Cell Differentiation

Section 2: Cell Differentiation

Structural and Functional Differentiation of Cells

As multicellular organisms grow from small to large, there is not only an increase in the number of cells but also differentiation in their structure and function. Even in mature individuals, some cells retain the ability to produce different types of new cells.

Cell Differentiation and Its Significance

In the early stages of embryo development, all cells are similar to each other. Through cell mitosis, the number of cells increases. At the same time, these cells gradually change in different directions.

Similar to animals, different types of cells also exist within the same plant body. For example, leaf mesophyll cells contain abundant chloroplasts capable of photosynthesis; epidermal cells lack chloroplasts but form a distinct cuticle layer on the cell wall, providing protection; storage cells lack chloroplasts and cuticle layers but store many nutrients (Figure 6-5). These cells all originate from a group of early embryonic cells that are similar to each other.

During individual development, the process where descendants produced by one or a type of cell through proliferation exhibit stable differences in morphology, structure, and physiological function is called cell differentiation. Cell differentiation is a persistent change; generally, differentiated cells will maintain their differentiated state until death.

Cell differentiation is a universal phenomenon in the biological world and forms the basis for the development of biological individuals. During the growth and development of multicellular organisms, without cell differentiation alongside cell proliferation, it is impossible to form tissues and organs with specific shapes, structures, and functions, and the organism cannot develop normally. Cell differentiation directs cells in multicellular organisms towards specialization, which enhances the efficiency of various physiological functions.

Why do various cells within an individual have significantly different morphologies, structures, and functions despite possessing identical genetic information? This is due to selective gene expression within cells during individual development. For example, genes related to hemoglobin synthesis are active in red blood cells, while genes related to antibody synthesis are inactive; the reverse is true for B cells (a type of immune cell).

Cell Totipotency

Early embryos gradually develop into various tissues and organs through cell division and differentiation. Under certain conditions, can highly differentiated cells within these tissues and organs, like early embryos, redifferentiate into other cells?

Experiments show that highly differentiated plant cells still retain the ability to develop into complete plants; this is cell totipotency. Cell totipotency refers to the potential and characteristics of cells, after division and differentiation, to produce complete organisms or differentiate into various other cell types. Of course, undifferentiated cells such as fertilized eggs, early embryo cells in animals and humans, and meristematic tissue cells in plants also possess totipotency. Nowadays, people can use the totipotency of plant cells to rapidly propagate flowers, vegetables, and other crops through plant tissue culture methods, cultivate miniature ornamental plants (Figure 6-6), and save endangered species. Consider the advantages of plant tissue culture compared to traditional hybridization techniques.

Comparatively, conducting similar experiments in animals is much more complex and difficult than in plant tissue culture. Apart from the experiment with African clawed frogs mentioned earlier, the cloned sheep "Dolly" born in 1996, and the world's first batch of somatic cell-cloned monkeys "Zhong Zhong" and "Hua Hua" obtained by Chinese scientists in 2017, were developed by transplanting somatic cells into enucleated egg cells, indicating that differentiated animal somatic cell nuclei possess totipotency. However, so far, people have not successfully cultured individual differentiated animal somatic cells into new individuals.

Within animals and humans, a few cells with the ability to divide and differentiate are retained, known as stem cells. Human bone marrow contains many hematopoietic stem cells that can continuously produce red blood cells, white blood cells, and platelets through proliferation and differentiation, replenishing the blood (Figure 6-7). Umbilical cord blood contains a large number of stem cells that can be cultured and differentiated into various blood cells in the human body. Currently, umbilical cord blood stem cells are used to treat hematological diseases.