Читать книгу The SAGE Encyclopedia of Stem Cell Research - Группа авторов - Страница 259

The general organization of the gonad of C. elegans is comparable to that of other species. Similar to Drosophila, the C. elegans gonad consists of a tube; however, C. elegans also has the unique capacity of developing into either a hermaphrodite or a male organism. In hermaphroditic nematodes, the distal tip cells are located at each end of the tube, whereas in the male nematodes, the distal tip cells are present in only one end of the tube. The distal end consists of highly proliferative germ cells, whereas the opposite end of the tube, called the proximal end, is populated by gametes.

The region between the distal and the proximal ends of the tube, also called the transition zone, is occupied by germ cells that are at various stages of differentiation. The entire transition zone is approximately 20 cell diameters in length. The organizational axis of germ cells in the C. elegans gonad is analogous to the seminiferous tubules of the mammalian testes, wherein germ cells located at the basement membrane are undifferentiated and those adjacent to the lumen are fully differentiated. Similar to other organisms, the germ cells of C. elegans interact with neighboring somatic cells, which influence their behavior. The distal tip cell of this nematode serves as a niche that fosters germ cell proliferation. The niche also secretes signaling molecules that enter the signal transduction pathways of germ cells. Niches have also been identified in various mammalian tissues of the integumentary, hematopoietic, gastrointestinal, neural, and reproductive systems. Research investigations using C. elegans thus allow analysts to better understand the behavior of cells during self-renewal and maintenance.

Anatomical drawing of a male C. elegans nematode with focus on the reproductive system. This free-living (not parasitic), colorless and transparent roundworm is approximately 1 mm in length, and lacks a respiratory and a circulatory system. (Wikimedia Commons)

Another interesting feature of the C. elegans gonad is that it is organized as a continuum, thus representing various stages of germ cell differentiation within the single entity. Research studies involving this nematode have thus allowed scientists to trace both major and subtle changes within the differentiation process, possibly identifying the behavior of specific cell populations undergoing the process of self-renewal. Germ cells that are situated close to the distal tip cell are in mitoses; these cells therefore incessantly divide, which is the hallmark feature of cell renewal.

Studies have shown that the intercellular communication, particularly that between cells residing in the niche, coupled with signaling factors, regulate the differentiation process of stem cells. In this scenario, a combination of conditions will induce stem cells at the distal tip to either continuously undergo mitosis or enter the differentiation process. Early experiments showed that the deletion of the distal tip cell or mutations induced in specific genes expressed in distal tip cells result in the premature entry into meiosis, indicating a loss of stem-cell identity. On the other hand, another protein, GLP-1, is expressed in distal tip cells and blocks their entry into meiosis, thus allowing the continuous division of the germ cells.