Patricia Hernandez

Craniofacial development using the zebrafish model system

Research in my lab centers around uncovering the developmental mechanisms involved in vertebrate head formation. I primarily use the zebrafish model system to examine the mechanisms involved in the morphogenesis of the pharyngeal arches and neurocranium. The vertebrate skull is composed of 3 basic units: the chondrocranium/neurocranium encases and protects the brain; the dermatocranium (composed of dermal bone) covers the neurocranium and makes up a significant portion of the cranial vault; and the viscerocranium, derived from seven pharyngeal arches, gives rise to the jaws and gill bearing structures in fishes. In mammals the viscerocranium consists of the jaws and associated structures, which support the throat and laryngeal structures. The pharyngeal arches in zebrafish embryos represent a series of relatively simple reiterated structures whose development depends on all three germ layers as well as neural crest cells. The relative simplicity of these embryonic structures affords an ideal situation in which to study the molecular mechanisms involved in the interactions among germ layers in these structures.

Presently I am investigating the role of the Hedgehog pathway in the proper growth and differentiation of the cartilaginous components of the pharyngeal arches. Using mutant analysis, overexpression, and pharmacological treatment with cyclopamine my lab is investigating the role Hedgehog proteins play in the differentiation of the branchial cartilages (which support the gills). Our previous findings suggest that the Hh signaling pathway is required for growth of the jaws, hyoid and branchial cartilages, and more importantly it is required for the differentiation of branchial cartilages. Work on this project is ongoing.

Evolutionary developmental biology of fishes

For many years I have been interested in the comparative cranial anatomy of fishes. It is clear that the skulls of fishes are much more complex than mammalian skulls, showing an enormous amount of structural diversity. Jaws and associated cranial structures differ from the small picking jaws of the butterflyfishes to the large straining branchial arches of the paddlefish. Such structural complexity coupled with amazing morphological diversity makes this group especially well suited for study within an evolutionary developmental perspective. How has this enormous morphological diversity evolved? Ultimately natural selection acts to cull failed experiments, but the only way that novel morphologies are generated is through modification of developmental mechanisms.

Thus a second area of study within my lab entails investigating the developmental mechanisms (as defined through molecular interactions/mechanisms) that are involved in the generation of different morphologies. While differential growth between ancestors and descendents (i.e. heterochrony) has been invoked to explain morphological differences among species, the molecular mechanisms involved in such shape/size changes have been poorly resolved. While heterochrony assuredly plays a key role in many of these morphological changes, to fully understand how these changes have evolved we must understand precisely which molecular/genetic mechanisms brought about changes in growth trajectories. Previous work using the comparative method has done much to establish the morphological patterns we see during the course of evolution, yet in order to specifically identify the factors responsible for morphological change we need to isolate the genetic factors involved.

Thus, in addition to our work on the development of the visceral arches in zebrafish, we are also examining the development and evolution of the neurocranium in zebrafish and a number of congeners. Previous work has shown that defects in the Hedgehog signaling pathway lead to defects associated with growth within the anterior neurocranium. This would suggest that during normal development, the Hh pathway plays an important role in the proper morphogenesis and growth of the anterior neurocranium. By conducting a morphometric analysis of shape changes in the neurocranium in a number of related cyprinids we hope to generate hypotheses concerning the role of Hh in neurocranial development within this group.

During the course of my postdoctoral work I became interested in the development and evolution of muscle fiber type proportion and distribution in cranial muscles. While an enormous amount of research effort has been placed on examining extensive differences in musculoskeletal architecture in the head, it is clear that relatively minor differences in fiber type composition and distribution may have a disproportionately large impact on animal performance. Moreover the genetic factors involved in fiber type specification are being uncovered. To this end my lab is investigating the evolution and development of muscle fiber type changes within cranial muscles. My previous work on zebrafish has shown that ontogenetic changes in cranial muscle fiber type proportion are correlated with changes in feeding mode. If this is true among ontogenetic stages, it is likely also true among different species of closely related fishes. Thus, we are presently analyzing fiber type distribution in a number of related Danios.

Finally my doctoral student, Nathan Bird, and I are examining the development and evolution of the Weberian apparatus, a novel bony connection between the inner ear and swim bladder, which characterizes the Otophysi. We believe that this probable key innovation has led to a major radiation (~27% of all fish species) within this group. We are mapping a number of gene expression patterns, as well as larval and adult morphological characters unto a known phylogeny to better understand how this complex structure has developed and evolved.