ORGANOGENESIS IN THE LEECH - DEVELOPMENT OF NEPHRIDIA, BLADDERS AND THEIR INNERVATION

The formation of the definitive excretory system (nephridium and bladd er complex) in Hirudo medicinalis during the last two thirds of embryo nic development was observed with light- and electron microscopy, immu nocytochemistry, and nuclear labeling. In jawed leeches, two excretory systems develop and function successively. The nephridia of the crypt olarva are associated with the larval sac and persist until the defini tive nephridia are sufficiently developed to be functional. Developmen t of the definitive excretory system begins with the differentiation o f the (ectodermal) bladder and urethra. The cells from which they aris e incorporate bacteria and are thereby recognizable at day 8. The (mes odermal) urine-forming tissues of the nephridium (canalicular cells an d central canal cells) appear a day later. By day 17, the nephridia ar e in contact with the outlet region and structurally able to function. Each nephridium is individually innervated by a peripheral neuron, th e nephridial nerve cell, which expresses FMRFamide-like immunoreactivi ty and begins growing into the nephridium on day 11. Organogenesis of the leech nephridium is compared with the formation of excretory organ s in other species. The temporal correlation of innervation and the de velopment of the transporting cells is discussed.