An ultrastructural study of oocyte growth within the endoderm and entry into the mesoglea in Actinia fragacea (Cnidaria, anthozoa)

Abstract

Sea anemone gametes arise in the endoderm but migrate into the mesoglea at an early stage. In order to observe this process, large individuals of Actinia fragacea were collected from the same intertidal location at regular intervals over a 2-year period, and their gonads were examined by light and electron microscopy. The cellular origin of the oocytes is unclear, but the smallest recognizable oocytes are rounded cells, 6-8 microns in diameter, with relatively large nuclei which may contain synaptinemal complexes. Their cytoplasm contains numerous ribosomes, a flagellar basal-body-rootlet complex, and distinctive dense structures also present in male germ cells but not found in anemone nongerminal cells. During the endodermal phase of growth, the density of the oocyte nucleus increases, a single nucleolus becomes prominent, and mitochondria and glycogen accumulate in the cytoplasm. Most oocytes, but not all, only begin major vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea before they attain a diameter of 25 microns. The oocytes migrate toward and enter the mesoglea by a process resembling amoeboid movement. During entry, the oocytes are constricted into a characteristic "hourglass" shape and become covered by a basal lamina continuous with that of the gonad epithelium. The last part of the oocyte to enter the mesoglea forms an intimate relationship with the surrounding endodermal cells, which is maintained after entry is complete, and is thought to be important in the establishment of the trophonema.

Ultrastructural features of the trophonema and oogenesis in the starlet sea anemone, Nematostella vectensis (Edwardsiidae)

“…As a result, the polarity of the embryo is established by the original polarity of the oocyte in the germinal epithelium of the adult. A very similar oogenic pattern occurs in some scyphozoan and anthozoan ovaries, in which oocytes show a consistent polarity in relation to the adjacent gastrodermis (Larkman 1983; Eckelbarger & Larson 1988, 1992). Holothurian oocytes also have an “apical protuberance” (also called a micropyle appendage, micropyle, and axial protuberance) that attaches the oocyte to the germinal epithelium and has been proposed as a structure to facilitate transport (Frick et al 1996) and ophiuroids have a similar structure (Moloney & Bryne 1994).…”

“…While the process of spermatogenesis and mature sperm morphology have proven to be taxonomically useful (e.g., Daly et al 2003), far less is known about anthozoan oogenesis, particularly on the ultrastructural level. Morphological studies of oocytes have been limited to descriptions of the unusual microvilli (termed “spines,”“cytospines,” or “spires”) (Clark & Dewel 1974; Dewel & Clark 1974; Spaulding 1974; Schmidt & Schäfer 1980; Schroeder 1982), the fine structure of specific ooplasmic organelles (Larkman 1980, 1984a; Schäfer & Schmidt 1980; Van‐Praët 1990; Van‐Praët et al 1990), the “cortical reaction” (Dewel & Clark 1974; Schroeder 1982; Schäfer 1984), and the migration of early oocytes from the endoderm into the mesoglea during early oogenesis (Larkman 1983). There have been no comprehensive ultrastructural studies of oogenesis in any sea anemone, especially the process of vitellogenesis.…”

Reproduction in Zoanthidea (Anthozoa: Hexacorallia)

“…) in oocytes of Parazoanthus parasiticus, a spongicolous zoanthid h m Bermuda, but the septa were so crowded and convoluted that it was impossible to determine whether there was a common orientation. There have been no ultrastructural studies on the trophonemata of Zoanthidea but, in the wax sections I have examined, they seem essentially similar to those described in actinians (Dunn, 1975;Larkman, 1983;Larkman and Carter, 1982).…”

Oocyte development in four species of scyphomedusa in the northern Adriatic Sea

“…Oocyte maturation, in P. noctiluca at least, appears to have an initial stage of exogenous vesiculated intake, followed by an endogenous vitellogenesis stage, during which it is possible that the POB performs a nutritive function, like the trophonema found in the actiniarian Actinia fragacea (Larkman & Carter, 1982;Larkman, 1983). In the other species examined, the presence of lipid vacuoles, though fewer and smaller, in the same position and in the same early oocyte maturation stage might indicate a similar process.…”