Primordial germ cells and oocytes of Branchiostoma virginiae (Cephalochordata, Acrania) are flagellated epithelial cells: relationship between epithelial and primary egg polarity.

Primordial germ cells (PGCs) are described from the gonad of c. 2 cm juvenile Branchiostoma virginiae; early oocytes (c. 10 microm) and enlarging, previtellogenic oocytes (c. 35 microm) are described from the ovary of c. 5 cm adults. The germinal epithelium of the juvenile gonad and adult ovary is composed of both germinal and somatic cells. In the juvenile, somatic cells retain contact with the basal lamina of the germinal epithelium though their perikarya may be displaced towards the lumen; the germinal epithelium is, therefore, a simple but pseudostratified epithelium. In the adult ovary, somatic cells may lose contact with the basal lamina and the epithelium appears to become stratified. PGCs and oocytes are identified as germ cells by the presence of nuage. PGCs and oocytes are polarised epithelial cells. They rest on a basal lamina, extend apically towards a lumen, form adhering junctions with neighbouring cells, and exhibit apical-basal polarity. PGCs and early oocytes have an apical flagellum with an associated basal body, accessory centriole, and one or more striated rootlet fibres. The flagellum is surrounded by a collar of microvilli. Once oocytes begin to enlarge and bulge basally into the connective tissue layer, the flagellum is lost, but the basal bodies and ciliary rootlets are present at the apex of 35 microm oocytes. Similarities of the oogenic pattern in cephalochordates and echinoderms indicate that the establishment of egg polarity in deuterostomes is influenced by the polarity of the germinal epithelium.

Primordial Germ Cells of Synaptula hydriformis (Holothuroidea; Echinodermata) Are Epithelial Flagellated-Collar Cells: Their Apical-Basal Polarity Becomes Primary Egg Polarity.

The primordial germ cells (PGCs) of a recently metamorphosed juvenile Synaptula hydriformis occur with somatic cells in the germinal epithelium of the gonad. As part of the epithelium, PGCs rest on a basal lamina, extend apically towards a lumen, are joined to other cells of the epithelium via apicolateral junctions, and express apical-basal polarity. Each PGC has an apical flagellum that is surrounded by a collar of microvilli. The apicolateral junctions of PGCs consist of apical adhering and subapical septate junctions. Hemidesmosomes attach the PGCs to the basal lamina. Although the somatic cells form an incomplete layer over the PGCs, both the PGCs and somatic cells remain exposed to the apical lumen and retain contact with the basal lamina. The peritoneum is the outermost layer of the gonad and faces the perivisceral coelom. The epithelial-cell characteristics expressed by cells of the peritoneum are identical to those of the germinal epithelium. PGCs of S. hydriformis are epithelial flagellated-collar cells and express the apical-basal polarity that is typical of epithelial cells. The apical-basal polarity of the oocyte, animal-vegetal axis of full-grown eggs, and anterior-posterior axis of larvae and adults are all in correspondence. Thus the polarity of the germinal epithelium may determine the primary body axis of the next generation.

Ultrastructure of the Coeloms of Auricularia Larvae (Holothuroidea: Echinodermata): Evidence for the Presence of an Axocoel.

A hallmark feature of echinoderm larvae is the development of the left anterior coelom. This coelom, called the axohydrocoel, consists of the morphologically distinct, but undivided, left axocoel and hydrocoel. The axocoelic portion forms a duct that opens to the exterior via a pore on the dorsal surface of the animal. Holothuroid larvae are thought to lack an axocoel, but develop an anterior coelom, duct, and pore that are regarded as parts of the hydrocoel. New ultrastructural data, however, show that holothuroid auricularia larvae possess an axocoel and hydrocoel united together into an axohydrocoel. During development the anterior coelom consists of an interconnected left somatocoel, hydrocoel, and axocoel. The left somatocoel separates from the axohydrocoel and subdivides into left and right somatocoels. The somatocoels and hydrocoel region of the axohydrocoel are lined by a monociliated mesothelium having characteristics of transporting epithelia. The axocoel epithelium, like that of asteroid larvae, is composed of mesothelial podocytes. A duct connects the axocoel directly to the open dorsal pore and is lined with a columnar transporting epithelium. The occurrence of a specialized podocyte-lined cavity between the surface pore and the hydrocoel in echinoderm larvae is indicative of an axocoel. That similar structures occur in auricularia larvae supports the identification of an axocoel in holothuroids.

Hemoglobin-containing cells of Neodasys (Gastrotricha, Chaetonotida). I. Morphology and ultrastructure.

The overall anatomy of Neodasys as well as data for hemoglobin-containing cells are described. Hemoglobin-containing cells are shown to be mesodermal specializations constituting approximately 14% of the animal's total body volume (4.87 ± 104 µl). These globular cells (10-14 µm) are situated in two longitudinal rows, each dorsolateral to the straight gut. Branches from the cells enwrap perikarya of muscle and nerve cells whose mitochondria are found just below their respective plasmalemmata in intimate association with the hemoglobin-containing cells. The ground substance of the cytoplasm and nucleoplasm of these nearly organelle-free cells is extremely electron-dense and is presumed to represent the hemoglobin molecules. Locomotion analyses indicate that the cells can undergo a threefold change in linear dimension in 0.25 seconds, raising the possibility of convective mixing in these cells. Structural and ultrastructural comparisons with similar cells in adults of other species of Gastrotricha indicate that the hemoglobin-containing cells of Neodasys may be homologous to the socalled Y cells of other species, some of which contain myofilaments. A muscle-cell origin is considered for the evolution of hemoglobin-containing cells of Neodasys.