Germ‐line cysts organisation in clitellate annelids – from electron microscopy to live cell imaging
Authors: Świątek Piotr , Urbisz Anna Z. , Małota Karol , Gorgoń Szymon , Jarosz Natalia , Świątek Piotr 
Published: December 2016

At the onset of gametogenesis in animals, the germ‐line cells usually form syncytial cysts (clusters). The germ cells in such cysts are interconnected by wide cytoplasmic channels known as intercellular (cytoplasmic) bridges that are modified contractile rings that do not close during late cytokinesis. These bridges ensure the cytoplasmic continuity between the interconnected cells and, as a result, the cytoplasm of the sister cells is common. It is believed that the interconnections of germ cells into syncytial clusters regulate and synchronise germ cell development. However, it seems that the actual role of the cell clustering is sex dependent.

The germ‐line cyst architecture (spatial organization) varies between taxa. Currently, we know several different modes of cysts organisation. The best characterised examples are: linear cysts in which the interconnected cells form chains (e.g. mammal spermatogenesis); branched cysts in which some cells may have more than two intercellular bridges and, as a result, “branchings” occur in these sites (e.g. male and female cysts in
Drosophila
); and cysts with germ cells that are clustered around a central anuclear mass of cytoplasm (e.g. in
C. elegans
).

Clitellate annelids (Clitellata) is a monophyletic taxon of segmented worms, which comprises earthworms and their allies and leeches. At the early stages of spermatogenesis and oogenesis in Clitellata, germ‐line cysts are also formed (with one known exception – female cells in
Capilloventer australis
). In both lines, male and female, the cysts have the same basal plan of organization – each germ cell is connected to the central and anuclear cytoplasmic mass, the cytophore via one cytoplasmic bridge (Fig. 1 and 2). However, the cyst morphology may differ substantially between taxa and even between sexes in the same specimen. These differences may be caused by: a different number of interconnected germ cells (from 16 in the pot worm
Enchytraeus albidus
(Fig. 2) to over 2 000 in the sludge worm
Tubifex tubifex
(Fig. 3)); a different shape and ratio of cytophore development (the cytophore in male cysts is usually spherical and well developed, while in female cysts the cytophore usually has the form of long cytoplasmic strands and is often poorly developed). Additionally, whereas male cysts are not associated with somatic cells, female clusters are usually enveloped by somatic cells and together form characteristic structures such as egg follicles or ovary cords.

During our long‐term studies, we have analysed the formation, architecture and functions in both male and female germ‐line clusters in several representatives of clitellate annelids (e.g. the earthworm
Dendrobaena veneta
, the pot worm
E. albidus
, a sludge worm
T. tubifex
and the European medicinal leech
Hirudo medicinalis
). Using classical (light and fluorescent microscopy, electron microscopy) and modern tools (live cell imaging see Fig. 4, 3D reconstructions in scanning electron microscopy – SBEM method), we have revealed, among others, the great plasticity in cyst architecture and sex‐dependent differences in their functions.