AN ELECTRON MICROSCOPIC STUDY OF THE MATURE MEGAGAMETOPHYTE IN ZEA MAYS

With light microscopy maize megagametophytes stained with Alcian blue-periodic acid-Schiff (AB-PAS) reveal acid or neutral polysaccharides in various cell walls. Comparative fine structural studies were made of permanganateor OsO4-fixed material. Organelle distribution is random in the vacuolate and multinucleate aiitipodal cells; organelles are abundant; starch is scarce. Antipodal cell walls have large openings forming several syncytia. Some walls are papillate. In the central cell (primary endosperm cell) a thin peripheral layer of cytoplasm surrounds the large vacuole; organelle number is moderate; starch is abundant. The central cell wall is also papillate adjacent to the antipodals and around the egg apparatus. In the synergids organelle distribution is non-random; nuclei and numerous organelles occupy the micropylar cytoplasm of each synergid; vacuoles dominate the chalazal cytoplasm of these cells. The filiform apparatus stains with AB-PAS and is composed of both lightly and darkly stained amorphous material. In the egg, organelle distribution is perinuclear with vacuoles proximal to the micropyle; mitochondria are large, abundant and polymorphic; starch is abundant. Nucleolar diameter is five times greater in the central cell and egg than in the antipodal cells and ten times greater than in the synergids. Plasmodesmata occur in all cell walls within the gametophyte, but none appear in the gametophyte wall itself. It is suggested that the antipodals and synergids might be secretory, the latter probably being involved in pollen tube attraction, and that stored metabolites in the central cell and egg cytoplasm support rapid increase in metabolism following fertilization. MiORPHOLOGICAL STUDIES (Weatherwax, 1919; Cooper, 1937; Kiesselbach, 1949) showed that the nmegagametophyte of maize is of the Polygonum type (Mlaheshwari, 1950). It consists of four highly differentiated cell types: antipodals, central cell, synergids and egg which develop from the functional megaspore. These and additional studies of maize and other taxa (Maheshwari, 1950) indicate possible physiological functions for each of the cell types. It is considered that antipodals may be secretory, possibly synthesizing enzymes to digest the enclosing nucellus. Brink and Cooper (1944) suggest the synthesis of a growth factor required for endosperm development. In some species the synergids may also be secretory, but in maize the synergid cytoplasm appears degenerate (Vazart, 1958). The polysaccharide filiform apparatus noted by Cooper (1937) in maize and described by Van der Pluijm (1964) and Jensen (1965) in other species is identified in the present study. In light microscopy the maize egg cell cytoplasm appears unspecialized, exhibiting a low degree of structural polarity (Vazart, 1958). 1 Received for publication May 24, 1965. This research, based in part upon a doctoral dissertation submitted by the first author to The University of Texas, was supported at The University of Texas with funds from grant NIH GM-07289 and at UCLA from NSF grant No. 68. The authors wish to thank Dr. Charles C. Tseng for the drawing of the mature megagametophyte. Few reports have appeared concerning the file structure of the angiosperm megagametophyte. Van der Pluijm (1964) reviewed the literature on synergid structure as he reported the fine structure of the mature megagametophyte of Torenia Journieri Fourn. Jensen (1964) correlated fine structural and cytochemical procedures in a study of the mature megagametophyte and young embryo in cotton. The present paper is a coiitinuation of fine structural studies of maize cytoplasms during stages of sexual reproductioll (Diboll, 1964). 1/IATERIALS AND METHODS-AIature megagametophytes were dissected from ovaries of inbred maize line 854 or from hybrids of lines 854 X 857 grown in the garden of the Cell Research Institute at The University of Texas. The megagametophyte and its cells are considered to be mature at anthesis. Fixation was for 2 hr in cold 2% OS04 buffered to pH 7.6 with S-Collidine; for 2 hr in cold 4% KMnO4 or for 12 hr in cold 10% LiMnO4. Following dehydration in a graded series of atcohol the material was embedded in Araldite 1\1 or Mollenhauer's plastic i\'Jixture II (Mollenhauer, 1964) and sectioned with a diamond knife on a Sorvall Porter-Blum ultramicrotome. These sections were photographed oii ani RCA EMU 3D or an RCA EMIT 2 electron microscone oneratinp