16.13: Regulating Gene Expression in Cell Development - Cellular Differentiation
- Discuss how differentiated cells can serve different functions
To develop a multicellular organisms, cells must differentiate to specialize for different functions. Three basic categories of cells make up the mammalian body: germ cells, somatic cells, and stem cells. Each of the approximately 100 trillion cells in an adult human has its own copy or copies of the genome except certain cell types, such as red blood cells, that lack nuclei in their fully differentiated state. Most cells are diploid; they have two copies of each chromosome. The process of cellular differentiation is regulated by transcription factors and growth factors, and results in expression or inhibition of various genes between the cell types, thereby resulting in varying proteomes between cell types. The variation in proteomes between cell types is what drives differentiation and thus, specialization of cells. The ability of transcription factors to control whether a gene will be transcribed or not that contributes to specialization and growth factors to aid in the division process are key components of cell differentiation.
Somatic cells are diploid cells that make up most of the human body, such as the skin and muscle. Germ cells are any line of cells that give rise to gametes—eggs and sperm—and thus are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development.
Embryonic Development
Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. These cells are referred to as pluripotent.
Pluripotent stem cells undergo further specialization into multipotent progenitor cells that then give rise to functional cells. Examples of stem and progenitor cells include:
- Hematopoietic stem cells (adult stem cells) from the bone marrow that give rise to red blood cells, white blood cells, and platelets
- Mesenchymal stem cells (adult stem cells) from the bone marrow that give rise to stromal cells, fat cells, and types of bone cells;
- Epithelial stem cells (progenitor cells) that give rise to the various types of skin cells
- Muscle satellite cells (progenitor cells) that contribute to differentiated muscle tissue
A pathway that is guided by the cell adhesion molecules is created as the cellular blastomere differentiates from the single-layered blastula to the three primary layers of germ cells in mammals, namely the ectoderm, mesoderm and endoderm (listed from most distal, or exterior, to the most proximal, or interior). The ectoderm ends up forming the skin and the nervous system, the mesoderm forms the bones and muscular tissue, and the endoderm forms the internal organ tissues.
Key Points
- The three major cell types in the mammalian body include germ cells (which develop into gametes), somatic cells ( diploid cells that develop into a majority of the human body) and stem cells (cells that can divide indefinitely).
- In human development, the inner cell mass exhibits the ability to differentiate and form all tissues of the body; however, they cannot form an organism.
- The various types of stem and progenitor cells included in the body that will differentiate to develop more specialized cells includes: hematopoietic stem cells, mesenchymal stem cells, epithelial stem cells and muscle satellite cells.
- To develop a multicellular oragnisms, cells must differentiate to specialize for different functions.
Key Terms
- blastocyst : the mammalian blastula formed during development where the inner cell mass can be found which forms the embryo
- inner cell mass : a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals
- proteome : the complete set of proteins encoded by a particular genome
- pluripotent : able to develop into more than one mature cell or tissue type, but not all