Mechanisms of cell migration in the vertebrate embryo

Requirement of Pax6 for the integration of guidance cues in cell migration

The intricate patterns of cell migration that are found throughout development are generated through a vast array of guidance cues. Responding integratively to distinct, often conflicting, migratory signals is probably crucial for cells to reach their correct destination. Pax6 is a master transcription factor with key roles in neural development that include the control of cell migration. In this study, we have investigated the ability of cells derived from cortical neurospheres from wild-type (WT) and Pax6^-/- mouse embryos to integrate diverging guidance cues. We used two different cues, either separately or in combination: substratum nanogrooves to induce contact guidance, and electric fields (EFs) to induce electrotaxis. In the absence of an EF, both WT and Pax6^-/- cells aligned and migrated parallel to grooves, and on a flat substrate both showed marked electrotaxis towards the cathode. When an EF was applied in a perpendicular orientation to grooves, WT cells responded significantly to both cues, migrating in highly oblique trajectories in the general direction of the cathode. However, Pax6^-/- cells had an impaired response to both cues simultaneously. Our results demonstrate that these neurosphere derived cells have the capacity to integrate diverging guidance cues, which requires Pax6 function.

Requirement of Pax6 for the integration of guidance cues in cell migration

The intricate patterns of cell migration that are found throughout development are generated through a vast array of guidance cues. Responding integratively to distinct, often conflicting, migratory signals is probably crucial for cells to reach their correct destination. Pax6 is a master transcription factor with key roles in neural development that include the control of cell migration. In this study, we have investigated the ability of cells derived from cortical neurospheres from wild-type (WT) and Pax6^-/- mouse embryos to integrate diverging guidance cues. We used two different cues, either separately or in combination: substratum nanogrooves to induce contact guidance, and electric fields (EFs) to induce electrotaxis. In the absence of an EF, both WT and Pax6^-/- cells aligned and migrated parallel to grooves, and on a flat substrate both showed marked electrotaxis towards the cathode. When an EF was applied in a perpendicular orientation to grooves, WT cells responded significantly to both cues, migrating in highly oblique trajectories in the general direction of the cathode. However, Pax6^-/- cells had an impaired response to both cues simultaneously. Our results demonstrate that these neurosphere derived cells have the capacity to integrate diverging guidance cues, which requires Pax6 function.

The emergence of extracellular matrix mechanics and cell traction forces as important regulators of cellular self-organization

Physical cues play a fundamental role in a wide range of biological processes, such as embryogenesis, wound healing, tumour invasion and connective tissue morphogenesis. Although it is well known that during these processes, cells continuously interact with the local extracellular matrix (ECM) through cell traction forces, the role of these mechanical interactions on large scale cellular and matrix organization remains largely unknown. In this study, we use a simple theoretical model to investigate cellular and matrix organization as a result of mechanical feedback signals between cells and the surrounding ECM. The model includes bi-directional coupling through cellular traction forces to deform the ECM and through matrix deformation to trigger cellular migration. In addition, we incorporate the mechanical contribution of matrix fibres and their reorganization by the cells. We show that a group of contractile cells will self-polarize at a large scale, even in homogeneous environments. In addition, our simulations mimic the experimentally observed alignment of cells in the direction of maximum stiffness and the building up of tension as a consequence of cell and fibre reorganization. Moreover, we demonstrate that cellular organization is tightly linked to the mechanical feedback loop between cells and matrix. Cells with a preference for stiff environments have a tendency to form chains, while cells with a tendency for soft environments tend to form clusters. The model presented here illustrates the potential of simple physical cues and their impact on cellular self-organization. It can be used in applications where cell-matrix interactions play a key role, such as in the design of tissue engineering scaffolds and to gain a basic understanding of pattern formation in organogenesis or tissue regeneration.

Arp2/3 is critical for lamellipodia and response to extracellular matrix cues but is dispensable for chemotaxis

Lamellipodia are sheet-like, leading edge protrusions in firmly adherent cells that contain Arp2/3-generated dendritic actin networks. Although lamellipodia are widely believed to be critical for directional cell motility, this notion has not been rigorously tested. Using fibroblasts derived from Ink4a/Arf-deficient mice, we generated a stable line depleted of Arp2/3 complex that lacks lamellipodia. This line shows defective random cell motility and relies on a filopodia-based protrusion system. Utilizing a microfluidic gradient generation system, we tested the role of Arp2/3 complex and lamellipodia in directional cell migration. Surprisingly, Arp2/3-depleted cells respond normally to shallow gradients of PDGF, indicating that lamellipodia are not required for fibroblast chemotaxis. Conversely, these cells cannot respond to a surface-bound gradient of extracellular matrix (haptotaxis). Consistent with this finding, cells depleted of Arp2/3 fail to globally align focal adhesions, suggesting that one principle function of lamellipodia is to organize cell-matrix adhesions in a spatially coherent manner.

Creating growth factor gradients in three dimensional porous matrix by centrifugation and surface immobilization

Polycaprolactone (PCL)/Pluronic F127 cylindrical scaffolds with gradually increasing growth factor concentrations were fabricated by the centrifugation of fibril-like PCLs and the subsequent fibril surface immobilization of growth factors. The cylindrical scaffolds exhibited gradually increasing surface areas along the longitudinal direction [from 3.17 ± 0.05 m(2)/g (top position) to 5.42 ± 0.01 m(2)/g (bottom position)]. The growth factors (BMP-7, TGF-β(2) and VEGF(165)) as model bioactive molecules were immobilized onto the fibril surfaces of the scaffolds via heparin binding to produce scaffolds with gradually increasing concentrations of growth factors from the top position (BMP-7, 60.89 ± 2.51; TGF-β(2), 42.85 ± 2.00; VEGF(165), 42.52 ± 3.22 ng/scaffold section) to the bottom position (BMP-7, 181.07 ± 3.21; TGF-β(2), 142.08 ± 2.91; VEGF(165), 112.00 ± 4.00 ng/scaffold section). The released amount of growth factor (VEGF(165)) from the cylindrical scaffold gradually decreased along the longitudinal direction in a sustained manner for up to 35 days, which can allow for a minutely controlled spatial distribution of growth factors in a 3D environment. The 3D porous scaffold with a concentration gradient of growth factors may become a useful tool for basic studies, including in vitro investigations of 3D chemotaxis/haptotaxis for the control of specific biological process. It may also be used as a tissue engineering scaffolding system for a variety of tissues/organs requiring the spatial regulation of growth factors for effective regeneration.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

Role of adenomatous polyposis coli (APC) and microtubules in directional cell migration and neuronal polarization

In response to extracellular signals during embryonic development, cells undergo directional movements to specific sites and establish proper connections to other cells to form organs and tissues. Cell extension and migration in the direction of extracellular cues is mediated by the actin and microtubule cytoskeletons, and recent results have shed new light on how these pathways are activated by neurotrophins, Wnt or extracellular matrix. These signals lead to modifications of microtubule-associated proteins (MAPs) and point to glycogen synthase kinase (GSK) 3beta as a key regulator of microtubule function during directional migration. This review will summarize these results and then focus on the role of microtubule-binding protein adenomatous polyposis coli (APC) in neuronal polarization and directed migration, and on its regulation by GSK3beta.

The effect of the RACK1 signalling protein on the regulation of cell adhesion and cell contact guidance on nanometric grooves

A wide variety of different cell types have been shown to respond to nanofabricated growth surfaces via the process of contact guidance, however little is known about the intracellular mechanisms that control these events. In the present study we have identified the multi-functional signalling adaptor protein, RACK1, as a novel negative regulator of contact guidance on custom-engineered nanometric grooves. We found that over-expression of RACK1 in human breast cancer cells leads to a pro-adherent morphology characterised by the formation of stress fibres and focal adhesions. Enforced expression of RACK1 also limits the response of cells to contact guidance on nanometric grooves. In contrast, ablation of RACK1 protein with specific anti-sense oligonucleotides led to a dramatic enhancement of bi-directional extension of cells on nanometrically deep grooved surfaces, with a corresponding loss of focal adhesions and stress fibres. RACK1 therefore exerts a tonic inhibitory effect on cell contact guidance, while positively promoting an adhesive phenotype. This is the first example of an intracellular signalling molecule involved in the regulation of cell contact guidance on nanometric growth surfaces.

Periodic direct current does not promote wound closure in an in vitro dynamic model of cell migration

BACKGROUND AND PURPOSE

A prevailing paradigm is that electrical fields can promote cell migration and tissue healing. To further validate this paradigm, we tested the hypothesis that periodic direct current (DC) can enhance wound closure using an in vitro dynamic model of cell migration.

METHODS AND RESULTS

Layers of primary fibroblasts were wounded and treated with DC under various voltages. Repair area, cell velocity, and directionality as well as lamellipodium area were evaluated at different times. Direct current had no beneficial effect on cell migration. Moreover, prolonged stimulation under the highest voltage led to significant reduction in wound closure and cell velocity. The reduction of membrane protusions in stimulated cells may be associated with the deleterious effect of DC.

DISCUSSION AND CONCLUSION

Contrary to the authors' expectations, they found that periodic DC did not promote wound closure, a finding that emphasizes the need to clarify the complex effects of electrical fields on migrating cells.

Effect of leukemia inhibitory factor (LIF) on in vitro produced bovine embryos and their outgrowth colonies

In vitro produced (IVP) bovine embryos were subjected to in vitro culture with or without 1000 U/ml human recombinant leukemia inhibitory factor (LIF) added to the culture medium from Days 5 to 8 post insemination (p.i.). Resulting blastocysts were subsequently plated intact on mouse feeder cells in a medium with or without LIF. Significantly more embryos reached the hatched blastocyst stage, and the number of blastocysts with excellent morphology was significantly higher, when LIF was omitted. At Day 8 p.i., total cell count (TCC) and inner cell mass (ICM) cell count was significantly higher in embryos cultured without LIF. In embryos cultured with LIF, cytoplasmic vesicles and lipid droplets were abundant and a decreased expression of both Oct4 and laminin could be observed. Initial hypoblast formation was revealed in almost 1/3 of the LIF-cultured blastocysts whereas this feature was evident in 2/3 of the blastocysts cultured in the absence of LIF. Overall, almost 60% of the blastocysts cultured without LIF formed outgrowth colonies (OCs) when plated on feeders, whereas this phenomenon was only observed in 30% of the blastocysts cultured in the presence of LIF. A tendency for retaining a tightly packed central growth of putative ICM-derived cells was observed, when attachment to the feeder layer was initiated close to the embryonic pole of the blastocyst. At Day 8 of outgrowth culture, approximately 20% of the colonies contained a central core of putative ICM-derived cells appearing large enough for mechanical isolation and further subculture. Immunohistochemical labeling for Oct4 revealed staining of both trophectodermal and ICM-derived cells. The presence of LIF in the outgrowth culture medium did not have any apparent effect on the plating efficiency or colony type. In conclusion, LIF had an adverse effect on in vitro embryonic development when added to the culture medium in the period from Days 5 to 8 p.i., whereas it had no apparent effect on the OCs subsequently formed from such embryos.

Distribution of N-cadherin in human cerebral cortex during prenatal development

An important subgroup of adhesion molecules is the superfamily of cadherins, which takes part in cell recognition and differentiation during development. To our knowledge only one study describing N-cadherin expression in developing human brain has been performed so far. Our aim is to identify N-cadherin expression to establish a relationship between its expression and function in human cerebral cortex during prenatal development. In the present study, localization and intensity of N-cadherin was investigated in developing cerebral cortex. Fetuses from spontaneous abortions (n=13) were obtained from first, second, and third trimesters. Western blot analysis revealed three bands and the third trimester samples showed the strongest bands for N-cadherin. Cell processes, axon bundles, and some of the developing neurons revealed immunoreactivity for N-cadherin throughout pregnancy. The immunoreactivity increased in the developing neocortex and expanded from the ventricular layer toward the marginal zone as development progressed. Moreover, the immunoreactivity was strong in vascular endothelium during all three trimesters. We conclude that N-cadherin is dynamically related to the organization of cerebral cortex layers during prenatal development. The dynamic expression pattern implicates N-cadherin as a potential regulator of cell migration, axon extension and fasciculation, the establishment of synaptic contacts, and neurovascular angiogenesis in the developing human cerebral cortex.

Role of cell surface metalloprotease MT1-MMP in epithelial cell migration over laminin-5

Laminin-5 (Ln-5) is an extracellular matrix substrate for cell adhesion and migration, which is found in many epithelial basement membranes. Mechanisms eliciting migration on Ln-5 need to be elucidated because of their relevance to tissue remodeling and cancer metastasis. We showed that exogenous addition of activated matrix metalloprotease (MMP) 2 stimulates migration onto Ln-5 in breast epithelial cells via cleavage of the gamma2 subunit. To investigate the biological scope of this proteolytic mechanism, we tested a panel of cells, including colon and breast carcinomas, hepatomas, and immortalized hepatocytes, selected because they migrated or scattered constitutively in the presence of Ln-5. We found that constitutive migration was inhibited by BB94 or TIMPs, known inhibitors of MMPs. Limited profiling by gelatin zymography and Western blotting indicated that the ability to constitutively migrate on Ln-5 correlated with expression of plasma membrane bound MT1-MMP metalloprotease, rather than secretion of MMP2, since MMP2 was not produced by three cell lines (one breast and two colon carcinomas) that constitutively migrated on Ln-5. Moreover, migration on Ln-5 was reduced by MT1-MMP antisense oligonucleotides both in MMP2+ and MMP2- cell lines. MT1-MMP directly cleaved Ln-5, with a pattern similar to that of MMP2. The hemopexin-like domain of MMP2, which interferes with MMP2 activation, reduced Ln-5 migration in MT1-MMP+, MMP2+ cells, but not in MT1-MMP+, MMP2- cells. These results suggest a model whereby expression of MT1-MMP is the primary trigger for migration over Ln-5, whereas MMP2, which is activated by MT1-MMP, may play an ancillary role, perhaps by amplifying the MT1-MMP effects. Codistribution of MT1-MMP with Ln-5 in colon and breast cancer tissue specimens suggested a role for this mechanism in invasion. Thus, Ln-5 cleavage by MMPs may be a widespread mechanism that triggers migration in cells contacting epithelial basement membranes.

Protein adsorption and cell attachment to patterned surfaces

To better understand the events involved in the generation of defined tissue architectures on biomaterials, we have examined the mechanism of attachment of human bone-derived cells (HBDC) to surfaces with patterned surface chemistry in vitro. Photolithography was used to generate alternating domains of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (EDS) and dimethyldichlorosilane (DMS). At 90 min after seeding, HBDC were localized preferentially to the EDS regions of the pattern. Using sera specifically depleted of adhesive glycoproteins, this spatial organization was found to be mediated by adsorption of vitronectin (Vn) from serum onto the EDS domains. In contrast, fibronectin (Fn) was unable to adsorb in the face of competition from other serum components. These results were confirmed by immunostaining, which also revealed that both Vn and Fn were able to adsorb to EDS and DMS regions when coated from pure solution, i.e., in the absence of competition. In this situation, each protein was able to mediate cell adhesion across a range of surface densities. Cell spreading was constrained on the EDS domains, as indicated by cell morphology and the lack of integrin receptor clustering and focal adhesion formation. This spatial constraint may have implications for the subsequent expression of differentiated function.

Effect of potent and selective inhibitors of the Grb2 SH2 domain on cell motility

Cell motility has been correlated both with oncogenic invasiveness and metastatic potential. The development of selective inhibitors of motility has thus great potential importance. Grb2 is a SH2/SH3 domain-containing adaptor protein that links growth factor receptor tyrosine kinases to the Ras signaling pathway. We have developed specific small molecule inhibitors of the Grb2 SH2 domain as potential leads for drug discovery. Synthesis of the inhibitors and their effects on growth factor-induced growth in cells have been reported previously. In the current study, we establish that these inhibitors inhibit hepatocyte growth factor/scatter factor-induced A431 and Madin-Darby canine kidney cell motility and various cell motility-related events, including epidermal growth factor-induced ruffling of A431 cells and epidermal growth factor-induced translocation of the small GTPase Rac in these cells. We demonstrate for the first time a direct role for Grb2 in cell motility and indicate a new avenue for cancer therapeutics.

Effect of thymosin fraction five (TF5) on early neuroembryogenesis in the newt Triturus cristatus

Thymosin fraction five (TF5), a well-characterized immunoregulatory thymic preparation, has been reported to stimulate corticotropin (ACTH) release. The present study explores the morphogenetic role of TF5 on early stages of nerve tissue formation during embryogenesis. In vivo TF5-treatment of newt embryos during neurulation results in stimulation of the process expressed at a macro-microscopical level by emphasizing the embryo's relief and by accelerating the neural tube closure. The stimulating effect of TF5 on neurulation is manifested ultrastructurally by enhanced apical endocytosis, a well-developed contractile microfilamentous layer, increased apical intercellular junctions and changes of the cytoplasmic organelles linked with the protein synthesis. The polymorphism of mitochondria and the morphological expression of enhanced yolk material assimilation confirm the idea of TF5-stimulating effect on the neuroepithelial cells.

Distribution of extracellular matrix components in nuchal skin from fetuses carrying trisomy 18 and trisomy 21

We have investigated histologically the elevations of the skin in dorsal and lateral neck (nuchal) regions of human fetuses carrying karyotypes of trisomy 18 (Edwards' syndrome) and trisomy 21 (Down's syndrome). Cavities filled with interstitial fluid were found in the dermis, epidermal basement membrane and occasionally in the epidermis of trisomy-18 fetuses, but were not delineated by an epithelium or basement membrane as judged by the absence of immunostaining for laminin, collagen IV and collagen VII. Dilated vessels were also found at the interface between dermis and subcutis. Neither normal fetal skin nor that of trisomy-21 fetuses contained cavities or dilated vessels. In order to detect possible alterations of the extracellular matrix in trisomy-18 and trisomy-21 skin, the distribution of glycoproteins, glycosaminoglycans and proteoglycans was studied immunohistochemically. In trisomy-21 and control skin, the dermis stained intensely for fibronectin, whereas the subcutis reacted only weakly. In trisomy-18 skin, the stronger staining for fibronectin appeared in the subcutis, and the prevailing collagen type was collagen III, collagen type I being absent. In the skin of trisomy-21 fetuses, collagen VI was more irregularly arranged and densely packed, whereas collagen I was more widely spaced than in normal fetuses. More hyaluronan was present in the dermis and subcutis of trisomy-21 fetuses than in that of trisomy-18 and control fetuses. A correlation seems to exist between undelimited cavities and collagen III in trisomy-18 skin, and between hyaluronan and the specific arrangement of collagen in trisomy-21 skin.

A large cell-adhesive scatter factor secreted by human gastric carcinoma cells

Human gastric carcinoma cell line STKM-1 secretes a large protein that induces scattering of a rat liver epithelial cell line (BRL) into disconnected individual cells in monolayer culture. This cell-scattering factor was purified from serum-free conditioned medium of STKM-1 cells and found to be composed of three disulfide-linked subunits of 140, 150, and 160 kDa. The 140-kDa peptide contains an amino acid sequence homologous to that of the laminin B2t chain. The native protein has an apparent molecular mass of > 1000 kDa and a pI of 5.0. In addition to the cell-scattering activity, the purified protein stimulates attachment of BRL cells to substrate and their migration. Similar effects have been observed toward various cell lines, including nontumorigenic epithelial, endothelial, and fibroblastic cell lines and human cancer cell lines. Similar cell-scattering activity was secreted by human squamous carcinoma and gastric carcinoma cell lines and nontumorigenic epithelial and endothelial cell lines. These results indicate that the protein, named "ladsin," is probably an extracellular matrix protein that regulates cell-cell and cell-substrate interactions and cell migration.

Gastrulation in the mouse embryo: ultrastructural and molecular aspects of germ layer morphogenesis

Ultrastructural studies and lineage analyses of gastrulating mouse embryos have revealed that different morphogenetic tissue movements are involved in the formation of the three definitive germ layers. Definitive ectoderm is formed by epibolic expansion of the pre-existing progenitor population in the embryonic ectoderm. Formation of the mesoderm and the endoderm is initiated by cellular ingression at the primitive streak. The mesodermal layer is established by cell migration and cell sheet spreading, but the endoderm is formed by replacing the original primitive endodermal population. To this date, genes that are expressed during mouse gastrulation mostly encode cell surface adhesion or signalling molecules, growth factors and their receptors, and putative transcriptional factors. Their precise role during gastrulation remains to be investigated.

Appearance and distribution of entactin in the early chick embryo

Entactin is a sulfated glycoprotein of basement membranes and recent data indicate that it may play a major role in extracellular matrix (ECM) assembly and in modulating the activities of the other molecular components. We investigated the time of appearance and subsequent distribution of entactin during the earliest stages of morphogenesis and its involvement in the first major cellular migrations and interactions in the chick embryo. Entactin is first detected in the epiblast and in the hypoblast at the blastula stage. The accumulating ECM displays intense presence of entactin in the space between the epiblast and the hypoblast at late blastula. Entactin is increasingly abundant in the neural plate and in the ECM and also at least transiently in many mesodermal tissues such as the notochord, the developing heart and somites in the early chick embryo. Immunogold labeling revealed a punctate pattern of entactin distribution in the ECM during the gastrula, neurula and at later stages and at all levels within the embryo. Because of its early appearance in more than one germ layer, entactin may be important in the formation of most embryonic structures. Entactin is detected at the same developmental time and co-localizes with laminin. Antibodies to entactin do not interfere with triggering of the first major cell movements but perturb directional migration of these cells. It would seem that entactin plays a functional role in the directed migration of cells and does not seem to affect cell adhesion during the period of the first morphogenetic events in the early chick embryo.

Osteogenic anaplastic carcinoma of the thyroid

An unusual osteogenic anaplastic carcinoma of the thyroid developed in a 68-year-old man and showed follicular and osteosarcomatous components. Seven months after surgery and 70 mCi 131I treatment, a local tumor recurrence was found and showed an intense uptake of 99mTc-MDP on the bone scan. After a second operation, pathologic and immunostaining analysis revealed no more thyroid carcinoma but only osteosarcomatous cells. Chemotherapy was ineffective and the patient died with diffuse pulmonary metastases 26 months after the diagnosis. The importance of osteogenic sarcomatous differentiation is proven by the bone scan. Osteosarcoma of the thyroid is a rare but well known tumor. Usually these tumors do not contain any cells originating from the thyroid epithelium and only comprise sarcomatous components. Ten cases of undifferentiated carcinoma of thyroid origin with osteogenic component have been reported. These tumors have been recently included in undifferentiated carcinomas in the second edition of the WHO classification. The evolution and pathologic findings favor the hypothesis of a transdifferentiation of the thyroid cell into osteogenic cells.

The human transient subpial granular layer: an optical, immunohistochemical, and ultrastructural analysis

The cytological features, origin, migration, and fate of the subpial granular layer cells of the human embryonic cerebral cortex are studied with light and electron microscopy, Golgi impregnations, and immunocytochemical staining with the microtubule associated protein 2 and glial fibrillary acidic protein antibodies. Subpial granular layer (SGL) cells form a distinct neuronal population in the molecular layer, characterized by a small dark nucleus with abundant chromatin clumps and prominent nucleoli, and a lightly stained cytoplasm containing few organelles. Somata and processes of SGL cells are intensively stained with microtubule-associated protein 2 antibody but do not express glial fibrillary acidic protein antibody. These cells apparently originate from the olfactory germinative zone. They follow two major strands from the olfactory subventricular zone to the subpial region. Subsequently, they migrate tangentially at the subpial level to all cortical regions, as is observed on Golgi and ultrastructural preparations. They constitute a transient population that penetrates the deep molecular layer and subsequently disappear from it. Several cytological features of these cells suggest an inward migration with growth of a radial process toward the cortical plate and subsequent nuclear translocation. The fate and the role of this new phylogenetic neuronal population has yet to be determined although the abundance of degenerating SGL cells in the deep molecular layer suggests at least partial degeneration.

Expression of tenascin mRNA in mesoderm during Xenopus laevis embryogenesis: the potential role of mesoderm patterning in tenascin regionalization

In Xenopus embryos, the extracellular matrix (ECM) protein tenascin (TN) is expressed dorsally in a very restricted pattern. We have studied the spatial and temporal expression of TN mRNA in tailbud-stage embryos by RNAase protection and in situ hybridization using a cDNA probe for Xenopus TN obtained by PCR amplification. We report that TN transcripts are principally expressed in cells dispersed around the neural tube and notochord as well as in myotome and sclerotome cells. No TN mRNA could be detected in lateral plate mesoderm, but expression was detectable beneath tail fin epidermis. In a second series of experiments, we studied the expression of TN mRNA and protein in combinations between animal and vegetal stage-6 blastomeres and in stage-8 blastula animal caps treated with activin A or basic fibroblastic growth factor (b-FGF). Isolated animal cap tissue cultured alone differentiates into epidermis, which expresses neither TN protein nor TN mRNA. TN expression is, however, elicited in response to isolated dorsal vegetal blastomeres and in response to high concentrations of activin, both of which treatments lead to formation of muscle and/or notochord. Low concentrations of activin, and ventral vegetal blastomeres, treatments that induce mesoderm of ventral character, are poor inducers of TN. However, b-FGF, which also induces ventral mesoderm, elicits strong expression. These results indicate that TN regionalization is a complex process, dependent both on the pattern of differentiation of mesodermal tissues and on the agent with which they are induced. The data further show that “ventral mesoderm” induced by low concentrations of activin is distinct from that induced by b-FGF, and imply that activin induces ventral mesoderm of the trunk while b-FGF induces posterior mesoderm of the tailbud.

Critical periods to ethanol exposure during early neuroembryogenesis in the chick embryo: cholinergic neurons

The acute and chronic effects of ethanol on cholinergic neuronal expression were studied in the chick embryo during early neuroembryogenesis using choline acetyltransferase (ChAT) activity as a cholinergic marker. Ethanol administered to embryos in ovo on day 1 (E1) produced a 30% decrease in ChAT activity, while ethanol administration on day 3 elicited no significant change. Similar effects were produced by ethanol on ChAT activity in the spinal cord. The decrease in ChAT activity in both brain and spinal cord was not accompanied by a significant change in protein content. Of significance were our findings with chronic ethanol treatment: in embryos treated from E1 to E5 and sacrificed at E6, ChAT activity was decreased. In contrast, in embryos treated similarly but sacrificed at E8 ChAT activity was increased. These findings establish that the critical period of cholinergic neuronal sensitivity to ethanol is confined to E. Moreover, the increase in ChAT activity observed after chronic ethanol treatment indicates that the developing neurons have the capability to adapt to ethanol. This apparent adaptation results in overcompensation, as reflected by the increase in ChAT activity. Whether this overcompensation is at the expanse of another neuronal population remains to be investigated.

A high-affinity receptor for urokinase plasminogen activator on human keratinocytes: characterization and potential modulation during migration

Low passage cultures of normal human keratinocytes produce several components of the plasminogen activator/plasmin proteolytic cascade, including urokinase plasminogen activator (uPA), tissue plasminogen activator (tPA), and two specific inhibitors. Studies here presented demonstrate that these cells also contain a high-affinity (Kd = 3 x 10(-10) M) plasma membrane-binding site for uPA. High molecular weight uPA, either as the single-chain precursor or two-chain activated form, bound to the receptor; however, low molecular weight (33 kD) uPA, tPA, or epidermal growth factor did not compete for binding, demonstrating specificity. Acid treatment, which removed endogenous uPA from the receptor, was required to detect maximal binding (45,000 sites per cell). To investigate the possibility that the uPA receptor on keratinocytes may be involved in epithelial migration during wound repair, cultures were wounded and allowed to migrate into the wounded site. Binding sites for uPA were localized by autoradiographic analysis of 125I-uPA binding as well as by immunocytochemical studies using anti-uPA IgG. With both techniques uPA binding sites were detected selectively on the plasma membrane of cells at the leading edge of the migrating epithelial sheet. This localization pattern suggests that uPA receptor expression on keratinocytes may be coupled to cell migration during cutaneous wounding.

Distribution and functional role of laminin during induction of the embryonic axis in the chick embryo

Laminin is a major glycoprotein of basement membranes and has been shown to promote cell adhesion, and movement of various nonepithelial cells and tumour cells. Using antibodies to laminin in paraffin sections and cultured embryos, we have studied the distribution of laminin and its involvement in the first morphogenetic events, beginning with the first extensive cellular migrations and interactions that result in the induction of the primitive streak (PS) and of the neural plate in the early chick embryo. Laminin immunogold labeling was not detected in the blastoderm at stage X. At stage XIII, laminin immunoreactivity was detected at the ventral surface of the epiblast and in the entire hypoblast. The intense labeling of the hypoblast indicated that these cells are active in laminin synthesis. Extracellular matrix (ECM) started accumulating as the first embryonic spaces were forming, before the morphogenetic movements of gastrulation were initiated. Immunogold labeling revealed a punctate pattern of laminin distribution in the ECM in the blastocoele, and in the space below the neural plate. Laminin, which is a multidomain molecule known to interact with other molecules of the ECM and with the cell surface, could serve as the scaffold for highly specific contact points of migrating cells and for the folding of epithelial sheets during this time in the developing embryo. We incubated blastoderms at stages X and XIII with laminin antibodies (1:30 dilution) for 4 h, then cultured the blastoderms further in plain egg albumin. The laminin antibodies did not interfere with triggering of PS cell movements, but perturbed the normal migration pattern of these cells. A normal PS did not form and, as a consequence, the embryonic axis was not induced.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous tenascin inhibits mesodermal cell migration during amphibian gastrulation

We have used amphibian gastrulation as a model system to study the action of the extracellular matrix (ECM) glycoprotein tenascin on mesodermal cell migration. Tenascin function was assayed in vitro during spreading of isolated cells from the dorsal marginal zone (DMZ) and during cell migration from DMZ explants. Plastic coated with bovine fibronectin or gastrula ECM was used as a substratum. In both cases, tenascin added to the medium inhibited spreading and migration of mesodermal cells. In addition, a substratum coated with a mixture of fibronectin and tenascin was found to prevent mesodermal cell migration. Tenascin was also microinjected into the blastocoel cavity of living embryos at the late blastula stage. This led to a complete arrest of gastrulation in more than 80% of the cases. Scanning electron microscopy of fractures from arrested gastrulae showed that mesodermal cell migration was blocked. Similar injection experiments carried out at the middle gastrula stage demonstrated that tenascin is able to inhibit cell migration after cells have already contacted the ECM. Mesodermal cell migration in the presence of tenascin could be restored in vitro and in vivo by the monoclonal antibody mAb Tn68 which is known to mask a cell binding site of the molecule. Finally, tenascin microinjected into the blastocoel of blastula or gastrula stage embryos bound within 15 min to the ECM fibrils at all the stages studied. Our results show that exogenous tenascin can be incorporated into embryonic ECM and interferes in vivo with the interactions of cells with a fibronectin-rich matrix.

Rearrangements of desmosomal and cytoskeletal proteins during the transition from epithelial to fibroblastoid organization in cultured rat bladder carcinoma cells

Changes of cell morphology and the state of differentiation are known to play important roles in embryogenesis as well as in carcinogenesis. Examples of particularly profound changes are the conversions of epithelial to mesenchymal cells; i.e., the dissociation of some or all polygonal, polar epithelial cells and their transformation into elongate, fibroblastoid cells of high motility. As an in vitro model system for such changes in cell morphology, we have used cell cultures of the rat bladder carcinoma-derived cell line NBT-II which, on exposure to inducing medium containing a commercial serum substitute (Ultroser G), show an extensive change in their organization (epithelial-mesenchymal transition): the junctions between the epithelial cells are split, the epithelial cell organization is lost, and the resulting individual cells become motile and assume a spindle-like fibroblastoid appearance. Using immunofluorescence microscopy and biochemical protein characterization techniques, we show that this change is accompanied by a redistribution of desmosomal plaque proteins (desmoplakins, desmoglein, plakoglobin) and by a reorganization of the cytokeratin and the actin-fodrin filament systems. Moreover, intermediate-sized filaments of the vimentin type are formed in the fibroblastoid cells. We demonstrate that the modulation of desmosomal proteins, specifically an increase in soluble desmoplakins, is a relatively early event in cell dissociation and in epithelial-mesenchymal transition. In this process, a latent period of 5 h upon addition of inducing medium precedes the removal of these desmosomal components from the plasma membrane. The transition, which is reversible, is dependent on continued protein synthesis and phosphorylation but not on the presence of the inducing medium beyond the initial 2-h period. We discuss the value of this experimental system as a physiologically relevant approach for studying the regulation of the assembly and disassembly of desmosomes and other intercellular adhesion structures, and as a model of the conversion of cells from one state of differentiation into another.

Distribution of fibronectins and laminin in the early pig embryo

Fibronectins (FN) and laminin (LN) distributions were studied in the pig embryo by indirect immunofluorescence using antiporcine FN and antimurine LN antibodies. Extracellular FN are first detected in the early blastocyst before endodermal cell migration. They appear between the cells and on the blastocoelic face of the inner cell mass; thus, they are located at the interface of the trophectoderm and extraembryonic endoderm. Mesodermal cells migrate in a tridimensional network of fibrillar FN. These glycoproteins are also in the extraembryonic membranes (chorion and yolk sac wall) contiguous to the FN-rich basement membranes of embryonic ectoderm and endoderm. Extracellular LN appears in the blastocyst when the endoderm is already established as a continuous cellular monolayer, and is located between the trophectoderm and the extraembryonic endoderm, which produces it. Laminin also accumulates at the basal surface of the embryonic ectoderm at the onset of gastrulation. In the extraembryonic membranes, LN appears at the interface of the endoderm and mesoderm and at the interface of the trophectoderm and mesoderm. It is produced and secreted by extraembryonic mesodermal cells. Analysis of the distribution of these glycoproteins suggests that FN allow the migration of endodermal and mesodermal cells by providing them with a suitable substrate. When these cells become immobilized, they synthesize LN, probably to stabilize their interactions with the underlying extracellular material and epithelia.

Extracellular matrix triggers a directed cell migratory response in sea urchin primary mesenchyme cells

The role of extracellular matrix in cell migration has generally been considered in terms of a substratum. However, when thin cell processes from migrating sea urchin primary mesenchyme cells contact small latex beads coated with extracellular matrix from the blastocoel, the cells migrate directly to the coated beads. Since the beads are not anchored, this result indicates that highly localized contact with the extracellular matrix can stimulate movement independently of any change in cell adhesion.

Fibronectin receptor exhibits high lateral mobility in embryonic locomoting cells but is immobile in focal contacts and fibrillar streaks in stationary cells

The dynamic process of embryonic cell motility was investigated by analyzing the lateral mobility of the fibronectin receptor in various locomotory or stationary avian embryonic cells, using the technique of fluorescence recovery after photobleaching. The lateral mobility of fibronectin receptors, labeled by a monoclonal antibody, was defined by the diffusion coefficient and mobile fraction of these receptors. Even though the lateral diffusion coefficient did not vary appreciably (2 X 10(-10) cm2/S less than or equal to D less than or equal to 4 X 10(-10) cm2/S) with the locomotory state and the cell type, the mobile fraction was highly dependent on the degree of cell motility. In locomoting cells, the population of fibronectin receptors, which was uniformly distributed on the cell surface, displayed a high mobile fraction of 66 +/- 19% at 25 degrees C (82 +/- 14% at 37 degrees C). In contrast, in nonmotile cells, the population of receptors was concentrated in focal contacts and fibrillar streaks associated with microfilament bundles and, in these sites, the mobile fraction was small (16 +/- 8%). When cells were in a stage intermediate between highly motile and stationary, the population of fibronectin receptors was distributed both in focal contacts with a small mobile fraction and in a diffuse pattern with a reduced mobile fraction (33 +/- 9%) relative to the diffuse population in highly locomotory cells. The mobile fraction of the fibronectin receptor was found to be temperature dependent in locomoting but not in stationary cells. The mobile fraction could be modulated by affecting the interaction between the receptor and the substratum. The strength of this interaction could be increased by growing cells on a substratum coated with polyclonal antibodies to the receptor. This caused the mobile fraction to decrease. The interaction could be decreased by using a probe, monoclonal antibodies to the receptor known to perturb the adhesion of certain cell types which caused the mobile fraction to increase. From these results, we conclude that in locomoting embryonic cells, most fibronectin receptors can readily diffuse in the plane of the membrane. This degree of lateral mobility may be correlated to the labile adhesions to the substratum presumably required for high motility. In contrast, fibronectin receptors in stationary cells are immobilized in focal contacts and fibrillar streaks which are in close association with both extracellular and cytoskeletal structures; these stable complexes appear to provide firm anchorage to the substratum.

Aspects of haemopoietic cell dynamics: ontogeny and targeted migration

In the developing avian and mammalian embryo, haemopoietic cells appear first in transient foci whose function is restricted to discrete periods of embryogenesis. These foci are essentially represented by the yolk sac, intraembryonic dispersed foci and the liver. Haemopoietic cells then repopulate the developing spleen, thymus and bone marrow, organs which persist and develop after birth. In the present review, we describe a number of possible mechanisms controlling specific adhesion, oriented migration and invasiveness of haemopoietic cells. One concerns the high specificity of the interactions of homing receptors on the surface of haemopoietic cells with determinants on vascular endothelium and/or thymic epithelium. A second is the importance of the presence of some macromolecules in the extracellular matrix, such as fibronectin, collagen, laminin and elastin. These components can interact with the haemopoietic cells (and/or induce chemotaxis) via the existence of specific receptors on the surface of the haemopoietic cells. Another mechanism is the activation of the haemopoietic cells through the interactions of cell-chemotactic factor, cell-extracellular matrix and/or cell-thymic epithelium. This activation can lead to: 1) the expression of new specific cell-surface receptors for the target foci; 2) the secretion of specific protease and glycosidase systems active upon the extracellular matrix; and 3) the differentiation of these cells in the thymus.

[Chimeras in biologic embryology]

Chimeras produced from amphibian, mammalian, and especially avian embryos have provided important insights into vertebrate development. Important contributions have led to new concepts in understanding the development of, for example, the nervous system, the vascular system, and the skeletal muscles. The migration of cells is particularly accessible in chimeras. More important results are to be expected from chimeras in the future, especially by combining this approach with other state-of-the-art techniques.

Roles of cell junctions in gametogenesis and in early embryonic development

Recent reviews of the role of cell junctions in development have focused primarily upon functions related to the relatively subtle physiological modulation of their subunits in relation to fundamental developmental processes in a wide variety of organisms. There is, however, considerable support from numerous laboratories that the more radical modulation of the presence and number of junctional subunits in many diverse tissues may play a pivotal role in a wide spectrum of developmental phenomena ranging from gametogenesis to organogenesis. Since a great deal of recent interest in this latter subject has concentrated upon vertebrate systems including mammals, this review will examine the functional significance of the modulation of gap junctions, tight junctions and desmosomes in a developing idealized mammalian system from gamete formation to tissue and organ differentiation during embryo-genesis.

Amphibian neural crest cell migration on purified extracellular matrix components: a chondroitin sulfate proteoglycan inhibits locomotion on fibronectin substrates

The ability of purified extracellular matrix components to promote the initial migration of amphibian neural crest (NC) cells was quantitatively investigated in vitro. NC cells migrated avidly on fibronectin (FN), displaying progressively more extensive dispersion at increasing amounts of material incorporated in the substrate. In contrast, dispersion on laminin substrates was optimal at low protein concentrations but strongly reduced at high concentrations. NC cells were unable to migrate on substrates containing a high molecular mass chondroitin sulfate proteoglycan (ChSP). When proteolytic peptides, representing isolated functional domains of the FN molecule, were tested as potential migration substrates, the cell binding region of the molecule (105 kD) was found to be as active as the intact FN. A 31-kD heparin-binding fragment also stimulated NC cell migration, whereas NC cells dispersed to a markedly lower extent on the isolated collagen-binding domain (40 kD), or the latter domain linked to the NH2-terminal part of the FN molecule. Migration on the intact FN was partially inhibited by antibodies directed against the 105- and 31-kD fragments, respectively; dispersion was further decreased when the antibodies were used in combination. Addition of the ChSP to the culture medium dramatically perturbed NC cell migration on substrates of FN, as well as of 105- or 31-kD fragments. However, preincubation of isolated cells or substrates with ChSP followed by washing did not affect NC cell movement. The use of substrates consisting of different relative amounts of ChSP and the 105-kD peptide revealed that ChSP counteracted the motility-promoting activity of the 105-kD FN fragment in a concentration-dependent manner also when bound to the substrate. Our results indicate that NC cell migration on FN involves two separate domains of the molecule, and that ChSP can modulate the migratory behavior of NC cells moving along FN-rich pathways and may therefore influence directionally and subsequent localization of NC cells in the embryo.

Neural crest development in the Xenopus laevis embryo, studied by interspecific transplantation and scanning electron microscopy

The Xenopus borealis quinacrine marker and scanning electron microscopy have been used to study the appearance, migration, and homing of neural crest cells in the embryo of Xenopus. The analysis shows that the primordium of the neural crest develops from the nervous layer of the ectoderm and consists of three segments at early neurula stages. This primordium is located in the lateral halves of the neural folds behind the prospective eye vesicles. The histological and experimental evidence shows that the neural crest cells also originate from the medial portion of the neural folds. The neural crest segments in the cephalic region start to migrate just before the closure of the neural tube. Isotopic and isochronic unilateral grafts of X. borealis neural crest into X. laevis embryos were performed in order to map the fate of the cranial crest segments and the vagal-truncal neural crest. The analysis of the X. laevis host embryos shows that the mandibular crest segment contributes to the lower jaw (Meckel's cartilage), quadrate, and ethmoid-trabecular cartilages, as well as to the ganglionic and Schwann cells of the trigeminus nerve, the connective tissues, the mesenchymal and choroid layers of the eye, and the cornea. The hyoid crest segment is located in the ceratohyal cartilage and in ganglia VII and VIII. The branchial crest segment migrates from the caudal part of the otic vesicle and divides into two portions which contribute to the cartilages of the gills. The vagal-truncal neural crest starts to migrate later at stage 25. It migrates by means of the vagus complex in a ventral direction and penetrates into the splanchnic layer of the digestive tract. The trunk neural crest cells disperse into three different pathways which differ from those of the avian embryo at this level.

The role of extracellular matrix in the migration and differentiation of parietal endoderm from teratocarcinoma embryoid bodies

Embryoid bodies formed from teratocarcinoma stem cells differentiate an outer layer consisting of parietal and visceral endoderm or of visceral endoderm exclusively. We have previously shown that when these embryoid bodies are plated on collagen-coated substrates a parietal endoderm-like cell migrates onto the substrate, whereas all of the visceral endoderm remains associated with the stem cell mass, suggesting a role for substrate contact in parietal endoderm differentiation. We now identify fibronectin as the migration-promoting component in these cultures, and note that laminin and collagen type IV are 10-fold less effective at promoting both attachment and endoderm outgrowth. The RGDS tetrapeptide (arg-gly-asp-ser) from the cell attachment domain of fibronectin can specifically block attachment and outgrowth on both fibronectin- and laminin-coated substrates. In addition, the involvement of the 140-kD fibronectin receptor is demonstrated using an antibody directed against this molecule.

Separation of the myogenic and chondrogenic progenitor cells of undifferentiated limb mesenchyme

Undifferentiated limb bud mesenchyme consists of at least two separate, possibly predetermined, populations of progenitor cells, one derived from somitic mesoderm that gives rise exclusively to skeletal muscle and one derived from somatopleural mesoderm that gives rise to the cartilage and connective tissue of the limb. In the present study, we demonstrate that the inherent migratory capacity of myogenic precursor cells can be used to physically separate the myogenic and chondrogenic progenitor cells of the undifferentiated limb mesenchyme at the earliest stages of limb development. When the undifferentiated mesenchyme of stage 18/19 chick embryo wing buds or from the distal subridge region of stage 22 wing buds is placed intact upon the surface of fibronectin (FN)-coated petri dishes, a large population of cells emigrates out of the explants onto the FN substrates and differentiates into an extensive interlacing network of bipolar spindle-shaped myoblasts and multinucleated myotubes that stain with monoclonal antibody against muscle-specific fast myosin light chain. In contrast, the cells of the explants that remain in place and do not migrate away undergo extensive cartilage differentiation. Significantly, there is no emigration of myogenic cells out of explants of stage 25 distal subridge mesenchyme, which lacks myogenic progenitor cells. Myogenic precursor cells stream out of mesenchyme explants in one or occasionally two discrete locations, suggesting they are spatially segregated in discrete regions of tissue at the time of its explantation. There are subtle overall differences in the morphologies of the myogenic cells that form in stage 18/19 and stage 22 distal subridge mesenchyme explants. Finally, groups of nonmyogenic nonfibroblastic cells which are fusiform-shaped and oriented in distinct parallel arrays characteristically are found along the periphery of stage 18/19 wing mesenchyme explants. Our observations provide support for the concept that undifferentiated limb mesenchyme consists of independent subpopulations of committed precursor cells and provides a system for studying the early determinative and regulatory events involved in myogenesis or chondrogenesis.

Migrating keratinocytes express urokinase-type plasminogen activator

When confluent keratinocyte cultures were wounded by cutting with a blade, the cells rapidly retracted from the wounded site, leaving an area denuded of cells. Within 3-4 h of wounding, keratinocytes began to migrate from the edges and gradually reepithelialized the entire denuded area. Mitomycin C did not prevent the reepithelialization but did dramatically inhibit [3H]thymidine incorporation into the leading edge of cells. These results indicate that cell proliferation was not required for reepithelialization. Using a rabbit antibody against urokinase-type plasminogen activator (u-PA) and an avidin-biotin-peroxidase detection method, we localized u-PA in the keratinocytes at the leading edge of the migrating cultures. Cytochalasin B dramatically inhibited the extent of migration and also altered cell morphology; nonetheless, urokinase was detected in the limited number of cells that moved into the wounded area, even in the presence of cytochalasin B. A small but consistent enhancement (36% +/- 9) of plasminogen activator activity was observed in the supernatant of wounded cultures. These data suggest that plasminogen activator may be involved in the migration of keratinocytes that occurs during wound healing.

Two modes of free migration of amacrine cell neuroblasts in the chick retina

The migration of amacrine neuroblasts toward the prospective amacrine cell layer in the chick embryo retina has been studied, in Golgi-stained sections, between days 5 and 9 of embryogenesis. Two distinct populations of presumptive amacrine neuroblasts have been identified on the basis of their shape and migratory behavior. One population (smooth amacrine neuroblasts) display smooth, monopolar or bipolar contours, moving freely across the retina without major changes in the original postmitotic shape, and give processes only after reaching the primitive inner plexiform layer. The second population (multipodial amacrine neuroblasts) includes multipolar neuroblasts with abundant filiform and/or lamelliform processes sprouting in various directions; these highly plastic cells begin modifying their shapes at the time of release from the ventricular lining and continue to do so as they move toward their definitive location. Thus, the well-known heterogeneity of adult amacrine cells seems to be preluded by differences in neuroblastic migratory patterns, suggesting the existence of at least two different subsets of amacrine cell precursors.

Homing of hemopoietic precursor cells to the embryonic thymus: characterization of an invasive mechanism induced by chemotactic peptides

During embryonic development, T cell precursors migrate to the thymus, where immunocompetency is acquired. Our previous studies have shown that avian hemopoietic precursor cells are recruited to the thymus by chemotactic peptides secreted by thymic epithelial cells (Champion, S., B. A. Imhof, P. Savagner, and J. P. Thiery, 1986, Cell, 44:781-790). In this study, we have characterized the homing of these precursor cells to the thymus in vivo by electron and light microscopy. Hemopoietic precursors could be seen to extravasate from blood or lymphatic vessels, migrate in the mesenchyme, traverse the perithymic basement membrane, and finally intercalate into the thymic epithelium. Labeled hemopoietic precursors injected into the blood circulation also followed the same pathway. Migrating hemopoietic precursor cells were found to express the fibronectin receptor complex. In the presence of thymic chemotactic peptides, hemopoietic precursors traverse a human amniotic basement membrane. This invasive process was inhibited by antibodies to laminin or to fibronectin, two major glycoproteins of the amniotic membrane, by monovalent Fab' fragments of antibodies to the fibronectin receptor, and, finally by synthetic peptides that contain the cell-binding sequence Arg-Gly-Asp-Ser of fibronectin. These results indicate that hemopoietic precursors respond to thymic chemotactic peptides by invasive behavior. Direct interactions between basement membrane components and fibronectin receptors appear to be required for this developmentally regulated invasion process.

Genetic programming of development: a model

Genetic programming of the developmental processes in multicellular organisms is proposed to be so intricate and vitally important that a large set of genes is dedicated solely to this end. It is further proposed that this set can be compartmentalized into subsets on the basis of the changes in gene activities that occur during ontogenesis, and that the genes in each subset transiently control the epigenetic activities of a small group of cells. Automatic subset activation is achieved by the product of a gene in each subset that transfers activity specifically to the subset next in the developmental sequence. This device can generate a unidirectional series of activations that cascade hierarchically through development like toppling dominoes. The model provides a basis for developmental phenomena, such as pattern formation, morphogenesis, and regeneration, and it makes testable predictions at the molecular level.