Maternal screening for hypothyroidism and thyroiditis using filter paper specimens.

BACKGROUND AND OBJECTIVE: Hypothyroidism and autoimmune thyroiditis are more prevalent than previously considered in women during pregnancy and the postpartum, and are associated with adverse effects on the mother and her fetus. We determined the efficacy and accuracy of screening women for primary hypothyroidism and autoimmune thyroiditis by testing TSH and two thyroid antibodies (TAb): thyroperoxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), in eluates of filter paper specimens collected during early pregnancy and the postpartum. METHODS: We enrolled 494 first-trimester pregnant women with no exclusion criteria into a prospective study to detect primary hypothyroidism and autoimmune thyroiditis. Finger stick blood was applied to filter paper, dried in room air, eluted, and promptly tested for TSH and TAb. A total of 178 of the pregnant women (36%) were tested in the early postpartum. Women with abnormal results had confirmatory serum tests. RESULTS: It was found that 91 pregnant women (18.4%) and 43 postpartum women (24.2%) had abnormal TSH values (>4.0 mU/L) and/or positive TAb; 140 pregnant women (28.3%) had TSH values >2.5 mU/L. All subjects with TSH values >4.0 mU/L tested positive for TAb. Eighteen women (3.6%) who tested normal during pregnancy tested abnormal in the postpartum. CONCLUSIONS: This study confirms that TSH and TPOAb measured in eluates of blood-spotted filter paper specimens are excellent screening tests to detect primary hypothyroidism and autoimmune thyroiditis in pregnant and postpartum women. Results are very comparable to serum data in this population published in the literature.

Assays for thyroid-stimulating hormone using dried blood spotted filter paper specimens to screen for hypothyroidism in older children and adults.

OBJECTIVE: Symptoms of hypothyroidism in adults can be mistaken for medical and psychiatric diseases, as well as for general signs of ageing such as weakness, lethargy and fatigue. The incidence of hypothyroidism is many-fold higher in adults than in newborn children. The latter have been routinely screened for the condition using filter paper dried blood spots (DBS) for nearly three decades but this cost-effective screening technique has only recently been applied to adults. This study was undertaken to show that DBS testing in adults and older children is an accurate way to screen for hypothyroidism. METHODS: Serum and DBS specimens were collected from adults and children. Assays were run on both specimens and the results correlated. In addition 972 specimens were collected from adults at community centres and nursing homes. Follow-up studies were performed on patients with positive results. RESULTS: The correlation coefficient for 118 matched serum and DBS specimens was 0.99. Thyroid-stimulating hormone (TSH) values were elevated in 50 of the 972 adults from nursing homes and community centres. Thirteen of these individuals were on thyroid medication and 28 had either high serum TSH or high thyroglobulin (TgAb) or thyroid peroxidase (TPOAb) antibody levels. CONCLUSIONS: Individuals can be screened for hypothyroidism by collecting finger stick DBS specimens at community centres, nursing homes and other locations which can be mailed by regular postal service to a central laboratory for accurate and inexpensive testing.

Cornea-lens transdifferentiation in the anuran, Xenopus tropicalis.

Previously, the only anuran amphibian known to regenerate the lens of the eye was Xenopus laevis. This occurs during larval stages through transdifferentiation of the outer cornea epithelium under control of factors presumably secreted by the neural retina. This study demonstrates that a distantly related species, X. tropicalis, is also able to regenerate lenses through this process. A transgenic line of X. tropicalis was used to examine the process of cornea-lens transdifferentiation in which green fluorescent protein (GFP) is expressed in differentiated lens cells under the control of the Xenopus gamma1-crystallin promoter element. Unlike X. laevis, the process of cornea-lens transdifferentiation typically occurs at a very low frequency in X. tropicalis due to the rapid rate at which the inner cornea endothelium heals to recover the pupillary opening. The inner cornea endothelium serves as a key physical barrier that normally prevents retinal signals from reaching the outer cornea epithelium. If this barrier is circumvented by implanting outer cornea epithelium of transgenic tadpoles directly into the vitreous chamber of non-transgenic X. tropicalis larval eyes, a higher percentage of cases formed lenses expressing GFP. Lenses were also formed if these tissues were implanted into X. laevis larval eyes, suggesting the same or similar inducing factors are present in both species. When pericorneal ectoderm and posteriolateral flank ectoderm were implanted into the vitreous chamber, only in rare cases did pericorneal ectoderm form lens cells. Thus, unlike the case in X. laevis, competence to respond to the inducing factors is tightly restricted to the cornea epithelium in X. tropicalis. As controls, all these tissues were implanted into the space located between the inner and outer corneas. None of these implants, including outer cornea epithelium, exhibited GFP expression. Thus, the essential inductive factors are normally contained within the vitreous chamber. One explanation why this type of lens regeneration is not seen in some other anurans could be due to the rapid rate at which the inner cornea endothelium heals to recover the pupillary opening once the original lens is removed. These findings are discussed in terms of the evolution of this developmental process within the anurans.

Magnetic resonance cholangiography in patients with biliary disease: its role in primary sclerosing cholangitis.

BACKGROUND/AIM: Magnetic resonance cholangiography (MRC) is a non-invasive diagnostic procedure whose role in the management of patients with primary sclerosing cholangitis (PSC) is unclear. The aim of this study was to determine the usefulness of MRC in the evaluation of the biliary tree in patients with suspected biliary disease, and in particular, PSC. METHODS: MRC and invasive cholangiography (ERCP or PTC) were both performed in 73 patients, (33 male, 40 female, mean age 56 years) with clinical and/or biochemical evidence of cholestasis. Images were interpreted by two radiologists unaware of the results of other studies. RESULTS: Forty-two patients (58%) had benign biliary disease, including 23 patients (32%) with PSC; 9 patients (12%) had malignant biliary disease; and 22 patients (30%) had a normal biliary tree. Diagnostic quality images were obtained in 73/73 (100%) of MRC, and in 70/73 (96%) of invasive cholangiography (68 ERCP's, 2 PTC's) procedures. Using ERCP/PTC findings as the reference standard, MRC had an accuracy greater than 90% in the diagnosis of normal bile ducts, biliary dilatation, biliary obstruction, bile duct stones, and PSC. Using the final diagnosis, MRC had an overall diagnostic accuracy of 90% in the detection of biliary disease compared to 97% for invasive cholangiography. Additional diagnostic/therapeutic interventions were performed during ERCP in 73% of patients with PSC and in 43% of patients without PSC (p=0.02). CONCLUSIONS: MRC has excellent diagnostic accuracy in the presence of biliary disease. Because of its noninvasive nature, MRC may have advantages over invasive cholangiography when diagnosis is the major goal of the procedure.

Xenopus laevis gelatinase B (Xmmp-9): development, regeneration, and wound healing.

It has been argued that matrix metalloproteinases play important roles in cellular differentiation and regeneration in certain systems. While studying changes in gene expression associated with the phenomena of cornea/lens transdifferentiation ("lens regeneration"), which takes place in the larva of Xenopus laevis, we identified the Xenopus gelatinase B gene. The open reading frame is homologous to other gelatinase B genes identified in other species and encodes all of the domains characteristic of this protein. Xenopus gelatinase B (Xmmp-9) is first expressed during early tail-bud stages in a subset of mesodermal cells scattered throughout the body. Expression is also seen in the peripheral tissues of the developing liver diverticulum, the hindgut/cloaca, and the paired caudal vein, and its dorsal branch in the larval tail. Given the significant role of matrix metalloproteinases in degrading components of the extracellular matrix, Xmmp-9 expression may be important in the morphogenesis of these structures. Xmmp-9 expression was also examined during the processes of cornea/lens transdifferentiation, epithelial wound healing, and limb regeneration in Xenopus larvae. Although Xmmp-9 is expressed very early during cornea/lens transdifferentiation, expression is restricted to the site of the peripheral wound created by removal of the original lens, which triggers transdifferentiation. Expression was not found in the central, uninjured area of the cornea where transdifferentiation takes place. Therefore, Xmmp-9 does not appear to play an important role in cornea/lens transdifferentiation. Xmmp-9 expression is associated with other epithelial wounds, indicating that gelatinase B is expressed in the general context of wound healing in Xenopus. Finally, Xmmp-9 is expressed in the ectoderm and mesoderm at the tip of the amputated limb, very early during limb regeneration, where it is argued to play a role in this process.

Conservation of gene expression during embryonic lens formation and cornea-lens transdifferentiation in Xenopus laevis.

Few molecular comparisons have been made between the processes of embryogenesis and regeneration or transdifferentiation that lead to the formation of the same structures. In the amphibian, Xenopus laevis, the cornea can undergo transdifferentiation to form a lens when the original lens is removed during tadpole larval stages. Unlike the process of embryonic lens induction, cornea-lens transdifferentiation is elicited via a single inductive interaction involving factors produced by the neural retina. In this study, we compared the expression of a number of genes known to be activated during various phases of embryonic lens formation, during the process of cornea-lens transdifferentiation. mRNA expression was monitored via in situ hybridization using digoxigenin-labeled riboprobes of pax-6, Xotx2, xSOX3, XProx1, and gamma6-cry. We found that all of the genes studied are expressed during both embryogenesis and cornea-lens transdifferentiation, though in some cases their relative temporal sequences are not maintained. The reiterated expression of these genes suggests that a large suite of genes activated during embryonic lens formation are also involved in cornea-lens transdifferentiation. Ultimately functional tests will be required to determine whether they actually play similar roles in these processes. It is significant that the single inductive event responsible for initiating cornea-lens transdifferentiation triggers the expression of genes activated during both the early and late phases of embryonic lens induction. These findings have significant implications in terms of our current understanding of the "multistep" process of lens induction. Dev Dyn 1999;215:308-318.

The cell lineage of a polyclad turbellarian embryo reveals close similarity to coelomate spiralians.

Recent molecular evidence suggests the turbellarian Platyhelminthes may represent the extant basal members of the Spiralia and therefore probably exhibit ancient features of the spiralian developmental program. The stereotypic quartet spiral cleavage pattern of the polyclad turbellarian embryo, among other features, indicates that this group may be closely related to the ancestral flatworm; however, polyclad embryos have been the subject of few experimental studies. Here we report the results of a cell lineage analysis of the embryo of the polyclad Hoploplana inquilina based on microinjection of DiI into cleavage-stage blastomeres following formation of each of the four quartets of micromeres. The first quartet gives rise to most of the lateral and anterior ectoderm of the Müller's larva; the second quartet forms largely dorsal and ventral ectoderm as well as the circular muscles; the third quartet forms only small clones of ectoderm; and only the 4d cell of the fourth quartet contributes to larval structure, forming the longitudinal muscles, mesenchyme, and probably endoderm. Our results demonstrate a striking similarity between the cell lineages of polyclad and higher spiralian embryos, in which the four quadrants also bear the same relationships to the larval axes and give rise to comparable larval structures, including derivation of mesoderm from both ectodermal (2b) and endodermal precursors (4d).

Conservation of the spiralian developmental program: cell lineage of the nemertean, Cerebratulus lacteus.

Lineage tracers were injected into individual blastomeres in embryos of the indirect-developing nemertean Cerebratulus lacteus through the formation of the fourth quartet of micromeres. Subsequent development was followed to the formation of feeding pilidium larvae to establish their ultimate fates. Results showed that these blastomeres have unique fates, and their clones give rise to highly predictable regions of the larval body. As in other spiralians, four discrete cell quadrants can be identified. For the most part, their identities are homologous to the typical spiralian A, B, C, and D cell quadrants. In some respects their fates differ from the typical spiralian fate map; however, these can be understood in terms of simple modifications of the early cleavage program. Unlike most spiralians, the first quartet micromeres in the eight-celled embryo are larger than the corresponding vegetal macromeres, and generate most of the larval ectoderm. All four of these micromeres contribute to the apical organ and generate four bilaterally situated domains of ectoderm, where the progeny of the 1a and 1d micromeres lie to the left of the median plane while those of 1b and 1c lie to the right. Unlike the progeny of the first quartet, those of the second quartet are situated in left (2a), ventral (2b), right (2c), and dorsal (2d) positions. The third quartet micromeres generate clones situated in a bilaterally symmetrical fashion similar to those of the first quartet. The alternating axial relationships exhibited by successive micromere quartets are a characteristic of spiralian development. Unlike other spiralian larvae possessing a ciliary band, the pilidium larval ciliary band is formed by all blastomeres of the first and second micromere quartets, as well as 3c and 3d. Ectomesoderm is derived from two blastomeres (3a and 3b), which give rise to the extensive array of the larval muscle cells. C. lacteus also possesses a true mesentoblast (4d) which gives rise to a pair of mesodermal bandlets, and scattered mesenchymal cells. The dual origin of the mesoderm, as both ectomesoderm and endomesoderm, appears to be a condition present in all spiralians. The gut is formed by all the fourth quartet micromeres as well as the vegetal macromeres (4A, 4B, 4C, 4D). Despite differences in the determination of axial properties and some modifications in quadrant fates, nemerteans appear to be constructed on the typical spiralian developmental platform.

The development of dorsoventral and bilateral axial properties in sea urchin embryos.

Sea urchin embryos have provided excellent material for experimental and molecular analyses of the processes of cell and axis specification during embryogenesis. These studies revealed the tremendous developmental plasticity of cells in early embryos and the roles that cell-cell interactions play in determining cell fates. Details concerning sea urchin development can be found in a number of excellent reviews. This review summarizes experimental and molecular studies relating to axis determination and cell fate specification in echinoid embryos. Correlations are drawn from research carried out on the development of axial systems in other organisms.

Inductive processes leading to inner ear formation during Xenopus development.

This study examines the spatial and temporal attributes of inner ear induction in Xenopus embryos. These results are compared to recent experiments concerning lens induction to assess whether head sensory structures share common ontogenetic features. Ectoderm from different regions and stages was transplanted to the presumptive ear region of hosts of either early (neural plate) or late (neural tube) stages. Explants of the presumptive ear ectoderm were also taken from embryos at these stages to establish the time of otic ectoderm specification. We find that ectodermal competence for otic vesicle formation extends through neural plate stages, far longer than for lens formation. Otic vesicle specification occurs substantially earlier, at neural plate stages, than lens specification. Competent ectoderm forms otic vesicles in a high fraction of cases when exposed to the ear-inducing environment of either neural plate stages or neural tube stages, a result which contrasts with lens induction where the neural tube stage embryo provides a much weaker inducing environment than earlier stages. Otic vesicles induced in neural tube stage hosts are primarily in contact with presumptive hindbrain, suggesting that this neural tissue may be sufficient for otic vesicle induction. These studies reveal overall similarities between lens and inner ear induction, but sufficient differences to propose that some facets of determination of these sensory tissues may occur by independent mechanisms and not via a common developmental state.

The matured eye of Xenopus laevis tadpoles produces factors that elicit a lens-forming response in embryonic ectoderm.

Previous studies have indicated that the outer cornea can undergo transdifferentiation to form a lens in the tadpole larva of Xenopus laevis following removal of the original lens. This transformation appears to require an interaction with the neural retina. In the present study, we carried out a series of experiments to determine if the matured tadpole eye can also elicit lens formation in embryonic ectoderm. Labeled embryonic ectoderm was removed from the presumptive lens-forming region, or from the belly region (ventral ectoderm), at various stages of development (stages 11-19, gastrula to neural tube stages) and implanted into the eye cavity (posterior chamber) of advanced stage 52-55 tadpoles. After 3 days, we examined the tadpoles and their implanted tissues for lens cell formation using lens-specific antibodies. Implanted presumptive lens ectoderm differentiated lens cells in a large number of cases. The percentage of cases forming lens cells and the extent of morphological differentiation increased with increasing age of the implanted tissue. Implanted ventral ectoderm also formed lens cells, although at a reduced frequency and with limited morphological differentiation. These results indicate that the environment of the matured tadpole eye cavity stimulates lens cell formation in both presumptive lens and nonlens ectoderm. The development of the implanted tissues was compared to that found in previous studies where these tissues were cultured as explants or transplanted to lens-forming regions during early development and subjected to various periods of embryonic lens induction. Together, these findings suggest that the process of embryonic lens formation is related to that involved in transdifferentiation of the tadpole cornea during "lens regeneration." However, the inductive effect of the matured tadpole eye is qualitatively different from that of the early period of embryonic lens induction and, while more intense, may be more closely related to that which takes place via the optic vesicle during the later phase of embryonic lens induction.

Progressive determination of cell fates along the dorsoventral axis in the sea urchin Heliocidaris erythrogramma.

In the direct-developing sea urchin Heliocidaris erythrogramma the first cleavage division bisects the dorsoventral axis of the developing embryo along a frontal plane. In the two-celled embryo one of the blastomeres, the ventral cell (V), gives rise to all pigmented mesenchyme, as well as to the vestibule of the echinus rudiment. Upon isolation, however, the dorsal blastomere (D) displays some regulation, and is able to form a small number of pigmented mesenchyme cells and even a vestibule. We have examined the spatial and temporal determination of cell fates along the dorsoventral axis during subsequent development. We demonstrate that the dorsoventral axis is resident within both cells of the two-celled embryo, but only the ventral pole of this axis has a rigidly fixed identity this early in development. The polarity of this axis remains the same in half-embryos developing from isolated ventral (V) blastomeres, but it can flip 180° in half-embryos developing from isolated dorsal (D) blastomeres. We find that cell fates are progressively determined along the dorsoventral axis up to the time of gastrulation. The ability of dorsal half-embryos to differentiate ventral cell fates diminishes as they are isolated at progressively later stages of development. These results suggest that the determination of cell fates along the dorsoventral axis in H. erythrogramma is regulated via inductive interactions organized by cells within the ventral half of the embryo.

Routine growth monitoring and assessment of growth disorders.

Pediatric nurse practitioners have an important role in the routine monitoring of growth and the assessment of growth disorders. The development of good history-taking skills and an accurate, repeatable measurement technique are central to the success of growth assessment and evaluation. Numerous causes exist for growth disorders. The pediatric nurse practitioner can be instrumental in identifying these disorders and in making appropriate referral to a pediatric endocrinologist for further evaluation and treatment.

Evolutionary dissociation between cleavage, cell lineage and embryonic axes in sea urchin embryos.

Using vital dye staining and the microinjection of fluorescent cell lineage-autonomous tracers, the relationship between the first cleavage plane and the prospective larval dorsoventral axis was examined in several sea urchin species, including: Strongylocentrotus purpuratus, S. droebachiensis, Lytechinus pictus, Clypeaster rosaceus, Heliocidaris tuberculata and H. erythrogramma. The results indicate that there is no single relationship between the early cleavage pattern and the dorsoventral axis for all sea urchins; however, specific relationships exist for individual species. In S. purpuratus the first cleavage plane occurs at an angle 45 degrees clockwise with respect to the prospective dorsoventral axis in most cases, as viewed from the animal pole. On the other hand, in S. droebachiensis, L. pictus and H. tuberculata, the first cleavage plane generally corresponds with the plane of bilateral symmetry. There does not appear to be a predominant relationship between the first cleavage plane and the dorsoventral axis in C. rosaceus. In the direct-developing sea urchin H. erythrogramma the first cleavage plane bisects the dorsoventral axis through the frontal plane. Clearly, evolutionary differences have arisen in the relationship between cleavage pattern and developmental axes. Therefore, the mechanism of cell determination is not necessarily tied to any particular pattern of cell cleavage, but to an underlying framework of axial systems resident within sea urchin eggs and embryos.

Recent progress on the mechanisms of embryonic lens formation.

Formation of the lens during embryonic development depends on tissue interactions as shown clearly both from teratological data and from extensive experimental analysis. Recent work has, however, altered our view of the importance of particular tissue interactions for lens formation. While earlier work emphasises the role of the optic vesicle in lens induction, more recent studies argue that lens-inducing signals important for determination act before optic vesicle formation. Evidence is given for a four stage model in which ectoderm first becomes competent to respond to lens inducers. It then receives inductive signals, at least in part emanating from the anterior neural plate, so that it gains a lens-forming bias and subsequently becomes specified for lens formation. Complete lens differentiation does require signals from the optic vesicle, and in addition an inhibitory signal from head neural crest may suppress any residual lens-forming bias in head ectoderm adjacent to the lens.

Chronic alcohol consumption enhances sepsis-induced cardiac dysfunction.

Chronic alcoholism causes a cardiac contractile dysfunction which, in rats, may occur after 6 mo to 1 yr of alcohol consumption. Sepsis, on a more acute basis, can also induce intrinsic cardiac dysfunction. We tested the hypothesis that 2 mo of chronic alcohol feeding, while not directly causing overt depression of the myocardium, might sensitize the heart to a known cardiac stress, i.e., sepsis. We proposed that sepsis, induced in an alcoholic animal, would cause a more severe myocardial depression than in a nonalcoholic rat. Thus rats were fed a liquid diet with 36% of the total calories as alcohol for 8-10 wk and were then anesthetized and received an injection of live Escherichia coli (approximately 10(10) E. coli) through a dorsal subcutaneous catheter followed by a second dose approximately 5 h later. The following day, hearts were removed and, using the isolated working heart preparation, intrinsic contractile performance was assessed by generating ventricular function curves. Four groups of animals were studied. Hearts from the nonalcoholic-nonseptic group and the alcoholic-nonseptic group showed identical cardiac work (cardiac output x peak systolic pressure at the highest preload was 6,113 +/- 324 and 5,955 +/- 406 ml.min-1.mmHg-1, respectively). Work in the nonalcoholic-septic and the alcoholic-septic groups was decreased by 30 and 50%, respectively (4,806 +/- 478 vs. 2,917 +/- 435 ml.min-1.mmHg-1 at the highest preload). Thus 2 mo chronic alcohol consumption caused no overt cardiac dysfunction by itself but did exacerbate the myocardial injury induced by sepsis.

The dorsoventral axis is specified prior to first cleavage in the direct developing sea urchin Heliocidaris erythrogramma.

Previous fate mapping studies as well as the culture of isolated blastomeres have revealed that the dorsoventral axis is specified as early as the 2-cell stage in the embryos of the direct developing echinoid, Heliocidaris erythrogramma. Normally, the first cleavage plane includes the animal-vegetal axis and bisects the embryo between future dorsal and ventral halves. Experiments were performed to establish whether the dorsoventral axis is set up prior to the first cleavage division in H. erythrogramma. Eggs were elongated and fertilized in silicone tubes of a small diameter in order to orient the cleavage spindle and thus the first plane of cell division. Following first cleavage, one of the two resulting blastomeres was then microinjected with a fluorescent cell lineage tracer dye. Fate maps were made after culturing these embryos to larval stages. The results indicate that the first cleavage division can be made to occur at virtually any angle relative to the animal-vegetal and dorsoventral axes. Therefore, the dorsoventral axis is specified prior to first cleavage. We argue that this axis resides in the unfertilized oocyte rather than being set up as a consequence of fertilization.

Differential cytokeratin gene expression reveals early dorsal-ventral regionalization in chick mesoderm.

The induction and spatial patterning of early mesoderm are known to be critical events in the establishment of the vertebrate body plan. However, it has been difficult to define precisely the steps by which mesoderm is initially subdivided into functionally discrete regions. Here we present evidence for a sharply defined distinction between presumptive dorsal and presumptive ventral regions in early chick mesoderm. Northern blot and in situ hybridization analyses reveal that transcripts corresponding to CKse1, a cytokeratin gene expressed during early development, are present at high levels in the presumptive ventral mesoderm, but are greatly reduced or undetectable in the future dorsal region of mesoderm, where the formation of axial structures occurs later in development. This distinction is present even while the mesoderm layer is being formed, and persists during the extensive cellular movements and tissue remodelling associated with morphogenesis. These results point to an early step in which two fundamentally distinct states are established along the presumptive dorsal-ventral axis in the mesoderm, and suggest that determination in this germ layer occurs in a hierarchical manner, rather than by direct specification of individual types of histological differentiation. The differential expression of CKse1 represents the earliest molecular index of dorsoventral regionalization detected thus far in the mesoderm.

Early tissue interactions leading to embryonic lens formation in Xenopus laevis.

Our previous research has demonstrated that lens induction in Xenopus laevis requires inductive interactions prior to contact with the optic vesicle, which classically had been thought to be the major lens inductor. The importance of these early interactions has been verified by demonstrating that lens ectoderm is specified by the time it comes into contact with the optic vesicle. It has been argued that the tissues which underlie the presumptive lens ectoderm during gastrulation and neurulation, dorsolateral endoderm and mesoderm, are the primary early inductors. We show here, however, that these tissues alone cannot elicit lens formation in Xenopus ectoderm. Evidence is presented that presumptive anterior neural plate tissue (which includes the early eye rudiment) is an essential early lens inductor in Xenopus. The presence of dorsolateral mesoderm appears to enhance this response. These findings support a model in which an essential inductive signal passes through the plane of ectoderm during gastrula and early neurula stages from presumptive anterior neural tissue to the presumptive lens ectoderm. Since there is evidence for such interactions within a tissue layer in mesodermal and neural induction as well, this may be a general feature of the initial stages of determination of many tissues.