Developmental autonomy of muscle fine structure in muscle lineage cells of ascidian embryos

We have observed ultrastructural features of muscle differentiation in the muscle lineage cells of cleavage-arrested whole embryos and partial embryos of ascidians. Whole embryos of Ciona intestinalis and Ascidia ceratodes were cleavage-arrested with cytochalasin B at the 8-cell stage and reared to an age equivalent to several hours after hatching; these embryos formed extensive myofilaments which were often further organized into myofibrils of different sizes and densities in the peripheral cytoplasm of the two muscle lineage blastomeres (B4.1 pair). Developing myofibrils in cleavage-arrested embryos resembled the muscle elements observed in normal hatched larvae, but were less uniformly organized. A similar development of myofilaments and myofibrils occurred in the muscle lineage cells of multicellular partial embryos reared to "hatching" age. These partial embryos resulted from the isolated muscle lineage pair (B4.1) of blastomeres of the 8-cell stage (Ciona and Ascidia), and from a muscle lineage blastomere pair (B5.2) isolated at the 16-cell stage (Ascidia). Muscle lineage cells in the partial embryos were readily identified by the dense aggregates of mitochondria in their cytoplasm. Taken together, these results from the two kinds of partial embryo effectively eliminate inductive interactions with embryonic tissues other than mesodermal as a necessary factor in the onset of self-differentiation in muscle lineage cells. The relative complexity of muscle phenotype expressed in cleavage-arrested and partial embryos attests to an unusually strong developmental autonomy in the ascidian muscle lineages. This autonomy lends further support to the theory that a localized and segregated egg cytoplasmic determinant is responsible for larval muscle development in ascidian embryos.     

Differentiation of histospecific ultrastructural features in cells of cleavage-arrested early ascidian embryos

Summary Ultrastructural features of histospecific differentiation were found in early cleavage stage ascidian embryos treated with cytochalasin B and held thereby in cleavagearrest until hatching time. Markers characteristic of tissue differentiation during normal embryonic and larval stages ofCiona intestinalis were expressed in muscle and two brain cell lineages of cleavage-arrested whole embryos and in epidermal and notochordal cell lineages of cleavage-arrested partial embryos. These features were muscle myofilaments and myofibrils, melanosomes of the brain pigment cells, cilium-derived structures present in a proprioceptive brain cell, extracellular test material of epidermal cell origin, and the sheath filaments, membrane leaflets, and vacuolar colloid associated with notochord cells. All of these ultrastructural markers of differentiation were blocked in their development by treatment of gastrula stage embryos with actinomycin D, an inhibitor of RNA synthesis, and presumably result from the expression of new gene activity. At the time of cleavage-arrest the five cell lineages studies still contained two or more unsegregated lineage pathways. Subsequent developmental autonomy within the lineages is consistent with the hypothesis of segregation during early development of functionally independent gene regulatory factors.     

Differentiation of tropomyosin-containing myofibrils in cleavage-arrested ascidian zygotes expressing acetylcholinesterase

Two muscle differentiation programs, acetylcholinesterase and tropomyosin-containing filaments and fibrils, occur together in the same cleavage-arrested zygotes (1-celled) of the ascidian Ciona intestinalis. Coexpression in such undivided but developing 'embryos' is consistent with the idea that separate elements of muscle differentiation are related at some regulatory level, perhaps through a single multi-gene regulatory factor. Fertilized Ciona eggs were exposed to cytochalasin B for 20 h and then briefly reacted histochemically for acetylcholinesterase activity. Strongly reacting specimens were selected and processed for transmission electron microscopy to reveal regions of muscle ultrastructure. Every acetylcholinesterase-reactive zygote tested contained muscle contractile elements; no example lacking acetylcholinesterase was found with myofilaments and myofibrils. As demonstrated by immunogold labelling, a polyclonal antibody to tropomyosin from Ciona adult body wall reacted differentially with the presumed ultrastructural muscle elements in cleavage-arrested zygotes. Site-specific reactions were also observed in larval tail muscle and the siphon muscles of postmetamorphic zooids.     

Cell contact induces multiple types of electrical excitability from ascidian two-cell embryos that are cleavage arrested and contain all cell fate determinants

Ascidian early embryonic cells undergo cell differentiation without cell cleavage, thus enabling mixture of cell fate determinants in single cells, which will not be possible in mammalian systems. Either cell in a two-cell embryo (2C cell) has multiple fates and develops into any cell types in a tadpole. To find the condition for controlled induction of a specific cell type, cleavage-arrested cell triplets were prepared in various combinations. They were 2C cells in contact with a pair of anterior neuroectoderm cells from eight-cell embryos (2C-aa triplet), with a pair of presumptive notochordal neural cells (2C-AA triplet), with a pair of presumptive posterior epidermal cells (2C-bb triplet), and with a pair of presumptive muscle cells (2C-BB triplet). The fate of the 2C cell was electrophysiologically identified. When two-cell embryos had been fertilized 3 h later than eight-cell embryos and triplets were formed, the 2C cells became either anterior-neuronal, posterior-neuronal or muscle cells, depending on the cell type of the contacting cell pair. When two-cell embryos had been fertilized earlier than eight-cell embryos, most 2C cells became epidermal. When two- and eight-cell embryos had been simultaneously fertilized, the 2C cells became any one of three cell types described above or the epidermal cell type. Differentiation of the ascidian 2C cell into major cell types was reproducibly induced by selecting the type of contacting cell pair and the developmental time difference between the contacting cell pair and 2C cell. We discuss similarities between cleavage-arrested 2C cells and vertebrate embryonic stem cells and propose the ascidian 2C cell as a simple model for toti-potent stem cells.     

Gray and Red Fragments of the Egg of the Ascidian Ciona savignyi: Preferential Development of Muscle Cells from Gray Fragments

The egg of the ascidian Ciona savignyi is pinkish red with brownish myoplasm that contains the putative determinants responsible for differentiation of muscle cells. When dechorionated unfertilized eggs were centrifuged at moderate speed, eggs were divided into centripetal, small gray fragments and centrifugal, large red fragments. The former contained the female pronucleus and clear cytoplasm, while most of the latter was filled with yolk granules. An antibody raised against the myoplasm of C. intestinalis eggs extensively stained the cortical region of gray fragments, while the antibody stained only small regions of the red fragments. After insemination, both fragments cleaved and gave rise to partial embryos. When development of muscle and epidermal cells in the partial embryos was examined with specific antibodies, muscle development was conspicuous in gray partial embryos, while epidermal differentiation was extensive in red partial embryos. Furthermore, when expression of markers of differentiation was examined in cleavage-arrested gray and red fragments, the number of arrested gray fragments exhibiting the muscle marker was about three-fold greater than in controls. These results suggest that putative muscle determinants are concentrated into gray fragments.     

Muscle lineage cytoplasm can change the developmental expression in epidermal lineage cells of ascidian embryos

Cytoplasm from muscle lineage blastomeres of an ascidian embryo can cause cells of a nonmuscle lineage to produce larval tail muscle acetylcholinesterase. Muscle cytoplasm was partitioned microsurgically into epidermal lineage blastomeres at the eight-cell stage. Posterior half-embryos (the two B3 cells) of Ascidia nigra were obtained first by separating the anterior and posterior blastomere pairs at the four-cell stage. At third cleavage, the two B3 cells divide into an ectodermal cell pair that gives rise solely to epidermal tissues, and a mesodermal-endodermal blastomere pair from which the tail muscle cells are derived. When the ectodermal and mesendodermal blastomere pairs were isolated from one another by microsurgery and reared as partial embryos, only cells originating from the mesendodermal blastomeres produced a histochemical acetylcholinesterase reaction. Immediately after cleavage of the isolated B3 cells into ectodermal and mesendodermal cell pairs, the cleavage furrows could be made to disappear by pressing firmly on the mesendodermal cells with a microneedle. Repeated up and down pressure with the microneedle at a new position across the mesendodermal cells caused furrows to reestablish in the new position, thereby incorporating mesodermal cytoplasm and increasing the size of the ectodermal cells. The cytoplasmically altered ectodermal blastomere pairs, which became detached from the mesendodermal cells by this microsurgical procedure, continued to divide and were reared to “larval” stages. One-third of these epidermal partial larvae produced patches of cells containing acetylcholinesterase. These results lend further support to the theory that choice of particular differentiation pathways (embryonic determination) in ascidian embryos is mediated by segregation of specific egg cytoplasmic determinants.     

Zygote shrinkage and subsequent development in some Hibiscus hybrids

Summary Early embryonic development was compared in self-fertilized embryos of the diploid species, Hibiscus costatus, and triploid hybrid embryos, H. costatus-aculeatus and H. costatus-furcellatus, the paternal parent in both hybrids being tetraploid. The self-fertilized zygotes shrank to 50% of the volume of the unfertilized egg. These young embryos showed marked polarity. There was a concentration of cytoplasm in the apical cells and large vacuoles in the basal cells. There was also a polar distribution of organelles within the embryo as a whole which probably reflected initial differentiation. In comparison, hybrid zygotes shrank only about 20% of their original volume but started division at about the same time as selffertilized zygotes. There appeared to be no polarization and little proliferation of the cytoplasm in the hybrids. Large vacuoles remained prominent throughout the hybrid embryos, while organelles were few in the scant cytoplasm and no polarization of these was evident. These highly divergent hybrid embryos had become necrotic and aborted by the time the normal, self-fertilized embryos had reached the late globular stage. This altered developmental sequence of the hybrids suggests that shrinkage and rearrangement of the zygote cytoplasm is essential for normal embryonic differentiation.     

Studies on the Cytoplasmic Determinant for Muscle Cell Differentiation in Ascidian Embryos: An Attempt at Transplantation of the Myoplasm

The 8-cell stage embryos of the ascidian Halocynthia roretzi which had been prevented from undergoing further divisions by continuous treatment with cytochalasin B could develop histospecific muscle acetylcholinesterase in two blastomeres (B4.1 and B4.1 cells). If the cytoplasm of a B4.1 or B4.1 cell was transplanted by microinjection into either an A4.1 or A4.1 cell of recipient embryos and the transplanted embryos were permanently cleavage-arrested with cytochalasin B, a few eventually developed AChE in three blastomeres instead of in just the two blastomeres found in cleavage-arrested control embryos. Judging from the relative positions of the blastomeres, the third AChE-producing cells appeared to be the A4.1 or A4.1 cells injected with the cytoplasm of B4.1 or B4.1. Although the success rate was considerably low, this result might indicate the presence in the cytoplasm of a determinant for the muscle-specific enzyme development.     

Isolation of cDNA Clones for Epidermis-Specific Genes of the Ascidian Embryo

The present investigation was conducted to isolate cDNA clones that correspond to epidermis-specific genes of the ascidian embryo. When cleavage of fertilized eggs of Halocynthia roretzi is blocked by treatment with cytochalasin B and the arrested eggs are reared as one-celled embryos for about 30 hr, they develop features of differentiation of the epidermis only. Translation in vitro of poly(A)⁺ RNA from cleavage-arrested embryos and analysis of the products by two-dimensional gel electrophoresis revealed several predominant polypeptides that were not detected in a similar analysis of fertilized eggs, suggesting the appearance of epidermis-specific mRNAs in cleavage-arrested embryos. A cDNA library was constructed from arrested one-celled embryos. Differential screening of the library with a total cDNA probe from cleavage-arrested embryos and with a similar probe from fertilized eggs yielded eight different cDNA clones specific for the cleavage-arrested embryos. Northern blot analysis revealed that the mRNAs that corresponded to these cDNAs were present in normal tailbud embryos. In addition, in situ hybridization of whole-mount specimens showed that the mRNAs were restricted to the epidermal cells of tailbud embryos.     

Two histospecific enzyme expressions in the same cleavage-arrested one-celled ascidian embryos

Fertilized eggs of the ascidian, Ciona intestinalis, were prevented from undergoing cytokinesis but not nuclear division by treatment with cytochalasin B. After appropriate times, such cleavage-arrested multinucleate zygotes developed acetylcholinesterase of larval tail muscle and an alkaline phosphatase ordinarily localized in the larval endoderm tissues. Separate histochemical reactions on one of a pair of samples taken from the eggs of single animals provided examples (6/34) in which the numbers of cytochalasin-treated embryos displaying the respective reaction product overlapped sufficiently (15-29%) to indicate that some of the zygotes had developed both enzymes in the same uncleaved single cell. With an actual dual-staining technique that can be applied to single cleavage-arrested zygotes, 62% of those developing a strong alkaline phosphatase reaction also had a strong acetylcholinesterase reaction. In other experiments, quantitative measurements of enzyme activity in homogenates of 114 single cleavage-arrested zygotes confirm directly that 18% of the zygotes produce both enzymes. There was no obligatory mutual exclusion of the potential for simultaneous expression of two tissue-specific characteristics that would ordinarily be segregated into different lineages during early cleavages. The cytoplasmic determinants believed responsible for these histotypic expressions can apparently function independently in the same cell.     

CYTOLOGICAL AND ULTRASTRUCTURAL STUDIES ON MUSCLE DIFFERENTIATION IN THE ASCIDIAN,PEROPHORA ORIENTALIS*,**

Muscle cell differentiation in the tail of the ascidian, Perophora orientalis, from early tail-bud embryos to swimming larvae, were studied cytologically and ultrastructurally. Myogenic cells did not form multinucleated myotubes, but remained as mononucleated cells. Nucleolar component increased prior to a marked increase in cytoplasmic RNA. Cytoplasmic RNA appeared first around nucleus and later concentrated in the peripheral cytoplasm. The fine filaments measuring 20–30 Å in their thin parts and 30–45 Å in their thick parts in diameter appeared initially, forming loose networks, in the peripheral cytoplasm where ribosome clusters had been concentrated. These filaments were tightly attached by particles of various size and density. These filaments tended to be arranged in parallel as they increased in their size. They seemed to be precursors of both actin and myosin filaments of formed myofibrils. Z band precursors were found as dense patches in association with loosely arranged myofilaments and consisted of particulate and filamentous materials. The myofibrils seemed to grow further by organizing free filaments into bundles and further by aligning bundles of myofilaments at both ends.     

The fine structure of differentiating muscle in the salamander tail

Summary Thin methacrylate sections of developing tails of Amblystoma opacum larvae were examined in the electron microscope and a series of stages in the differentiation of the myotome musculature was reconstructed from electron micrographs and earlier light microscopic studies of living muscle. The earliest muscle cell precursor that can be clearly identified is a round or oval cell with abundant cytoplasm containing scattered myofilaments and free ribonucleoprotein granules, but little endoplasmic reticulum. These cells sometimes form a syncytium and they may also be fused with adjacent formed muscle fibers by lateral processes. Nuclei are large and nucleoli are prominent. This cell, called a myoblast here, is distinctly different in its appearance from the adjacent mesenchymal cells which have abundant granular endoplasmic reticulum. The earliest myofilaments are of both the thick and thin varieties and are distributed in a disorganized fashion in the cytoplasm. These filaments are similar to the actin and myosin filaments described by Huxley and they are present in the cytoplasm at an earlier stage of differentiation than heretofore suspected from light microscopy studies. The first myofibrils are a heterogeneous combination of thick and thin filaments and dense Z bands and are not homogeneous as so many light microscopists have contended. As development progresses, cross striations become more orderly and definitive sarcomeres are formed. Thereafter, new myofilaments and Z bands seem to be added to the lateral surfaces and distal ends of existing myofibrils.Free ribonucleoprotein granules are a prominent part of the myoblast cytoplasm and are found in close association with the differentiating myofilaments in all stages of development. In early muscle fibers and some of the formed fibers, similar granules are often concentrated in the I bands. A theory of myofilament differentiation based on current concepts of the role of ribonucleoprotein in protein synthesis is presented in the discussion. Stages in myofibril formation and possible relationships of the filaments in developing muscle cells to other types of cytoplasmic filaments are also discussed.Supported by grant C-5196 from the United States Public Health Service.     

Mapping the distribution of differentiation potential for intestine, muscle, and hypodermis during early development in Caenorhabditis elegans

We have analyzed the differentiation potential of cells in early embryos of Caenorhabditis elegans by assessing the production of markers for intestinal, muscle, and hypodermal cell differentiation in cleavage-arrested blastomeres. Our results show that differentiation potential does not always segregate during cleavage in a linear fashion, i.e., a blastomere can express a differentiation potential that is absent in its parent blastomere and vice versa. Furthermore, the expression of a particular differentiation program by certain cleavage-arrested blastomeres is an exclusive event in that each cell will express only one program of differentiation, even though it may have the potential to express several.     

Differentiation Expression in Blastomeres of Cleavage-Arrested Embryos of the Ascidian Halocynthia roretzi

The question whether two or more different genetic programs are expressed in the common cytoplasm of single blastomeres or the expression of one genetic program somehow excludes expression of the other, was analyzed by assessing the occurrence of a muscle-specific and two epidermis-specific antigens in cleavage-arrested blastomeres in early embryos of the ascidian Halocynthia roretzi . Blastomeres which had been arrested in 1- to 4-cell stages expressed only the epidermis markers. Arrested 8-cell to 32-cell embryos produced both epidermis and muscle markers, but each cell expressed only one program of differentiation, even though some possessed the potential to express both. The differentiation expressions followed their cell lineages. These results indicate that at least in this experimental system differentiation markers of the two different cell-types are expressed exclusively.
A distinct order was noticed in expression of the two epidermis markers in a single blastomeres; a marker is always superiorly expressed, whereas the other appears only when the superior marker is expressed.     

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 was 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 II-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.     

Requirement of cell division for muscle actin expression in the primary muscle cell lineage of ascidian embryos

The role of cell division in the expression of muscle actin and its relationship to acetylcholinesterase (AChE) development was examined in cleavage-arrested embryos of the ascidian Styela. Muscle actin expression was detected by two-dimensional gel electrophoresis of radioactively labelled proteins and by in situ hybridization with a cDNA probe, whereas AChE activity was assayed by enzyme histochemistry. In the majority of cases, muscle actin expression was first detected in embryos arrested after the 16-cell stage. Some embryos showed muscle actin expression after arrest at the 8-cell stage, however, muscle actin mRNA did not accumulate in embryos arrested at earlier cleavages. The cells that expressed muscle actin in 8- to 64-cell cleavage-arrested embryos belonged to the primary muscle lineage; secondary muscle cell precursors did not express muscle actin. Zygotic muscle actin mRNA appeared to accumulate with myoplasmic pigment granules in the perinuclear region of cleavage-arrested embryos, suggesting that the myoplasm may have a role in the organization of muscle cells. In contrast to muscle actin, AChE was detected in a small proportion of embryos treated with cytochalasin as early as the 1- or 2-cell stage, and most embryos treated with cytochalasin at later cleavages expressed this enzyme in some of their cells. Most primary muscle lineage cells expressed both muscle actin mRNA and AChE, however, some cells expressed only muscle actin mRNA or AChE. The results suggest that at least three cleavages are required for muscle actin expression and that muscle actin and AChE expression can be uncoupled in cleavage-arrested embryos.     

The ascidian embryo as a prototype of vertebrate neurogenesis

Ascidian tadpole larvae, composed of only about 2500 cells, have a primitive nervous system which is derived from the neural plate. The stereotyped cell cleavage pattern and well characterized cell lineage in these animals allow the isolation and culture of identified blastomeres in variable combinations. Ascidian embryos express cell-type-specific markers corresponding to their cell fates, even when cultured under cleavage-arrest by cytochalasin B. This system provides us with a unique opportunity to study the roles of cell lineage and cell contact in early neuronal differentiation in the absence of events associated with complex morphogenesis. In addition, the isolated, cleavage-arrested blastomeres are ideally suited to electrical recording, permitting the use of ionic channels as specific markers for differentiation. In the cleavage-arrested embryos, suppression of one type of K+ channel, and induction of two types of Na+ channels, occur following cell contact with the vegetal blastomere. The combination of molecular and electrophysiological analyses on this simple animal system may provide insights into the nature of the cell interactions important in early neurogenesis, both in ascidians and in vertebrates.     

Sepsis stimulates release of myofilaments in skeletal muscle by a calcium-dependent mechanism.

Sepsis is associated with a pronounced catabolic response in skeletal muscle, mainly reflecting degradation of the myofibrillar proteins actin and myosin. Recent studies suggest that sepsis-induced muscle proteolysis may reflect ubiquitin-proteasome-dependent protein breakdown. An apparently conflicting observation is that the ubiquitin-proteasome pathway does not degrade intact myofibrils. Thus, it is possible that actin and myosin need to be released from the myofibrils before they can be ubiquitinated and degraded by the proteasome. We tested the hypothesis that sepsis results in disruption of Z-bands, increased expression of calpains, and calcium-dependent release of myofilaments in skeletal muscle. Sepsis induced in rats by cecal ligation and puncture resulted in increased gene expression of micro-calpain, m-calpain, and p94 and in Z-band disintegration in the extensor digitorum longus muscle. The release of myofilaments from myofibrillar proteins was increased in septic muscle. This response to sepsis was blocked by treating the rats with dantrolene, a substance that inhibits the release of calcium from intracellular stores to the cytoplasm. The present results provide evidence that sepsis is associated with Z-band disintegration and a calcium-dependent release of myofilaments in skeletal muscle. Release of myofilaments may be an initial and perhaps rate-limiting component of sepsis-induced muscle breakdown.     

长链非编码RNA与细胞多向分化

本文重点概述LncRNA参与细胞分化的相关机制,列举了LncRNA在不同细胞系中参与调控的各种作用方式,如LncRNA参与维持胚胎干细胞多潜能状态、胚胎干细胞分化、神经细胞分化、肌肉分化、表皮细胞分化、成骨细胞分化、脂肪生成的机制等.     

Determinative properties of muscle lineages in ascidian embryos

Blastomeres removed from early cleavage stage ascidian embryos and reared to 'maturity' as partial embryos often elaborate tissue-specific features typical of their constituent cell lineages. We used this property to study recent corrections of the ascidian larval muscle lineage and to compare the ways in which different lineages give rise to muscle. Our evaluation of muscle differentiation was based on histochemical localization and quantitative radiometric measurement of a muscle-specific acetylcholinesterase activity, and the development of myofilaments and myofibrils as observed by electron microscopy. Although the posterior-vegetal blastomeres (B4.1 pair) of the 8-cell embryo have long been believed to be the sole precursors of larval muscle, recent studies using horseradish peroxidase to mark cell lineages have shown that small numbers of muscle cells originate from the anterior-vegetal (A4.1) and posterior-animal (b4.2) blastomeres of this stage. Fully differentiated muscle expression in isolated partial embryos of A4.1-derived cells requires an association with cells from other lineages whereas muscle from B4.1 blastomeres develops autonomously. Clear differences also occurred in the time acetylcholinesterase activity was first detected in partial embryos from these two sources. Isolated b4.2 cells failed to show any muscle development even in combination with anterior-animal cells (a4.2) and are presumably even more dependent on normal cell interactions and associations. Others have noted an additional distinction between the different sources of muscle: muscle cells from non-B4.1 lineages occur exclusively in the distal part of the tail, while the B4.1 descendants contribute those cells in the proximal and middle regions. During the course of ascidian larval evolution tail muscle probably had two origins: the primary lineage (B4.1) whose fate was set rigidly at early cleavage stages and secondarily evolved lineages which arose later by recruitment of cells from other tissues resulting in increased tail length. In contrast to the B4.1 lineage, muscle development in the secondary lineages is controlled less rigidly by processes that depend on cell interactions.