The braincase and palate of the tetrapodomorph sarcopterygian mandageria fairfaxi: morphological variability near the fish–tetrapod transition

The braincase of the Late Devonian tristichopterid sarcopterygian Mandageria fairfaxi , from Canowindra, NSW, Australia, differs radically from the conservative pattern present in other 'osteolepiforms' (stem–group tetrapodomorph fishes) and non–dipnoan sarcopterygian fishes in general. The basioccipital region is short, displaced anteriorly, and either unossified or loosely articulated to the exoccipital, leaving most or all of the notochordal tunnel open ventrally. The exoccipital complex, which is developed into a large saddle that would have rested on top of the notochord, carries large, triangular articular facets on its posterior face and appears to have formed part of a functional neck joint, a synovial articulation between the skull and vertebral column that allows the former to rotate against the latter. Such a joint is characteristic of post–Devonian tetrapods, but unknown in other sarcopterygians. We infer that the ventrally open notochordal tunnel allowed gentle flexion of the cranial notochord during (predominantly vertical) rotational movement at the occiput; this is a mechanically unique solution to the problem of creating a mobile neck. Other unusual features of Mandageria include a posteriorly located lateral commissure, and structures on the entopterygoid and lateral commissure that may have been associated with an elaborate spiracular tract.     

In vitro uptake of particles by lysosomes

Isolated rat liver lysosomes were incubated with Percoll particles in vitro at 25 and 37 °C. On morphological examination the incubated lysosomes contain vesicles some of which enclose Percoll particles, indicating that invagination of the lysosomal membrane occurs in vitro by means of microautophagy. Vesiculation occurs by formation of flaplike processes or cuplike invaginations. At later time points of incubation Percoll particles can be seen free in the lysosomal matrix indicating rupture or digestion of the vesicular membrane. The uptake of isotopically pre-labelled Percoll particles increases with incubation time and temperature.It is concluded that lysosomes show microautophagic activity in vitro and that this may be a mechanism for degradation of soluble cytoplasmic proteins also in vivo.     

Lysosomal uptake of isolated cell organelles microinjected into HeLa cells

Lysosomes and microsomes were isolated from rat liver and microinjected into the cytoplasm of HeLa cells. The fate of the transplanted organelles and their effects on the recipient cells were followed in the electron microscope at various time intervals after administration. Needle injection with buffer or sucrose did not seem to evoke any ultrastructural alterations, such as induced autophagy or other signs of sublethal cell injury. Recipients of microinjected cell organelles elicited a rapid and conspicuous increase in membrane-bounded cytoplasmic vacuoles, concomitant with the disappearance of the injected material. Golgi complexes became abundant with many small vesicles clustering around their cisternae. The volume density of the lysosomal compartment increased 2-3-fold after organelle injection as compared with control-injected (0.3 M sucrose) or noninjected cells. Our preliminary results show that isolated cell organelles can be microinjected into cells n culture and indicate that the microinjected organelles were segregated from the cytoplasm into membrane-bounded vacuoles probably through autophagolysosome formation. Thus, this technique offers an additional approach for studies on the segregation and degradation of cell organelles in somatic cells and may enable more detailed analyses on the mechanisms of autophagic sequestration of specific cell organelles.     

3D Microstructural Architecture of Muscle Attachments in Extant and Fossil Vertebrates Revealed by Synchrotron Microtomography

Background

Firm attachments binding muscles to skeleton are crucial mechanical components of the vertebrate body. These attachments (entheses) are complex three-dimensional structures, containing distinctive arrangements of cells and fibre systems embedded in the bone, which can be modified during ontogeny. Until recently it has only been possible to obtain 2D surface and thin section images of entheses, leaving their 3D histology largely unstudied except by extrapolation from 2D data. Entheses are frequently preserved in fossil bones, but sectioning is inappropriate for rare or unique fossil material.

Methodology/Principal Findings

Here we present the first non-destructive 3D investigation, by propagation phase contrast synchrotron microtomography (PPC-SRµCT), of enthesis histology in extant and fossil vertebrates. We are able to identify entheses in the humerus of the salamander Desmognathus from the organization of bone-cell lacunae and extrinsic fibres. Statistical analysis of the lacunae differentiates types of attachments, and the orientation of the fibres, reflect the approximate alignment of the muscle. Similar histological structures, including ontogenetically related pattern changes, are perfectly preserved in two 380 million year old fossil vertebrates, the placoderm Compagopiscis croucheri and the sarcopterygian fish Eusthenopteron foordi.

Conclusions/Significance

We are able to determine the position of entheses in fossil vertebrates, the approximate orientation of the attached muscles, and aspects of their ontogenetic histories, from PPC-SRµCT data. Sub-micron microtomography thus provides a powerful tool for studying the structure, development, evolution and palaeobiology of muscle attachments.     

The first specimen of Archaeopteryx from the Upper Jurassic Mörnsheim Formation of Germany

From an initial isolated position as the oldest evolutionary prototype of a bird, Archaeopteryx has, as a result of recent fossil discoveries, become embedded in a rich phylogenetic context of both more and less crownward stem-group birds. This has prompted debate over whether Archaeopteryx is simply a convergently bird-like non-avialan theropod. Here we show, using the first synchrotron microtomographic examination of the genus, that the eighth or Daiting specimen of Archaeopteryx possesses a character suite that robustly constrains it as a basal avialan (primitive bird). The specimen, which comes from the Mörnsheim Formation and is thus younger than the other specimens from the underlying Solnhofen Formation, is distinctive enough to merit designation as a new species, Archaeopteryx albersdoerferi sp. nov., but is recovered in close phylogenetic proximity to Archaeopteryx lithographica. Skeletal innovations of the Daiting specimen, such as fusion and pneumatization of the cranial bones, well vascularized pectoral girdle and wing elements, and a reinforced configuration of carpals and metacarpals, suggest that it may have had more characters seen in flying birds than the older Archaeopteryx lithographica. These innovations appear to be convergent on those of more crownward avialans, suggesting that Bavarian archaeopterygids independently acquired increasingly bird-like traits over time. Such mosaic evolution and iterative exploration of adaptive space may be typical for major functional transitions like the origin of flight.