LESSON 2: THE DEVELOPMENT OF ORGANISMS

From previous lessons, you now have a general idea of how cells communicate and change in embryos and adult organisms. The next phase of understanding involves putting these processes in the bigger picture of how cells work together to form a functional individual. We will take a chronological look at development starting with the early stages, such as fertilization and then move on to larger morphological changes that occur later on in development. We will be using the typical examples of plants and animals, but keep in mind that there are a number of other multicellular eukaryotic organisms that have similar processes and controls over development.

Learning Objectives

• Understand the hierarchical organization of developmental genes.

• Become familiar with common stages of early development.

• Understand the factors affecting the development of shape, or morphology, in organisms.

Topics covered in this Lesson

• Regulation of Development
• Early Stages of Development
• Pattern Formation

Regulation of Development

In previous topics and lessons we have looked at how genes are transcribed and translated, but the question keeps coming up - how do cells and organisms “know” what genes to transcribe when? Differential gene expression during development has been studied in some detail for a number of organisms. It turns out that development often occurs as a cascade of events, with proteins from one phase regulating the activity of the genes of the next phase.

The pathways found in pattern formation and organismal development can shed light on the evolutionary history of very diverse organisms. As you work through the details of segmentation and homeobox genes in animals, and organ identity genes in plants, keep in mind what these processes are telling us about the way evolution works and how genetic “solutions” are conserved.

Learning Objectives

• Understand the hierarchical arrangement of regulatory cascades.
• Know what homeobox genes are and why they are important in organismal development.
• Think about the similarities in the genes controlling development in diverse organisms.
• Understand the process of organ identity genes creating various plant structures.

Early Stages of Development

As you have seen from previous lessons, multicellular organisms begin as a single-celled zygote. The next few hours in the development of an individual involve a series of dynamic and critical processes such as cleavage, and major morphological changes in the embryo. Most cells are determined by the end of these early stages, and the basic layout for the organism has been made. Keep in mind through this lesson the similarities of developmental processes and reflect back on the regulatory cascades from the previous module.

Learning Objectives

• Know which stage of development represents the first incidence of cell signaling processes and why.

• What are some of the key differences in the development of animals versus plants?

• How are the developmental processes in animals and plants similar?

• Understand what processes cause morphological changes in the developing embryo.

• Develop an understanding of how a zygote becomes a late-stage embryo in both plants and animals.

Pattern Formation

We have explored the early stages of development in multicellular organisms. But we know that development does not end there. Pattern formation and morphogenesis are processes that “sculpt” and refine the late-stage embryo into a fully-functional organism. As we will see in this module, developmental changes don’t stop here either. In fact, phenotypic changes occur throughout the lifetime of most organisms. Sometimes these changes are drastic, such as the regeneration of a limb, and some are more subtle but equally important, like seasonal color change.

Learning Objectives

• Understand how cell induction and morphogens give cells in the developing embryo positional information that leads to the formation of particular structures.
• What role does apoptosis play in the formation of organisms?
• Know the difference between dedifferentiated and undifferentiated cells and how each of them plays a role in pattern formation.
• What is phenotypic plasticity, and how does it relate to changes in adult organisms?