DEVELOPMENT AND EARLY DIFFERENTIATION OF GONAD IN THE EUROPEAN EEL (ANGUILLA-ANGUILLA [L], ANGUILLIFORMES, TELEOSTEI) - A CYTOLOGICAL AND ULTRASTRUCTURAL-STUDY

The structure of the gonad of the European eel (Anguilla anguilla [L.] ), an ''undifferentiated'' gonochoristic teleost, was investigated by transmission electron microscopy from 6-8 cm elvers to 22 cm yellow ee ls with juvenile hermaphroditic gonads. The pear-shaped gonads of 6-8 cm elvers assume, in 12-15 cm eels, a lamellar shape and enlarge by mi gration of germ cells, which we refer to as primary primordial germ ce lls. In the gonads of similar to 16 cm eels, the primary primordial ge rm cells multiply, giving rise to clusters of germ cells that have ult rastructural characteristics of the primary primordial germ cells but show giant mitochondria, enlarged Golgi complexes, and round bodies no t limited by membranes. We refer to these as secondary primordial germ cells. In 16-18 cm eels, syncytial clones of oogonia interconnected b y cytoplasmic bridges are also observed. In 18-22-cm-long eels, the go nads contain primordial germ cells, oogonial clones, early oocyte cyst s, single oocytes in early growth stages, and primary spermatogonia. S uch germ cells are present in the same cross section where they are ei ther intermingled or are in areas of predominantly female germ cells c lose to areas with predominantly male germ cells. These gonads are juv enile hermaphroditic and should be considered ambisexual because in la rger eels they differentiate either into an ovary or into a testis. So matic cells always envelop the germ cells following their migration in to the gonad. These somatic cells first show similar ultrastructural f eatures and then differentiate either into early Sertoli cells investi ng spermatogonia, or into early follicular (granulosa) cells investing the early previtellogenic oocytes. In eels similar to 14 cm long, pri mitive steroid-producing cells also migrate into the gonad. In the amb isexual gonad they differentiate either into immature Leydig cells in the male areas, or into early special cells of the theca in the female areas. Nerve fibers are joined to the steroid-producing cells. Gonad development and differentiation are also associated with structural ch anges of the connective tissue characterized by the progressive appear ance and deposition of collagen fibrils first in the mesogonadium, the n in the gonad vascular region, and then in the germinal region. The c ollagen-rich areas are massive in the male areas and reduced in the fe male ones. (C) 1997 Wiley-Liss, Inc.