The Questions of Developmental Biology

The first evidence suggesting the importance of sperm in reproduction
came from a series of experiments performed by Lazzaro Spallanzani in the
late 1700s. Spallanzani demonstrated that filtered toad semen devoid of sperm
would not fertilize eggs. He concluded, however, that the viscous fluid retained
by the filter paper, and not the sperm, was the agent of fertilization.
He, like many others, felt that the spermatic "animals" were parasites.
The combination of better microscopic lenses and the cell theory led to
a new appreciation of spermatic function. In 1824, J. L. Prevost and J. B.
Dumas claimed that sperm were not parasites, but rather the active agents of
fertilization. They noted the universal existence of sperm in sexually mature
males and their absence in immature and aged individuals. These observations,
coupled with the known absence of spermatozoa in the sterile mule, convinced
them that "there exists an intimate relation between their presence in the organs
and the fecundating capacity of the animal." They proposed that the sperm
entered the egg and contributed materially to the next generation.
These claims were largely disregarded until the 1840s, when A. von
Kolliker described the formation of sperm from cells within the adult testes. He
ridiculed the idea that the semen could be normal and yet support such an
enormous number of parasites. Even so, von Kolliker denied that there was any physical contact
between sperm and egg. He believed that the sperm excited the egg to develop, much as a magnet
communicates its presence to iron. It was only in 1876 that Oscar Hertwig and Herman Fol
independently demonstrated sperm entry into the egg and the union of the two cells' nuclei.
Hertwig had sought an organism suitable for detailed microscopic observations, and he found that
the Mediterranean sea urchin, Toxopneustes lividus, was perfect. Not only was it common
throughout the region and sexually mature throughout most of the year, but its eggs were
available in large numbers and were transparent even at high magnifications. After mixing sperm
and egg suspensions together, Hertwig repeatedly observed a sperm entering an egg and saw the
two nuclei unite. He also noted that only one sperm was seen to enter each egg, and that all the
nuclei of the embryo were derived from the fused nucleus created at fertilization. Fol made
similar observations and detailed the mechanism of sperm entry. Fertilization was at last
recognized as the union of sperm and egg, and the union of sea urchin gametes remains one of the
best-studied examples of fertilization.
Each sperm consists of a haploid nucleus, a propulsion system to move the nucleus, and a
sac of enzymes that enable the nucleus to enter the egg. Most of the sperm's cytoplasm is
eliminated during maturation, leaving only certain organelles that are modified for spermatic
function (Figure 7.2). During the course of sperm maturation, the haploid nucleus becomes very
streamlined, and its DNA becomes tightly compressed. In front of this compressed haploid
nucleus lies the acrosomal vesicle, or acrosome, which is derived from the Golgi apparatus and
contains enzymes that digest proteins and complex sugars; thus, it can be considered a modified
secretory vesicle. These stored enzymes are used to lyse the outer coverings of the egg. In many
species, such as sea urchins, a region of globular actin molecules lies between the nucleus and the
acrosomal vesicle. These proteins are used to extend a fingerlike acrosomal process from the
sperm during the early stages of fertilization. In sea urchins and several other species, recognition
between sperm and egg involves molecules on the acrosomal process. Together, the acrosome
and nucleus constitute the head of the sperm.