Origin of cells giving rise to mesoderm and endoderm in chick embryo

In amniotes, all of the tissues of the adult arise from the epiblast, one of the two layers of cells present in the early embryo; the mesoderm and gut endoderm arise from an epiblast-derived structure known as the primitive streak. The monoclonal antibody HNK-1 recognizes the cells of the primitive streak in the chick embryo. Before streak formation, HNK-1 identifies cells that are randomly distributed within the epiblast. We have now used two novel ways to study cell lineage and commitment to show that the epiblast of the early chick embryo contains two distinct populations of cells with different developmental fates at a stage during which 'mesodermal induction' is believed to occur. One cell population, recognized by monoclonal antibody HNK-1, is destined to form mesoderm and endoderm; the rest of the epiblast is unable to give rise to mesoderm if this population of cells is removed.

Isolation from chick somites of a glycoprotein fraction that causes collapse of dorsal root ganglion growth cones

The segmented pattern of peripheral spinal nerves in higher vertebrates is generated by interactions between nerve cells and somites. Neural crest cells, motor axons, and sensory axons grow exclusively through anterior-half sclerotome. In chick embryos, posterior cells bind the lectins peanut agglutinin (PNA) and Jacalin. When liposomes containing somite extracts are applied to cultures of chick sensory neurons, growth cones collapse abruptly, recovering within 4 hr of liposome removal. Collapse activity is eliminated by immobilized PNA, and SDS-PAGE demonstrates two major components (48K and 55K), which are absent from anterior-half sclerotome. Rabbit polyclonal antibodies against these components recognize only posterior cells and may also be used to eliminate collapse activity. We suggest that spinal nerve segmentation is produced by inhibitory interactions between these components and growth cones.

J1/tenascin-related molecules are not responsible for the segmented pattern of neural crest cells or motor axons in the chick embryo

It has been suggested that substrate adhesion molecules of the tenascin family may be responsible for the segmented outgrowth of motor axons and neural crest cells during formation of the peripheral nervous system. We have used two monoclonal antibodies (M1B4 and 578) and an antiserum [KAF9(1)] to study the expression of J1/tenascin-related molecules within the somites of the chick embryo. Neural crest cells were identified with monoclonal antibodies HNK-1 and 20B4. Young somites are surrounded by J1/tenascin immunoreactive material, while old sclerotomes are immunoreactive predominantly in their rostral halves, as described by other authors (Tan et al. 1987—Proc. natn. Acad. Sci. U.S.A. 84, 7977; Mackie et al. 1988—Development 102, 237). At intermediate stages of development, however, immunoreactivity is found mainly in the caudal half of each sclerotome. After ablation of the neural crest, the pattern of immunoreactivity is no longer localised to the rostral halves of the older, neural-crest-free sclerotomes. SDS-polyacrylamide gel electrophoresis of affinity-purified somite tissue, extracted using M1B4 antibody, shows a characteristic set of bands, including one of about 230 × 10(3), as described for cytotactin, J1-200/220 and the monomeric form of tenascin. Affinity-purified somite material obtained from neural-crest-ablated somites reveals some of the bands seen in older control embryos, but the high molecular weight components (120–230 × 10(3] are missing. Young epithelial somites also lack the higher molecular mass components. The neural crest may therefore participate in the expression of J1/tenascin-related molecules in the chick embryo. These results suggest that these molecules are not directly responsible for the segmented outgrowth of precursors of the peripheral nervous system.

Comparative carcinogenicity of the PAHs as a basis for acceptable exposure levels (AELs) in drinking water

The carcinogenicity of various polynuclear aromatic hydrocarbons (PAHs) has generally been demonstrated by their ability to act as complete carcinogens in the development of cancers in rodent skin tests. In order to develop proposed acceptable concentration levels for various PAHs in drinking water, we reviewed the studies that formed the basis for determining that these specific PAHs were carcinogenic in animals. We found that the relative potency of these PAHs varied over a range of many orders of magnitude. For example, the carcinogenic strength of benz[a]anthracene (BaA) is found to be about 1/2000th that of benzo[a]pyrene (BaP). We have used the calculated carcinogenic potency of the various PAHs relative to that of BaP as a means for proposing specific acceptable concentration levels in drinking water for each of the specific PAHs. BaP is the only carcinogenic PAH for which EPA has published an acceptable concentration level based on carcinogenicity. Based on the level EPA set for BaP (0.028 micrograms/liter), this methodology has provided for the specific PAHs a determination of proposed acceptable concentration levels quantitatively based on the same data that were used to qualitatively determine them to be animal carcinogens. We have proposed acceptable concentration levels for the carcinogenic PAHs in drinking water that range from 0.03 micrograms/liter for BaP to 6.5 micrograms/liter for BaA. We recommend that acceptable concentration levels for the various PAHs be based on their relative carcinogenic potencies rather than the EPA method of using the potency of only one specific PAH, BaP, to serve as the exposure level determinant for all PAHs. We further suggest that this methodology may be applicable to other classes of carcinogenic compounds. We have also found useful for the determination of acceptable concentration levels for the noncarcinogenic PAHs an analogous methodology based on the relative toxicities of the noncarcinogenic PAHs.

Molecular differences between the rostral and caudal halves of the sclerotome in the chick embryo

It is known that both neural crest cell migration and motor axon outgrowth in most vertebrate embryos are segmented because of restrictions imposed upon their distribution by the neighbouring sclerotomes, each of which is divided into a rostral and a caudal half. The caudal half does not allow crest migration or axon outgrowth, while the rostral half does. In this paper, we investigate the expression of proteins and glycoproteins in the two halves of the sclerotome of the chick embryo at stages between 20 and 32 pairs of somites by two-dimensional SDS-polyacrylamide gel electrophoresis. We find that the patterns of expression are complex, and that polypeptides and glycoproteins vary both spatially and temporally: of those that are expressed differentially by the sclerotome, some differ quantitatively and others qualitatively. Some macromolecules change their spatial distribution with developmental age, and some appear or disappear as the embryos become older.

Periodic segmental anomalies induced by heat shock in the chick embryo are associated with the cell cycle

This study provides evidence that cells destined to segment together into somites have a degree of cell division synchrony. We have measured the duration of the cell division cycle in somite and segmental plate cells of the chick embryo as 9.5 h using [3H]thymidine pulse- and-chase. Treatment of embryos with any of a variety of inhibitors known to affect the cell division cycle causes discrete periodic segmental anomalies: these anomalies appear about 6–7 somites after treatment and, in some cases, a second anomaly is observed 6 to 7 somites after the first. Since somites take 1.5 h to form, the 6- to 7- somite interval corresponds to about 9–10 h, which is the duration of the cell cycle as determined in these experiments. The anomalies are similar to those seen after heat shock of 2-day chick embryos. Heat shock and some of the other treatments induce the expression of heat-shock proteins (hsp); however, since neither the expression nor the distribution of these proteins relate to the presence or distribution of anomalies seen, we conclude that hsps are not responsible for the pattern of segmental anomalies observed. The production of periodic segmental anomalies appears to be linked to the cell cycle. A simple model is proposed, in which we suggest that the cell division cycle is involved directly in gating cells that will segment together.

Changes in the expression of the carbohydrate epitope HNK-1 associated with mesoderm induction in the chick embryo

We report that a monoclonal antibody, HNK-1, identifies specific regions and cell types during primitive streak formation in the chick blastoderm. Immunohistochemical studies show that the cells of the forming hypoblast are HNK-1 positive from the earliest time at which they can be identified. Some cells of the margin of the blastoderm are also positive. The mesoderm cells of the primitive streak stain strongly with the antibody from the time of their initial appearance. In the epiblast, some cells are positive and some negative at pre-primitive-streak stages, but as the primitive streak develops a gradient of staining intensity is seen within the upper layer, increasing towards the primitive streak. At later stages of development, the notochord and the mesenchyme of the headfold are positive, while the rest of the mesoderm (lateral plate) no longer expresses HNK-1 immunoreactivity. This antibody therefore reveals changes associated with mesodermal induction: before induction, it recognizes the ‘inducing’ tissue (the hypoblast) and reveals a mosaic pattern in the responding tissue (the epiblast); after primitive streak formation, the mesoderm of the primitive streak that results from the inductive interactions expresses the epitope strongly. Affinity purification of HNK-1-related proteins in various tissues was carried out, followed by SDS-PAGE to identify them. The hypoblast, mesoderm and epiblast of gastrulating chick embryos have some HNK-1-related proteins in common, while others are unique to specific tissues. Attempts have been made to identify these proteins using Western blots and antibodies known to recognize HNK-1-related molecules, but none of the antibodies used identify the bands unique to any of the tissues studied. We conclude that these proteins may be novel members of the HNK-1/L2 family, and that they may have a role in cell interactions during early development.

Heat shock causes repeated segmental anomalies in the chick embryo

A single heat shock, given to 2-day-old chick embryos, can generate multiple but discrete somite and skeletal anomalies. Each of these anomalies is restricted to one, or at the most two, consecutive segments. The anomalies are separated from each other by a distance of 6–7 somites or vertebrae, or a multiple of this distance. These results argue against the ‘clock and wavefront’ model; while they support the idea of a cellular clock, they are not consistent with a single propagating wave gating cells destined to form each segment. Heat shock also alters the size and number of segments, as well as the rostrocaudal proportions of the sclerotome. The results are consistent with the rostrocaudal fate of sclerotome cells being determined during segmentation. From our observations, we speculate on the implications for regionalization of the vertebral column.

A cell lineage analysis of segmentation in the chick embryo

We have studied the lineage history of the progenitors of the somite mesoderm and of the neural tube in the chick embryo by injecting single cells with the fluorescent tracer, rhodamine-lysine-dextran. We find that, although single cells within the segmental plate give rise to discrete clones in the somites to which they contribute, neither the somites nor their component parts (sclerotome, dermatome, myotome or their rostral and caudal halves) are `compartments' in the sense defined in insects. Cells in the rostral two thirds or so of the segmental plate contribute only to somite tissue and divide about every 10 h, while those in the caudal portions of this structure contribute both to the somites and to intermediate and lateral plate mesoderm derivatives. In the neural tube, the descendants of individual prospective ventral horn cells remain together within the horn, with a cycle time of 10 h.

We have also investigated the role of the cell division cycle in the formation and subsequent development of somites. A single treatment of 2-day chick embryos with heat shock or a variety of drugs that affect the cell cycle all produce repeated anomalies in the pattern of somites and vertebrae that develop subsequent to the treatment. The interval between anomalies is 6-7 somites (or a multiple of this distance), which corresponds to 10 h. This interval is identical to that measured for the cell division cycle. Given that cell division synchrony is seen in the presomitic mesoderm, we suggest that the cell division cycle plays a role in somite formation.

Finally, we consider the mechanisms responsible for regionalization of derivatives of the somite, and conclude that it is likely that both cell interactions and cell lineage history are important in the determination of cell fates.

The neural tube origin of ventral root sheath cells in the chick embryo

The embryonic origin of peripheral nerve Schwann/sheath cells is still uncertain. Although the neural crest is known to be an important source, it is not clear whether the ventral neural tube also contributes a progenitor population for motor axons. We have used the techniques of immunohistochemistry, electron microscopy and quail-chick grafting to examine this problem. Immunohistochemistry with monoclonal antibody HNK-1 identified a cluster of immunoreactive cells in the sclerotome, at the site of the future ventral root. With the electron microscope, nucleated cells could not be seen breaching the basal lamina of the neural tube, exclusively in the region of the ventral root and preceding axon outgrowth. After grafting a length of crest-ablated quail neural tube in place of host chick neural tube, a population of quail cells was found localized to the ventral root exit zone, associated with the ventral root axons. Taken together, these observations support the possibility of a neural tube origin for ventral root sheath cells, although we found no evidence for a more extensive migration of these cells. The ventral root cells share certain phenotypic traits, such as HNK-1 immunoreactivity, with neural-crest-derived Schwann cells, but are not necessarily identical to them. We argue that while they may help motor axons to exit the neural tube at the correct position, they are unlikely to guide axons beyond the immediate vicinity of the neural tube.

The differing effects of occipital and trunk somites on neural development in the chick embryo

In all higher vertebrate embryos the sensory ganglia of the trunk develop adjacent to the neural tube, in the cranial halves of the somite-derived sclerotomes. It has been known for many years that ganglia do not develop in the most cranial (occipital) sclerotomes, caudal to the first somite. Here we have investigated whether this is due to craniocaudal variation in the neural tube or crest, or to an unusual property of the sclerotomes at occipital levels. Using the monoclonal antibody HNK-1 as a marker for neural crest cells in the chick embryo, we find that the crest does enter the cranial halves of the occipital sclerotomes. Furthermore, staining with zinc iodide/osmium tetroxide shows that some of these crest-derived cells sprout axons within these sclerotomes. By stage 23, however, no dorsal root ganglia are present within the five occipital sclerotomes, as assessed both by haematoxylin/eosin and zinc iodide/osmium tetroxide staining. Moreover, despite this loss of sensory cells, motor axons grow out in these segments, many of them later fasciculating to form the hypoglossal nerve. The sclerotomes remain visible until stages 27/28, when they dissociate to form the base of the skull and the atlas and axis vertebrae. After grafting occipital neural tube from quail donor embryos in place of trunk neural tube in host chick embryos, quail-derived ganglia do develop in the trunk sclerotomes. This shows that the failure of occipital ganglion development is not the result of some fixed local property of the neural crest or neural tube at occipital levels. We therefore suggest that in the chick embryo the cranial halves of the five occipital sclerotomes lack factors essential for normal sensory ganglion development, and that these factors are correspondingly present in all the more caudal sclerotomes.

The specificity of motor innervation of the chick wing does not depend upon the segmental origin of muscles

In vertebrate embryos, motor axons originating from a particular craniocaudal position in the neural tube innervate limb muscles derived from myoblasts of the same segmental level. We have investigated whether this relationship is important for the formation of specific nerve-muscle connections, by altering the segmental origin of muscles and examining their resulting innervation. First, by grafting quail wing somites to a new craniocaudal position opposite the chick wing, we established that the segmental origin of a muscle can be altered: presumptive muscle cells migrated according to their new, rather than their original, somitic level, colonizing a different subset of muscles. However, after reversal of a length of brachial somitic mesoderm along the craniocaudal axis, or exchange or shift of brachial somites, the craniocaudal position of wing muscle motoneurone pools within the spinal cord was undisturbed, despite the new segmental origin of the muscles themselves. While not excluding the possibility that muscles and their motor nerves are labelled segmentally, we conclude that specific motor axon guidance in the wing does not depend upon the existence of such labels.

Interactions between somite cells: the formation and maintenance of segment boundaries in the chick embryo

We have investigated the interactions between the cells of the rostral and caudal halves of the chick somite by carrying out grafting experiments. The rostral half-sclerotome was identified by its ability to support axon outgrowth and neural crest cell migration, and the caudal half by the binding of peanut agglutinin and the absence of motor axons and neural crest cells. Using the chick-quail chimaera technique we also studied the fate of each half-somite. It was found that when half-somites are placed adjacent to one another, their interactions obey a precise rule: sclerotome cells from like halves mix with each other, while those from unlike halves do not; when cells from unlike halves are adjacent to one another, a border is formed. Grafting quail half-somites into chicks showed that the fates of the rostral and caudal sclerotome halves are similar: both give rise to bone and cartilage of the vertebral column, as well as to intervertebral connective tissue. We suggest that the rostrocaudal subdivision serves to maintain the segmental arrangement when the mesenchymal sclerotome dissociates, so that the nervous system, vasculature and possibly vertebrae are patterned correctly.

Interactions between neurites and somite cells: inhibition and stimulation of nerve growth in the chick embryo

After neural processes emerge from the neural tube in the chick embryo, their growth is restricted to the cranial halves of the neighbouring somites. In this study we have developed an in vitro system to model the interactions between these tissue types. Pioneer neurites display a hierarchy of preferences in terms of the substrates they can grow on. As expected, tissue culture plastic does not support neural outgrowth, but this can be overcome by coating the plastic substrate with either collagen or poly-L-lysine. Neural crest, cranial half somite, and a number of other tissues support growth well, while caudal half somite and tail bud mesenchyme do so to a much smaller extent. The binding pattern of a variety of lectins was assessed in cryostat sections of embryos and in cultured cells of the above tissues. It was found that peanut agglutinin can discriminate between cranial and caudal sclerotome both in vitro and in the embryo, since it binds preferentially to caudal sclerotome in both cases. This difference is expressed as soon as the sclerotome forms. The significance of these findings is twofold: first, they show that the interactions that take place during peripheral neural segmentation can be modelled in vitro; second, they represent the first instance of a molecular difference between the cranial and caudal halves of the sclerotome, detectable both in culture and in the embryo.

Interactions between neurites and somite cells: inhibition and stimulation of nerve growth in the chick embryo

After neural processes emerge from the neural tube in the chick embryo, their growth is restricted to the cranial halves of the neighbouring somites. In this study we have developed an in vitro system to model the interactions between these tissue types. Pioneer neurites display a hierarchy of preferences in terms of the substrates they can grow on. As expected, tissue culture plastic does not support neural outgrowth, but this can be overcome by coating the plastic substrate with either collagen or poly-L-lysine. Neural crest, cranial half somite, and a number of other tissues support growth well, while caudal half somite and tail bud mesenchyme do so to a much smaller extent.

The binding pattern of a variety of lectins was assessed in cryostat sections of embryos and in cultured cells of the above tissues. It was found that peanut agglutinin can discriminate between cranial and caudal sclerotome both in vitro and in the embryo, since it binds preferentially to caudal sclerotome in both cases. This difference is expressed as soon as the sclerotome forms.

The significance of these findings is twofold: first, they show that the interactions that take place during peripheral neural segmentation can be modelled in vitro; second, they represent the first instance of a molecular difference between the cranial and caudal halves of the sclerotome, detectable both in culture and in the embryo.

A historical perspective on the discovery of the accessory duct of the pancreas, the ampulla 'of Vater' and pancreas divisum

The discovery of the accessory duct of the pancreas is usually ascribed to Giovanni Domenico Santorini (1681-1737), after whom this structure is named. The papilla duodeni (ampulla 'of Vater', or papilla 'of Santorini') is named after Abraham Vater (1684-1751) or after GD Santorini. Pancreas divisum, a persistence through non-fusion of the embryonic dorsal and ventral pancreas is a relatively common clinical condition, the discovery of which is usually ascribed to Joseph Hyrtl (1810-1894). In this review I report that pancreas divisum, the accessory duct and the papilla duodeni (ampulla 'of Vater') had all been observed and the observations published during the 17th century by at least seven anatomists before Santorini, Vater, and Hyrtl. I further suggest, in the light of frequent anatomical misattributions in common usage, that anatomical structures be referred to only by their proper anatomical names.

Fluid transport across the epiblast of the early chick embryo

A simple method is described which allows quantitation of the rate of fluid transport across the isolated epiblast of the early chick embryo. This method consists of allowing the tissue to form spheres, which then spontaneously undergo a large volume increase. The rate of fluid uptake into the spheres can be estimated by measuring the dimensions of the spheres. Pharmacological and electrophysiological studies were performed on the spheres to determine the mechanisms of fluid transport. It was found that fluid is driven into the interior of the spheres by the osmotic gradient generated by unidirectional sodium transport and to a lesser extent by another mechanism, as yet unknown. We discuss possible candidates for this mechanism, and consider the significance of these findings to early development.

Fluid transport across the epiblast of the early chick embryo

A simple method is described which allows quantitation of the rate of fluid transport across the isolated epiblast of the early chick embryo. This method consists of allowing the tissue to form spheres, which then spontaneously undergo a large volume increase. The rate of fluid uptake into the spheres can be estimated by measuring the dimensions of the spheres. Pharmacological and electrophysiological studies were performed on the spheres to determine the mechanisms of fluid transport. It was found that fluid is driven into the interior of the spheres by the osmotic gradient generated by unidirectional sodium transport and to a lesser extent by another mechanism, as yet unknown. We discuss possible candidates for this mechanism, and consider the significance of these findings to early development.

Mini-review: hyaluronidases in early embryonic development

The foregoing discussion indicates that hyaluronidases probably play an important part in the control of development. In morphogenesis, they may be involved in epithelial-mesenchymal inductive interactions, in non-malignant invasion when one tissue displaces another in normal development, in controlling cell movements, in modulating changes of shape of cells and sheets of cells, in controlling the permeability of tissues and regulating the ionic environment within the embryo. There is also evidence indicating that hyaluronidases are involved in the initiation of cytodifferentiation pathways, perhaps via direct or indirect effects upon the cell division cycle and histone-DNA interactions. The evidence presented indicates that hyaluronidases are important repeatedly at different stages of embryonic development and differentiation, where periods of high activity follow others of reduced activity in localized regions of the embryo. Some new results were also presented, showing the presence of different hyaluronidase activities at early stages of chick embryo development. The highest levels of hyaluronidase activity were found in the primitive streak and mesoderm.