Cellular components of Paramecium - conserved and specialized organelles.

(A) Scheme of a Paramecium cell (based on Allen and Fok, 2000 [26], Plattner and Kissmehl, 2003 [33]). The cell is lined with cortical Ca²⁺-stores, the alveolar sacs (as) attached to the cell membrane, from where cilia (ci) and dense-core secretory vesicles (trichocysts, tr) emerge. “gh” indicates membrane “ghosts” from released trichocysts. Also outlined are components of the phagocytic cycle, starting with the terminal invagination of the oral cavity (oc), the cytopharynx, from where food vacuoles (fv) are released. Acidosomes (as), discoidal vesicles (dv) and other recycling vesicles (rv) deliver membrane material for phagosome formation. Defecation occurs at the cytoproct (cp). The endosomal system is represented by parasomal sacs (ps; corresponding to coated pits) and early endosomes (ee; designated as terminal cisternae). Additionally, the Golgi apparatus (ga), the endoplasmic reticulum (er) and the two distinct types of nuclei, the germline micronuclei (mic) and the somatic macronucleus (mac) are outlined. Also included are elements of the contractile vacuole complex with the contractile vacuole (cv), the ampullae (a), the decorated spongiome (ds) and the smooth spongiome (ss). Cell length is ∼100 to 120 µm. (B) In detail the cortical organization contains alternating arrangements of trichocysts (tr), cilia (ci) and basal bodies (bb) as well as alveolar sacs (as). Those underlie the plasma membrane, except at sites occupied by trichocysts and cilia. Parasomal sacs (ps) are positioned at the anterior sites of cilia, whereas early endosomes (ee) are located below basal bodies. (C) Adjacent alveolar sacs are in contact with each other at longitudinal and perpendicular elevations of the cell surface (upper graph) and at depressions (lower graph) in the vicinity of cilia.