Variation for host range within and among populations of the parasitic plant Striga hermonthica.

Striga hermonthica is an angiosperm parasite that causes substantial damage to a wide variety of cereal crop species, and to the livelihoods of subsistence farmers in sub-Saharan Africa. The broad host range of this parasite makes it a fascinating model for the study of host-parasite interactions, and suggests that effective long-term control strategies for the parasite will require an understanding of the potential for host range adaptation in parasite populations. We used a controlled experiment to test the extent to which the success or failure of S. hermonthica parasites to develop on a particular host cultivar (host resistance/compatibility) depends upon the identity of interacting host genotypes and parasite populations. We also tested the hypothesis that there is a genetic component to host range within individual S. hermonthica populations, using three rice cultivars with known, contrasting abilities to resist infection. The developmental success of S. hermonthica parasites growing on different rice-host cultivars (genotypes) depended significantly on a parasite population by host-genotype interaction. Genetic analysis using amplified fragment length polymorphism (AFLP) markers revealed that a small subset of AFLP markers showed 'outlier' genetic differentiation among sub-populations of S. hermonthica attached to different host cultivars. We suggest that, this indicates a genetic component to host range within populations of S. hermonthica, and that a detailed understanding of the genomic loci involved will be crucial in understanding host-parasite specificity and in breeding crop cultivars with broad spectrum resistance to S. hermonthica.

Global patterns of gene expression in rice cultivars undergoing a susceptible or resistant interaction with the parasitic plant Striga hermonthica.

Striga hermonthica is a root hemiparasite of cereals that causes devastating loss of yield. Recently, a rice cultivar, Nipponbare, was discovered, which exhibits post-attachment resistance to this parasite and quantitative trait loci (QTL) associated with the resistance were identified. Changes in gene expression in susceptible (IAC 165) and resistant (Nipponbare) rice cultivars were profiled using rice whole-genome microarrays. In addition to a functional categorization of changes in gene expression, genes that were significantly up-regulated within resistance QTL were identified. The resistance reaction was characterized by up-regulation of defence genes, including pathogenesis-related proteins, pleiotropic drug resistance ABC transporters, genes involved in phenylpropanoid metabolism and WRKY transcription factors. These changes in gene expression resemble those associated with resistance to microbial pathogens. Three genes encoding proteins of unknown function, within a major resistance QTL on chromosome 12, were highly up-regulated and are excellent candidate resistance genes. The susceptible interaction was characterized by large-scale down-regulation of gene expression, particularly within the functional categories plant growth regulator signalling and metabolism, biogenesis of cellular components and cell division. Up-regulated genes included nutrient transporters, enzymes of amino acid metabolism and some abiotic stress genes.

A novel form of resistance in rice to the angiosperm parasite Striga hermonthica.

The root hemiparasitic weed Striga hermonthica is a serious constraint to grain production of economically important cereals in sub-Saharan Africa. Breeding for parasite resistance in cereals is widely recognized as the most sustainable form of long-term control; however, advances have been limited owing to a lack of cereal germplasm demonstrating postattachment resistance to Striga. Here, we identify a cultivar of rice (Nipponbare) that exhibits strong postattachment resistance to S. hermonthica; the parasite penetrates the host root cortex but does not form parasite-host xylem-xylem connections. In order to identify the genomic regions contributing to this resistance, a mapping population of backcross inbred lines between the resistant (Nipponbare) and susceptible (Kasalath) parents were evaluated for resistance to S. hermonthica. Composite interval mapping located seven putative quantitative trait loci (QTL) explaining 31% of the overall phenotypic variance; a second, independent, screen confirmed four of these QTL. Relative to the parental lines, allelic substitutions at these QTL altered the phenotype by at least 0.5 of a phenotypic standard deviation. Thus, they should be regarded as major genes and are likely to be useful in breeding programmes to enhance host resistance.

Interaction between Orobanche crenata and its host legumes: unsuccessful haustorial penetration and necrosis of the developing parasite.

BACKGROUND AND AIMS: Orobanche species represent major constraints to crop production in many parts of the world as they reduce yield and alter root/shoot allometry. Although much is known about the histology and effect of Orobanche spp. on susceptible hosts, less is known about the basis of host resistance to these parasites. In this work, histological aspects related to the resistance of some legumes to Orobanche crenata have been investigated in order to determine which types of resistance responses are involved in the unsuccessful penetration of O. crenata. METHODS: Samples of resistance reactions against O. crenata on different genotypes of resistant legumes were collected. The samples were fixed, sectioned and stained using different procedures. Sections were observed using a transmission light microscope and by epi-fluorescence. KEY RESULTS: Lignification of endodermal and pericycle host cells seems to prevent parasite intrusion into the root vascular cylinder at early infection stages. But in other cases, established tubercles became necrotic and died. Contrary to some previous studies, it was found that darkening at the infection site in these latter cases does not correspond to death of host tissues, but to the secretion of substances that fill the apoplast in the host-parasite interface and in much of the infected host tissues. The secretions block neighbouring host vessels. This may interfere with the nutrient flux between host and parasite, and may lead to necrosis and death of the developing parasite. CONCLUSIONS: The unsuccessful penetration of O. crenata seedlings into legume roots cannot be attributed to cell death in the host. It seems to be associated with lignification of host endodermis and pericycle cells at the penetration site. The accumulation of secretions at the infection site, may lead to the activation of xylem occlusion, another defence mechanism, which may cause further necrosis of established tubercles.

Wheat (Triticum aestivum) Is Susceptible to the Parasitic Angiosperm Striga hermonthica, a Major Cereal Pathogen in Africa.

ABSTRACT Striga hermonthica is a parasitic weed endemic to sub-Saharan Africa. It most commonly parasitizes sorghum, maize, pearl millet, and upland rice, lowering yields and affecting the welfare of over 100 million people, principally subsistence farmers. Cereal crops with complete resistance to this pathogen have not been reported. In southern and eastern Africa, where Striga spp. are endemic, 5.6 million ha of wheat are cultivated annually. Despite this, there are only isolated field reports of wheat infected with Striga spp. It is not clear whether this is due to resistance in this cereal or to environmental factors. In this article, we examined the ability of root exudates from five cultivars of wheat (Chablis, Cadenza, Hereward, Riband, and Brigadier) to trigger germination of S. hermonthica seed. A study of the development of S. hermonthica on two cultivars of wheat (Hereward and Chablis) and on a range of ancestral relatives of wheat (Triticum and Aegilops spp.) then was conducted. Last, the effect of Striga spp. on host growth and yield was examined using wheat cv. Chablis and compared with that of a highly susceptible sorghum cultivar (CSH-1). Wheat was able to support the germination, attachment, and subsequent development of Striga spp. All wheat cultivars and ancestral species of modern wheat (Triticum and Aegilops spp.) were susceptible to S. hermonthica. In addition, in wheat, infection severely lowered plant height (-24%) and biomass accumulation (-33%); a small parasite biomass elicited a large host response. In conclusion, wheat is highly susceptible to S. hermonthica and, in light of global climate change, this may have implications for wheat-producing areas of Africa.

Physiological and ecological significance of sunflecks for dipterocarp seedlings.

Irradiance is highly dynamic in many plant canopies. Photosynthesis during sunflecks provides 10-90% of daily carbon gain. The survivorship of tree seedlings in the deeply shaded understorey of tropical rain forests is limited by their ability to maintain a positive carbon balance. Dipterocarp seedlings from the SE Asian rain forest were used as a model system to test novel aspects of the physiological and ecological significance of sunflecks. First, understorey seedlings experienced leaf temperatures up to 38 degrees C in association with sunflecks. Under controlled environment conditions, the inhibition of carbon gain at 38 degrees C, compared with 28 degrees C, was significantly greater during a sequence of sunflecks (-59%), than under uniform irradiance (-40%), providing the same total photosynthetic photon flux density (PPFD). Second, the relative enhancement effects of elevated [CO2] were greater under sunflecks (growth +60%, carbon gain +89%), compared with uniform irradiance (growth +25%, carbon gain +59%), supplying the same daily PPFD. Third, seedling growth rates in the forest understorey were 4-fold greater under a dynamic irradiance treatment characterized by long flecks, compared with a regime of short flecks. Therefore, stresses associated with dynamic irradiance may constrain photosynthetic carbon gain. Additionally, seedling photosynthesis and growth may be more responsive to interactions with abiotic factors, including future changes in climate, than previously estimated. The sensitivity of seedling growth to varying patterns of dynamic irradiance, and the increased likelihood of species-specific responses through interactions with environmental factors, indicates the potential for sunflecks to influence regeneration processes, and hence forest structure and composition.

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings.

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.

Novel sources of resistance to Striga hermonthica in Tripsacum dactyloides, a wild relative of maize.

• The parasitic weed Striga hermonthica lowers cereal yield in small-holder farms in Africa. Complete resistance in maize to S. hermonthica infection has not been identified. A valuable source of resistance to S. hermonthica may lie in the genetic potential of wild germplasm. • The susceptibility of a wild relative of maize, Tripsacum dactyloides and a Zea mays-T. dactyloides hybrid to S. hermonthica infection was determined. Striga hermonthica development was arrested after attachment to T. dactyloides. Vascular continuity was established between parasite and host but there was poor primary haustorial tissue differentiation on T. dactyloides compared with Z. mays. Partial resistance was inherited in the hybrid. • Striga hermonthica attached to Z. mays was manipulated such that different secondary haustoria could attach to different hosts. Secondary haustoria formation was inhibited on T. dactyloides, moreover, subsequent haustoria formation on Z. mays was also impaired. • Results suggest that T. dactyloides produces a signal that inhibits haustorial development: this signal may be mobile within the parasite haustorial root system.

p53 Mutation in adenocarcinoma arising in retrorectal cyst hamartoma (tailgut cyst): report of 2 cases--an immunohistochemistry/immunoperoxidase study.

Retrorectal cyst hamartoma (RCH) is a rare benign cystic lesion located in the retrorectal space. Malignancy arising in such lesions is very uncommon. In this study, 2 cases of mucinous adenocarcinoma arising in RCH are presented. In one case, dysplastic epithelium lined the cyst wall, surrounding the area of carcinoma and suggesting a dysplasia-carcinoma progression in RCH. Adenocarcinoma and the dysplastic epithelium were strongly positive for p53 and Ki-67 and showed negative staining for p21 by immunohistochemistry. These findings are suggestive of a mutation in the p53 gene in the adenocarcinoma and in dysplastic epithelium lining the cysts, similar to the dysplasia-carcinoma sequence described for the development of colonic adenocarcinoma.

Unusually located left atrial papillary fibroelastoma with probable embolization to the spinal cord.

This report presents a patient with flaccid paraplegia, most probably secondary to embolization from a left atrial papillary fibroelastoma. The unique location of the tumor, attached to the ridge in the left atrium between the left atrial appendage and pulmonary vein, was well documented on transesophageal echocardiography, and the diagnosis was confirmed by histopathology. Although benign, these tumors may develop in all four cardiac chambers and result in pulmonary or systemic embolization. Paraplegia is a rare embolic complication.

Differential expression of the cell-adhesion molecule Nr-CAM in hyperplastic and neoplastic human pancreatic tissue.

Nr-CAM is a member of the immunoglobulin superfamily of neural cell-adhesion molecules initially thought to be expressed mainly in the brain. Here we show the presence of Nr-CAM protein in normal human pancreas and characterize its expression in hyperplastic and neoplastic human pancreatic tissue. Nr-CAM is expressed on the cell surface in normal pancreatic acini with enhanced staining at cell-cell junctions, and weak or no surface staining is seen on normal ductal cells. Nr-CAM expression is markedly up-regulated in intraductal hyperplasia. Expression was well maintained in well or moderately differentiated carcinoma but was reduced or absent from most poorly differentiated tumors. In addition, 4 of 4 human pancreatic adenocarcinoma cell lines tested demonstrated little or no Nr-CAM expression. This differential regulation of Nr-CAM expression suggests that it may be involved in the pathogenesis and invasive/metastatic behavior of pancreatic cancers. HUM PATHOL 32:396-400.

PNET-like features of synovial sarcoma of the lung: a pitfall in the cytologic diagnosis of soft-tissue tumors.

Fine-needle aspiration (FNA) cytology of soft-tissue tumors is evolving. As more experience is gained, we are becoming aware of potential pitfalls. We describe 2 cases of synovial sarcoma of the lung, primary and metastatic, in patients who had FNA biopsy performed on a lung mass. The cytologic smears showed extremely cellular groups of malignant small round cells, intersected by small blood vessels, with numerous loose single cells, in a background of macrophages and mature lymphocytes. The tumors displayed monomorphic cells forming rosettes and displaying occasional mitoses. A diagnosis of neuroendocrine tumor/primitive neuroepithelial tumor (PNET) was suspected. Furthermore, this suspicion was supported by immunohistochemical stains, which showed positivity for a neuroendocrine marker, Leu 7 (case 1), and for a neural marker, CD 99 (O 13 or HBA 71) (both cases); and negativity for cytokeratins (case 1). The resection specimen of case 1 had mostly tightly packed small round cells, with occasional rosettes, similar to the FNA biopsy, and focal areas composed of spindle cells, organized in a focal fibrosarcoma-like and hemangiopericytoma-like pattern. A balanced translocation between chromosomes X and 18, demonstrated by both karyotyping and fluorescent in situ hybridization (FISH), enabled us to make a diagnosis of synovial sarcoma, which was histologically classified as poorly differentiated. Case 2 was a metastatic biphasic synovial sarcoma of the arm, with a prominent epithelial component. Synovial sarcoma, when composed mainly of small round cells on cytologic smears, is a great mimicker of neuroendocrine/PNET tumors, with light microscopic and immunohistochemical overlap. Awareness of this potential pitfall may aid in preventing a misdiagnosis. Its recognition is of major concern, especially for the poorly differentiated variant, because it is associated with a worse prognosis.

Signal transduction targets in androgen-independent prostate cancer.

Prostate cancer (PCa) first manifests as an androgen-dependent disease. Thus, androgen-deprivation therapy is a standard regimen for patients with metastatic PCa. Despite the initial success of androgen-deprivation therapy, PCa inevitably progresses from being androgen dependent (AD) to androgen independent (AI), and this marks the poor prognosis of this disease. Relapse of AIPCa becomes life threatening and accounts for the majority of mortality of PCa patients. Currently, no effective therapy is available for controlling AIPCa. Therefore, the challenge in providing a new intervention is to understand the fundamental changes that occur in AIPCa. Increasing evidence indicates that, under androgen-deprived milieu, several signal networks elicited by peptide growth factors dictate the AI phenotype of PCa. This review covers the latest studies investigating the potential involvement of autocrine growth factors in cell proliferation, survival, metastasis, and the reciprocal interaction with the androgen receptor pathway. In addition, loss of the negative feedback mechanism of the signal cascade further amplifies the effect of growth factors, and thus contributes significantly to the onset of AIPCa. The understanding of the signal target(s) in AIPCa should provide the new markers for prognosis and a new strategy for prevention and therapy.

Cystic pancreatic masses: cross-sectional imaging observations and serial follow-up.

BACKGROUND: We retrospectively reviewed the imaging features of a series of patients with cystic pancreatic masses, the majority of whom underwent imaging surveillance. METHODS: Imaging data from 30 patients with known cystic pancreatic masses were reviewed. Nine patients had surgical and/or cytologic classification. Of the 21 who were not operated on, all underwent serial imaging surveillance. Of these, five had corroborative endoscopic retrograde cholangiopancreatography and 16 were followed by only computed tomography and/or magnetic resonance imaging. RESULTS: In the nonoperated group, mean follow-up time was 30 months (3-144 months). Two patients demonstrated growth, and the remainder remain stable. In the patients who underwent surgery, invasive carcinoma was found in those with lesions larger than 4 cm, involvement of the main pancreatic duct, or visible solid components on the imaging study. Smaller lesions were benign. CONCLUSION: In patients with suspected cystic pancreatic neoplasms, surveillance might be possible if lesions are smaller than 2.5 cm, spare the main pancreatic duct, and demonstrate no solid components.

Clinical exchange: one model to achieve culturally sensitive care.

This paper reports on a clinical exchange programme that formed part of a pre-registration European nursing degree run by three collaborating institutions in England, Holland and Spain. The course included: common and shared learning including two summer schools; and the development of a second language before the students went on a three-month clinical placement in one of the other base institutions' clinical environments. The aim of the course was to enable students to become culturally sensitive carers. This was achieved by developing a programme based on transcultural nursing principles in theory and practice. Data were gathered by interview, focus groups, and questionnaires from 79 exchange students, fostering the strategies of illuminative evaluation. The paper examines: how the aims of the course were met; the factors that inhibited the attainment of certain goals; and how the acquisition of a second language influenced the students' learning about nursing. A model is presented to illustrate the process of transformative learning from the exchange experience.

Differences between the clearance of apoptotic cells by professional and non-professional phagocytes.

Both professional and non-professional phagocytes [1] participate in clearing the massive numbers of cells that undergo apoptosis during animal development [2], but it is not known how they divide this task. Using time-lapse recordings of cells in culture, we show that professional phagocytes (brain macrophages or microglia) are highly motile, ingest apoptotic cells immediately, and digest them quickly. Non-professionals such as BHK and lens epithelial cells are sessile, often recognize apoptotic cells as soon as they die by showing characteristic palpating movements, but delay ingestion until several hours later. By pre-ageing apoptotic cells, we show that this delay is because the apoptotic cells must undergo further changes before non-professionals can ingest them. The difference was also apparent in vivo, using immunofluorescence and electron microscopy of the developing central nervous system. This arrangement favours prompt clearance by professionals if present in adequate numbers; if they are scarce, however, non-professional bystanders will reluctantly clear the apoptotic cells.

Balancing stakeholder needs: a review of ENB 100 and 415 courses. English National Board for Nursing, Midwifery and Health Visiting.

This paper reports the findings of a documentary analysis and literature review of general and paediatric intensive care unit (ICU) courses (ENB 100 and ENB 415). The findings are part of a larger review of critical care courses commissioned by the English National Board for Nursing, Midwifery and Health Visiting (ENB), also incorporating operating department, coronary care and accident and emergency courses. It was important to set the curriculum review in the context of intensive care practice and education, hence the study also comprised interviews with lecturers and ICU managers. The study findings reveal diversity in major aspects of the critical care courses, including the academic level of the programmes and credits they attracted; the assessment strategies for theory and practice, the extent of shared learning and the amount of student effort. Many factors influenced this diversity including contrary opinion among stakeholders about the purpose of the course: to prime the students for working in the specialty; or to consolidate previous experience (in some cases up to 15 years). Course structure and content have changed in response to local university requirements and directives from the statutory bodies, as well as in response to the higher level of academic credit awarded for pre-registration programmes (qualification inflation). The perceived shift in course content as well as the diversity across programmes had led a group of ICU managers to define their own list of competencies (Crunden 1998). However, the majority of the managers interviewed for this study (63% of General ICU managers (n = 19) and 83% (n = 6) of Paediatric ICU managers) were generally satisfied with the competencies and skills of the nurses who had undertaken the ENB course. The authors conclude from the diverse nature of the courses that there is little national comparability in the courses although this finding might be an artefact of documentary analysis. The extent to which this (apparent) diversity results in different levels of competence in practice requires further exploration.

Obesity potentiates AOM-induced colon cancer.

Obesity and diet affect the incidence and severity of various types of cancer, including colon cancer. It is not known whether obesity, independent of diet, is a risk factor for colon adenocarcinoma. We used azoxymethane (AOM) to induce colon cancer in mature genetically obese male Zucker rats (fa/fa) on low-fat crude diet (LFC, 10% fat) and their lean counterparts (Fa/fa and Fa/fa) on high-fat crude diet (HFC, 40% fat) for three months. At death visible tumors, histopathology, and colonic aberrant crypt (AC) formation were studied by blinded investigators. At death the obese animals were heavier (719 +/- 19 g; mean +/- SEM) than lean animals regardless of diet or genotype (Fa/fa-LFC:451 = 6 g; Fa/fa-HFC:441 +/-10 g; Fa/Fa-HFC:412 +/- 9 g; P < 0.001 vs fa/fa by ANOVA). All AOM-treated rats developed AC, compared to none of the saline-injected controls. Macroscopic adenocarcinoma developed in 8/9 obese rats on LFC (P < 0.001), compared to none in lean rats regardless of diet. Obese rats had significantly more AC (876 +/- 116) than any of the lean rats (Fa/fa-LFC:550 +/- 99; Fa/fa-HFC:325 +/- 37; Fa/Fa-HFC:360 +/- 36; P < 0.05 vs fa/fa). We conclude that obesity more than exposure to high-fat diet was associated with colon carcinogenesis in these rats.

Cd44: a marker of squamous differentiation in adenosquamous neoplasms.

OBJECTIVE: To test the hypothesis that CD44 standard (CD44[s]) and its other variants, CD44v6 and CD44v7-8, might be useful markers of squamous differentiation in epithelial tumors. DESIGN: We studied expression of CD44(s), CD44v6, and CD44v7-8 using immunohistochemistry in human tumors that had squamous differentiation, glandular differentiation, or both arising in the colon, stomach, esophagus, lung, pancreas, gallbladder, or uterus/cervix, as well as in adjacent nonneoplastic tissues. Formalin-fixed, paraffin-embedded archival tissue specimens of 33 adenosquamous tumors were used. All were stained with monoclonal antibodies against a conserved portion of CD44(s) and its variants, CD44v6 and CD44v7-8, using the avidin-biotin peroxidase method. RESULTS: CD44(s) and its variants consistently and strongly stained areas of tumors with well-developed squamous differentiation. These markers also consistently and strongly stained normal squamous mucosa. Reactivity for CD44 and its variants was lacking in normal glandular type epithelium and in adenocarcinomas composed entirely of well-differentiated mucin-producing glands. Areas of well-differentiated carcinoma, both squamous and adenocarcinoma, were consistent with respect to both extent and intensity of staining. Staining in lymph nodes was similar to that in the primary tumors, with well-differentiated squamous foci being consistently positive, well-differentiated mucin-producing adenocarcinoma foci consistently negative, and poorly differentiated foci showing variable staining. Although staining was less intense with the variants, it followed the same staining pattern as found for CD44(s). No differences in the extent or intensity of staining were identified in the metastatic versus primary tumor foci, nor was any difference identified between superficial and deeply invasive areas of primary tumors. CONCLUSIONS: Our study shows that CD44(s) and its variants are good markers of squamous epithelial differentiation in several types of normal epithelium and tumors, and that these markers can identify areas of well- to moderately differentiated elements in adenosquamous neoplasms. However, poorly differentiated tumors show an inconsistent staining pattern with CD44, such that it cannot be used as a reliable and practical marker of squamous differentiation in poorly differentiated neoplasms.

Schwann cells in the regenerating fish optic nerve: evidence that CNS axons, not the glia, determine when myelin formation begins.

Fish optic nerve fibres quickly regenerate after injury, but the onset of remyelination is delayed until they reach the brain. This recapitulates the timetable of CNS myelinogenesis during development in vertebrate animals generally, and we have used the regenerating fish optic nerve to obtain evidence that it is the axons, not the myelinating glial cells, that determine when myelin formation begins. In fish, the site of an optic nerve injury becomes remyelinated by ectopic Schwann cells of unknown origin. We allowed these cells to become established and then used them as reporters to indicate the time course of pro-myelin signalling during a further round of axonal outgrowth following a second upstream lesion. Unlike in the mammalian PNS, the ectopic Schwann cells failed to respond to axotomy and to the initial outgrowth of new optic axons. They only began to divide after the axons had reached the brain. Shortly afterwards, small numbers of Schwann cells began to leave the dividing pool and form myelin sheaths. More followed gradually, so that by 3 months remyelination was almost completed and few dividing cells were left. Moreover, remyelination occurred synchronously throughout the optic nerve, with the same time course in the pre-existing Schwann cells, the new ones that colonised the second injury, and the CNS oligodendrocytes elsewhere. The optic axons are the only common structures that could synchronise myelin formation in these disparate glial populations. The responses of the ectopic Schwann cells suggest that they are controlled by the regenerating optic axons in two consecutive steps. First, they begin to proliferate when the growing axons reach the brain. Second, they leave the cell cycle to differentiate individually at widely different times during the ensuing 2 months, during the critical period when the initial rough pattern of axon terminals in the optic tectum becomes refined into an accurate map. We suggest that each axon signals individually for myelin ensheathment once it completes this process.