is observed whether these test explants remain on the surface of their in vivo substratum, or sink into it.

Embryogenesis in the reef-building coral Acropora spp.

Embryogenesis in the reef building corals Acropora intermedia, A. solitaryensis, A. hyacinthus, A. digitifera, and A. tenuis was studied in detail at the morphological level, and the relationships among the animal pole, blastopore, and mouth were investigated for the first time in corals. These species showed essentially the same sequence of development. The embryo undergoes spiral-like holoblastic cleavage despite the presence of a dense isolecithal yolk. After the morula stage, the embryo enters the "prawn-chip" stage, which consists of an irregularly shaped cellular bilayer. The embryo begins to roll inward to form the bowl stage; the round shape observed during this stage suggests that it may be the beginning of gastrulation. However, the blastopore closes and the stomodeum (mouth and pharynx) is formed via invagination at a site near the closed blastopore. During the planula stage, a concavity forms in the aboral region in conjunction with numerous spirocysts, suggesting that spirocysts are used to attach to the substrate before the onset of metamorphosis.

Induction and the organization of the body plan in Xenopus development.

Various experiments are surveyed in this paper that may throw light on how the degree of spatial organization of the Xenopus embryo increases during development. The events of the 100 minutes or so that follow fertilization may do little more than orientate and give proportions, within the egg's yolky vegetal region, to the system that originates patterned inductive signals for organization of the mesodermal and ectodermal regions during blastula stages (10(2)-10(4) cells). By onset of gastrulation, an outline plan for mediolateral and anteroposterior body organization has developed within induced tissue around the equator of the embryo, which seems to control subsequent development in two ways. It sets the spatial and temporal pattern of mechanical activities whereby the mesoderm rudiment drives the crucial shape changes that lay it and the neural rudiment out correctly. It is also the probable starting point for positionally specific gene transcription that begins immediately after gastrulation. Experiments with known inducing factors that explore the possible bases for the early 'pre-organization' in mesoderm are informative, but leave us far from a complete understanding. Evidence that inhibitory or modulating, as well as activating, signals are involved is surveyed.

The twist gene: isolation of a Drosophila zygotic gene necessary for the establishment of dorsoventral pattern.

The twist zygotic gene appears to be involved in the establishment of the dorso-ventral pattern in Drosophila embryos. Homozygous twist embryos are partially dorsalized, their gastrulation is abnormal, and they fail to differentiate mesoderm. We determined the temperature-sensitive period of twist around the gastrulation time, and we isolated the gene. A 300 kb chromosomic walk allowed the detection of the 70 kb deletion that delimits the twist region in Df(2R)twiS60. Southern blot analyses of 21 EMS induced twist allele DNAs and systematic Northern blot analyses all over this 70 kb region lead to the localization of the twist gene: within about 10 kb at the left border of the deletion, 2 twist alleles show each a small deletion that uncover a transcription unit whose expression occurs about at the time of gastrulation.

Induction of gastrulation in the chick embryo.

Interaction between the epiblast and the primary hypoblast in chick blastula results in induction of the primitive streak (PS) in the epiblast. Alpha-amanitin, a specific inhibitor of poly A-containing RNA synthesis, inhibits formation of the definitive PS. This inhibition is associated with qualitative changes in the pattern of protein synthesis in the hypoblast but not in the epiblast. The protein pattern of the component areas of the epiblast shows increase in some polypeptides after treatment with alpha-amanitin. By contrast, alpha-amanitin resulted in a decrease in synthesis of several polypeptides, which are either undetectable or weakly present in the hypoblast. The alpha-amanitin-sensitive translational products of the embryonic genome that are observed in the hypoblast may have specific functions in the control of PS induction and stabilization.

Differential distribution of cell protrusions on the ventral surface of the deep layer in gastrulating quail and chick embryos.

The ventral surface of the deep layer of gastrulating quail and chick embryos was examined using scanning electron microscopy. On the basis of cell protrusions, three or four different cell types were recognized. Cells covered with microplicae were found in the caudal region of the germ and as a narrow band extending along the lateral and anterior borders of the area pellucida. Cells covered with microvilli were found in a horseshoe-shaped zone in the anterior part of the germ. Beneath the rostral end of the primitive streak, the flattened deep-layer cells exhibited intercellular ridges and few microvilli. This area was surrounded by cells that usually had extended microvilli. The pattern of these cell types is discussed in relation to the formation of the different tissues that compose the deep layer in gastrulating embryos.

Requirements for zygotic gene activity during gastrulation in Drosophila melanogaster.

Mutations at the folded gastrulation (fog) and twisted gastrulation (tsg) loci interfere with early morphogenetic movements in Drosophila melanogaster. fog embryos do not form a normal posterior midgut and although their germbands do elongate, they do not extend dorsally. As a result, when normal embryos have fully extended germbands, the germbands in mutant embryos are folded into the interior on the ventral side of the embryo. tsg embryos have abnormally deep dorsal folds during early gastrulation, associated with the failure of dorsal cells to slip laterally to make way for the expanding germband. Both fog and tsg embryos continue to develop, but form disorganized first instar larvae. fog and tsg are zygotically active genes expressed at least by 10 and 20 min after the onset of gastrulation. Both mutations are viable in homozygous germ cells and the wild-type genes need not be expressed during oogenesis for survival of heterozygous progeny. Elimination of fog+ gene product from maternal germ cells does, however, affect the extent of folding observed during gastrulation in viable heterozygotes. Analysis of fog adult and larval gynandromorphs indicates that normal folded gastrulation gene function is only required at the posterior region of the embryo, most probably in the cells giving rise to the posterior midgut or proctodeum. The relative survival of fog mosaics suggests that embryos with mosaic "lethal foci" also die during embryogenesis, although the typical fog phenotype is only produced when the entire focus is mutant. In contrast to the fog focus, no particular cell must be wild type in tsg mosaics for survival. Wild-type cells on the dorsal side of the embryo, however, are most effective in rescuing the embryo. This indicates that normal tsg gene product may be required only on the dorsal side of the embryo, potentially in the region which gives rise to the amnion serosa.

Newly synthesized extracellular ribonucleic acids in the amphibian gastrula.

The possible synthesis of RNA located in the extracellular compartment of Bufo arenarum gastrula was studied using a biochemical method. [3H]adenosine was microinjected into the blastocoel of late blastulae or early gastrulae, which were then dissociated at advanced gastrula stage. RNA was extracted from both, the cellular supernatant and the disaggregated cells, by the Kirby-phenol procedure. Most of the ethanol-precipitable radioactivity was sensitive to RNase and alkaline treatment. The partial characterization of these molecules indicate that the radioactive pattern of total RNA, found in sucrose gradients, the ratio Poly(A)+RNA/Poly(A)-RNA as well as the radioactive pattern of Poly(A) fraction in acrylamide gels were different in samples from cellular and from extracellular origin. Although not conclusive, these results are proposed as a new argument for the existence of an extracellular RNA in the amphibian embryo.

The establishment of a somitomeric pattern in the mesoderm of the gastrulating mouse embryo.

Mesoderm formation in the mouse embryo begins at 6.5-6.75 days p.c. (postcoitum) when a primitive streak is formed along the posterior side of the egg cylinder. Epiblast cells in a localized region separate from one another and spread laterally between the primitive endoderm and the rest of the epiblast. The newly formed mesoderm contributes to both embryonic and extraembryonic regions. When the endoderm is removed, a definitive somitomeric pattern is first observed in the lateral sings of mesoderm of the mid-primitive-streak-stage embryo. The sequential appearance and the placement of somitomeres in the gastrulating mouse embryo are closely related to the general changes in physical dimensions and to the pattern of tissue growth which occur during the maturation of the egg cylinder. By the late-primitive-streak stage, about four somitomeres are present in the paraxial mesoderm on either side of the embryonic axis. These somitomeres will undergo morphogenesis and give rise to the cranial segments and head mesenchyme of neurulating embryos (Meier and Tam, 1982). The midline or axial mesoderm, consisting of prechordal plate and notochord, is derived from the head process mesoderm originating from the anterior end of the primitive streak. Cells of the head process are compact and adherent to the endoderm. The early presence of a somitomeric pattern which persists and is added to throughout subsequent phases of mesoderm formation suggests that spreading mesodermal cells have relatively stable neighbor relationships. This morphological evidence supports the idea that the expansion of the mesoderm during gastrulation results from tissue growth and progressive deposition of cells from the primitive streak. Cell migration may be limited principally to nonsomitomeric mesodermal cells found in the leading edge of the spreading lateral wings.

Development and phylogenetic significance of the fetal membranes and placenta of the African hystricognathous rodents Bathyergus and Hystrix.

Morphogenesis of fetal membranes in African and American hystricognathous rodents is essentially the same, even in the possession of such uniquely derived features as a subplacenta and a capillary ring on the inverted yolk sac, features that are unknown in any other rodent group. This is good evidence that the African and American hystricognaths comprise a monophyletic group. Certain interesting features of the decidua, decidual giant cells, periplacental bilaminar omphalopleure, and epamnion are described in Bathyergus and Hystrix. Of special interest in Bathyergus is the widespread decidualization of the uterine connective tissue, even that of the myometrium and perimetrium; and the presence of numerous, usually binucleate, decidual giant cells.