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Multicellular Life B-2.4 Explain the process of cell differentiation as the basis for the hierarchical organization of organisms (including cells, tissues, organs, and organ systems). Multicellular Life •In the development of most multicellular organisms, a single cell (fertilized egg) gives rise to many different types of cells, each with a different structure and corresponding function. •As cell division proceeds, the cells not only increase in number but also undergo differentiation becoming specialized in structure and function. Multicellular Life •The fertilized egg gives rise to a large number of cells through cell division, but the process of cell division alone could only lead to increasing numbers of identical cells. Multicellular Life •The various types of cells (such as blood, muscle, or epithelial cells) arrange into tissues which are organized into organs, and, ultimately, into organ systems. Multicellular Life Multicellular organisms depend on interactions among different cell types. CELL TISSUE leaf stem vascular tissue ORGAN lateral roots primary root root system • Tissues are groups of cells that perform a similar function. • Organs are groups of tissues that perform a specific or related function. • Organ systems are groups of organs that carry out similar functions. shoot system SYSTEMS Multicellular Life •Nearly all of the cells of a multicellular organism have exactly the same chromosomes and DNA. •During the process of differentiation, only specific parts of the DNA are activated; the parts of the DNA that are activated determine the function and specialized structure of a cell. Multicellular Life Specialized cells perform specific functions. • Cells develop into their mature forms through the process of cell differentiation. • Cells differ because different combinations of genes are expressed. • A cell’s location in an embryo helps determine how it will differentiate. Outer: skin cells Middle: bone cells Inner: intestines Multicellular Life •Because all cells contain the same DNA, all cells initially have the potential to become any type of cell. •Once a cell differentiates, the process can not be reversed. •Stem cells are unspecialized cells that continually reproduce themselves and have, under appropriate conditions, the ability to differentiate into one or more types of specialized cells. Multicellular Life Stem cells are unique body cells. • Stem cells have the ability to – divide and renew themselves – remain undifferentiated in form – develop into a variety of specialized cell types Multicellular Life • Stem cells are classified into three types. – totipotent, or growing into any other cell type – pluripotent, or growing into any cell type but a totipotent cell – multipotent, or growing into cells of a closely related cell family Multicellular Life •Embryonic cells, which have not yet differentiated into various cell types, are called embryonic stem cells. •Stem cells found in adult organisms, for instance in bone marrow, are called adult stem cells. •Scientists have recently demonstrated that stem cells, both embryonic and adult, with the right laboratory culture conditions, differentiate into specialized cells. Multicellular Life • Stem cells come from adults and embryos. – Adult stem cells can be hard to isolate and grow. – The use of adult stem cells may prevent transplant rejection. – The use of embryonic stem cells raises ethical issues – Embryonic stem cells are pluripotent and can be grown indefinitely in culture. First, an egg is fertilized by a sperm cell in a petri dish. The egg divides, forming an inner cell mass. These cells are then removed and grown with nutrients. Scientists try to control how the cells specialize by adding or removing certain molecules. Multicellular Life • The use of stem cells offers many currently realized and potential benefits. – Stem cells are used to treat leukemia and lymphoma. – Stem cells may cure disease or replace damaged organs. – Stem cells may revolutionize the drug development process. Multicellular Life HW •Ch 5 Notes Quiz Tomorrow •Ch 5 Study Guide pages due