Overview of Hydra: a concurrent language for synchronous digital circuit design

Hydra is a computer hardware description language that integrates several kinds of software tool (simulation, netlist generation and timing analysis) within a single circuit specification. The design language is inherently concurrent, and it offers black box abstraction and general design patterns that simplify the design of circuits with regular structure. Hydra specifications are concise, allowing the complete design of a computer system as a digital circuit within a few pages. This paper discusses the motivations behind Hydra, and illustrates the system with a significant portion of the design of a basic RISC processor

Nerve cell differentiation in hydra requires two signals

Endogenous signals controlling nerve cell commitment in hydra were investigated using an assay for committed nerve precursors. Extracts of hydra tissue were prepared and tested for their ability to induce nerve cell commitment. The active component in such extracts was identified as a neuropeptide, the head activator [H. C. Schaller and H. Bodenmüller (1981) Proc. Natl. Acad. Sci. USA 78, 7000–7004], based on its chromatographic properties and reaction with anti-head activator antibody. In addition, synthetic head activator (10−13–10−11 M) was shown to cause nerve cell commitment. Additional experiments demonstrated that committed nerve precursors require a second signal to differentiate nerve cells. Committed precursors induced by treatment of hydra with head activator do not differentiate in whole hydra; but do differentiate when pieces of treated tissue are explanted or when whole animals are simply injured with transverse cuts. The injury stimulus is long-lived. It cannot be replaced with head activator (10−12–10−10 M) but is contained in a methanol extract of hydra tissue

Horizontal gene transfer contributed to the evolution of extracellular surface structures

The single-cell layered ectoderm of the fresh water polyp Hydra fulfills the function of an epidermis by protecting the animals from the surrounding medium. Its outer surface is covered by a fibrous structure termed the cuticle layer, with similarity to the extracellular surface coats of mammalian epithelia. In this paper we have identified molecular components of the cuticle. We show that its outermost layer contains glycoproteins and glycosaminoglycans and we have identified chondroitin and chondroitin-6-sulfate chains. In a search for proteins that could be involved in organising this structure we found PPOD proteins and several members of a protein family containing only SWT (sweet tooth) domains. Structural analyses indicate that PPODs consist of two tandem β-trefoil domains with similarity to carbohydrate-binding sites found in lectins. Experimental evidence confirmed that PPODs can bind sulfated glycans and are secreted into the cuticle layer from granules localized under the apical surface of the ectodermal epithelial cells. PPODs are taxon-specific proteins which appear to have entered the Hydra genome by horizontal gene transfer from bacteria. Their acquisition at the time Hydra evolved from a marine ancestor may have been critical for the transition to the freshwater environment

Cloned interstitial stem cells grow as contiguous patches in hydra

The migration of interstitial cells was analyzed during the growth of stem cell clones in vivo. The spatial distribution of cloned cells was analyzed at a time by which extensive migration of interstitial cells could have occurred. All interstitial cell clones were found to form large contiguous patches of cells. The results indicate that there is little migration of large interstitial cells in undisturbed tissue during normal growth. This finding is surprising since numerous grafting experiments have shown extensive migration of these cells. The implications of finding nonrandomly distributed stem cells are discussed