Spiral patterns in the micropylar wall and filaments on the chorion in eggs of the medaka, Oryzias latipes

Abstract

The relationship between spiral patterns in the inner wall of the micropyle and attaching and non-attaching filaments on the chorion was investigated by electron microscopy in eggs of the medaka, Oryzias latipes. There were both right-and left-handed spiral patterns of attaching filaments that arose from the chorion at the vegetal pole area. In the majority of eggs examined, however, the prominent folds in the inner wall of the micropylar vestibule descended toward the micropylar canal in a left-handed spiral. This spiral pattern in the micropyle may reflect surface indentations in the cytoplasmic extension of the micropylar cell during formation of the micropyle apparatus. The present observations suggest that the patterns of these spiral structures of the chorion are caused by the rotation of the oocyte due to the movement of follicular cells during oogenesis. On the other hand, there is no unique structure located at the vegetal pole in most teleosts. In the medaka egg, however, a cluster of many long attaching filaments identify the chorion overlying the vegetal pole (Hart et al., '84; Iwamatsu, '75; Kubo, '35). It has been found that the long attaching filaments are wound spirally in one direction on the axis between the animal and vegetal poles of the egg (Iwamatsu, '75, '92a). Medaka eggs exhibit spherical symmetry around the animal-vegetal axis that can be drawn between the micropyle and the center of the cluster of the attaching filaments. The relationship between the right-and left-handed orientation of the spiral folds in the inner wall of the 0 1993 WILEY-LISS, INC.micropyle and the attaching filaments is very interesting because of its association with formation of the chorion and especially with determination of egg polarity during oogenesis.In order to gain a clue as to the mechanism for determination of egg polarity, the morphology of the attaching filaments and micropyles of mature eggs was investigated using scanning and transmission electron microscopy. The results suggest that the spiral orientation of such chorionic structures as the inner wall of the micropyle and the attaching and non-attaching filaments reflects the dynamic development of the oocyte within the basement membrane during oogenesis. MATERIALS AND METHODSThe medakas, Oryzias latipes (orange-red variety) used in the present study, were purchased from a fish farm (Yamato-koriyama, Nara Pref.) and kept for more than 2 weeks under conditions suitable for reproduction (light 14 hr, darkness 10 hr; temperature 26-28OC). After the females' brains were pithed, the ovaries were removed from their body cavities and placed in saline by a routine procedure. Unfertilized mature eggs were released from the ovarian lumen, and the long attaching filaments on each egg were cut short to facilitate observation.For observations by electron microscopy, eggs were prefixed for more than 6 hr with buffered 4%

Medaka Oocytes Rotate Within the Ovarian Follicles During Oogenesis

“…As reported in the previous study [12], the animal-vegetal axis of medaka oocytes is first recognized by the unilateral bending of the nonattaching filaments prior to the initiation of vitellogenesis. The polarity of small pre-vitellogenic (St. V) oocytes seems to be determined by the formation of the VPA.…”

“…As described elsewhere [12], rotation of the oocyte must cause bending of the attaching and non-attaching filaments that are directly in contact with the basement membrane, and the bending must be in the direction opposite to that of the rotation. When the rotation was disturbed by impaling the oocytes with glass needles at right angles to the oocyte axis, the bending direction or the shape of the attaching and non-attaching filaments was altered, as seen in Figs.…”

“…Structures of the micropylar cell become apparent in stage V [23]. In oocytes between stage Ill and stage V, the animal-vegetal axis is recognized by the unilateral bending of elongating filaments on the chorion [12]. Oocytes surrounded by a granulosa cell layer may begin to rotate within the basement membrane before or during determination of the oocyte axis in the very early stage of oogenesis.…”

Morphology of unfertilized mature and fertilized developing marine pelagic eggs in four types of multiple spawning flounders

“…3d). Several authors have reported that, until the initiation of oocyte maturation, the micropylar canal is plugged with cytoplasmic protrusion of a large, mushroomshaped micropylar cell (Takano and Ohta 1982;Kobayashi and Yamamoto 1985;Iwamatsu et al 1993b;Nakashima and Iwamatsu 1994). Bodies of the micropylar cell and nearby granulosa cells exert mechanical pressure on the external surface of the growing oocyte and thus participate in the formation of the micropylar vestibule.…”

Reproduction of landlocked Aplochiton zebra Jenyns (Pisces, Galaxiidae)

“…Histological analyses revealed the presence of filaments located on the chorionic surface of vitellogenic oocytes. Other species [e.g, Oryzias latipes (Temminck and Schlegel) and Odontesthes bonariensis (Valenciennes), Iwamatsu & Keino 1978; Iwamatsu 1992; Iwamatsu et al. 1993; Guandalini et al.…”