Biology: Developmental Biology

Introduction

Developmental biology is the study of how living organisms develop from their earliest stages and grow to maturity. Rather than studying the adult organism, developmental biologists study the juvenile stages, starting with the embryo. All living things, including plants, have some sort of embryonic stage. Comparing them has led to many important advances in developmental biology.

Some developmental biologists study a whole organism for only a short period of its growth. Others are interested in development on a smaller scale. Some study the cell growth, or how specialized cell types differentiate from stem cells. Others are interested in how tissues grow into complex and specific shapes, eventually forming organs. At its core, developmental biology seeks to understand how an organism grows from a simple, undifferentiated form to a complex, specialized living thing.

Historical Background and Scientific Foundations

Biology, the study of life, is divided into three major disciplines—botany, microbiology, and zoology—with many smaller specialties within each. Much work in these fields seeks to understand the ways an organism interacts with its environment, finding food, seeking shelter, and communicating with other organisms. Biologists also investigate the inner workings of cells to determine how they metabolize energy, reproduce, or become diseased.

Developmental biology, however, looks in a different direction, seeking to discover how an organism—plant, animal, or bacteria—came to be. The stages that an organism undergoes from single cell to adult are many, complicated, and in constant danger of failure. By studying these stages, developmental biologists gain insight into the origin of species, the relationships between them, and many of the diseases of growth or deterioration that can affect both animals and humans.

The origins of developmental biology lie in the search to understand the phenomenon of evolution. Before English naturalist Charles Darwin (1809–1882) put forth his theory of natural selection, the ideas surrounding evolution were in their infancy. Some scientists wondered whether organisms changed from one kind to another over time and generations; other more orthodox minds rejected the idea completely. Early philosophers from many cultures, including the ancient Greeks, thought about the development of the world, and eventually of living things. However, by the early nineteenth century most Western scientists were convinced that the shape of all life was fixed and unchanging. This view was reinforced by the teachings of fundamentalist Christianity, which discouraged scientific inquiry that was not in line with its religious principles.

IN CONTEXT: PREFORMATIONISM

Until the mid- 1800s, many naturalists supported a theory of development called epigenesis, which held that the eggs of organisms were undifferentiated, but had a developmental potential which could be directed by certain external forces. Other naturalists supported a theory of development called preformationism, which held that the entire complex morphology of a mature organism is present in miniature form in the egg, a developmental form called the homunculus.

These early theories of development relied on little experimental evidence. Thus, biologists often criticized the original theory of epigenesis because it seemed to propose that mystical forces somehow directed development, a view clearly outside the realm of science. Biologists also rejected preformationism, since studies of cytology and embryology clearly showed that development is much more than the simple growth of a preformed organism.

The modern view is that developmental processes have certain general features of both concepts. Thus, we know that the simple cells of an egg are preformed in the sense that they contain a preformed instruction set for development which is encoded in their genes. Similarly, we know that the egg is relatively formless, but has the potential to develop into a complex organism as it grows. Thus modern developmental biology views development as the expression of a preformed genetic program which controls the epigenetic development of an undifferentiated egg into a morphologically complex adult.

At one point the study of embryology was used to argue against evolution. Then known as “transformationism,” this early evolutionary idea held that the embryos of more advanced animals at first resembled and then surpassed the form of more primitive adult organisms. This misconception, namely that today's more advanced organisms evolved directly from less-advanced organisms still living today, has caused much confusion and brought ridicule upon the entire theory of evolution. Overcoming this erroneous idea was one of the most important advances in evolutionary thought.

In the course of his research, Karl Ernst von Baer, (1792–1876) a German biologist who studied embryos extensively, noted that the embryos of fish and humans each passed through a stage where they appeared to have similar features, but then diverged to assume their adult forms. Von Baer used this observation to argue against the incorrect assumptions of transformationism that were prevalent at his time. However, its basic ideas were difficult to eradicate, and even as evolutionary theories advanced and Darwin's ideas became more influential, embryology was still generally thought to argue against evolution.

Darwin himself, however, asserted that embryonic forms and development of organisms strongly supported his theories. Agreeing with von Baer, Darwin reiterated that advanced organisms do not pass through the shapes of less-developed ones, but that both share similar stages on the way to their respective adult forms. Though the idea was met with great opposition, it was eventually accepted by biologists that evolution was not linear, i.e., that humans evolved from modern apes, who themselves had evolved from fish. The study of embryos allowed Darwin to see that the embryos of fish, humans, and other vertebrates appeared similar because they all descended from a common ancestor, albeit a very distant one. In fact, evolution occurs because of changes in the way embryos develop, not because of changes in the adult organism.

Developmental biology had other faulty ideas to overcome. Scientists and physicians had long wondered how a tiny sperm and egg could become a fully formed organism. One theory, known as preformationism, stated that the organism was present, fully formed and in miniature, within the sperm cell. During the Renaissance, this miniature person was known as a “homunculus,” Latin for “little man”; several physicians even reported seeing it under magnification.

Fertilization and Development

It is often helpful to begin the study of developmental biology at the cellular level, specifically at the fertilization of an egg, because many important processes and reactions must occur for fertilization to be successful. First, sperm and egg must recognize one another as compatible members of the same species. Then, they fuse to keep more than one sperm from fertilizing the egg (which would cause chromosomal problems). Each male and female reproductive cell has only half the number of chromosomes necessary for life. By combining the genetic material of both, the offspring will have the correct number of chromosomes and its genetic diversity will be assured. The egg cell then awakens its metabolism to power the reactions that make up the earliest stages of the new zygote's development.

The next important development is known as cell cleavage. In this stage the cell makes another cell by duplicating its genetic material and then dividing in two. First, the nucleus must duplicate and divide its genetic material. Then the cell wall will pinch inward and seal the two new cells, each of which can soon repeat the process. In mammals, some new cells will go on to form the organism, while others will form the placenta. After a certain number of cells is reached, the organism, now known as a blastula, undergoes gastrulation, a process that forms the layered structure of the new organism and establishes the cells in the correct locations to form organs and other structures. Gastrulation is one of the most dramatic actions studied by developmental biologists. The cells of the blastula move from their original positions established into their new locations based on their future functions. Errors at this stage can cause serious flaws in the organism, some so dangerous that it might not survive. Different kinds of animals exhibit many different kinds of gastrulation, giving developmental biologists much to study.

An important milestone of vertebrate development is the formation of the neural tube that will become the brain and spinal cord. As the cells on the back of the animal grow, they begin to curl into a tube, eventually closing completely. If this process is unsuccessful, the organism will develop the birth defect known as spina

IN CONTEXT: RECAPITULATION THEORY

People have long been interested in the connection between the development of an organism (its ontogeny) and the evolutionary ancestry of the species (its phylogeny). Anaximander (c.610–546 BC), a philosopher of ancient Greece, noted that human embryos develop inside fluid-filled wombs and proposed that human beings evolved from fish as creatures of the water.

This early idea was a progenitor to recapitulation theory, proposed in the 1800s by Ernst Haeckel (1834–1919), a German scientist. Recapitulation theory is summarized by the idea that the embryological development of an individual is a quick replay of its evolutionary history. As applied to humans, recapitulation theory was accepted by many evolutionary biologists in the 1800s. It also influenced the intellectual development of other disciplines outside of biology, including philosophy, politics, and psychology.

By the early 1900s, developmental biologists had disproved recapitulation theory and had shown that the relationship between ontogeny and phylogeny is more complex than proposed by Haeckel.

bifida. This process is closely linked with the formation of the envelope of cells that will become the skin. At the same time, the middle layers of the embryo differentiate to eventually form the face, bone, muscles, limbs, circulatory system, urinary system, and reproductive tract. The innermost layer forms much of the digestive and respiratory tracts.

Developmental biologists also study how different types of cells make up organs and tissues. First, cells undergo a process called determination, in which their future type is selected. This happens either during cleavage or by interaction between cell types. The process of changing into these many different cell types is called differentiation. While early physicians relied on the idea of preformationism to explain embryonic growth, scientists now understand that many complex processes work in a complicated sequence to build tissues and organs. Some developmental biologists study genes to see how this happens, trying to determine, for instance, how the liver cells “know” how to become the liver and the teeth cells “know” how to become the teeth. Other researchers study problems in the transmission, expression, or control of genes and how this affects the developing organism.

A major factor in the development of many organisms is the interactions between different types of cells. Sometimes these occur on the cells' surfaces; other times they occur at a greater distance. These ultimately lead to cells gathering into tissues, and tissues assembling into organs, with many different tissue types joined in an intricate structure. Understanding these complex interactions is an exciting field of study, one that may give doctors the ability to regenerate damaged organs, or even grow new ones.

Many organisms undergo metamorphosis from one stage of life to another. Simple organisms, such as insects, amphibians, and sea creatures such as corals and urchins hatch from eggs, spend time as larvae or pupae, and then assume adult form. Often, the juvenile forms of these animals appear very different from the adult. This is usually because the larval stage has evolved to exploit a different resource or serve a different purpose than the adult. For example, butterfly larvae, known as caterpillars, are best suited for eating leaves and storing great quantities of energy. The adult is formed using this energy, but it eats very different foods, such as nectar, and its main purpose is to reproduce. Hormones, substances produced to regulate or cause development, are usually responsible for the changes that occur during metamorphoses. Hormones can cause changes in organisms at all stages of life, from embryo to adult, and are therefore relevant to the study of developmental biology in many ways.

The growth of an organism can be as important as its development in other ways. Even while an animal is growing, it still needs to perform all the functions that keep it alive. For this reason, growth is strictly limited by the organism's needs—too much growth can sicken or kill the organism.

Growth also dictates the development of some structures associated with more advanced organisms. Very small organisms such as bacteria, corals, and some kinds of worms obtain nutrients and oxygen by living in fluid that constantly provides them. Larger animals have had to develop extensive circulatory systems to bring these materials to cells deep within the body. As an animal's size increases, its structure must become exponentially heavier and stronger to bear its weight, requiring more energy to produce and maintain.

Modern Cultural Connections

The principal topics of developmental biology are closely related to the political and scientific controversies surrounding the use of stem cells, which have the potential to become many or even all types of adult cells, holding out promise that scientists may some day be able to regenerate damaged tissues and organs. Since the embryo is constantly undergoing these kinds of changes, it contains many more stem cells than adults do, especially those that are able to change into more types of cells. Many scientists and physicians view stem cell research as a promising field with the potential to offer cures for diseases that are currently untreatable. The controversy lies in the use of human embryos, which are destroyed in the process. This is perhaps the most compelling problem in developmental biology today: to find a way to control and use stem cells for medical treatments.

See Also Biology: Botany; Biology: Classification Systems; Biology: Comparative Morphology: Studies of Structure and Function; Biology: Concepts of Heredity and Change Prior to the Rise of Evolutionary Theory; Biology: Evolutionary Theory; Biology: Zoology.

bibliography

Books

Gilbert, Scott F. Developmental Biology. 2nd ed. Sunderland, MA: Sinauer Associates, 1988.

Web Sites

Anders, Ralf. Developmental Biology. “Developmental Biology of Plants and Animals.” May 14, 2007. http://developmentalbiology.de/en (accessed January 29, 2008).

Sinauer Associates. “Developmental Biology. Vol. 6.” http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio (accessed January 29, 2008).

Kenneth T. LaPensee