Biology 133 lecture AY 2021- 2022

Good day Ma’am Leonardo! Thank you very much for your comments! Apologies po for the background music. Here are the following answers to your inquiries:

One example of an extrinsic signal is intervention of limb organogenesis done by the somitic mesoderm. In the study of Chevallier et al. (1977), it was seen in quail-chick chimeras that the somitic mesoderm exerts an inductive influence on the adjacent limb somatopleural mesoderm, causing limb development to begin. The somitic mesoderm then gives the growing limb bud a number of cells that will turn into myoblasts. Another example of an extrinsic signal is connective tissue of somatopleure origin. The study of Jacob & Christ (1980) also utilizes quail-chick chimeras to show that the regional somatopleure influences the corresponding regional muscle patterns. This connective tissue causes the myogenic cells of somitic origin to establish a specific regional muscle pattern.

To answer your second question, one developmental innovation integral to the evolution of the musculoskeletal system is the regulatory sequences that drive the expression of the genes Tbx5 and Tbx4. These genes play an important role in the evolution of the paired appendages. The expression of Tbx5 is limited in the forelimbs while the expression of Tbx4 is limited in the hindlimbs (Ruvinsky et al., 2000), and the expression of both of these genes allows for the initiation of limb outgrowth. The study by Minguillon et al. (2009) then further expounds on this innovation by analyzing the amphioxus Tbx4/5 gene. Through assaying the amphioxus locus for the presence of limb-forming regulatory regions, it was proposed that the regulatory sequences that cause the difference in the levels of Tbx5 and Tbx4 are what cause the acquisition of paired appendages during vertebrate evolution. This was seen as amphioxus posses a single Tbx4/5 gene and lack paired appendages, while vertebrates with paired appendages express Tbx5 in the rostral pair and Tbx4 in the caudal pair.

References:
Chevallier, A., Kieny, M. and Mauger, A. (1977). Limb-somite relationship: origin of the limb musculature. J. Embryol. Exp. Morphol. 41, 245-258.
Jacob, H. J. and Christ, B. (1980). On the formation of muscle pattern in the chick limb. In Teratology of the Limbs (ed. H. J. Merker, H. Nau and D. Neubert), pp. 89-97. Berlin: Walter de Gruyter and Co.
Minguillon, C., Gibson-Brown, J. J., & Logan, M. P. (2009). Tbx4/5 gene duplication and the origin of vertebrate paired appendages. Proceedings of the National Academy of Sciences, 106(51), 21726–21730. doi:10.1073/pnas.0910153106 
Ruvinsky, I., Oates, A. C., Silver, L. M., & Ho, R. K. (2000). The evolution of paired appendages in vertebrates: T-box genes in the zebrafish. Development Genes and Evolution, 210(2), 82–91. doi:10.1007/s004270050014 

Hi there! Happy New Years to your group (Agustin, Bisquera, Punzalan) too!
To answer your first question, basically, the two genes, EFF-1 and AFF-1 are necessary to fuse cells. They can mediate developmental cell-cell fusion to form syncytial tissues, fuse gametes, and permit the viral infection of host cells. Furthermore, these genes can mediate cell auto-fusion to form or repair dendrites and axons and give rise to narrow seamless tubes with intracellular lumens (Soulavie et al., 2018).

For the second question, a study found that non-canonical Wnt signaling for osteoblast maturation can be an alternative. These non-canonical pathway ligands, receptors, and intracellular signaling molecules were upregulated in the study. Specifically, Wnt5a (non-canonical pathway) and Wnt3a (canonical pathway) were cultured in cells. The results found that the non-canonical calcium-dependent Wnt pathway played a major role in the differentiation of osteoblast during implant osseointegration (Olivares-Navarrete et al., 2011).

References:
Olivares-Navarrete, R., Hyzy, S. L., Hutton, D. L., Dunn, G. R., Appert, C., Boyan, B. D., & Schwartz, Z. (2011). Role of non-canonical Wnt signaling in osteoblast maturation on microstructured titanium surfaces. Acta Biomaterialia, 7(6), 2740–2750. https://doi.org/10.1016/j.actbio.2011.02.030
Soulavie, F., Hall, D. H., & Sundaram, M. V. (2018). The AFF-1 exoplasmic fusogen is required for endocytic scission and seamless tube elongation. Nature Communications, 9(1). https://doi.org/10.1038/s41467-018-04091-1