The histology of Nanomia bijuga (Hydrozoa: Siphonophora)

Abstract

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, the stem, and the 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. 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 N. bijuga, including functional assays and gene expression analyses.

Figure 1

Schematic of Nanomia bijuga colony. Adapted from http://commons.wikimedia.org/wiki/File:Nanomia_bijuga_whole_animal_and_growth_zones.svg, which was drawn by Freya Goetz. (A)–(C) are oriented with anterior to the top and ventral to the left. (D)–(F) are oriented with anterior to the left and ventral up. (A) Overview of the mature colony. All zooids are produced from two growth zones, one at the anterior end of the siphosome, immediately posterior to the pneumatophore, and one at the anterior end of the siphosome, immediately posterior to the nectosome. Zooids are organized on the siphosome into reiterated units known as cormidia. (B) The pneumatophore and nectosomal growth zone at the anterior of the colony, showing forming nectophores. (C) The siphosomal growth zone and budding cormidia, with gastrozooids representing cormidial boundaries. (D) Gastrozooid, connected at the base to the siphosomal stem, showing labelled regions of the mouth (buccal hypostome), the mid‐region of the hypostome, and the basigaster. (E) Palpon, connected to the stem by its peduncle. (F) Female and male gonodendra. Multiple individual gonophores are borne on stalks which connect to the siphosomal stem.

Figure 2

Pneumatophore. (A) Schematic of pneumatophore showing five tissues: external ectoderm (ec1), endoderm resting on the shared mesoglea with external ectoderm (en1), invaginated endoderm (en2), invaginated ectoderm which produces chitin surrounding the gas chamber (ec2), and an ectodermally derived aeriform cell population (ae). Blind tubes connected to the base of the gas chamber are shown. (B) Longitudinal thick section of the pneumatophore base stained with toluidine blue (composite image). Small schematic at bottom left shows orientation of panels (B)–(D) and (G). All five tissues and multiple acellular regions are visible, including ectoderm (ec1) resting on the mesoglea (m), endodermal cells (en1) packed tightly surrounding the gastric cavity (g), invaginated endodermal cells (en2), invaginated ectodermal cells (ec2) which produce chitin (c), and the aeriform cell population (ae). At the base of the gas chamber, blind tubes (t) connect and insinuate between the endodermal septa (s). (C) Enlarged image of region shown in B, showing internal endoderm, with nuclei (nu) close to the gastric membrane of the cell as well as large vacuoles (v), invaginated ectoderm with extracellular projection (pr) from mesoglea to chitin layer, and aeriform ectoderm. (D) Transverse section of the pneumatophore, showing septa composed of endodermal cells, and blind tubes. (E) Transmission electron micrograph of the transverse section of the external ectoderm (ec1), showing cilia (ci) and granules (gr). (F) Transverse electron micrograph of chitin producing cells (ec2) and chitin layer (c) and extracellular projections. (G) Thick section showing tightly packed endodermal cells, as well as the branching connections between septum mesoglea and the outermost mesoglea. (H) Transverse electron micrograph of the aeriform ectoderm cell with electron dense granules. (I) Electron micrograph of the aeriform ectoderm cells showing granules and envelope (ev) between cells and gas. Panels (B)–(D) and G are thick sections (0.5–0.75 μm) stained with toluidine blue, and panels (E) and (F) and (H) and (I) are electron micrographs (90 nm).

Figure 3

Gastrozooid. (A) Transverse section of mouth region of hypostome. Schematic in bottom left shows orientation of panels (A)–(E) and (H). Visible are ectoderm cells (ec) with cilia (ci), ectodermal gland cells (eg), and club shaped gland cells of the endoderm (en), which line the gastric cavity (g) in folds known as taeniolae (ta) represented with a dashed line. (B) Transverse section of lower buccal region of hypostome, showing compact taeniolae of endodermal tissue. (C) Transverse section of mid‐region of hypostome showing taeniolae of endodermal tissue. (D) Enlarged image of mouth region from (A), showing ectoderm, mesoglea (m), and endoderm. Two types of endodermal gland cells are visible, one with more darkly stained granule (gc1) and one with larger granules (gc2). Ectodermal cilia and ectodermal gland cells are also visible. (E) Enlarged image of mid‐region of hypostome from (C), showing third type of gland cell (gc3) which contains granules which stain lightly and are much larger than those of gc2. The cilia on the ectoderm are less dense in the mid region of the hypostome. (F) Longitudinal section of the hypostome showing extended gland cells of all three types, with visible nuclei (nu). (G) Transverse electron micrograph of the mid region of the hypostome, showing gland cells types gc1 and gc3. (H) Transverse thick section of the basigaster region. Nematocysts (n) at multiple stages of formation are visible, with undifferentiated cells near the mesoglea and capsules (ca) present in cells in middle of the cell population and those near the ectoderm. Only the undifferentiated cells nearest the gastric cavity are in contact with mesoglea; the ectodermal cell layer and all other capsule bearing cells do not appear to have a connection to the basement membrane. Absorptive cells (ab) are present endodermally. (I) Transverse electron micrograph of the hypostome showing gland cell type gc2. (J) Transverse electron micrograph of the mid‐region of the hypostome, showing the ciliated surface of the gland cells, with cilia oriented perpendicular to gastric surface. Panels (A)–(F) and (H) are thick sections (0.5–0.75 μm) stained with toluidine blue, and panels (G) and (I) and (J) are electron micrographs (90 nm).

Figure 4

Palpon. (A) Longitudinal thick section of the base of the palpon. Schematic in the top left shows the orientation of panels (A), (B), (D), and (F). Visible are funnel cells (fc) and absorptive cells (ab) in the endoderm (en) lining the gastric cavity (g), the ectoderm (ec) including the reduced basigaster region with forming capsules (ca), and the peduncle (pe) that connects to the stem. (B) Enlarged image of region boxed in (A) showing the absorptive and funnel cells with microvilli (mi). Pairs of nuclei (nu) within a single cell are visible. Ectodermal cilia (ci) are also visible. (C) Longitudinal electron micrograph of the ectoderm showing cilia and beta granules (gr), and mesoglea (m). (D) Transverse thick section of the mid‐region of the palpon, showing funnel cells endodermally and thin ectoderm. (E) Longitudinal electron micrograph of the endoderm, showing funnel cells with microvilli arranged in clusters at the apex, and containing a large vacuole (v). (F) Longitudinal thick section of the base of the palpon, showing the mesoglea (m) partially surrounding a lipid droplet (li). A reduced basigaster can be observed basal to the droplet. (G) Longitudinal electron micrograph of endoderm cells from surface to mesoglea, showing rows of microvilli, a well developed endoplasmic reticulum (er), beta granules, and lipid droplets. Panels (A), (B), (D), and (F) are thick sections (0.5–0.75 μm) stained with toluidine blue, and panels (C), (E), and (G) are electron micrographs (90 nm).

Figure 5

Gonodendra and stem. (A) Longitudinal thick section of the ectoderm of the male gonophore. Schematic in the bottom left shows orientation of panels (A)–(F). Sperm progenitor cells (sp) are located between a thin layer of ectodermal cells (ec) and the mesoglea (m). Compact endodermal cells (en) line the gastric cavity (g) of the spadix. (B) Longitudinal thick section of the surface of a male gonophore, showing cilia (ci) at regular intervals on thin ectodermal cells. Beneath the ectodermal cells are sperm progenitor cells with nuclei (nu) that fill most of the cellular space. (C) Longitudinal section showing the tip of a large male gonophore. The spadix reaches nearly the surface of the ectoderm but no opening is visible. The nuclei of the sperm progenitor cells of the large gonophore are elongated toward the ectodermal surface. (D) Longitudinal section of the female gonodendron stalk showing the tissue organization of the oocytes (o) which form ectodermally. (E) Transverse section of young oocytes showing endodermal canal cells (cn) and large nuclei. (F) Transverse section of the siphosomal stem, showing radial extentions of mesoglea lined with ectodermal cells containing myofibrils (my). The large axon (ax) is dorsal. Endodermal cells (en) are compact and thickened at the apical and dorsal sides of the gastric cavity (g). All panels are thick sections (0.5–0.75 μm) stained with toluidine blue.

Figure 6

Nerve cells. In situ nb‐rfamide hybridization. (A) Table showing amino acid structures of selected cnidarian neuropeptides, modified from Grimmelikhuijzen (2004). Amino acids shared with Nb‐RFamide are shaded. (B) The pneumatophore with the anterior facing upward, showing apical pore (ap). nb‐rfamide positive cells are scattered most densely in a ring posterior to the apical pore, and less densely toward the mid region of the pneumatophore. (C) The cormidia of the siphosome, anterior to the left, with gastrozooids (ga) and palpons (pa) ventrally attached to the stem (st), and the giant axon (ax) on the dorsal side. Bracteal scars (bsc) are visible. nb‐rfamide expression can be observed in rings (r) around palpon and gastrozooid peduncles above their point of attachment on the siphosomal stems. (D) Palpon showing rings of nb‐rfamide positive cells at the base of the peduncle (pe) and at the base of the palpacle (pl). No nb‐rfamide positive cells are observed in the palpon body or palpacle. (E) Gastrozooid hypostome showing ectodermal nb‐rfamide positive cells with higher densities near the mouth (mo) and lower density in the mid‐region (mr) of the hypostome. (F) Enlarged image of ectodermal (ec) neuronal cells in the hypostome of the gastrozooid. (G) Whole mount of the siphosomal stem showing transverse collars of nb‐rfamide positive cells. (H) Whole mount of ectoderm, mesoglea (m), and endoderm (en) in the hypostome of the gastrozooid. nb‐rfamide signal is absent in the endoderm. Morphology of ectodermal nb‐rfamide positive cell (arrow) suggests sensory function.