C100 Biology : Nature Reviews Molecular Cell Biology

Briefly explain cell differentiation and its importance in the formation of tissue types.

Using the Electron micrographs of tissues provided in Task P4, explain how the presence of certain numbers of cell components influence the functions of tissues i.e: effect of large number of mitochondria in a specific tissue type.

(i) Cell differentiation can be defined as the process by which an immature and young immature cell evolves and develops to form a specialized cell. This leads to the cell attaining a mature form and function. It also leads to development of stable differences between cells. All essential activities of metabolism, transport of molecules and reproduction occur within the single cell of unicellular organisms. However, several cell types are required by multicellular organisms to carry out their functions. There occurs a division of labour. A classic example of such differentiation is the process of development of a zygote to a multicellular embryo from a single cell, and its subsequent maturation into a fetus .

Cell differentiation is regulated by cell shape, size, density, polarity, extracellular composition of the matrix and metabolism. The influence of such external factors instructs each cell to differentiate and mature into a specialized structure such as muscles, skin, heart and neurons. Differentiation occurs by turning on or repressing the expression of certain genes. Transcription factors regulate this gene expression . Transcription factors are a class of proteins, which bind to specific genes on DNA sequences and either inhibit or promote the transcription of the gene of interest. These molecules convey messages to target cells to facilitate biological processes. The molecules carry the signal to cell receptors. The receptors interpret the messages by the action of signal transduction pathways. Activation of these pathways translates the messages and governs the functioning of the cells.

A cell, which is capable of differentiating into any specialized type, is known as a totipotent cell. The zygote and first few cells that are formed after initial cell division stages are totipotent . On the other hand, cells that get differentiated into many specialized cell types are known as pluripotent cells. These cells retain their ability to divide and are responsible for maintaining the stem cell population. During the cell differentiation process, most of the cells become committed towards transforming into another cell type. This state of commitment is described as specification or determination .

Thus, it can be stated that cell differentiation is essential because all higher organisms develop from a single celled zygote. Differentiation of the zygote gives rise to different tissues and organs. 

(ii) Presence of more mitochondria in the liver cells is due to the fact that a liver has to perform several tasks like metabolism, deamination and protein synthesis. More mitochondria (1000-2000) per liver cell provide the energy required to carry out its functions . Moreover, enzymes present in liver cell mitochondria assist in detoxifying ammonia. The Leydig cells contain more number of smooth endoplasmic reticulum due to the property of SER of storing an enzyme that participates in testosterone synthesis chemical pathway. The SER in Leydig cells provide an environment where the enzymes can complete testosterone synthesis.

References:

Baumann, K., 2017. Stem cells: A key to totipotency. Nature Reviews Molecular Cell Biology, 18(3), pp.137-137.

De Paepe, C., Krivega, M., Cauffman, G., Geens, M. and Van de Velde, H., 2014. Totipotency and lineage segregation in the human embryo. Molecular human reproduction, 20(7), pp.599-618.

Dixon, J.R., Jung, I., Selvaraj, S., Shen, Y., Antosiewicz-Bourget, J.E., Lee, A.Y., Ye, Z., Kim, A., Rajagopal, N., Xie, W. and Diao, Y., 2015. Chromatin architecture reorganization during stem cell differentiation. Nature, 518(7539), p.331.

Hall, J.E., 2015. Guyton and Hall Textbook of Medical Physiology E-Book. Elsevier Health Sciences.

Wang, J.T. and Seydoux, G., 2013. Germ cell specification. In Germ Cell Development in C. elegans (pp. 17-39). Springer New York.