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1. Anterior trunk muscle shows mix of axial and appendicular developmental patterns

   https://doi.org/10.1002/dvdy.95  

   Sagarin, Kathleen A. ; Redgrave, Anna C. ; Mosimann, Christian ; Burke, Ann C. ; Devoto, Stephen H. ( January 2019 , Developmental dynamics)

   null (Ed.)

   Background: Skeletal muscle in the trunk derives from the somites, paired segments of paraxial mesoderm. Whereas axial musculature develops within the somite, appendicular muscle develops following migration of muscle precursors into lateral plate mesoderm. The development of muscles bridging axial and appendicular systems appears mixed. Results: We examine development of three migratory muscle precursor-derived muscles in zebrafish: the sternohyoideus (SH), pectoral fin (PF), and posterior hypaxial (PHM) muscles. We show there is an anterior to posterior gradient to the developmental gene expression and maturation of these three muscles. SH muscle precursors exhibit a long delay between migration and differentiation, PF muscle precursors exhibit a moderate delay in differentiation, and PHM muscle precursors show virtually no delay between migration and differentiation. Using lineage tracing, we show that lateral plate contribution to the PHM muscle is minor, unlike its known extensive contribution to the PF muscle and absence in the ventral extension of axial musculature. Conclusions: We propose that PHM development is intermediate between a migratory muscle mode and an axial muscle mode of development, wherein the PHM differentiates after a very short migration of its precursors and becomes more anterior primarily by elongation of differentiated muscle fibers. 

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2. Gastrocnemius Muscle Structural and Functional Changes Associated with Domestication in the Turkey

   https://doi.org/10.3390/ani11071850  

   Stover, Kristin K. ; Sleboda, David A. ; Brainerd, Elizabeth L. ; Roberts, Thomas J. ( July 2021 , Animals)

   Selection for increased muscle mass in domestic turkeys has resulted in muscles twice the size of those found in wild turkeys. This study characterizes muscle structural changes as well as functional differences in muscle performance associated with selection for increased muscle mass. We compared peak isometric force production, whole muscle and individual fiber cross-sectional area (CSA), connective tissue collagen concentration and structure of the lateral gastrocnemius (LG) muscle in wild and adult domestic turkeys. We also explored changes with age between juvenile and adult domestic turkeys. We found that the domestic turkey’s LG muscle can produce the same force per cross-sectional area as a wild turkey; however, due to scaling, domestic adults produce less force per unit body mass. Domestic turkey muscle fibers were slightly smaller in CSA (3802 ± 2223 μm2) than those of the wild turkey (4014 ± 1831 μm2, p = 0.013), indicating that the absolutely larger domestic turkey muscles are a result of an increased number of smaller fibers. Collagen concentration in domestic turkey muscle (4.19 ± 1.58 μg hydroxyproline/mg muscle) was significantly lower than in the wild turkeys (6.23 ± 0.63 μg/mg, p = 0.0275), with visible differences in endomysium texture, observed via scanning electron microscopy. Selection for increased muscle mass has altered the structure of the LG muscle; however, scaling likely contributes more to hind limb functional differences observed in the domestic turkey. 

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3. Worms and gills, plates and spines: the evolutionary origins and incredible disparity of deuterostomes revealed by fossils, genes, and development

   https://doi.org/10.1111/brv.12908  

   Nanglu, Karma ; Cole, Selina R ; Wright, David F ; Souto, Camilla ( February 2023 , Biological Reviews)

   Deuterostomes are the major division of animal life which includes sea stars, acorn worms, and humans, among a wide variety of ecologically and morphologically disparate taxa. However, their early evolution is poorly understood, due in part to their disparity, which makes identifying commonalities difficult, as well as their relatively poor early fossil record. Here, we review the available morphological, palaeontological, developmental, and molecular data to establish a framework for exploring the origins of this important and enigmatic group. Recent fossil discoveries strongly support a vermiform ancestor to the group Hemichordata, and a fusiform active swimmer as ancestor to Chordata. The diverse and anatomically bewildering variety of forms among the early echinoderms show evidence of both bilateral and radial symmetry. We consider four characteristics most critical for understanding the form and function of the last common ancestor to Deuterostomia: Hox gene expression patterns, larval morphology, the capacity for biomineralization, and the morphology of the pharyngeal region. We posit a deuterostome last common ancestor with a similar antero‐posterior gene regulatory system to that found in modern acorn worms and cephalochordates, a simple planktonic larval form, which was later elaborated in the ambulacrarian lineage, the ability to secrete calcium minerals in a limited fashion, and a pharyngeal respiratory region composed of simple pores. This animal was likely to be motile in adult form, as opposed to the sessile origins that have been historically suggested. Recent debates regarding deuterostome monophyly as well as the wide array of deuterostome‐affiliated problematica further suggest the possibility that those features were not only present in the last common ancestor of Deuterostomia, but potentially in the ur‐bilaterian. The morphology and development of the early deuterostomes, therefore, underpin some of the most significant questions in the study of metazoan evolution.

    

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4. Anatomical and ontogenetic influences on muscle density

   https://doi.org/10.1038/s41598-021-81489-w  

   Leonard, Kaitlyn C. ; Worden, Nikole ; Boettcher, Marissa L. ; Dickinson, Edwin ; Omstead, Kailey M. ; Burrows, Anne M. ; Hartstone-Rose, Adam ( January 2021 , Scientific Reports)

   Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle’s contractile force potential and is derived from dividing muscle mass by the product of a muscle’s average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle. This density constant is usually taken from experimental studies of small samples of several model taxa using tissues collected predominantly from the lower limbs of adult animals. The generalized application of this constant to broader analyses of mammalian myology assumes that muscle density (1) is consistent across anatomical regions and (2) is unaffected by the aging process. To investigate the validity of these assumptions, we studied muscles of rabbits (Oryctolagus cuniculus) in the largest sample heretofore investigated explicitly for these variables, and we did so from numerous anatomical regions and from three different age-cohorts. Differences in muscle density and histology as a consequence of age and anatomical region were evaluated using Tukey’s HSD tests. Overall, we observed that older individuals tend to have denser muscles than younger individuals. Our findings also demonstrated significant differences in muscle density between anatomic regions within the older cohorts, though none in the youngest cohort. Approximately 50% of the variation in muscle density can be explained histologically by the average muscle fiber area and the average percent fiber area. That is, muscles with larger average fiber areas and a higher proportion of fiber area tend to be denser. Importantly, using the age and region dependent measurements of muscle density that we provide may increase the accuracy of PCSA estimations. Although we found statistically significant differences related to ontogeny and anatomical region, if density cannot be measured directly, the specific values presented herein should be used to improve accuracy. If a single muscle density constant that has been better validated than the ones presented in the previous literature is preferred, then 1.0558 and 1.0502 g/cm³would be reasonable constants to use across all adult and juvenile muscles respectively.

    

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5. Hidden limbs in the “limbless skink” Brachymeles lukbani : Developmental observations

   https://doi.org/10.1111/joa.13447  

   Smith‐Paredes, Daniel ; Griffith, Oliver ; Fabbri, Matteo ; Yohe, Laurel ; Blackburn, Daniel G. ; Siler, Cameron D. ; Bhullar, Bhart‐Anjan S. ; Wagner, Günter P. ( April 2021 , Journal of Anatomy)

   Reduced limbs and limblessness have evolved independently in many lizard clades. Scincidae exhibit a wide range of limb‐reduced morphologies, but only some species have been used to study the embryology of limb reduction (e.g., digit reduction inChalcidesand limb reduction inScelotes). The genusBrachymeles, a Southeast Asian clade of skinks, includes species with a range of limb morphologies, from pentadactyl to functionally and structurally limbless species. Adults of the small, snake‐like speciesBrachymeles lukbanishow no sign of external limbs in the adult except for small depressions where they might be expected to occur. Here, we show that embryos ofB. lukbaniin early stages of development, on the other hand, show a truncated but well‐developed limb with a stylopod and a zeugopod, but no signs of an autopod. As development proceeds, the limb's small size persists even while the embryo elongates. These observations are made based on external morphology. We used florescent whole‐mount immunofluorescence to visualize the morphology of skeletal elements and muscles within the embryonic limb ofB. lukabni. Early stages have a humerus and separated ulna and radius cartilages; associated with these structures are dorsal and ventral muscle masses as those found in the embryos of other limbed species. While the limb remains small, the pectoral girdle grows in proportion to the rest of the body, with well‐developed skeletal elements and their associated muscles. In later stages of development, we find the small limb is still present under the skin, but there are few indications of its presence, save for the morphology of the scale covering it. By use of CT scanning, we find that the adult morphology consists of a well‐developed pectoral girdle, small humerus, extremely reduced ulna and radius, and well‐developed limb musculature connected to the pectoral girdle. These muscles form in association with a developing limb during embryonic stages, a hint that “limbless” lizards that possess these muscles may have or have had at least transient developing limbs, as we find inB. lukbani. Overall, this newly observed pattern of ontogenetic reduction leads to an externally limbless adult in which a limb rudiment is hidden and covered under the trunk skin, a situation calledcryptomelia. The results of this work add to our growing understanding of clade‐specific patterns of limb reduction and the convergent evolution of limbless phenotypes through different developmental processes.

    

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