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Primordial germ cells are among the first lineages that are established in development [1] and they are the precursors for gametes. [2] It is thought that the process of primordial germ cell migration itself has been conserved rather than the specific mechanisms within it, as chemoattraction and repulsion seem to have been borrowed from blood cells, neurones, and the mesoderm. [1]
Cleavage in most animals segregates cells containing germ plasm from other cells. The germ plasm effectively turns off gene expression to render the genome of the cell inert. Cells expressing germ plasm become primordial germ cells (PGCs) which will then give rise to the gametes. The germ line development in mammals, on the other hand, occurs ...
In mammals, a few cells of the early embryo are induced by signals of neighboring cells to become primordial germ cells. Mammalian eggs are somewhat symmetrical and after the first divisions of the fertilized egg, the produced cells are all totipotent. This means that they can differentiate in any cell type in the body and thus germ cells.
Later studies found that the process from primordial germ cell to spermatogonial development is gradual, without clear gene expression markers to distinguish the precursor cells. [2] A 2006 study found that some gonocytes differentiate straight into committed spermatogonia (type B) rather than spermatogonial stem cells (type A).
These cell divisions are usually rapid with no growth so the daughter cells are half the size of the mother cell and the whole embryo stays about the same size. They are called cleavage divisions. Mouse epiblast primordial germ cells (see Figure: "The initial stages of human embryogenesis") undergo extensive epigenetic reprogramming. [16]
In the mouse, primordial germ cells arise from the inner cell mass (the epiblast) as a result of extensive genome-wide reprogramming. [22] Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, and results in cellular totipotency. [23] [20]
They differentiate in the gonads from primordial germ cells into gametogonia, which develop into gametocytes, which develop into the final gametes. [1] This process is known as gametogenesis. Germ cells pass on genetic material through the process of sexual reproduction. This includes fertilization, recombination and meiosis. These processes ...
Germ cells lead to the production of gametes, while somatic cells perform all other functions within the body. Within the broad category of somatic cells, there is further specialization as cells become specified to certain tissues and functions. In addition, stem cell are undifferentiated cells which can develop into a specialized cell and are ...