Intrafusal muscle fibers of duck muscle spindles.

Serial transverse paraffin sections of intrafusal muscle fibers of spindles from the extensor pollicis and the extensor digitorum communis of ducks show that only one type of intrafusal muscle fiber exists, based on the mid-equatorial nucleation pattern, diameter, and length. Although the overall range in fiber diameter at the mid-equatorial region is between 4.2-20.0 microns, the average caliber is 10.4 +/- 3.18 microns (S.D.) for spindles of the extensor pollicis and 9.3 +/- 2.11 microns (S.D.) for spindles of the extensor digitorum communis muscles. The range in spindle length for the extensor pollicis is 290-2,090 microns, average 1,120 +/- 569 microns (S.D.), and for the extensor digitorum communis 1,160-2,500 microns, average 1,745 +/- 367 microns (S.D.). The range in number of fibers per spindle for the extensor pollicis muscle is 5-12, average 8.2, and for the extensor digitorum muscle it is 1-11. In the extensor digitorum communis, there appear to be two groups, based on fiber number. Spindles of one group have a range of 5-11 fibers per spindle with an average of 7.2, whereas the second group has a range of 1-4 with an average of 2.7 fibers per spindle. The second group of spindles constitutes 52.5% of the 40 spindles studied, and of these 7.5% were monofibril spindles, 15.0% difibril, 17.5% trifibril, 12.5% quadrifibril spindles.

The transverse tubular system of cat intrafusal muscle fibres.

A modified staining technique for transverse tubular and sarcoplasmic reticular systems was used to investigate their occurrence in different types of intrafusal muscle fibres in cat tail spindles. Intrafusal muscle fibres can be divided into three basic regions, namely, the periaxial space (A-region), intracapsular area (B-region) and the extracapsular area (C-region); the components of these systems were seen to vary in structure and distribution. The occurrence of these systems also varied among the different types of intrafusal muscle fibres, namely, the bag1, bag2 and chain fibres. In bag1 fibres components were sparse in the A-region, increased slightly in the B-region, but were most developed in the C-region; triads were consistently located at the border between A- and I-bands. In bag2 fibres membrane components were noted in the A-region but were more abundant in the B-region where some tubular components showed transverse and longitudinal branches linked together in the form of a network; membrane systems diminished towards the C-region. The majority of triads were located within the A-bands. In chain fibres the membrane systems occurred more commonly in the A-region, while in the B- and C-regions, the transverse tubular system possessed numerous transverse and longitudinal branches forming irregularly distributed tubular networks. Some tubular branches were dilated, while other branches terminated as sacs among arrays of the sarcoplasmic reticular system in I-bands. Some transverse tubules bifurcated into two branches with numbers of dilated sarcoplasmic reticular cisternae lying on either side, or between, the branches. Triads sometimes occurred between A- and I-bands, but, generally, were situated well within A-bands.

The ultrastructural and biosynthetic characteristics of steroidogenic cells in the gonad of Monopterus albus (Teleostei) during natural sex reversal.

The ultrastructural and biosynthetic characteristics of the steroid cells in the gonad of Monopterus albus have been studied. Ultrastructural features related to steroidogenesis have been identified in the interstitial Leydig cells, Sertoli cells, granulosa cells and thecal cells, and are especially abundant in the Leydig cells during the mid-intersexual phase. Steroidogenic ultrastructures in the Sertoli cells develop only during the maturation of the spermatogenic cysts, whereas in the granulosa and thecal cells, these features become obvious only during the maturation of the large oocytes. EM evidence also suggests a nutritive function for the Sertoli cells and the granulosa cells. Results of in vitro steroidogenic studies, using either testosterone or progesterone as a precursor, show a predominant conversion to androstenedione and 5 alpha-reduced compounds, and suggest a change in biosynthesis from 5 alpha-reduced products to androstenedione during sex reversal. 11-Ketotestosterone (11KT) has been identified, but not 11 beta-hydroxytestosterone. Production of 11KT is high in the late intersexual and the male phases, but a lack of a marked variation in 11KT production between the early and the mid-intersexual phase suggests that this steroid is not a trigger for natural sex reversal in Monopterus.

The sensory and motor innervation of muscle spindles in cat tail dorsolateral muscles.

The sensory and motor innervation of a sample of 202 muscle spindles from the dorsolateral tail muscles of eight cats were studied by silver and gold staining techniques. The range of diameters of Group Ia afferent nerve fibres to primary (P) sensory endings was 2.0-14.0 micron with a peak at 8.5 micron. The range of Group II fibres to secondary (S) sensory endings was 2.0-8.5 micron with a peak at 5.0 micron. In a sample of 234 spindle capsules, 171 (84.2%) were single capsule spindles and 31 (15.8%) were tandem spindles. With respect to the sensory endings of single capsule spindles, 19.7% showed only a P-ending, 32.5% had an additional S-ending and 15.8%, 3.0% and 2.1% had two, three and four S-endings, respectively. In the tandem spindle capsules, 0.8% of the larger proximal capsules showed only a P-ending, 5.1% had an additional S-ending and 4.7%, 2.6% and 0.4% had two, three and four S-endings, respectively. In the 31 tandem spindles, one of which had a triple capsule, there were 14 types of sensory ending combination in the proximal and distal capsules. The most common (44.4%) innervation was by a P-ending only, while 27.0% had an additional S-ending and 17.5%, 9.5% and 1.6% had two, three and four S-endings, respectively. Of the distal capsules, 92.9% showed only a P-ending. There were three types of motor nerve ending, namely the p1-plate, the p2-plate and the trail (tr) ending, in spindle polar regions. Spindle poles without a motor innervation also occurred. There were 16 types of motor ending combination in the two polar regions of muscle spindles. The most common type of combination was the trp2-ending (41.8%), receiving an average of 5.8 fusimotor fibre branches and the next common was the tr-ending only (38.7%), innervated by an average of 3.2 fibres. The least common combination was with the p1-plate ending (9.5%), receiving an average of 6.9 fibres. The mean number of fusimotor fibre branches per spindle pole was 4.2.

The sensory ending of duck muscle spindles.

Thin and thick types of muscle spindles have been demonstrated from the extensor pollicis (EP) and the extensor digitorum communis (EDC) wing muscles of the duck. The thin type shows a smaller capsule, innervated by a long and narrow sensory ending; the thick type consists of a larger capsule innervated by a truncated and wide sensory ending. Each spindle receives an afferent nerve fibre within the range of 2.1-8.4 microns diameter and the ratio between fibre and sensory ending is always 1:1. There is no secondary sensory ending in duck spindles. By electron microscopy, sensory terminals of different sizes may be seen to lie in depressions of various depths on intrafusal muscle fibres, usually without disturbing the external contour of the muscle fibre, and are covered externally by a rather thick layer of basement membrane.

The number, distribution and density of muscle spindles in two wing muscles of the domestic duck.

A study by serial longitudinal sections of two dorsal wing muscles of the duck, the extensor pollicis and the extensor digitorum communis, has shown that: (1) The extensor pollicis muscle shows a spindle content range of 19--27, mean 23.2, giving a spindle density of 76.7/g; the extensor digitorum communis muscle shows a spindle content range of 37--59, mean 47.6, and a spindle density of 119.0/g. (2) Spindles in the extensor pollicis muscle are randomly distributed, while those in the extensor digitorum muscle are located primarily in the proximal two thirds of the muscle. (3) The present data have been discussed in relation to previous observations of muscle spindles occurring in some avian and mammalian skeletal muscles.

The blood supply of duck muscle spindles.

The blood supply of duck muscle spindles from the extensor pollicis and the extensor digitorum comminis wing muscles has been studied by light and electron microscopy. Capillaries usually accompany the nerve bundle that innervates the spindle, approaching at an oblique angle around the midequatorial region. Capillaries may run for some distance along the surface of the outer capsule, or penetrate partially into the outer capsule and lie between layers of the capsule cell processes. Some capillaries also penetrate the outer capsule, running into the periaxial space and continuing further towards the polar region. They have been shown to be in close contact with intrafusal muscle fibres, from the juxta-equatorial to the polar region, but have not been encountered among sensory terminals in the mid-equatorial region.

Capsules of duck muscle spindles.

Duck muscle spindles show a large outer capsule enclosing a number of intrafusal muscle fibres which are individually encircled by an inner capsule. The outer capsule consists of a multilayer of 4-6 flattened cells with elongated nuclei, and are usually lined by a basement membrane. They resemble and are linked with perineural cells of the nerve bundle that innervates the spindle. There is overlapping and interdigitation between branching processes of these cells. Some apposing surfaces develop junctional complexes in the form of hemi-desmosomes and desmosomes. The cytoplasmic content shows numerous pinocytotic vesicles, some polyribosomes, lamellated cisternae of rough endoplasmic reticulum and microfilaments. The inner capsule consists of cells showing, at the mid-equatorial region, a large ovoid nucleus and extensive branching of thin and long processes that overlap, forming numerous layers around each intrafusal muscle fibre. Bundles of collagen fibrils in a crescent-shaped configuration occur circumferentially between the multilayer of processes and the muscle fibre. Some linkages between the inner and the outer capsule cell processes provide a network that subdivides the large periaxial space into compartments. There is no basement material lining the inner capsule cells and the processes. Some cytoplasmic area occurs around the ovoid nucleus and the cytoplasm varies in density, showing some dilated short profiles of rough endoplasmic reticulum, a few pinocytotic vesicles and microfilaments.

Cephalic anatomy of a gnathostomatid nematode Echinocephalus sinensis, parasite of oysters and rays.

Light, transmission, and scanning electron microscopy of both adult and third-stage Echinocephalus sinensis shows that the head is characterized by a circular muscle layer in the headbulb wall, a highly unusual arrangement in nematodes. The basal region of cephalic spines is enclosed by all three cuticular zones but the distal region of its shaft is lined only by the cortical layer. The so-called "ballonet-cervical sac" system is actually formed by four modified cervical muscle cells of the circomyarian type, each with a liquid cytoplasmic matrix serving as a hydrostatic chamber. The headbulb is deflated by contraction of the circular wall muscle and six pairs of specialized longitudinal and oblique muscles. Two pairs of oblique somato-oesophageal muscles also serve to shorten the oesophagus. Relaxation of the muscles and an anterior flow of fluid into the cephalic region of the cervical muscle cells inflate the headbulb. In adult worms, the trilobed pseudolabia are lined internally mostly by the oesophageal cuticle. Four radially arranged muscles help to dilate the buccal cavity and the pseudolabia can be retracted into the headbulb by two pairs of oblique muscles inserted at their base. The radial musculature at the anteriormost oesophagus has more abundant and tightly packed myofilaments than other regions. Four pitlike structures of unknown function are located near the base of the pseudolabium. In the third-stage worm the pyriform pseudolabium is internally lined mostly by the body cuticle. Two rows of bulbous structures each with a central process are located on the headbulb a short distance from the pseudolabium. Two pairs of oblique buccal dilatory muscles help to dilate the oral opening and draw the pseudolabia towards the headbulb. Two bands of oblique myofilaments are present within the anterior-most region of the oesophagus. The functional adaptation of the cephalic system in relation to the biology of the parasite is discussed.

Leptofibrils in intrafusal muscle fibres of muscle spindles in the domestic fowl.

Leptofibrils consisting of narrow dark and wide light bands at regular periods are commonly found in intrafusal muscle fibres of chicken muscle spindles. They are particularly abundant in intrafusal muscle fibres with the loose type of myofilaments. They occur either at the periphery of intrafusal muscle fibres or in deeper regions, or even close to sensory nerve terminals. Dark bands of some peripheral leptofibrils vary considerably in size and appear less regular in configuration. Lateral extensions from the dark bands may occur with or without interconnections. Lateral attachments to myofilaments at the immediate neighbourhood may also occur.

Fusimotor innervation in the cat.

The motor innervation of cat spindles was examined in hindlimb muscles using a variety of techniques employed in light and electron microscopy. Observations were made on teased, silver preparations of 267 spindles sampled from the peroneal, flexor hallucis longus, and soleus muscles, hereafter referred to as the PER/FHL/SOL series. The γ innervation. Trail endings are almost invariably present, and innervate both bag and chain muscle fibres. Trail fibres accounted for 64.6 to 74.8% of the total fusimotor supply to samples of spindle poles in the PER/FHL/SOL series, the mean number of fibres per pole varying from 2.7 to 5.0 in the different muscles, and the mean number of ramifications (areas of synaptic contact) per fibre being 3.7. By contrast, the p2innervation of a spindle pole generally consists of a single fibre supplying only one plate. In the above samples p(2) fibres accounted for 4.1 to 28.0% of the total fusimotor supply, and the mean number of fibres per pole varied from 0.3 to 1.2 in the different muscles. Ninety per cent of p(2) plates innervate bag fibres. The α innervation. The structure of p1plates as seen in both light and electron microscopy compares very closely with that of extrafusal plates. After nerve section p1plates degenerate at the same time as extrafusal plates, being the first of the three types of fusimotor ending to disappear. The frequency of the p1innervation is similar to that of the p2innervation. In the same samples of PER/FHL/SOL spindle poles as above p1 fibres accounted for 6.0 to 28.8% of the total fusimotor supply, the mean number of fibres per pole varying from 0.25 to 2.1 in the different muscles. The majority of p1 fibres enter a pole to terminate in one plate only. Seventy-five per cent of the plates innervate bag fibres. The three types of fusimotor ending are thus not selectively distributed to the two types of intrafusal muscle fibre. All three types of fusimotor fibre may branch within the spindle so as to innervate both bag and chain fibres. Bag fibres receive both types of plate ending as well as trail endings. Most chain fibres receive trail endings only; the rest receive either a p1or a p2plate innervation in addition, 25% of the p1and 10% of the p2innervation being distributed to chain fibres. The significance of this nonselective innervation is interpreted as indicating that the type of contraction elicited by stimulating a fusimotor fibre depends upon the type of ending initiating it rather than upon the type of muscle fibre executing it. Reasons are given for concluding that the dynamic response is controlled via the p1and p2plates, and that the static response is controlled by the trail endings. The participation of the α fibres in mammalian fusimotor innervation, previously regarded as a vestigial feature, proved to be widespread in the muscles studied and more prevalent in fast muscles (FHL, peroneus digiti quinti) than slow (soleus). A low frequency of p1innervation is offset by a high frequency of p2(as in peroneus longus), and vice versa (as in FHL). It is unlikely that collaterals from slow α fibres innervating type B muscle fibres are wholly responsible for the high frequency of the p1innervation in FHL, and it is suggested that collaterals may also be derived from fast α fibres innervating type C muscle fibres. The possibility of there being some motor fibres of α conduction velocity and with an exclusively fusimotor distribution is also taken into account.