Pathikrit Bhattacharyya | Search bioRxiv

bioRxiv2014

The Histology of Nanomia bijuga (Hydrozoa: Siphonophora)

The siphonophore Nanomia bijuga is a pelagic hydrozoan (Cnidaria) with complex morphological organization. Each siphonophore is made up of many asexually produced, genetically identical zooids that are functionally specialized and morphologically distinct. These zooids predominantly arise by budding in two growth zones, and are arranged in precise patterns. This study describes the cellular anatomy of several zooid types as well as of the stem and gas-filled float, called the pneumatophore. The distribution of cellular morphologies across zooid types enhances our understanding of zooid function. The unique absorptive cells in the palpon, for example, indicate specialized intracellular digestive processing in this zooid type. Furthermore, there are multiple areas of both endodermal and ectodermal epithelial complexity. Though cnidarians are usually thought of as mono-epithelial, we characterize at least two cellular populations in this species which are not connected to a basement membrane. This work provides a greater understanding of epithelial diversity within the cnidarians, and will be a foundation for future studies on Nanomia bijuga, including functional assays and gene expression analyses.

Zoology↗

bioRxiv2014

Stem cells in Nanomia bijuga (Siphonophora), a colonial animal with localized growth zones

BackgroundSiphonophores (Hydrozoa) have unparalleled colony-level complexity, precision of colony organization, and functional specialization between zooids (i.e., the units that make up colonies) Previous work has shown that, unlike other colonial animals, most growth in siphonophores is restricted to one or two well-defined growth zones that are the sites of both elongation and zooid budding. It remained unknown, however, how this unique colony growth and development is realized at the cellular level.\n\nResultsTo understand the colony-level growth and development of siphonophores at the cellular level, we characterize the distribution of proliferating cells and interstitial stem cells (i-cells) in the siphonophore Nanomia bijuga. Within the colony we find that i-cells are present at the tip of the horn, the structure within the growth zone that gives rise to new zooids. They persist in the youngest zooid buds, but as each zooid matures i-cells become progressively restricted to specific regions within the zooids until they are mostly absent from the oldest zooids. I-cell marker-gene expression remained in gametogenic regions. I-cells are not found in the stem between maturing zooids. Domains of high cell proliferation include regions where i-cells can be found, but also include some areas without i-cells such as the stem within the growth zones. Cell proliferation in regions devoid of marker gene expression indicates the presence of mitotically active epithelial cell lineages and, potentially, progenitor cell populations.\n\nConclusionsRestriction of stem cells to particular regions in the colony may play a major role in facilitating the precision of siphonophore growth, and also lead to a reduced developmental plasticity in other, typically older, parts of the colony. This helps explain why siphonophore colonies have such precise colony-level organization.

Developmental Biology↗