Seasonal muscle ultrastructure plasticity and resistance of muscle structural changes during temperature increases in resident black-capped chickadees and rock pigeons.

Resident birds in temperate zones respond to seasonally fluctuating temperatures by adjusting their physiology, such as changes in basal metabolic rate or peak metabolic rate during cold exposure, or altering their organ sizes, so as to match the thermogenic requirements of their current environment. Climate change is predicted to cause increases in the frequency of heat and cold wave events, which could increase the likelihood that birds will face an environmental mismatch. Here, we examined seasonality and the effects of acute and chronic heat shock to 33°C and subsequent recovery from heat shock on the ultrastructure of the superficial pectoralis muscle fiber diameter, myonuclear domain (MND) and capillary density in two temperate bird species of differing body mass, the black-capped chickadee (Poecile atricapillus) and the rock pigeon (Columba livia). We found that muscle fiber ultrastructure did not change with heat treatment. However, in black-capped chickadees, there was a significant increase in fiber diameter in spring phenotype birds compared with summer phenotype birds. In rock pigeons, we saw no differences in fiber diameter across seasons. Capillary density did not change as a function of fiber diameter in black-capped chickadees, but did change seasonally, as did MND. Across seasons, as fiber diameter decreased, capillary density increased in the pectoralis muscle of rock pigeons. For both species in this study, we found that as fiber diameter increased, so did MND. Our findings imply that these two temperate birds employ different muscular growth strategies that may be metabolically beneficial to each.

The effects of electrical stunning methods on broiler meat quality: effect on stress, glycolysis, water distribution, and myofibrillar ultrastructures.

This study was designed to compare the effects of different stunning systems on the meat quality of broilers. This was done by investigating meat water-holding capacity, meat color, muscle glycogen, and lactate concentrations, as well as blood parameters, low-field nuclear magnetic resonance (NMR) transverse relaxation, and myofibrillar ultrastructures. A total of 160 broilers were divided into 4 treatment groups: a low-voltage stunning (LS) with a constant voltage of 15 V at 750 Hz for 10 s; a midvoltage stunning (MS) with a constant voltage of 50 V at 50 Hz for 10 s; a high-voltage stunning (HS) with a constant voltage of 100 V at 50 Hz for 5 s; and a control group with no stunning (NS). Blood samples were collected immediately after cutting the neck. Pectoralis major muscles were removed from the carcass after chilling and placed in ice. Breast muscle pH, meat color, glycogen, and lactate contents were determined at both 2 and 24 h postmortem. Drip loss, cooking loss, pressing loss, cooked breast meat shear values, low-field NMR, and ultrastructures of myofibrils were determined 24 h postmortem. The NS and MS treatments significantly increased (P < 0.05) blood plasma corticosterone, initial rate of glycolysis, and drip loss, and significantly reduced (P < 0.05) initial muscle pH and shear force values when compared with LS and HS. The results of low-field NMR reflect that NS and MS significantly decreased (P < 0.05) NMR transverse relaxation population 1 (T21) and increased (P < 0.05) NMR transverse relaxation population 2 (T22). The myofibrils of NS and MS samples showed significantly (P < 0.05) longer sarcomere length when compared with the LS and HS samples. The meat color, cooking loss, pressing loss, and final pH were not affected by the stunning methods. This study indicates that NS and MS treatments reduce meat water-holding capacity and decrease meat shear force when compared with LS and HS.

Evidence of vascular endothelial dysfunction in Wooden Breast disorder in chickens: Insights through gene expression analysis, ultra-structural evaluation and supervised machine learning methods.

Several gene expression studies have been previously conducted to characterize molecular basis of Wooden Breast myopathy in commercial broiler chickens. These studies have generally used a limited sample size and relied on a binary disease outcome (unaffected or affected by Wooden Breast), which are appropriate for an initial investigation. However, to identify biomarkers of disease severity and development, it is necessary to use a large number of samples with a varying degree of disease severity. Therefore, in this study, we assayed a relatively large number of samples (n = 96) harvested from the pectoralis major muscle of unaffected (U), partially affected (P) and markedly affected (A) chickens. Gene expression analysis was conducted using the nCounter MAX Analysis System and data were analyzed using four different supervised machine-learning methods, including support vector machines (SVM), random forests (RF), elastic net logistic regression (ENET) and Lasso logistic regression (LASSO). The SVM method achieved the highest prediction accuracy for both three-class (U, P and A) and two-class (U and P+A) classifications with 94% prediction accuracy for two-class classification and 85% for three-class classification. The results also identified biomarkers of Wooden Breast severity and development. Additionally, gene expression analysis and ultrastructural evaluations provided evidence of vascular endothelial cell dysfunction in the early pathogenesis of Wooden Breast.

Microsomal T system: a stereological analysis of purified microsomes derived from normal and dystrophic skeletal muscle.

Heterogeneous populations of microsomes obtained from normal and dystrophic chicken pectoralis muscle were separated into two subfractions by an iterative loading technique. The buoyant density of the sarcoplasmic reticulum (SR) microsomes was increased after loading them with calcium oxalate. Several incubations in the transport medium were necessary to load all of the SR. The fraction that did not form a pellet contained microsomes which displayed freeze-fracture faces that had a low density of particles. A stereological analysis was used on membrane fracture faces of intact muscle to generate reference particle density distributions, which were compared with the distributions measured on the microsomal fracture faces. The concave microsomal fracture faces of purified microsomes which did not load calcium oxalate had particle distributions nearly identical to the distributions of intact P-face T tubules. The morphological data suggest that this subfraction is microsomal T system. Biochemical measurements show negligible amounts of specific Na+, K+-ATPase activity, suggesting that there was little contamination from the surface membrane in this subfraction. Furthermore, an active Ca2+-ATPase is demonstrated in both normal and dystrophic T-tubular membranes.

Pectoral Vessel Density and Early Ultrastructural Changes in Broiler Chicken Wooden Breast Myopathy.

In wooden breast myopathy (WBM) of broiler chickens, the pectoralis major muscles show abnormally hard consistency and microscopical myodegeneration of unknown aetiology. To date, previous studies have focused primarily on chronic WBM and ultrastructural descriptions of early WBM are lacking. The aim of this study was to elucidate the pathogenesis of WBM by light microscopical morphometry of vessel density and the ultrastructural description of early WBM changes with transmission electron microscopy. The pectoral vessel density was compared between unaffected chickens (n = 14) and two areas of focal WBM in affected chickens (n = 14). The transverse myofibre area per vessel was highest in the unaffected area of muscle from cases of focal WBM, significantly higher (P = 0.01) than in macroscopically unaffected tissue, indicating that relatively decreased blood supply may trigger the development of WBM. The ultrastructural study included unaffected chickens (n = 3), two areas of focal WBM from affected chickens (n = 3) and areas of diffuse WBM from affected chickens (n = 3). The morphologically least affected myofibres within the WBM lesion areas in light microscopy exhibited ultrastructural changes of increased sarcoplasmic reticulum diameter and mitochondrial hyperplasia. Such changes originate typically from osmotic imbalance, for which the most likely aetiologies in WBM include tissue hypoxia or myodegeneration of the surrounding myofibres. The findings suggest that a relative reduction of blood supply in the major pectoral muscle occurs in the early phase of WBM, which may be linked to the ultrastructural changes of osmotic imbalance.

The Effect of the Wooden Breast Myopathy on Sarcomere Structure and Organization.

The wooden breast (WB) has been classically identified by the phenotypic presence of a wood-like pectoralis major (p. major) muscle. The WB-affected p. major muscle is characterized by necrotic muscle fibers and the replacement of muscle with connective tissue, water, and fat. The objective of the current study was to determine the effect of the WB myopathy on sarcomere organization by transmission electron microscopy. Sarcomere structure and organization were examined in two broiler lines with a high incidence of WB (Lines A and B) and another broiler line without WB (Line C). Affected muscle had an increase in smaller myofibers with diameters of 20 µm or less. Sarcomere organization decreased with fiber diameter in both Lines A and B. The structure and organization of sarcomeres in Line C were similar to WB-unaffected muscle in Lines A and B. Taken together, these data demonstrate that the WB myopathy detrimentally affects sarcomere organization in a broiler line-specific manner. Disorganization of sarcomere structure will affect the function of the p. major muscle as well as meat quality.

Ultrastructural localization of M-band proteins in chicken breast muscle as revealed by combined immunocytochemistry and ultramicrotomy.

Cryo-ultramicrotomy and "conventional" plastic sectioning have been used in combination with extraction and immunolabeling techniques to determine the location of the two M-band proteins characterized to date, MM-creatine kinase (MM-CK: Mr, 80,000) and M-protein "myomesin" (Mr, 165,000) within the M-region of chicken pectoralis muscle. The following main results were obtained. (1) The M-band in chicken pectoralis muscle contains five major striations (M1, M4 and M4', M6 and M6' in the terminology of Sjöström & Squire, 1977a). (2) Extraction of the bulk of the electron-dense M-band with low ionic strength removes the M-striations M1, M4 and M4' while M6 and M6' are retained. Cross-sections through the M-region of such muscles lack primary M-bridges connecting the thick myosin filaments. (3) Labeling with antibodies against MM-CK enhances the M-striations M4 and M4'; sometimes the whole region between M4 and M4' is labeled. (4) Incubation with antibodies against myomesin results in the labeling of the whole M-band from M6 to M6'; no label is found in the rest of the bare zone outside M6 and M6'. (5) Incubation of low ionic strength extracted muscle fibers with antibodies against myomesin leads to an "incomplete" labeling of the M-band between M6 and M6'; lines M6 and M6' are sometimes seen to be enhanced presumably due to antibody labeling. From these results it is concluded that MM-CK is the major protein of the M4 and M4' (and possibly also of the M1) M-bridges. Myomesin is bound within the M-band along the thick filaments from M6 to M6'. Two hypothetical models for the possible location of myomesin are discussed. According to these models myomesin would either make up the M-filaments or be directly attached to and along the central bare zone of thick myosin filaments.

Novel thick filament protein of chicken pectoralis muscle: the 86 kd protein. II. Distribution and localization.

Antibodies specific for the novel 86 kd protein purified from chicken pectoralis myofibrils stained by indirect immunofluorescence the middle third of each half A-band of isolated myofibrils and myotubes. Pectoralis muscle 86 kd protein, like pectoralis C-protein, displayed a fibre-type specific distribution by being restricted to fast twitch fibres and absent in slow tonic and heart muscle fibres. This was demonstrated by immunoblotting experiments with tissue extracts and by immunofluorescence labelling of cryosections. In primary cell cultures prepared from embryonic chicken breast muscle, 86 kd protein, C-protein and myomesin were all detected in post-mitotic myoblasts where fluorescence was found in a cross-striated pattern along strands of nascent myofibrils. Fluorescence due to the 86 kd protein was restricted to myofibrils within myotubes and no significant labelling of the sarcoplasm was evident. Glycerinated fast twitch muscle fibres, after incubation with antibodies to 86 kd protein, revealed in each half of the A-band nine distinctly labelled stripes, spaced about 43 nm apart. Simultaneous incubation of fibres with antibodies against 86 kd protein and C-protein showed a co-localization of the seven C-protein stripes (stripes 5 to 11), with seven stripes of 86 kd protein. The two additional stripes (stripes 3 and 4) labelled by anti-86 kd antibody continued towards the M-band at the same periodicity from the last C-protein stripe (stripe 5). Thus, partial co-localization of two different thick filament proteins is demonstrated and the identity of transverse stripes at positions 3 and 4 attributed in part to the presence of the new 86 kd protein.

Thyroidal involvement in the expression of avian muscular dystrophy.

We showed previously that propylthiouracil (PTU), a thyroid inhibitor, could alleviate several major signs of hereditary muscular dystrophy in chickens. The goals of the present investigation were to: (1) determine whether a nearly athyroid condition (achieved within two days after hatching by surgical thyroidectomy plus PTU) during an 11-day period beneficially affects the dystrophic condition when followed by triiodothyronine (T3) replacement to 33 days of age; (2) determine the beneficial effects on the expression of avian dystrophy when the thyroidectomized-PTU-treated chickens received a wide range of moderate to low T3 replacement doses beginning by two days after thyroidectomy; and (3) examine the thyroid hormone receptor system in dystrophic muscle for a possible abnormality. Thyroid deprivation increased muscle function (righting ability) and reduced plasma creatine kinase activity in dystrophic chickens. The major thyroid-related abnormality in dystrophic pectoralis muscles was an increased maximum binding capacity of solubilized nuclear T3 receptors.

Benign spindle cell tumor of the male breast.

Benign spindle cell tumors of the male breast are definitely rare. They have a subareolar localization and are circumscribed but not encapsulated. Ultrastructurally the neoplasm consists of histiocytic cells, fibroblasts, myofibroblasts and their transitional stages. Myoepithelial cells or smooth muscle cells are lacking. Although myofibroblasts contain lots of filaments, aniline reaction, PTAH stain and iron hematoxylin impregnation remain negative.

Muscle fiber types in the dystrophic puboischiofemoralis of commercial broilers.

Muscle fiber types, their diameters, lipid content, and nuclei distributions were studied in normal and necrotic puboischiofemoral muscles from commercial broiler chickens of the Hubbard strain. Three categories of puboischiofemoral fiber morphology (alpha R, beta R, and alpha W) occur in the cranial and caudal portions of the pars lateralis and in the cranial, medial, and caudal portions of the pars medialis. The greatest amount of necrosis occurred in the caudal portion of the pars medialis of the puboischiofemoralis. A fiber type characterized by a greatly enlarged and rounded or oval cross-section, decreased fat content, and internalized and clustered nuclei was present in necrotic muscle but not in normal muscle.

Skeletal muscle ultrastructural pathology in Serinus canarius infected with Plasmodium cathemerium.

Serinus canarius infected with Plasmodium cathemerium was used as an animal model in order to study the skeletal muscle compromise in malaria. Pectoral muscle biopsies were obtained from 7 infected female birds. The transmission electron microscopic study showed alterations of contractile and sarcotubular systems, mitochondrial abnormalities, lysosomal proliferation and nuclear pyknosis. The sarcolemma looked disrupted and separated from the necrotic fibres. Capillary abnormalities included endothelial degeneration with proliferation of lysosomal structures, penetration of endothelial cell by the parasites and necrosis. A mononuclear cell infiltrate formed by plasmocytes, lymphocytes and macrophages was observed. This investigation shows that skeletal muscle is an important target tissue for some malaria parasites.

Possible maternal inheritance of breast muscle morphology in turkeys at sixteen weeks of age.

During the study of inheritance of breast muscle morphology of turkeys during embryonic development and posthatch in two experiments, interesting results were obtained at 16 wk of age. In experiment 1, an experimental line (F) selected long term for increased 16-wk BW was crossed reciprocally with a commercial sire line (B). Samples of pectoralis major muscle were obtained from three males and three females of each genetic group. The F line was reciprocally crossed with a randombred control line (RBC2) in experiment 2, and p. major muscle samples were taken from 10 males and 10 females of each genetic group. The RBC2 line was the base population for the F line. The muscle samples were obtained in a manner to avoid contraction. After fixing, the muscle samples were stained with hematoxylin and eosin to view muscle morphology. Distinct morphological types were observed in the muscle samples for the B, F, and RBC2 lines. The pectoralis major muscle from the B line was, in general, characterized by large fibers with a well-defined extracellular matrix (perimysial and endomysial spaces). Similar to the B line, representative samples of the RBC2 line had a well-defined extracellular matrix but muscle fiber size was not as large as that of the B line. Representative samples of the F line were characterized by a greatly reduced extracellular matrix with the muscle fibers not well defined. For the reciprocal crosses in both experiments, representative samples indicated the offspring had breast muscle morphology similar to that of the female parent, suggesting maternal inheritance. There was no sex effect on breast muscle morphology in either experiment. In order to study the consistency of the results, muscle morphology of representative sections of muscle were subjectively rated by four individuals. The ratings ranged from 1 (little extracellar matrix and indistinct muscle fibers) to 5 (large extracellular space and distinct muscle fibers). Ratings of 2 to 4 were intermediate to these extremes. In experiment 1, ratings of the pure B line and the F sire x B dam cross were similar and higher than that of the pure F line and B sire x F dam cross, which did not differ in average rating. The results for experiment 2 were similar to those for experiment 1 in that the average ratings of the reciprocal crosses were similar to that of the female parental line. The results from the ratings support maternal inheritance of breast muscle morphology.

Inheritance of breast muscle morphology in turkeys at sixteen weeks of age.

The inheritance of morphology of the pectoralis major muscle in turkeys at 16 wk of age was studied in a randombred control line (RBC2), a subline (F) of RBC2 selected long term for increased 16 wk BW only, and F1 and F2 crosses of the F and RBC2 lines. Samples of pectoralis major muscle were obtained from 10 males and 10 females of each genetic group in a manner to avoid muscle contraction. After being fixed and cross sectioned, the muscle samples were stained with hematoxylin and eosin to view muscle morphology. The stained sections were analyzed for muscle fiber width, number of fibers in a 136-microm2 area, and extracellular matrix perimysial (PW) and endomysial (EW) width in areas of sections in which accurate measurements could be made. Because muscle damage was evident in some sections and, therefore, morphological measurements might not have provided a complete overview of muscle morphology, sections of the F2 crosses were subjectively rated by 4 people. The ratings ranged from 1 (little extracellular matrix and indistinct muscle fibers) to 5 (large extracellular space and distinct muscle fibers). Ratings of 2 to 4 were intermediate to these extremes. Creatine kinase concentrations of blood samples taken immediately prior to collecting muscle tissue were obtained and correlated with muscle section ratings within genetic group and sex. The F and RBC2 lines differed in PW and EW but not in individual fiber measurements. In the F1 generation, heterosis was -10.4% (P < or = 0.01), 19.7% (P < or = 0.05), -25.2% (P < or = 0.01), and -34.3% (P < or = 0.01), respectively, for fiber width, number of muscle fibers, PW, and EW. The F2 crosses differed only in EW based on measurements of sections in which accurate measurements could be made. However, based on subjective ratings of the muscle sections, possible maternal inheritance was suggested, as the orthogonal contrast was significant (P < or = 0.01) for crosses with F dams as F1 parents vs. those with RBC2 dams as F1 parents, confirming a previous study. The correlation coefficient between creatine kinase concentration and muscle section ratings was -0.282 (P < or = 0.01) after adjustment for line and sex effects.

Is ribosomal capacity a potential metabolic marker of muscle development? Measurement by muscular biopsy.

Metabolic markers of muscle metabolism could help geneticists and nutritionists predict the breast meat development of chickens. The aim of the current study was to test the ribosomal capacity (CS), a potential metabolic marker, and to evaluate a simple biopsy method on the pectoralis major muscle. Ribosomal capacity was measured in three commercial meat chicken genotypes differing in their growth rate. Fast-, medium-, and slow-growing male chickens were fed using three commercial dietary programs of increasing energy and protein concentration (nine treatments). Biopsy was performed at 4 wk of age on the p. major 12 chickens per treatment. Fast-, medium-, and slow-growing chickens were slaughtered at market weight, i.e., 6, 8, and 12 wk of age, respectively, and breast meat (right and left p. major + minor) was dissected. A significant reduction in BW at slaughter (-1 to -3%) and breast meat yield (-4%) occurred only in biopsied medium-growing chickens but not in the other two types. Slow-growing chickens had a significantly lower CS (8.75 microg/mg) than the two other chicken types (9.40 and 9.46 microg/mg for fast- and medium-growing chickens, respectively). No significant dietary effect or interaction of dietary treatment with genotype was measured. The CS was not significantly correlated to breast meat development. Under conditions of the present experiment, CS may not be a relevant marker of subsequent breast meat development at 4 wk of age. The biopsy technique can easily be applied to other markers.

Breast muscle development in commercial broiler chickens.

Genetic and gender-related variations in breast muscle yield of broiler chickens may be attributed to differences in number and size of muscle cells (myofibers). In this study, male and female broilers from eight commercial strain crosses (SC) were compared for body and breast muscle weight with adjustment of the Gompertz function. Additionally, breast fillet dimensions (length, width, and depth) and myofiber density (myofiber number/area; MFD) were assessed. Live weight and breast muscle development was determined to 56 d of age at weekly intervals. MFD was assessed at 8 d of age. As expected, SC differed in BW, breast weight and yield, and breast fillet dimensions and had variations in growth curves. Maximal growth rate for breast weight was reached approximately 4 d after that of BW. Males and females showed different growth curves, with males having slower growth rate maturity parameter and reaching the maximal growth rate later than females for BW and breast weight. Breast depth was the breast measure with highest positive correlation to breast yield. SC differences could not be explained by MFD, but males had higher MFD density than females. The possible relationship of the MFD observations to total myofiber number is discussed.

Characteristics of the pectoralis superficialis and semimembranosus of broiler strain chickens, bantam chickens, and the reciprocal crosses.

At hatch, chicks from bantam sire x broiler dam (BaBr) matings were as heavy as those from broiler sire x broiler dam (BrBr) matings. Both were heavier than pure bantam chicks (BaBa) or chicks from broiler strain sires and bantam dams (BrBa), but weights of the latter two genotypes were identical. The Pectoralis superficialis and Semimembranosus of pure bantam chicks weighed less than those of pure broiler chicks throughout the studies. Muscle weights in the BaBr chicks were markedly heavier than those of genetically similar BrBa chicks at hatch. Genetic influences were quickly expressed and weights of muscles in these two groups had converged by 14 d of age. Myofiber cross-sectional areas at 1 d of age were not clearly associated with genotype or maternal environment, but clear patterns had begun to develop by 4 d of age. Myofiber area of both muscles was greater in BrBr than BaBa chicks by this age and after. Myofiber areas in the reciprocal crosses converged toward each other and away from the pure line chicks. By 11 d, the area of Pectoralis myofibers of BrBr chicks was greatest, those of the reciprocal crosses was intermediate and equal to each other, and that of BaBa chicks was least. A similar trend was apparent in Semimembranosus fibers by 11 d but the differences did not become significant until 21 d of age. Significant differences in myofiber number of the Semimembranosus were present (BrBr > BaBr > BrBa > BaBa). Thus, differences in muscle mass were the result of differences in both fiber number and fiber size.

Comparison of chicken genotypes: myofiber number in pectoralis muscle and myostatin ontogeny.

This study was performed to evaluate breast muscle development in chicken genotypes divergently selected for muscularity. In the first experiment, 2 commercial broiler lines (a high breast yield, HBY, and a normal breast yield broiler strain-cross, NBY) and a Leghorn line were grown up to 35 d to evaluate BW, breast weight, and breast yield. At 7 and 21 d of age, pectoralis muscle was used to estimate myofiber density (MFD, number of myofibers per mm2) and total apparent myofiber number (MFN). In the second experiment, the ontogeny of myostatin was determined from broiler- and Leghorn-type chick embryos, at embryonic days 1 to 20 (E1 to E20), using reverse transcription (RT)-PCR. As expected, the Leghorn line had lower BW, breast weight, and breast yield than broiler lines. The HBY line showed higher breast yield at all ages evaluated, but lower BW at 21 and 35 d than the NBY line. The Leghorn line had 45% higher MFD than broilers, which indicates an increased cross-sectional area of the myofibers in broiler lines. No MFD difference was observed between the broiler strains (P > 0.05). The myofiber number of broilers was more than twice that of Leghorns and HBY had 10% higher MFN than the NBY line. Myofiber number was correlated to BW (r = 0.58), breast weight (r = 0.58), and breast yield (r = 0.69). Conversely, MFD showed negative correlation with BW, breast weight, and breast yield (r = -0.85, -0.83, and -0.88, respectively). No effect of genotype or interaction between genotype and embryonic age was observed for myostatin expression. This study showed that broilers have higher MFN in the breast muscles than Leghorn-type chickens, and that high breast yield of broiler strains may be due to increased MFN. Higher muscularity of broilers, as compared with Leghorns, was not attributed to lower expression of myostatin during embryonic development.

Collagen content and architecture of the pectoralis muscle in male chicks and broilers reared under various nutritional conditions.

Varying chicken growth rates were induced with different nutritional regimes, and the collagen content and architecture of M. pectoralis (PT) were compared among 21-day-old chicks and broilers at 80 or 95 days of age. The percentage of muscle weight to live weight was higher in rapid growing chicks (8.4%) than slow growing chicks (6.3%). The 80-day-old broilers engaged in compensatory growth after the early slow growth period producing PT muscle at 11% of live weight. The 80- and 95-day-old chicks with restricted late growth after an early rapid growth period showed PT weight at 8% and 9% of live weight, respectively. Collagen content of the PT muscle markedly decreased from the chicks to the broilers. The collagen concentration was higher in the late-growth restricted broilers (1.67-1.88 mg/g) than the compensatory growth broilers (1.01-1.10 mg/g). Collagen concentration did not differ between the rapid and slow growing chicks (2.72 and 2.94 mg/g). Scanning electron micrographs showed thick and thin perimysia, and honeycomb endomysia. In the perimysia, a stack layer of collagen platelets and a reticular layer of collagen fiber cords were distinguished and collagen baskets of adipocytes were observed. The perimysial collagen fibers became thicker during growth of the chicks to broilers. However, in the late-growth restricted broilers, the perimysial collagen fibers seemed to have retarded development compared with the compensatory growth birds. The PT muscle of chickens develops optimally when body growth is enhanced. The PT muscle of the compensatory growth broilers had improved collagen architecture regardless of the marked decrease in collagen content.