Cell Dissociation from the Tongue Epithelium and Mesenchyme/Connective Tissue of Embryonic-Day 12.5 and 8-Week-Old Mice.

Cell dissociation has been an essential procedure for studies at the individual-cell level and/or at a cell-population level (e.g., single cell RNA sequencing and primary cell culture). Yielding viable, healthy cells in large quantities is critical, and the optimal conditions to do so are tissue dependent. Cell populations in the tongue epithelium and underlying mesenchyme/connective tissue are heterogeneous and tissue structures vary in different regions and at different developmental stages. We have tested protocols for isolating cells from the mouse tongue epithelium and mesenchyme/connective tissue in the early developmental [embryonic day 12.5 (E12.5)] and young adult (8-week) stages. A clean separation between the epithelium and underlying mesenchyme/connective tissue was easy to accomplish. However, to further process and isolate cells, yielding viable healthy cells in large quantities, and careful selection of enzymatic digestion buffer, incubation time, and centrifugation speed and time are critical. Incubation of separated epithelium or underlying mesenchyme/connective tissue in 0.25% Trypsin-EDTA for 30 min at 37 °C, followed by centrifugation at 200 x g for 8 min resulted in a high yield of cells at a high viability rate (>90%) regardless of the mouse stages and tongue regions. Moreover, we found that both dissociated epithelial and mesenchymal/connective tissue cells from embryonic and adult tongues could survive in the cell culture-based medium for at least 3 h without a significant decrease of cell viability. The protocols will be useful for studies that require the preparation of isolated cells from mouse tongues at early developmental (E12.5) and young adult (8-week) stages requiring cell dissociation from different tissue compartments.

Cardiopharyngeal mesoderm origins of musculoskeletal and connective tissues in the mammalian pharynx.

Cardiopharyngeal mesoderm (CPM) gives rise to muscles of the head and heart. Using genetic lineage analysis in mice, we show that CPM develops into a broad range of pharyngeal structures and cell types encompassing musculoskeletal and connective tissues. We demonstrate that CPM contributes to medial pharyngeal skeletal and connective tissues associated with both branchiomeric and somite-derived neck muscles. CPM and neural crest cells (NCC) make complementary mediolateral contributions to pharyngeal structures, in a distribution established in the early embryo. We further show that biallelic expression of the CPM regulatory gene Tbx1, haploinsufficient in 22q11.2 deletion syndrome patients, is required for the correct patterning of muscles with CPM-derived connective tissue. Our results suggest that CPM plays a patterning role during muscle development, similar to that of NCC during craniofacial myogenesis. The broad lineage contributions of CPM to pharyngeal structures provide new insights into congenital disorders and evolution of the mammalian pharynx.

Examination of the Topographical Anatomy and Fetal Development of the Tendinous Annulus of Zinn for a Common Origin of the Extraocular Recti.

Purpose: The aim was to clarify the topographical anatomy of the common tendinous ring for the four rectus muscles in both adults and fetuses. Methods: We histologically examined the annular ligament for a common origin of the extraocular rectus muscles using 10 specimens from elderly individuals and 31 embryonic and fetal specimens. Results: At 6 to 8 weeks, each rectus carried an independent long tendon, individually originating from the sphenoid. Notably, we found additional origins from the optic or oculomotor nerve sheath. At 12 to 15 weeks, the lateral, inferior, and medial recti muscles were united to provide a C-shaped musculofibrous mass that was separated from the superior rectus originating from the edge of the optic canal opening. Morphologic features at 31 to 38 weeks were almost the same as those at 12 to 15 weeks, but the long and thick common tendon of the three recti reached the sphenoid body in the parasellar area. In adults, a ring-like arrangement of the rectus muscles ended at a site 8.1 to 12.0 mm anterior to the optic canal opening and independent of the superior rectus origin, the lateral, inferior, and medial recti formed a C-shaped muscle mass. The united origins of the three recti changed to a fibrous band extending along the superomedial wall of the orbital fissure. Conclusions: Consequently, none of the specimens we examined exhibited an annular tendon representing a common origin of the four recti, suggesting that the common tendinous ring includes only medial, lateral, and inferior rectus muscles with the superior rectus taking its origin independently.

Connecting muscle development, birth defects, and evolution: An essential role for muscle connective tissue.

Skeletal muscle powers all movement of the vertebrate body and is distributed in multiple regions that have evolved distinct functions. Axial muscles are ancestral muscles essential for support and locomotion of the whole body. The evolution of the head was accompanied by development of cranial muscles essential for eye movement, feeding, vocalization, and facial expression. With the evolution of paired fins and limbs and their associated muscles, vertebrates gained increased locomotor agility, populated the land, and acquired fine motor skills. Finally, unique muscles with specialized functions have evolved in some groups, and the diaphragm which solely evolved in mammals to increase respiratory capacity is one such example. The function of all these muscles requires their integration with the other components of the musculoskeletal system: muscle connective tissue (MCT), tendons, bones as well as nerves and vasculature. MCT is muscle's closest anatomical and functional partner. Not only is MCT critical in the adult for muscle structure and function, but recently MCT in the embryo has been found to be crucial for muscle development. In this review, we examine the important role of the MCT in axial, head, limb, and diaphragm muscles for regulating normal muscle development, discuss how defects in MCT-muscle interactions during development underlie the etiology of a range of birth defects, and explore how changes in MCT development or communication with muscle may have led to the modification and acquisition of new muscles during vertebrate evolution.

How to make a tongue: Cellular and molecular regulation of muscle and connective tissue formation during mammalian tongue development.

The vertebrate tongue is a complex muscular organ situated in the oral cavity and involved in multiple functions including mastication, taste sensation, articulation and the maintenance of oral health. Although the gross embryological contributions to tongue formation have been known for many years, it is only relatively recently that the molecular pathways regulating these processes have begun to be discovered. In particular, there is now evidence that the Hedgehog, TGF-Beta, Wnt and Notch signaling pathways all play an important role in mediating appropriate signaling interactions between the epithelial, cranial neural crest and mesodermal cell populations that are required to form the tongue. In humans, a number of congenital abnormalities that affect gross morphology of the tongue have also been described, occurring in isolation or as part of a developmental syndrome, which can greatly impact on the health and well-being of affected individuals. These anomalies can range from an absence of tongue formation (aglossia) through to diminutive (microglossia), enlarged (macroglossia) or bifid tongue. Here, we present an overview of the gross anatomy and embryology of mammalian tongue development, focusing on the molecular processes underlying formation of the musculature and connective tissues within this organ. We also survey the clinical presentation of tongue anomalies seen in human populations, whilst considering their developmental and genetic etiology.

Unique morphogenetic signatures define mammalian neck muscles and associated connective tissues.

In vertebrates, head and trunk muscles develop from different mesodermal populations and are regulated by distinct genetic networks. Neck muscles at the head-trunk interface remain poorly defined due to their complex morphogenesis and dual mesodermal origins. Here, we use genetically modified mice to establish a 3D model that integrates regulatory genes, cell populations and morphogenetic events that define this transition zone. We show that the evolutionary conserved cucullaris-derived muscles originate from posterior cardiopharyngeal mesoderm, not lateral plate mesoderm, and we define new boundaries for neural crest and mesodermal contributions to neck connective tissue. Furthermore, lineage studies and functional analysis of Tbx1- and Pax3-null mice reveal a unique developmental program for somitic neck muscles that is distinct from that of somitic trunk muscles. Our findings unveil the embryological and developmental requirements underlying tetrapod neck myogenesis and provide a blueprint to investigate how muscle subsets are selectively affected in some human myopathies.

Formation of intramuscular connective tissue network in fish: first insight from the rainbow trout (Oncorhynchus mykiss).

The formation of the intramuscular connective tissue was investigated in rainbow trout Oncorhynchus mykiss by combining histological and in situ gene-expression analysis. Laminin, a primary component of basement membranes, surrounded superficial slow and deep fast muscle fibres in O. mykiss as soon as the hatching stage (c. 30 days post fertilization (dpf)). In contrast, type I collagen, the primary fibrillar collagen in muscle of vertebrates, appeared at the surface of individual slow and fast muscle fibres only at c. 90 and 110 dpf, respectively. The deposition of type I collagen in laminin-rich endomysium ensheathing individual muscle fibres correlated with the late appearance of collagen type 1 α 1 chain (col1α1) expressing fibroblasts inside slow and then fast-muscle masses. Double in situ hybridization indicated that coll1α1 expressing muscle resident fibroblasts also expressed collagen type 5 α 2 chain (col5α2) transcripts, showing that these cells are a major cellular source of fibrillar collagens within O. mykiss muscle. At c. 140 dpf, the formation of perimysium-like structure was manifested by the increase of type I collagen deposition around bundles of myofibres concomitantly with the alignment and elongation of some collagen-expressing fibroblasts. Overall, this study shows that the formation of O. mykiss intramuscular connective tissue network is completed only in aged fry when fibroblast-like cells expressing type I and V collagens arise inside of the growing myotome.

Developmental origin and morphogenesis of the diaphragm, an essential mammalian muscle.

The diaphragm is a mammalian skeletal muscle essential for respiration and for separating the thoracic and abdominal cavities. Development of the diaphragm requires the coordinated development of muscle, muscle connective tissue, tendon, nerves, and vasculature that derive from different embryonic sources. However, defects in diaphragm development are common and the cause of an often deadly birth defect, Congenital Diaphragmatic Hernia (CDH). Here we comprehensively describe the normal developmental origin and complex spatial-temporal relationship between the different developing tissues to form a functional diaphragm using a developmental series of mouse embryos genetically and immunofluorescently labeled and analyzed in whole mount. We find that the earliest developmental events are the emigration of muscle progenitors from cervical somites followed by the projection of phrenic nerve axons from the cervical neural tube. Muscle progenitors and phrenic nerve target the pleuroperitoneal folds (PPFs), transient pyramidal-shaped structures that form between the thoracic and abdominal cavities. Subsequently, the PPFs expand across the surface of the liver to give rise to the muscle connective tissue and central tendon, and the leading edge of their expansion precedes muscle morphogenesis, formation of the vascular network, and outgrowth and branching of the phrenic nerve. Thus development and morphogenesis of the PPFs is critical for diaphragm formation. In addition, our data indicate that the earliest events in diaphragm development are critical for the etiology of CDH and instrumental to the evolution of the diaphragm. CDH initiates prior to E12.5 in mouse and suggests that defects in the early PPF formation or their ability to recruit muscle are an important source of CDH. Also, the recruitment of muscle progenitors from cervical somites to the nascent PPFs is uniquely mammalian and a key developmental innovation essential for the evolution of the muscularized diaphragm.

Four and a half domain 2 (FHL2) scaffolding protein is a marker of connective tissues of developing digits and regulates fibrogenic differentiation of limb mesodermal progenitors.

Four and a half LIM domain 2 (FHL2) is a multifunctional scaffolding protein of well-known function regulating cell signalling cascades and gene transcription in cancer tissues. However, its function in embryonic systems is poorly characterized. Here, we show that Fhl2 is involved in the differentiation of connective tissues of developing limb autopod. We show that Fhl2 exhibits spatially restricted and temporally dynamic expression around the tendons of developing digits, interphalangeal joint capsules, and fibrous peridigital tissue. Immunolabelling analysis of the skeletal progenitors identified a predominant, but not exclusive, cytoplasmic distribution of FHL2 being associated with focal adhesions and actin cytoskeleton. In the course of chondrogenic differentiation of cultures of limb skeletal progenitors, the expression of Fhl2 is down-regulated. Furthermore, cultures of skeletal progenitors overexpressing Fhl2 take on a predominant fibrogenic appearance. Both gain-of-function and loss-of-function experiments in the micromass culture assays revealed a positive transcriptional influence of Fhl2 in the expression of fibrogenic markers including Scleraxis, Tenomodulin, Tenascin C, ßig-h3, and Tgif1. We further show that the expression of Fhl2 is positively regulated by profibrogenic signals including Tgfß2, all-trans-retinoic acid, and canonical Wnt signalling molecules and negatively regulated by prochondrogenic factors of the bone morphogenetic protein family. Expression of Fhl2 is also regulated negatively in immobilized limbs, but this influence appears to be mediated by other connective tissue markers, such as Tgfßs and Scleraxis.

Fibrillin-1 Expression Is Decreased in the Diaphragmatic Muscle Connective Tissue of Nitrofen-Induced Congenital Diaphragmatic Hernia.

Introduction Diaphragmatic morphogenesis depends on proper formation of muscle connective tissue (MCT) and underlying extracellular matrix (ECM). Fibrillin-1 is an essential ECM protein and crucial for the structural integrity of MCT in the developing diaphragm. Recently, mutations in the fibrillin-1 gene (FBN1) have been identified in cases of congenital diaphragmatic hernia (CDH), thus suggesting that alterations in FBN1 gene expression may lead to diaphragmatic defects. We designed this study to investigate the hypothesis that the diaphragmatic expression of fibrillin-1 is decreased in the MCT of nitrofen-induced CDH. Materials and Methods Time-mated rats were exposed to nitrofen or vehicle on gestational day 9 (D9). Fetal diaphragms (n = 72) were harvested on D13, D15, and D18, and divided into control and nitrofen-exposed specimens. Laser-capture microdissection was used to obtain diaphragmatic tissue cells. Gene expression levels of FBN1 were analyzed by qRT-PCR. Immunofluorescence-double-staining for fibrillin-1 and the mesenchymal marker Gata4 was performed to evaluate protein expression and localization. Results Relative mRNA expression of FBN1 was significantly decreased in pleuroperitoneal folds on D13 (3.39 ± 1.29 vs. 5.47 ± 1.92; p < 0.05), developing diaphragms on D15 (2.48 ± 0.89 vs. 4.03 ± 1.62; p < 0.05), and fully muscularized diaphragms on D18 (2.49 ± 0.69 vs. 3.93 ± 1.55; p < 0.05) of nitrofen-exposed fetuses compared with controls. Confocal-laser-scanning microscopy revealed markedly diminished fibrillin-1 immunofluorescence mainly in MCT, associated with a reduction of proliferating mesenchymal cells in nitrofen-exposed fetuses on D13, D15, and D18 compared with controls. Conclusions Decreased expression of fibrillin-1 during morphogenesis of the fetal diaphragm may disrupt mesenchymal cell proliferation, causing malformed MCT and thus resulting in diaphragmatic defects in the nitrofen-induced CDH model.

Expression of Prx1 and Tcf4 is decreased in the diaphragmatic muscle connective tissue of nitrofen-induced congenital diaphragmatic hernia.

BACKGROUND/PURPOSE: Pleuroperitoneal folds (PPFs) are the source of the primordial diaphragm's muscle connective tissue (MCT), and developmental mutations have been shown to result in congenital diaphragmatic hernia (CDH). The protein paired-related homeobox 1 (Prx1) labels migrating PPF cells and stimulates expression of transcription factor 4 (Tcf4), a novel MCT marker that controls morphogenesis of the fetal diaphragm. We hypothesized that diaphragmatic Prx1 and Tcf4 expression is decreased in the nitrofen-induced CDH model. METHODS: Time-mated rats were exposed to either nitrofen or vehicle on gestational day 9 (D9). Fetal diaphragms were microdissected on D13, D15, and D18, and divided into control and nitrofen-exposed specimens. Gene expression levels of Prx1 and Tcf4 were analyzed by qRT-PCR. Immunofluorescence double staining for Prx1 and Tcf4 was performed to evaluate protein expression and localization. RESULTS: Relative mRNA expression of Prx1 and Tcf4 was significantly downregulated in PPFs (D13), developing diaphragms (D15) and fully muscularized diaphragms (D18) of nitrofen-exposed fetuses compared to controls. Confocal laser scanning microscopy revealed markedly diminished Prx1 and Tcf4 expression in diaphragmatic MCT of nitrofen-exposed fetuses on D13, D15, and D18 compared to controls. CONCLUSIONS: Decreased expression of Prx1 and Tcf4 in the fetal diaphragm may cause defects in the PPF-derived MCT, leading to development of CDH in the nitrofen model. LEVEL OF EVIDENCE: Level 2c (Centre for Evidence-Based Medicine, Oxford).

Muscle composition is regulated by a Lox-TGFß feedback loop.

Muscle is an integrated tissue composed of distinct cell types and extracellular matrix. While much emphasis has been placed on the factors required for the specification of the cells that comprise muscle, little is known about the crosstalk between them that enables the development of a patterned and functional tissue. We find in mice that deletion of lysyl oxidase (Lox), an extracellular enzyme regulating collagen maturation and organization, uncouples the balance between the amount of myofibers and that of muscle connective tissue (MCT). We show that Lox secreted from the myofibers attenuates TGFß signaling, an inhibitor of myofiber differentiation and promoter of MCT development. We further demonstrate that a TGFß-Lox feedback loop between the MCT and myofibers maintains the dynamic developmental homeostasis between muscle components while also regulating MCT organization. Our results allow a better understanding of diseases such as Duchenne muscular dystrophy, in which LOX and TGFß signaling have been implicated and the balance between muscle constituents is disturbed.

Regulatory mechanisms of anthrax toxin receptor 1-dependent vascular and connective tissue homeostasis.

It is well known that angiogenesis is linked to fibrotic processes in fibroproliferative diseases, but insights into pathophysiological processes are limited, due to lack of understanding of molecular mechanisms controlling endothelial and fibroblastic homeostasis. We demonstrate here that the matrix receptor anthrax toxin receptor 1 (ANTXR1), also known as tumor endothelial marker 8 (TEM8), is an essential component of these mechanisms. Loss of TEM8 function in mice causes reduced synthesis of endothelial basement membrane components and hyperproliferative and leaky blood vessels in skin. In addition, endothelial cell alterations in mutants are almost identical to those of endothelial cells in infantile hemangioma lesions, including activated VEGF receptor signaling in endothelial cells, increased expression of the downstream targets VEGF and CXCL12, and increased numbers of macrophages and mast cells. In contrast, loss of TEM8 in fibroblasts leads to increased rates of synthesis of fiber-forming collagens, resulting in progressive fibrosis in skin and other organs. Compromised interactions between TEM8-deficient endothelial and fibroblastic cells cause dramatic reduction in the activity of the matrix-degrading enzyme MMP2. In addition to insights into mechanisms of connective tissue homeostasis, our data provide molecular explanations for vascular and connective tissue abnormalities in GAPO syndrome, caused by loss-of-function mutations in ANTXR1. Furthermore, the loss of MMP2 activity suggests that fibrotic skin abnormalities in GAPO syndrome are, in part, the consequence of pathophysiological mechanisms underlying syndromes (NAO, Torg and Winchester) with multicentric skin nodulosis and osteolysis caused by homozygous loss-of-function mutations in MMP2.

Anatomical and histological study of human deep fasciae development.

PURPOSE: To characterize the connective tissue found between the subcutaneous adipose tissue and the underlying muscle tissue in different regions and at different stages of human fetal development. We aim to identify its structural similarities to adult deep fascia, and to establish its role in myofascial development. METHODS: Samples from the arm, forearm, low back and thigh regions (from sites topographically homologous to the adult deep fascia) of five fetus body donors were obtained to perform gross anatomy dissection and histologic sections. Sections were stained with hematoxylin-eosin and Masson trichrome stain to observe their overall structure. Antiserum to protein S100 was used to analyze the presence and distribution of nerve fibers, and immunohistochemistry processing with Tcf4 marker was used to ensure fibroblast activity. RESULTS: Gross anatomy and histological sections of fetal samples showed the presence of connective tissue topographically and morphologically equivalent to adult deep fasciae. Developing blood vessels and nerves were found evenly distributed within the connective tissue during early development and in the portion adjacent to the muscle at later stages. The presence of Tcf4+ fibroblasts was confirmed in all analyzed mesenchymal connective tissue. CONCLUSIONS: Deep fascia is present from week 21 of human development in the lower back and upper and lower limbs. Blood vessels and nerves develop parallel to it and occasionally cross it from the deep to superficial plane. The presence of Tcf4+ fibroblasts in the deep fascia suggests a crucial role for this structure in muscle morphogenesis.

Structural study of gubernaculum testis in fetuses with prune belly syndrome.

PURPOSE: We compared and contrasted the structure of the gubernaculum testis in fetuses with prune belly syndrome and normal controls. MATERIALS AND METHODS: We studied a total of 6 gubernacula from 3 male fetuses with prune belly syndrome and a total of 14 from 7 male fetuses without an anomaly. Gubernacular specimens were cut into 5 µm sections and stained with Masson trichrome to quantify connective tissue and smooth muscle cells, with Weigert stain to observe elastic fibers and with picrosirius red with polarization to observe collagen. Immunohistochemical analysis was done with tubulin to observe the nerves. Images were captured with a BX51 microscope and DP70 camera (Olympus®). Stereological analysis was done with Image-Pro and ImageJ (MediaCybernetics®) using a grid to determine volumetric density. Means were statistically compared with the Mann-Whitney test. All tests were 2-sided with p <0.05 considered statistically significant. RESULTS: Prune belly syndrome fetuses were at 17 to 31 weeks of gestation and control fetuses were at 12 to 35 weeks of gestation. Quantitative analysis showed no difference in the volumetric density of smooth muscle cells in prune belly syndrome vs control gubernacula (mean 15.70% vs 19%, p = 0.2321). Collagen fiber analysis revealed a predominance of green areas in prune belly syndrome gubernacula, suggesting collagen type III, and a predominance of red areas in control gubernacula, suggesting collagen type I. Elastic fibers were significantly smaller in prune belly syndrome gubernacula than in control gubernacula (mean 14.06% vs 24.6%, p = 0.0190). Quantitative analysis demonstrated no difference in the volumetric density of nerves in prune belly syndrome or control gubernacula (mean 5.200% vs 3.158%, p = 0.2302). CONCLUSIONS: The gubernaculum in fetuses with prune belly syndrome had altered concentrations of collagen and elastic fibers. These structural alterations could be one of the factors involved in cryptorchidism in prune belly syndrome.

Anatomy research of nasolabial muscle structure in fetus with cleft lip: an iodine staining technique based on microcomputed tomography.

A thorough knowledge of the anatomic structure of the orbicularis oris of the upper lip and the nasalis in fetus with cleft lip is the key for the success of cleft lip repair. To understand the anatomic structure of the muscles of nasolabial region in fetus with cleft lip, the nasolabial tissues in 4 aborted fetuses with cleft lip were soaked for 7 days with iodine solution (Lugol solution of 3.75%) and were given micro-computed tomography. After the iodine solution permeated into the soft tissues, a good contrast was showed between muscle fibers and other fibrillar connective tissues. Through the observation of the obtained images, we found that most orbicularis oris fibers gathered into bundles with clear outline and only had slight deformation and displacement on the health side of the cleft of the unilateral incomplete cleft lip; however, in the lateral cleft, the muscle fibers not only had deformation and displacement but also were immature, disorganized, and not gathered into bundles. After being restored in Digital Imaging and Communications in Medicine format, the obtained images were then transferred into Materialise's interactive medical image control system, edited, and reconstructed into three-dimensional models. The models clearly showed the spatial relationship between the muscular tissues of the nasolabial region and the nasolabial outline in fetus with cleft lip.

Connective tissue alterations in Fkbp10-/- mice.

Osteogenesis imperfecta (OI) is an inherited brittle bone disorder characterized by bone fragility and low bone mass. Loss of function mutations in FK506-binding protein 10 (FKBP10), encoding the FKBP65 protein, result in recessive OI and Bruck syndrome, of which the latter is additionally characterized by joint contractures. FKBP65 is thought to act as a collagen chaperone, but it is unknown how loss of FKBP65 affects collagen synthesis and extracellular matrix formation. We evaluated the developmental and postnatal expression of Fkbp10 and analyzed the consequences of its generalized loss of function. Fkbp10 is expressed at low levels in E13.5 mouse embryos, particularly in skeletal tissues, and steadily increases through E17.5 with expression in not only skeletal tissues, but also in visceral tissues. Postnatally, expression is limited to developing bone and ligaments. In contrast to humans, with complete loss of function mutations, Fkbp10(-/-) mice do not survive birth, and embryos present with growth delay and tissue fragility. Type I calvarial collagen isolated from these mice showed reduced stable crosslink formation at telopeptide lysines. Furthermore, Fkbp10(-/-) mouse embryonic fibroblasts show retention of procollagen in the cell layer and associated dilated endoplasmic reticulum. These data suggest a requirement for FKBP65 function during embryonic connective tissue development in mice, but the restricted expression postnatally in bone, ligaments and tendons correlates with the bone fragility and contracture phenotype in humans.

Early fish myoseptal cells: insights from the trout and relationships with amniote axial tenocytes.

The trunk muscle in fish is organized as longitudinal series of myomeres which are separated by sheets of connective tissue called myoseptum to which myofibers attach. In this study we show in the trout that the myoseptum separating two somites is initially acellular and composed of matricial components such as fibronectin, laminin and collagen I. However, myoseptal cells forming a continuum with skeletogenic cells surrounding axial structures are observed between adjacent myotomes after the completion of somitogenesis. The myoseptal cells do not express myogenic markers such as Pax3, Pax7 and myogenin but express several tendon-associated collagens including col1a1, col5a2 and col12a1 and angiopoietin-like 7, which is a secreted molecule involved in matrix remodelling. Using col1a1 as a marker gene, we observed in developing trout embryo an initial labelling in disseminating cells ventral to the myotome. Later, labelled cells were found more dorsally encircling the notochord or invading the intermyotomal space. This opens the possibility that the sclerotome gives rise not only to skeletogenic mesenchymal cells, as previously reported, but also to myoseptal cells. We furthermore show that myoseptal cells differ from skeletogenic cells found around the notochord by the specific expression of Scleraxis, a distinctive marker of tendon cells in amniotes. In conclusion, the location, the molecular signature and the possible sclerotomal origin of the myoseptal cells suggest that the fish myoseptal cells are homologous to the axial tenocytes in amniotes.

Hox11 genes are required for regional patterning and integration of muscle, tendon and bone.

Development of the musculoskeletal system requires precise integration of muscles, tendons and bones. The molecular mechanisms involved in the differentiation of each of these tissues have been the focus of significant research; however, much less is known about how these tissues are integrated into a functional unit appropriate for each body position and role. Previous reports have demonstrated crucial roles for Hox genes in patterning the axial and limb skeleton. Loss of Hox11 paralogous gene function results in dramatic malformation of limb zeugopod skeletal elements, the radius/ulna and tibia/fibula, as well as transformation of the sacral region to a lumbar phenotype. Utilizing a Hoxa11eGFP knock-in allele, we show that Hox11 genes are expressed in the connective tissue fibroblasts of the outer perichondrium, tendons and muscle connective tissue of the zeugopod region throughout all stages of development. Hox11 genes are not expressed in differentiated cartilage or bone, or in vascular or muscle cells in these regions. Loss of Hox11 genes disrupts regional muscle and tendon patterning of the limb in addition to affecting skeletal patterning. The tendon and muscle defects in Hox11 mutants are independent of skeletal patterning events as disruption of tendon and muscle patterning is observed in Hox11 compound mutants that do not have a skeletal phenotype. Thus, Hox genes are not simply regulators of skeletal morphology as previously thought, but are key factors that regulate regional patterning and integration of the musculoskeletal system.

Anatomy and age estimation of an early blue whale (Balaenoptera musculus) fetus.

The external anatomy of a 130-mm blue whale fetus (Balaenoptera musculus) is described, and its internal anatomy is reconstructed noninvasively from microCT scans. The specimen lies developmentally at the junction of the embryonic and fetal periods. Similarly to the embryos of many odontocetes, it lacks a caudal fluke and dorsal fin, but it also exhibits an elongated rostrum, resorbed umbilical hernia, partially exposed cornea, and spatial separation of the anus and genitalia seen in early odontocete fetuses. Dermal ossification of the cranial bones has begun, but the endochondral skeleton is completely cartilaginous. The shape and position of the maxilla suggest that the earliest stages of anterior skull telescoping have begun, but there is no indication of occipital overlap posteriorly. The nasopharynx, larynx, and heart already display the distinctive morphology characteristic of Balaenoptera. This study develops a model of body length changes during blue whale development by integrating the large International Whaling Statistics (IWS) database, historical observations of blue whale migration and reproduction, and descriptions of fetal growth trends in other mammals. The model predicts an age of 65 days postconception for the specimen. The early developmental milestones of Balaenoptera mirror those of the odontocete Stenella to a remarkable extent, but the first appearance of the caudal fluke and dorsal fin are delayed relative to other morphological transitions. The accelerated prenatal growth characteristic of Balaenoptera occurs during fetal, not embryonic, development.