Formation of the diverticular lumen that enables oocyte fertilization in katoikogenic scorpions, Heterometrus spinifer and Opistophthalmus boehmi (Scorpionidae).

The structure of diverticula of the female gonads was analyzed in two scorpions from the family Scorpionidae by means of standard microscopic techniques (light microscopy, histochemistry, transmission electron microscopy). In scorpions, the female gonad, termed the ovariuterus, participates in two consecutive processes: oogenesis and embryogenesis. In sexually reproducing scorpions, the ovariuterus is also involved in fertilization. Both scorpions under study reproduce sexually. They also represent the katoikogenic type of development, which means that oogenesis, fertilization, and embryogenesis take place in the diverticula, which are sac-like outpocketings of the ovariuterine tubules. Formation of a lumen in the diverticulum is indispensable for sperm entry to enable fertilization and subsequent embryogenesis. The aim of the study was to test our hypothesis that the diverticulum lumen forms due to the engagement of the centrally located stalk cells. In this report, we show that in two species of katoikogenic scorpions, at the final stages of oogenesis, the cytoplasm of the stalk cells contains secretory organelles. In the stages preceding fertilization, secretory activity of the stalk cells and fragmentation of their apical parts lead to formation of the diverticulum lumen by a process similar to "cord hollowing" that commonly occurs for lumen formation in other morphogenesis model systems.