Bio Worldwide
Why polyps regenerate and we don't: Towards a cellular and molecular framework for Hydra regeneration
Thomas C.G. Bosch
2006.12.012
The basis for Hydra's enormous regeneration capacity is the “stem cellness” of its epithelium which continuously undergoes self-renewing mitotic divisions and also has the option to follow differentiation pathways. Now, emerging molecular tools have shed light on the molecular processes controlling these pathways.[1]
In this review I discuss how the modular tissue architecture may allow continuous replacement of cells in Hydra. I also describe the discovery and regulation of factors controlling the transition from self-renewing epithelial stem cells to differentiated cells.[1]
Hydra is made up of two cell layers – the ectoderm and endoderm – separated by a thin extracellular matrix (ECM) called the mesoglea. [1]
The polar body plan has a head and tentacles on one end and a foot on the opposite end of a hollow column . The cells either belong to the ectodermal or endodermal epithelial cell lineage, or to the interstitial cell lineage. [1]
Epithelial cells are epitheliomuscular cells covering the outside of the animal or lining the gastric cavity. Interstitial cells are mostly localized in the interstitial space between ectodermal epithelial cells and differentiate into nerve cells, cnidocytes, gland cells, and – during sexual differentiation – into gametes ( Bosch and David, 1987 and Bosch, 2006).[1]
Hydra has chosen a life cycle in which proliferation occurs mostly asexual by budding. That requires that each bud obtains the complete cellular repertoire from the mother polyp.[1]
By giving all the epithelial cells in the budding region stem cell properties and by filling the interstitial space with multipotent interstitial stem cells with the potential to differentiate not only into somatic cells but also into gametes, buds obtain all what they need.[1]
Thus, it is the stem cellness of the tissue which allows Hydra its unique life cycle. It seems that this feature alone is sufficient to explain Hydra's unprecedented regeneration capacity. Does this reflect a particularly simple or even “primitive” molecular and cellular tissue architecture? [1]
Are regeneration studies in Hydra telling us anything relevant with respect to regeneration in man? Analysis of one of the most extensively studied mammalian epithelial stem cell systems, the crypt of the small intestine, has revealed that stem cells in vertebrates are present only in extremely low numbers ( Moore and Lemischka, 2006 ) and – due to the complexity of the niche microenvironment – difficult to study directly.[1]
Biological Systems
Are Staggeringly Complex
Finn Pond
2006,May–June
Biological systems are staggeringly complex. Professional biologists devote their careers to describing those complexities, dissecting those systems by chemical and physical methods, and characterizing their structural components and functional interactions.[2]
How can such complex systems evolve? We understand the ways in which the individual components of a complex system can be altered in structure and function by mutation, and the way in which natural selection favors one form over another. Furthermore, in many cases we have traced the family relationships among different nucleic acid and protein variants.[2]
Envisioning ways by which natural selection can construct biochemical and molecular systems that involve dozens of proteins integrated in complex and highly specific ways is much more difficult. [2]
How could all the necessary proteins be selected simultaneously with a common endpoint as the goal? Unless each intermediate construct possessed at least partial function, how could natural selection act?[2]
Biologists recognize that integrated system complexity is a feature of living systems. That is, some biological systems consist of component parts that interact in a coordinated way so that the system as a whole exhibits a specific function.[2]
It is questionable, however, whether any such systems are irreducibly complex as Behe claims (see Coyne 1996; Doolittle 1997; Miller 1999; Shanks and Joplin 1999).[2]
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