Intestinal transport of antibodies in the newborn rat

Coated pits, coated vesicles, and receptor-mediated endocytosis

Proteins and peptides can enter cells by receptor-mediated endocytosis, a coupled process by which selected extracellular proteins or peptides are first bound to specific cell surface receptors and then rapidly internalised by the cell. Internalisation follows clustering of the receptors in specialised regions of the cell surface called coated pits that invaginate to form intracellular coated vesicles. It is now recognised that receptor-mediated endocytosis has a fundamental role in the growth, nutrition and differentiation of animal cells.

Direct evidence for pH-dependent Fc receptors on proximal enterocytes of suckling rat gut

By means of an erythrocyte-antibody rosette technique, Fc receptors, functional at pH 6.0 but not at 7.2, were shown to be present on enterocytes isolated from duodenum and jejunum (but absent from ileum) of 12-20-day-old suckling rats.

Protein transmission in the intestine of the newborn lamb: the involvement of acid and alkaline phosphatase activity

The localization of acid phosphatase activity was differentiated from that of alkaline phosphatase in the foregut of the newborn lamb by light and electron microscopy. The examination of samples from fed and unfed lambs indicated the presence of alkaline phosphatase activity in endocytic vesicles originating from the brush-border. These vesicles, associated with protein absorption, were particularly numerous in fed lambs and occurred throughout the cytoplasm of the enterocytes. Acid phosphatase activity was absent from vesicles in the apical cytoplasm but it was localized in most sub-nuclear vesicles, also in the Golgi apparatus and the lysosomes of macrophages. The sub-nuclear vesicles were often observed in close proximity to the lateral and basement membranes of the enterocytes, also in continuity with the intercellular space. It is suggested that these results indicate the mechanism for transmission of brush-border and lysosomal enzymes, along with the immunoglobulins, into the lymph of the newborn lamb.

Biosynthesis and secretion of milk proteins: a review

Recent years have seen a great increase in the knowledge and understanding of milk proteins. Arising from several origins including the blood stream and various cellular sources, many of the proteins found in milk are products of the secretory cells directly involved in the synthesis and secretion processes of various milk components. The lactation-specific proteins present in major amounts are synthesized in the rough endoplasmic reticulum (RER) under genetic control and undergo further post-translational modifications in their secretory route from the RER through the Golgi apparatus and secretory vesicles before ejection into the lumen with other milk components. Various molecular aspects of these mechanisms and their control are now understood, but many remain to be described.

Morphogenesis of the small intestine during fetal development

During the last five days ('last trimester') of the 22-day gestation period of the rat the mucosa of the small intestine changes from undifferentiated stratified epithelium without villi to a mucosa with villi covered with simple columnar epithelium. During this process many secondary lumina form in the primitive stratified epithelium; these lumina enlarge and eventually fuse with the main intestinal lumen as degenerating superficial epithelial cells are exfoliated and as upward growth of mesenchyme towards the main lumen takes place. Proliferation of intestinal epithelial cells occurs along the entire length of the newly formed villi until one or two days before birth, when proliferating epithelial cells become confined to incompletely formed crypts at the base of the villi. In contrast, differentiation of the small intestinal mucosa in human fetuses begins much earlier in gestation. Villi form at 9 to 10 weeks and crypts are well developed by 12 weeks (first trimester). By 17 weeks, all epithelial cells types seen in intestinal crypts of adults are present. Absorptive cells on villi have a prominent apical tubular system, large meconium-filled lysosomes and abundant glycogen between 10 and 22 weeks' gestation. Whereas there is uptake of the macromolecular marker, ferritin, into the apical tubular system after eight or more minutes of in vitro exposure, there is no evidence of transport of ferritin across the absorptive cells after up to 40 minutes of exposure between 11 and 12 weeks' gestation.

The differentiation and redifferentiation of the intestinal epithelium and its brush border membrane

This paper briefly reviews the progress of investigations during the past 30 years into the development of the intestinal epithelium, and surveys the directions in which research in this field is now advancing. Early studies showed that the differentiation of the epithelium is a discontinuous process, and also focused attention on the enzyme specialization of the brush border. The phases of differentiation were found to be under the control of the pituitary--adrenal system, with the outflow of glucocorticoids influencing both structural and biochemical aspects of enterocyte development in fetal and postnatal stages. More recent evidence has shown that thyroid hormones can also regulate epithelial differentiation, even in the absence of adrenocortical function. Organ culture is now becoming an important means of probing into the processes underlying overt differentiation, and methods for culturing pure populations of enterocytes are being perfected. Studies of developmental alterations in the constituents of the brush border membrane, and of synthesis and glycosylation of membrane constituents, are contributing to the development of an integrated account of the differentiation of this membrane, through which all nutrients must pass.

Gastrointestinal host defence: importance of gut closure in control of macromolecular transport

An important adaptation of the gastrointestinal tract to the extrauterine environment is its development of a mucosal barrier against the penetration of harmful substances (bacteria, toxins and antigens) present within the intestinal lumen. At birth, the newborn infant must be prepared to deal with bacterial colonization of the gut, with formation of toxic byproducts of bacteria and viruses (enterotoxins and endotoxins) and with the ingestion of antigens (milk proteins). These potentially noxious substances if allowed to penetrate the mucosal epithelial barrier under pathological conditions can cause inflammatory and allergic reactions which may result in gastrointestinal and systemic disease states. To combat the potential danger of invasion across the mucosal barrier the infant must develop an elaborate system of defence mechanisms within the lumen and on the luminal mucosal surface which act to control and maintain the epithelium as an impermeable barrier to uptake of macromolecular antigens. These defences include a unique immunological system adapted to function in the complicated milieu of the intestine as well as other non-immunological processes such as a gastric barrier, intestinal surface secretions, peristaltic movement and natural antibacterial substances (lysozyme, bile salts) which also help to provide maximum protection for the intestinal surface. Unfortunately, during the immediate postpartum period, particularly for premature and small-for-dates infants, this elaborate local defence system is incompletely developed. As a result of the delay in the maturation of the mucosal barrier newborn infants are particularly vulnerable to pathological penetration by harmful intraluminal substances. The consequences of altered defence are susceptibility to infection and the potential for hypersensitivity reactions and for formation of immune complexes. With these reactions comes the potential for developing life-threatening diseases such as necrotizing enterocolitis, sepsis and hepatitis. Fortunately, 'nature' has provided a means for passively protecting the 'vulnerable' newborn against dangers of a deficient intestinal defence system, namely human milk. It is now increasingly apparent that human milk contains not only antibodies and viable leucocytes but many other substances which can interfere with bacterial colonization and prevent antigen penetration.

Selective protein transport: characterization and solubilization of the phosvitin receptor from chicken oocytes

Phosvitin (PV), a subunit of a female-specific protein, vitellogenin, binds to oocyte membranes with a KD of 10(-6) M. Binding reaches equilibrium within 30 min after incubation at 25 degrees C. Bound 125I-PV dissociates from the membrane with a t1/2 of 13 h when incubated in buffer. However, when 125I-PV-labeled membranes are incubated in buffer containing 10(-5) M unlabeled PV, 50% of the initially bound 125I-PV dissociates from the membrane within 10 min. These results support the conclusion that PV binds to a membrane-associated receptor. Solubilization studies show that Triton X-100 solubilizes up to 45% of the total membrane-bound 125I-PV. Gel-exclusion chromatography of the solubilized material yields a 500,000 dalton 125 I-PV-containing complex separated from free 125I-PV. The 500,000 dalton complex completely dissociates to yield free 125I-PV when incubated with excess unlabeled PV. However, when incubated with 1) no addition, 2) IgG, or 3) serum albumin, the extent of dissociation is significantly reduced and is consistent with that which would be predicted on the basis of the observed dissociation rate in the absence of unlabeled PV. These results suggest that bound 125I-PV can only be displaced by unlabeled PV. These results also indicate that the 500,000 dalton species is a solubilized PV-receptor complex and that it is possible to solubilize the PV-receptor in an active form.

Ultrastructural cytochemistry of iron absorption

Conventional ultrastructural autoradiographic and morphologic studies of the duodenal mucosal cell have generally corroborated physiologic observations of iron absorption, but such methods have limited resolution and fail to distinguish ferric and ferrous iron. This study describes the application of the Prussian blue reaction as an electron microscopic cytochemical stain to the investigation of inorganic iron absorption in iron-deficient, normal, and iron-loaded rats. Ferrous iron is converted to ferric iron at the microvillus membrane. Subsequently intraepithelial ferric iron appears bound to a non-heme acceptor substance in microvilli and later appears as small non-membrane-bound stain deposits which are concentrated in the apical cytoplasm. The appearance of larger stain deposits in the lateral intercellular spaces, in the basal extracellular spaces, and along the intraluminal and extraluminal outer plasmalemma of adjacent endothelial cells of the lamina propria suggests passage of iron from epithelial cells through the lamina propria to blood vessels. The extreme sensitivity of the method compared with simultaneous ultrastructural autoradiographic techniques is demonstrated and suggests usefulness of the method in further studies of iron metabolism.

The structure of the germinal disc region of the hen's ovarian follicle during the rapid growth phase

The structure of the ovarian follicle in the region of the germinal disc, which appears as a white plaque at the surface of the oocyte, was examined by electron microscopy and compared with the non-disc region which overlies the yellow yolk mass of the oocyte in the final growth phase. The main differences concerned the granulosa cell layer and the surface layer of the oocyte. In the disc the granulosa cells were less regularly arranged and the spaces between them varied in width. Their mitotic rate was higher than that in the non-disc region, where cell division was seldom observed at maturity. The perivitelline layer was comparatively poorly developed at the periphery of the germinal vesicle in 15 mm follicles, but eventually attained a uniform thickness throughout the follicle. In the intercellular and perivitelline spaces there were smaller amounts of granular material. Marked differences were observed in the ooxyte surface layer. In 15 mm follicles the surface of the germinal disc was thrown into numerous microvilli and some narrow indentations containing macrovilli from the granulosa cells. Coated vesicles, 120 nm diameter, appeared to be invaginating from the oolemma, whereas 70 nm coated vesicles were present in the deeper cytoplasm. In follicles of more than 25 nm diameter these structural conformations were evident only at the periphery of the disc; for the most part the 120 nm coated vesicles were absent, and over the germinal vesicle microvilli were of rare occurrence. On the other hand, the bulk of the oocyte surface was highly convoluted throughout this period of growth, numerous granulosa cell macrovilli extended into deep pouches associated with 300 nm coated vesicles, and the oolemma possessed a coating of fuzzy material. These observations suggest that there is a restricted passage of yolk precursors to the surface of the germinal disc, and that the inability to transport yellow yolk into the disc is related to differences in the oolemmal surface coat and the population of coated vesicles. The surface modifications, as well as the proliferation of the granulosa cells, are likely to be influenced by the presence of the germinal vesicle.

Structure of occluding junctions in ileal epithelial cells of suckling rats

Two kinds of occluding junctions are found between ileal epithelial cells of suckling rats: apical zonulae occludentes (ZO) and fasciae occludentes (FO) which are associated with the lateral plasma membranes of many epithelial cells. In unfixed preparations, glycerol treatment induces the further proliferation of extensive fasciae occludentes. Both kinds of junction have identical structural elements when visualized in freeze fracture replicas, although the arrangement of these elements differs. Zonulae occludentes consist of networks of branching and anastomosing linear ridges or rows of approximately 10 nm particles with 20--30 nm spaces between the rows which form narrow belt-like structures around the apical region of adjacent cells. Fasciae occludentes, on the other hand, consist of similar linear ridges or rows of particles but the junction strands are often discontinuous, open ended and only occasionally intersect with each other. Several different fracture planes through the plasma membrane in the region of the occluding junctions have been observed and these provide further evidence that two components, one from each membrane, fused at the level of the extracellular space, form the junction sealing element. Furthermore, we present evidence which indicates a staggered rather than an in-register arrangement of these two components.

Relationships between immunoglobulins and the intestinal epithelium

The intestinal epithelium is intimately associated with immunoglobulins. This association may begin in neonatal life with the ingestion of large quantities of immunoglobulins in breast fluids. These ingested immunoglobulins probably have a local protective action in the intestinal lumen. In some mammalian species a large portion of the maternal immunoglobulins is translocated intact across the intestinal epithelium into the circulation, providing additional immunological protection. In rodents, the transepithelial translocation of IgG from breast fluids is initiated and critically dependent upon receptors on enterocyte surface membranes for the Fc region of IgG. Close epithelial-immunoglobulin relationships continue throughout life with the transfer of various classes of immunoglobulins across the epithelium into the intestinal fluids. In man and other mammalian species, IgA and IgM are selectively transported through enterocytes, principally in the crypts of intestinal glands. This transfer may involve binding of polymeric forms of these immunoglobulins to receptors on the abluminal surfaces of the enterocytes. The secretory component, a glycoprotein synthesized by enterocytes, may be such a receptor. IgE and IgG enter the gut lumen by mechanisms that are not defined but seem to be distinct from those involved in the translocation of IgA and IgM. Secreted antibodies in intestinal fluids and mucus bathe the luminal surfaces of intestinal epithelial cells but appear not to be firmly bound to their apical plasma membranes or glycocalyces. The intimate association of immunoglobulins with intestinal epithelial cells illustrates the close relationships that exist between the gut and lymphoid cells and their products. These relationships suggest the possibility that the gut epithelium is affected by a large variety of immunological reactions in health and disease; these possibilities, which have been explored only minimally, warrant much attention in the future. Studies on the binding, uptake, and intracellular transport of immunoglobulins by enterocytes could contribute much to the understanding of receptors for immunoglobulins on many other types of cells, such as lymphocytes, macrophages, mast cells, and the lining cells of placental or yolk sac membranes.

Morphological factors influencing transepithelial permeability: a model for the resistance of the zonula occludens

Epithelial cells are joined at their apical surfaces by zonulae occludentes. Claude and Goodenough (1973) demonstrated a correlation between the structure of the zonula occludens as seen in freeze-fracture preparations and the passive electrical permeability of several simple epithelia. In epithelia with high transepithelial resistance, the zonula occludens consisted of many strands. In epithelia with low transepithelial resistance the zonula occludens was much reduced, sometimes consisting of only one strand. Evidence is reviewed here that indicates that in a number of simple epithelia the structure of the zonula occludens is largely responsible for the magnitude of transepithelial conductance. An equation is derived relating transepithelial junctional resistance to the number of junctional strands: R = RminP-N where R is the transepithelial resistance of the zonula occludens, Rmin is the minimum resistance of the junction (as when there are no strands in the zonula occludens), p is the probability a given strand is "open" and n is the number of strands in the junction. Using published experimental values of R and n for different epithelia, the calculated value of p was found to be as high as 0.4, which suggests that the strands in the zonula occludens are remarkably labile. Other morphological parameters relevant to transepithelial permeability are also considered, such as the width and depth of the intercelllar spaces, and the size of the epithelial cells themselves.

Electron microscope observations on the ovarian follicle of the domestic fowl during the rapid growth phase

A survey has been made of the ultrastructural features of the oocyte surface and its enveloping layers, comprising the theca, the granulosa and the perivitelline layer, during the final growth phase of 'yellow yolk' deposition. The following observations suggest that many macromolecular components of the blood plasma have free access from the vasculature to the oolemma: the fenestrated structure of the endothelium of the thecal capillaries, the intercellular spaces containing several erythrocytes in the theca interna, the 20-30 nm particles in the granulosa basal lamina, the wide spaces between the granulosa cells, and the open meshwork of fibres in the perivitelline layer. Numerous coated pits and vesicles, of 0.25-0.35 micron diameter, in the highly convoluted surface layer of the oocyte provide a mechanism for the incorporation of yolk precursors by pinocytosis. Such large coated vesicles and wide spaces between the granulosa cells are lacking in follicles in the preceding growth phase, which is concerned with the deposition of 'white yolk'. Considerable metabolic activity of the granulosa cells is indicated by their prominent Golgi elements, diverse granules, vesicles and villus processes. Cell junctions at the tips of the macrovilli anchor the granulosa to the oocyte. The theca externa, which provides mechanical support for the oocyte and its vascular and nervous elements, consists of sheets of collagen fibrils and fibroblast-like cells. Microfilaments in these cells may contribute to the contractility of the theca. Groups of interstitial cells associated with nerve fibres are present in the theca interna.

Uptake of horseradish peroxidase from CSF into the choroid plexus of the rat, with special reference to transepithelial transport

Protein uptake from cerebral ventricles into the epithelium of the choroid plexus, and transport across the epithelium were studied ultrastructurally in rats. Horseradish peroxidase (HRP, MW 40,000) was used as protein tracer. Steady-state ventriculo-cisternal perfusion with subatmospheric pressure (-10cm of water) in the ventricular system was applied. HRP dissolved in artificial CSF was perfused from the lateral ventricles to cisterna magna for various times, and ventriculo-cisternal perfusion, vascular perfusion or immersion fixation with a formaldehyde-glutaraldehyde solution was performed. Coated micropinocytic vesicles containing HRP were seen both connected with the apical, lateral and basal epithelial surface and within the cells. Heavily HRP-labeled vesicles were often fused with the lining membrane of slightly labeled or unlabeled intercellular spaces. Since the apical tight junctions of the epithelium never appeared open or never contained HRP in the spaces between the fusion points, and since the intercellular spaces between adjacent epithelial cells below the junctions only infrequently contained tracer after 5 min, by increasing amounts after 15-60 min of HRP perfusion, a vesicular transport of HRP from the apical epithelial surface to the intercellular spaces, bypassing the tight junctions, is suggested. In addition to the transepithelial transport, micropinocytic vesicles also transported HRP to the lysosomal apparatus of the epithelial cells. With increasing length of exposure to HRP, a sequence of HRP-labeled structures could be evaluated, from slightly labeled apical vacuoles and multivesicular bodies to very heavily labeled dense bodies.

Uptake and fate of luminally administered horseradish peroxidase in resting and isoproterenol-stimulated rat parotid acinar cells

The formation and fate of apical endocytic vesicles in resting and isoproterenol-stimulated rat parotid acinar cells were studied using luminally administered horseradish peroxidase (HRP) to mark the vesicles. The tracer was taken up from the lumen by endocytosis in small, smooth-surfaces "c"- or ring-shaped vesicles. About 1 h after HRP administration the vesicles could be found adjacent to the Golgi apparatus. At later times HRP reaction product was localized in multivesicular bodies and lysosomes; in isoproterenol-stimulated cells it was also present in autophagic vacuoles. HRP reaction product was never localized in any structure associated with secretory granule formation. These results suggest that the apical endocytic vesicles play a role in membrane recovery, but that they are degraded and not reutilized directly in secretory granule formation. Additionally, it was found that when isoproterenol was injected before HRP administration, the apical junctional complexes became permeable to the tracer, allowing it to gain access to the lateral and basal intercellular spaces. This permeability may provide an additional route whereby substances in the extracellular fluid could reach the saliva.

[Peptidases. I. Histochemical investigations with 2-naphthylamides and hexazonium-p-rosanilin (author's transl)]

Using fresh frozen (with and without semipermeable membranes), freeze-dried or sections from aldehyde fixed material and hexazotized p-rosaniline for simultaneous coupling more than 20 different unsubstituted or substituted L-amino acid naphthylamides are split especially in the microvilli and/or stereocilia of the small intestine, kidney and epididymis from rats. Further sites of positive reactions can be revealed by L-alanyl, L-leucyl, L-lysyl, alpha,L-glutamyl, gamma,L-glutamyl, L-asparaginyl, N-benzoyl-L-arginyl, N-carbobenzoxy-L-arginyl and N-benzoyl-L-phenylalanyl 2-naphthylamide. Among the substituted and unsubstituted peptide 2-naphthylamides L-prolyl-L-arginyl 2-naphthylamide is not hydrolysed in visible amounts; L-arginyl-L-arginyl, L-alanyl-L-arginyl-L-arginyl, L-alanyl-L-leucyl-L-tyrosyl, L-histidyl-L-seryl, L-seryl-L-tyrosyl and L-glycy-L-phenylalanyl 2-naphthylamide are metabolized in the renal and intestinal brush border; the reaction pattern obtained with N-carbobenzoxy-L-glycyl-L-glycyl-L-arginyl 2-naphthylamide differs from that of N-carbobenzoxy-L-arginyl 2-naphthylamide. In addition L-glycyl-L-prolyl, L-leucyl-L-alanyl, L-lysyl-L-alanyl and L-alanyl-L-phenylalanyl-L-prolyl 2-naphthylamide are also split in the lysosomes of many organs and the secretion granules of gland cells.

The digestion and transmission of labelled immunoglobulin G by enterocytes of the proximal distal regions of the small intestine of young rats

Density gradient centrifugation of samples prepared from proximal gut homogenates after intra-lumenal injection of 125I-labelled IgG, was used to prepare batches of IgG fragments according to sedimentation coefficients. 2. Ultrafiltration was employed to partition the radioactivity in the vascular compartments, viscera and carcasses of rats aged 14-15 days, 2 hr after the injection of standard doses of labelled IgG into the proximal and distal regions of the small intestine. 3. Radioactive samples prepared by these methods were re-introduced into young rats by intra-cardiac injection, and the rate at which they were removed from the vascular compartment was assessed. 4. Proximal enterocytes transmitted about 39% of the IgG which had been removed from the intestine in intact form. Most of this was retained in the vascular compartment; they degraded up to about 57% of the total removed into fragments less than 1000 mol. wt. and about 4% into intermediate sized fragments. 5. Distal enterocytes degraded almost 90% of the IgG processed into fragments less than 1000 mol. wt., about 8% as fragments greater than 100,000 mol. wt. 6. Fragments, of all sizes, were cleared rapidly from the circulation into the viscera and carcass. 7. The relevance of these results to protein transmission and digestion by the rat small intestine is discussed.

Endocytosis and immunoglobulin transport across the small intestine of the new-born pig

1. Measurements of porcine and human IgG transport across the small intestine of the new-born pig have been made in vivo and related to the amount of endocytosis taking place.2. The amount of immunoglobulins transported, following intraduodenal administration to conscious animals, is directly related to the degree of endocytosis which these proteins produce. Administration of protein in amounts sufficient to cause a maximal endocytotic response causes saturation of the protein transporting process.3. Absorption of small amounts of human IgG from the small intestine can be accelerated by the addition of sow colostrum or porcine IgG in quantities sufficient to stimulate endocytosis. These effects disappear when the amount of human IgG administered is itself sufficient to fully stimulate endocytosis.4. Preferential transport of porcine over human IgG can be demonstrated when both are given as a single solution to individual pigs. The degree of preference is, however, small in relation to the total amount of immunoglobulin transported.5. The initial formation of endocytotic vacuoles in the pig intestine seems, unlike the situation in rats and mice, to provide a major route for the trans-cellular movement of macromolecules including immunoglobulins. Present evidence suggests that, though sow colostrum can on occasion stimulate such transport, it does so merely by acting as a protein-containing solution to increase the amount of endocytosis taking place.

Quantitative and visualization studies of the transport of rat and bovine IgG and ferritin across the segments of the small intestine of the suckling rat

Quantitative and visualization studies have been made of the transport of [ 125 I]rat IgG and [ 131 I]ferritin across the duodenum, jejunum and ileum of the 14-day-old rat. The most active transport of both isotopes into the carcase occurs across the ileum, and although there is more breakdown of the proteins in the ileum, a great deal of this activity in the carcase is protein-bound. Ferritin is also transported in considerable amount across the duodenum. The selective ratio is slightly greater in the duodenum than in the ileum. These results are confirmed by visualization studies, which show ferritin entering and leaving both duodenal and ileal cells in quantity. Both markers are seen in the cytoplasm of both types of cell.

An electron microscopic study of absorption of peroxidase-conjugated immunoglobulin G by guinea pig visceral yolk sac in vitro

In the guinea pig and some other animals, passive immunity is conferred on the developing fetus by passage of immunoglobulin from mother to fetus across the yolk sac. In order to examine the cytological pathway involved in immunoglobulin transport, guinea pig visceral yolk sacs from late in gestation were exposed in vitro to peroxidase-conjugated guinea pig immunoglobulin G (IgG-HRP). Tissue was then fixed, incubated to show the site of localization of peroxidase reaction product and prepared for electron microscopy. The results suggested that the first step in the uptake of IgG-HRP by yolk sac is attachment of the protein to the surface coats of endocytic invaginations at the apical surfaces of the endodermal cells. The endocytic vesicles then appear to pinch off from the surface and move deeper into the cytoplasm. Some of the small endocytic vesicles fuse with large apical vacuoles, which often contain large amounts of reaction product. Other small endocytic vesicles pinch off from the surface, move deeper into the cytoplasm and fuse with the lateral plasmalemma; their protein content is emptied into the intercellular space by exocytosis. From the intercellular spaces the protein presumably diffuses across the basement membrane and connective tissue spaces and enters the vitelline capillary bed. It is postulated that the latter cellular pathway, involving small vesicles and the intercellular spaces, is utilized by those immunoglobulins which are transferred intact across the yolk sac endoderm.

Kinetic analysis of the binding of immunoglobulins IgG1 and IgG2 to bovine mammary cells

Previous reports were confirmed that specific binding sites exist on bovine mammary cells near parturition presumably involved in the transfer of immunoglobulins IgG1 and IgG2 across the mammary gland at the time of colostrum formation. Determination of the kinetic parameters of these binding sites using 125I-labeled IgG1 and IgG2 immunoglobulins indicated the presence of sites with association constants (Ka) of about 5 - 10(8)--10 - 10(8)M-1 for both subclasses during normal lactation with about 9000 and 3000 sites per cell for each, respectively. The number of IgG1 sites tended to increase as the time of parturition approached. In addition, a new group of sites numbering about 5000 per cell with very strong binding for IgG1 (Ka about 45 - 10(8)M-1) appeared on the cells about a week before parturition. The numbers and affinity of the IgG1 and IgG2 binding sites bear a relationship to the approximate 7 : 1 ratio of these immunglobulin subclasses found in colostrum and normal milk and to the time of maximum colostrum formation. The results support the premise that a highly selective transport mechanism exists in the bovine mammary epithelial cell for the transfer of IgG1 and IgG2 immunoglobulins from blood to the lacteal secretions.

Fractionation studies on the absorption of labelled immunoglobulin G by the gut of young rats

1. Centrifugation in density gradients was used to study the fragments produced during intraluminal and intracellular digestion, after the injection of 125I-labelled immunoglobulin G (IgG) into different regions of the small intestine of 14 to 15-day-old (pre-closure) and 24-day-old (post-closure/ rats. 2. After injection into the proximal small intestine and into the ileum of pre-closure animals, the bulk of the radioactivity recorded for gut washes and gut homogenates was located at 4S-7S. The serum from animals which had received injections into the proximal small intestine had high radioactivity and one peak at 7S; the serum from animals which had received injections into the ileum had low radioactivity and no activity in the 7S region. 3. After injection into the proximal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash samples was located at 3-5S--5S. Gut homogenates had peak activity at 2-5S--4S. Thus large molecular weight products can be absorbed by the proximal enterocytes of post-closure rats and degraded. The sera of these animals had low radioactivity. 4. After injection into the distal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash and gut homogenate samples was located at 4S-7S and in this respect the radioactivity plots resembled those for (2) above. Serum radioactivity was low. 5. The effect of precipitation with trichloroacetic acid and incubation with specific antiserum upon the radioactivity of gut washes, gut homogenates and serum samples was recorded. 6. The relevance of these findings to studies on the transmission of protein by the rat small intestine is discussed.

The selective uptake and transmission of proteins to the circulation from the small intestine of the suckling rat

The relative concentration of various immunoglobulins attain in the sera of 14-day-old rats have been compared 4 h after their oral and intraluminal administration. The circulatory concentration of these proteins attained higher values when they were administered into the isolated proximal small intestina than when they were administered into the distal small intestine. Selective uptake of proteins was shown to predominate in the proximal small intestine.

pH-dependent binding of immunoglobulins to intestinal cells of the neonatal rat

Rat and rabbit IgG immunoglobulins conjugated to horseradiah peroxidase as a histochemical marker bind at 0 degrees C to the luminal surface of absorptive cells in isolated segments of jejunum from 10-12-day old rats. Binding is observed at pH 6.0, near the normal luminal pH of the duodenum and jejunum at this age, but not at pH 7.4. Furthermore, no binding occurs when cells are exposed at pH 6.0 to either free peroxidase or peroxidase conjugated to chicken or sheep IgG immunoglobulins or bovine serum albumin. The sensitivity of binding to pH suggests a means whereby immunoglobulins which are selectively absorbed by the cells can be released efficiently at the abluminal surface.

Localisation of proteins in coated micropinocytotic vesicles during transport across rabbit yolk sac endoderm

Rabbit yolk sac splanchnopleur exposed in utero to IgG-HRP and IgG-ferritin conjugates, rabbit and bovine anti-HRP antibodies, free HRP, ferritin and human IgG, was examined ultrastructurally in an attempt to determine whether or not coated micropinocytotic vesicles are involved in selectively transporting immunoglobulins across yolk sac endodermal cells. Human, rabbit and bovine IgG-HRP conjugates, rabbit anti-HRP antibodies, free HRP and human IgG, all become localised in coated micropinocytotic vesicles. Differences were observed in that only human IgG and rabbit anti-HRP antibodies could be located in the intercellular space and bovine IgG-HRP conjugate could not be detected in coated micropinocytotic vesicles in confluence with the lateral and basal plasmalemma. Bovine anti-HRP anti-bodies, IgG-ferritin conjugates, and free ferritin, could not be observed in coated micropinocytotic vesicles. All proteins were detected in macropinocytotic vesicles, and dense bodies resembling phagolysosomes. Results are discussed in the light of a proposal that selection occurs at the cell surface during formation of coated micropinocytotic vesicles and is not linked to intracellular proteolysis.

Collagen deposition on a preformed grid

Appearance of collagen fibrils in the cuticle was seen by electron microscopy to be preceded by formation of a finely filamentous matrix material. At first, the fine filaments of the matrix are unorganized. However, signs of orthogonal ordering soon appear in the most superficial portion of the cuticle, and subsequently appear more basally and closer to the underlying epidermis. Meanwhile, fibrils of different staining properties and identifiable as collagen begin to be deposited in the superficial portion of the cuticle, the same region which first showed organized fine filaments. Then, like the fine filaments before them, the collagen fibrils polymerize more basally. Collagen appears to polymerize on the preformed skeleton of fine filaments as though the fine filaments lagen fibrils seems to require direct cellular intervention but occur first in that portion of the cuticle which is furthest away from the underlying epidermis. The fine filaments may be self ordering, extracellular macromolecules which in turn determine the polymerization of collagen fibrils.

Quantitative assessment of the transmission of labelled protein by the proximal and distal regions of the small intestine of young rats

1. The plasma volume in rats aged 15-16 days was measured by dilution analysis using homologous, 125I-labelled immunoglobulin G. A mean plasma volume of 5-53 ml./100 g and a mean blood volume of 8-01 ml./100 g were obtained.2. After the injection of labelled immunoglobulin G into the heart, homogenates of various abdominal organs and of the carcass were prepared. Labelled immunoglobulin G left the vascular compartment at a rate of about 9-10%/hr over a 3 hr period. About 11% of the labelled immunoglobulin G was catabolized in 2 hr. 3. The data obtained from these studies was used to make quantitative estimates of the amount of intact immunoglobulin G transmitted from the proximal intestine and from the ileum after the intra-intestinal injection of 1000 mug of labelled immunoglobulin G. Homogenates of the experimental animals were prepared and it was estimated that over 40% of the labelled immunoglobulin G was transmitted as intact protein from the proximal intestine. The results suggest that no intact immunoglobulin G was transmitted from the ileum, but about 15% of the protein removed from the ileum was recovered in the whole body as degraded fragments precipitable with trichloroacetic acid. 4. These observations are discussed in the context of the transmission of antibodies, and their relevance to the receptor hypothesis is considered.

Morphological changes in the neuritic growth cone and target neuron during synaptic junction development in culture

Our object was to characterize the morphological changes occurring in pre- and postsynaptic elements during their initial contact and subsequent maturation into typical synaptic profiles. Neurons from superior cervical ganglia (SCG) of perinatal rats were freed of their supporting cells and established as isolated cells in culture. To these were added explants of embryonic rat thoracic spinal cord to allow interaction between outgrowing cord neurites and the isolated autonomic neurons. Time of initial contact was assessed by light microscopy; at timed intervals thereafter, cultures were fixed for electron microscopy. Upon contact, growth cone filopodia became extensively applied to the SCG neuronal plasmalemma and manifested numerous punctate regions in which the apposing plasma membranes were separated by only 7-10 nm. The Golgi apparatus of the target neuron hypertrophied, and its production of coated vesicles increased. Similar vesicles were seen in continuity with the SCG plasmalemma near the close contact site; their apparent contribution of a region of postsynaptic membrane with undercoating was considered to be the first definitive sign of synapse formation. Tracer work with peroxidase and ferritin confirmed that the traffic of coated vesicles within the neuronal soma is largely from Golgi region to somal surface. Subsequent to the appearance of postsynaptic density, the form and content of the growth cone was altered by the loss of filopodia and the appearance of synaptic vesicles which gradually became clustered opposite the postsynaptic density. As the synapse matured, synaptic vesicles increased in number, cleft width and content increased, presynaptic density appeared, branched membranous reticulum became greatly diminished, and most lysosomal structures disappeared. Coated vesicles continued to be associated with the postsynaptic membrane at all stages of maturation. The incorporation of Golgi-derived vesicles into discrete regions of the cell membrane could provide the mechanism for confining specific characteristics of the neuronal membrane to the synaptic region.

Membrane receptors in the specific transfer of immunoglobulins from mother to young

Transfer of immunity from mother to young takes place prenatally, postnacally, or both depending on the animal species. Where prenatal transfer occurs the fetal tissues across which the immunoglobulins pass are the yolk sac or the hemochorioplacenta. Postnatal transfer is effected via the gut of the newborn. Transfer mechanisms are discussed and evaluated as is the evidence from both in vivo and in vitro studies that support the view that membrane receptors specific for IgG and its Fc fragment mediate the transfer process. This is followed by a brief discussion of the properties of the receptors.

The effects of corticosterone and cortisone on the uptake of polyvinyl pyrrolidone and the transmission of immunoglobulin G by the small intestine in young rats

1. The distribution of polyvinyl pyrrolidone along the intestinal lumen and in the intestinal wall, following oral administration to normal and corticosterone treated rats, was found to be extremely variable. Valid comparisons between the two groups of animals could not be made using this technique. 2. Three, 4 and 5 days after corticosterone treatment there was no significant change in the uptake of 125I-labelled polyvinyl pyrrolidone from standard doses injected into ligated segments of the distal small intestine; nor did the treatment induce precocious replacement of the absorptive cells in this region. Cortisone induced precocious cell replacement, a process which took up to 4 days to complete, and also led to a marked reduction in the uptake of 125I-labelled polyvinyl pyrrolidone from ligated segments of the distal intestine. 3. Three days after treatment with corticosterone (5 mg I.P. at 12 days) there was a marked reduction of labelled immunoglobulin G transport into the blood. Four and 5 days after treatment there was some recovery of the immunoglobulin G transport function. Three days after treatment with cortisone (5 mg I.P. at 12 days) there was closure of the gut to labelled immunoglobulin G. 4. The relevance of these results to antibody transmission and the termination of immunoglobulin transport is discussed.

Protein transport: a selective membrane mechanism

Proteins are selectively sequestered by a number of cell types. However, only in oocytes is the process sufficiently aggravated and specific to be readily studied. In these cells certain serum proteins are taken up in proportions different from those found in the serum. In vitro incubations of hormonally stimulated and synchronous mosquito oocytes show that the only protein capable of initiating the transport process is the female specific yolk protein. Heterologous proteins such as IgG, bovine serum albumin, cytochrome C, and ferritin are inactive. The female specific protein is a phosphoglycolipoprotein. It is synthesized in the fat body, a liver analog in the insect, and passed into the serum before being transported into the oocytes. Preliminary kinetic analysis shows the uptake process to be specific with an apparent Km of about 10(-7) M. Glycolytic inhibitors stop protein uptake. The receptor-mediated binding steps in the transport process are most easily studied in the chicken because of the enormous amount of oocyte membrane available from a given oocyte and because up to 1 gm of protein is normally transported per day per oocyte. IgG and the hen specific phosvitin lipovitellin are two of the physiologically important proteins that are transported intact into the chicken oocytes. The uptake appears selective as shown by studies with iodinated proteins. Ferritin conjugated to IgG is shown by electron microscopy to bind to isolated plasma membranes only where coated pits have formed, whereas ferritin alone is not seen localized on any membrane surface. These very specialized regions of the membrane are similar to micropinocytotic pits but, in addition, possess on their cytoplasmic side dense ridges that form the coat. Transport involves binding to the coated pits, the pinching off of the pits, and the subsequent movement of the coated vesicles in the cytoplasm.

Role of the cell surface in selection during transport of proteins from mother to foetus and newly born

The transport of immunoglobulins from mother to foetus and newly born mammal involves selective events which are independent of molecular size, related to immunoglobulin class, structure, and species of origin, and involve considerable protein degradation. Such events are briefly described as background information to a discussion of how selection of proteins might take place during transport across the cellular barriers concerned, namely the yolk sac splanchnopleur, chorio-allantoic placenta, and small intesting. Until recently the Brambell hypothesis has been the most favoured explanation. This implies that selection occurs intracellularly, within endodermal cells of the yolk sac splanchnopleur and small intestine, and within the syncytiotrophoblast of the chorio-allantoic placenta, of certain species. It also suggests that specific receptors are present which give attached proteins protection from degradation when the vesicles containing them fuse with lysosomes; such protected proteins are then liberated from the vesicle by exocytosis. This hypothesis is examined in the light of what is now known about the mechanism of uptake and transport of proteins by the endodermal cells and syncytiotrophoblast. It is suggested that rather than being an intracellular event, involving protection from proteolytic degradation, selection takes place at the cell surface. Evidence is presented, some direct and some circumstantial, that proteins may be selectively endocytosed by coated micropinocytotic vesicles, and non-selectively endocytosed through a complex apical canalicular system leading to macropinocytotic vesicle formation. In the small intesting of the suckling rat these two processes appear to be segregated, selective uptake occurring in the proximal half and non-selective uptake occurring in the distal half. In the endodermal cells of the rabbit yolk sac splanchnopleur, and by implication in the syncytiotrophoblast of man and monkey, it is suggested that both selective, and non-selective, uptake of protein occurs. Non-selective uptake into macropinocytotic vesicles is regarded as an event leading to complete degradation of all contained protein and functioning so as to supply the foetus and newly born mammal with essential amino acids. Selective uptake into coated micropinocytotic vesicles is regarded as an event leading to the transport of immunoglobulins across the cell without any contact with lysosomes, and functioning so as to supply the newly born mammal with protection against invasive organism. Specific receptors are still required but only for the initial uptake and segregation of proteins into coated micropinocytotic vesicles. The role which the glycocalyx might have in such selective binding of proteins is considered and possible difficulties in characterization of specific receptors brought to light in view of the likely overwhelming need for non-specific binding to effect non-selective uptake.

Macromolecular transport by the middle ear and its lymphatic system

The macromolecular transport in the middle ear mucosa of the guinea pig was investigated by means of light and electron microscopy using tracer substances such as Evans blue, India ink, and horseradish peroxidase (HRP). HRP particles were readily taken up by the middle ear mucosa and Eustachian tube. They were absorbed by all cell types, including ciliated, secretory and nonsecretory cells. The particles were first taken up by the pinocytotic vesicles and then transported into the intercellular spaces by reverse pinocytosis. These particles were transported toward the connective tissue through the basilar membrane. Some of the tracer particles were engulfed by the tissue histiocytes, but most of the particles entered into lymphatic and blood capillaries. By Evans blue instillation of the bulla, the retroauricular and junctional lymph nodes were positively identified as those draining the middle ear and the Eustachian tube. When HRP was used, the tracer particles were detected in the ipsilateral retroauricular and junctional lymph nodes as early as 5 minutes following the tympanic instillation. HRP in these lymph nodes were mainly found in the specific granules of the macrophages.

Localization of secretory IgA, secretory component, and alpha chain in the mammary gland of lactating rabbits by immunoelectron microscopy

The different steps in the processing of secretory IgA (sIgA) of the mammary gland of lactating rabbits have been investigated with immunoelectron microscopy. The method employed consists of an indirect localization sequence, in which the immunologic reagents are allowed to diffuse into the fixed tissue prior to embedding and thin sectioning. In the first step, goat F(ab')2 fragments directed against sIgA, secretory component (SC), or alpha chain were used. To visualize the location of these first-step antibody fragments, a second Fab fragment directed against goat F(ab')2 antibody was prepared and coupled to a small heme octapeptide that possessed peroxidatic activity, the reaction product of which is visible by electron microscopy. Both the epithelial cell and the local plasmacytes of the mammary gland contain alpha chain, whereas SC is exclusively located in the epithelial cells. In the plasmacytes, both the cisternae of the rough endoplasmic reticulum and the saccules and vesicles of the Golgi complex contain alpha chain. In the epithelial cells, the elements of the Golgi complex and large apical vacuoles situated in the apical region of the cell. Based on these results, a model for processing of sIgA is proposed and discussed.