Location of the integrin complex and extracellular matrix molecules at the chicken myotendinous junction

Summary The distribution of several extracellular matrix macromolecules was investigated at the myotendinous junction of adult chicken gastrocnemius muscle. Localization using monoclonal antibodies specific for 3 basal lamina components (type IV collagen, laminin, and a basement membrane form of heparan sulfate proteoglycan) showed strong fluorescent staining of the myotendinous junction for heparan sulfate proteoglycan and laminin, but not for type IV collagen. In addition, a strong fluorescent stain was observed at the myotendinous junction using a monoclonal antibody against the subunit of the chicken integrin complex (antibody JG 22). Neither fibronectin nor tenascin were concentrated at the myotendinous junction, but instead were present in a fibrillar staining pattern throughout the connective tissue which was closely associated with the myotendinous junction. Tenascin also gave bright fluorescent staining of tendon, but no detectable staining of the perimysium or endomysium. Type I collagen was observed throughout the tendon and in the perimysium, but only faintly in the endomysium. In contrast, type III collagen was present brightly in the endomysium and in the perimysium, but could not be detected in the tendon except when associated with blood vessels and in the epitendineum, which stained intensely. Type VI collagen was found throughout the tendon and in all connective tissue partitions of skeletal muscle. The results indicate that one or more molecules of the integrin family may play an important role in the attachment of muscle to the tendon. This interaction does not appear to involve extensive binding to fibronectin or tenascin, but may involve laminin and heparan sulfate proteoglycan.     

Expression of basement membrane components through morphological changes in the hair growth cycle

The amount and distribution of fibronectin associated with hair follicles was found to vary during the hair growth cycle in the rat. Immunocytochemical staining of follicles in mid-late anagen (the growth stage) revealed the presence of fibronectin in the dermal papilla matrix, in the basement membrane separating this from the epithelial cells of the hair bulb, and in the basement membrane and connective tissue sheath which underly the cells of the outer root sheath. Early in catagen, the transitional stage, staining of the dermal papilla matrix disappeared. Fibronectin persisted in the basement membrane and connective tissue sheath, which undergo corrugation and apparent thickening in catagen. After follicle shortening, the telogen (resting) stage is reached, at which point fibronectin staining was found to be minimal, being restricted to the basement membrane around the secondary germ. The onset of anagen, involving cell division and follicle elongation, was associated with a great increase in the amount of fibronectin in this zone and in and around the dermal papilla. Analysis of entry into anagen by [3H]thymidine incorporation and autoradiography revealed that growth could be detected before the increase in fibronectin expression. However, growing cells, even in a suprabasal position, always had some fibronectin at their surface. Immunoelectron microscopy of early anagen follicles confirmed the light microscopic findings and also showed that fibronectin was present in small vesicles close to the surface of dermal papilla and some epithelial cells. Increased deposition of laminin and type IV collagen in early anagen follicles was also noted, emphasizing the importance of basement membrane components during morphogenetic events in vivo.     

The localization of laminin mRNA and protein in the postimplantation embryo and placenta of the mouse: an in situ hybridization and immunocytochemical study

In situ hybridization (ISH) and immunocytochemistry were used to localize sites of synthesis and deposition of the basement membrane glycoprotein laminin during development in the postimplantation mouse embryo and extraembryonic membranes. In addition, similar studies were performed on postnatal viscera during the first 20 days after birth. Up to 10 days post coitum, embryonic laminin synthesis was confined to parietal endoderm. In maternal tissue, intense laminin mRNA expression was detected in decidual cells in the mesometrial and antimesometrial endometrium at 5-7 days. At 10 days, uniform expression was still seen within the mesometrial endometrium, with higher levels around migrating trophoblast, but in the antimesometrial aspect expression was restricted to the basal zone. High levels of mRNA expression persisted in parietal endoderm throughout gestation but much lower levels were detected in visceral yolk sac. In the mature placenta, laminin mRNA expression was also found associated with fetal vessels in the labyrinth and giant cells at the fetal/maternal boundary. In the embryo, the external limiting membrane of the cerebral vesicles and spinal cord stained for laminin protein and detectable mRNA was found in the pia mater. Growing peripheral nerves and dorsal and ventral root fibres expressed laminin mRNA and stained for laminin protein. Laminin mRNA expression was found in ureteric buds and nephrogenic vesicles (but not in metanephric blastema) during early prenatal kidney development, and in glomeruli, Bowman's capsule, loops of Henle and collecting duct cells at later stages of development, and after birth. All these structures possessed laminin-rich basement membrane (BM). Laminin mRNA expression fell to below detectable levels in the kidney around weaning. In the gut, laminin expression and protein staining was confined to the muscularis externa and the lamina propria during embryogenesis. After birth, the muscularis externa, muscularis mucosa and lamina propria cells corresponding to fibroblasts had detectable laminin mRNA, but in adult gut no laminin mRNA could be demonstrated in any cell type. In liver, low levels of laminin mRNA were seen in the capsule and in periportal connective tissue. After birth, laminin mRNA was associated with intrahepatic bile channels; no laminin mRNA was detected in the parenchyma and protein deposition was restricted to blood sinus BM. In the adult liver, no laminin mRNA was detected in any cell type. The developing heart showed uniform expression of laminin mRNA from 12 days to before birth. Postnatally, labelling was restricted to connective tissue cells.     

Immunohistochemical distribution of laminin-5 gamma2 chain and its developmental change in human embryonic and foetal tissues

Immunohistochemical distribution of laminin gamma2 chain, a subunit of the basement membrane protein laminin-5, was examined in 19 cases of human embryos and foetuses ranging from 4 to 25 weeks of gestation. Laminin gamma2 was first detected in the basement membranes underlying ectodermal epithelial tissues, such as the skin and tooth, as early as 5-6 weeks of gestation. Between 6-7 and 12-13 weeks, laminin gamma2 was detected in the basement membranes of various endodermal epithelial tissues, such as the bronchus, oesophagus, stomach, intestines, urinary bladder, gallbladder and hepatopancreatic duct. The deposition of laminin gamma2 in basement membrane was associated with the process of morphogenesis. In the small intestine, laminin gamma2 first appeared in the basement membrane of the primitive short villi, and its level gradually increased in the villus region but decreased in the cryptic region during the maturation of the organ. In addition, non-basement membrane immunoreactivity for laminin gamma2 was detected in some mesoderm-derived tissues, such as the cartilage and skeletal and smooth muscle fibres. These results suggest a common role of laminin-5 and some specific roles of its gamma2 chain in the morphogenesis of human tissues.     

Abnormal deposition of basement membrane and connective tissue components in dimethylbenzanthracene-induced rat mammary tumours: an immunocytochemical and ultrastructural study

Summary Dimethylbenzanthracene-induced rat mammary tumours consist of lobules of tumours cells surrounded by connective tissue. The interstitial connective tissue proteins, collagen types I, III and V, fibronectin and elastin are largely restricted to the interlobular connective tissue. The tumour lobules are surrounded by a basement membrane that stains with antiserum to laminin. Electron microscopy reveals a greatly thickened basement membrane to which striated interstitial collagen fibres are closely juxtaposed. The lumina within the tumour lobules are of two types. In the first type, the luminal surface is characterized by the presence of microvilli and tight junctions are reacts with antiserum to rat milk fat globule membrane. In the second type, the luminal surface is flattened and lined by a thickened basement membrane that stains with antiserum to laminin and type IV collagen. These abnormal patterns of growth and differentiation may be partly a consequence of the disorganization of extracellular matrix components at the interface between the tumour epithelial cells and the surrounding stroma.     

Immunohistochemical Distribution of Laminin-5 γ 2 Chain and its Developmental Change in Human Embryonic and Foetal Tissues

Immunohistochemical distribution of laminin γ 2 chain, a subunit of the basement membrane protein laminin-5, was examined in 19 cases of human embryos and foetuses ranging from 4 to 25 weeks of gestation. Laminin γ 2 was first detected in the basement membranes underlying ectodermal epithelial tissues, such as the skin and tooth, as early as 5–6 weeks of gestation. Between 6–7 and 12–13 weeks, laminin γ 2 was detected in the basement membranes of various endodermal epithelial tissues, such as the bronchus, oesophagus, stomach, intestines, urinary bladder, gallbladder and hepatopancreatic duct. The deposition of laminin γ 2 in basement membrane was associated with the process of morphogenesis. In the small intestine, laminin γ 2 first appeared in the basement membrane of the primitive short villi, and its level gradually increased in the villus region but decreased in the cryptic region during the maturation of the organ. In addition, non-basement membrane immunoreactivity for laminin γ 2 was detected in some mesoderm-derived tissues, such as the cartilage and skeletal and smooth muscle fibres. These results suggest a common role of laminin-5 and some specific roles of its γ 2 chain in the morphogenesis of human tissues.     

Changes in the distribution of type IV collagen,laminin, proteoglycan,and fibronectin during mouse tooth development

The distribution of certain basement membrane (BM) components including type IV collagen, laminin, BM proteoglycan, and fibronectin was studied in developing mouse molar teeth, using antibodies or antisera specific for these substances in indirect immunofluorescence. At the onset of cuspal morphogenesis, type IV collagen, laminin, and BM proteoglycan were found to be present throughout the basement membranes of the tooth. Fibronectin was abundant under the inner enamel epithelium at the region of differentiating odontoblasts and also in the mesenchymal tissues. After the first layer of predentin had been secreted by the odontoblasts at the epithelial-mesenchymal interface, laminin remained in close association with the epithelial cells whereas type IV collagen, BM proteoglycan, and fibronectin were distributed uniformly throughout this area. Later when dentin had been produced and the epithelial cells had differentiated into ameloblasts, basement membrane components disappeared from the cuspal area. These matrix components were not detected in dentin while BM proteoglycan and fibronectin were present in predentin. The observed changes in the collagenous and noncollagenous glycoproteins and the proteoglycan appear to be closely associated with cell differentiation and matrix secretion in the developing tooth.     

The ultrastructural localization of two basement membrane components: entactin and laminin in rat tissues

The localization of two noncollagenous components of basement membranes, laminin and entactin, was determined in rat kidney, muscle, and small intestine using electron immunohistochemistry. In the renal glomerulus anti-laminin antibodies reacted with the basement membrane of peripheral capillary loops and with mesangial matrix. In the peripheral capillary loop laminin was preferentially distributed in both laminae rarae. This was in contrast to anti-entactin that localized in peripheral capillary loops but not in mesangial matrix. Even in the peripheral capillary loops it had a different distribution than laminin. Entactin was found predominantly in the lamina rara interna. In renal tubular basement membranes both antibodies localized throughout the full thickness of the basement membranes, with laminin having a preferential distribution in the lamina rara, whereas entactin was more evenly distributed. In the basement membrane of the duodenal mucosa entactin localized in the lamina densa, whereas laminin was present in both laminae. In skeletal muscle both antibodies had similar localization in all basement membranes. These results demonstrate that entactin is an intrinsic component of basement membranes. They also demonstrate that basement membranes from different tissues have subtle variations in content and/or assembly of the different components. It is likely that these variations may be reflected in different functional properties.     

Developmental expression and cellular origin of the laminin alpha2, alpha4, and alpha5 chains in the intestine.

Laminins are extracellular matrix glycoproteins that are involved in various cellular functions, including adhesion, proliferation, and differentiation. In this study, we examine the expression patterns and the cellular origins of the laminin alpha2, alpha4, and alpha5 chains in the developing mouse intestine and in in vitro mouse/chick or chick/mouse interspecies hybrid intestines. In situ hybridization and Northern blot analysis revealed that mRNA levels for all three laminin alpha chains are highest in the fetal intestine undergoing intense morphogenetic movements. Laminin alpha4 mRNA and polypeptide are associated with mesenchyme-derived cell populations such as endothelium and smooth muscle. In contrast, laminin alpha2 and alpha5 chains participate in the structural organization of the subepithelial basement membrane and, in the mature intestine, show a complementary pattern of expression. All three laminin alpha chains occur in the smooth muscle basement membrane, with a differential expression of laminin alpha5 chain in the circular and longitudinal smooth muscle layers. The cellular origin of laminin alpha2 and alpha5 chains found in the subepithelial cell basement membrane was studied by immunocytochemical analysis of mouse/chick or chick/mouse interspecies hybrid intestines at various stages of development using mouse-specific antibodies. Laminin alpha2 was found to be deposited into the basement membrane exclusively by mesenchymal cells, while the laminin alpha5 chain was deposited by both epithelial and mesenchymal cells in an apparently developmentally regulated pattern. We conclude that (1) multiple laminin alpha chains are expressed in the intestine, implying specific roles for individual laminin isoforms during intestinal development, and (2) reciprocal epithelial/mesenchymal interactions are essential for the formation of a structured subepithelial basement membrane.     

Role of laminin and fibronectin in selecting myogenic versus fibrogenic cells from skeletal muscle cells in vitro

Growth of embryonic skeletal muscle occurs by fusion of multinucleated myotubes with differentiated, fusion-capable myoblasts. Selective recognition seems to prevent fusion of myotubes with nonmyogenic cells such as muscle fibroblasts, endothelial cells, or nerve cells, but the nature of the signal is as yet unknown. Here we provide evidence that one of the selection mechanisms may be the enhanced affinity for laminin of myogenic cells as compared to fibrogenic cells. Growing myotubes in myoblast cultures accumulate laminin and type IV collagen on their surface in patches and strands as the first step in assembling a continuous basal lamina on mature myofibers (U. Kühl, R. Timpl, and K. von der Mark (1982), Dev. Biol. 93, 344-359). Fibronectin, on the other hand, assembles into an intercellular fibrous meshwork not associated with the free myotube surface. Over a brief time period (10-20 min) myoblasts from embryonic mouse thigh muscle adhere faster to laminin than do fibroblasts from the same tissue; these adhere faster to fibronectin. When a mixture of the cells is plated for 20 min on laminin/type IV collagen substrates, only myogenic cells adhere, giving rise to cultures with more than 90% fusion after 2 weeks; on fibronectin/type I collagen in the same time primarily fibroblastic cells adhere, giving rise to cultures with less than 10% nuclei in myotubes. The differential affinities of myoblasts for basement membrane constituents and of fibroblasts for interstitial connective tissue components may play a role in sorting out myoblasts from fibroblasts in skeletal muscle development.     

Distribution of myoepithelial cells and basement membrane proteins in the resting, pregnant, lactating, and involuting rat mammary gland

Using antisera to specific proteins, the localization of the rat mammary parenchymal cells (both epithelial and myoepithelial), the basement membrane, and connective tissue components has been studied during the four physiological stages of the adult rat mammary gland, viz. resting, pregnant, lactating, and involuting glands. Antisera to myosin and prekeratin were used to localize myoepithelial cells, antisera to rat milk fat globule membrane for epithelial cells, antisera to laminin and type IV collagen to delineate the basement membrane and antisera to type I collagen and fibronectin as markers for connective tissue. In the resting, virgin mammary gland, myoepithelial cells appear to form a continuous layer around the epithelial cells and are in turn surrounded by a continuous basement membrane. Antiserum to fibronectin does not delineate the basement membrane in the resting gland. The ductal system is surrounded by connective tissue. Only the basal or myoepithelial cells in the terminal end buds of neonatal animals demonstrate cytoplasmic staining for basement membrane proteins, indicating active synthesis of these proteins during this period. In the secretory alveoli of the lactating rat, the myoepithelial cells no longer appear to form a continuous layer beneath the epithelial cells and in many areas the epithelial cells appear to be in contact with the basement membrane. The basement membrane in the lactating gland is still continuous around the ducts and alveoli. In the lactating gland, fibronectin appears to be located in the basement membrane region in addition to being a component of the stroma. During involution, the alveoli collapse, and appear to be in a state of dissolution. The basement membrane is thicker and is occasionally incomplete, as also are the basket-like myoepithelial structures. Basement membrane components can also be demonstrated throughout the collapsed alveoli.     

Appearance and distribution of collagens and laminin in the early mouse embryo

Appearance and distribution of the different collagen types and the noncollagenous glycoprotein laminin was studied during early mouse development from unfertilized ova to 8-day embryos using indirect immunofluorescence techniques. Laminin was first detected intracellularly in the 16-cell compacted morula and appeared also intercellularly along cell contours. Type IV collagen was first seen in the blastocyst mainly in the inner cell mass. After implantation intense fluorescence for both of these proteins was found in all the embryonic and extraembryonic basement membranes. The interstitial collagens type I and III were first detected in the 8-day embryo closely codistributed in tissues of mesodermal origin including the head and heart mesenchymes and in basement membranes bounded by mesodermal structures. The results establish a developmental sequence for the appearance of basement membrane and extracellular matrix glycoproteins in early mouse development. The distribution of laminin suggests the presence of extracellular matrix material already in compacted morulae. The appearance of type IV collagen coincides with differentiation of the primitive endoderm and assembly of the first embryonal basement membrane. The appearance of the interstitial collagens during mesoderm differentiation indicates a stage when mesoderm acquires connective tissue characteristics.     

Light microscopic immunolocalization of type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin in the basement membranes of a variety of rat organs

Immunohistochemical methods were used to determine whether type IV collagen, laminin, fibronectin, and heparan sulfate proteoglycan were present in diverse basement membranes. Antisera or antibodies against each substance were prepared, tested by enzyme-linked immunosorbent assay, and exposed to frozen sections of duodenum, trachea, kidney, spinal cord, cerebrum, and incisor tooth from rats aged 20 days to 34 months. Bound antibodies were then localized by indirect or direct peroxidase methods for examination in the light microscope. Immunostaining for type IV collagen, laminin, fibronectin, and heparan sulfate proteoglycan was observed in all of the basement membranes encountered. Fibronectin was also found in connective tissue. In general, the intensity of immunostaining was strong for type IV collagen and laminin, moderate for heparan sulfate proteoglycan, and weak for fibronectin. The pattern was similar in the age groups under study. Very recently the sulfated glycoprotein, entactin, was also detected in the basement membranes of the listed tissues in 20-day-old rats. It is accordingly proposed that, at least in the organs examined, type IV collagen, laminin, fibronectin, heparan sulfate proteoglycan, and entactin are present together in basement membranes.     

Changes in the matrix proteins, fibronectin and collagen, during differentiation of mouse tooth germ.

The distribution of the matrix protein fibronectin was studied by indirect immunofluorescence in differentiating mouse molars from bud stage to the stage of dentin and enamel secretion, and compared to that of collagenous proteins procollagen type III and collagen type I. Fibronectin was seen in mesenchymal tissue, basement membranes, and predentin. The dental mesenchyme lost fibronectin staining when differentiating into odontoblasts. Fibronectin was not detected in mineralized dentin. Epithelial tissues were negative except for the stellate reticulum within the enamel organ. Particularly intense staining was seen at the epithelio-mesenchymal interface between the dental epithelium and mesenchyme. Fibronectin may here be involved in anchorage of the mesenchymal cells during their differentiation into odontoblasts. Procollagen type III was lost from the dental mesenchyme during odontoblast differentiation but reappeared with advancing vascularization of the dental papilla. Similarly, procollagen type III present in the dental basement membrane during the bud and cap stages disappeared from the cuspal area along with odontoblast differentiation. Weak staining was seen in predentin but not in mineralized dentin. The staining with anti-collagen type I antibodies was weak in dental mesenchyme but intense in predentin as well as in mineralized dentin.     

Extracellular matrix organization in developing muscle: correlation with acetylcholine receptor aggregates

Monoclonal antibodies recognizing laminin, heparan sulfate proteoglycan, fibronectin, and two apparently novel connective tissue components have been used to examine the organization of extracellular matrix of skeletal muscle in vivo and in vitro. Four of the five monoclonal antibodies are described for the first time here. Immunocytochemical experiments with frozen-sectioned muscle demonstrated that both the heparan sulfate proteoglycan and laminin exhibited staining patterns identical to that expected for components of the basal lamina. In contrast, the remaining matrix constituents were detected in all regions of muscle connective tissue: the endomysium, perimysium, and epimysium. Embryonic muscle cells developing in culture elaborated an extracellular matrix, each antigen exhibiting a unique distribution. Of particular interest was the organization of extracellular matrix on myotubes: the build-up of matrix components was most apparent in plaques overlying clusters of an integral membrane protein, the acetylcholine receptor (AChR). The heparan sulfate proteoglycan was concentrated at virtually all AChR clusters and showed a remarkable level of congruence with receptor organization; laminin was detected at 70-95% of AChR clusters but often was not completely co-distributed with AChR within the cluster; fibronectin and the two other extracellular matrix antigens occurred at approximately 20, 8, and 2% of the AChR clusters, respectively, and showed little or no congruence with AChR. From observations on the distribution of extracellular matrix components in tissue cultured fibroblasts and myogenic cells, several ideas about the organization of extracellular matrix are suggested. (a) Congruence between AChR clusters and heparan sulfate proteoglycan suggests the existence of some linkage between the two molecules, possibly important for regulation of AChR distribution within the muscle membrane. (b) The qualitatively different patterns of extracellular matrix organization over myotubes and fibroblasts suggest that each of these cell types uses somewhat different means to regulate the assembly of extracellular matrix components within its domain. (c) The limited co-distribution of different components within the extracellular matrix in vitro and the selective immune precipitation of each antigen from conditioned medium suggest that each extracellular matrix component is secreted in a form that is not complexed with other matrix constituents.     

Immunocytochemical study of fibronectin in the sea lamprey,Petromyzon marinus,and the Atlantic hagfish,Myxine glutinosa

Summary Immunoreactive fibronectin-like material was localized within tissues of agnathans (hagfishes and lampreys) by an immunoperoxidase technique. Fibronectin was detected in basement membranes and in loose and dense connective tissues throughout the agnathan body. A fibronectin-like component was also identified in the plasma of both lampreys and hagfishes. The results indicate that fibronectin or a fibronectin-like material is a major component of agnathan connective tissues. Although there were some variations in the localization of fibronectin both between the lamprey and the hagfish and between agnathan and other vertebrate tissues, the generalized pattern of distribution of fibronectin in the agnathans supports the view that this protein, like that in higher vertebrates, plays a role in cellmatrix adhesion and tissue organization.     

Changing patterns of fibronectin, laminin, type IV collagen, and a basement membrane proteoglycan during rat Mullerian duct regression

Antibodies to type IV collagen, laminin, heparan sulfate proteoglycan, and fibronectin were used to study the regression of the rat Mullerian duct. All four of these matrix constituents are located at the perimeter of the Mullerian duct within the ductal basement membrane. As the Mullerian duct regresses, the staining of all of these basement membrane constituents becomes irregular and discontinuous. Fibronectin, which is also present in the interstitium, becomes undetectable in the mesenchyme which condenses around the regressing Mullerian duct. These data indicate that degradation of the extracellular matrix around the male Mullerian duct is a central event in the regression of this structure.     

FETAL RAT INTESTINAL ABSORPTION OF HORSERADISH PEROXIDASE FROM SWALLOWED AMNIOTIC FLUID

Horseradish peroxidase (HRP) injected into amniotic fluid is swallowed by rat fetuses and within 3–6 h reaches the gut lumen. This macromolecular protein is then absorbed by the columnar lining cells via a system of apical cytoplasmic tubules formed by invaginations of the plasma membrane. From cytoplasm subjacent to the brush border HRP is transported, within vacuoles, to the supranuclear region, where some is retained for at least 18 h, and to interepithelial spaces. Extracellular enzyme is then found throughout the epithelial basement membrane and between connective tissue cells of the mucosal and submucosal layers Finally, HRP can be detected within lumina of blood and lymphatic capillaries, strongly suggesting that it is transported from the intestine to the circulation.     

C3d fragment of complement interacts with laminin and binds to basement membranes of glomerulus and trophoblast

Two mouse monoclonal antibodies generated against human placental homogenate were found to react specifically with human complement component C3. In immunofluorescence of human tissues, these antibodies gave a bright linear staining outlining the glomerular basement membrane of the adult kidney and the trophoblast basement membrane of placenta. An identical staining pattern was observed with a rabbit C3d antiserum which also prevented binding of the monoclonal antibodies to tissue sections. Only negligible basement membrane staining was observed in the same tissues with antisera to human C3c, C5, IgG, IgA, or IgM. When interactions of C3 with basement membrane proteins were tested in enzyme immunoassays and column chromatography, C3(H2O) was found to bind efficiently to solid-phase laminin. Native C3 from fresh plasma did not bind to laminin but C3 from plasma treated with methylamine bound efficiently. When C3 was cleaved with trypsin, C3b and C3d but not C3c bound to laminin-Sepharose. The interaction of C3 and laminin was inhibited by soluble laminin and by high ionic strength. The results indicate that C3d, a biologically active breakdown product of C3, can be found in glomerular and placental basement membranes in the absence of signs for ongoing local complement activation or immune complex deposition. It is possible that binding affinities between C3 and basement membrane molecules, especially laminin, are involved in the retention of C3d at these sites. Such interactions between C3 and components of the glomerular basement membrane could play important roles in complement-related pathological processes of the glomerulus.     

Cytological and Morphological Analyses Reveal Distinct Features of Intestinal Development during Xenopus tropicalis Metamorphosis

Background

The formation and/or maturation of adult organs in vertebrates often takes place during postembryonic development, a period around birth in mammals when thyroid hormone (T3) levels are high. The T3-dependent anuran metamorphosis serves as a model to study postembryonic development. Studies on the remodeling of the intestine during Xenopus (X.) laevis metamorphosis have shown that the development of the adult intestine involves de novo formation of adult stem cells in a process controlled by T3. On the other hand, X. tropicalis, highly related to X. laevis, offers a number of advantages for studying developmental mechanisms, especially at genome-wide level, over X. laevis, largely due to its shorter life cycle and sequenced genome. To establish X. tropicalis intestinal metamorphosis as a model for adult organogenesis, we analyzed the morphological and cytological changes in X. tropicalis intestine during metamorphosis.

Methodology/Principal Findings

We observed that in X. tropicalis, the premetamorphic intestine was made of mainly a monolayer of larval epithelial cells surrounded by little connective tissue except in the single epithelial fold, the typhlosole. During metamorphosis, the larval epithelium degenerates and adult epithelium develops to form a multi-folded structure with elaborate connective tissue and muscles. Interestingly, typhlosole, which is likely critical for adult epithelial development, is present along the entire length of the small intestine in premetamorphic tadpoles, in contrast to X. laevis, where it is present only in the anterior 1/3. T3-treatment induces intestinal remodeling, including the shortening of the intestine and the typhlosole, just like in X. laevis.

Conclusions/Significance

Our observations indicate that the intestine undergoes similar metamorphic changes in X. laevis and X. tropicalis, making it possible to use the large amount of information available on X. laevis intestinal metamorphosis and the genome sequence information and genetic advantages of X. tropicalis to dissect the pathways governing adult intestinal development.