FINE STRUCTURE OF CELL SURFACE SPECIALIZATIONS IN THE MATURING DUODENAL MUCOSA OF THE CHICK

Contributions of Myosin Light Chain Kinase to Regulation of Epithelial Paracellular Permeability and Mucosal Homeostasis

Intestinal barrier function is required for the maintenance of mucosal homeostasis. Barrier dysfunction is thought to promote progression of both intestinal and systemic diseases. In many cases, this barrier loss reflects increased permeability of the paracellular tight junction as a consequence of myosin light chain kinase (MLCK) activation and myosin II regulatory light chain (MLC) phosphorylation. Although some details about MLCK activation remain to be defined, it is clear that this triggers perijunctional actomyosin ring (PAMR) contraction that leads to molecular reorganization of tight junction structure and composition, including occludin endocytosis. In disease states, this process can be triggered by pro-inflammatory cytokines including tumor necrosis factor-α (TNF), interleukin-1β (IL-1β), and several related molecules. Of these, TNF has been studied in the greatest detail and is known to activate long MLCK transcription, expression, enzymatic activity, and recruitment to the PAMR. Unfortunately, toxicities associated with inhibition of MLCK expression or enzymatic activity make these unsuitable as therapeutic targets. Recent work has, however, identified a small molecule that prevents MLCK1 recruitment to the PAMR without inhibiting enzymatic function. This small molecule, termed Divertin, restores barrier function after TNF-induced barrier loss and prevents disease progression in experimental chronic inflammatory bowel disease.

Chick midgut morphogenesis

The gastrointestinal tract is an essential system of organs required for nutrient absorption. As a simple tube early in development, the primitive gut is patterned along its anterior-posterior axis into discrete compartments with unique morphologies relevant to their functions in the digestive process. These morphologies are acquired gradually through development as the gut is patterned by tissue interactions, both molecular and mechanical in nature, involving all three germ layers. With a focus on midgut morphogenesis, we review work in the chick embryo demonstrating how these molecular signals and mechanical forces sculpt the developing gut tube into its mature form. In particular, we highlight two mechanisms by which the midgut increases its absorptive surface area: looping and villification. Additionally, we review the differentiation and patterning of the intestinal mesoderm into the layers of smooth muscle that mechanically drive peristalsis and the villification process itself. Where relevant, we discuss the mechanisms of chick midgut morphogenesis in the context of experimental data from other model systems.

Factors controlling the reassembly of the microvillous border of the small intestine of the salamander

Hydrostatic pressure, when applied to segments of the small intestine of the salamander, causes a tremendous reduction in number of microvilli and a loss of the terminal web. The intestinal epithelium strips off from its deeper layers at the level of the basement membrane. When the pressure is released and this epithelial sheet is allowed to recover, the microvilli and its terminal web reappear. Stages in the reformation of microvilli are described. In the earliest stages, foci of dense material seem to associate with the cytoplasmic surface of the apical plasma membrane. From this material, filaments appear and their regrowth is correlated with the extension of the microvilli. We suggest that the dense material nucleates the assembly of the filaments which, in turn, appear instrumental in the redevelopment of microvilli. This concept is supported by the existing literature. Further, since neither the microvilli nor the terminal web reappear on any surface but the apical surface, even though the apical and basal surfaces are bathed with the same medium, we suggest that information in the membrane itself or directly associated with the membrane dictates the distribution of the dense material which leads to the formation of the microvilli and ultimately to the polarity of the cell.

STUDIES ON THE ORGANIZATION OF THE BRUSH BORDER IN INTESTINAL EPITHELIAL CELLS : II. Fine Structure of Fractions of Tris-Disrupted Hamster Brush Borders

Two of the fractions obtained by density gradient centrifugation of Tris-disrupted brush borders from hamster intestinal mucosa have been identified as the microvillus cores and their surrounding membranous coats, respectively. This identification has the following morphological basis. In shadowed preparations one fraction (cores) appears as rounded, compact rods, and the other fraction (coats) appears as flattened sheets. Both rods and sheets have dimensions appropriate to the identities assigned to them. In addition, negative staining shows that the rods are composed of aligned particles of roughly 60 A, consistent with the appearance of the core in tissue section, where 60-A fibrils are characteristic. The sheets are covered by non-aligned particles of approximately the same size. Sectioned preparations show that the core fraction contains predominantly fibrous material with some membranous contamination and that the coat fraction is apparently composed exclusively of elongated sacs with a unit membrane structure. Some details of the structure of the core are evident in cases where the compact rod appears to be loosened, revealing a doubled strand. The strand is approximately 350 A wide; the compact rod is roughly twice this width. With negative staining the strand shows a dense central region. The morphological identification presented here is consistent with the distribution of enzymic activity among the density gradient fractions described in the preceding paper.

Cotransport systems in the brush border membrane of the human placenta

The cotransport systems present in the brush border membrane of the human placental syncytiotrophoblast are reviewed. Attention is focused on the systems that are powered by the electrochemical gradient of Na+ (for example, for neutral amino acids but not for glucose), and on recently described systems in which K+ flux is coupled to C1- flux or to that for Na+ and C1-. These systems are similar to those found in other tissues and may be significant for net trans-placental transport and its control, as well as for the regulation of placental trophoblast volume.

Enterocyte dynamics and mucosal development in the posthatch chick

Changes in the morphology of the small intestinal mucosa and enterocyte dynamics were examined in posthatch chicks through 12 d. At hatch, enterocytes were round and nonpolar; however, within 24 h posthatch, enterocytes lengthened and exhibited more typical morphology. Crypts were rudimentary at hatch and by 48 h invagination was completed and crypt numbers increased by branching and fission, with the number of crypts per villus reaching plateau after 72 h posthatch. All epithelial cells were proliferative at hatch. In the crypts, the proportion of proliferating enterocytes decreased to 50 to 60% within 2 d posthatch, whereas along the villus the proportion of proliferating cells decreased to 10 to 20% by 6 d. Different patterns of temporal development of villi were observed in the duodenum, jejunum and ileum. Individual villus surface area increased steadily in the duodenum throughout the experiment, whereas individual jejunal and ileal villus surface areas increased more slowly after 4 d posthatch. The number of villi per cross-section of intestine increased in the duodenum and jejunum but not in the ileum. The total segment villus surface area increased similarly in all segments until 3 d posthatch, after which the jejunum increased considerably in absorptive area, whereas the duodenum and ileum increased more slowly. This study shows that, in the hatching chick, the small intestine matures in a manner similar to neonatal mammals, with specific ontogenetic timetables in the different small intestinal segments, however, the most dramatic changes occur within the first 24 h posthatch.

Small intestinal development in the young chick: crypt formation and enterocyte proliferation and migration

1. Post-natch mucosal development was examined in the chick small intestinal epithelium using immunostaining with proliferating cell nuclear antigen (PCNA) and 5-bromo-2-deoxyuridine (BrdU). 2. On the day of hatching jejunal crypts were small and a single crypt per villus was observed. However, during the 108 h post-hatch crypts developed rapidly branching and increasing in size, cell numbers and cell size. 3. Almost all epithelial cells in the small intestine of the hatching chick were proliferating, as indicated by PCNA and BrdU, while more than 80% of proliferating cells were localised in the crypts after 108 h post hatch. 4. Estimate of villus cell transit time using BrdU was only possible from 48 h post-hatch when villus transit time was 72 h in the jejunum, whereas at 336 h transit time was 96 h. 5. In the 108 h post hatch a rapid transition occurs from total jejunal epithelial cell proliferation and immature crypts to a defined proliferative zone in the crypts, with constant division and migration.

Kinetic constants of alpha-methyl-D-glucoside transport in the chick small intestine during perinatal development

The kinetic parameters of alpha-methyl-D-glucoside (alpha Glc1Me) have been determined in the small intestine in order to establish developmental and regional changes in the apical transport in embryos and newly hatched chicks. Results show that the apparent Michaelis constant (Km) values did not change during the period studied in each region of the small intestine. However, the ileum showed a smaller Km than that of the duodenum and jejunum, indicating an increase of the affinity of the carrier in the distal portion of the intestine and a significant contribution of the ileum to overall sugar absorption in the perinatal period. There were important changes in the diffusional component and in the mediated transport system capacity during this period, as well as in the different regions of the intestine. Significant increases in the maximal rate of transport (Vmax) were observed in all regions during the embryonic period until the second day after hatching, followed by a decline during the first week. During all the period studied, Vmax values from the jejunum were significantly greater than those from both the duodenum and the ileum at every age studied showing that the jejunum is the segment that is best suited for Na(+)-mediated uptake. Such changes which occur when the need for nutrients for rapid development are at their highest are not solely a result of diet composition, but rather in accordance with a genetic programme.

Uptake capacity in vitro for glucose and methionine and in situ for oleic acid in the proximal small intestine of posthatch chicks

The uptake capacity of the duodenum and jejunum was determined in vitro for glucose and methionine using 14C-inulin as an unabsorbed reference substance in posthatch chicks. Fatty acid uptake in situ was also determined in the duodenum. Methionine uptake capacity increased in both duodenum and jejunum between hatch and 7 d and was constant between 7 and 14 d. Calculation of apparent Michaelis constant (Kt) for both substrates indicated little change with age and thus the amount of carrier in the intestine may be increasing. Uptake capacity of the duodenum for glucose increased between hatch and 7 d of age but no changes were found between 7 and 14 d. In the jejunum, uptake capacity was constant from hatch to 14 d of age. In situ uptake of oleic acid in the duodenum did not change between hatch and 14 d of age. These results suggest that intestinal uptake capacity changes little after 7 d of age, and thus feed intake may be the major factor controlling nutrient uptake in chicks.

Development of the small intestine in heavy and light strain chicks before and after hatching

1. Intestinal development was examined in Arbor Acres and Lohmann chicks from one week before hatching until one week after. Changes in morphology and concentrations of DNA, RNA and protein in the duodenal tissue were determined. 2. Villus height and perimeter increased 9 to 11 fold from day 14 of incubation until 7 d after hatching. Arbor Acres chicks had values which were consistently higher than Lohmann chicks. 3. DNA concentration of duodenal tissue increased with age in parallel to the increase in the number of enterocytes per villus. In the pre-hatch period tissue activity as indicated by RNA/DNA, and ribosomal capacity as shown by the RNA/protein ratios, were high for both strains; values for Arbor Acres embryos and chicks were greater than for Lohmann. 4. DNA concentrations, RNA/DNA, RNA/protein and protein/DNA ratios correlate with morphological measurements and can be used as additional criteria for evaluating development in chick intestine. 5. In the last week of incubation and immediately after hatching, intestinal growth appears to arise mainly from cellular hyperplasia and not from cellular hypertrophy. Intestinal development patterns were similar for both strains but growth was more rapid in Arbor Acres chicks.

Cell apical surface area in enterocytes from chicken small and large intestine during development

The absorptive surface of epithelial cells from chicken small and large intestine was studied at the day of hatch (1 d group) and at 2 and 6 wk after hatch. The segments considered were duodenum, jejunum, ileum, cecum (proximal, medial, and distal regions), and rectum. The length, diameter, and density of microvilli as well as cell apical diameter were measured in tip-villous enterocytes by transmission electron microscopy. The results obtained showed that during development: 1) microvillus length remained constant in duodenum and jejunum and decreased in the other segments; 2) microvillus diameter increased only in the jejunum and the rectum; 3) microvillus density increased in duodenum, ileum, distal cecum, and rectum (especially from 1 d to 2 wk) and did not change in the other segments; 4) cell apical diameter did not change; 5) apical surface area increased both in the duodenum (2nd to 6th wk) and in the jejunum (1 d to 2 wk) but did not change in the ileum. In the proximal-medial cecum and in the rectum there was a decrease in apical surface, whereas no changes were observed in distal cecum. Results indicated that microvillus length and density are the variables that best explain the changes observed in apical surface that occurred during development.

Posthatch changes in morphology and function of the small intestines in heavy- and light-strain chicks

The morphology of the small intestines of heavy (Arbor Acres) and light (Lohman) chicks was determined posthatch in parallel with digestion, enzyme secretion, and passage time. Villus height and volume increased from 4 to 10 d, particularly in the jejunum and ileum. The number of enterocytes per villus increased with age, but enterocyte density was greater in jejunum than ileum. Villus volume and enterocyte density was greater in Arbor Acres than Lohman chicks from hatching and the rate of change with age was similar in both strains. Enzyme secretion to the duodenum was higher per gram of feed intake in heavy-strain birds on Day 4 after hatching but thereafter no differences were apparent. Passage time was 50% shorter in light-strain chicks on Day 4 but the difference between the strains was not significant from Day 10. No differences in fatty acid digestion were observed with age or between strains. Nitrogen digestion increased in both strains from approximately 70% on Day 4 to close to 90% on Day 14. Starch digestion was 90 to 95% from 4 to 14 d in Arbor Acres birds, but increased from approximately 80% on Day 4 in Lohman chicks to 93% on Day 14. Feed intake may determine the amount of uptake in posthatch chicks.

Structural and compositional analysis of early stages in microvillus assembly in the enterocyte of the chick embryo

Morphological and immunocytochemical techniques were used to examine the distribution of villin, with respect to actin, during the early events of brush border morphogenesis in the embryonic chicken intestine. Immunolocalization studies indicate that actin and villin exist as a cortical array in the apical domain of embryonic enterocytes at a time when few surface microvilli are visible by scanning and transmission electron microscopic techniques. A population of villin is also localized at the level of the junctional complex. With time, the density of microvilli increases and the cells begin to flatten. In these cells, villin is detected in the newly formed microvilli and also in the subjacent cortex, where microvillar rootlets are beginning to appear. The significance of actin-villin associations in the process of brush border assembly is discussed in the light of the functional properties of villin.

Assembly of the brush border cytoskeleton: changes in the distribution of microvillar core proteins during enterocyte differentiation in adult chicken intestine

The assembly of the intestinal microvillus cytoskeleton was examined during the differentiation of enterocytes along the crypt-villus axis in adult chicken duodenum using light and electron microscopic immunolocalization techniques. Using antibodies reactive with villin, fimbrin, and the heavy chain (hc) of brush border (BB) myosin I (110K-calmodulin complex) and rhodamine-conjugated phalloidin as a probe for F-actin, we determined that while actin, villin, and fimbrin were all localized apically along the entire axis, BB myosin I (hc) did not assume this localization until the crypt-villus transition zone. In addition to their localization at the BB surface, all four proteins were present at significant levels along the lateral margins of enterocytes along the entire crypt-villus axis, suggesting that these proteins may be involved in the organization and function of the basolateral membrane cytoskeleton as well. The pattern of expression of the microvillar core proteins along the crypt-villus axis in the adult was comparable to that seen in the intestine of the late stage chicken embryo and suggests that a common program for brush border assembly may be used in both modes of enterocyte differentiation.

Intestinal crypt stem cells possess high levels of cytoskeletal-associated phosphotyrosine-containing proteins and tyrosine kinase activity relative to differentiated enterocytes

Growth and differentiation of stem cells is thought to be regulated by growth factors and responding protein tyrosine kinase activities. Comparing mitotic stem cells from the adult intestinal epithelium, isolated from the crypts of Lieberkuhn, with isolated differentiated absorbtive cells we find major differences in the levels of phosphotyrosine-containing proteins. Crypt stem cells possess two major phosphotyrosine-containing polypeptides of 36 and 17 kD which have greater than 15 times more phosphotyrosine than that present in the polypeptides of differentiated enterocytes. Tyrosine kinase activity and similar phosphotyrosine-containing proteins are associated with the Triton cytoskeleton. Moreover, crypt tyrosine kinase(s) is active in vitro in phosphorylating similar cytoskeleton-associated substrates. These results suggest that cytoskeleton-associated phosphotyrosine kinase(s) and their substrates may play a role in growth and differentiation of adult intestinal epithelial cells.

The ultrastructure of fiber cells in primate lenses: a model for studying membrane senescence

We have compared the surface morphology of the youngest (cortical) fiber cells with that of the most senescent (nuclear) fiber cells in monkey and baboon crystalline lenses by stereo scanning electron microscopy (SEM) and thick-section stereo transmission electron microscopy (TEM). Both the broad and the narrow faces of the most senescent fiber cells featured distinctive, polygonal areas (domains) of furrowed cell membrane. The domains ranged in size from 2.42 to 8.78 microns2. Stereopair SEM and TEM micrographs demonstrated precisely oriented microvilli measuring approximately 0.14 micron in diameter and ranging in length from 1.27 to 4.65 microns overlying each ridge in the domains. Formation of microvilli on senescent cells has been noted in other types of aging cells but they are imprecisely arranged and their function is unknown. Since every fiber cell remains in a fixed location (relative to other fiber cells) throughout life, the lens provides a unique model to study structure-function relationships of senescent microvilli in situ. The discovery of an age-related elaboration of numerous microvilli on senescent fiber cells of noncataractous lenses invalidates the currently accepted theory that close, parallel apposition of the broad faces of lens fiber cells is necessary for the lens to be transparent.

Phosphotyrosine-modified proteins are concentrated at the membranes of epithelial and endothelial cells during tissue development in chick embryos

We have used high affinity polyclonal antibodies specific for phosphotyrosine (PTyr) residues to examine the localization in various chick embryonic tissues in situ of PTyr-modified proteins by immunocytochemical methods. During the period from 9 to 21 d of development, most tissues exhibit elevated levels of PTyr-modified proteins as determined by immunoblotting experiments of tissue extracts with the anti-PTyr antibodies (Maher, P. A., and E. B. Pasquale. 1988. J. Cell Biol. 106:1747-1755). By immunofluorescence labeling of semithin frozen sections, the highest concentrations of PTyr immunolabeling in all of the embryonic tissues examined were localized to the membranes of the epithelial and endothelial cells with other cells showing no detectable labeling. These results were confirmed by immunoelectron microscopic labeling, which showed particularly high concentrations of PTyr-modified proteins close to the membranes at the apical junctions. The corresponding adult tissues showed no labeling. It is proposed that these results reflect the molecular basis for the functional plasticity of epithelial and endothelial cell junctions during embryonic development.

Developmental organization of the intestinal brush-border cytoskeleton

At the terminal web of chicken intestinal epithelial cell, the actin bundles are cross-linked by a fine filamentous network of actin-associated cross-linkers. Myosin, fodrin, and TW 260/240 have been identified as major components of the cross-linkers. We studied the development of the cross-linkers by quick-freeze, deep-etch electron microscopy, and the expression of cross-linker proteins (myosin, fodrin 240, and TW 260) by immunofluorescence and immunoblotting analysis during the embryogenesis. Microvilli start to form at 5-7 days, and the rootlets begin to elongate at 10 days. At an early stage of the development of the terminal web (13 days), fodrin 240 and a small amount of myosin are expressed, and a few actin-associated cross-linkers are present between the rootlets. However, TW 260 is not expressed at this stage. At an intermediate stage (19 days), the amount of myosin increases, and TW 260 begins to be expressed. The number of cross-linkers associated with the unit length of the rootlets is 24/microns. At the final stage of the terminal web formation (2 days after hatching), the amount of fodrin 240, myosin, and TW 260 is similar to the adult level, and the number of the actin-associated cross-linkers per unit length of the rootlet is 27/microns (approximately 85% of the adult). These results suggest that the synthesis of cross-linker proteins may be intricately regulated to achieve the desired density of cross-linkages at each developmental stage: at early and intermediate stages, sufficient and not an excess of cross-linkages are formed; and at a final stage, a higher complexity of cross-linkages is achieved. In addition, there is a differential expression of the components of the actin-associated cross-linkers: myosin and fodrin could be early components of the cross-linkers involved in the basic stabilization of the terminal web structure, whereas TW 260/240 becomes incorporated later, possibly involved in the stabilization preparatory to the rapid elongation of microvilli, which occurs after the formation of the terminal web.

Development of glucose active transport in embryonic chick intestine. Influence of thyroxine and hydrocortisone

1. Glucose active transport is detectable in 12-day-old embryonic chick duodenum and increases at least 11-fold after 4 days of postnatal life. 2. Glucose active transport develops at the in vivo rate in 72-hr cultures of 14-day embryonic duodenum. 3. In the presence of either 1 nM thyroxine or 1 microM hydrocortisone in vitro, glucose active transport reaches levels approximately 200% of control values (equivalent to 18-19 day levels in vivo). 4. Thyroxine and hydrocortisone act by different mechanisms based on their antagonistic interaction and differences in time course of action, requirement for protein synthesis and modulation by extracellular calcium.

Assembly of the intestinal brush border: appearance and redistribution of microvillar core proteins in developing chick enterocytes

The assembly of the intestinal microvillus cytoskeleton during embryogenesis in the chick was examined by immunochemical and light microscopic immunolocalization techniques. For these studies, affinity-purified antibodies reactive with three major cytoskeletal proteins of the adult intestinal microvillus, fimbrin, villin, and the 110-kD subunit of the 110K-calmodulin protein complex were prepared. Immunocytochemical staining of frozen sections of embryonic duodena revealed that all three proteins were present at detectable levels at the earliest stages examined, day 7-8 of incubation (Hamilton/Hamburger stages 25-30). Although initially all three proteins were diffusely distributed throughout the cytoplasm, there was a marked asynchrony in the accumulation of these core proteins within the apical domain of the enterocyte. Villin displayed concentrated apical staining by embryonic day 8 (stage 28), while the apical concentration of fimbrin was first observed at embryonic day 10 (stage 37). Diffuse staining of the enterocyte cytoplasm with the anti-110K was observed throughout development until a few days before hatch. By embryonic day 19-21 110K staining was concentrated at the cell periphery (apical and basolateral). The restricted apical localization characteristic of 110K in the adult brush border was not observed until the day of hatching. Immunoblot analysis of whole, solubilized embryonic duodena confirmed the presence of 110K, villin, and fimbrin throughout development and indicated substantial increases in all three proteins, particularly late in development. Immunoblot staining with anti-110K also revealed the presence of a high molecular mass (200 kD) immunoreactive species in embryonic intestine. This 200-kD form was absent from isolated embryonic enterocytes and may be a component of intestinal smooth muscle.

Regulation of intestinal brush border microvillus length during development by the G- to F-actin ratio

We examined ultrastructural changes in developing chicken intestinal microvilli and correlated these with changes in the G- to F-actin ratio and the amount of actin per milligram cell protein. Three discrete morphological and temporal changes occur during late microvillus morphogenesis: an increase in microvillus number associated with microvilli becoming hexagonally packed on the cell surface; an increase in core actin filament number; and an increase in the length of microvilli. Dramatic rises in the amount of cell actin occur at the time of the first two morphological changes. Changes in the G- to F-actin ratio suggest that increases in the level of monomeric actin drive the elongation phase of microvillus growth since immediately prior to growth the G- to F-actin ratio shifts from its embryonic and adult 3:7 ratio to a 1:1. Our results also indicate, but do not prove, that an increase in the amount of G-actin precedes the rise in level of F-actin and growth of microvilli by 1 day, implying that an increase in the content of G-actin stimulates actin polymerization. Our findings also suggest that the G- to F-actin ratio and their absolute amounts, perhaps in combination with cytoskeletal protein turnover and/or the pool size of actin binding proteins, plays a role in restricting the mature constant length of microvilli.

The cellular basis of epithelial morphogenesis

Epithelial tissues are ubiquitous in metazoan organisms, performing many different functions and assuming a variety of shapes. This diversity of form and function is ultimately dependent on the behavior of the cells within the epithelia. For example, it is intercellular adhesion and the control of paracellular permeability by cell junctions that permit epithelia to form barriers and act as selective filters. It is cellular polarity that enables absorptive epithelia to extract materials from a particular side of the sheet; it is the collective contributions of cell proliferation, cellular translocation, and changes in cell shape that sculpt epithelia from simple sheets into folds, pouches and tubes. Clearly, a complete understanding of epithelial morphogenesis is inextricably entwined with questions of cell behavior in general, such as how any cell adheres, moves, and maintains its shape. The study of epithelial systems has lent considerable insight into these problems and should continue to do so, just as examination of the behavior and architecture of nonepithelial cells will undoubtedly clarify many aspects of the cellular events underlying epithelial morphogenesis. Although the action of individual cells ultimately shapes epithelial, coordination of that action is necessary for the development of a coherent tissue. Attention must therefore be given to integrative mechanisms in epithelial morphogenesis. How do the many cells in an epithelial sheet act in virtual unison during folding? What defines the boundaries of epithelial invaginations? How does an individual cell detect its position within, and thereby know its role in the morphogenesis of, the epithelial whole of which it is a part? At the most elementary level, epithelial cells interact via their physical attachments to one other. Even such rudimentary communication affects cell shape, movement, and possibly proliferation and also plays a part in the maintenance of epithelial polarity. Additional signals pass among epithelial cells by a number of other mechanisms as well, most notably electrical coupling. However, many questions remain regarding the quality and quantity of what is communicated between epithelial cells. Accordingly, elucidating the means by which supracellular order is maintained in epithelial tissues may still be regarded as the major problem in the study of epithelial morphogenesis.

Supplemental thyroid hormones and hatchability of turkey eggs

Fertile turkey eggs were injected with thyroid hormones to test the effects of exogenous hormones on hatchability. Physiological doses of thyroxine (T4) and triiodothyronine (T3) of 50 and 25 ng, respectively, injected at setting, depressed hatchability. Conversely, injections of T4 and T3 at 25 days of incubation significantly (P less than .05) improved hatchability. Injections with thyroid-stimulating hormone (TSH) or tyrosine had no significant effect on hatchability when injected at setting or at 25 days of incubation. Injections of thyroid-releasing hormone (TRH) had no effect on hatchability when injected at setting but significantly (P less than .05) improved hatchability when injected at 25 days of incubation. Because T4 and T3 significantly (P less than .05) depressed hatchability when injected whereas TSH and TRH had no effect on hatchability when injected at setting, these data suggest classic negative feedback mechanisms function very early in the embryonic turkey. The improvement in hatchability observed when thyroid hormones were injected at 25 days of incubation offers further evidence that hypothyroidism may be a cause of poor hatchability among turkey eggs.

Development of sucrase in the chick small intestine

Development of sucrase in the chick small intestine was studied biochemically and immunologically using antiserum prepared against purified chick intestinal sucrase. Sucrase activity was first detectable at 10 days of incubation and increased with age. After a transient drop at 20 days, the activity rapidly increased to the adult level. Immunodiffusion and polyacrylamide gel electrophoretic studies suggested that the sucrase of the embryonic and hatched chick intestines was identical except for a difference in the content of sialic acids. In immunofluorescence and immunoelectron microscopy, sucrase was found to appear on the luminal surface of epithelial cells at 8-10 days of incubation, soon after the start of morphological differentiation from an undifferentiated thick epithelium to a thin simple epithelium.

Epithelial locomotion and differentiation in frog skin cultures

Small explants from the medioventral skin of the green frog were maintained in culture for 5 days. During the first hours, a rapid outgrowth of the stratum germinativum was observed at the periphery of the fragment (2 X 3 X 0.75 mm). The Malpighian cells stretched and emitted long lamellipodia following the cut edges of the dermis. These cells acquired a fibroblastic shape and were covered by other flattened cells from the stratum spinosum and even from the stratum granulosum. This progression of cells simulated an epiboly; cell divisions occurred and were revealed by autoradiography. The underlying dermis could be totally covered after 3 days. An increase in the number of cellular layers was observed. The migrating cells redifferentiated, in particular at the lower side of the dermis, which provided a suitable substratum for the newly formed epidermis. A new basal lamina was formed. Some exfoliations of keratinized layers were seen. After 5 or 6 days of culture, epiboly was complete, the epithelium formed a closed system, and degenerative processes occurred, probably by non-penetration of the nutritive medium into the deeper regions of the explant.

Fodrin is the general spectrin-like protein found in most cells whereas spectrin and the TW protein have a restricted distribution

Spectrin and related proteins are made up of a common calmodulin-binding subunit tightly associated with a variant subunit. We have analyzed the distribution of the variant subunits in various cell types using subunit-specific antibodies in immunofluorescence as well as western blotting and in some cases have compared the subunits by two-dimensional peptide mapping. We have found that in the majority of cell types (lymphocytes, hepatocytes, neurons, fibroblasts) fodrin 235 K is present in the absence of the other two variant subunits, spectrin 220 K and TW260. Two cell types were found (skeletal muscle and erythrocytes) which contained only the spectrin variant. Two cell types display two distinct variant subunits. Both fodrin 235 K and spectrin 220 K are detected in cardiac muscle whereas TW260 is present in addition to fodrin 235 K in intestinal epithelial cells. During the early stages of embryonic development of the chicken intestine, fodrin 235 K is expressed in the epithelial cells whereas TW260 and spectrin are not detectable. TW260 is expressed relatively late in development (15-16 days) and is inserted only in the apical (brush border) membrane compartment whereas fodrin 235 K is present in these same cells and underlies the entire plasma membrane. These results suggest that fodrin provides the general linkage system between microfilaments and the membrane in nonerythroid and nonmuscle cells.

Vitamin D stimulates (Na+ + K+)-ATPase activity in chick small intestine

Vitamin D3 and 1,25-dihydroxyvitamin D3 raise (Na+ + K+)-ATPase activity (ouabain-sensitive 86Rb+ uptake) in cultured embryonic and 4-week-old chick small intestine. Vitamin D stimulation of the sodium pump, which requires genomic action of the sterol, may lead to enhanced Ca2+ extrusion via a basolateral Na+/Ca2+ exchange mechanism, and, in addition, may provide a proliferative signal in undifferentiated enterocytes.

Inductive properties of fibroblastic cell cultures derived from rat intestinal mucosa on epithelial differentiation

The present study represents a first attempt to elucidate the regulatory properties displayed by the non-epithelial portion of the intestinal mucosa, growing as fibroblasts in monolayer cultures. Thus, we compared the inductive action of 6-day suckling rat duodenal fibroblasts with that displayed by chick embryonic intestinal mesenchyme on the heterotypic cytodifferentiation of 5 1/2-day chick embryonic gizzard endoderm. The latter, isolated by 0.03% collagenase, was surrounded by intestinal intramucosal fibroblastic cell sheets. As control experiments, fibroblastic cells derived from the intestinal muscle or from 20-day fetal rat skin and lung were used. Every type of association was grafted into the coelomic cavity of 3-day chick embryos for 11 to 12 days, a system providing their vascularization and growth. The results clearly demonstrate that the mucosal fibroblastic cells of rat intestine were as potent as embryonic intestinal mesenchyme in inducing brush-border enzymes like sucrase and maltase, in conformity with an induced intestinal morphology. In contrast, the control fibroblastic cells were completely ineffective.

Development of the crayfish retina: a light and electron microscopic study

The development of the crayfish retina was examined in embryos and first, second and third instars with both and light and electron microscope. Light microscopic observations indicate that differentiation begins at the posterior portion of the optic disc and progresses in an anterior direction. Development of screening pigment, dioptric elements, and rhabdoms all parallel this posterior to anterior gradient in the retina. Tracer studies in early embryos reveal that the retina is separated from the proximal neuropil regions by a distinct vascular space. This observation suggests that the source of new cells for the retina may not be the more proximal cell proliferation zone as previously indicated. It is proposed that mitotic activity within the retina and/or differentiation of cells from the anterior surface layer of the eye may be sources for addition of new cells to the retina. Proto-ommatidial clusters of seven retinula cells occur very early at the posterior region of the embryonic retina. Initially the receptor cells extend throughout the entire thickness of the retina, but later they withdraw from beneath the cornea to occupy only the proximal portion of the retina. Microvilli of the rhabdom arise from the centrally opposed membranes of the retinula cells in each cell cluster. Each new microvillus contains a core of fine filaments which extend out into the cytoplasm at its base. As development of the microvilli continues, the core filaments appear to be lost or altered, but the cytoplasmic bundles at the base of the microvilli persist.

Developmental aspects ofL-valine uptake by mouse intestine

¹⁴C-L-Valine uptake by intestinal segments of mice of various ages, ranging between 20-day fetuses and adults, was studied in vitro. 1 mML-Valine was accumulated against a concentration gradient by processes which showed saturation kinetics. There appeared to be a two-fold increase ofL-valine accumulation after the 2nd postnatal day and a three-fold increase in adult mice. Fetal transport of valine only occurred at pH 7.4 but was not Na⁺ dependent. In contrast, valine transport became increasingly Na⁺ dependent and the pH optimum widened, ranging between 5-8. A series of amino acids, including representatives of the imino acid and dibasic groups, failed to inhibit valine uptake while leucine and isoleucine manifested mutual inhibition with valine. It is speculated that in the mouse intestine,L-valine is transported by at least two mechanisms, one functioning in the fetus, not requiring Na⁺, but pH dependent and another which developes postnatally, is Na-dependent and functions over a wide pH optimum.

Selection of a chemically defined medium for culturing fetal mouse small intestine

We evaluated six commercially available tissue culture media in their capacity to support villi morphogenesis and enterocyte differentiation during duodenal development of the fetal mouse in vitro: McCoy's 5A, Medium 199, Swim's S77, Trowell T8, Leibovitz L-15, and RPMI-1640. The duodenal segments were resected at 15 d gestation, before the formation of intestinal villi. In the segments cultured with the first four media, no villi differentiated even at 72 h culture. The number of epithelial cells per transverse section of the explants did not increase at 24 h and thereafter the number of epithelial cells decreased, except with McCoy's 5A. With the Leibovitz and RPMI media, rudimentary villi differentiated at 24 h of culture and they attained their longest length at 48 h. With the RPMI medium, the number of epithelial cells doubled at 24 h of culture and with Leibovitz medium it doubled at 48 h. At the fine structural level absorptive cells remained poorly differentiated with all the media studied. Goblet cells were easily identified after 24 h culture; they had a well developed rough endoplasmic reticulum and numerous mucous granules. Endocrine cells differentiated in culture and they were loaded with secretion granules. It was concluded that the small intestine of the fetal mouse can be kept in organ culture for at least 72 h. Full maturation of absorptive cells seemed to require some additional factor(s) as they remained poorly differentiated with all the media studied. Because well differentiated endocrine cells were present in all the explants, it appeared that gastrointestinal hormones do not affect villi morphogenesis and absorptive cells differentiation.

Establishment of regional differences in brush border enzymatic activities during the development of fetal mouse small intestine

In order to study the establishment of regional differences in brush border enzymic activities during the development of fetal mouse small intestine we have followed (1) the differentiation of microvilli by morphometry, and (2) the developmental pattern of three brush border enzymes (lactase, glucoamylase and alkaline phosphatase). From day 16 to day 19 of gestation, the height of duodenal microvilli increases 2.4 times on the absorptive cells located near the tip of the villi. During the same period in the upper half of the duodenal villi, the number of microvilli per square micrometer rises by a factor of 2.4 and the microvillous surface area increases by a factor of 5.2. The differentiation of ileal microvilli follows a similar pattern but they are always shorter and less numerous than those of the duodenum. Lactase activity appears at 18 days of gestation; the other two brush border enzymes are first detected at 16 days of gestation. Afterwards all three enzyme activities increase rapidly and a decreasing gradient of activity is established from the proximal to the distal segment of the small intestine. Hence, the structural development of the microvilli and the appearance of brush border enzyme activities occur simultaneously and a proximo-distal gradient is already established at 16 days of gestation.