Understanding How Eggs Work 
Allan C. Spradling, Ph.D. — Investigator 
Dr. Spradling is also a staff member of the Department of Embryology at the Carnegie Institution of 
Washington in Baltimore and Adjunct Professor of Biology and Microbiology at the fohns Hopkins 
University. He earned his B.A. degree in physics from the University of Chicago and his Ph.D. in cell biology 
from the Massachusetts Institute of Technology. His postdoctoral study was done at Indiana University 
with Anthony Mahowald. Dr. Spradling is a member of the National Academy of Sciences and has received 
many honors for his work. 
ALTHOUGH the union of egg and sperm initi- 
ates the complex processes that ultimately 
result in a new animal, the roles played by the 
two cells in embryonic development are by no 
means equivalent. Even nonspecific treatments, 
such as pricking with a needle, will stimulate 
many types of eggs to develop in the absence of 
sperm. Without an egg, however, neither a sperm 
cell nor any other can even begin the processes 
that lead to an embryo and ultimately an adult. In 
fact, eggs have undergone extensive and intricate 
preparations that allow them to support and di- 
rect embryogenesis. If the mystery of a new life 
could be condensed into a single cell, that cell 
would surely be an egg. 
Eggs Pose Exceptional Biological Problems 
Egg cells are very different in structure and in 
biological capacity from other cells. Most 
chicken cells weigh less than one millionth of an 
ounce, whereas a single chicken egg makes a nice 
breakfast. Eggs not only differ in size but in many 
other important ways. For example, the egg's 
genes function differently from those of other 
cells; so extensive are these differences that the 
chromosomes appear in the microscope quite 
unlike those of other tissues. In eggs the products 
of many genes accumulate and are stored in spe- 
cial forms so that they can be utilized at precisely 
the right times during embryonic development. 
Indeed, storage of material is one of the reasons 
many eggs are enormous. 
A great deal remains to be learned about egg 
structure. How are gene products specially 
stored, and what are their specific functions later 
in development? Is each product located in a par- 
ticular place? What are the mechanisms that al- 
low these materials to be produced and stock- 
piled in the appropriate manner? The sheer 
complexity of an egg, with its tens of thousands 
of specific, highly organized components, has 
until recently blocked all attempts to describe 
egg structure in molecular detail, much less to 
understand the logic that allows this structure to 
develop into an even more intricate organism. 
However, over the past 10 years or so, powerful 
new methods in molecular biology, such as gene 
cloning, gene transfer, and fruit fly genetics, have 
begun to unravel the fascinating secrets stored 
within an eggshell. 
Using Genetics to Study Eggs 
It is now possible to study structures as com- 
plex as eggs because techniques of both genetics 
and molecular biology allow us to take eggs apart 
gene by gene. Each component of an egg is speci- 
fied by a gene carried on the chromosomes. Much 
research on eggs utilizes the fruit fly Drosophila, 
since far more has been revealed about its genes 
during 70 years of genetic studies than about 
those of any other complex organism, including 
humans. A genetic mutation in the fly inactivates 
one gene among the 20,000 to 50,000, and if that 
gene specifies an egg component, then the mu- 
tant fly will produce defective eggs. 
Frequently, however, the eggs can still par- 
tially function. For example, in a particular mu- 
tant, development might begin normally, but 
then stop because of a lack of stored food. By 
studying what goes wrong with the mutant eggs, 
biologists can deduce the normal function of a 
particular gene's product. In this case, one might 
conclude that the gene was involved in produc- 
ing the stored food. To understand eggs, it will be 
necessary, at a minimum, to create mutations and 
carry out such studies on all the fruit fly genes, 
one by one. 
If researchers were limited to looking at egg 
defects in a microscope, progress would still be 
slow. Many of the defective eggs would not con- 
tain any detectable problems; they just wouldn't 
work. It is at this point that the ability to clone the 
gene molecularly becomes essential to further ad- 
vancement. Cloning is simply a way of purifying 
an individual gene so that its DNA structure can 
be determined in the laboratory. 
There are two major benefits. First, the struc- 
ture of the gene's product, a specific protein that 
actually becomes part of the egg, can now be de- 
termined by the process of DNA sequencing. Sec- 
ond, specific antibodies can be prepared that 
only bind to the product of that particular gene. 
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