Question

• What ultimately characterizes different cells from each other? What makes neurons different from immune cells vs. kidney cells, etc.?

Answer 1

Ans. When an egg fertilizes an ovum, the fusion of nucleus of these two cells form zygote. The zygote undergoes repeated divisions to form mass of cells called blastocyst. It then differentiates into three layers of cells which ultimately give rise to various parts of the body. these three layers are ectoderm, mesoderm and endoderm. So, at first the cells behave as stem cells which can give rise to any cell of the body. The process of differentiation and morphogenesis begins at later stages when a cell develops receptor for various triggering growth factors and hormones. For instance, the epidermal growth factor (EGF) binds with receptor on a cell surface and triggers it to differentiate into epidermal tissue. The cells of this tissue now are developed to differentiate into various kinds of epidermal cells.

Neurons are a unique type of cell of the body as they do not regenerate or differentiate further into various kinds of cells. They are the longest cells of the body and once they undergo the process of degeneration, they cannot be regrown. Each cell of a body undergoes the different phases of cell cycle. These phases are first growth phase (G₁ phase), synthetic phase (S phase), second growth phase (G₂ phase) and mitotic phase (M phase). When a cell stops division, it enters G₀ phase. Neurons are different from other cells like kidney cells or immune cells as a mature neuron permanently enters into G₀ phase. Immune cells differentiate into various types like B and T lymphocytes. These cells are again divided into helper cells and memory cells. This kind of differentiation is not possible in case of neurons. Again, when certain area of kidney is damaged, the organ can regenerate its functions to certain extent by greater division and differentiation of existing cells . This is also not possible in case of neurons.