Stem cell autotomy and niche interaction in different systems

Figure 3 Schematized longitudinal sections through the apices of testicular follicles of four insect species. The order of the images from (A) to (D) is arranged according to increasing complexity of the structural relationships between germline stem cells (GSCs) and their niche. It should be noted that the order is not in congruence with the positions of the species in the natural insect system based on evolutionary progress. A: Drosophila melanogaster. The ACs (hub cells) are located in a small terminal appendix of the follicle, the hub (HUB), where many of them border the testicular wall (TW). (The testicular wall consists of an outer pigment layer resting on a basal lamina and a muscle cell layer sitting on an inner basal lamina.) The testicular wall does not provide a hemolymph-testis barrier. A spermatogonial is constituted by two CCs which protect the spermatogonia against hemolymph. Both GSCs and CySCs contact ACs (Adapted from Hardy et al[61]); B: Locusta migratoria. Depicted is the rosette-like arrangement of GSCs and CySCs with the single AC in the center. The AC of this example includes two phagocytized GSCs (see above). Besides GSCs also CySCs contact extensions of the star-like AC. A plug of CySC-like cells (CySC-L) is located below the apical rosette. The wall of the spermatogonial cysts is composed of numerous CCs (Adapted from Szöllösi et al[154] and Dorn et al[64]); C: Oncopeltus fasciatus. The cellular extensions of GSCs and segregated vesicles surround the surface of ACs. CySCs cover only the distal part of the apical rosette; they do not make contacts with the ACs. Proximal to the rosette, remnants of degraded GSCs and probably young cysts amass (RDC). Cysts form at the lateral parts of the rosette (Adapted from Schmidt et al[65]); D: Lymantria dispar. Note that the cellular extensions of GSCs protrude into the large singular AC. The extensions autotomize and the segregated vesicles are phagocytized by the AC. Each GSC is affiliated with one CySC. During early larval development the AC is attached to the TW [comparable to the situation in Drosophila, where ACs (i.e., hub cells) are lifelong attached to the TW], but separates with progressing development. The apical complex then adopts a spherical organization. (Adapted from Klein[66]). AC: Apical cell (often called hub cell in Drosophila) (green); CC: Cyst cell (orange); CySC: Cyst stem cell (yellow); GB: Gonialblast (purple); GSC: Germline stem cell (red); SPG: Spermatogonia (purple); TW: Testicular wall (grey).