Lectin binding pattern in the embryonal and early fetal human vertebral column

Topographical variation in the distributions of versican, aggrecan and perlecan in the foetal human spine reflects their diverse functional roles in spinal development

We evaluated the immunohistochemical distribution of three major proteoglycans of cartilage, i.e., aggrecan, versican and perlecan vis-a-vis collagens I and II in the developing human spine of first-trimester foetuses. Aggrecan and perlecan were prominently immunolocalised in the cartilaginous vertebral body rudiments and to a lesser extent within the foetal intervertebral disc. In contrast, versican was only expressed in the developing intervertebral disc interspace. Using domain-specific monoclonal antibodies against the various modules of versican, we discovered the V0 isoform as the predominant form present. Versican immunolocalisations conducted with antibodies directed to epitopes in its N and C termini and GAG-alpha and GAG-beta core protein domains provided evidence that versican in the nucleus pulposus was either synthesised devoid of a G3 domain or this domain was proteolytically removed in situ. The V0 versican isoform was localised with prominent fibrillar components in the annular lamellae of the outer annulus fibrosus. Perlecan was a notable pericellular proteoglycan in the annulus fibrosus and nucleus pulposus but poorly immunolocalised in the marginal tissues of the developing intervertebral disc, apparently delineating the intervertebral disc-vertebral body interface region destined to become the cartilaginous endplate in the mature intervertebral disc. The distribution of collagens I and II in the foetal spine was mutually exclusive with type I present in the outer annulus fibrosus, marginal tissues around the vertebral body rudiment and throughout the developing intervertebral disc, and type II prominent in the vertebral rudiment, absent in the outer annulus fibrosus and diffusely distributed in the inner annulus fibrosus and nucleus pulposus. Collectively, our findings suggest the existence of an intricate and finely balanced interplay between various proteoglycans and collagens and the spinal cell populations which synthesise and assemble these components during spinal development.

Lectin binding studies on C-28/I2 and T/C-28a2 chondrocytes provide a basis for new tissue engineering and drug delivery perspectives in cartilage research

The present study was performed to evaluate the applicability of plant lectins as mediators of bioadhesion in cartilage research using human chondrocyte cell lines C-28/I2 and T/C-28a2. The bioadhesive properties of fluorescein-labelled lectins with different carbohydrate specificities were investigated by flow cytometry. Specificity of the lectin-cell interactions was ascertained by competitive inhibition using complementary carbohydrates. As compared to that of other lectins, the interaction between wheat germ agglutinin (WGA) and chondrocytic cells was characterised by remarkable cytoadhesion, adequate binding strength and a high degree of specificity for N-acetyl-glucosamine as contained in hyaluronan chains. We therefore suggest WGA to be a promising candidate for mediating bioadhesion to low-adhesive scaffolds in cartilage tissue engineering. Moreover, the WGA-association rate of C-28/I2 and T/C-28a2 cells was dependent on temperature indicating cellular uptake of membrane-bound WGA. Intracellular enrichment was confirmed by confocal microscopy. Equilibration of intracellular pH gradients with monensin resulted in the reversal of quenching effects indicating accumulation of WGA within acid compartments of chondrocytic cells. Thus, WGA might be internalised into chondrocytes together with hyaluronan via the CD44 receptor-mediated endocytosis pathway and accumulated within lysosomes. This physiological process could represent a feasible pathway to target WGA-functionalised drug delivery devices into chondrocytes.

Biology of developmental and regenerative skeletogenesis

Embryonic skeletal development involves the recruitment, commitment, differentiation, and maturation of mesenchymal cells into those in the skeletal tissue lineage, specifically cartilage and bone along the intramembranous and endochondral ossification pathways. The exquisite control of skeletal development is regulated at the level of gene transcription, cellular signaling, cell-cell and cell-matrix interactions, as well as systemic modulation. Mediators include transcription factors, growth factors, cytokines, metabolites, hormones, and environmentally derived influences. Understanding the mechanisms underlying developmental skeletogenesis is crucial to harnessing the inherent regenerative potential of skeletal tissues for wound healing and repair, as well as for functional skeletal tissue engineering. In this review, a number of key issues are discussed concerning the current and future challenges of the scientific investigation of developmental skeletogenesis in the embryo, specifically limb cartilage development, and how these challenges relate to regenerative or reparative skeletogenesis in the adult. Specifically, a more complete understanding the biology of skeletogenic progenitor cells and the cellular and molecular mechanisms governing tissue patterning and morphogenesis should greatly facilitate the development of regenerative approaches to cartilage repair.

Glycoconjugate distribution in early human notochord and axial mesenchyme

Glycosylation patterns of cells and tissues give insights into spatially and temporally regulated developmental processes and can be detected histochemically using plant lectins with specific affinities for sugar moieties. The early development of the vertebral column in man is a process which has never been investigated by lectin histochemistry. Therefore, we studied binding of several lectins (AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA) in formaldehyde-fixed sections of the axial mesenchyme of 5 human embryos in Carnegie stages 12-15. During these developmental stages, an unsegmented mesenchyme covers the notochord. Staining patterns did not show striking temporal variations except for SBA which stained the cranial axial mesenchyme only in the early stage 12 embryo and for PNA, of which the staining intensity in the mesenchyme decreased with age. The notochord appeared as a highly glycosylated tissue. Carbohydrates detected may correspond to adhesion molecules or to secreted substances like proteoglycans or proteins which could play an inductive role, for example, for the neural tube. The axial perinotochordal unsegmented mesenchyme showed strong PNA binding. Therefore, its function as a PNA-positive "barrier" tissue is discussed. The endoderm of the primitive gut showed a lectin-binding pattern that was similar to that of the notochord, which may correlate with interactions between these tissues during earlier developmental stages.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Extensive glycosylation changes revealed by lectin histochemistry in morphologically normal prenatal tissues of the mouse mutant undulated (un/un)

Recently we observed that in human embryos and fetuses with a variety of malformations, not only malformed tissues, but also several non-malformed tissues displayed alterations in the glycosylation pattern. It was the aim of this work to investigate this more or less inexplicable phenomenon under experimental conditions. To this end, we studied a well known mouse model, the mouse mutant undulated, which has an exactly defined genetic defect (substitution in the pax-1 gene) leading to a localized malformation in the vertebral column. The glycosylation pattern was studied using lectin histochemistry. Distribution of binding sites for the lectins RCA I, Con A, SNA, SBA, PNA, LTA and WGA was studied during the organogenesis stages (i.e., days 11-18). It was striking that in mutants, changes in the glycosylation pattern were found not only in the malformed organ (i.e., vertebral anlage), but also in other embryonic tissues, which showed normal morphology. This suggests that the altered glycosylation seems to be a part of genetically determined phenomena throughout the entire organism. Our results show that a defect in a gene with a very restricted expression can cause universal changes in the glycosylation pattern during development.

Comparison of lectin binding patterns in malformed and normal human embryos and fetuses

Altered glycosylation in the course of disease detectable by changes in lectin binding patterns has been well established for adult tissues, but only a few authors have described carbohydrate entities during normal human embryonic and fetal development. Whether alterations in carbohydrate patterns occur in human embryonic and fetal tissues, affected by malformations, remains to be investigated. We, therefore, examined human embryos and fetuses at corresponding developmental stages with and without malformations (spina bifida, exencephaly, cleft lip and cleft palate, and dysmelia) with respect to their lectin binding patterns for the lectins RCA I, PNA, WGA, SBA, SNA, Con A, and LTA. Our results demonstrated that during the development of malformations, the affected tissue sites exhibited a different carbohydrate pattern from normally developed specimens. Furthermore, tissues known to be sites of secondary malformation, accompanying the primary defect, although displaying a histologically normal appearance, also showed an altered carbohydrate pattern. This might indicate a possible general alteration in the carbohydrate pattern in the course of development of malformations in man.

In vitro characterization of chondrogenic cells isolated from chick embryonic muscle using peanut agglutinin affinity chromatography

Specific binding to the lectin, peanut agglutinin (PNA), has been reported in embryonic precartilage tissues, including the condensing limb bud blastema and the caudal half of the developing somite. The present study aimed to test the hypothesis that PNA-binding may be a surface characteristic of chondroprogenitor cells residing within noncartilage tissues, such as muscle, which have the potential of being induced to form cartilage, e.g., in the presence of bone matrix-derived factors. Day-14 chick embryonic pectoral muscle, which contained histochemically detectable PNA-binding cells, was dissociated into single cells (TM cells) and fractionated by PNA affinity chromatography into PNA-binding (PNA+) and nonbinding (PNA-) cells by PNA-Sepharose 6 MB affinity chromatography. The differentiation potential of the PNA-affinity fractionated cells in vitro was analyzed as a function of culture plating cell density. Immunohistochemistry of a number of cell-type-specific differentiation markers, including sarcomeric actin, collagen type II, and aggrecan core protein, demonstrated that PNA+ cells, when cultured as a micromass at high density (20 x 10(6) cells/ml), exhibited a chondrocyte-like phenotype, whereas the PNA-cells remained myogenic; however, both PNA+ and PNA- monolayer cultures (4 x 10(4) cells/ml) behaved as myoblastic cells. The expression of collagen type II mRNA was also confirmed by coupled reverse transcription/polymerase chain reaction analysis. These observations suggest that PNA binding, i.e., the presence of specific galactose-containing cell surface moieties, is likely to be one of the characteristics of chondrogenic cells residing in mesenchymally derived embryonic tissues.

Genetic interaction between hoxb-5 and hoxb-6 is revealed by nonallelic noncomplementation

hoxb-5 and hoxb-6 are adjacent genes in the mouse HoxB locus and are members of the homeotic transcription factor complex that governs establishment of the mammalian body plan. To determine the roles of these genes during development, we generated mice with a targeted disruption in each gene. Three phenotypes affecting brachiocervicothoracic structures were found in the mutant mice. First, hoxb-5- homozygotes have a rostral shift of the shoulder girdle, analogous to what is seen in the human Sprengel anomaly. This suggests a role for hoxb-5 in specifying the position of limbs along the anteroposterior axis of the vertebrate body. Second, hoxb-6- homozygotes frequently have a missing first rib and a bifid second rib. The third phenotype, an anteriorizing homeotic transformation of the cervicothoracic vertebrae from C6 through T1, is common to both hoxb-5- and hoxb-6- homozygotes. Quite unexpectedly, hoxb-5, hoxb-6 transheterozygotes (hoxb-5-hoxb-6+/hoxb-5+ hoxb-6-) also show the third phenotype. By this classical genetic complementation test, these two mutations appear as alleles of the same gene. This phenomenon is termed nonallelic noncomplementation and suggests that these two genes function together to specify this region of the mammalian vertebral column.

Lectin-binding patterns in the embryonic human paraxial mesenchyme

The paraxial mesenchyme in seven human embryos aged between Carnegie stages 12 and 17 was studied by lectin histochemistry with the lectins AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA. The paraxial mesenchyme was found to be segmented into sclerotomes by intersegmental vessels and from late stage 12 by intrasclerotomal clefts dividing each sclerotome into a cranial and caudal half. The lectins Con A, GSA II, LFA, LTA, SBA and SNA did not react at all in the paraxial mesenchyme. Staining for AIA, PNA, RCA I and WGA was found in the developing sclerotomes. However, no differences in the staining pattern between the two sclerotomal halves could be seen. It was striking that in contrast to the chick embryo no differences in binding for PNA between the cranial and caudal sclerotomal parts was observed. These findings reveal that PNA-binding sites do not play the same functional role in segmented axonal outgrowth and neural crest immigration into cranial sclerotomal halves in the human embryo, as found in chick embryonic development. Beginning with the stage 16-embryo, the already condensed caudal sclerotomal halves express Con A-, RCA- and PNA-binding sites. The staining for PNA in particular marked the differentiation of chondrogenous structures developing in this half. From the late stage 12 or stage 13, the walls of intersegmental and other vessels showed binding sites for AIA, PNA, RCA I, SNA and WGA.

Distribution of type I collagen, type II collagen and PNA binding glycoconjugates during chondrogenesis of three distinct embryonic cartilages

Previous studies of chondrogenesis have been focused on limb bud cartilage, whereas little is known about chondrogenic processes of other cartilages with different developmental fates. We hypothesize that cartilages with various developmental fates might show identical characteristics of chondrogenesis. The chondrogenic processes in the nasal septum, the mandible, and the limb bud of the mouse were examined by means of PNA-binding glycoconjugate, and types I and II collagen expression. Swiss-Webster mouse embryos of 11 days (E11) to 14 days (E14) gestation were fixed and processed for immuno- and lectin histochemistry. The blastema of mesenchymal cell aggregates stained positively with anti-type I collagen, but very weakly with anti-type II collagen in all three models at E12, whereas PNA bound to the blastema in the limb bud but not in nasal septum or mandible. Types I and II collagens coexisted in cartilages at E13. Type II collagen was predominant in E14; type I collagen was confined to the peripheral region. The synchronized transitional expression of the collagen phenotypes in all three embryonic cartilages may be systemically regulated. The presence or absence of the PNA-binding glycoconjugates may be involved in characterizing the nature of the cartilages.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.