Congenital splayleg of piglets: myofibrillar hypoplasia

Homer 1 genotype AA variant relates to congenital splay leg syndrome in piglets by repressing Pax7 in myogenic progenitors

Introduction

Porcine congenital splay leg syndrome (PCS) is a major birth defect in piglets, resulting in lameness and high mortality rates. The multifactorial pathogenesis of PSC is not well understood but includes a polygenic inheritance.

Methods

Here, in addition to morphological investigations, we characterized the expression of myogenic genes and functional (proliferation and differentiation) properties of myogenic precursor/satellite cells (SATCs) in 1 day-old PCS piglets, non-affected littermates (LCs), and piglets from PCS-free healthy litters (HCs). In addition, PCS phenotypes were related to the SNP Homer1_rs325197091 within the Homer1 locus, which has been identified as a potential hereditary cause of PCS.

Results and discussion

Samples from musculus semitendinosus (ST) of PCS piglets had a higher proportion of type II fibers, reflecting myofiber immaturity. In addition, myofiber atrophy, a lower number of myonuclei per fiber (ST), and a higher apoptotic activity (in ST and longissimus dorsi muscle; LD) were found in the PCS group. A higher proportion of cycling committed myoblasts (Pax7+/Ki67+ cells) occurred in samples from PCS-affected piglets, and on the other hand, the mRNA expression of genes involved in differentiation (muscle differentiation 1; MyoD, myogenin; MyoG) was repressed compared with HCs. Cultured SATCs from PCS-affected animals showed a temporal shift in peak expression of Pax7, MyoD, and MyoG toward days 3 and 4 of their 7 days differentiation regime. In vitro experiments with isolated SATCs confirmed the lower differentiation potential and the delayed progression of the myogenic processes in cells from piglets with PCS phenotype. In addition, Pax7 and desmin were differently expressed in Homer1_rs325197091 genotype variants (GG, GA, and AA). Both genes showed the lowest expression in the homozygous AA-variant, which was most frequently found in PCS-affected animals. The homozygous AA-variant was also associated with lower expression of the truncated Homer1-subtype 205. Thus, we hypothesize that in PCS, the balance between Homer1 proteins and its signaling functions is changed in a way detrimental to the myogenic differentiation program. Our results demonstrated direct negative effects of the Homer1 AA genotype on Pax7 expression, but the exact mode of action still needs to be elucidated.

Congenital tremor and splay leg in piglets – insights into the virome, local cytokine response, and histology

Background

Atypical porcine pestivirus (APPV) is a neurotropic virus associated with congenital tremor type A-II. A few experimental studies also indicate an association between APPV and splay leg. The overarching aim of the present study was to provide insights into the virome, local cytokine response, and histology of the CNS in piglets with signs of congenital tremor or splay leg.

Results

Characterization of the cytokine profile and virome of the brain in piglets with signs of congenital tremor revealed an APPV-associated upregulation of Stimulator of interferon genes (STING). The upregulation of STING was associated with an increased expression of the gene encoding IFN-α but no differential expression was recorded for the genes encoding CXCL8, IFN-β, IFN-γ, IL-1β, IL-6, or IL-10. No viral agents or cytokine upregulation could be detected in the spinal cord of piglets with signs of splay leg or in the brain of piglets without an APPV-infection. The histopathological examination showed no lesions in the CNS that could be attributed to the APPV-infection, as no difference between sick and healthy piglets could be seen.

Conclusion

The results from this study provide evidence of an APPV-induced antiviral cytokine response but found no lesions related to the infection nor any support for a common causative agent.

Congenital Splay Leg Syndrome in Piglets-Current Knowledge and a New Approach to Etiology

The porcine congenital splay leg syndrome (PCS), even though being of transient nature, is still one of the most important causes for piglet losses due to its high incidence and mortality. Although, described decades ago, the pathogenetic mechanism is still elusive. Numerous, mostly descriptive studies characterized the syndrome at clinical, histological and cellular levels but resulted in a highly diverse picture of the syndrome. Broad variability in phenotypical expression and, in case of proper care, the rapid recovery of affected animals complicated a systematical analysis of the underlying pathogenesis. Although, several environmental factors were discussed as potential causes of PCS, most of the evidence points to a hereditary basis of PCS. Nevertheless, only few of the suggested candidate genes from transcriptome and mapping analyses, like F-box protein 32 (FBXO32), could be confirmed so far. Only recently, a genome wide association study revealed genomic regions on five porcine chromosomes and named a number of potential candidate genes, among them homer scaffold protein 1 (HOMER1). This new candidate-a cellular scaffold protein-plays a role in a plethora of cellular signaling cascades, and is not only involved in skeletal muscle differentiation but also critical for muscular function. In this review, we critically elucidate the current state of knowledge in the field and evaluate current achievements in the identification of the pathogenetic mechanism for the syndrome.

Detection of atypical porcine pestivirus in Swedish piglets with congenital tremor type A-II

Background

Congenital tremor (CT) type A-II is a neurological disorder characterized by tremor of the head and body of newborn piglets. The suggested causative agent of the disease is the recently found atypical porcine pestivirus (APPV). The virus has been detected in piglets suffering from congenital tremor in central Europe, South and North America and in China but no studies has so far been performed in the Nordic countries. The overarching goal of this study was to investigate if APPV is present in the brain tissue of Swedish piglets suffering from congenital tremor.

From June 2017 – June 2018, 15 piglets from four Swedish farms with ongoing outbreaks of congenital tremor and 13 piglets with splay leg originating from four different farms, were investigated for presence of APPV RNA in brain tissue. Matched healthy control piglets (n = 8) were also investigated. Two APPV-specific RT-qPCR methods targeting the NS3 and NS5B region, respectively, were used. A retrospective study was performed on material from Swedish piglets with congenital tremor sampled in 2004 (n = 11) and 2011/2012 (n = 3) using the described APPV-specific RT-qPCR methods. The total number of piglets with signs of CT in this study was 29.

Results

Atypical porcine pestivirus-RNA was detected in 93% (27/29) of the piglets suffering from congenital tremor. All piglets with congenital tremor from 2004 (n = 11) and 2012 (n = 3) were PCR-positive with respect to APPV, whereas, all of the healthy controls (n = 11) were negative. The piglets with congenital tremor sampled 2017–2018 had an odds ratio of 91.8 (95% CI 3.9128 to 2153.7842, z = 2.807, P = 0.0050) to test positive for APPV by qRT-PCR compared to the healthy piglets (Fishers exact test p < 0.0001). These findings make it interesting to continue investigating APPV in the Swedish pig-population.

Conclusion

This is the first description of atypical porcine pestivirus in piglets suffering from congenital tremor type A-II in Sweden and the Nordic countries. The virus has been present in the Swedish pig population since at least 2004.

A review of congenital tremor type A-II in piglets

Congenital tremor (CT) is a neurological disease that affects new-born piglets. It was described in 1922 and six different forms, designated type AI-V and type B, are described based on the causative agents, as well as specific histological findings in the central nervous system (CNS). The various forms present with identical clinical signs consisting of mild to severe tremor of the head and body, sometimes complicated with ataxia. By definition, all A-forms have hypomyelination of the CNS, whereas there are no histopathological lesions with the B-form. The cause of the A-II form was long unknown, however, at present several different viruses have been proposed as the causative agent: porcine circovirus-II (PCV-II), astrovirus, PCV-like virus P1, and atypical porcine pestivirus (APPV). Currently, APPV is the only virus that has been proven to fulfill Mokili's Metagenomic Koch's Postulates. Following infection of the pregnant sow, the virus passes the placental barrier and infects the fetus. Interestingly, no clinical signs of disease have been associated with APPV in adult pigs. Furthermore, other viruses cannot be ruled out as additional potential causes of CT. Given the increased interest and research in CT type A-II, the aim of this review is to summarize current knowledge.

Transcriptome analysis reveals candidate genes involved in splay leg syndrome in piglets

Splay leg is frequently observed in newborn piglets and leads to economic loss as well as welfare concerns. However, the etiology and pathogenesis of splay leg syndrome in piglets are still poorly understood. The aims of this paper were to characterize changes in the transcriptome of splay leg piglets and identify candidate genes responsible for this disease. We chose three splay leg piglets and their healthy full sibs, and constructed six RNA libraries using skeletal muscle samples from both groups and identified the differentially expressed genes between the two groups using RNA-seq. A total of 555 differentially expressed genes were identified, of which 216 were up-regulated and 339 genes were down-regulated in the splay leg group relative to the healthy group. In addition, 321 significantly enriched GO terms and 12 significantly enriched KEGG pathways were identified. FBXO32 is one of the ten most differentially expressed genes in our experiment, and it is regulated by the significantly enriched pathway (PI3K-Akt). The overexpression of FBXO32 which leads to the process of muscle atrophy might be responsible for congenital splay leg in piglets. The result of this study could help improve understanding of the molecular mechanism of congenital splay leg syndrome.

Detection of atypical porcine pestivirus in Brazil in the central nervous system of suckling piglets with congenital tremor

Atypical porcine pestivirus (APPV) has been detected in piglets with congenital tremor (CT) from three different continents including North America, Europe and Asia. Thirteen piglets from four farms in two different states in Brazil with CT were sampled. Viral RNA was detected by quantitative real-time PCR in the cerebellum or cerebellum and spinal cord in the 100% of the piglets with CT, and APPV was not detected in any tissue sample from clinically non-affected piglets with the exception of the cerebellum of one piglet from Farm A. Piglets with CT had an odds ratio of 99.0 (95% CI 3.4, 2823.8; p = .0072) compared to piglets without CT to test positive for APPV by qRT-PCR. A subset of positive samples was selected for sequencing of the NS3 gene. Phylogenetic analysis revealed that Brazilian sequences of the NS3 formed an independent cluster and had the highest sequence identity with a sequence from the United States. This is the first identification of APPV infection in piglets with CT in South America.

Genome-wide association study identifies candidate genes for piglet splay leg syndrome in different populations

BACKGROUND

Piglet splay leg syndrome (PSL) is one of the most frequent genetic defects, and can cause considerable economic loss in pig production. The present understanding of etiology and pathogenesis of PSL is poor. The current study focused on identifying loci associated with PSL through a genome-wide association study (GWAS) performed with the Illumina Porcine60 SNP Beadchip v2.0. The study was a case/control design with four pig populations (Duroc, Landrace, Yorkshire and one crossbred of Landrace × Yorkshire).

RESULT

After quality control of the genotyping data, 185 animals (73 cases, 112 controls) and 43,495 SNPs were retained for further analysis. Principal components (PCs) identified from the genomic kinship matrix were included in the statistical model for correcting the effect of population structure. Seven chromosome-wide significant SNPs were identified on Sus scrofa chromosome 1 (SSC1), SSC2 (2 SNPs), SSC7, SSC15 (2 SNPs) and SSC16 after strict Bonferroni correction. Four genes (HOMER1 and JMY on SSC2, ITGA1 on SSC16, and RAB32 on SSC1) related to muscle development, glycogen metabolism and mitochondrial dynamics were identified as potential candidate genes for PSL.

CONCLUSIONS

We identified seven chromosome-wide significant SNPs associated with PSL and four potential candidate genes for PSL. To our knowledge, this is the first pilot study aiming to identify the loci associated with PSL using GWAS. Further investigations and validations for those findings are encouraged.

Pilot study of large-scale production of mutant pigs by ENU mutagenesis

N-ethyl-N-nitrosourea (ENU) mutagenesis is a powerful tool to generate mutants on a large scale efficiently, and to discover genes with novel functions at the whole-genome level in Caenorhabditis elegans, flies, zebrafish and mice, but it has never been tried in large model animals. We describe a successful systematic three-generation ENU mutagenesis screening in pigs with the establishment of the Chinese Swine Mutagenesis Consortium. A total of 6,770 G1 and 6,800 G3 pigs were screened, 36 dominant and 91 recessive novel pig families with various phenotypes were established. The causative mutations in 10 mutant families were further mapped. As examples, the mutation of SOX10 (R109W) in pig causes inner ear malfunctions and mimics human Mondini dysplasia, and upregulated expression of FBXO32 is associated with congenital splay legs. This study demonstrates the feasibility of artificial random mutagenesis in pigs and opens an avenue for generating a reservoir of mutants for agricultural production and biomedical research.

DOI:http://dx.doi.org/10.7554/eLife.26248.001

Selection for carcass length in Large White pigs

A sib selection programme for length of carcass was carried out for five generations within a closed herd of Large White pigs. A generation interval of one year was obtained.

Litter production was good throughout the project and the numbers of pigs born and reared in the final generation were 11·2 and 10·3 respectively.

Overall response to selection was predicted with reasonable success. The increase in length was associated with an increase in number of vertebrae.

Heritability for length of carcass computed from the pooled components of variance was 0·53 ± 0·12. Realized heritability was 0·5 ± 0·1. No control strain was maintained.

Correlated traits moved in the expected directions and responses were beneficial for traits such as backfat measurements and chest depth, but unfavourable for belly thickness and ham conformation. Carcass acceptability was not good initially and this deteriorated during the project. The difficulties involved in maintaining selection pressure and herd size are discussed. Abnormalities and lameness were recorded and did not increase systematically over the experiment.

It is suggested that the specific relationships between ‘conformation’, carcass value and viability require further investigation with particular reference to length of leg and length of carcass.

Identification of candidate genes for congenital splay leg in piglets by alternative analysis of DNA microarray data

The congenital splay leg syndrome in piglets is characterized by a temporarily impaired functionality of the hind leg muscles immediately after birth. Etiology and pathogenetic mechanisms for the disease are still not well understood. We compared genome wide gene expression of three hind leg muscles (M. adductores, M. gracilis and M. sartorius) between affected piglets and their healthy littermates with the GeneChip Porcine Genome Array (Affymetrix) in order to identify candidate genes for the disease. Data analysis with standard algorithms revealed no significant differences between both groups. By application of an alternative approach, we identified 63 transcripts with differences in two muscles and 5 genes differing between the groups in three muscles. The expression of six selected genes (SQSTM1, SSRP1, DDIT4, ENAH, MAF, and PDK4) was investigated with SYBRGreen RT-Real time PCR. The differences obtained with the microarray analysis could be confirmed and demonstrate the validity of the alternative approach to microarray data analysis. Four genes with different expression levels in at least two muscles (SQSTM1, SSRP1, DDIT4, and MAF) are assigned to transcriptional cascades related to cell death and may thus indicate pathways for further investigations on congenital splay leg in piglets.

Porcine congenital splayleg is characterised by muscle fibre atrophy associated with relative rise in MAFbx and fall in P311 expression

BACKGROUND

Porcine congenital splayleg (PCS) is the most important congenital condition of piglets, associated with lameness and immobility, of unknown aetiology and pathogenesis, hence the need to better understand the condition by defining, in the first instance, its histopathology and molecular pathology.

RESULTS

Semitendinosus, longissimus dorsi, and gastrocnemius muscles were removed from 4 sets of 2-day-old splayleg piglets, each with a corresponding normal litter mate. Based on immunohistochemistry and histological image analysis, PCS piglets showed significantly smaller fibre size without any accompanying sign of inflammation. Although there was no dramatic change in fibre type composition in affected muscles, several structural myosin heavy chain genes were significantly down-regulated. MAFbx, a major atrophy marker, was highly up-regulated in nearly all PCS muscles, in comparison with controls from normal litter mates. In contrast, P311, a novel 8 kDa protein, was relatively down-regulated in all the PCS muscles. To investigate a functional role of P311 in skeletal muscle, its full-length cDNA was over-expressed in murine C2C12 muscle cells, which resulted in enhanced cell proliferation with reduced myotube formation. Hence, reduced P311 expression in PCS piglets might contribute to atrophy through reduced muscle cell proliferation. P311, predictably, was down-regulated by the over-expression of calcineurin, a key signalling factor of muscle differentiation.

CONCLUSION

We demonstrated that PCS is a condition characterised by extensive fibre atrophy and raised fibre density, and propose that the combined differential expression of MAFbx and P311 is of potential in the diagnosis of subclinical PCS.

Incidence of splayleg pigs in Nebraska litter size selection lines1

Genetic parameters for the splayleg (SL) condition were estimated from 37,673 records of pigs from six lines derived from a Large White-Land-race base population. Random selection for 22 generations was practiced in Lines C1 and C2. Line C2 was derived from C1 at Generation 8. Selection lines were as follows: 1) Line I, selected 11 generations for an index of ovulation rate and embryonic survival followed by 11 generations of selection for litter size; 2) Line IOL, derived from Line I at Generation 8 and which underwent eight generations of two-stage selection for ovulation rate and number of fully formed pigs per litter followed by four generations of litter size selection; 3) Line COL, derived from Line C1 at Generation 8 and selected eight generations in two stages for ovulation rate and number of fully formed pigs followed by four generations of litter size selection; and 4) Line T, selected 12 generations for increased testis size. From logistic models, it was found that boars were 224% more likely to have SL than gilts (P < 0.01). Decreases in birth weight, dam age at puberty, dam nipple number, and dam embryonic survival, and increases in dam litter size and inbreeding increased the odds of SL (P < 0.05). Direct and maternal heritabilities of SL were 0.07 and 0.16, respectively, and the correlation between direct and maternal effects was -0.24. Correlations between direct genetic effects for SL and number born alive, nipple number, birth weight, age at puberty, and embryonic survival were -0.19, -0.36, 0.23, -0.19, and -0.32, respectively. Except for the correlation of 0.32 between maternal effects for SL and direct effects for number of live pigs, correlations of SL maternal genetic effects with direct genetic effects of other traits were less than 0.11. Annual direct genetic trends (%) for SL in I, IOL, COL, T, C1, and C2 were -0.003 +/- 0.003, 0.121 +/- 0.012, -0.273 +/-0.009, 0.243 +/-0.014, -0.274 +/-0.004, and 0.086 +/-0.008, respectively; annual maternal genetic trends (%) were 0.106 +/-0.004, 0.508 +/-0.019, 0.383 +/-0.015, 0.527 +/-0.024, 0.188 +/-0.005, and 0.113 +/-0.012, respectively. Annual genetic maternal trend in Line I after Generation 12 was 0.339 +/-0.014. Maternal breeding value for SL is expected to increase as a correlated response to selection for increased litter size and increased size of testes.

Isolation of expressed sequence tags of skeletal muscle of neonatal healthy and splay leg piglets and mapping by somatic cell hybrid analysis

We have isolated 14 differentially displayed and 10 further expressed sequence tags (ESTs) from Musculus biceps femoris of newborn healthy and splay leg piglets. By comparison with EMBL/GenBank data we could identify nine porcine homologues to human genes (TATA box binding protein associated factor B TAF1B; B-cell CLL/lymphoma 7B BCL7B; pyruvate dehydrogenase kinase, isoenzyme 4 PDK4; ribosomal protein S10 RPS10; SPARC-like 1 SPARCL1; epithelial protein lost in neoplasm beta EPLIN; N-myc downstream-regulated gene 2 NDRG2; pleiomorphic adenoma gene like 2 PLAGL and, BCL-2 associated transcription factor short form BTFS). Eight fragments correspond to uncharacterized ESTs and 7 ESTs had no significant match with database sequences. These data provide the first expression profiles in skeletal muscle of neonatal piglets and are a basis for candidate gene investigations for congenital splay leg in piglets. Eleven ESTs were physically mapped to porcine chromosomes 1, 3, 4, 5, 7, 8, 9 and 10 and contribute to the comparative map of humans and pigs.

Determination of beta-adrenoceptors and cAMP in muscle from normal and splayleg Belgian Landrace pigs

beta-Adrenoceptors were identified and characterized by [3H]dihydroalprenolol ([ 3H]DHA) binding experiments in muscle membrane preparations from piglets. The [3H]DHA binding was rapid, reversible and stereoselective. Catecholamines competed for specific binding with a rank order of potency (-)-isopropylnoradrenaline greater than (-)-epinephrine much greater than (-)-norepinephrine, indicating a beta 2-subtype of adrenoceptor. Saturation binding experiments with [3H]DHA showed no significant difference for either the number of binding sites or the equilibrium dissociation constants in normal and splayleg pigs. Adenylate cyclase assays indicated that the basal adenylate cyclase activity and the prostaglandin E1 (PGE1)-, (-)-isopropylnoradrenaline (ISO)-, 5'-guanylyl-imidodiphosphate (GppNHp)- and sodium fluoride (NaF)-stimulated values were not significantly different in normal and splayleg pigs. In both groups, PGE1 did not affect basal activity, whereas ISO and GppNHp stimulated adenylate cyclase activity significantly (p less than 0.001) to about 40% above basal level. NaF induced a significant (p less than 0.001) increase of cAMP in normal and splayleg pigs amounting to 48% and 61% respectively. Significant correlations between absolute adenylate cyclase responses to ISO (r = 0.83***), NaF (r = 0.72**) and GppNHp (r = 0.61*) and basal activity in the splayleg pigs were the most striking findings. In contrast, these correlations could not be detected in the normal pigs. Whether or not this observation reflects an alteration in the signal transduction system needs to be further investigated. To the best of our knowledge this is the first biochemical study which relates an altered beta-adrenoceptor function and porcine splayleg.

Spontaneous primary myopathies in domestic mammals: a review

By reviewing the literature concerning primary myopathies in man and animals, a classification of spontaneous primary myopathies in domestic mammals is made. This classification is based on aetiological factors. Primary myopathies are divided into metabolic, immunologic, toxic, nutritional, congenital, inherited, and inflammatory myopathies. Muscular dystrophies, in accordance with the definition in human cases, are considered as a separate entity. In the description of the different muscle disorders the emphasis is on the structural changes and the pathogenesis. Clinical signs and diagnostic criteria are considered only briefly.

Spontaneous and experimental myofibrillar hypoplasia and its relation to splayleg in newborn pigs

The relation of myofibrillar hypoplasia to clinical splayleg was studied. A strain of Belgian Landrace sows was selected for this study because they produced pigs which had no myofibrillar hypoplasia. Myofibrillar hypoplasia could nevertheless by induced experimentally in these animals by dexamethasone treatment of the sows during late pregnancy. The lesion was observed without clinical signs and was compared to the myofibrillar hypoplasia in clinical cases of splayleg. The differences between these 2 groups may account for the appearance of clinical signs. These differences included the maturity of the myofibrils and the degree of autophagolysosomal glycogen breakdown.

Use of the Goettingen miniature pig for studying pyrimethamine teratogenesis

Although pigs are suitable animals for experimental teratology, use of miniature pigs developed as a medicobiological experimental animal is essential for this purpose. Miniature pigs are established genetically so that the background data on the naturally occurring birth defects are reliable, whereas the commercial breeds are always improved genetically to seek higher productivity. In this review the authors summarized pyrimethamine teratogenesis in Goettingen miniature pigs. Pyrimethamine administration to a pregnant sow during the early stage of the organogenetic period caused cleft palate and other birth defects, and also hind leg paralysis, a functional defect in newborns.

The splayleg disease: a form of congenital glucocorticoid myopathy?

In countries with modern pig industries an increasing percentage of newborn piglets is affected by congenital myofibrillar hypoplasia, also called splayleg or spraddleleg. The main pathomorphological features of this disease are myofibrillar hypoplasia and degeneration. Glucocorticoid myopathy induced in immature muscles of early postnatal rats and rabbits has a pathomorphology very similar to that of splayleg disease. Also in other respects the splayleg disease resembles the glucocorticoid myopathy, as summarized in this contribution. It is suggested that splayleg might represent a congenital form of glucocorticoid myopathy resulting from stress and hormonal imbalance of pregnant sows which affect the fetuses.

Splayleg: a spontaneous limb defect in rabbits. Genetics, gross anatomy, and microscopy

A limb defect in rabbits known as splayleg arose in a closed breeding colony of Dutch rabbits. Since the causes of this postural deformity are not fully understood, an effort was made to clarify the mode of inheritance and the etiology of the disease. Pedigree analysis and genetic mating tests showed that, in contrast to published reports, the disease is not determined by a single recessive gene with complete expressivity. The underlying genetic system is composed of one recessive gene with reduced expressivity or of more genes with the probable involvement of environmental factors. Femurs of abnormal limbs displayed endotorsion of the shaft and anteversion of the neck as the main features. Endotorsion of the femur was associated with exotorsion of the tibia. Dislocation of the hip was not observed. There was no abnormnal laxity of the joint capsule or the ligamentaum teres. Only minor differences in the acetabulum between control and splayleg rabbits were observed. Enzyme histochemistry and electron microscopy of the gluteus medius, adductor longus- and magnus, sartorius, gracilis, and semitendinosus muscles and the femoral, peroneal, and tibial nerves did not reveal any qualitative differences between splayleg and control rabbits. A study on muscle fiber diameter showed that fibers of splayleg animals were in general smaller, particularly in the semitendinosus. Atypical mitochondria of muscles were observed in all rabbits, but more frequently in splayleg animals. A hypothesis is presented in which imbalanced development of the neural, muscular and skeletal systems of the limb is the primary defect. This results in torsion of the femur. It depends on the degree of femoral torsion and the capacity of the young rabbit for compensation by exorotation of the limb in the hip whether a persistent splayleg posture will develop.

Histochemical patterns in normal and splaylegged piglet muscle fibers

The histochemical pattern of muscle fiber types of the longissimus dorsi and biceps femoris muscles was investigated in normal and splaylegged piglets at birth and seven days later. Only slight differences between the muscle fibers at birth were found using histochemical reactions for alkaline adenosine triphosphatase (ATPase), succinate dehydrogenase (SDH), phosphorylase (PH) activities, and for the periodic acid-Schiff (PAS) reaction. With the method for acid-preincubated ATPase activity, high activity was observed in Type I muscle fibers and low activity in Type II muscle fibers in animals of both groups investigated. However, a higher number of Type I fibers was found in muscles of normal piglets, suggesting a faster and more advanced process of transformation of Type II into Type I muscle fibers in unaffected animals. Thus the histochemical conversion appears to be retarded in muscles of splaylegged animals, which have a histochemical pattern similar to that of normal prenatal animals. Cholinesterase activity in motor endplates was well developed; its staining revealed smaller sized and irregularly arranged endplates in muscles of affected piglets. Fiber type differentiation in muscles of animals which recovered from splayleg becomes fully developed and comparable to normal piglets seven days after birth. The number of fibers which became converted from Type II to Type I was increased; the fiber types were differentiated with regard to the PAS reaction and to their ATPase, SDH and PH activities. Morphological features of motor endplates in muscles of normal and surviving splaylegged piglets are similar. Histochemical investigation of the fiber type differentiation thus suggests that full recovery occurs within the first week of postnatal life in muscles affected by pathological changes accompanying splayleg.

Abnormalities of the nuclear envelope in porcine muscle affected with congenital myofibrillar hypoplasia

The nuclear ultrastructure of muscle fibres in congenital myofibrillar hypoplasia of piglets was studied in order to provide more information on the pathological changes present. In the initial stages of the disease, muscle fibres with a small reduction of myofibrils often had an increased number of nuclear pores. In the more advanced stages of the disease, the following nuclear abnormalities were observed in muscle fibres showing an extensive disintegration of myofibrils: 1) large dilations of the perinuclear cisternae, 2) small nuclear protrusions projecting into these dilations and 3) appositions of stray membranes of the disrupted endoplasmic reticulum to the nuclear envelope. The increase in the number of nuclear pores may be significant in the development of pathological changes in congenital myofibrillar hypoplasia which is thought to be a hereditary disease. The observed abnormalities of the nuclear envelope appear to be characteristic for muscle fibres undergoing degeneration.

Relationships between mitochondrial content and glycogen distribution in porcine muscle fibres

Serial sections of longissimus dorsi and rectus femoris muscles from 15 Yorkshire breed pigs (live weights 24-46 and 49-139 kg) were stained for glycogen (PAS) and a mitochondrial enzyme (NAD tetrazolium reductase). Muscle fibres with a low mitochondrial content in both muscles were more frequently PAS-positive than fibres with a high or intermediate mitochondrial content. However, some pigs had all their muscle fibres PAS-positive while one pig with a high post-mortem muscle pH had all rectus femoris fibres PAS-negative. Relative to lighter weight pigs, longissimus dorsi muscles of heavy pigs tended to have less fibres with a high mitochondrial content and less fibres with a positive PAS reaction. Compared to longissimus dorsi muscles, rectus femoris muscles had more fibres with a high mitochondrial content and less with a positive PAS reaction. All fibres in both muscles became PAS-negative with an accompanying decrease in pH by 24 hr post-mortem. Fibres from longissimus dorsi muscles frequently had PAS-positive sarcoplasmic cores between their myofibrils. Heavy pigs tended to have larger cores (up to a mean maximum diameter of 13.4 mum), more fibres with cores, and more cores per fibre. The pigs involved exhibited no other ante- or post-mortem muscle abnormalities.