Leech embryogenesis

Leech embryogenesis is the process by which the embryo of the leech forms and develops. The embryonic development of the larva occurs as a series of stages. During stage 1, the first cleavage occurs, which gives rise to an AB and a CD blastomere, and is in the interphase of this cell division when a yolk-free cytoplasm called teloplasm is formed.[1] The teloplasm is known to be a determinant for the specification of the D cell fate.[2] In stage 3, during the second cleavage, an unequal division occurs in the CD blastomere. As a consequence, it creates a large D cell on the left and a smaller C cell to the right. This unequal division process is dependent on actomyosin,[3] and by the end of stage 3 the AB cell divides. On stage 4 of development, the micromeres and teloblast stem cells are formed and subsequently, the D quadrant divides to form the DM and the DNOPQ teloblast precursor cells. By the end stage 6, the zygote contains a set of 25 micromeres, 3 macromeres (A, B and C) and 10 teloblasts derived from the D quadrant.[4]

The teloblasts are pairs of five different types (M, N, O, P, and Q) of embryonic stem cells that form segmented columns of cells (germinal band) in the surface of the embryo.[5] The M-derived cells make mesoderm and some small set of neurons, N results in neural tissues and some ventral ectoderm, Q contributes to the dorsal ectoderm and O and P in the leech are equipotent cells (same developmental potential) that produce lateral ectoderm; however the difference between the two of them is that P creates bigger batches of dorsolateral epidermis than O.[2] The sludgeworm Tubifex, unlike the leech, specifies the O and P lineages early in development and therefore, these two cells are not equipotent.[6] Each segment of the body of the leech is generated from one M, O, P cell types and two N and two Q cells types.[2]

The ectoderm and mesoderm of the body trunk are exclusively derived from the teloblast cells in a region called the posterior progress zone.[7][8] The head of the leech that comes from an unsegmented region, is formed by the first set of micromeres derived from A, B, C and D cells, keeping the bilateral symmetry between the AD and BC cells.[8]

References
  1. Fernandez, J.; Olea, N.; Tellez, V.; Matte, C. (1990). "Structure and development of the egg of the glossiphoniid leech Theromyzon rude: reorganization of the fertilized egg during completion of the first meiotic division". Developmental Biology. 137 (1): 142–154. doi:10.1016/0012-1606(90)90015-B. PMID 2295361.
  2. Weisblat, D. A.; Shankland, M. (1985). "Cell lineage and segmentation in the leech". Philosophical Transactions of the Royal Society B: Biological Sciences. 312 (1153): 39–56. doi:10.1098/rstb.1985.0176. JSTOR 2396301. PMID 2869529.
  3. Lyons, D. C.; Weisblat, D. A. (2009). "D quadrant specification in the leech Helobdella: actomyosin contractility controls the unequal cleavage of the CD blastomere". Developmental Biology. 334 (1): 46–58. doi:10.1016/j.ydbio.2009.07.007. PMC 3077801. PMID 19607823.
  4. Sandig, M.; Dohle, W. (1988). "The cleavage pattern in the leech Theromyzon tessulatum (Hirudinea, Glossiphoniidae)". Journal of Morphology. 196 (2): 217–252. doi:10.1002/jmor.1051960210. PMID 3385778. S2CID 46511971.
  5. Berezovskii, V. K.; Shankland, M. (1996). "Segmental diversification of an identified leech neuron correlates with the segmental domain in which it expresses Lox2, a member of the Hox gene family". Journal of Neurobiology. 29 (3): 319–329. doi:10.1002/(SICI)1097-4695(199603)29:3<319::AID-NEU4>3.0.CO;2-C. PMID 8907161.
  6. Arai, A.; Nakamoto, A.; Shimizu, T. (2001). "Specification of ectodermal teloblast lineages in embryos of the oligochaete annelid Tubifex: involvement of novel cell-cell interactions". Development. 128 (7): 1211–1219. PMID 11245587.
  7. Nardelli-Haefliger, D.; Shankland, M. (1993). "Lox10, a member of the NK-2 homeobox gene class, is expressed in a segmental pattern in the endoderm and in the cephalic nervous system of the leech Helobdella". Development. 118 (3): 877–892. PMID 7915671.
  8. Shankland, M.; Bruce, A. E. (1998). "Axial patterning in the leech: developmental mechanisms and evolutionary implications". Biological Bulletin. 195 (3): 370–372. doi:10.2307/1543150. JSTOR 1543150. PMID 9924777.
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