Structure and development of the egg of the glossiphoniid leech Theromyzon rude: characterization of developmental stages and structure of the early uncleaved egg

Some aspects of the reproductive biology of the glossiphoniid leech, Theromyzon rude, under laboratory conditions, and the staging and structure of its uncleaved egg were studied. Sexually mature animals form breeding communities and fertilization occurs in the ovisacs, presumably around the time of egg laying. Oviposition may be postponed for hours or days, but the eggs in the ovisacs remain blocked at first meiotic metaphase. Development of the uncleaved egg, from the time of oviposition to completion of the first cleavage division, has been sub-divided into six stages. At 20¡C, the six developmental stages take 5-6h. Characterization of the stages is based on observations of both live and fixed/cleared eggs. Discharge of the first pole cell, by the end of stage 1a, is associated with the movement of a ring of contraction between the equator and the animal pole. Discharge of the second pole cell, by the end of stage 1b, is accompanied by contraction of the animal hemisphere which becomes a cone-shaped structure. Polar rings and meridional bands of contraction make their appearance by stages 1c and 1d, respectively. Constriction of the polar rings and shortening of the meridional bands, during stage 1e, lead to accumulation of ooplasm at both egg poles. In this manner, the teloplasm or pole plasm forms. Completion of the first cleavage furrow, by the end of stage 1f, is preceded by dorsoventral flattening of the egg and rearrangement of its teloplasm and perinuclear plasm. Structure of the early uncleaved egg has been studied with transmission and scanning electron microscopy of intact or permeabilized preparations. The plasmalemma forms numerous long and some short microvilli evenly distributed across the egg surface. The ectoplasm includes many vesicles, mitochondria, granules and an elaborate network of filament bundles. Staining of this network with rhodamine-labelled phalloidin indicates that it contains F-actin. A less complex network of similar filaments is also present throughout the yolky endoplasm. The meiotic spindle lies close to the egg surface and not far from the presumptive animal pole ectoplasm. It comprises highly developed poles whose structure and relationships are described.

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				B. H. NELSON and D. A. WEISBLAT Conversion of Ectoderm to Mesoderm by Cytoplasmic Extrusion in Leech Embryos Science, July 12, 1991; 253(5018): 435 - 438. [Abstract] [PDF]