Last update: Oct 2020
Let us look at how the scientists used to look at the differentiation of a cell over 60 years ago. In 1957, Conrad Waddington described this process using a ball rolling down a "developmental" landscape (Fig.1.A), starting off as a stem cell at the top of the hill and becoming a differentiated cell once it reaches the bottom. When at the top, the cell has the potential to become any somatic cell. You can think of that as having a lot of potential energy. As the ball representing the cell rolls downhill, it gradually loses that potential energy, until it reaches its most stable point- the differentiated state.
As you can see, there are various grooves in the hillside, meaning the ball can't just roll in any way. It can only go down along the valleys, each valley representing a certain developmental path. Once it enters a valley, it cannot move into another valley because of ridges. It also cannot move uphill because it does not have enough energy. This is a representation of how cell differentiation was thought to be a one-way process only.
While Waddington's model was popular in the past, over the years, accumulating amount of evidence suggested that the developmental progress of the cell is not unidirectional. Indeed, it has become clear that a cell can be reprogrammed and move back up its path or move between ridges, that is to say, between the different differentiation states. I will explain both processes in the next sections.
Figure 1. Developmental landscapes
Another theory of the distant past, 1892 to be specific, is the Weismann Barrier. According to Weismann, information that can be inherited moves from germline cells to somatic cells, but not from somatic cells to germline cells. In other words, the acquired characteristics of the somatic cells are not passed on to the next generation. This draws a sharp line between the soma and the germ plasm.
While the theory normally holds in mammals, it was challenged by more recent findings.
 Proteintech. 2018. Cell Fate Commitment and the Waddington Landscape Model. https://www.ptglab.com/news/blog/cell-fate-commitment-and-the-waddington-landscape-model/. [Accessed 5 May 2020].
 History of Information. 2020. The Weissmann Barrier. https://www.historyofinformation.com/detail.php?id=3843. [Accessed 5 May 2020].
 Surani, M. Breaking the germ line–soma barrier. Nat Rev Mol Cell Biol 17, 136 (2016). https://doi.org/10.1038/nrm.2016.12
Figure 1: Proteintech, (2020), The Waddington’s epigenetic landscape model https://www.ptglab.com/news/blog/cell-fate-commitment-and-the-waddington-landscape-model/ [Accessed 5 May 2020].