Structure and function of the nematosome C. D. OCKLEFORD*, C. H. F. NEVARD, I. INDANS Department of Anatomy, University of Leicester Medical School, University Rd, Leicester LEI 7RH, UK and C. J. P. JONES Department of Pathology, Stopford Building, University of Manchester, Oxford Rd, Manchester M13 9PT, UK •Author for correspondence Summary The ultrastructural morphology of human pla- cental and mouse placental nematosomes has been investigated. The description includes a three-dimensional analysis of the shape of the organelles based on serial sectioning, measure- ments of the repeat distance of the subunit fibre of the organelle derived by optical diffraction analysis and the results of an ultrastructural cytochemical study designed to test whether the organelle contains nucleic acid. Key words: nematosome, placenta, organelle ultrastructure. Introduction General morphology The nematosome is a non-membrane-bound, cytoplas- mic organelle of unknown function. It is spherical or ovoid, about 1 fim in mean cross-sectional diameter and consisting of fibres approximately 40-60 nm in diameter. Each fibre appears to be made up of smaller filaments 2-5-6-0 nm in diameter. The whole struc- ture seems to be embedded in a homogeneous ground substance of lower density. Distribution This inclusion was first described as a 'peculiar fibrous structure', when found in rat and mouse placentae by Enders (1965) during a comparative study of various haemomonochorial placentae. He described the area in which the organelle was found as the 'middle layer' of the trophoblast. Since this time, the nematosome has been reported again in mouse placenta (Toro & Rohlich, 1966; Bjorkman, 1970; Hernandez-Verdun, 1971, 1972; Hernandez-Verdun & Bouteille, 1976; King & Hast- ings, 1977) and rat placenta (Toro & Rohlich, 1966). It has also been observed in baboon placenta (Wynn et al. 1971) and human placenta (Martin & Spicer, 1973; Szvaras, 1977; Jones & Ockleford, 1985), while dragonfly oocytes (Halkka & Halkka, 1975, 1977), rat embryo primitive ectodermal and yolk sac endodermal Journal of Cell Science 87, 27-44 (1987) Printed in Great Britain © The Company of Biologists Limited 1987 cells (Takeuchi, 1980) and even pollen of campanulae (Dunbar, 1973) also contain nematosomes. In addition, the organelle has been found in rat sympathetic neurones (Grillo, 1970), paracervical ganglia (Kanerva & Teravainen, 1972), trigeminal ganglia (Peach, 1972), cerebellum (Chan-Palay, 1973), cerebral cortex (Routtenberg & Tarrant, 1974; Knox et al. 1980), dorsal root ganglia (Jacobs et al. 1975), spinal ganglia (Volk, 1980; Van den Bosch de Aguilar & Vanneste, 1981), superior cervical ganglia (Heym & Addicks, 1982), medial accessory olive (Bourrat & Sotelo, 1983), reeler mutant mouse cer- ebellum (Mariani et al. 1977), ped mutant mouse Purkinje cells (Landis $: Mullen, 1978) and dog spinal ganglion neurons (Fercacova & Marsela, 1983). Study of these published locations may be useful in giving some clue to the function of the nematosome if it is assumed that this is an accurate reflection of the true distribution. The nematosome is found in two main classes of tissue: embryonic and neural. The embryonic localization may be of trophic origin, as placental tissue, which may be syncytiotrophoblastic (rat and mouse) or cytotrophoblastic (baboon and human), or from tissue developed from the inner cell mass (primitive ectodermal and yolk sac endodermal cells). The tissue involved may be of male germinal origin (pollen) or from the female germline (oocytes). 27