The Questions of Developmental Biology

The sperm released during ejaculation are able to move, yet they do not yet have the capacity to
bind to and fertilize an egg. These final stages of sperm maturation (called capacitation) do not
occur until the sperm has been inside the female reproductive tract for a certain period of time.
The egg
All the material necessary for the
beginning of growth and development must
be stored in the mature egg (the ovum).
Whereas the sperm has eliminated most of
its cytoplasm, the developing egg (called
the oocyte before it reaches the stage of
meiosis at which it is fertilized) not only
conserves its material, but is actively involved
in accumulating more. The meiotic divisions
that form the oocyte conserve its cytoplasm
(rather than giving half of it away), and the oocyte
either synthesizes or absorbs proteins, such as yolk,
that act as food reservoirs for the developing embryo.
Thus, birds' eggs are enormous single cells, swollen with their accumulated yolk. Even eggs with
relatively sparse yolk are comparatively large. The volume of a sea urchin egg is about 200
picoliters (2 × 10 4 mm 3 , more than 10,000 times the volume of the sperm) (Figure 7.4). So,
while sperm and egg have equal haploid nuclear components, the egg also has a remarkable
cytoplasmic storehouse that it has accumulated during its maturation. This cytoplasmic trove
includes the following:*
Proteins. It will be a long while before the embryo is able to feed itself or obtain food from its
mother. The early embryonic cells need a supply of energy and amino acids. In many species, this
is accomplished by accumulating yolk proteins in the egg. Many of the yolk proteins are made in
other organs (liver, fat body) and travel through the maternal blood to the egg.
Ribosomes and tRNA. The early embryo needs to make many of its own proteins, and in some
species, there is a burst of protein synthesis soon after fertilization. Protein synthesis is
accomplished by ribosomes and tRNA, which exist in the egg. The developing egg has special
mechanisms to synthesize ribosomes, and certain amphibian oocytes produce as many as 10 12
ribosomes during their meiotic prophase.
Messenger RNA. In most organisms, the instructions for proteins made during early
development are already packaged in the oocyte. It is estimated that the eggs of sea urchins
contain 25,000 to 50,000 different types of mRNA. This mRNA, however, remains dormant until
after fertilization (see Chapter 5).
Morphogenetic factors. Molecules that direct the differentiation of cells into certain cell types
are present in the egg. They appear to be localized in different regions of the egg and become
segregated into different cells during cleavage (see Chapter 8).
Protective chemicals. The embryo cannot run away from predators or move to a safer
environment, so it must come equipped to deal with threats. Many eggs contain ultraviolet filters
and DNA repair enzymes that protect them from sunlight; some eggs contain molecules that
potential predators find distasteful; and the yolk of bird eggs even contains antibodies.