Oral-aboral Axis Research Articles

The oral-aboral axis of a sea urchin embryo is specified by first cleavage

Several lines of evidence suggest that the oral-aboral axis in Strongylocentrotus purpuratus embryos is specified at or before the 8-cell stage. Were the oral-aboral axis specified independently of the first cleavage plane, then a random association of this plane with the blastomeres of the four embryo quadrants in the oral-aboral plane (viz. oral, aboral, right and left) would be expected. Lineage tracer dye injection into one blastomere at the 2-cell stage and observation of the resultant labeling patterns demonstrates instead a strongly nonrandom association. In at least ninety percent of cases, the progeny of the aboral blastomeres are associated with those of the left lateral blastomeres and the progeny of the oral blastomeres with the right lateral ones, respectively. Thus, ninety percent of the time the oral pole of the future oral-aboral axis lies 45 degrees clockwise from the first cleavage plane as viewed from the animal pole. The nonrandom association of blastomeres after labeling of the 2-cell stage implies that there is a mechanistic relation between axis specification and the positioning of the first cleavage plane.

1 Intrinsic control of the gut

In most mammals (except ruminants) activity in the gastrointestinal (GI) tract depends upon the condition or state of the animal, namely, fasted or fed. The fasted state is characterized by a caudally migrating, cycling motor complex, showing periods of intense contractile and secretory activity alternating with periods of quiescence. Although the mechanisms involved in the transition from the fasted to the fed state are not fully understood it seems likely that both states utilize intrinsically located neural control mechanisms and common neuronal pathways to the effector tissues. We have commented on the reported properties of the myenteric neurones and their projections to the muscle layers. The data suggests that there are both cholinergic and non-cholinergic excitatory motor neurones supplying the muscle layers. In the guinea-pig, at least, the projections of the neurones to the circular muscle layer run for relatively short distances in oral-aboral axis of the gut. The non-cholinergic excitatory transmitter substance may be Substance P or a similar tachykinin. Other excitatory nerves may well be present. There are at least two mechanisms used by non-cholinergic nonadrenergic inhibitory nerves supplying the muscle layers. In the guinea-pig ileum, there are at least two distinct projections of inhibitory motor neurones; both have aborally directed projections. The first of these is relatively short and the other long (>10 mm). Individual myenteric neurones appear to contain unique and perhaps identifying groups of peptides. The functional role of many of these peptides, either within the myenteric plexus or their projections to the muscle layers, remains to be elucidated. The projections of the neurones of the submucous plexus run primarily to the mucosa. Both cholinergic and non-cholinergic secretomotor neurones appear to be present. The activation of local neural reflexes, which results in secretomotor activity, may involve submucous sensory neurones containing acetylcholine and Substance P together with cholinergic interneurones. Projections from the myenteric to the submucous plexus are likely to be involved in the coordination of intestinal movement and secretomotor activity. A simplified schematic diagram of some of the neuronal circuitry of the submucous plexus has been developed and includes the findings from immunocytochemical and electrophysiological studies.

Determination and morphogenesis in the sea urchin embryo.

The study of the sea urchin embryo has contributed importantly to our ideas about embryogenesis. This essay re-examines some issues where the concerns of classical experimental embryology and cell and molecular biology converge. The sea urchin egg has an inherent animal-vegetal polarity. An egg fragment that contains both animal and vegetal material will produce a fairly normal larva. However, it is not clear to what extent the oral-aboral axis is specified in embryos developing from meridional fragments. Newly available markers of the oral-aboral axis allow this issue to be settled. When equatorial halves, in which animal and vegetal hemispheres are separated, are allowed to develop, the animal half forms a ciliated hollow ball. The vegetal half, however, often forms a complete embryo. This result is not in accord with the double gradient model of animal and vegetal characteristics that has been used to interpret almost all defect, isolation and transplantation experiments using sea urchin embryos. The effects of agents used to animalize and vegetalize embryos are also due for re-examination. The classical animalizing agent, Zn2+, causes developmental arrest, not expression of animal characters. On the other hand, Li+, a vegetalizing agent, probably changes the determination of animal cells. The stability of these early determinative steps may be examined in dissociation-reaggregation experiments, but this technique has not been exploited extensively. The morphogenetic movements of primary mesenchyme are complex and involve a number of interactions. It is curious that primary mesenchyme is dispensable in skeleton formation since in embryos devoid of primary mesenchyme, the secondary mesenchyme cells will form skeletal elements. It is likely that during its differentiation the primary mesenchyme provides some of its own extracellular microenvironment in the form of collagen and proteoglycans. The detailed form of spicules made by primary mesenchyme is determined by cooperation between the epithelial body wall, the extracellular material and the inherent properties of primary mesenchyme cells. Gastrulation in sea urchins is a two-step process. The first invagination is a buckling, the mechanism of which is not understood. The secondary phase in which the archenteron elongates across the blastocoel is probably driven primarily by active cell repacking. The extracellular matrix is important for this repacking to occur, but the basis of the cellular-environmental interaction is not understood.(ABSTRACT TRUNCATED AT 400 WORDS)

The ontogeny and maintenance of adult symmetry properties in the ctenophore, Mnemiopsis mccradyi

Ctenophores are biradially symmetrical animals. The body is composed of four identical quadrants which are organized along an oral-aboral axis. Most species have eight comb rows, two tentacles, and an apical organ (located on the aboral surface). During embryogenesis there is a fixed pattern of cleavage, a precocious specification of blastomere developmental potential, and an inability to regulate for portions of the embryo that have been removed. When blastomeres are separated at the two-cell stage each blastomere develops into a “half-animal” with four comb rows, one tentacle, and half an apical organ. In contrast, adult ctenophores regenerate readily. When an adult ctenophore is cut in half to produce “half-animals,” in most cases each half regenerates the missing half. In some cases, however, bisected animals remain as “half-animals” which repair the wound site but do not replace all of the missing structures. When animals are cut in half along the tentacular or esophageal axis at different stages of embryogenesis a transition period is detected when the capacity for adult regeneration begins. This transition occurs at the time when the formation of the apical organ is complete and comb row function becomes coordinated. Embryos bisected prior to this time remain as “half-animals” even after growing to large reproductive sizes, while animals bisected after the transition period usually regenerate the missing structures within 2–3 days. When adult “half-animals” (produced by bisection either before or after the transition period) are cut into “quarter-pieces,” the pieces regenerate to form either “half-animals” or whole animals. Thus, “half-animals” produced prior to the transition period—although they failed to undergo embryonic regulation—have not irreversibly lost the capacity to form whole animals if challenged to regenerate during adult stages. When aboral blastomeres destined to form the apical organ, tentacles, and comb rows are removed from early cleavage stages (prior to the transition period), the embryo does not form these structures at the appropriate time. However, the resulting deficient adults spontaneously form these structures from remaining blastomere lineages soon after hatching. These experiments suggest that as long as some quadrant-specific cells of the oral pole are present at the time of the transition period, the structures of that quadrant will be spontaneously replaced during the adult period. However, if the appropriate oral cells that make up that quadrant are not present prior to the transition period the information for the formation of that quadrant from existing quadrants can only be elicited by operations performed during adulthood.

Fertilization and early development in Beroe ovata.

Fertilization in the clear egg (1 mm in diameter) of the ctenophore Beroe ovata and, in particular, the positioning and movements of pronuclei, and their relationship to the larval oral-aboral axis have been observed. Fertilization can take place anywhere on the egg surface. The sperm pronucleus remains at its entry site and becomes surrounded by a specialized zone (30-50 micron in diameter) beneath the surface referred to as the sperm pronuclear zone or SPZ and devoid of large cortical granules. Polyspermy has been observed to be frequent; each pronucleus is surrounded by its own SPZ. Only the egg pronucleus migrates with a continuous velocity (averaging 18 micron/min) and moves beneath the surface directly toward the immobile sperm pronucleus. In polyspermic eggs, the egg pronucleus can probe several SPZ, each containing a single sperm nucleus, before it finally enters one SPZ and fuses with the chosen sperm pronucleus. These migrations of the egg pronucleus occur over several millimeters and take hours, but the mechanism underlying the motion or how the egg pronucleus decides which SPZ to enter is not yet known. Under our experimental conditions the mitotic apparatus and the first cleavage plane which defines the oral-aboral axis of the larva (see Reverberi (1971). "Experimental Embryology of Marine and Fresh-Water Invertebrates." North-Holland, Amsterdam. for review) pass through the point of sperm entry. During fertilization and cleavage, movements of a cortical autofluorescent material are clearly seen. This material is segregated into micromeres as cleavage progresses.

EXPERIMENTAL STUDIES ON EMBRYOGENESIS IN HYDROZOANS (TRACHYLINA AND SIPHONOPHORA) WITH DIRECT DEVELOPMENT.

The normal embryology of the trachymedusa Aglantha digitale and the siphonophores Nanomia cara and Muggiaea atlantica is described. Marking experiments on these embryos indicate that the site of first cleavage initiation corresponds to the oral pole of the oral-aboral axis. In Muggiaea the plane of the first cleavage corresponds to the plane of bilateral symmetry. Experiments in which presumptive aboral and oral regions are isolated from these embryos at different stages of development indicate that there is an early determination of different regions along this axis. Only the oral region of the Muggiaea embryo has the ability to regulate. These eggs have a pronounced centrolecithal organization. As a consequence of cleavage, the outer ectoplasmic layer of the egg ends up in the cells that form the ectoderm, while the inner or endoplasmic region of the egg ends up in the cells that form the endoderm. Experimentally created fragments of fertilized eggs that contain only ectoplasm differentiate to form an unorganized ectodermal cell mass, indicating that endoplasm is necessary in order to differentiate endoderm. The process of embryogenesis in these animals and the developmental mechanisms they use are very different from those used by hydrozoans with indirect development. These embryos use a suite of developmental mechanisms which are very similar to those used by ctenophores. The significance of this similarity is discussed.

Distinctive features of the cortex and cell surface of micromeres: observations and cautions

The molecular regulatory mechanisms underlying primary axis formation during sea urchin development have recently been identified. Two opposing maternally inherited systems, one animalizing and one vegetalizing, set up the animal-vegetal (A–V) axis. The vegetal system relies in part on the Wnt–β-catenin-Tcf/Lef signaling pathway and the animal system is based on a cohort of animalizing transcription factors that includes members of the Ets and Sox classes. The two systems autonomously define three zones of cell-type specification along the A–V axis. The vegetalmost zone gives rise to the skeletogenic mesenchyme lineage; the animalmost zone gives rise to ectoderm; and the zone in which the two systems overlap generates endoderm, secondary mesenchyme, and ectoderm. Patterning along the A–V also depends on cellular interactions involving Wnt, Notch, and BMP signaling. We discuss how these systems impact the formation of the second axis, the oral-aboral axis; how they connect to later developmental events; and how they lead to cell-type-specific gene expression via cis-regulatory networks associated with transcriptional control regions. We also discuss how these systems may confer on the embryo its spectacular regulatory capacity to replace missing parts.

The epitheliomuscular cell of hydra: Its fine structure, three-dimensional architecture and relation to morphogenesis

Ectodermal epitheliomuscular cells of Hydra attenuata were studied by transmission and scanning electron microscopy, and a three-dimensional model was constructed. These cells are cuboidal to columnar, and each cell has one muscle process arising from the basal portion of the oral-facing surface and one from the aboralsurface. Adjacent epitheliomuscular cells are joined apicolaterally by septate junctions. Numerous gap junctions occur between adjacent epitheliomuscular cells and are irregularly distributed along the lateral and basal aspects. Finger-like interdigitations and specialized folds (couplers) also occur along the basal and lateral aspects and interlock adjacent epitheliomuscular cells. In the basal portion of these cells, myofilaments are aggregated into myonemes which are oriented in the oral-aboral axis of the polyp. Myonemes dominate the cytoplasm of muscle processes. Myofilaments are also aggregated in the basal cytoplasm of the cell body when the cell body is in contact with the mesoglea but are sparse or absent when the cell body rests upon other muscle processes. Epitheliomuscular cells and associated muscle processes rest upon other processes and the mesoglea and show variations in these relations. A muscle process and associated cell may rest upon another process; the process may then extend under the preceding process and cell body. This configuration, and variations, present a woven or braided network of muscle processes which collectively form a sheet of muscle on the mesoglea. The interdigitations, couplers and gap junctions between epitheliomuscular cells and the woven network of muscle processes present a cytological basis for the observations that the ectoderm in hydra behaves as a coherent sheet along the body column.

The establishment of the oral-aboral axis in the ctenophore embryo

ABSTRACTIn a small percentage of normal embryos and in a higher percentage of embryos centrifuged prior to the first cleavage the positions of the polar bodies and the site of the first cleavage furrow do not coincide. These cases have been used to establish whether polar body formation sites or first cleavage initiation sites correlate best with the oral-aboral axis of the embryo. In all cases when the first cleavage is initiated at a site different from the site where the polar bodies were given off, the pattern of the first four cleavages is normal, the segregation of comb plate potential at these stages is normal, and the larvae that form are normal. The extent to which comb plate potential is localized along the oral-aboral axis of the embryo prior to the first cleavage, during the first cleavage and at the 2-cell stage was also examined. These experiments demonstrate that the oral-aboral axis is established at the time of the first cleavage, that cleavage plays a causal role in setting up the axis, and that comb plate-forming potential begins to be localized in the aboral region of the embryo at this time.

The development of bioluminescence in the ctenophore Mnemiopsis leidyi

The photocytes of the ctenophore Mnemiopsis have a discontinuous distribution along the radial canal between the sites where the comb plate cilia cells are located on the side of the canal which contains the testes. They are separated from the lumen of the canal by a population of gastric cells. Cytologically these cells are characterized by a condensed nucleus and cytoplasm which stains lightly with basophilic dyes. The ability of the ctenophore embryo to produce light appears at the developmental stage when the comb plate cilia first begin to grow out. At this stage four light-producing areas are present; each area corresponds to one quadrant of the adult animal. At the sites of light production, a population of cells can be identified that have some of the cytological properties of the photocytes of the adult animal. Within 8–10 hr after light production begins there is a 10-fold increase in the amount of light produced by an embryo and a cytological maturation of its photocytes; during this time period there is no increase in photocyte number. At about the time the embryo begins to feed, each light-producing region splits into two regions, each of which corresponds to a radial canal. During the process of embryogenesis the photocyte cell lineage is first segregated from non-photocytes at the differential division which gives the 8-cell stage embryo. The M macromere lineage goes on to form photocytes, but the E macromere lineage does not. The M macromeres form a micromere at the aboral pole of the embryo at each of the next two cleavages; during these cleavages the potential for photocyte differentiation continues to segregate with the M macromeres. During the division which gives the 64-cell stage the M macromeres divide equally; the potential for photocyte differentiation segregates with the M macromeres nearest the oral-aboral axis. M macromeres which are isolated from the embryo at the 8-, 16-, or 32-cell stage of development will continue to cleave as though they were part of a normal embryo and differentiate to form photocytes. The events that are responsible for the differential division during the formation of the 8-cell stage embryo have been studied by centrifuging eggs to produce fragments of different cytoplasmic composition. Egg fragments which contain only cortical cytoplasm differentiate comb plate cilia cells, but do not produce photocytes. Cortical fragments with a small amount of yolk differentiate comb plate cilia cells and photocytes. Both the M and E macromeres from cortical fragments with no yolk produce comb plate cilia. Only M macromeres containing yolk form photocytes; if an M macromere forms photocytes it does not form comb plate cilia.

Tetraplatia, a coronate scyphomedusan.

The true relationships of the pelagic coelenterate Tetraplatia have long been a matter of controversy. The most recent and widely held view, that it is a highly modified trachyline medusa, is here demonstrated to be untenable because it has no velum, it has gastric filaments, a sense organ that is a rhopalium with an endodermal statolith, and gonads that originate in the endoderm. A reinvestigation based on adequate samples shows that it is a scyphomedusan. It is suggested first, that Tetraplatia has no ephyral stage and the planula develops directly into a juvenile, and secondly, that Tetraplatia evolved from a coronate scyphomedusan in which the normal radial expansion of the ephyra larva was arrested at an early stage and replaced by accelerated growth in the oral-aboral axis. Later, or accompanying the increased growth rate on the oral-aboral axis, four pairs of pouches were formed causing displacement and crowding of the eight ephyran lappets into four pairs and finally their fusion into a single locomotory organ. Thus, there were four locomotory lappets separated by four pairs of pouches. T. chuni remains essentially unaltered from this basic pattern, but in T. volitans the pouches fuse to form the well-known flying buttresses. Tetraplatia should now be placed in the Coronate Scyphomedusae, near the Ephyropsidae and Periphyllidae, in a family of its own, the Tetraplatidae. For this study, supplies of material of both T. volitans and T. chuni have been available.