Evolution of antero-posterior patterning of the limb: insights from the chick

The developing limbs of chicken embryos have served as pioneering models for understanding pattern formation for over a century. The ease with which chick wing and leg buds can be experimentally manipulated, while the embryo is still in the egg, has resulted in the discovery of important developmental organisers, and subsequently, the signals that they produce. Sonic hedgehog (Shh) is produced by mesenchyme cells of the polarizing region at the posterior margin of the limb bud and specifies positional values across the antero-posterior axis (the axis running from the thumb to the little finger). Detailed experimental embryology has revealed the fundamental parameters required to specify antero-posterior positional values in response to Shh signaling in chick wing and leg buds. In this review, the evolution of the avian wing and leg will be discussed in the broad context of tetrapod paleontology, and more specifically, ancestral theropod dinosaur paleontology. How the parameters that dictate antero-posterior patterning could have been modulated to produce the avian wing and leg digit patterns will be considered. Finally, broader speculations will be made regarding what the antero-posterior patterning of chick limbs can tell us about the evolution of other digit patterns, including those that were found in the limbs of the earliest tetrapods

Injury and Hematocyst of the Penis

September 29, 1948, a 2- year-old Stanradrbred stallion was admitted to Stange Memorial Clinic for treatment of an injury of the penis which had occurred the night before. The history, as offered by the owner and the groom, indicated the stallion had escaped from his box stall into a runway in the barn. A Shetland pony was kept in a box stall near that of the stallion. The groom stated that the pony had remained within the stall throughout the night and therefore could not have caused the injury. The clinicians at the clinic were of the opinion that the stallion had become excited while free and running, and erection had occurred. The constant pummeling of the erected penis against the hocks while running was probably the cause of the injury to the penis

Memory and mutualism in species sustainability: a time-fractional Lotka-Volterra model with harvesting

We first present a predator-prey model for two species and then extend the model to three species where the two predator species engage in mutualistic predation. Constant effort harvesting and the impact of by-catch issue are also incorporated. Necessary sufficient conditions for the existence and stability of positive equilibrium points are examined. It is shown that harvesting is sustainable, and the memory concept of the fractional derivative damps out oscillations in the population numbers so that the system as a whole settles on an equilibrium quicker than it would with integer time derivatives. Finally, some possible physical explanations are given for the obtained results. It is shown that the stability requires the memory concept in the model

Research and development of high temperature resistant polymeric film forming material final summary report, 1 mar. 1961 - 17 apr. 1962

Preparation of poly-organometallosiloxane polymers from reactions between bis-dialkylamino metal derivatives and silanediol

Inhibition of Shh signalling in the chick wing gives insights into digit patterning and evolution.

In an influential model of pattern formation, a gradient of Sonic hedgehog (Shh) signalling in the chick wing bud specifies cells with three antero-posterior positional values, which give rise to three morphologically different digits by a self-organizing mechanism with Turing-like properties. However, as four of the five digits of the mouse limb are morphologically similar in terms of phalangeal pattern, it has been suggested that self-organization alone could be sufficient. Here, we show that inhibition of Shh signalling at a specific stage of chick wing development results in a pattern of four digits, three of which can have the same number of phalanges. These patterning changes are dependent on a posterior extension of the apical ectodermal ridge, and this also allows the additional digit to arise from the Shh-producing cells of the polarizing region - an ability lost in ancestral theropod dinosaurs. Our analyses reveal that, if the specification of antero-posterior positional values is curtailed, self-organization can then produce several digits with the same number of phalanges. We present a model that may give important insights into how the number of digits and phalanges has diverged during the evolution of avian and mammalian limbs

Establishing the pattern of the vertebrate limb

The vertebrate limb continues to serve as an influential model of growth, morphogenesis and pattern formation. With this Review, we aim to give an up-to-date picture of how a population of undifferentiated cells develops into the complex pattern of the limb. Focussing largely on mouse and chick studies, we concentrate on the positioning of the limbs, the formation of the limb bud, the establishment of the principal limb axes, the specification of pattern, the integration of pattern formation with growth and the determination of digit number. We also discuss the important, but little understood, topic of how gene expression is interpreted into morphology

Sonic hedgehog-expressing cells in the developing limb measure time by an intrinsic cell cycle clock

How time is measured is an enduring issue in developmental biology. Classical models of somitogenesis and limb development implicated intrinsic cell cycle clocks, but their existence remains controversial. Here we show that an intrinsic cell cycle clock in polarizing region cells of the chick limb bud times the duration of Sonic hedgehog (Shh) expression, which encodes the morphogen specifying digit pattern across the antero-posterior axis (thumb to little finger). Timing by this clock starts when polarizing region cells fall out of range of retinoic acid signalling. We found that timing of Shh transcription by the cell cycle clock can be reset, thus revealing an embryonic form of self-renewal. In contrast, antero-posterior positional values cannot be reset, suggesting that this may be an important constraint on digit regeneration. Our findings provide the first evidence for an intrinsic cell cycle timer controlling duration and patterning activity of a major embryonic signalling centre

Truncation of TRIM5 in the <i>Feliformia</i> explains the absence of retroviral restriction in cells of the domestic cat

TRIM5[alpha] mediates a potent retroviral restriction phenotype in diverse mammalian species. Here, we identify a TRIM5 transcript in cat cells with a truncated B30.2 capsid binding domain and ablated restrictive function which, remarkably, is conserved across the &lt;i&gt;Feliformia&lt;/i&gt;. Cat TRIM5 displayed no restriction activity, but ectopic expression conferred a dominant negative effect against human TRIM5[alpha]. Our findings explain the absence of retroviral restriction in cat cells and suggest that disruption of the TRIM5 locus has arisen independently at least twice in the &lt;i&gt;Carnivora&lt;/i&gt;, with implications concerning the evolution of the host and pathogen in this taxon