Lecture 6 - Germ Cell Definition and Migration PDF

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Germ Cell Definition and Migration

ANAT334 Lecture 6 27/7/18

Lecture Objectives: - Review the germline and primordial germ cells - Describe the migration of germ cells to the gonad - Describe the differences between the eternal and broken germline o Effect of germline definition on body plan evolution o Germline reprogramming. The germline is often considered non-terminal, because the germ cells go on to create the next generation. Weissman’s View (1889) was that germ cells represent a continuous cellular lineage and the body represents temporary housing for the ‘immortal’ germline. Primordial germ cells are defined before the gonads even exist, usually kept at the periphery of the developing embryo until they migrate to the gonad.

Germ Cell Migration to Genital Ridge The PGCs arrive in the germinal crescent (rostral end of embryo) at stages 4-5 (1822 hours) and remain there until the region is vascularised at about stage 10 (3338 hours) and they can begin to spread out. Formation of the genital ridge (precursors to the gonads) does not happen until about stage 21 (3 days). Why are PGCs found where they are? PGCs have latent pluripotency, because they can give rise to embryonic stem cells in culture. They also show transcription of the pluripotency network (Oct4). Because of this, they could potentially form embryonal carcinomas, which is one possible reason why they are kept away from the embryo around the periphery. PGC Migration In chicks, PGCs travel to the gonads by hijacking vasculature, using blood vessels to travel to the genital ridge by about day 4. They’re larger than blood cells so very easy to identify. In mammals, PGCs travel into the embryo through the gut tube, exiting the hindgut at the level of the gonad at about day 9.5 and migrating into the genital ridge at about day 10.5. They have filopedia projections that help with motility, but the cause of directionality is unknown. Their movement is initially quite chaotic and appears to be disordered, but when they encounter a signal from the genital ridge, their movement becomes much more directed.

Eternal vs Broken Germline The eternal germline is defined by ‘germ plasm’ (ribonucleo-complex in cells marking their development into germ cells) carried through from the mother, which is ‘preformed’. Zebrafish and chickens have eternal germlines. The broken germline is defined by signals in the embryo (FGF and BMP4 expressed by trophectoderm), which is ‘induced’ or ‘epigenesis’ i.e. somatic cells are induced to become germ cells. In mammals, no cells express ‘germ plasm’ initially therefore the germ cells have not been passed on from the parents. Proof for Eternal: moving the germ plasm alters the formation of the germline. Proof for Broken: moving regular embryonic cells into the region experiencing the signalling will create PGCs. Additionally, PGCs can be made in culture from ESCs by mimicking these signals. Monkey PGCs arise in the amniotic cavity, which may provide some insight into where human PGCs arise from because we are so closely related to primates.

Germline Reprogramming If something goes wrong with this migration process of PGCs to the genital ridge, miscarriage is most likely to occur. Clearly this is of medical interest, but PGCs are likely more useful in transgenics. If researchers can target the chicken PGCs, there is a higher chance that any genetic alterations will be carried into the next generation. This could be a good way of accessing the germline, which is actually very difficult in chickens. Chick PGCs can be cultured, therefore they can be extensively modified. These cells can be put back into a host embryo to produce transgenic founder offspring. However, you need to match the sexes of

the donor PGCs and host embryo in order to prevent rejection. A simpler way of reprogramming the germline to avoid the need for a chimera is the injection of ‘piggyBAC’ transposon directly into live chick blood. This could potentially be used to modify other animals, such as possums in NZ. We could alter their characteristics to reduce their ability to support offspring, contributing to the government’s Predator Free 2050 goal. This idea has already been used to control mosquito populations in Brazil, the Cayman Islands and Panama....