Cranial ontogeny in the direct-developing frog, Eleutherodactylus coqui (Anura: Leptodactylidae), analyzed using whole-mount immunohistochemistry.

Direct development in amphibians is an evolutionarily derived life-history mode that involves the loss of the free-living, aquatic larval stage. We examined embryos of the direct-developing anuran Eleutherodactylus coqui (Leptodactylidae) to evaluate how the biphasic pattern of cranial ontogeny of metamorphosing species has been modified in the evolution of direct development in this lineage. We employed whole-mount immunohistochemistry using a monoclonal antibody against the extracellular matrix component Type II collagen, which allows visualization of the morphology of cartilages earlier and more effectively than traditional histological procedures; these latter procedures were also used where appropriate. This represents the first time that initial chondrogenic stages of cranial development of any vertebrate have been depicted in whole-mounts. Many cranial cartilages typical of larval anurans, e.g., suprarostrals, cornua trabeculae, never form in Eleutherodactylus coqui. Consequently, many regions of the skull assume an adult, or postmetamorphic, morphology from the inception of their development. Other components, e.g., the lower jaw, jaw suspensorium, and the hyobranchial skeleton, initially assume a mid-metamorphic configuration, which is subsequently remodeled before hatching. Thirteen of the adult complement of 17 bones form in the embryo, beginning with two bones of the jaw and jaw suspensorium, the angulosplenial and squamosal. Precocious ossification of these and other jaw elements is an evolutionarily derived feature not found in metamorphosing anurans, but shared with some direct-developing caecilians. Thus, in Eleutherodactylus cranial development involves both recapitulation and repatterning of the ancestral metamorphic ontogeny.(ABSTRACT TRUNCATED AT 250 WORDS)

Anatomical architecture and electrical activity of the heart.

In most early studies of cardiac electrophysiology, the correlation between propagation of excitation and the architecture of cardiac fibers was not addressed. More recently, it has become apparent that the spread of excitation, the sequence of recovery, the associated time-varying potential distributions and the intra- and extracardiac electrocardiograms are strongly affected by the complex orientation of myocardial fibers. This article is a review of older and very recent, partly unpublished, mathematical simulations and experimental findings that document the relationships between cardiac electrophysiology and fiber structure. Important anatomical factors that affect propagation and recovery are: the elongated shape of myocardial fibers which is the basis for electrical anisotropy; the epi-endocardial rotation of fiber direction in the ventricular walls; the epi-endocardial obliqueness of the fibers ("imbrication angle"), and the conduction system. Due to the complex architecture of the fibers, many different pathways are available to an excitation wavefront as it spreads from a pacing site: the straight line; the multiple, bent pathways resulting from the epi-endocardial rotation of fiber direction; the coiling intramural pathways associated with the "imbrication" angles (Streeter) and the pathways involving the Purkinje network. Only in a few cases is the straight line the fastest pathway. The shape of an excitation wavefront at a given time instant results from the competition between all possible pathways. To compute the potential distributions and ECG waveforms generated by a spreading excitation wave we must know the successive shapes and positions of the wavefront, the architecture of the fibers through which it propagates and the spatial distribution of their anisotropic electrical properties.