Sialic acid of mammalian cell lines

Sialylation: fate decision of mammalian sperm development, fertilization, and male fertility†

Sperm development, maturation, and successful fertilization within the female reproductive tract are intricate and orderly processes that involve protein translation and post-translational modifications. Among these modifications, sialylation plays a crucial role. Any disruptions occurring throughout the sperm's life cycle can result in male infertility, yet our current understanding of this process remains limited. Conventional semen analysis often fails to diagnose some infertility cases associated with sperm sialylation, emphasizing the need to comprehend and investigate the characteristics of sperm sialylation. This review reanalyzes the significance of sialylation in sperm development and fertilization and evaluates the impact of sialylation damage on male fertility under pathological conditions. Sialylation serves a vital role in the life journey of sperm, providing a negatively charged glycocalyx and enriching the molecular structure of the sperm surface, which is beneficial to sperm reversible recognition and immune interaction. These characteristics are particularly crucial during sperm maturation and fertilization within the female reproductive tract. Moreover, enhancing the understanding of the mechanism underlying sperm sialylation can promote the development of relevant clinical indicators for infertility detection and treatment.

Engineered molecular sensors for quantifying cell surface crowding

Cells mediate interactions with the extracellular environment through a crowded assembly of transmembrane proteins, glycoproteins and glycolipids on their plasma membrane. The extent to which surface crowding modulates the biophysical interactions of ligands, receptors, and other macromolecules is poorly understood due to the lack of methods to quantify surface crowding on native cell membranes. In this work, we demonstrate that physical crowding on reconstituted membranes and live cell surfaces attenuates the effective binding affinity of macromolecules such as IgG antibodies in a surface crowding-dependent manner. We combine experiment and simulation to design a crowding sensor based on this principle that provides a quantitative readout of cell surface crowding. Our measurements reveal that surface crowding decreases IgG antibody binding by 2 to 20 fold in live cells compared to a bare membrane surface. Our sensors show that sialic acid, a negatively charged monosaccharide, contributes disproportionately to red blood cell surface crowding via electrostatic repulsion, despite occupying only ~1% of the total cell membrane by mass. We also observe significant differences in surface crowding for different cell types and find that expression of single oncogenes can both increase and decrease crowding, suggesting that surface crowding may be an indicator of both cell type and state. Our high-throughput, single-cell measurement of cell surface crowding may be combined with functional assays to enable further biophysical dissection of the cell surfaceome.

Surface Modification of Macrophages with Nucleic Acid Aptamers for Enhancing the Immune Response against Tumor Cells

Antigen-presenting cells play a dominant role in cancer immunotherapy. Tumor cells, however, can still resort to several mechanisms of immune evasion that ultimately lead to the development of tumor tissues. In the current study, we performed surface modification of live macrophages with nucleic acid aptamers with the aim to enhance their affinity for tumor cells. Intercellular adhesion of tumor cells to surface-modified macrophages and the functions of the macrophages when in contact with tumor cells were investigated. To immobilize thiol-terminated nucleic acid aptamers that showed high affinity for the membrane protein of the tumor cells, methacryloyl groups were delivered into the sialic acids of the macrophages via metabolic glycoengineering (MGE). The proposed surface modification was cytocompatible and did not induce any undesirable activation of macrophages. According to the cell proliferation assay, the density of aptamers immobilized on a macrophage was found to decrease over time. However, the presence of aptamers on the cell surface was observed for more than 24 h after the immobilization. The number of adherent tumor cells on aptamer-immobilized macrophages was significantly larger than that of non-immobilized macrophages. Although the number of adherent tumor cells on aptamer-immobilized macrophages was not influenced by the pretreatment of doxorubicin to induce apoptosis in tumor cells, the apoptosis-induced tumor cells were highly phagocytosed by the aptamer-immobilized macrophages. The secretion amount of proinflammatory cytokines (TNF-α and IL-12) from the macrophages was coincident with the phagocytic index, which increased with the phagocytic uptake of tumor cells by the macrophages. In addition, the expression level of the major histocompatibility complex (MHC) class I and II molecules, required for antigen presentation, increased in nucleic acid aptamer-immobilized macrophages. Overall, the surface modification of macrophages with nucleic acid aptamers improved the tumor cell recognition of macrophages, indicating that the combination of cell surface engineering and anticancer drug treatment could constitute a promising strategy for tumor cell elimination.

Sialic Acids in Nonenveloped Virus Infections

Sialic acid-based glycoconjugates cover the surfaces of many different cell types, defining key properties of the cell surface such as overall charge or likely interaction partners. Because of this prominence, sialic acids play prominent roles in mediating attachment and entry to viruses belonging to many different families. In this review, we first describe how interactions between viruses and sialic acid-based glycan structures can be identified and characterized using a range of techniques. We then highlight interactions between sialic acids and virus capsid proteins in four different viruses, and discuss what these interactions have taught us about sialic acid engagement and opportunities to interfere with binding.

Removal of sialic acid from bull sperm decreases motility and mucus penetration ability but increases zona pellucida binding and polyspermic penetration in vitro

This study tested the hypothesis that sperm sialic acid (Sia) is required to reach the site of fertilization, and that successful fertilization requires recognition of Sia from both the sperm and oocyte to occur. In addition, it has recently been reported that Siglecs (Sia-binding-immunoglobulin-like lectins) are present on the sperm surface. Thus, the possibility that the recognition of oocyte Sia was sperm-Siglec-mediated was also addressed. Sperm exposed to neuraminidase (NMase) exhibited lower overall and progressive motility, which translated to a decreased ability to swim through cervical mucus from cows in oestrus. In addition, when either sperm or cumulus-oocyte complexes (COCs) were treated with NMase, a decrease in cleavage and blastocyst rate was observed. However, incubation of sperm with increasing concentrations of anti-Siglec-2, -5, -6 and -10 antibodies prior to fertilization had no effect on their fertilizing ability. Interestingly, treatment with NMase increased the number of sperm bound to the ZP but also the rate of polyspermic fertilization. Flow cytometry analysis revealed no differences in the percentage of capacitated or acrosome-reacted sperm. These results suggest that Sia are required to reach the site of fertilization but need to be removed for sperm-oocyte interaction. However, fine regulation is needed to avoid abnormal fertilization which can lead to impaired embryo development.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

SANTAVAC ™: A Novel Universal Antigen Composition for Developing Cancer Vaccines

Background:

Development of a universal cancer vaccine for the prevention of all cancers has been under development for many years. Antiangiogenic cancer vaccines elicit immune responses with the potential of destroying tumor vasculature endothelial cells without affecting vasculature integrity in normal tissues. The methods used in the development of antigen compositions comprising these vaccines have been recently improved and described in this report in the context of SANTAVAC ™ development - the first cancer vaccine based on endothelial cell heterogeneity.

Methods:

The present report summarizes data related to SANTAVAC™ development, including technical key points associated with optimal SANTAVAC™ production, a description of the composition required for preparing cancer vaccines with the highest predicted efficacy and safety, and a strategy for SANTAVAC™ large-scale implementation. Patents related to SANTAVAC™ and other universal cancer vaccines are also described.

Results:

SANTAVAC ™ was shown to be the most promising antigen composition for anti-cancer vaccination, allowing for immune targeting of the tumor vasculature in experimental models with a high predicted efficacy (up to 60), where efficacy represents the fold decrease in the number of endothelial cells with a tumor-induced phenotype and directly related to predicted arrest of tumor growth.

Conclusion:

The use of SANTAVAC ™ as a universal antigenic composition may spur vaccine development activities resulting in a set of therapeutic or prophylactic vaccines against different types of solid cancers.

Design of universal cancer vaccines using natural tumor vessel-specific antigens (SANTAVAC)

Vaccination against endothelial cells (ECs) lining the tumor vasculature represents one of the most attractive potential cancer immunotherapy options due to its ability to prevent solid tumor growth. Using this approach, target antigens can be derived from ECs and used to develop a universal cancer vaccine. Unfortunately, direct immunization with EC preparations can elicit autoimmune vasculitis in normal tissues. Recently, tumor-induced changes to the human EC surface were described that provided a basis for designing efficient EC-based vaccines capable of eliciting immune responses that targeted the tumor endothelium directly. This review examines these data from the perspective of designing EC-based cancer vaccines for the treatment of all solid tumors, including the antigen composition of vaccine formulations, the selection ECs for antigen derivation, the production and control of antigens, and the method for estimating vaccine efficacy and safety. As the vaccine preparation requires a specifically derived set of natural cell surface antigens, a new vaccine preparation concept was formulated. Antigen compositions prepared according to this concept were named SANTAVAC (Set of All Natural Target Antigens for Vaccination Against Cancer).

Sexual selection by female immunity against paternal antigens can fix loss of function alleles

Humans lack the common mammalian cell surface molecule N-glycolylneuraminic acid (Neu5Gc) due to a CMAH gene inactivation, which occurred approximately three million years ago. Modern humans produce antibodies specific for Neu5Gc. We hypothesized that anti-Neu5Gc antibodies could enter the female reproductive tract and target Neu5Gc-positive sperm or fetal tissues, reducing reproductive compatibility. Indeed, female mice with a human-like Cmah(-/-) mutation and immunized to express anti-Neu5Gc antibodies show lower fertility with Neu5Gc-positive males, due to prezygotic incompatibilities. Human anti-Neu5Gc antibodies are also capable of targeting paternally derived antigens and mediate cytotoxicity against Neu5Gc-bearing chimpanzee sperm in vitro. Models of populations polymorphic for such antigens show that reproductive incompatibility by female immunity can drive loss-of-function alleles to fixation from moderate initial frequencies. Initially, the loss of a cell-surface antigen can occur due to drift in isolated populations or when natural selection favors the loss of a receptor exploited by pathogens, subsequently the same loss-of-function allele can come under sexual selection because it avoids being targeted by the female immune system. Thus, we provide evidence of a link between sexual selection and immune function: Antigenicity in females can select against foreign paternal antigens on sperm and rapidly fix loss-of-function alleles. Similar circumstances existed when the CMAH null allele was polymorphic in ancestral hominins, just before the divergence of Homo from australopithecines.

Cellular cancer vaccines: an update on the development of vaccines generated from cell surface antigens

A recent advance in anti-cancer therapies has been the use of cancer cells to develop vaccines. However, immunization with cancer cell-based vaccines has not resulted in significant long-term therapeutic benefits. A possible reason for this is that while cancer cells provide surface antigens that are targets for a desired immune response, they also contain a high abundance of housekeeping proteins, carbohydrates, nucleic acids, lipids, and other intracellular contents that are ubiquitous in all mammalian cells. These ubiquitous molecules are not the intended targets of this therapy approach, and thus, the immune response generated is not sufficient to eliminate the cancer cells present. In this review, a discussion of the cell surface of cancer cells is presented in relation to the goals of improving antigen composition of cancer cell-based vaccines. Strategies to enrich vaccines for cancer-specific antigens are also discussed.

Hydrocortisone-induced DNA endoreplication in human trabecular cells in vitro

Primary monolayer cultures of trabecular cells and organ cultures of trabecular meshwork, obtained from normal eyes of human subjects (age range, 40 to 70 yr) were exposed to hydrocortisone at concentrations ranging from 10(-4) M to 10(-6) M for periods up to 4 weeks. Phase-contrast microscopy of cultured cells showed an increase in the size of the nuclei (up to three times) and in the extent of the cell cytoplasm compared to those in control cultures, and vesicular structures frequently accumulated in the cytoplasm. Microdensitometry of Feulgen-stained cell nuclei indicated that the cells of the trabecular meshwork in vivo have predominantly diploid levels (2C) of DNA. Many nuclei in the trabecular cell cultures were polyploid and contained DNA values of 4C, 8C, and 16C. Cultures which had been exposed to hydrocortisone showed a significant shift toward the higher DNA classes, in contrast to the untreated control cultures; the average increase in the amount of DNA per nucleus was 36%. We discuss the relevance of these findings to the disease glaucoma in man.

Stimulation of receptor-mediated low density lipoprotein endocytosis in neuraminidase-treated cultured bovine aortic endothelial cells

Sialic acids, occupying a terminal position in cell surface glycoconjugates, are major contributors to the net negative charge of the vascular endothelial cell surface. As integral membrane glycoproteins, LDL receptors also bear terminal sialic acid residues. Pretreatment of near-confluent, cultured bovine aortic endothelial cells (BAEC) with neuraminidase (50 mU/ml, 30 min, 37 degrees C) stimulated a significant increase in receptor-mediated 125I-LDL internalization and degradation relative to PBS-treated control cells. Binding studies at 4 degrees C revealed an increased affinity of LDL receptor sites on neuraminidase-treated cells compared to control BAEC (6.9 vs. 16.2 nM/10(6) BAEC) without a change in receptor site number. This enhanced LDL endocytosis in neuraminidase-treated cells was dependent upon the enzymatic activity of the neuraminidase and the removal of sialic acid from the cell surface. Furthermore, enhanced endocytosis due to enzymatic alteration of the 125I-LDL molecules was excluded. In contrast to BAEC, neuraminidase pretreatment of LDL receptor-upregulated cultured normal human fibroblasts resulted in an inhibition of 125I-LDL binding, internalization, and degradation. Specifically, a significant inhibition in 125I-LDL internalization was observed at 1 hr after neuraminidase treatment, which was associated with a decrease in the number of cell surface LDL receptor sites. Like BAEC, neuraminidase pretreatment of human umbilical vein endothelial cells resulted in enhanced receptor-mediated 125I-LDL endocytosis. These results indicate that sialic acid associated with either adjacent endothelial cell surface molecules or the endothelial LDL receptor itself may modulate LDL receptor-mediated endocytosis and suggest that this regulatory mechanism may be of particular importance to endothelial cells.

Surface modification of retinal pigment epithelial cells: effects on phagocytosis and glycoprotein composition

Proteases have been used as a tool to investigate the role of cell-surface molecules of cultured retinal pigment epithelial cells (RPE) in the phagocytosis of rod outer segments (ROS). Proteolytic digestion of RPE cells by pronase, thermolysin and Staphylococcus aureus V8 protease (V8 protease) inhibited the phagocytosis of ROS without affecting the viability of the RPE cells. A particular feature of RPE cell proteolysis was that those macromolecules responsible for ROS ingestion were susceptible, while those macromolecules that mediated ROS binding were resistant to cleavage by all three proteases. By taking advantage of this phenomenon, ROS were used as affinity particles to obtain a plasma membrane-enriched fraction of RPE cells before and after proteolytic digestion. All three proteases partially or completely removed several glycoproteins from the cell surfaces. Removal of these glycoproteins was correlated with a loss in phagocytic ability by RPE cells. Two high-molecular-weight (MW) glycoproteins of MWs 160,000 and 214,000 were consistently removed by all proteases tested. Protease-treated RPE cells restored their phagocytic capabilities and normal glycoprotein composition within 24 hr after proteolytic treatment. These data suggest that glycoproteins located on the surfaces of RPE cells may be involved in mediating the phagocytosis of ROS by these cells.

Teichoic acids of Streptococcus agalactiae: chemistry, cytotoxicity, and effect on bacterial adherence to human cells in tissue culture

The ratio of teichoic acid to lipoteichoic acid (LTA) in a strain of Streptococcus agalactiae type III was found to be 8:1, with the total amount of LTA being 0.1% of the dry weight of the organism. Purified teichoic acid contained D-alanine and possibly a small amount of D-glucose and was approximately 22 glycerol phosphate units in length. The linkage between each of these units was 1-3. In addition, LTA contained a complex lipid, more glucose, and an unusually high content of a short-chain fatty acid, tridecanoic acid. This LTA was cytotoxic for a variety of human cell monolayers in tissue culture, including one derived from the human central nervous system. Established human cells were more sensitive than primary cell monolayers to this LTA, with as little as 12.5 micrograms of LTA per ml being cytotoxic for HeLa cells. Teichoic acid (250 micrograms/ml) was nontoxic under identical conditions. These cytotoxicity results suggest an LTA involvement in group B streptococcal pathogenesis. Also, the first model system for the study of group B streptococcal adherence to primary human embryonic amnion cells in tissue culture is detailed. This system was used to quantitate pronounced differences in tissue tropism between S. agalactiae and Streptococcus pyogenes and showed enhanced binding by this group A coccus over that of S. agalactiae for amnion cell monolayers. The adherence of both streptococcal species to only a portion (40%) of these amnion cells suggested that host cell receptor expression may vary for primary cells in vitro. Finally, this strain of S. agalactiae was shown to adhere to amnion cells by a non-LTA-mediated mechanism. The possibility of an LTA-mediated versus a protein-mediated adherence mechanism for host cells that is related to the virulence of S. agalactiae is discussed.

The in vitro transport of 14C-alpha-aminoisobuturic acid into blastocysts from mice in delay and after activation for implantation

The uptake of 14C-AIB (14C-alpha-aminoisobuturic acid) into mouse blastocysts was studied in vitro. The flushed blastocysts fell into two size categories, namely large or "expanded" and small or "contracted". The arithmetic mean volume for the large blastocysts grew from 0.58 nl for delayed blastocysts to 0.76 nl for 8 h activated ones, while both delayed and activated small blastocysts had a mean volume of 0.22 nl. Delayed and 8 h activated large blastocysts showed an enhanced uptake of 14C-AIB with time of incubation, the uptake increasing 3-fold from 15 min to 60 min. The small blastocysts generally showed a higher uptake than the large ones, but the activated small blastocysts were atypical in that they unexpectedly displayed a 3-fold decrease in uptake at 60 min incubation as compared with 15 min. The early peaks of 14C-AIB uptake in vivo by uterine tissue and blastocysts were shown to be well correlated to the uptake in vitro. The presence of a working System A carrier in the activated blastocysts was demonstrated by showing that the uptake of AIB was sodium dependent and could be inhibited by N-methyl-AIB. Ultrastructural studies by scanning and transmission electron microscopy revealed that the trophoblast of large blastocysts was elongated and covered by numerous microvilli on a slightly bulging surface. The small blastocysts on the other hand displayed cuboidal trophoblast cells, sometimes in a pseudostratified arrangement, with the outer surface covered by many ridges.

Specific antigens of chicken thymus

Purified plasma membranes from chicken thymus and bursa cells were prepared and solubilized with sarkosyl (sodium salt of N-methyl-N-(1-oxodecyl)-glycine). Antisera to solubilized thymus plasma membrane (TPM) were produced in rabbits and the globulin fraction obtained by ammonium sulfate precipitation. Four precipitating antigens were detected in solubilized TPM by immunoelectrophoresis. Following absorption with chicken serum and bursa plasma membrane (BPM) immunosorbents, three antigens, designated T1, T2, T3, were specific for the TPM fraction, and one antigen, T1, was found in soluble extracts of thymus tissue. Absorption with isolated plasma membrane and whole cells indicated that the T1 and T2 antigens in solubilized TPM are associated with the plasma membrane but not expressed on the surface of the cell. A common antigen, designated BT, was detected in BPM and TPM fractions and in membrane preparations of spleen. The antigens were not detected in any other tissues or cells including brain, circulating lymphocytes and erythrocytes.

The in vivo transport of 14C-alpha aminoisobuturic acid into mouse blastocysts during activation for implantation

The in vivo transport of 14C-AIB (14C-alpha-aminoisobuturic acid) into mouse blastocysts was studied during activation for implantation. Mice, kept in experimentally delayed implantation, were given estrogen to induce implantation and then injections of 14C-AIB i.v. at 0, 4 and 8 h after the estrogen. After in vivo incubation times for 1/3 or 4 h with the labeled amino acid the blastocysts were flushed out of the uterus and collected. A distinct uptake of 14C-AIB occurred in the blastocysts 8 h (the highest uptake) and 12 h after the induction, provided that the in vivo incubation time was 4 h. At these times the blastocysts are lying free in the uterine lumen and consequently there is a transport of 14C-AIB from the epithelium via the uterine secretion into the blastocysts. This uptake indicates that amino acids transported by the system A are important nutrients during early activation. The uptake and retention of 14C-AIB in the uterus was tested at 4 and 8 h after the induction of implantation. The highest uptake was observed when the labeled amino acid was given at 8 h while the longest retention time occurred when 14C-AIB was given at 4 h. Since the transport ratio between the blastocysts and the uterine tissue is not maintained constant it is concluded that the metabolic rates for 14C-AIB transport are different for the uterus and the blastocysts. The AIB transport into uterine tissue preceeds that into the blastocysts. The AIB transport into the blastocysts is maintained as long as they have a negative surface charge.

The repair of the surface structure of animal cells

Experiments were performed to determine if animal cells in culture possess specific mechanisms to repair surface molecules damaged by enzymes. The surface membranes of a primary cell culture, chick fibroblasts, a permanent hamster cell line, BHK21/C13, and its virally transformed counterpart, C13/B4 were damaged by exposure to trypsin or to neuraminidase. Following digestion with trypsin, the incorporation of radioactive amino acids or sugars into purified surface membrane of cells was monitored. No differences were noted in rates of incorporation when control and trypsin-damaged cells were compared. Neuraminidase damage to the surface of BHK21/C13 and C13/B4 cells was evidenced by altered gel filtration profiles of surface glycopeptides, i.e., delayed elution because of reduction in size. By labelling cells with 14C-L-fucose prior to neuraminidase treatment and following the incorporation of 3H-L-fucose into cell surface glycopeptides after neuraminidase digestion, we were able to monitor the synthesis and turnover of fucose-containing glycopeptides in the same cells. Gel filtration profiles indicated that little or no desialylated glycoproteins were resialylated (repaired) by specific replacement of sialic acid. Comparing neuraminidase-digested and control cells we observed no difference in rates of 3H-L-fucose incorporation or of 14C-L-fucose loss from these cells; nor did we find differences in the rate of incorporation of isotopic glucosamine into sialic acid. Neuraminidase treatment failed to alter the rate of cell growth or the pattern of isotopic incorporation into various cell surface components. These results support the suggestion that return of sialic acid (repair) was effected by turnover which serves as a non-specific repair mechanism to replace damaged cell surface molecules (Warren and Glick '68; Warren, '69).

Changes in membrane structure associated with cell contact

Ultrastructural analysis of 3T3 fibroblasts by freeze-cleavage has demonstrated significant changes in cell-membrane structure associated with cell-to-cell contact and malignant transformation. These changes consist of a rearrangement and redistribution of intramembranous particles on the membrane fracture faces exposed by freeze-cleavage. The results show that noncontacted 3T3 cells in low density contain randomly distributed intramembranous particles. With the development of cell-to-cell contacts during the logarithmic phase of growth however, a pronounced aggregation of intramembranous particles is seen. A direct correlation between the degree of cell contact and the percentage of cells showing intramembranous-particle aggregation has been established. By contrast, transformed SV3T3 and SP3T3 cells show no evidence of intramembranous-particle aggregation even at cell densities where cell-to-cell contact is extensive. In view of recent reports that intramembranous particles represent foci of interaction between certain intrinsic membrane proteins and lipids, we propose that cell-to-cell contact of nontransformed 3T3 cells may initiate a change in the distribution of intrinsic membrane proteins associated with intramembranous particles and that these changes may influence control of cell proliferation.

Cell-surface mucosubstances from trypsin diaggregation of normal and virus-transformed lines of baby-hamster kidney cells

Cell disaggregation by trypsin solubilizes significantly less mucosubstance from the surface of polyoma-virus-transformed baby-hamster kidney cells than from the same non-transformed cell line. The mucosubstance, which consists of both acid mucopolysaccharides and mucoproteins, also differs qualitatively in the two cell lines.

Dependence of salivary epithelial morphology and branching morphogenesis upon acid mucopolysaccharide-protein (proteoglycan) at the epithelial surface

The morphogenetic role of the acid mucopolysaccharide (glycosaminoglycan) at the epithelial surface of mouse embryo submandibular glands has been studied by comparing the in vitro morphogenesis of epithelia from which the mucopolysaccharide was removed with that of those that retained the mucopolysaccharide. Epithelia isolated free of mesenchyme by procedures which retain the bulk of surface mucopolysaccharide maintain their lobular shape and undergo uninterrupted branching morphogenesis in culture in direct combination with fresh mesenchyme. Under identical culture conditions, epithelia from which surface mucopolysaccharide was removed lose their lobules and become spherical masses of tissue. During continued culture, the spherical epithelia produce outgrowths from which branching morphogenesis resumes. The morphogenetically active mucopolysaccharide is localized within the basal lamina of the epithelial basement membrane and appears to be bound to protein. During culture in combination with mesenchyme, epithelia undergoing uninterrupted morphogenesis show maximal accumulation of newly synthesized surface mucopolysaccharide at the distal ends of the lobules, the sites of incipient branching. In contrast, the material accumulates nearly equivalently over the surface of the spherical epithelia, with the exception that there is greater accumulation of the material at the surfaces of the budding outgrowths, the sites where morphogenesis will resume. Rapidly proliferating cells are localized within the lobules of epithelia undergoing uninterrupted morphogenesis, but are distributed uniformly in the cortex of the spherical epithelia, except for the outgrowths which show a greater localization of proliferating cells. It is concluded that normal salivary epithelial morphology and branching morphegenesis require the presence of acid mucopolysaccharide-protein within the epithelial basal lamina.

Bilirubin retinopathy

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