The fine structure of differentiating muscle in the salamander tail

Ca2+ as a coordinator of skeletal muscle differentiation, fusion and contraction

Muscle regeneration is essential for vertebrate muscle homeostasis and recovery after injury. During regeneration, muscle stem cells differentiate into myocytes, which then fuse with pre-existing muscle fibres. Hence, differentiation, fusion and contraction must be tightly regulated during regeneration to avoid the disastrous consequences of premature fusion of myocytes to actively contracting fibres. Cytosolic calcium (Ca²⁺ ), which is coupled to both induction of myogenic differentiation and contraction, has more recently been implicated in the regulation of myocyte-to-myotube fusion. In this viewpoint, we propose that Ca²⁺ -mediated coordination of differentiation, fusion and contraction is a feature selected in the amniotes to facilitate muscle regeneration.

3D in vitro models of skeletal muscle: myopshere, myobundle and bioprinted muscle construct

Typical two-dimensional (2D) culture models of skeletal muscle-derived cells cannot fully recapitulate the organization and function of living muscle tissues, restricting their usefulness in in-depth physiological studies. The development of functional 3D culture models offers a major opportunity to mimic the living tissues and to model muscle diseases. In this respect, this new type of in vitro model significantly increases our understanding of the involvement of the different cell types present in the formation of skeletal muscle and their interactions, as well as the modalities of response of a pathological muscle to new therapies. This second point could lead to the identification of effective treatments. Here, we report the significant progresses that have been made the last years to engineer muscle tissue-like structures, providing useful tools to investigate the behavior of resident cells. Specifically, we interest in the development of myopshere- and myobundle-based systems as well as the bioprinting constructs. The electrical/mechanical stimulation protocols and the co-culture systems developed to improve tissue maturation process and functionalities are presented. The formation of these biomimetic engineered muscle tissues represents a new platform to study skeletal muscle function and spatial organization in large number of physiological and pathological contexts.

Regenerative Models for the Integration and Regeneration of Head Skeletal Tissues

Disease of, or trauma to, the human jaw account for thousands of reconstructive surgeries performed every year. One of the most popular and successful treatment options in this context involves the transplantation of bone tissue from a different anatomical region into the affected jaw. Although, this method has been largely successful, the integration of the new bone into the existing bone is often imperfect, and the integration of the host soft tissues with the transplanted bone can be inconsistent, resulting in impaired function. Unlike humans, several vertebrate species, including fish and amphibians, demonstrate remarkable regenerative capabilities in response to jaw injury. Therefore, with the objective of identifying biological targets to promote and engineer improved outcomes in the context of jaw reconstructive surgery, we explore, compare and contrast the natural mechanisms of endogenous jaw and limb repair and regeneration in regenerative model organisms. We focus on the role of different cell types as they contribute to the regenerating structure; how mature cells acquire plasticity in vivo; the role of positional information in pattern formation and tissue integration, and limitations to endogenous regenerative and repair mechanisms.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

A Titan but not necessarily a ruler: assessing the role of titin during thick filament patterning and assembly

The sarcomeres of striated muscle are among the most elaborate and dynamic eukaryotic cellular protein machinery, and the mechanisms by which these semicrystalline filament networks are initially patterned and assembled remain contentious. In addition to the acto-myosin filaments that provide motor function, the sarcomere contains titin filaments, comprised of individual molecules of the giant Ig- and fibronectin domain-rich protein titin. Titin is the largest known protein, containing many structurally distinct domains with a variety of proposed functions, including sarcomere stabilization, the prevention of over-stretching, and returning to resting length after contraction. One molecule of titin, which binds to both the Z-disk and the M-line, spans a half-sarcomere, and is proposed to serve as a "molecular ruler" that dictates the spacing of sarcomeres. The semirigid rod-like A-band region of titin has also been proposed to act as a scaffold for thick filament formation during muscle development, but despite decades of research, this hypothesis has not been rigorously tested. Recent studies in zebrafish have brought into question the necessity for the A-band region of titin during the early stages of sarcomere patterning. In this review, we give an overview of the many different roles of titin in the development and function of striated muscle, and address the validity of the "molecular ruler" model of myofibrillogenesis in light of the current literature.

Differentiation of the sarcoplasmic reticulum and T system in developing chick skeletal muscle in vitro

The electron microscope was used to investigate the first 10 days of differentiation of the SR and the T system in skeletal muscle cultured from the breast muscle of 11-day chick embryos. The T-system tubules could be clearly distinguished from the SR in developing muscle cells fixed with glutaraldehyde and osmium tetroxide. Ferritin diffusion confirmed this finding: the ferritin particles were found only in the tubules identified as T system. The proliferation of both membranous systems seemed to start almost simultaneously at the earliest myotube stage. Observations suggested that the new SR membranes developed from the rough-surfaced ER as tubular projections. The SR tubules connected with one another to form a network around the myofibril. The T-system tubules were formed by invagination of the sarcolemma. The early extension of the T system by branching and budding was seen only in subsarcolemmal regions. Subsequently the T-system tubules could be seen deep within the muscle cells. Immediately after invaginating, the T-system tubule formed, along its course, specialized connections with the SR or ER: triadic structures showing various degrees of differentiation. The simultaneous occurrence of myofibril formation and membrane proliferation is considered to be important in understanding the coordinated events resulting in the differentiated myotube.

Vertebrates that regenerate as models for guiding stem cels

There are several animal model organisms that have the ability to regenerate severe injuries by stimulating local cells to restore damaged and lost organs and appendages. In this chapter, we will describe how various vertebrate animals regenerate different structures (central nervous system, heart and appendages) as well as detail specific cellular and molecular features concerning the regeneration of these structures.

Embryonic ultrastructure as a guide in the diagnosis of tumors

The purpose of this article has been to illustrate the applications and limitations of comparing the ultrastructural morphology of certain poorly differentiated neoplasms with normal human embryonic tissues. The examples used--embryonal rhabdomyosarcoma, Wilms' tumor, and fibrous mesothelioma, in comparison with embryonic mesoderm and its early derivatives--are but a few of the entities that can be investigated in this manner. Ewing's sarcoma was included in the report because of the long-standing and unsolved mystery concerning its histogenesis and because we believe it probably fits into the same category of poorly differentiated mesodermal neoplasms as the other lesions described. However, unlike the other more definite examples of primitive neoplasms manifesting focal markers of secondary mesenchymal differentiation, Ewing's sarcomas exhibit no ultrastructural evidence for myogeneous, nephrogenic, or mesothelial lines of maturation. Nor are there any morphologic indications of ectodermal or endodermal differentiation. The questions arise as to whether the category of Ewing's sarcoma has been perpetuated by so classifying all small-cell neoplasms having no recognizable line of differentiation, and whether this category may include cells of more than one type, that is, cells that are biologically determined to develop along several different pathways. This concept would be one explanation for the occurrence of Ewing's tumors in soft tissue as well as in bone, and we know that the cells contained in somites are ultrastructurally very similar to the least-differentiated cells making up the myotomes and sclerotomes. Apparently, questions such as these cannot be answered by morphologic studies alone. However, a number of other useful diagnostic and academic points of information can be gleaned.

Morphogenesis of motor endplates along the proximodistal axis of the mouse hindlimb

The morphogenesis of motor endplates along the proximodistal hindlimb axis is described for the mouse using nonspecific cholinesterase histochemistry and electron microscopy. There is a two day lag in relative stages of development between a proximal muscle (rectus femoris, RF) and a distal muscle (flexor hallucis brevis, FHB). Cholinesterase activity first appears in the RF on embryonic day 15 and the FHB on embryonic day 17. In the following days, faint wisps of reaction product thicken, form small ovals on myotubes, and finally enlarge with internal ramifications as the muscle fibers increase in diameter. Axons first enter the RF between embryonic days 12 and 13, and contact both embryonic cells (most likely myoblasts) and cells assumed to be Schwann cells. Myotubes are present in the RF the following day. The first signs of synapse formation-appearance of symmetrical electron opaque membrane patches, and dense cored and synaptic vesicles--occur between axons and myotubes in the RF on embryonic day 15. During the following days basal lamina material accumulates in the synaptic cleft, coated vesicles and postjunctional folds appear in the myotubes, and synaptic vesicles accumulate in the axon terminals. By postnatal day 42 the axon terminals lay in primary gutters opposite deep secondary postjunctional folds, and are separated and capped by Schwann cell processes.

An ultrastructural study of the differentiation of skeletal muscle in the bovine fetus

The differentiation of skeletal muscle was studied by electron microscopy in bovine fetuses from 47 days gestation to neonatal calves 3 days of age. Initially, the muscle was composed of clusters of myotubes with mononucleated myoblasts between them. In 2-month-old fetuses these myoblasts became apposed to the differentiating muscle cells and were enclosed within the rudimentary basal lamina of the myotubes. At this stage the clusters of myotubes consisted of central, larger diameter, more differentiated myotubes and also the mononucleated satellite cells. The differentiated myotubes separated from the clusters accompanied by satellite cells which continued proliferating and fused together to form new generations of satellite myotubes. In this manner new clusters of myotubes were formed. By 4-5 months some of the separating myotubes began to form individual myotubes and independent myofibers were prominent in fetuses of 5-8 months of age. The myofibers in the 8-month-old fetuses showed diversification into fiber types by differences in the thickness of the Z-line, the prominence of the sarcotubular system, the amount of glycogen and lipid droplets and also the number of mitochondria.

Fine structure of the nigrostriatal anlage in fetal rat brain by immunocytochemical localization of tyrosine hydroxylase

The developmental morphology and synaptic associations of neurons in the nigrostriatal anlage are examined by the electron microscopic immunocytochemical localization of tyrosine hydroxylase at embryonic (E) day 13.5 and 14.5 in rat brain. At E 13.5, immunoreactivity for the enzyme is localized throughout the cytoplasm of neuronal perikarya and processes including somatic, dendritic, and axonal growth cones. The cytoplasmic organelles in perikarya include primarily ribonucleic-protein particles, mitochondria and an immature Golgi apparatus. At E 14.5, the tyrosine hydroxylase labeled processes are detected in the lateral hypothalamus and ventrolateral caudate-putamen. The axonal processes showing immunoreactivity in the ventral mesencephalon and more rostral portions of the nigrostriatal bundle are frequently attached to unlabeled neurites by puncta adherentia. In the hypothalamus and caudate-putamen presumably transient synaptic junctions are also detected between the labeled axons and unlabeled neurons. The immature morphological features of neurons showing immunoreactivity for tyrosine hydroxylase thus indicate, that the biochemical differentiation of the nigrostriatal neurons precedes complete cytological differentiation.

Motility, linear arrangement and cell-to-cell contact of myogenic cells prior to fusion

Time-lapse cinematography elucidates the genesis of a uniform and approximately linearly arranged myogenic cell aggregate, stemming from two larger cell groups. The ultimate aggregate is created by continuous movement of one cell group toward the other. Following this motion, the angle between the cell groups is reduced as they approach each other. Different patterns of cell motility can be recognized. Some cells move in a preferred direction in relation to the aggregate as a whole, whereas others alter their direction of movement. The myogenic cells are aligned end-to-end and side-by-side. The latter is often accomplished in the following manner: two cells in end-to-end contact form as crescent-shaped free space with their polar extensions; a neighboring spindle-shaped cell then settles in this space. An arrangement of cells such that their greatest cytoplasmic widths lie at the same level can also be seen. During the recording period, two cells in one of the groups were replicating. One of them realized karyo- and cytokinesis in approximately 80 min. The daughter cells moved apart in opposite directions, but never lost contact to the aggregate. This observation shows that contact between presumptive myoblasts and myoblasts is established.

On the origin of Z-band material and myofilaments in myoblasts from the human atrial wall

The origin of cardiac myofibrils in cells from the atrial wall in human embryos was studied, Z-band substance appears throughout the cytoplasm as irregular electron dense patches in a network of thin filaments. The thin and thick filaments are synthesized as separate units in the sarcoplasm and are later aggregated into myofibrils. Complexes of Z substance and thin filaments occur numerously at different stages of myofibrillar organisation. Thick filaments are formed in close proximity to free ribosomes and are later incorporated in an hexagonal pattern into the Z-band/thin filament complex.

Nickel sub-sulphide-induced leiomyosarcoma in rabbit white skeletal muscle: a light microscopical and ultrastructural study

Nickel sub-sulphide-induced leiomyosarcomas in rabbit white skeletal muscle were studied by both light and electron microscopy. Two types of tumor cells, small spindle cells and elongated smooth muscle cells, are revealed by light microscopy. Nevertheless, their ultrastructure displays the same general feature. The most differentiated cells have abundant cytoplasmic filaments 7 nm in diameter, kept together in bundles by dense bodies. There also exist many 10 nm filaments and a large number of microtubules. The nuclei have prominent nucleoli with an extensive nucleolonema which form an irregular tridimensional network. Distinct fibrillar nuclear bodies were observed. Sometimes there exist desmosomes or gap junctions. The Golgi apparatus produces coated vesicles with secretory function. In the tumors were generally found the Ni3S2 implantations surrounded by a capsule, the major component of which were collagen fibers, degenerated nuclei and rod-like structures with a transverse periodicity of 15.5 nm. From these observations, several characteristics should be pointed out: 1) Many tumor cells contain large nucleoli and distinct intranuclear inclusions of undetermined nature. 2) The coated vesicles represent a secretory activity of the tumor cells; the coat material is probably used during the formation of cell membranes. Another possible function of coated vesicles could be the sequestering of calcium ions. 3) The rod-like structures in the Ni3S2-including capsule are not of Z-line material. 4) The tumoral stem myoblast in heart and skeletal muscle arise from mesenchymal cells.

Membrane dynamics of differentiating cultured embryonic chick skeletal muscle cells by fluorescence microscopy techniques

Changes in membrane fluidity during myogenesis have been studied by fluorescence microscopy of individual cells growing in monolayer cultures of embryonic chick skeletal muscle cells. Membrane fluidity was determined by the techniques of fluorescence photobleaching recovery (FPR), with the use of a lipid-soluble carbocyanine dye, and by fluorescence depolarization (FD), with perylene used as the lipid probe. The fluidity of myoblast plasma membranes, as determined from FPR measurements in membrane areas above nuclei, increased during the period of myoblast fusion and then returned to its initial level. The membrane fluidity of fibroblasts, also found in these primary cultures, remained constant. The fluidity in specific regions along the length of the myoblast membrane was studied by FD, and it was observed that the extended arms of the myoblast have the highest fluidity on the cell and that the tips at the ends of the arms had the lowest fluidity. However, since the perylene probe used in the FD experiments appeared to label cytoplasmic components, changes in fluidity measured with this probe reflect changes in membrane fluidity as well as in cytoplasmic fluidity. The relative change in each of these compartments cannot yet be ascertained. Tips have specialized surface structures, filopodia and lamellipodia, which may be accompanied by a more immobile membrane as well as a more rigid cytoplasm. Rounded cells, which may also have a more convoluted surface structure, show a lower apparent membrane fluidity than extended cells.

Ultrastructural investigation of NI3S2-induced rhabdomyosarcoma in Wistar rat: comparative study with emphasis on myofibrillar differentiation and ciliar formation

Nickel-sulfid-induced rhabdomyosarcomas were studied by both light and electron microscopy. The successive stages differentiating tumor cells were described, and two differentiation types of rhabdomyoblasts could be observed 1) with the characteristic pattern of fetal differentiation--i.e., myofilament apparation before Z-line formation--and 2) with synthesis of these elements in the reverse order. An organized T-system is not evident. The sarcoplasmic reticulum is irregular and its cisternae often contain a granular substance or microcrystals. The only well-developed element of tumoral myofibrils is the Z-line; the other zones of sarcomeres are seldom clearly defined. Several unusual granular structures were observed. No virus particles were found. The formation of cilia appears only in interphase rhabdomyoblasts and has to be considered as aberrant and temporary formations from centrioles. They generally possess a "9 + 0" microtubular pattern, but often could be observed as rudimentary forms with a "7 + 2" microtubular arrangement. This case is another example demonstrating the relationship of cilia formation with cell division, especially after suppression of mitotic control. The histology and the electron microscopy results are discussed in relation to the differentiation pattern and the ultrastructural features of embryonic, regenerating and pathological muscle differentiating in vitro.

The ultrastructure of normal myogenesis in the limb of the mouse

The ultrastructure of myogenesis in the mouse hind limb has been studied from day 12 to birth. Mononucleated cells with myofilaments are small and infrequent during myogenesis and never line myotubes. Only mononucleated cells without myofilaments cover the myotubes are involved in mass fusion. These mononucleated cells are pleomorphic and undifferentiated during early myogenesis. They have a heterochromatic nucleus and many accumulate mitochondria and lengthen during days 16-18. After day 18 they are normally elongate with a heterochromatic nucleus and packed with free ribosomes. The multinucleated cell development may be divided into four stages. 1. Immature myotube (days 14-15). Their fibrils are small, out of register and some poorly defined. Many myofilaments are not in rigid hexagonal alignment. The sarcoplasmic reticulum is simple and randomly oriented and triads are absent. Large clumps of glycogen occur between euchromatic nuclei which are in chain formation. 2. Mature myotube (days 16-18). Small groups of fibrils show alignment. Glycogen granules and sarcoplasmic reticulum become numerous between fibrils. Triads are sparse and mitochondria cluster in subsarcolemmal regions and between nuclei. 3. Young myofibers are present by day 19. More fibrils are aligned and compact. The filaments are in a rigid hexagonal array, the glycogen is dispersed and nuclei are peripherally located and moderately heterochromatic. Triads are frequent but often obliquely oriented and mitochondria are elongate and numerous between fibrils. 4. Mature myofibers occur postnatally (2 weeks).

The ultrastructure of the normal human thymus: a study of 36 cases

Electron microscopy of the normal human thymus demonstrates a supporting framework of epithelial-reticular cells with long branchticular cell processes lie lymphocytes, macrophages, and rare myoid cells. Both small and large lymphocytes are evident. No desmosomes are observed between the lymphocytes and the epithelial-reticular cells. Macrophages are most numerous in the cort(x were they often contain phagocytized nuclear debris. The possible functional significance of the above-described fine structural features is discussed.

Ultrastructural studies on the formation of myoflilaments and myofibrils in the human embryonic and adult hypertrophied heart

The myofibrillogenesis in the human embryonic heart is described. The synthesis of thin filaments, which are the first to appear, takes place in close proximity to smooth surfaced SR tubules. Z-band material is closely related to the thin filaments and appears first as irregularly distributed patches in the filamenteous mass. Further cellular differentiation includes an organization of the thin filaments/Z-band material. The synthesis of thick filaments, which follows that of the thin filaments, takes place in ribosome rich areas of the cell. They are rapidly incorporated into the strings of organized thin filaments/Z-band material. The periodic binding sites on both kinds of filaments are believed to play an important role in the precise ordering of the filaments. The formation of myofilaments in the adult hypertrophied human heart is also described. The similarities between this process and that observed in the embryonic heart are striking, and we believe it to be the same process.

Cell-to-cell communication and myogenesis

Cell-to-cell communication was characterized in prefusion chick embryo myoblast cultures, and it was determined that the prefusion myoblasts can interact via gap junctions, ionic coupling, and metabolic coupling. The biological relevance of this communication was supported by the detection of gap junctions between myoblasts in embryonic muscle. Communication was also examined in fusion-arrested cultures to determine its potential relationship to fusion competency. In cultures that were fusion arrested by treatment with either 1.8 mM ethyleneglycolbis-(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA), 3.3 X 10(-6) M 5-bromodeoxyuridine (BUdR), or 1 microgram/ml cycloheximide (CHX), both gap junctions and ionic coupling were present. Therefore, it is possible to conclude that cell communication is not a sufficient property by itself, to generate fusion between myob-asts. The potential role of communication in myogenesis is discusssed with respect to these observations.

Postnatal development of mitral cell perikaryon in the olfactory bulb of the rat. A light and ultrastructural study

The differentiation of the mitral cell perikaryon in postnatal rat olfactory bulb was studied with the light and electron microscope. At birth the mitral cell was distinguishable and occupied a definitive position in the mitral cell layer. The cell contained a large oval nucleus surrounded by a thin rim of cytoplasm. Ribosomes, free and clustered, were scattered in the cell cytoplasm. Rough endoplasmic reticulum was relatively scarce. The Golgi complexes were made up of stacks of smooth-surfaced cisternae and associated vesicles. In certain cases the Golgi complexes projected into cellular processes. Mitochondria were present in all regions of the cytoplasm and contained well developed cristae. At the end of the first week, the mitral cell had developed significantly in size, and the cytoplasm contained well-developed rough endoplasmic reticulum. The Golgi complexes were made up of several stacks of smooth-surfaced cisternae with the association of vesicles and electron dense bodies. The apical dendrites of mitral cells at this period had increased significantly in length. Subsequently, during the second and third week, the rough endoplasmic reticulum and Golgi complexes became well developed. Associated with the Golgi complexes were electron dense lysosomal bodies. At three weeks and in older cells it was observed that dense lipofuschin granules increased significantly. It is suggested that the mitral cell matures and differentiates earlier than cells in the cerebral cortex.

[Histochemical, quantitative and ultrastructural maturation of human fetal muscle]

Histochemical studies of muscle specimens from human fetuses showed: (a) a uniform fiber type population having the properties of Type IIC fibres up to 19 weeks of development; (b) a progressive appearance of Type I fibres after that age; (c) a decrease in number of Type IIC fibres during the last 3 months of pregnancy, accompanied by the appearance of Type IIB and Type IIA fibres; (d) the presence after the myotube stage of fibers with a light peripheral halo in sections stained for mitochondrial dehydrogenases. Electron-microscopic examination of the muscle fibres confirmed the existence of a peripheral halo devoid of myofibrils and mitochondria and showed: (a) scarcity of myofibrils in comparison with mature muscle fibres and (b) irregularity in shape of the myofibrils. In addition, quantitative studies demonstrated an important variation of the fibre diameters up to 21 weeks and the increase of the mean diameter after this age. It is suggested that the persistence after birth of some features of immaturity identical to those described in this work may be considered as a pathological finding.

Observations on the ultrastructure of developing myocardium of rat embryos

Timed pregnancies were obtained in Sprague-Dawley rats and early ultrastructural differentiation of myocardium of embryos of 10, 11, 12, 13, and 14 days were investigated and compared with that of newborn. Ten-day myocardium is characterized by loosely packed cells; the cytoplasm is typified by a dearth of organelles. Both thick (myosin) and thin (actin) filaments become identifiable for the first time in the 10-day myocardium where the heart is pulsating but circulation is not established. These filaments are not visible in the embryos of 9-day-old myocardium. The formation of these filaments is observed to continue throughout the period covered in this investigation. Concomitant with the appearance of the myofilaments is the synthesis of Z band material. By the eleventh day of gestation and during the subsequent days there is a rapid proliferation and differentiation of most of the organelles. The myofilaments become organized into fully formed striated fibrils. Intercalated discs appear as small wavy lines on the eleventh day and become plicated in later stages and serve as cell boundaries and points of attachment for myofilaments and fibrils. There is a perceptible change in the number and morphology of mitochondria from the tenth to eleventh day and later stages of development when the heart becomes functional. Similarly, there is a rapid proliferation and differentiation of granular endoplasmic reticulum and Golgi bodies. Large quantities of free ribosomes are dispersed in the cytoplasm of 10-day myocardium; however, in later stages there is a progressive reduction in the distribution of these particles. An intimate association of ribosomes and polysomes with the developing myofibrils is discernible. The T-system and sarcoplasmic reticulum begin to appear in 11-day myocardium. The embryonic myocardium displays intense mitotic activity throughout its development and a unique feature of embryonic myocardial cells is the simultaneous occurrence of myofilament synthesis and mitotic activity within the same cells.