Primordial and mature germ cells overview
Embryonic germ cells are a new pluripotent cell line derived from primordial germ cells. In recent years, it has been found that it has many of the same characteristics as embryonic stem cells (SE cells, derived from pre-embryo embryos such as mulberry embryos, blastocyst inner cell masses), and is therefore also called embryonic stem cells. The concept of therapeutic cloning provides a new way of thinking for overcoming rejection in organ transplantation, giving special value to pluripotent stem cell research.
Primordial germ cells
Primordial germ cells (PGC): There are few reports on the morphology of PGCs. The horse's PGCs are about 8μm in diameter. In addition to the nucleus, the whole cell cytoplasm has many deeply stained circular regions. The shape of the cells is rounded, and pseudopods are common. They are similar in morphology and staining to other species of PGCs. The kangaroo's PGCs are mononuclear large cells with characteristic elliptic mitochondria and abundant ribosomes. Therefore, the cytoplasm appears dense, and the cytoplasm contains very few contents, which are in PGCs and surrounding somatic cells. There is no permanent connection complex between them. The migrated PGCs are mainly located in the tissues of the dorsal mesenteric and near gonads of the gonads. The PGCs in migration has many finger-like, blunt pseudo-foods, micro-filaments in the pseudo-food, and many PGCs are dumbbell-shaped. Polar cytoplasmic, indicating that PGCs move toward the genital warts in the form of amoebic movement, and once the PGCs reach the reproductive ridge, they are oval. There are different opinions about the origin of PGCs. The currently accepted viewpoints of PGCs are derived from the tail yolk sac endoderm, which then migrates from the posterior intestine germ layer and the base of the allantoic sac to the left and right genital warts through the dorsal mesenteric. Ginsburg and his colleagues confirmed that the mice were born 7 days after mating (7dpc), when the rats were born to the midgut. The PGCs are 8 large alkaline phosphatase positive cells, and the cells gradually migrate into the embryo and pass through the allantois to the endoderm. 8 dpc is located in the hindgut endoderm and the base of the allantoic sac, and then the PGCs migrate to the left and right genital warts through the posterior mesentery in two parts. Most PGCs reach genital warts at 10 dpc and then lose their ability to migrate, stopping in the mitosis (testis) or meiosis (ovary). However, they did not observe the situation of PGCs before 7 dpc. Mouse PGCs proliferated very actively during the migration and within two days after reaching genital warts, increasing from less than 100 to 25,000. Alkaline phosphatase staining followed the migration of 8.5 to 13.5 dpc mouse embryo PGCs and found that most of the PGCs were located at the yolk sac layer of the urinary sac and the end of the primitive stalk, ie the edge of the posterior intestine combined with it. A small amount of PGCs are located at the bottom of the yolk sac and the allantoic sac, and most of the cells are buried in the hindgut (usually in the mesoderm). The cells begin to migrate from the hindgut into the developing mesentery that hangs from the dorsal wall and then enter the bilateral genital warts. A small number of PGCs reached genital warts at 10.5 dpc, mostly at 11. 5 dpc. At 12 to 13 dpc, PGCs begin to differentiate into identifiable male or female gametes. The alkaline phosphatase activity of PGCs in females decreased by 14 dpc, while in males it was 15 dpc. In vitro studies have shown that it takes about 4 days (8-12 dpc) for mouse PGCs to migrate, and eventually, all PGCs are in genital warts. Currently, PGCs gradually lose their ability to exercise, and their morphology is also different from that of migrated cells. Significant changes in cell surface properties; reduced adhesion to fibronectin many cell surface antigens, such as stage-specific embryonic antigen-1 (SSEA-1), have lost alkaline phosphatase activity. There are limited data on the dynamics of mouse PGCs, and the results are not the same due to the differences in mouse species and counting methods. The study on Q mice found that 8.5 dpc, the number of PGCs was about 145, the number of cells increased to 364 at 9.5 dpc, 10.5 dpc, 1012 for PGCs, and 2999 for PGCs. At 13.5 dpc, the number of cells reached 26791, and the germ cells of the reproductive genital cloning were formed. At this time, 8 replication cycles were completed and the proliferation of PGCs was completed. The migration of PGCs is affected by many factors, and its mechanism of action is still unclear. It is generally believed that before the formation of the hindgut, the migration of PGCs is mainly carried out passively through morphological changes. From the hindgut to the genital warts, it is an active migration process. This migration is closely related to the cells passing through the migration pathway. Through the arima movement, over the single or multi-layer cells below. This deformation movement is directional and is carried out in strict accordance with the above route. The exact factors that control and guide the migration of germ cells are as follows. Chemotaxis (development of the gonads produces a substance that attracts PGCs); Studies have shown that genital ticks release chemotactic substances that attract PGCs to move toward genital warts. The mouse embryonic genital cells produce a diffusible transfer growth factor TGF-β1 for PGCs, and the PGCs carry the TGF-β1 receptor, so they can migrate along the concentration gradient of the growth factor; TGF-β1in vitro can block the chemotaxis of genital warts. Another chemokine is the steel factor, and keshet et al. describe the expression of SLF during PGC migration, regulating PGCs and “homing” through the chemotaxis of SLF. Contact guidance (PGCs move along pre-formed molecular pathways): During the migration process, surrounding cells and matrices affect the movement of PGCs, and laminin and fibronectin guide the movement of PGCs. Experimental observations revealed that PGCs began to migrate out of the intestine once they adhered to fibronectin, and this effect was attenuated at the end of the migration. This suggests that fibronectin plays an important role in the migration of PGCs. In situ hybridization and histological methods have demonstrated that PGCs in migration does not contain fibronectin mRNA and cannot synthesize and secrete fibronectin. This protein is synthesized by the endoplasmic reticulum of the mesenteric cells and is rarely present in genital warts. Therefore, PGCs do not migrate after reaching genital warts. PGCs gradually lose their ability to adhere to fibronectin during development, thereby exhibiting active migration from the passive migration of the posterior intestine to the endoderm. Differential adhesion (adhesion of PGCs to intercellular contact): Contact between PGCs cells has the greatest effect on cell migration.
Mature germ cells
Germ cells, also known as gametes, are a general term for cells that can produce offspring in multicellular organisms, from primordial germ cells to finally differentiated germ cells (sperm and egg cells), all haploid cells containing a sex chromosome. This term was proposed by A. Engler and K. Planter in 1897 to distinguish it from somatic cells. Somatic cells will eventually die, and only germ cells have the opportunity to survive to the next generation. Species rely on germ cells for continuation and reproduction. Long-term natural selection allows each organism's structure to provide the best conditions for its germ cell survival. There are signs of germ cell differentiation in the single-celled organism population. For example, the four small cells of the genus algae have lost their ability to divide, specializing in exercise and metabolism, called nutrition individuals, and the remaining have the ability to divide, called reproductive individuals; A small number of large germ cells appear between cells. Extremely large, in which small particles rich in ribonucleic acid are called polar particles. After fertilization, cleavage, cells containing polar particles are called polar cells, which are the primordial germ cells of Drosophila. If the original posterior pole is injected into the anterior part of the egg, the cells that are intended to develop into somatic cells can be developed into germ cells. A similar situation can be seen in the eggs of the locust. There is evidence that amphibious germ cells may be determined in the same way. But it is not known what factors determine the development of certain cells into germ cells in mammalian embryos. Only know that the germ cells are determined, they need to migrate to the gonads and differentiate there.
Tada H, Mochii M, Orii H, et al. Ectopic formation of primordial germ cells by transplantation of the germ plasm: direct evidence for germ cell determinant in Xenopus. Developmental Biology. 2012, 371(1):86-93.