How are cells in the body arranged to be functional and not randomly placed?

Each individual, human, and animal is built up by more than 70 organs and there are different types of cells that form each organ (Tortora & Derrickson, 2012). Each type of cell has different roles and characteristics to support the organ to be fully functional. For example, the cells found in the bone are different from the cells in the brain. Their main difference could be identified from its function and shape. According to current knowledge, there are more than 200 different types of cells in the body (Gartner & Hiatt, 2006). However, how do these cells know how to arrange themselves to the position where it can perform its function? How do cells on the skin know that their position is supposed to be at the outermost layer of an individual? How do cells in the kidney or intestine are positioned at the innermost location and not outside the skeleton?

To answer all these questions, scientists revealed that the specification of body formation has been set very early since the development of an embryo. In the initial stage, cells in the embryo are specified into three germ layers: ectoderm, mesoderm, and endoderm. These three different germ layers will further give rise to various organs in the body. The outer part of the body including the epidermis of skin and hair was derived from ectoderm. The middle layer, mesoderm, will form skeletal, muscular, and circulatory system. The innermost layer, the endoderm, gives rise to the innermost organ and body cavity, including lung and digestive organs (Barresi & Gilbert, 2019). The cells that have been specified give rise to certain types of cells depending on the location.

Fig 1. Specification of three different germ lines that gives to various organs (Barresi & Gilbert, 2019)

An experiment was done to figure out whether cells from the same germ layer will assemble. Cells were dissociated from the ectoderm and mesoderm into single cells and then combined to observe how these cells will organize themselves. Interestingly, cells from the same origin will adhere together. In the final formation, the ectoderm was positioned on the outer part and the mesoderm in the center. These cells are somehow able to organize as in the development of an embryo. What drives these cells to be organized in such a position?

Aggregation of different type of cells and organization of different type of cells (Winklbauer & Parent, 2017)

A hypothesis underlying this mechanism was different types of cells will rearrange themselves to be stable thermodynamically. It was assumed that the same type of cells has the lowest energy, thus they adhere together. Meanwhile, different types of cells will arrange themselves in a formation that has the lowest possible energy. Further on, these thermodynamic differences were speculated caused by different types of adhesion molecules expressed as a cell surface marker (Goldschneider & Moscona, 1972).

Cell adhesion between two cells are connected by protein known as cadherin (Mui et al., 2016)

It was then revealed that one of the reasons is indeed because there is a protein on the cell surface that connects the cells. This protein is known as cadherin, and different types of cells express different cadherin. The same type of cadherin binds the same type of cells together, thus separating different types of cells (Takeichi et al., 1988). During embryonic development, cadherin is crucial for the organization of cells in the body.

The presence of cell surface marker cadherin is only one of the known parameters for cell organization. Scientists are still researching other causes and some other reasons that have been revealed are cell polarity, surface tension on the cells, and the role of other protein (Yanagida et al., 2020). It is also possible that certain chemical or physical properties could activate signal for the cells to migrate and rearrange themselves. The mechanism of cell organization is still investigated since it could improve the advancement in the medical field. This knowledge might bring us the possibility to reconstruct the whole organ in the future.

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