The fine structure of perinotochordal microfibrils in control and enzyme-treated chick embryos

Microfibrils from the arterial subendothelium

Microfibrillar structures of the subendothelium are represented by either type VI collagen or elastin-associated microfibrils which are also referred to as fibrillin-containing microfibrils. These structures are present throughout the subendothelium irrespective of the presence of elastin. The localization, structure, and protein composition of microfibrils are reviewed. The arterial subendothelium is thrombogenic despite its very low content in fibrillar collagens. This thrombogenicity is linked to the microfibrillar structures, essentially to type VI collagen and to thrombospondin-containing microfibrils. Their respective ability to bind the von Willebrand factor and to activate blood platelets is discussed.

Changes in the expression of intermediate filaments and desmoplakins during development of human notochord

Indirect immunofluorescence was used to study the expression of desmosomal and intermediate filament (IF) proteins in the human notochord between the 4th and 12th weeks of embryonic development. Towards the end of this period, the development of the notochord is characterized by its gradual physiological atrophy and disappearance inside the vertebral bodies. In all of our embryos, the notochord cells expressed cytokeratin and vimentin but not desmin, neurofilament protein or glial fibrillary acidic protein. Throughout the stages studied, the expression of cytokeratin was strong. Vimentin expression, on the other hand, changed during the stages studied. In our youngest embryos, vimentin could be detected only in the peripheral cells of the notochord. During development, a distinct increase occurred in vimentin expression, and in the oldest embryos, all notochord cells showed bright vimentin-specific fluorescence. Simultaneously with this modification, a change occurred in the expression of desmosomal proteins: The notochord cells expressed desmoplakins abundantly during early stages, but weakly or not at all during later stages. Correspondingly, electron microscopy of the same stages showed a striking decrease in the number of desmosomes between notochord cells. Our results confirm that, during early development, the notochord displays features specific for epithelial cells. This accords with the view that notochord is of epithelial origin. The modifications observed in the expression of IF and desmosomal proteins were temporally correlated with developmentally regulated atrophy of the notochord. The programmed regression of the notochord cells is thus associated with a switch from a predominantly epithelial phenotype to a more mesenchymal one.

Morphological and immunohistochemical characteristics of axial structures in the transitory human tail

Ultrastructural relationships between the notochord and neighboring spinal cord were examined during the regression of the human tail. Also, the presence of certain extracellular matrix components in the notochord was immuno-histochemically analysed in the 4th to 12th week old embryos. At the early stages, a close apposition of the notochord to the spinal cord exists in the entire tail region. The external surface of both structures is covered with a continuous basal lamina. The narrow tissue interspace contains interdigitating cell processes and both amorphous and fibrillar extracellular matrix material. With advancing embryonic age, separation of the two structures occurs in craniocaudal direction and the widening interspace becomes occupied by mesenchymal cells. During tail regression and spinal cord retraction, the appearance of large intercellular spaces and cell degeneration takes place in both tissues. With age, the extracellular matrix of the notochord, predominantly the perinotochordal sheath, increases in amount and antigenic complexity. While the intensity of laminin, collagen type IV and type III expression rises continuously during the period examined, the expression of fibronectin begins first at later stages, after the separation of the notochord from the spinal cord. The possible developmental significance of the described phenomena in the regression of the posterior end of the human tail remains to be elucidated.

Microdissection by ultrasonication: application to early chick embryos

A technique utilizing microdissection by ultrasonication was applied to scanning electron microscopy of chick embryos during the first three days of incubation. Using a tank cleaner operating at 80 kHz, whole embryos immersed in pure acetone were sonicated until fragmentation became evident. At 12 hr incubation disintegration occurred by one second or less. At 18 hr, three sonic bursts of one second each produced only partial fragmentation. All three germ layers retained their original relationships to each other. During the second day of incubation, large pieces of integument were removed and somites began to microdissect after 10-20 seconds of sonication. Late in the third day of incubation, sonication for 1 min or more was required to produce significant microdissection. Living embryos exposed to 0.1% collagenase for 10 min prior to standard fixation fragmented in a different manner. Lamellipodia and filopodia were most sensitive and were largely destroyed. The three major germ layers (ectoderm, endoderm, mesoderm), however, retained their structural integrity and original relationships to each other. Factors contributing to the results reported here include: 1) extracellular fibrils of varying chemical composition, 2) primitive cell junctions, 3) biomechanical stability in the nonfibrillar portions of the extracellular matrix, and 4) effects of technical procedures performed prior to sonication. Sonicated tissues of early embryos reveal features that are difficult to demonstrate in other ways and may be unrecognized in conventional preparations.

Cytochemical analysis of the notochord in early rhesus monkey embryos

Functional differentiation of the notochord in rhesus monkey embryos at stages 11-12 (25-28 days of gestation) was analyzed by means of ultrastructural cytochemistry. The notochordal cells exhibited well developed Golgi complexes, rough endoplasmic reticulum, mitochondria, and numerous coated vesicles. Large irregular intercellular spaces were common, and some contained fibrils and particulate matter similar to that observed in the perinotochordal space immediately surrounding the notochord. With the glycogen-specific thiocarbohydrazide-silver proteinate technique, solitary particles as well as large aggregates of glycogen were present within the notochordal cells. The center of some aggregates was electron lucent and contained collapsed membranous structures. The results indicate that as early as stage 11 the nonhuman primate notochord exhibits ultrastructural features suggestive of secretory activity and cytological complexity.

Macrophage mobilization and morphology during lens regeneration from the iris epithelium in newts: studies with correlated scanning and transmission electron microscopy

The lens was removed from both eyes of adult newts (Notophthalmus viridescens), and the eyes were fixed in Karnovsky's fixative every 2 days 0-20 days after operation. Anterior half-eyes were prepared by standard procedures for scanning electron microscopy of the surface. Before fixation, the posterior iris surface was cleaned of adhering vitreous mechanically with forceps or by treatment with bovine testicular hyaluronidase or with hyaluronidase and collagenase. Some specimens were cryofractured in buffer or ethanol transverse to the mid-dorsal iris, and the fractured surface viewed with scanning electron microscopy (SEM). Cells with various combinations of ridges, blebs, filopodia, and lamellipodia were observed adhering to the posterior surface of the iris by 6 days after lentectomy. These cells, which exhibited the surface characteristics of macrophages, became more numerous in specimens fixed after longer intervals. Invasion of the iris epithelium was observed in a cryofractured specimen. After observations with SEM, selected specimens were embedded in plastic and sectioned for study with transmission electron microscopy (TEM). The cells on the iris surface had the cytological characteristics of macrophages, and other macrophages were located within the iris epithelium. In specimens fixed 16 or more days after lentectomy, a bulging lens vesicle was regenerating from the dorsal pupillary margin of the iris. Macrophages were absent or few on the surface of this developing lens but remained scattered over the adjoining iris. Roles that might be played by these macrophages during the transdifferentiation of iris epithelium into lens are discussed.

Distribution of hyaluronic acid and chondroitin sulfate proteoglycans in the presumptive aganglionic terminal bowel of ls/ls fetal mice: an ultrastructural analysis

The terminal colon of the ls/ls mouse is aganglionic because an intrinsic defect prevents its colonization by cells migrating from the neural crest. Previous studies showed that laminin, type IV collagen, and glycosaminoglycans accumulate in the region of the presumptive aganglionic ls/ls bowel through which crest-derived cells would be expected to migrate. It was suggested that crest-derived cells might fail to enter the abnormal bowel because they receive inappropriate signals from a defective extracellular matrix. This hypothesis was evaluated by analyzing the ultrastructure of the extracellular matrix in mutant and control gut. Tissue was fixed in the presence of ruthenium red before or after selective enzymatic digestion. Heparan sulfate proteoglycan (diameter approximately equal to 15 nm) and chondroitin sulfate proteoglycan (diameter approximately equal to 20-50 nm) granules were found in both control and presumptive aganglionic gut. The heparan sulfate proteoglycan granules were primarily located within formed basal laminae, while chondroitin sulfate proteoglycan granules decorated plasma membranes and 5 nm hyaluronic acid microfibrils that formed a network in the extracellular matrix. At day E11.5, the mutant gut differed from the control in the following: 1) Hyaluronic acid microfibrils were longer and more numerous. 2) There were larger numbers of chondroitin sulfate proteoglycan granules associated with cell membranes and with hyaluronic acid microfibrils. By day E13 the spaces between mesenchymal cells of the outer wall of the control bowel contained a regular lattice of hyaluronic acid microfibrils studded with chondroitin sulfate proteoglycan granules. Instead of this lattice, tangles of excessively long hyaluronic acid microfibrils, coated more heavily than in the control with chondroitin sulfate proteoglycan granules, were found in the presumptive aganglionic gut. These results confirm that the extracellular matrix is abnormal in the presumptive aganglionic bowel of the ls/ls mouse; moreover, they also indicate that the defect involves not one, but several components of the extracellular matrix, as well as their distribution. The defective extracellular matrix is apparent at a time when crest-derived cells would be expected to be migrating in the terminal bowel and is located in their path. The observations thus support the idea that a localized abnormality of the extracellular matrix interferes with the colonization of the terminal bowel by crest-derived cells in the ls/ls mouse.

Development of the notochord in human embryos: ultrastructural, histochemical, and immunohistochemical studies

In the present study of the notochord, the specimens were 54 externally normal human embryos ranging between Carnegie stages 13 and 23. The following staining procedures were used: periodic acid-Schiff (PAS), modified method of PAS, alcian blue, colloidal iron, and toluidine blue. Routine electron microscopic techniques were used. Immunoreactivity of the notochord to alpha-enolase was also examined. The notochord cells were undifferentiated in stage 13 with few intracellular organelles. The microfibrils and deposition of acid mucopolysaccharides appeared in the notochordal sheath in stage 14. The characteristic relation of mitochondria with rough endoplasmic reticulum was observed. Golgi complexes increased in the perinuclear region in stage 15. The layer of microfibrils in the notochordal sheath initially separated from the notochord in stage 16. Glycogen, mucoprotein, neutral mucopolysaccharides, and glycolipids began to increase in the mesenchymal cells around the notochord, starting at stage 16. Acid mucopolysaccharides increased in the notochordal sheath and in the matrix of the precartilage area around the notochord as this embryonic stage advanced. It was also revealed that the immunoreactivity of the notochord to alpha-enolase remained constant during the embryonic period. The results show that the notochord is transformed from an apparently undifferentiated organ into an organ with secretory activity in stage 14, producing microfibrils and depositing acid mucoplysaccharides in the notochordal sheath. The immunoreactivity of the notochord to alpha- and gamma-enolase isoenzymes and the development of the notochord are discussed. This study was undertaken to provide additional information on the development of tumors of notochordal origin.

Fibrillin, a new 350-kD glycoprotein, is a component of extracellular microfibrils

A new connective tissue protein, which we call fibrillin, has been isolated from the medium of human fibroblast cell cultures. Electrophoresis of the disulfide bond-reduced protein gave a single band with an estimated molecular mass of 350,000 D. This 350-kD protein appeared to possess intrachain disulfide bonds. It could be stained with periodic acid-Schiff reagent, and after metabolic labeling, it contained [3H]glucosamine. It could not be labeled with [35S]sulfate. It was resistant to digestion by bacterial collagenase. Using mAbs specific for fibrillin, we demonstrated its widespread distribution in the connective tissue matrices of skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule. Electron microscopic immunolocalization with colloidal gold conjugates specified its location to a class of extracellular structural elements described as microfibrils. These microfibrils possessed a characteristic appearance and averaged 10 nm in diameter. Microfibrils around the amorphous cores of the elastic fiber system as well as bundles of microfibrils without elastin cores were labeled equally well with antibody. Immunolocalization suggested that fibrillin is arrayed periodically along the individual microfibril and that individual microfibrils may be aligned within bundles. The periodicity of the epitope appeared to match the interstitial collagen band periodicity. In contrast, type VI collagen, which has been proposed as a possible microfibrillar component, was immunolocalized with a specific mAb to small diameter microfilaments that interweave among the large, banded collagen fibers; it was not associated with the system of microfibrils identified by the presence of fibrillin.

The retention and ultrastructural appearances of various extracellular matrix molecules incorporated into three-dimensional hydrated collagen lattices

Artificial extracellular matrices composed of collagen, glycosaminoglycans (GAG), proteoglycans (PG), plasma fibronectin (FN), and a hyaluronate-binding protein (HABP) have been prepared that morphologically resemble embryonic extracellular matrices in vivo at the light and electron microscope level. The effect of each of the above matrix molecules on the structure and "self-assembly" of these artificial matrices was delineated. (1) Matrix components assembled in vitro morphologically resemble their counterparts in vivo, for the most part. Scanning and transmission electron microscopy indicate that under our assembly and fixation conditions, collagen forms striated fibrils that are 125 nm in diameter, FN forms 30- to 60-nm granules, chondroitin sulfate proteoglycan (CSPG) forms 27- to 37-nm granules, chondroitin sulfate (CS) assembles into 100- to 250-nm spheres, and hyaluronate (HA) appears either as granular mats when fixed with cetylpyridinium chloride (CPC) or as 1.5- to 3-nm microfibrils when preserved with ruthenium red plus tannic acid. These molecules are known to assume the same configurations in embryonic matrices when the same preservation techniques are used with the exception of FN, which generally forms fibrillar arrays. (2) Addition of various matrix molecules can radically change the appearance of the collage gels. HA greatly expands the volume of the gel and increases the space between collagen fibrils. CSPG at low concentrations (less than 1 mg/ml) and CS at high concentrations (greater than 20 mg/ml) bundle the collagen fibrils into twisted ropes. (3) A variety of assays were used to examine binding between various matrix components and retention of these components in the hydrated collagen lattices. These assays included solid-phase binding assays, negative staining of spread mixtures of matrix components, cryostat sections of unfixed mixtures of matrix components, and retention of radiolabeled matrix molecules in fixed and washed gels. A number of these binding interactions may play a role in the assembly and stabilization of the matrix. (a) HA, CSPG, and FN bind to collagen. CS appears to only weakly bind to collagen, if at all. (b) FN promotes the increased retention of HA, CSPG, and to a very small degrees, CS, in collagen gels. Conversely, the GAG increase the retention of 3H-FN in the gels. Furthermore, FN binds to HA, CS, and CSPG as demonstrated by solid surface binding assays and morphological criteria. The increased retention of GAG and CSPG by the addition of FN may be due to both stabilization of binding to the collagen and trapping of matrix complexes within the gel. (c) HA binds to both CS and CSPG.(ABSTRACT TRUNCATED AT 400 WORDS)

Notochordal development as influenced by the insecticide dicrotophos (Bidrin)

White Leghorn chicken embryos were treated at different ages with the insecticide dicrotophos to determine the time period of maximum effect upon notochordal development. Doses of insecticide ranging from 250 micrograms to 2.0 mg were injected into eggs at 8, 16, 24, 32, 40, 48, 72, or 96 hr of incubation and the eggs allowed to incubate for an additional 48 hr. Dicrotophos treatment caused dorsoventral and lateral folding of the notochord, with the cervical region being most severely affected. Although there was no apparent difference in dose responsiveness at any one age, there was an obvious age relationship. Notochordal responsiveness, expressed as both the number and severity of folds, was low among the 8- and 16-hr treated embryos, increased to a maximum in the 48-hr treatment group, and then declined among the older embryos. The time of maximum effect correlates closely with the time of sheath deposition and vacuolization of the notochord, but not to initial formation of the notochord from the mesoblast or later extracellular matrix production by sclerotome cells. It is proposed that dicrotophos interferes with some aspect of sheath formation. The pressure exerted by the vacuolization upon a structurally weakened sheath is thought to cause the observed folding.

Morphology and behavior of quail neural crest cells in artificial three-dimensional extracellular matrices

Neural crest cells migrate extensively through a complex extracellular matrix (ECM) to sites of terminal differentiation. To determine what role the various components of the ECM may play in crest morphogenesis, quail (Coturnix coturnix japonica) neural crest cells have been cultured in three-dimensional hydrated collagen lattices containing various combinations of macromolecules known to be present in the crest migratory pathways. Neural crest cells migrate readily in native collagen gels whereas the cells are unable to use denatured collagen as a migratory substratum. The speed of movement decreases linearly as the concentration of collagen in the gel increases. Speed of movement of crest cells is stimulated in gels containing 10% fetal calf serum and chick embryo extract, 33 micrograms/ml fibronectin cell-binding fragments, 3 mg/ml chondroitin sulfate, or 3 mg/ml chondroitin sulfate proteoglycan when compared to rates of movement through collagen lattices alone. Low concentrations of hyaluronate (250-500 micrograms/ml) in a 750 micrograms/ml collagen gel do not alter rates of movement over collagen alone, but higher concentrations (4 mg/ml) greatly inhibit migration. Conversely, hyaluronate (250 micrograms/ml) significantly increases speed of movement if the crest cells are cultured in high concentration collagen gels (2.5 mg/ml), suggesting that hyaluronate is expanding spaces and consequently enhancing migration. The morphology and mode of movement of neural crest cells vary with the matrix in which they are grown and can be correlated with their speed of movement. Light and scanning electron microscopy reveal rounded, blebbing cells in matrices associated with slower translocation, whereas rounded cells with branching filopodia or lamellipodia are associated with rapid translocation. Bipolar cells with long processes are observed in cultures of rapidly moving cells that appear to be adhering strongly, as well as in cultures of cells that are stationary for long periods. These data, considered with the known distribution of macromolecules in the early embryo, suggest the following: (1) Both collagen and fibronectin can act as preferred substrata for migration. (2) Chondroitin sulfate and chondroitin sulfate proteoglycan increase speed of movement, but probably do so by decreasing adhesiveness and thereby producing more frequent detachment. In the embryo, crest cells would most likely avoid regions containing high concentrations of chondroitin sulfate. (3) Hyaluronate cannot act as a substratum for migration, but in low concentrations it can open spaces in the matrix and consequently may stimulate movement. The complex interactions of combined matr

Fibronectin in basement membrane of Hertwig's epithelial sheath. Light and electron immunohistochemical localization

The distribution of fibronectin throughout the basement membrane of Hertwig's epithelial sheath was studied using specific antibodies with the immunoperoxidase technique in both light and electron microscopy. Our results demonstrate that, after collagenase digestion in situ, the basement membrane was strongly labelled by antifibronectin antibodies on the lamina lucida, the lamina densa and the lamina (pars) fibroreticularis which contained aperiodic fibrils of 5-10 nm in diameter.

Ultrastructural changes associated with the mineralization of deer antler cartilage

The maturation and mineralization of deer antler cartilage were investigated ultrastructurally by using enzymatic digestions and subsequent staining with ruthenium red (RR) or phosphotungstic acid (PTA). RR staining of matrix granules was observed in the immature prechondroblastic matrix and became more intense as the cartilage matured into a mineralized tissue. The granules got larger and more numerically dense in the mature matrix. There were matrix granules that coalesced around matrix vesicles or remnants of such in the mineralized zone. These granules were observed after demineralization, and they were RR and acidic PTA-positive (they were not susceptible to hyaluronidase nor trypsin digestion, however). It appears that the granules were modified such that the matrix vesicle formed a centralized nidus for mineralization. The growth of hydroxyapatite crystals along matrix granules (which in this zone may or may not represent proteoglycan monomers) may have caused the coalescence. Microfibrils associated with matrix granules probably represented the hyaluronic acid core of the large proteoglycan complexes because of their susceptibility to hyaluronidase digestion.

Adhesion to extracellular materials by neural crest cells at the stage of initial migration

Trunk-level neural anlagen bearing neural crest cells at the stage of initiation of migration were isolated from chick embryos and explanted in serum-free medium onto glass substrates which had previously been treated with extracellular materials. After 0.5-2 h incubation, the explants were dislodged with a stream of culture medium and the substrate examined for adherent crest cells. Crest cells adhered to collagen gels, and adhered to and spread on adsorbed fibronectin; antiserum to fibronectin prevented adhesion to fibronectin but not to collagen gels. Air-dried collagen gels and collagen solutions were less adhesive, the adhesivity declining with longer drying time and lower collagen concentration. Crest cells adhered poorly to dried gelatin and not at all to adsorbed collagen. Fibronectin increased the adhesion to dried collagen and gelatin. Pretreatment of collagen gels with hyaluronate retarded adhesion. Hyaluronate pretreatment also retarded adhesion to adsorbed fibronectin but only when adsorbed collagen was also present. Pretreatment of collagen gels with the proteoglycan monomer from bovine nasal cartilage had no effect of the adhesion of crest cells, but the proteoglycan almost completely inhibited adhesion to adsorbed fibronectin, but only when absorbed collagen was also present. The results are discussed in terms of the control of migration of neural crest cells by extracellular materials.

The fine localization of acid phosphatase activity in the unvacuolated notochordal cells of the early chick embryo

The electron microscopical localization of acid phosphatase activity was investigated in ultra-thin and semi-thin sections of unvacuolated notochordal cells of chick embryos from stages 9 to 14 (as defined by Hamburger & Hamilton). At stage 9, many notochordal cells show a lightly positive reaction for acid phosphatase activity. Thereafter, the acid phosphatase-positive cells of the notochord increase in number and, at stage 14, the reaction products for the enzymes are distributed throughout almost all the cisternae of the nuclear envelope and a well-differentiated endoplasmic reticulum, the parallel cisternal and reticular parts of the Golgi complex, and various lysosomes in nearly all notochordal cells. In the cisternae of the nuclear envelope and endoplasmic reticulum, the acid phosphatase reaction products are in a fine granular form. In the outermost layer of the cisternal parts of the Golgi complex, faint lead deposits similar to those in the endoplasmic reticulum are found, but in other cisternal and reticular regions which may correspond to the GERL, considerable amounts of reaction products are present. Knob-like projections are also seen protruding from the reticular parts of the Golgi complex. These results suggest that, at least up to stage 14, the notochordal cells are actively synthesizing acid phosphatase which is directly transported from the endoplasmic reticulum to the Golgi complex. The enzyme may be accumulated by the Golgi complex from which primary lysosomes are formed. Furthermore, the pattern of the ultrastructural localization of acid phosphatase activity in embryonic notochordal cells of the chick differs from that of adult cells of other animals.

Microfibril formation in chick notochordal cells

The central parts of the chick notochord at Hamburger and Hamilton's stages 20-22 were investigated by electron microscopy. Electron-dense bodies of various sizes and shapes and bounded by a limiting membrane were found in the central cells the notochord. These dense bodies contained fibrous material or microfibrils which ranged from 120 to 600 A in diameter. The large microfibrils often exhibited a typical repeating period with an interval of about 320 A. These dense bodies were always located near the cell membrane, which is rough or irregular in the central parts of the notochord at these stages. The fibrous core material of the dense body frequently shows striking similarities to amorphous fibrous material in the intercellular space of the central parts of the notochord, where they are situated at a considerable distance from the perinotochordal sheath space. From these results, it seems reasonable to suggest that the central cells as well as the peripheral cells of the notochord are capable of forming microfibrils similar to those observed in the perinotochordal sheath space. Moreover, they may play an important role in the total fibrillogenesis of the notochord.

Ultrastructural and tissue-culture studies on the role of fibronectin, collagen and glycosaminoglycans in the migration of neural crest cells in the fowl embryo

The initial migration of neural crest (NC) cells into cell-free space was studied by transmission electron microscopy at trunk levels of fowl embryos, some of which were fixed in the presence of ruthenium red. Migrating NC cells occurred in zones which contained fewer ruthenium-red stained 15-40nm diameter granules than other regions. The ruthenium-red stained granules were linked by similarly stained thin (greater than 3nm diameter) microfibrils. The granules resemble proteoglycan and the microfibrils may be hyaluronate. NC cells contacted thicker (greater than 10 nm diameter) fibrils and interstitial bodies, which did not require ruthenium red for visualization. Cytoplasmic microfilaments were sometimes aligned at the point of contact with the extracellular fibrils, which may be fibronectin and collagen. Phase-contrast time-lapse videotaping and scanning electron microscopy showed that NC cells of the fowl embryo in vitro migrated earlier and more extensively on glass coated with fibronectin-rich fibrous material and adsorbed fibronectin molecules than on glass coated with collagen type I (fibres and adsorbed molecules). NC cells became completely enmeshed in fibronectin-rich fibres, but generally remained on the surface of collagen-fibre gels. When given a choice, NC cells strongly preferred fibronectin coatings to plain glass, and plain glass to dried collagen gels. NC cells showed a slight preference for plain glass over glass to which collagen was adsorbed. Addition to the culture medium of hyaluronate (initial conc. 20 mg/ml), chondroitin (5 mg/ml) and fully sulphated chondroitin sulphate and dermatan sulphate (up to 10 mg/ml) did not drastically alter NC cell migration on fibronectin-rich fibrous substrates.

The role of epithelial collagen and proteoglycan in the initiation of osteogenesis by avian neural crest cells

Osteogenesis was inhibited when mandibular processes from 3 1/2-day-old embryos were cultured in BUdR, LACA, alpha, alpha'-Dipyridyl, 4-Methylumbelliferone, and 4-Methylumbelliferyl-beta-D-glucoside or beta-D-xyloside. Mandibular processes were then cultured in the test substances for 3 days, enzymatically separated into their epithelial and ectomesenchymal components, combined with mandibular components from untreated embros, and either organ-cultured or grafted to chorioallantoic membranes of host embryos. Osteogenesis was inhibited when treated epithelium, but not when treated ectomesenchyme, was present in the tissue recombinations. Analysis of the known action of these inhibitors indicates that proliferation, hydroxylation of collagen, and synthesis of proteoglycans by epithelial cells are all necessary components of this osteogenic epithelial-ectomesenchymal interaction.

Surface ultrastructure of the isolated avian notochord in vitro: the effect of the perinotochordal sheath

The perinotochordal sheath (PNS) is a "tube" of extracellular matrix (ECM) that surrounds the avian notochord beginning in the second day of development. Somites, like the notochord, derive from chordamesoblast but are encased by a less substantial perisomitic matrix (PSM). Initially both tissue types exhibit epithelioid characteristics. Somitic cells subsequently disperse, however, while notochordal histoarchitecture is maintained until much later. To test the possible shape-preserving role of the PNS, otochords were isolated from chick embryos by homogenization (which retains the sheath) or by trypsinization (which removes the sheath). Somites were similarly isolated. Tissues were cultured 12-72 hours and studied by LM, SEM and TEM. Mechanically isolated notochords are initially rigid with smooth surfaces. During the culture period a few cells grow outward from cut ends of the notochord, but its overall rod shape and intact PNS are maintained. In contrast, uncultured trypsinized notochords are flaccid, denuded cylinders with numerous cytoplasmic blebs. They adhere to the substratum within 12 hours of culture when a few cells break away from the central tissue rod, migrate laterally, and appear mesenchymal. This cellular dispersion is directional (perpendicular to the long notochordal axis) and continuous (up to 72 hours). At this time a flattened ovoid growth area is formed. Cultured somites form flat circular growth areas within 12 hours of culture irrespective of the isolation method. These data suggest that the maintenance of an epithelial configuration by notochords in vivo may be due in part to physical restraints of the PNS. It seems possible that notochordal secretions (manifested by the formation of a PNS) could result in its compartmentation and axial confinement while its unrestrained somitic relatives are free to disperse.

Structure and orientation of extracellular matrix in developing chick optic tectum

The mechanisms underlying directed axonal movement in the developing central nervous system are largely unknown. Histochemical methods for transmission and scanning electron microscopy were used to study the surface of the developing optic tectum in the chick embryo at the time of optic fiber ingrowth. A highly structured extracellular matrix consisting of fibrillar and granular components was seen in normal and in uninnervated specimens that had been fixed in solutions containing the cationic dyes Alcian blue, ruthenium red, or safranin O. The strong affinity of these stains for glycosaminoglycans suggests that the matrix contains such macromolecular aggregates. With routine fixation methods the matrix was not seen, but empty extracellular spaces were apparent. The tectal matrix was particularly prominent ahead of the growing front of optic fibers. Its location was thus appropriate for interacting with pioneering axons that cross the surface of the developing tectum along its anterior-posterior axis. Matrix fibrils were organized in a stacked alignment predominantly parallel to the tectal surface, but otherwise their orientation appeared random. The matrix possibly bears on the guidance of optic fibers. However, its geometry suggests that this may involve a mechanism more specific than mechanical contact guidance.

A histochemical analysis of polyanoinic compounds found in the extracellular matrix encountered by migrating cephalic neural crest cells

Neural crest cells destined to form craniofacial primordia initially are "seeded" into and subsequently migrate through the extracellular matrix (ECM) of a cell free space (CFS) between the surface ectoderm and the underlying mesoderm. Utilizing histochemical procedures for polyanionic compounds, we have demonstrated that both sulfated and nonsulfated glycosaminoglycans (GAG) are present in the CFS of the cephalic region of the chick embryo and that their distribution and structural organization vary with the passage of neural crest or mesodermally derived (MD) mesenchymal cells through it. In stages 7 and 8 embryos a predominance of fine filamentous strands composed primarily on nonsulfated, carboxyl-rich GAG is seen spanning intercellular spaces between adjacent tissues and MD mesenchymal cells. In older embryos (stages 9 and 10) much of the filamentous material is replaced by coarse fibrillar strands or amorphous material which coats the surfaces of MD mesenchymal and neural crest cells as they invade the CFS. Using enzymatic digestions (Streptomyces and testicular hyaluronidase) and the critical electrolyte concentration procedure, data suggest that the fine filamentous matrix onto which the neural crest cells migrate consists mainly of hyaluronate with lesser amounts of chondroitin and some sulfated GAG present. The coarse fibrillar matrix that appears after passage of either neural crest or MD mesenchymal cells through the original CFS contains strongly sulfated polyanionic material, predominantly chondroitin sulfates A, C. Since GAG is located ubiquitously within the ECM of embryos at various stages, the role of GAG, if any, in the transfer of developmental information may be of a general nature (ie. stimulus of motility) rather than of specific morphogenetic cues (for specific differentiation into craniofacial primordia).

Attraction of primordial germ cells by notochord in seven somites chick embryo

Chemical studies in chick embryo have indicated the existence of proteoglycan in notochordal sheath. Primordial germ cells were observed with scanning electron microscope on the notochord dorsal face, surrounded with perichordal material. We postulate the identification of such a material with proteoglycan which could attract primordial germ cells to the notochord.

Studies on intercellular LETS glycoprotein matrices

Intercellular matrices secreted by chick embryo fibroblasts in culture were studied by scanning electron microscopy. Cell-cell contact is a prerequisite for the expression of such matrices. The smallest fiber detected by transmission electron microscopy is 5--10 nm in diameter. These matrix fibers tend to cluster to form bundles. Immunofluorescence and immunoferritin procedures reveal that LETS protein is one of the components of the matrices. The matrices are isolated from other cellular organelles by detergent treatment. More than 90% of the proteins in cell-free matrices are LETS protein, suggesting that the matrices are probably made of only one component--LETS protein. Since the solubilization of matrices requires beta-mercaptoethanol, LETS protein matrices may be the first known polymer system in nature to use disulfide linkage as an intermolecular polymerization vehicle. Collagen does not appear to be involved in such matrices. The LETS protein matrix supports the morphological conversion of rounded cells into spindle-shaped, and also promotes myoblast fusion. It does not, however, exert an effect upon cell growth, the rate of glucose uptake or protease production.

Ultrastructural identification of collagen and glycosaminoglycans in notochordal extracellular matrix in vivo and in vitro

Notochordal extracellular matrix consists of a continuous basal lamina, amorphous materials and microfibrils embedded in the ground substance of low electron density. Together they comprise the notochord sheath and are of considerable interest because of their suspected role in early embryonic tissue interactions. The notochord is particularly well-suited to morphological investigation of extracellular matrix because it is one of the few embryonic epithelia which produces ultrastructurally recognizable stroma in vitro without the advantage of a collagenous substratum. Furthermore, these matrix components produced in vitro are morphologically identical to those observed in vivo. The present study used ruthenium red staining to demonstrate that notochordal microfibrils exhibit collagen-like cross-banding patterns both in vivo and in vitro. Collagenase and testicular hyaluronidase digestion studies designed to localize collagen and glycosaminoglycans show a reduction of microfibrillar diameters by 30-35%. Furthermore, these enzyme treatments frequently result in enhanced striations of microfibrils. When cis-hydroxyproline (a proline analog) or beta-aminoproprionitrile (BAPN, a lathyrogenic compound) is added to the culture medium, a similar reduction in microfibrillar diameters is seen. Moreover, increased ruthenium red-positive surface coats and large collagen fibrils are frequently present in BAPN-treated cultures, implying a stimulatory metabolic effect. We conclude that most, if not all, notochordal extracellular matrix components are composed of both collagen and glycosaminoglycans and suggest that the entire extracellular matrix should be considered a macromolecular composite which acts in concert to induce or stabilize developmental interactions.

High-voltage electron microscopy of extracellular fibrillogenesis

High-voltage electron microscopy was employed to observe developing extracellular connective tissue elements in the cervical perinotochordal and perivertebral regions in the chick embryo from 2 through 15 days' incubation. During days 2 and 3, small (10 nm) and large (18-20 nm) microfibrils surrounded the notochord, becoming evident around fibroblast-like cells in day 4. Amorphous material, globular granules and microfibrillar bundles were present at this time. Microfibrillar length increased as did the total population of microfibrils. At four days microfibrils 3-5 nm in diameter arose in all directions from globular granules. During day 9 and thereafter to day 15, microfibrillar diameters increased. This growth formed unit collagenous fibrils 30 nm in diameter or greater. Axial periodicity became evident at day 14. Small microfibrils appear to be composed largely of glycoproteins and do not contain a significant amount of collagen. The globular granules and associated filaments are probably proteoglycans. The amorphous material is believed to provide molecular collagen to developing fibrils. Large microfibrils and unit collagenous fibrils contain significant amounts of molecular collagen.

Ultrastructural distinction between reticular and collagenous fibers with an ammoniacal silver stain

Reticular and collagenous fibers stain differently when subjected to ammoniacal silver reduction. A variety of tissues were subjected to such a "reticulin" technique and the association of reaction product with intercellular connective tissue elements was studied with the electron microscope. The reaction with reticular fibers was primarily associated with the interfibrillar matrix, and was globular in form having a wide variety of particle sizes. Conversely, in dermal collagen the unit fibrils were stained rather than the interfibrillar matrix. The precipitate was punctate in form and was associated with the cross striations of unit collagen fibrils. Large microfibrils also reacted positively with the stain, imparting a faint periodicity. Basement membranes were stained uniquely. The underlying plasmalemma and the lamina densa were heavily stained with silver while the lamina lucida was relatively unstained. The unit fibrils of the lamina reticularis stained in the same manner as dermal unit collagen while the ground substance remained unstained. This represents a clear distinction between the argentophilic characteristics of collagenous fibers, reticular fibers, and basement membranes.

Extracellular matrix fibrils and cell contacts in the chick embryo. Possible roles in orientation of cell migration and axon extension

The migration of neural crest and sclerotome cells and the extension of ventral root axons in chick embryos at stages 16-20 were studied by light microscopy as well as scanning and transmission electron microscopy at the leg bud level of fixed specimens. Extensive cellular movements take place in association with an extracellular matrix consisting of microfibrils. The neural crest and sclerotome cells migrate into the large matrix-filled extracellular space surrounding the neural tube and notochord, apparently using microfibril microfibril bundles as substratum. The cells exhibit pseudopodia which are closely associated with the matrix fibrils. The fibrils around the notochord show a spatial arrangement indicating that the sclerotome cells are contact-guided to their subsequent positions. Mutual cell contacts, including those established by cell processes, frequently show cytoplasmic electron dense plaques at adjacent membranes. These small "plaque contacts" might be correlated to contact inhibition of locomotion between the cells and participate in the guidance of cells. The growth cones of extending axons exhibit filopodia contacting both surrounding mesenchyme cells and extracellular fibrils. The orientation of the axons might thus be affected by contacts with cell surfaces as well as with extracellular material.

[The early differentiation of the perinotochordal connective tissue. A scanning and transmission electron microscopic study on chick embryos (author's transl)]

The early differentiation of the connective tissue was investigated in the perinotochordal zone of 2-3 day-old chick embryos. After characterizing the different tissue components by transmission electron microscopy, their arrangement and distribution were examined by SEM. The results are discussed with regard to the role of the extracellular material in embryonic tissue interactions.

Tissue culture study of a sacrococcygeal chordoma with further ultrastructural study

This report concerns an electron microscopic study of a sacrococcygeal chordoma and its in vitro cultured cells. In vitro, the cells that proliferated in the early phase were predominantly non-vacuolated stellate cells, which were later transformed into vacuolated cells. This suggests that various cell types seen in vivo represent variants of the same cell type at different stages of differentiation and cellular activity. The in vitro tumor cells also show the origin of their vacuoles from both rough endoplasmic reticulum and Golgi membranes. The finding of amorphous and granular material and collagen fibrils in the extracellular spaces of cultured cells seems to suggest that chordoma cells have certain synthetic and secretory activity.

Characterization of developing antler cartilage matrix, II, An ultrastructural study

Cartilage from the main beams and tines of deer antler was examined with the electron microscope. The material studied included prechondroblastic, chondroblastic and chondrocytic matrices. Exdysial microfibrils (5-10 nm in diameter) were observed in the matrix of the prechondroblastic zone. These microfibrils and associated amorphous material were continuous with electron-dense material that probably represented extracellular units of collagen polymers. Matrix (proteoglycan) granules were first observed in the chondroblastic zone. They stained positively with colloidal iron and therefore probably represented proteinpolysaccharides. The matrix granules of the chondroxytic (unmineralized and mineralized) zone were twice the diameter of those in the chondroblastic zone. Matrix vesicles were present in all three stages of development. They were in contact with cellular extensions and also arose directly from cell membranes in the immature zones. As in somatic mineralizing cartilage, these vesicles served as the foci for early mineralization. The initial mineralization process was associated with the membrane of the vesicles.

The development of proline-containing extracellular connective tissue fibrils by chick notochordal epithelium in vitro

Notochords were isolated from Hamburger-Hamilton stages 13-15 chick embryos by trypsinization and microdissection. These were shown by electron microscopy to be completely devoid of extracellular materials or mesenchymal contaminants. Cultivation of notochordal isolates was carried out on a non-collagenous (Falcon Plastic) substratum for 0 to 48 hours. At 12 hours of in vitro incubation, a discontinuous basal lamina could be demonstrated on the surface of notochordal cells. This was followed by the appearance of microfibrils of various sizes and other components of the extracellular matrix. By 48 hours of in vitro incubation, the same extracellular materials which surround the notochord in vivo (notochord sheath) could be demonstrated in vitro. Autoradiographic studies show that tritiated proline is taken up by notochordal cells and secreted to the extracellular space where label is associated with basal lamina, microfibrils and ground substance. When cis-hydroxyproline, a known collagen-specific inhibitor is added to the system, tritiated proline label is located primarily intracellularly and fewer areas of active fibrillogenesis are noted. This suggests that ultrastructurally recognizable materials produced by notochordal cells in vitro may be at least partially collagenous. Significantly, these materials are produced in vivo at the same time (following stage 10) that notochordal tissues actively induce somite differentiation and cartilage formation. It seems reasonable that a biochemically or ultrastructurally identifiable component of the extracellular matrix may possibly mediate such induction.

Electron microscopic visualization of proteoglycans with Alcian Blue

The intercellular matrices of bovine nasal cartilage, chick embryo perichordal cartilage, and chick embryo mesenchymal cells cultured in vitro have been examined by electron microscopy after staining them with Alcian Blue in salt solutions according to Scott & Dorling (1965). Matrix granules, which are typical components of cartilage at the ultrastructural level, are not visible after Alcian Blue staining and are replaced by alcianophilic rod-like particles, varying in length and width. With tissue cultures, Alcian Blue stains 40-120 A thick filaments which display an orthogonal and longitudinal relationship to collagen fibrils. We assume that cartilage matrix granules represent linear proteoglycans that are coiled as a consequence of the usual glutaraldehyde-osmium fixation. It is thought that Alcian Blue, on the other hand, contributes to the stabilization of the proteoglycans in their original structural arrangement. This stabilizing property presumably also results in the sharp visualization of fine filaments in the tissue culture matrix.

Somite chondrogenesis. A structural analysis

Light and electron microscopy are used in this study to compare chondrogenesis in cultured somites with vertebral chondrogenesis These studies have also characterized some of the effects of inducer tissues (notochord and spinal cord), and different nutrient media, on chondrogenesis in cultured somites Somites from stage 17 (54-60 h) chick embryos were cultured, with or without inducer tissues, and were fed nutrient medium containing either horse serum (HS) and embryo extract (EE), or fetal calf serum (FCS) and F12X Amino acid analyses were also utilized to determine the collagen content of vertebral body cartilage in which the fibrils are homogeneously thin (ca. 150 A) and unbanded. These analyses provide strong evidence that the thin unbanded fibrils in embryonic cartilage matrix are collagen. These thin unbanded collagen fibrils, and prominent 200-800 A protein polysaccharide granules, constitute the structured matrix components of both developing vertebral cartilage and the cartilage formed in cultured somites Similar matrix components accumulate around the inducer tissues notochord and spinal cord. These matrix components are structurally distinct from those in embryonic fibrous tissue The synthesis of matrix by the inducer tissues is associated with the inductive interaction of these tissues with somitic mesenchyme. Due to the deleterious effects of tissue isolation and culture procedures many cells die in somitic mesenchyme during the first 24 h in culture. In spite of this cell death, chondrogenic areas are recognized after 12 h in induced cultures, and through the first 2 days in all cultures there are larger accumulations of structured matrix than are present in equivalently aged somitic mesenchyme in vivo. Surviving chondrogenic areas develop into nodules of hyaline cartilage in all induced cultures, and in most non-induced cultures fed medium containing FCS and F12X There is more cell death, less matrix accumulation, and less cartilage formed in cultures fed medium containing HS and EE. The inducer tissues, as well as nutrient medium containing FCS and F12X, facilitate cell survival, the synthesis and accumulation of cartilage matrix, and the formation of cartilage nodules in cultured somites.