Abele Saita

Muscle system organization in the echinoderms: II. Microscopic anatomy and functional significance of the muscle‐ligament‐skeleton system in the arm of the comatulids (Antedon mediterranea)

Comatulids are able to perform quick and complex movements of the arms which are used to swim, creep, walk, and also form a rigid, feeding‐filtration fan. To perform such versatile movements, the arms of these animals are equipped with a classical endoskeletal system, with joints, muscles, ligaments, and a hydroskeleton of three different coelomic channels. Light microscopic study of the detailed anatomical organization of the arm clarifies both the complex relations between the parts involved in the movements and their functional responsibilities. In particular, (1) the ventral muscle bundles show a heterogeneous structure that consists of different and variously arranged populations of fibers, which allows the different flexing movements of the arms (i.e., flexion and maintaining the flexed state); (2) the ligaments (both dorsal and interarticular) consist only of collagen fibrils and, therefore, have a passive function in binding the skeletal pieces together: their possible active engagement in the extending movements of the arms is thus excluded; (3) owing to the absence of other suitable antagonists to the flexor muscles, the only efficient antagonist system seems to be the coelomic cavities, which are well separated from each other and are also provided with muscular valves. They thus function as typical hydraulic systems, which allows the arm to perform both simple extensions and very complex combined movements and to maintain some rigid straight or twisted positions.

Electron microscope study of spermiogenesis in Caiman crocodylus L.

Abstract Spermiogenesis in Caiman crocodylus was examined by electron microscopy. The acrosomal complex, the perforatorium and the growing axoneme of young spermatids are described. During the nuclear elongation of intermediate spermatids, two ≪manchettes≫ of microtubules were observed in addition to the acrosomal complex, perforatorium growth and chromatin condensation. At the onset of spermatid elongation, there is a set of microtubules around the nucleus and, at the end of elongation, a second set of microtubules parallel to the main axis of the cell appears. Both circular and longitudinal manchettes were observed together. In the neck‐region a striated structure of electron‐dense material is found on one side of the distal centriole. The descent of the mitochondria into che middle‐piece of the tail occurs only in the last stage of differentiation. The mitochondria are small, their cristae are irregularly arranged and the centres of the mitochondria consist of an electrondense material. Unlike most oth...

Ultrastructural analysis of spermiogenesis in Admetus pomilio (Arachnida, amblypygi)

The mature spermatozoon of Admetus pomilio is a spherical cell containing nucleus and tightly coiled flagellum. In early spermatids the Golgi apparatus forms the acrosomal vesicle and at the opposite side the distal centriole gives rise to the axonemal complex of the sperm tail. As the nucleus elongates, chromatin forms twisted filaments and the spermatid nucleus takes on a helical form. Microtubules are juxtaposed with the nucleus envelope, which is separated from a central chromatin mass by an electron lucid region. A long perforatorium, located on the border of the chromatin mass, runs helically in the nucleus from the centriolar region to subacrosomal space. During tail elongation, the anterior part of the axoneme is surrounded by a long, spiral mitochondrial sheath. In the late spermatid, chromatin filaments appear twisted and become aggregated. The nucleus and flagellum undergo further contortions in which the nucleus coils and the flagellum winds up into the body of the cell and coils in a regular fashion. The mitochondrial sheath surrounds about 2/3 of the 9 + 3 axoneme. These features of spermatid ultrastructure resemble those in the primitive Liphistiomorpha.

Muscle system organization in the echinoderms: III. Fine structure of the contractile apparatus of the arm flexor muscles of the comatulids (Antedon mediterranea)

The crinoid arm muscles consist of obliquely striated fibers with striking differences in function and in ultrastructural features. These fibers can be distinguished mainly on the basis of different myofilament arrangements (A‐ and B‐type patterns) and are variously combined at different levels (proximal, intermediate, and distal portions) of the arm. Some rare smooth fibers (C‐type) are irregularly distributed in the periphery and in the core of the bundle. The characteristic features of the A‐ and B‐type obliquely striated fibers are (1) a continuous and homogeneous structure of the Z line and (2) a very heterogeneous arrangement of myosin filaments which vary widely in size, number, and distribution from section to section. The significance of such an atypical, obliquely striated muscle may be related to the double skeletal system combination (endoskeleton and hydroskeleton) of the crinoid arms.

An unusual Z-system in the obliquely striated muscles of crinoids: three-dimensional structure and computer simulations

SummaryThe peculiar functional structure of the Z-line in the obliquely striated muscles of some feather stars is described. It is known that cross-striated muscles are characterized by linear and continuous Z-bands, and obliquely striated muscles by disconnected, obliquely aligned Z-elements. Owing to this discontinuous organization, the sarcomere can perform wide active lengthenings, shortenings, and even ‘super-elongations’ in the helical fibres. In contrast, the obliquely striated fibres of crinoids show markedly continuous and homogeneous oblique Z-lines; such a structure is not compatible with ‘super-performances’ like sliding and shearing of the sarcomere elements, but instead could allow functions comparable to those characteristic of a cross-striated muscle (quick, short movements, mechanically amplifiable by bone levers). This odd situation, only interpretable in terms of evolutionary constraint, could be considered opposite and symmetrical to that of cross-striated ‘super-contracting’ muscles, where the Z-line is exceptionally fragmented to allow the sarcomere to super-contract.The possible architecture of a significant parameter such as the Z-line, which determines muscle fibre potential capacities, is analysed in detail: (1) through qualitative-quantitative evaluation of electron micrographs, supported by statistical analysis of the data; and (2) bycomputer simulations. The data obtained suggest that the most realistic conformation of the whole Z-complex in these muscles consists of a multiple system of continuous, ribbon-like helical planes running in parallel along the fibre from end to end and regularly cutting it with a constant thickness. The proposed model seems morphologically compatible with the experimentally verified situations and functionally compatible with the mechanical requirements for a normal contraction and for a balanced distribution of the involved strengths.

Ultrastructural investigation of spermiogenesis in Peripatopsis capensis (Onychophora)

Early spermatids of the onychophoran Peripatopsis capensis are spherical cells with a centrally located nucleus, numerous mitochondria, Golgi complexes, microtubules and two centrioles. During spermiogenesis, Golgi vesicles migrate to one side of the cell where they form a tight aggregate, which is later shed. The mature spermatozoon has no acrosome. Several mitochondria fuse to form a middle piece containing three large mitochondria. Nucleus and middle‐piece elongate, presumably under the influence of helically twisted microtubules. Outside this set of microtubules a continuous layer of endoplasmic reticulum cisternae is formed which separates the interior portion of the cell from an external cytoplasmic rim, which is later shed. Outside the 9 + 2 complex, the tail presents nine accessory microtubules, and a peripheral layer of microtubules beneath the plasma membrane. The enforcement of the tail structure may be related to the fertilization biology of this animal, which is by “hypodermal” impregnation.

ULTRASTRUCTURAL ANALYSIS OF MUSCLE FIBRES IN GLOSSOBALANUS MINUTUS (KOVALEVSKIJ, 1866) (ENTEROPNEUSTA)

SUMMARY Observations of the muscle fibres of the Enteropneusta have been carried out at the electron microscope. In this Class only smooth muscle exists but such a similar definition is generic, as the smooth fibres present different organizations in the various zoological groups at the electron microscope level. The observations deal with the muscle fibres of the trunk, the collar and the proboscis of the Glossobalanus minutus (Kovalevskij, 1866); in particular the longitudinal and circular fibres of the parietal musculature have been studied, together with the visceral musculature of the collar. The parietal muscle fibres present a contractile apparatus made up of two types of filaments, thick and thin, measuring 250 A and 70 A in diameter. The thin filaments, much more numerous than the thick ones, are arranged among the latter in an irregular fashion. In the longitudinal sections the presence of transversally alligned elements, which would seem to be a structure analogous to the «Z» band, are not dete...