LESSON 1: THE DEVELOPMENT OF CELLS

Multicellular organisms are composed of not only many cells, but many different types of cells. Yet all of the cells of an organism develop from one, original cell, either a spore or a zygote. How do these cells become differentiated from one another, so that they may perform specific roles for an organism?

The way in which these cells become specialized starts out with some type of signal, usually a chemical signal. In response to a given signal or signals, cells can change the way they function and develop, most often by controlling what genes are expressed at what time. This process of cellular determination and differentiation happens most commonly in developing embryos, where the cells don’t yet have a specific function, but can occur in certain cells of adult organisms.

Learning Objectives

• Understand the processes of signal transduction, and how they are important for all cells.

• Understand the concepts of cell determination and differentiation, and how these processes are effected in developing cells.

• Learn how developmental genes are expressed in a hierarchical manner.

• Be familiar with common early stages of development.

• Understand the factors influencing the development of shape in tissues, organs, and bodies.

Topics covered in this Lesson

• Cell Differentiation and Morphogenesis
• Cell Signaling
• Cell Determination and Differentiation

Cell Differentiation and Morphogenesis

As you go through the next few lessons, try to paint a picture in your head of the ways in which cells change and develop in embryos and organisms. Basically, organisms begin life as a cell whose only developmental restrictions are based on the genome it carries. For instance, an early embryonic animal cell does not have the proper genes to develop into a leaf, but it can develop into any cell type found in the mature animal. Cells with such unlimited developmental potential, like cell of very early embryos, are often referred to as totipotent. As an organism develops, the processes of cell signaling, cell determination and cellular differentiation lead to the maturation of cells and tissues that perform specific functions for an organism.

Learning Objectives

• Why do most all individuals in a species have the same basic body structure?
• Understand the cellular causes of organismal morphogenesis.
• Be able to describe the general process by which expression of genes is controlled during development.
• Understand some of the key differences and similarities among very different species.

Cell Signaling

Cells have developed complex ways of receiving and responding to signals from other cells and from their environment. In this lesson we will look at the process of signal transduction, in which signals are received by cells and translated into an appropriate cellular response. Cells receive signals at the cell membrane or within the cytoplasm, amplify the signals, and finally respond to the signal in one of several ways. Cell signaling plays a vital role in the development of organisms, and in the regulation and activity of mature cells and tissues.

Learning Objectives

• What is a signal transduction pathway?

• Know what a ligand is.

• Understand the four basic receptors and how they work to receive and transmit signals.

• Understand the general differences between membrane-bound receptors and cytoplasmic receptors.

• Be able to describe three common cellular responses to cell signals.

• Be able to describe how a protein kinase cascade works and why it is useful.

• Understand what second messengers are and what they do. Be familiar with several examples of second messengers.
   

Cell Determination and Differentiation

With few exceptions, all of the cells in a multicellular organism contain the same DNA. This is not surprising, given that organisms develop from mitotic divisions of one original cell, called a zygote. However, it is clear that there are many different cell types in the bodies of multicellular organisms. How do these cells ‘mature’ to take on specific roles for an organism? This module will explore the processes by which cells mature, commonly referred to as cell determination and differentiation.

Learning Objectives

• Understand the difference between determination and differentiation as it pertains to cell development.

• Be able to explain how the developmental fate of cells becomes less flexible over time in a developing embryo.

• What molecular processes lead to cell determination and differentiation? In other words, how can cells with the same genetic information look and function differently?

• Understand what stem cells are, know the different types of stem cells, and why they are so useful/important to medical research.