Postnatal growth and differentiation in three hindlimb muscles of the rat. Characterization with biochemical and enzyme-histochemical methods

Human tissue-engineered skeletal muscle: a novel 3D in vitro model for drug disposition and toxicity after intramuscular injection

The development of laboratory-grown tissues, referred to as organoids, bio-artificial tissue or tissue-engineered constructs, is clearly expanding. We describe for the first time how engineered human muscles can be applied as a pre- or non-clinical model for intramuscular drug injection to further decrease and complement the use of in vivo animal studies. The human bio-artificial muscle (BAM) is formed in a seven day tissue engineering procedure during which human myoblasts fuse and differentiate to aligned myofibers in an extracellular matrix. The dimensions of the BAM constructs allow for injection and follow-up during several days after injection. A stereotactic setup allows controllable injection at multiple sites in the BAM. We injected several compounds; a dye, a hydrolysable compound, a reducible substrate and a wasp venom toxin. Afterwards, direct reflux, release and metabolism were assessed in the BAM constructs in comparison to 2D cell culture and isolated human muscle strips. Spectrophotometry and luminescence allowed to measure the release of the injected compounds and their metabolites over time. A release profile over 40 hours was observed in the BAM model in contrast to 2D cell culture, showing the capacity of the BAM model to function as a drug depot. We also determined compound toxicity on the BAMs by measuring creatine kinase release in the medium, which increased with increasing toxic insult. Taken together, we show that the BAM is an injectable human 3D cell culture model that can be used to measure release and metabolism of injected compounds in vitro.

Differences in lectin-binding properties between the common mucosal epithelium and follicle-associated epithelium in the rabbit small intestine

Differences in sugar distribution between the villous epithelium and follicle-associated epithelium (FAE) were compared using lectins in the rabbit small intestine. In every portion, villous columnar epithelial cells primarily exhibited a positive reaction to the GalNAc, GlcNAc, galactose, and oligosaccharide. In the ileal Peyer's patch (PP), whereas microvillous epithelial cells exhibited positive reactions, M cells tended to be negative. The villous epithelial reaction to the fucose group was negative, but M cells and microvillous epithelial cells showed a positive to the fucose. No epithelium had a positive reaction to the mannose and glucose. The variety of lectin-binding properties of villous epithelial cells and M cells may reflect specificity for the recognizing luminal substances such as antigenic molecules and bacterial elements.

Mechanical components of rat intestinal villi as revealed by ultrastructural analysis with special reference to the axial smooth muscle cells in the villi

The ultrastructure of the rat intestinal interstitium with regard to the mechanical components was analyzed from a functional viewpoint utilizing serial horizontal as well as longitudinal sections through the lamina propria mucosae, including both villi and crypts. The axial smooth muscle cells in the villi (villus-axial SMs) exhibited different configurations at various levels of the wall. They were separated from the voluminous fluid-filled spaces by sheet-like processes of fibroblasts in the upper part of the intravillous interstitium, formed a sheet around the central lymphatics, and were covered by the sheet-like processes of fibroblasts in the lower part of the intravillous interstitium. These villus-axial SMs were poorly developed and associated with the lymphatic walls in the upper part of the pericryptal interstitium; they were tapered and connected to microtendons composed of fascicles of longitudinal collagen fibrils in the lower part of pericryptal interstitium. At the apical termination, the villus-axial SMs were connected to myofibloblasts, which sent off many processes into the subepithelial meshwork layer of fine cell processes and extracellular matrices. The villus-axial SMs possibly develop longitudinal tension against the intravillous hydraulic pressure developing from the transepithelial absorption through the intestinal epithelium.

Presence of (phospho)creatine in developing and adult skeletal muscle of mice without mitochondrial and cytosolic muscle creatine kinase isoforms

We assessed the relationship between phosphocreatine (PCr) and creatine (Cr) content and creatine kinase (CK) activity in skeletal muscle of mice. The PCr and total Cr (tCr) concentrations, as well as CK activity, in hindlimb muscles of mice, with or without the cytosolic and mitochondrial isoforms of muscle creatine kinase (wild-type or CK--/-- mice), were determined by in vivo magnetic resonance (MR) spectroscopy and by biochemical means during postnatal growth and adulthood. In wild-type muscle the [tCr], PCr/ATP ratio and CK activity increased rapidly in the first 4-7 weeks. Remarkably, CK--/-- mice showed a similar increase in the PCr/ATP ratio during the first month in the presence of only minor brain-type BB-CK activity. Uptake of Cr in muscle was seemingly unrelated to CK activity as tCr increased in the same way in the muscles of both mouse types. At older ages the PCr/ATP ratio decreased in CK--/-- muscles, in contrast to wild-type where it still slowly increased, whereas [tCr] was similar for muscle of both mouse types. Using a new in vivo MR approach with application of [4-13C]Cr, a lower PCr/tCr ratio was also observed in CK--/-- muscle. From these data it follows that in vivo global ATP levels at rest are similar in the presence or absence of CK. Although Cr could still be converted to PCr in mature CK--/-- muscle, the immediate availability of PCr decreased, and PCr became partly inconvertible at older ages. Apparently, catalysis of the CK reaction by BB-CK, although significant in muscles of newborn mice, gradually declines to very low levels in adulthood. Part or all of this BB-CK may arise from satellite cells fusing with myotubes, a process that is most active during the first months of life. Finally, our observation that the MR and chemical assessment of muscle [tCr] and PCr/tCr ratio were similar for all mice does not support the existence of a significant MR-invisible or immobile pool of Cr, with a role for CK in this phenomenon.

The organization of the communication routes between the epithelium and lamina propria mucosae in the human esophagus

Morphological studies examined communication routes between the epithelium and lamina propria mucosae in the human esophagus, using a series of techniques including silver staining, immunohistochemistry, transmission electron microscopy, and scanning electron microscopy (SEM). For SEM, tissue blocks were treated with either osmium/ultrasonication or NaOH. Observations showed the esophageal papillae to be arranged regularly in a mostly longitudinal row. The reticular fibers, consisting of fibrils approximately 40 nm in diameter, were situated just beneath the epithelial basal lamina. They showed a positive reaction with a type III collagen antibody, and formed a continuous sheet 2-3 microm thick with dense networks. This sheet as well as the epithelial basal lamina had numerous foramina of diameters of 3-5 microm. Immune cells such as lymphocytes and Langerhans cells were situated around these foramina. The foramina were situated both around papillae and the duct orifice of the esophageal gland. In addition, lymphoid follicles surrounded the duct of the esophageal gland. The structural characteristics around the duct appear to be those of duct-associated lymphoid tissue (DALT). Thus, these foramina in the epithelial basal lamina and reticular fiber sheet may represent important communication routes between the epithelium and lamina propria mucosae. In addition, they may play an important role in the mucosal immune response in the human esophagus.

Differential maturation of motoneurons innervating ankle flexor and extensor muscles in the neonatal rat

The first postnatal week is a critical period for the development of posture in the rat. The use of ankle extensor muscles in postural reactions increases during this period. Changes in excitability of motoneurons are probably an important factor underlying this maturation. The aim of this study was to identify whether variations in the maturation exist between motor pools innervating antagonistic muscles. Intracellular recordings in the in vitro brain stem-spinal cord preparation of neonatal rats (from postnatal day 0-5) were used to examine the developmental changes in excitability of motoneurons innervating the ankle flexors (F-MNs) and the antigravity ankle extensors (E-MNs). No significant difference in resting potential, action potential threshold, input resistance or rheobase was observed at birth. The age-related increase in rheobase was more pronounced for F-MNs than for E-MNs. The development of discharge properties of E-MNs lagged behind that of F-MNs. More F-MNs than E-MNs were able to fire repetitively in response to current injection at birth. F-MNs discharged at a higher frequency than E-MNs at all ages. Differences in the duration of action potential afterhyperpolarization accounted, at least partly, for the differences in discharge frequency between E-MNs and F-MNs at birth, and for the age-related increase in firing rate. These results suggest that E-MNs are more immature at birth than F-MNs and that there is a differential development of motoneurons innervating antagonistic muscles. This may be a critical factor in the development of posture and locomotion.

Early ontogeny of locomotor behaviour: a comparison between altricial and precocial animals

The focus of this review is to examine the physiological and behavioural differences between the early ontogeny of locomotion in precocial and altricial species. Both groups of animals are capable of performing alternating stepping movements upon birth or hatching, indicating that the basic elements underlying locomotor synergy are present prior to expression of mature overground gait. Nevertheless, the notable difference between precocial and altricial animals is the ability of the former to walk and run soon after birth or hatching. The weight of experimental evidence suggests that postural constraints play an important role in preventing early expression of locomotor behaviour in altricial species. Even some precocial animals, however, need time to develop sufficient stability and balance to walk as an adult. Therefore, components of locomotor behaviour involving the maintenance of equilibrium need a period of maturation in both precocial and altricial species, possibly requiring locomotor experience to become fully mature.

Porosity of the epithelial basement membrane as an indicator of macrophage-enterocyte interaction in the intestinal mucosa

The epithelial basement membrane of intestinal villi is perforated with numerous small pores, through which free cells in the lamina propria communicate with the enterocytes. This study was a comparative analysis of the pores in the basement membrane by SEM after removal of the gut epithelium with OsO4 maceration. The porosity as represented by the area fraction of the pores varied along the baso-apical axis of villi in patterns specific for each animal species examined: consistent scantiness along the entire length of villi in mice, acute elevation in the second and third distal one-sixths of villi in rats, and gradual augmentation toward the villus tips in guinea pigs. Size distribution analyses of the pores indicated their heterogeneous enlargement in the regions of elevated porosity. Concomitant observation of lamina propria macrophages by histochemical labelings and by conventional TEM showed that the cells specifically clustered beneath the hyperporous basement membrane, with their thick processes penetrating it. The spatially-regulated patterns of perforation of the epithelial basement membrane indicate phase-specific interventions of lamina propria macrophages in the maturation or aging of enterocytes, which steadily proliferate in crypts and exfoliate at the villus tips.

Development of hindlimb postural control during the first postnatal week in the rat

The development of the postural control of hindlimbs was investigated during the first postnatal week in the rat. The whole body was tilted in a vertical plane with the nose up. The proportion of animals producing a complete extension of both hindlimbs increased with age until the end of the first postnatal week. Motor responses were evoked by the pitch tilt already at birth with a slight extension of the hips, the knees and the ankles remaining bent in most cases. The extension produced at the ankle level increased gradually during the first postnatal week. This was correlated with a change in the EMG activity recorded from the triceps surae muscles (ankle extensors) during this postural reaction. There was a gradual acquisition of a tonic pattern. Characteristics of EMG responses changed significantly with age demonstrating an important increase in the use of triceps surae muscles in this postural task. These data demonstrate that the first postnatal week is a critical period for the development of postural reactions in the hindlimbs. They also suggest the existence of a proximo-distal gradient in the maturation of postural control. The mechanisms responsible for this reflex and for the maturation of posture are discussed.

Regulation of glycogen accumulation in L6 myotubes cultured under optimized differentiation conditions

The differentiation of the L6 myogenic cell line was enhanced by the addition of dexamethasone, retinoic acid, insulin-like growth factor I (IGF-I), and creatine. Spontaneous contractions appeared from day 10 or 11 and persisted to day 14 or 15. Glucose transport was increased by insulin (100 nM) and IGF-I (5 nM) by approximately 60%. The highest level of glycogen was measured in myotubes differentiated under the influence of a combination of 5 nM dexamethasone, 100 nM retinoic acid, 5 nM IGF-I, and 10 mM creatine with glucose as substrate. The glycogen accumulation rate was constant from 0 to 2 h of incubation and decreased gradually to zero at 4 h. From 0 to 0.5 h of the glycogen accumulation, the glycogen synthase a (GSa) activity was 30-35% of the total activity, with a subsequent gradual decline to 2.5% after 6 h. The glycogen phosphorylase a (GPha) activity was constant at approximately 80% from 0 to 0.5 h, increasing to approximately 100% after 6 h. The activity ratio of GSa to GPha decreased about sixfold without significant change in the rate of glycogen accumulation. This indicates that factors other than phosphorylation/dephosphorylation play a decisive role in the regulation of glycogen metabolism in L6 myotubes. Intracellular glucose (glucosei) and glucose 6-phosphate (G-6-P) may be such factors. The observed values of these parameters may in fact explain an activation of GSa (G-6-P) and an inhibition of GPha (glucosei).

Decay of calcium transients after electrical stimulation in rat fast- and slow-twitch skeletal muscle fibres

1. Calcium transients were calculated from fura-2 fluorescence signals (corrected for kinetic delays in the Ca(2+)-fura-2 reaction) from single rat skeletal muscle fibres, either fully dissociated from the fast-twitch flexor digitorum brevis (FDB) muscle or in small bundles from the slow-twitch soleus muscle. Fibres or bundles were embedded in agarose gel to inhibit movement and stimulated by single or trains of 1-2 ms electrical pulses (100 Hz, 2-400 ms train duration). 2. The rate constant of decay of [Ca2+] determined from single-exponential fits to the final decay phase of [Ca2+] after a single action potential was considerably faster in FDB fibres than in soleus fibres. As the stimulation duration increased, the rate constant of [Ca2+] decay decreased for both the FDB and soleus fibres, but the effect was greater in FDB than in soleus fibres. 3. Using the magnitude of the decline in the rate constant of [Ca2+] decay with increasing stimulation duration as an index of relative contribution of the saturable Ca2+ binding sites on parvalbumin, subpopulations termed 'high', 'medium' and 'low', referring to estimated parvalbumin content, were determined within each group of FDB and soleus fibres. In fibres assigned to the 'high' and 'medium' groups, parvalbumin was the major contributor (50-73%) to the [Ca2+] decay rate constant after a single action potential. In fibres in the 'low' group, parvalbumin contributed only 0-28% to the rate constant of [Ca2+] decay. 4. Fluorescence recordings using mag-fura-2, a lower-affinity Ca2+ indicator expected to be in equilibrium with myoplasmic Ca2+, gave similar values for both the [Ca2+] decay rate constant after a single action potential and the decrease in this rate constant with increased stimulation duration, as found for the fura-2 [Ca2+] transients from FDB and soleus fibres. Thus, the observed differences in decay rate of Ca2+ were not introduced by kinetic correction of the fura-2 recordings, but are attributed to differences in the Ca2+ binding and transport properties of fast- and slow-twitch mammalian fibres.

Differences in ultrastructural and metabolic profiles within the same type of fibres in various muscles of young and adult rats

Single fibres from tibialis anterior, extensor digitorum longus, gastrocnemius and soleus muscles in young (4-week-old) and adult (35-week-old) Wistar male rats were classified into three types on the basis of their enzyme-histochemical features: slow-twitch oxidative (SO), fast-twitch oxidative and glycolytic (FOG) and fast-twitch glycolytic (FG) fibres. Ultrastructural (volume density of mitochondria: Vmt and Z line width) and metabolic (phosphofructokinase: PFK and succinate dehydrogenase: SDH activities) profiles were measured. PFK activity in all types of fibres was higher in adult rats, and the difference between the two age-groups (adult/young) was largest between FG, FOG and SO fibres respectively. SDH activity and Vmt were lower in adult rats in a similar way in all fibres. A significant positive correlation was observed between the Vmt and SDH activity in both age-groups. This positive correlation was very specific in fast-twitch and slow-twitch fibres. Changes in the Vmt did not relate directly to the changes in fibre cross-sectional area. The overall pattern indicates that glycolytic capacity of fast-twitch fibres in flexor muscles (TA and EDL) is higher than in extensor muscles (GC and SOL), and that oxidative capacity of all types of fibre in extensor muscles is higher than in flexor muscles. These profiles were changed by growth, and may be related to the specific differences in pattern of activity of each skeletal muscle, and may reflect differences in the recruitment order of different muscles.

Effects of growth medium, electrical stimulation and paralysis on various enzyme activities in cultured rat muscle cells. Comparison with activities in rat muscles in vivo

1. Replacement of fetal calf serum and chicken embryo extract by Ultroser G and rat brain extract during the proliferation phase resulted in a higher maturation grade of cultured rat muscle cells after 7 days of differentiation, on base of the percentage of the muscle specific isoenzyme of creatine kinase (CK-MM). 2. Furthermore, the activities of creatine kinase, citrate synthase, cytochrome c oxidase and hexokinase were significantly higher. 3. Compared to the enzyme activities in m. quadriceps of 10 day-old rat and m. quadriceps, m. soleus and m. extensor digitorum longus of young adult rats, the metabolic capacity of cultured myotubes most closely resembles that of the first muscle. 4. Paralysis with tetrodotoxin caused a slight decrease of the creatine kinase activity and the percentage of CK-MM of cultured myotubes and an increase of the activities of hexokinase, phosphorylase and AMP deaminase. 5. Electrical stimulation performed at different frequencies and time periods had no effect on the enzyme activities of cultured rat muscle cells. 6. Only the AMP deaminase activity was decreased after intense electrical stimulation.

Detection, tissue distribution and (sub)cellular localization of fatty acid-binding protein types

This overview of recent work on FABP types is focussed on their detection and expression in various tissues, their cellular and subcellular distribution and their binding properties. Besides the 3 well-known liver, heart and intestinal types, new types as the adipose tissue, myelin and (rat) renal FABPs have been described. Recent observations suggest the occurrence of more tissue-specific types, e.g. in placenta and adrenals. Heart FABP is widely distributed and present in skeletal muscles, kidney, lung, brain and endothelial cells. The cellular distribution of FABP types appears to be related to the function of the cells in liver, muscle and kidney. The presence of FABP in cellular organelles requires more evidence. The functional significance of the occurrence of more FABP types is unclear, in spite of the observed differences in their ligand-protein interaction.

The fatty acid-binding protein from human skeletal muscle

Fatty acid-binding protein (FABP) was isolated from human skeletal muscle by gel filtration and anion- and cation-exchange chromatography. The isolation procedure, however, with rat and pig skeletal muscle gave mostly inactive preparations. Rat muscle FABP preparations contained parvalbumin as a contaminant. FABP from human muscle had a Mr of about 15 kDa, a pI value of 5.2, and a Kd value with oleic acid of 0.50 microM. Skeletal muscle and heart FABPs and their antisera showed a strong cross-reactivity on Western blots and in enzyme-linked immunosorbent assays (ELISA). No cross-reactivity was observed with liver FABP and its antiserum. On the basis of amino acid composition, electrophoretic behavior, fatty acid binding, and immunochemical properties, human skeletal muscle FABP must be similar or closely related to human heart FABP. The FABP content determined by ELISA was comparable in various human muscles and cultured muscle cells, but lower than that in rat muscles.

Quantitative analysis of single muscle fibre action potentials recorded at known distances

In vivo records of single fibre action potentials (SFAPs) have always been obtained at unknown distance from the active muscle fibre. A new experimental method has been developed enabling the derivation of the recording distance in animal experiments. A single fibre is stimulated with an intracellular micropipette electrode. The same electrode is used thereafter for labelling with an auto-fluorescent dye, Lucifer Yellow. In this method there is no use of chemical fixation. The tissue structure is kept as well as possible. In cross-sections the fluorescent fibre is seen and its position is quantitized with respect to the tip of one or more recording wire electrodes. Morphometric data, such as the recording distance and the fibre cross-sectional area, are used for the interpretation of parameters of the SFAPs (peak-peak amplitude, time between the first positive and negative peaks). The present results show that within 300 microns recording distance is not as dominant for the SFAP shape as expected. The method offers also a direct check of the relation between the muscle fibre; diameter and the conduction velocity of the action potential. In the present small set of data there is no simple linear relationship.

Postnatal development of the actual and total activity of the branched-chain 2-oxo acid dehydrogenase complex in rat tissues

Actual and total activities of the branched-chain 2-oxo acid dehydrogenase complex were determined in homogenates of quadriceps muscle, heart, liver, kidney and brain from rats of 0-70 days age. All rat tissues except quadriceps muscle showed a marked increase of total activity between 0 and 21 days, heart and kidney also after weaning. The actual activity rose after birth in liver, kidney and brain and after weaning in liver, kidney and heart. The activity state was always about 100% in liver and varied between 40-60% in kidney and brain, 10-23% in heart and 6-12% in quadriceps muscle. The actual activities measured indicate, that the degradation of branched-chain 2-oxo acids mainly takes place in the liver of the newborn, suckling and young-adult rat.

Effect of dietary fat on total and peroxisomal fatty acid oxidation in rat tissues

In this study the effect of dietary trans fatty acids on the peroxisomal and mitochondrial beta-oxidation is compared with that of saturated or cis-monounsaturated fatty acids. Oxidation of [1-14C]- and [16-14C]palmitate was assayed in the absence as well as in the presence of antimycin plus rotenone in homogenates of liver, heart and skeletal muscle of four groups of rats fed diets containing 40 energy% fat of different fatty acid composition. Three groups were given fat blends rich in C16, C18 saturated (cocoa butter), cis-monounsaturated (low-linoleic-acid olive oil) or trans fatty acids (partially hydrogenated soybean oil), respectively. The fourth group received a mixture of these fats with half the amount of trans fatty acids of the third group. Total oxidation rates of [1-14C]- and [16-14C]palmitate in the absence of antimycin were not significantly influenced by the type of dietary fat in the investigated tissues. The antimycin-insensitive [1-14C]palmitate oxidation rate and the proportion of peroxisomal oxidation of the total oxidation were lower in all tissues of those animals fed the mixed dietary fat than in those fed the other diets; both parameters were higher in the liver of cocoa butter-fed rats than in those of the other groups. Comparison of the results with literature data and with previous results obtained with a low-fat diet (Veerkamp and Van Moerkerk (1986) Biochim. Biophys. Acta 875, 301-310) indicates that high-fat diets only induce peroxisomal beta-oxidation activity if they also contain C20, C22 fatty acids. High dietary concentrations of trans C18 fatty acids do not result in a higher peroxisomal activity than that observed for other fatty acids with the same chain length.