Chapter 21

The Genetic Basis of Development

“Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think,

of my reviewers has alluded to this.”

-Charles Darwin, 1860

“Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think,

of my reviewers has alluded to this.”

-Charles Darwin, 1860

Zygote and Cell Division

When the zygote divides, it undergoes 3 major changes:

1. Cell division

2. Cell differentiation

3. Morphogenesis

1. Cell Division

Cell division gives rise to numerous cells.

2. Cell Differentiation

Cell differentiation is the process by which cells become specialized in form and function. These cells undergo changes that organize them into tissues and organs.

3. Morphogenesis

As the dividing cells begin to take form, they are undergoing morphogenesis which means the “creation of form.”

Morphogenetic events lay out the development very early on in development as cell division, cell differentiation and morphogenesis overlap.

3. Morphogenesis

These morphogenetic events “tell” the organism where the head and tail are, which is the front and back, and what is left and right.

As time progresses, later morphogenetic events will give instructions as to where certain appendages will be located.

QuickTime™ and a decompressor

are needed to see this picture.

Morphogenetic Events Morphogenetic events, as well as cell

division and differentiation, take place in all multicellular organisms.

Morphogenesis differs in 2 major ways in plants and animals:

1. In animals, movements of cells and tissues are required for the transformation of the early embryo into the characteristic 3D form of the organism.

2. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods, they occur throughout the life of the plant.

The Experiments of F.C. Steward

In the 1950’s, Steward was working with carrot plants.

He showed that cells taken from the root of the plant would grow into an adult carrot when cultured in growth medium. These plants were clones of the original.

It demonstrated that differentiation doesn’t involve irreversible changes in DNA; that cells can dedifferentiate; some cells are totipotent while other cells are pluripotent.

Stem Cells

Stem cells are cells that have the ability to divide and differentiate along many different pathways.

There are two main types:

Totipotent

Pluripotent

Totipotent Cells

Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism.

Totipotent cells are formed as a result of sexual reproduction.

Pluripotent In cell biology, the definition of pluripotency has

come to refer to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm, mesoderm, or ectoderm.

Pluripotent stem cells can give rise to any fetal or adult cell type. However, alone they cannot develop into a fetal or adult animal because they lack the potential to contribute to extraembryonic tissue, such as the placenta.

Animals

The ongoing development in adult animals is normally restricted to the generation of cells that need to be continually replenished: blood cells, skin cells and the cells lining the intestine for example.

Reflection

1. Explain the difference between pluripotent and totipotent stem cells.

2. What is morphogenesis?

3. What is a zygote?

4. How do plants and animals differ in their morphogenesis?

Multicellular Organisms

The cells of multicellular organisms come almost entirely from differences in gene expression. Regulatory mechanisms turn certain genes on and off during development.

These regulatory mechanisms are what makes cells different because nearly all cells have the same genetic complement.

Early division of the cells divides up the intracellular contents. These contents are called transcription factors--the combination of which results in the differential gene expression unique to each type of cell.

Stem Cells

The capacity of cells to divide and differentiate along different pathways is necessary in embryonic development. This feature is also what makes stem cells suitable for therapeutic uses.

The use of stem cells, especially embryonic stem cells, has many obvious medical applications.

Stem Cells

There are obvious ethical dilemmas that arise from the research.

There are moral issues on both sides:

One is that it is immoral to tamper with human embryos for medical purposes.

The other is that it is immoral not to because the benefits outweigh the cost of doing nothing.

Ethics of Stem Cell Use

Embryo Cord Blood Adult

Ease of Extraction

Can be obtained from excess

embryos generated by IVF

programs

Easily obtained and stored.

Though limited quantities are

available

Difficult to obtain as there

are very few and are buried deep

in tissues

Ethics of Extraction

Can only be obtained by

destruction of an embryo

Umbilical cord is removed at birth

and discarded whether or not stem cells are

harvested

Adult patient can give

permission for cells to be extracted

Growth Potential

Almost unlimited Reduced Reduced

Tumor Risk High Low Low

Cell Differentiation

There are 2 major things telling a cell when and how to differentiate:

1. The “stuff” found within the egg at the time of conception.

2. The environment in which the embryo develops.

1. The “Stuff” in the Egg

The egg cell’s cytoplasm contains RNA and protein molecules encoded by the mother’s DNA.

mRNA, proteins, organelles, and other substances are scattered unevenly throughout the cytoplasm of an unfertilized egg.

These maternal substances influence the course of early development called cytoplasmic determinants.

Cytoplasmic Determinants

Following fertilization, mitotic divisions distribute the zygote’s cytoplasm into separate cells.

The nuclei of these cells are subjected to many different cytoplasmic determinants.

What has been received will determine the developmental fate of each of the cells.

Cytoplasmic Determinants

Cytoplasmic determinants help to create an animal’s 3D arrangement before morphogenesis can shape the animal.

2. The Environment The environment in which the embryo

develops plays an important factor in outcome of the developing organism.

The surface contact of cell-to-cell interaction helps to signal development.

By the process of induction, an embryo’s genes signal the expression of proteins that cause changes in nearby target cells.

These signals send a cell down a specific developmental pathway--inducing further differentiation of the many specialized cells within the new organism.

Reflection

1. What is the difference between embryonic and adult stem cells?

2. The differences in the cells of multicellular organisms is almost entirely due to differences in gene expression. Explain.

3. What are cytoplasmic determinants? Where do they come from?

4. What are the two major things that tell a cell how to differentiate?

Pattern Formation

Pattern formation is the development of spatial organization in which the tissues and organs of an organism are all in their characteristic places.

In plants, pattern formation occurs through the life of the plant.

In animals, it is restricted to the embryonic or juvenile stage.

Pattern Formation Pattern formation in animals begins in the

embryo when the major axes are determined.

Before tissues and organs within an animal can be formed, the 3D arrangement must be established. Recall that this occurs as a result of cytoplasmic determinants.

This process has been extensively studied in many animals such as the fruit fly, sea urchin, frog, nematode, and chicken.

Pattern Formation, An Example Here is an

example of pattern formation and cell signaling as seen in the fruit fly.

Movie

Apoptosis Apoptosis is the programmed cell death that

occurs through the normal course of development.

It is usually triggered by signals that activate a cascade of signal proteins in cells that are to die.

During the process, the cell shrinks, the nucleus breaks down and the nearby cells quickly engulf and break down the contents of the cell.

Apoptosis

Apoptosis is essential to the development of all cells. The process helps in the growth and development of the major structures and systems of an organism.

It controls cell division helping to slow or stop division in certain cells.

Questions:

1. What is pattern formation? How is it controlled?

2. What are homeotic genes?

3. What is a homeobox?

4. What is a homeodomain?

5. What are Hox genes?

Homeotic Genes

Looking across species, there are many similarities in the genes controlling development.

Homeotic Genes A homeobox is a DNA

sequence found within genes that are involved in the regulation of patterns of development (morphogenesis).

Genes that have a homeobox are called homeobox genes and form the homeobox gene family.

Homeotic GenesF The most studied

and the most conserved group of homeodomain proteins are the Hox genes, which control segmental patterning during development.

F Not all homeodomain proteins are Hox proteins.

Homeotic Genes

Many distantly related eukaryotes such as plants and yeasts also have these Hox genes (regulatory sequences).

Such similarities indicate that the homeobox sequence is very useful in development and arose very early on in evolution and has been conserved for hundreds of millions of years.

Homeotic Genes

Not all Hox genes are homeotic genes--not all of them control body parts. However, most are involved in development.

Homeotic Genes Research has revealed that the homeobox-

encoded region is part of the protein that functions as a transcription regulator.

The shape of the encoded region allows it to bind to any DNA segment, but by itself, it cannot select a specific sequence. The variable regions within the whole protein allow it to interact with other transcription factors and enhancers within the DNA.

In this way, the homeobox genes work to switch certain developmental genes on and off.

Homeotic Genes There are many other regions of DNA

that are highly conserved among species.

The common question is how can the same genes code for different body forms?

It is likely that the small changes in the regulatory sequences lead to major changes in body form--the basis of the next unit.