Hensen's node,but not other biological signallers,can induce supernumerary digits in the developing chick limb bud

Summary The purpose of this study was to determine whether the organizer regions of early avian and amphibian embryos could induce supernumerary (SN) wing structures to develop when they were grafted to a slit in the anterior side of stage 19–23 chick wing buds. Supernumerary digits developed in 43% of the wings that received anterior grafts of Hensen's node from stage 4–6 quail or chick embryos; in addition, 16% of the wings had rods of SN cartilage, but not recognizable SN digits. The grafted quail tissue did not contribute to the SN structures. When tissue anterior or lateral to Hensen's node or lateral pieces of the area pellucida caudal to Hensen's node were grafted to anterior slits, the wings usually developed normally. No SN structures developed when Hensen's nodes were grafted to posterior slits in chick wing buds. Wings developed normally when pieces of the dorsal lip of the blastopore from stage 10–11.5 frog (Xenopus laevis and Rana pipiens) embryos were grafted to anterior slits. No SN digits developed when other tissues that have limb-inducing activity in adult urodele amphibians [chick otic vesicle, frog (Rana pipiens) lung and kidney] or that can act as heteroinductors in neural induction (rat kidney, lung, submaxillary gland and urinary bladder; mouse liver and submaxillary gland) were grafted to anterior slits in chick wing buds. SN digits also failed to develop following preaxial grafts of chick optic vesicles. These results suggest that although the anteroposterior polarity of the chick wing bud can be influenced by factors other than the ZPA (e.g., Hensen's node, retinoids), the wing is not so labile that it can respond to a wide variety of inductively-active tissues.     

A fate map of superficial and deep circumblastoporal cells in the early gastrula of Pleurodeles waltl.

We have determined the fate of presumptive mesodermal cells in the early Pleurodeles waltl gastrula. We labeled all cells in a gastrula with RLDx cell lineage tracer and superficial cells with 125I and then grafted small pieces of the marginal zone orthotopically into unlabeled host embryos. Labeled progeny were identified in sectioned embryos at the tail bud stage. The use of double-labeled grafts allowed us to study the relative contributions by superficial and deep cells to different derivatives. We found that the presumptive regions are generally distributed according to classical fate maps for urodeles but that the boundaries between presumptive regions are indistinct, due to extensive intermingling between cells at the edges of grafted regions. We have shown that there is a high dorsal to low ventral gradient of mixing between superficial and deep cells.     

The factors that promote the development of symmetry properties in aggregates from dissociated echinoid embryos

Summary Embryos of Hemicentrotus pulcherrimus at the 16 cell, 400 cell or mesenchyme blastula stage of development were dissociated into single cells. The cells were reaggregated, and the development of individual aggregates was monitored. Only aggregates from 16 cell embryos developed into pluteus-like larvae with radial or bilateral symmetry. When embryos at these three developmental stages were incompletely dissociated so that there were mixtures of single cells and groups of undissociated cells, the percentage of aggregates from 16 cell embryos that developed in a pluteus-like manner was greater than in aggregates from completely dissociated 16 cell embryos. Also a small percentage of aggregates from 400 cell embryos now developed into pluteus-like larvae. In both of these experiments small aggregates tend to develop in a more normal manner than larger aggregates.In order to test the role of undissociated cells in promoting pluteus-like development in aggregates from incompletely dissociated blastula stage embryos, pieces of intact animal, lateral, or vegetal blastula wall were grafted to aggregates formed from completely dissociated embryos. While each kind of graft improved the ability of the aggregate to develop in a pluteus-like manner, grafts of vegetal blastula wall were most effective. In an aggregate, a graft differentiates according to its presumptive fate and influences the cells of the aggregate to differentiate in an appropriate manner. The ability of the graft to influence the development of the other cells in the aggregate depends on the developmental stage of the cells that make up the aggregate and the size of the aggregate.     

States of determination of single cells transplanted between 512-cell Xenopus embryos

Single cells from 512-cell Xenopus embryos, totally labeled with intracellular horseradish peroxidase, were transplanted orthotopically (from either dorsal or lateral marginal zone) or heterotopically (between these two marginal zones) to unlabeled host embryos at the 512-cell stage. At tailbud stage 23 the locations, numbers, and histotypes of labeled cells were recorded. The transplanted cell had divided many times, giving rise to labeled progeny that expressed a wide range of cell types and were located in several different organs. Locations and cell types of progeny derived from orthotopic grafts to the dorsal marginal zone were different from those derived from grafts to the lateral marginal zone. Single cells grafted heterotopically to either dorsal or lateral positions expressed fates that were appropriate to their final grafted positions, and not to their original positions. We conclude that individual cells of the marginal zone at the 512-cell stage have multiple presumptive fates and have not been committed to any single fate.     

Evidence for fusion of myoblasts in amphibian embryos. I. Homoplastic transplantations of somitic material labeled with tritiated thymidine

In order to obtain more direct evidence for the occurrence of myoblast fusion in the developing amphibian embryo, the following transplantations were performed in vitro. The nuclei of early embryos. Ambystoma tigrinum and A. maculatum, were labeled with tritiated thymidine. Portions of prospective somite areas from these labeled donors were grafted homoplastically and orthotopically into unlabeled hosts of the same, or nearly the same, stage. The stages employed were: neurula, early tail bud, and late tail bud. Hosts were raised until they had developed into more advanced larval forms, fixed, sectioned, and prepared for radioautographic processing according to the customary procedures. The histological preparations contained varying numbers of multinucleate myotubes of a “composite” nature: that is, individual myotubes contained labeled nuclei of the donor, side by side with unlabeled nuclei of the host. There was no doubt that the mononucleate myoblasts of the grafts had fused with those of the host species to form the mutlinucleate composite myotubes. In addition to the above determinations, the method of thymidine labeling has proven to be a satisfactory method of tracing, in the context of the intact organism, somitic cell derivatives up to the feeding larval stage. Mesenchymal cells from the grafted labeled somitic tissues were consequently found in: dermatomic, sclerotomic and intermyotomic locations; the matrix of the dorsal fin; the limb bud; the abdominal muscles.     

Some histochemical observations on the effects of chorioallantoic grafts on the spleen, bursa, and peripheral blood of chicken embryos

Hatching eggs from inbred lines of chickens (inbreeding coefficient exceeds 95%) which show various degrees of resistance and susceptibility to Rous sarcoma, were used for experimentation. Adult tissues were grafted onto the chorioallantois on the tenth day of incubation and tissues of host and control embryos were harvested on the twentieth day of incubation. Enzymes were localized in tissues by histochemical procedures. Small pieces of tissue (thymus or bursa), when grafted onto the chorioallantois, increased the size of the spleen in host embryos although splenomegaly did not invariably occur. Two types of reactions were observed in the spleen, i.e., enlarged spleens with cysts or enlarged spleens which from a morphological point of view were normal. Grafts of either thymus or bursa decreased the size of the host embryo's bursa or were without effect. When weight of the bursa of host embryos was significantly less than that of control embryos on the twentieth day of incubation, this size relationship persisted in chicks four weeks post hatching. Intensity of dehydrogenase and acid phosphatase reactions in cysts of enlarged spleens and in the multinucleated giant cells investing them suggests that they consist of groups of degenerating cells. Intensity of enzyme reaction indicates that enlarged spleens of host embryos in which cysts were absent were normal. Enzyme reactions in the bursae of experimental embryos were more intense than those identified in the same tissues of control embryos. Catabolic reactions were the predominant type in grafts ten days subsequent to implantation. Grafts increased the number of erythrocytes in the peripheral blood of host embryos.     

Mesoderm induction in the future tail region of Xenopus

Summary We have used interspecific grafts between Xenopus borealis and Xenopus laevis to study the signalling system that produces tail mesoderm. Early gastrula ectoderm grafted into the posterior neural plate region of neurulae responds to a mesodermal inducing signal in this region and forms mainly tail somites; this signal persists until at least the early tail bud stage. Ventral ectoderm grafted into the posterior neural plate loses its competence to respond to this signal after stage 10 1/2. We have established the specification of anterior and posterior neural plate ectoderm. In ectodermal sandwiches or when grafted into unusual positions, anterior regions gave rise to mainly nervous system and posterior regions to large amounts of muscle, together with some nervous system. Thus it was impossible to assess the competence of posterior neural plate ectoderm to form further mesoderm and hence to establish if mesodermal induction continues during neurulation in unmanipulated embryos.     

Differentiation of peptidergic neurones in quail-chick chimaeric embryos

The problem raised in this work was whether peptidergic neurones with vasoactive intestinal peptide (VIP)-and substance P-like immunoreactivity could develop in chimaeric embryos in which quail neural crest cells had been implanted into chick at an early developmental stage. Differentiation of peptide-containing nerve somas was looked for in different situations: i) when the quail neural primordium had been grafted orthotopically and isochronically into the chick host either at the adrenomedullary (level of somites 18-24) or at the vagal (level of somites 1-7) levels of the neural axis; ii) when the quail adrenomedullary neural primordium had been heterotopically implanted at the vagal level of the chick host. In all conditions, VIP- and substance P-like immunoreactivity were observed in a number of quail neurones located either in the peripheral ganglia of the trunk at the level of the graft (in orthotopic grafts of the adrenomedullary neural primordium) or in the enteric ganglia of the chick gut (in the other types of grafts). The developmental stage at which the first neurones become detectable in the host conforms to the genetic characteristics of the effector cells, i.e. they differentiate at the same stage in normal quail neuroblasts and in quail neuroblasts transplanted into the chick host. In contrast, the distribution of the peptidergic neurones in the host depends on the tissue into which the neural crest cells migrate and not on their origin in the neural axis and their fate in normal development.     

The determination of myogenic and cartilage cells in the early chick embryo and the modifying effect of retinoic acid

Summary The time of determination of cartilage and skeletal muscle was studied by making chimeric grafts or explants of small tissue pieces from several stages of early chick or quail embryos. Chondrogenesis was assessed by histology or with antibodies directed against type II collagen or cartilage proteoglycan, while myogenesis was detected immunohistochemically with antibodies directed against 3 different muscle markers, including muscle myosin. Grafts from Hensen's node, primitive streak and segmental plate of donor embryos of Stage 3–5 (Hamburger and Hamilton) were transplanted under the ectoderm in the extraembryonic area of Stage 12 host embryos. In addition, explants and mesodermal cells were cultured on glass in DMEM+F12 medium supplemented with 10% FCS. The results showed that determined myogenic cells could first be detected in Hensen's node and the primitive streak at Stage 3⁺–4 and that they developed from mesodermal cells located between the epiblast and hypoblast. Myogenic cells also appeared in grafted and explanted segmental plate with or without notochord from Stage 5 embryos. On the other hand, cartilage cells only formed in grafted and explanted segmental plate that also contained notochord. RA (1 ng/ml) could induce the formation of cartilage cells in the explanted primitive streak without Hensen's node or notochord taken from Stage 3–5 embryos and could also promote the differentiation of myogenic cells in primitive streak from Stage 3 embryo. Thus RA can substitute for Hensen's node or the notochord in the induction of cartilage cells and has some stimulatory effects on the differentiation of myogenic cells. Additional evidence indicates that the hypoblast might play an inductive role in the formation of the notochord which may subsequently promote the differentiation of cartilage cells. Offprint requests to: M. Solursh     

Mitogenic effect of muscle on the neuroepithelium of the developing spinal cord

A previous study revealed that segments of bowel grafted between the neural tube and somites of a younger chick host embryo would induce a unilateral increase in cellularity of the host's neural tube. The current experiments were done to test the hypotheses that muscle tissue in the wall of the gut is responsible for this growth-promoting effect and that the spinal cord enlargement is the result of a mitogenic action on the neuroepithelium. Fragments of skeletal (E8-15) or cardiac muscle (E4-14) were removed from quail embryos and grafted between the neural tube and somites of chick host embryos (E2). Both skeletal and cardiac muscle grafts mimicked the effect of bowel and induced an increase in cell number as well as a unilateral enlargement of the region of the host's neural tube immediately adjacent to the grafts. The growth-promoting effect of muscle-containing grafts was restricted to the neural tube itself and was not seen in proximate dorsal root or sympathetic ganglia. The action of the grafts of muscle was neither species- nor class-specific, since enlargement of the neural tube was observed following implantation of fetal mouse skeletal muscle into quail hosts. Grafts of skeletal muscle or gut increased the number of cells taking up [3H]thymidine in the host's neuroepithelium as early as 9 h following implantation of a graft. The increase in the number of cells entering the S phase of the cell cycle preceded the increase in cell number. These observations demonstrate that muscle-containing tissues can increase the rate of proliferation of neuroepithelial cells when these tissues are experimentally placed together.     

Split tolerance induced by chick embryo thymic epithelium allografted to embryonic recipients

To test the capacity of the epithelial component of the chick embryo thymus to induce tolerance to major histocompatibility complex (MHC) antigens, pre-colonized thymic rudiments were grafted into chick embryonic recipients. Semi-allogeneic or allogeneic transplantations were done between two lines of chickens histocompatible at the MHC locus. Approximately 10% of these thymic chimeras hatched and were studied 3 mo after hatching. Thymic grafts were not rejected by the allogeneic host. The tolerance of chimeric chickens to thymus donor MHC antigens was tested by using a skin graft rejection test and a graft-vs-host (GvH) assay. Chimeric chickens that received an MHC-incompatible thymic graft during the embryonic life tolerated skin graft with the MHC haplotype of the thymus donor. Nevertheless, the lymphocytes within the thymic graft, the host thymus, and the blood were tolerant to the host MHC antigens but were alloreactive in GvH reaction for the MHC antigens of the thymic graft type. These results suggest that the epithelial component of the thymus when taken before the starting of the colonization by hemopoietic precursors and grafted into an early chick embryonic host can induce a tolerance for the MHC determinants involved in allograft rejection but not in the GvH reaction.     

Induction of Secondary Embryos by Intra- and Interspecific Grafts of Center Cells under the blastopore in Horseshoe Crabs

When the cell mass (center cells) of the early gastrulae in both American and Asian horseshoe crabs was grafted into the embryo of the homologous species, secondary embryos were formed as a result of these grafts. Secondary embryos were also formed in a similar way when the center cells of heterologous embryos were grafted between the American and Asian horseshoe crab embryos. The characteristics of the secondary embryos were similar to the host embryos in both cases, indicating that the center cells played the roles same as those by the amphibian organizer. The homogenate of center cells also induced the formation of secondary embryos. This is the first published study in which secondary embryos of horseshoe crabs have been induced by grafting. The fact may mean that this type of embryonic induction is widespread in the animal kingdom.     

A test of positional properties of avian wing-bud mesoderm

Supernumerary wing structures are readily produced by grafting pieces of wing-bud mesoderm into different locations of host wing buds, but the mechanism underlying their formation remains obscure. The major aim of this study was to examine the ability of posterior quail wing-bud mesoderm, cultured in vitro long enough to lose ZPA (zone of polarizing activity) activity, to stimulate or participate in the formation of supernumerary structures when grafted into anterior slits of host chick wing buds. Small pieces of anterior and posterior quail wing-bud mesoderm (HH stages 21-23) were placed in in vitro culture for up to 3 days. After 2 days, ZPA activity of cultured mesoderm was lost. After the grafting of 2- to 3-day cultured anterior quail wing-bud mesoderm into posterior slits of host chick wing-buds, a consistently high percentage (70%-90%) of grafts result in formation of supernumerary cartilage; in this experiment, however, only a low percentage of grafts resulted in supernumerary cartilage when 2- to 3-day cultured posterior mesoderm was grafted into anterior slits. Taken with controls, these results show that positional differences exist between cultured anterior and posterior wing-bud mesoderm. Serial-section analysis of numerous operated wings has shown several patterns of contribution to supernumerary structures by cells of graft and host. Single supernumerary digits induced by grafts of ZPA mesoderm into anterior slits were normally composed entirely of host cells, but graft cells regularly contributed to skeletal elements of more complex supernumerary structures. Cartilage rods produced by anterior-to-posterior grafts were composed mostly of graft cells, but cartilage nodules and the bases of some rods were often mosaics of chick and quail cells. The results support the proposition that mesodermal cells of the quail wing-bud possess a form of anteroposterior positional memory, but its nature and the means by which the memory of grafted cells interacts with host mesoderm are still not clear.     

Experimental analysis of the extension of the dorsal marginal zone in Pleurodeles waltl gastrulae

The capacity for extension of the dorsal marginal zone (DMZ) in Pleurodeles waltl gastrulae was studied by scanning electron microscopy and grafting experiments. At the onset of gastrulation, the cells of the animal pole (AP) undergo important changes in shape and form a single layer. As gastrulation proceeds, the arrangement of cells also changes in the noninvoluted DMZ: radial intercalation leads to a single layer of cells. Grafting experiments involving either AP or DMZ explants were performed using a cell lineage tracer. When rotated 90 degrees or 180 degrees, grafted DMZ explants were able to involute normally and there was extension according to the animal-vegetal axis of the host. In contrast, neither single nor bilayered explants from AP involutes completely, and neither extends when grafted in place of the DMZ. Furthermore, when inside of the host, these AP grafts curl up and inhibit the closure of the blastopore. Once transplanted to the AP region, the DMZ showed no obvious autonomous extension. DMZs cultured in vitro showed little extension and this only from the late gastrula stage onward. Removal of blastocoel roof blocked involution to a varied extent, depending on the developmental stage of the embryos. From these results, it is argued that differences could well exist in the mechanism of gastrulation between anuran and urodele embryos. That migrating mesodermal cells play a major role in urodele gastrulation is discussed.     

Regulation in the neural plate of Xenopus laevis demonstrated by genetic markers

To follow the subsequent history of grafted tissue in experiments designed to study regulation and commitment in the amphibian neural plate, previous workers have relied on graft scars, vital dyes applied externally to cells, or xenoplastic grafts. Each of these methods has been criticized on the grounds that they do not indicate unambiguously the origins of individual cells within the operated host. To overcome these difficulties, homoplastic, genetically marked embryonic grafts were taken from the prospective spinal neuroectoderm of triploid and tetraploid Xenopus laevis frogs and transplanted to presumptive eye and prosencephalic regions of the neural plate of diploid X. laevis embryos. Orthotopic presumptive eye grafts also were done. Marked cells were scored in section either by nucleolar number or computerized nuclear size analysis. Of 28 heterotopically grafted embryos that survived to stage 41, when the retina has differentiated, prospective spinal cord neuroectoderm in eight animals gave rise to cell types unique to the eye. The remaining 20 survivors appeared to be mosaic. These results substantiate claims of regulation in the neural plate and extend these observations to the level of individual cell types, a level of resolution not previously obtained in other studies.     

The preservation of the ability of cultured quail wing bud mesoderm to elicit a position-related differentiative response

A previous study showed that grafting wedges of fresh anterior quail wing mesoderm into posterior slits of chick wing buds resulted in the formation of rods and nodules of cartilage in a high percentage of cases (B. Carlson, 1983, Dev. Biol. 101, 97-105). The purpose of the present study was to determine if a similar response could be elicited by grafting pieces of mesoderm that had been cultured in vitro. When pieces of 1-day cultured anterior mesoderm from stage 17-24 donors were grafted into standard posterior slits of chick wing buds, the percentages of supernumerary structures differed little from those which formed after the grafting of pieces of fresh mesoderm. In a time series, grafts of stage 22-23 anterior mesoderm which had been cultured for 1-4 days retained the ability to form cartilage after being grafted into posterior locations. A time series showed that the duration of this retention was longer in cultured mesoderm than it was in mesoderm that remains in the donor wing bud.     

Functional integration of grafted neural stem cell-derived dopaminergic neurons monitored by optogenetics in an in vitro Parkinson model

Intrastriatal grafts of stem cell-derived dopamine (DA) neurons induce behavioral recovery in animal models of Parkinson''s disease (PD), but how they functionally integrate in host neural circuitries is poorly understood. Here, Wnt5a-overexpressing neural stem cells derived from embryonic ventral mesencephalon of tyrosine hydroxylase-GFP transgenic mice were expanded as neurospheres and transplanted into organotypic cultures of wild type mouse striatum. Differentiated GFP-labeled DA neurons in the grafts exhibited mature neuronal properties, including spontaneous firing of action potentials, presence of post-synaptic currents, and functional expression of DA D₂ autoreceptors. These properties resembled those recorded from identical cells in acute slices of intrastriatal grafts in the 6-hydroxy-DA-induced mouse PD model and from DA neurons in intact substantia nigra. Optogenetic activation or inhibition of grafted cells and host neurons using channelrhodopsin-2 (ChR2) and halorhodopsin (NpHR), respectively, revealed complex, bi-directional synaptic interactions between grafted cells and host neurons and extensive synaptic connectivity within the graft. Our data demonstrate for the first time using optogenetics that ectopically grafted stem cell-derived DA neurons become functionally integrated in the DA-denervated striatum. Further optogenetic dissection of the synaptic wiring between grafted and host neurons will be crucial to clarify the cellular and synaptic mechanisms underlying behavioral recovery as well as adverse effects following stem cell-based DA cell replacement strategies in PD.     

Phase Relations between Host and Grafted SCN Depend on Graft Location in Rats

Fetal SCN grafts into intact rats were used as a model to study coupling among circadian oscillators. The phase relation in metabolic activity was analyzed between host SCN and fetal SCN grafted into the third (n = 24) or lateral (n = 18) ventricles. Host and third ventricle SCN grafts showed metabolic activity rhythms with peak values at CT 6, as extrapolated from drinking rhythmicity. In lateral ventricle grafted animals only the host SCN showed rhythmicity, although peak values oc curred as indicated by the drinking rhythms at CT 9. The present data suggest asymmetrical communication between host and grafted SCN depending on graft location, which may be related to different types of signal transmission.     

Responses to amputation of denervated ambystoma limbs containing aneurogenic limb grafts

The developing neural tubes and associated neural crest cells were removed from stage 30 Ambystoma maculatum embryos to obtain larvae with aneurogenic forelimbs. Forelimbs were allowed to develop to late 3 digit or early 4 digit stages. Limbs amputated through the mid radius-ulna regenerated typically in the aneurogenic condition. Experiments were designed to test whether grafts of aneurogenic limb tissues would rescue denervated host limb stumps into a regeneration response. In Experiment 1, aneurogenic limbs were removed at the body wall and grafted under the dorsal skin of the distal end of amputated forelimbs of control, normally innervated limbs of locally collected Ambystoma maculatum or axolotl (Ambystoma mexicanum) larvae. In Experiment 1, at the time of grafting or 1, 2, 3, 4, 5, 7, or 8 days after grafting, aneurogenic limbs were amputated level with the original host stump. At 7 and 8 days, this amputation included removing the host blastema adjacent to the graft. The host limb was denervated either one day after grafting or on the day of graft amputation. These chimeric limbs only infrequently exhibited delayed blastema formation. Thus, not only did the graft not rescue the host, denervated limb, but the aneurogenic limb tissues themselves could not mount a regeneration response. In Experiment 2, the grafted aneurogenic limb was amputated through its mid-stylopodium at 3, 4, 5, 7, or 8 days after grafting. By 7 and 8 days after grafting, the host limb stump exhibited blastema formation even with the graft extending out from under the dorsal skin. The host limb was denervated at the time of graft amputation. When graft limbs of Experiment 2 were amputated and host limbs were denervated on days 3, 4, or 5, host regeneration did not progress and graft regeneration did not occur. But, when graft limbs were amputated on days 7 or 8 with concomitant denervation of the host limb, regeneration of the host continued and graft regeneration occurred. Thus, regeneration of the graft was correlated with acquisition of nerve-independence by the host limb blastema. In Experiment 3, aneurogenic limbs were grafted with minimal injury to the dorsal skin of neurogenic hosts. When neurogenic host limbs were denervated and the aneurogenic limbs were amputated through the radius/ulna, regeneration of the aneurogenic limb occurred if the neurogenic limb host was not amputated, but did not occur if the neurogenic limb host was amputated. Results of Experiment 3 indicate that the inhibition of aneurogenic graft limb regeneration on a denervated host limb is correlated with substantial injury to the host limb. In Experiment 4, aneurogenic forelimbs were amputated through the mid-radius ulna and pieces of either peripheral nerve, muscle, blood vessel, or cartilage were grafted into the distal limb stump or under the body skin immediately adjacent to the limb at the body wall. In most cases, peripheral nerve inhibited regeneration, blood vessel tissue sometimes inhibited, but other tissues had no effect on regeneration. Taken together, the results suggest: (1) Aneurogenic limb tissues do not produce the neurotrophic factor and do not need it for regeneration, and (2) there is a regeneration-inhibiting factor produced by the nerve-dependent limb stump/blastema after denervation that prevents regeneration of aneurogenic limbs.     

Phase Relations between Host and Grafted SCN Depend on Graft Location in Rats

Fetal SCN grafts into intact rats were used as a model to study coupling among circadian oscillators. The phase relation in metabolic activity was analyzed between host SCN and fetal SCN grafted into the third ( n = 24) or lateral ( n = 18) ventricles. Host and third ventricle SCN grafts showed metabolic activity rhythms with peak values at CT 6, as extrapolated from drinking rhythmicity. In lateral ventricle grafted animals only the host SCN showed rhythmicity, although peak values oc curred as indicated by the drinking rhythms at CT 9. The present data suggest asymmetrical communication between host and grafted SCN depending on graft location, which may be related to different types of signal transmission.