The hagfish oocyte at late stages of oogenesis: Structural and metabolic events at the micropylar region

Acrosome reaction in spermatozoa from hagfish (Agnatha) Eptatretus burgeri and Eptatretus stouti: acrosomal exocytosis and identification of filamentous actin

Spermatozoa of the hagfishes Eptatretus burgeri and Eptatretus stouti, caught in the sea near Japan and North America, respectively, were found to undergo the acrosome reaction, which resulted in the formation of an acrosomal process with a filamentous core. The acrosomal region of spermatozoa of E. stouti exhibited immunofluorescent labeling using an actin antibody. The midpiece also labeled with the antibody. The acrosomal region showed a similar labeling pattern when sperm were probed with tetramethylrhodamine isothyocyanate (TRITC)-phalloidin; the midpiece did not label. Following induction of the acrosome reaction with the calcium (Ca2+) ionophore ionomycin, TRITC-phalloidin labeling was more intense in the acrosomal region, suggesting that the polymerization of actin occurs during formation of the acrosomal process, as seen in many invertebrates. The potential for sperm to undergo acrosomal exocytosis was already acquired by late spermatids. During acrosomal exocytosis, the outer acrosomal membrane and the overlying plasma membrane disappeared and were replaced by an array of vesicles; these resembled an early stage of the acrosome reaction in spermatozoa of higher vertebrates in which no formation of an acrosomal process occurs. It is phylogenetically interesting that such phenomena occur in spermatozoa of hagfish, a primitive vertebrate positioning between invertebrates and high vertebrates.

Fine structure of micropylar region during late oogenesis in eggs of the hagfish Eptatretus burgeri (Agnatha)

Using the hagfish, Eptatretus burgeri, the fine structure of formation of the micropylar region in hagfish eggs during the late stages of oogenesis was investigated for the first time, focusing on the bottom region of the micropyle and the egg surface. During these stages, many cells penetrated through the chorion and reached a pit of the egg surface, forming a shovel-like structure in two-dimensional sections. The cells, which we called micropylar cells, were separated from the chorion by a wall of amorphous material. In the pit, another fibrous layer filled the space between the egg surface and the anterior portion of the shovel-like structure. Microvilli coming from the egg surface were embedded in this layer. In later stages, the stack of micropylar cells loosened, and a space appeared between the anterior region of the shovel-like structure and the layer on the egg surface. Microvilli decreased in length and number. The pit region appeared likely to have a role in fertilization. The structures associated with the forming micropyle were markedly different from those observed in the same region of teleost fishes. A hypothesis that hagfish might show transitional structures in gametes from protochordates to teleosts is suggested.

An unusual intermediate filament subunit from the cytoskeletal biopolymer released extracellularly into seawater by the primitive hagfish (Eptatretus stouti)

Each slime gland thread cell from the primitive Pacific hagfish (Eptatretus stouti) contains a massive, conical, intermediate filament (IF)-rich biopolymer (‘thread,’ approximately 60 cm length, approximately 3 microns width). In view of the unusual ultrastructure of the thread, its extracellular role in modulation of the viscoelastic properties of mucus, and the ancient lineage of this primitive vertebrate, we report the nucleotide and deduced amino acid sequences of one major thread IF subunit, alpha (pI 7.5), which is coexpressed with a second polypeptide, gamma (pI 5.3). These two polypeptides coassemble in vitro into approximately 10 nm filaments. The alpha-thread chain, a 66.6 kDa polypeptide, has an unusual central rod domain containing 318 residues flanked by N- and C-terminal domains of 192 and 133 residues, respectively. Each peripheral region exhibits some epidermal keratin-like features including peptide repeats and a high total content of glycine and serine residues. The terminal domains, however, lack the H1 and H2 subdomains characteristic of known keratins. Moreover, when the central rod is aligned either in relation to established homology profiles (J. F. Conway and D. A. D. Parry (1988) Int. J. Biol. Macromol. 10, 79–98) of other IF subunits (types I-V, nestin, non-neuronal invertebrate), or by computer-based homology searches of the GenBank/EMBL Data Bank, a low identity (< 30%) is evident, with no preferred identity to keratins or other known IF types. Although the central rod of 318 residues consists of the canonical apolar heptad repeats interspersed with three linker regions, a discontinuity in phasing of the heptad substructure in rod 2B, and conserved sequences at either end of the rod domain, other collective characteristics are atypical: overall high threonine content (13.2% vs 2.3-5.4% for other IFs), high threonine content in rod 1B (18.8% vs 1–6%), five Thr-Thr repeats in coiled coil segments, L12 of length greater than in keratins, substitution of phenylalanine for a highly conserved glutamate in the sixth position of L2, and a glycine-proline sequence in segment 2B. Possibly as a result of the high threonine content, the percentage of both acidic and basic residues in most helical subdomains is reduced relative to type I and II chains. Fast Fourier transform analyses show that only the acidic residues in segment 1B and basic residues in segment 2 have near typical IF periods.(ABSTRACT TRUNCATED AT 400 WORDS)