Main Page Dynamic Development
The Foundations of Developmental Biology
From Sperm and Egg to Embryo
Genetic Regulation of Development
Organizing the Multicellular Embryo
Generating Cell Diversity
Dynamic Development at a Glance
The Developmental Biology Journal Club
Developmental Biology Tutorial
The Ark of Life: The Germ Line
What's so special about germ cells?
Development of multicellular organisms is usually initiated by
fertilization of an egg by a sperm. The sperm and the egg have distinct roles to play in
launching development of a new generation. The sperm must seek out the egg, fuse with it
and deliver its precious cargo consisting of a haploid nucleus. The egg, on the other
hand, must make itself apparent to the sperm and be capable of responding appropriately to
its contact. That response includes allowing a sperm of its own species to fuse with it
while ignoring attempts by foreign sperm to fuse. Once fusion has occurred, the egg
marshals its defenses to prevent the fusion of additional sperm. It also initiates a
sequence of events that enable the sperm nucleus to fuse with the egg nucleus to form the
zygote nucleus and begin the complicated process that produces an embryo that develops
into an organism consisting of many different cell types that are organized into tissues
and organs to enable the organism to function like its parents. Ultimately, the organism
becomes an adult that may produce gametes that enable the process to be repeated.
Gametes are derived from the primordial germ cells, which enter the gonads during
development. The primordial germ cells may arise at some distance
from the presumptive gonads, to which they migrate and become established. The
formation of the germ line is dependent upon the presence of the germ plasm, which is a
cytoplasmic component that causes these cells to become distinct from the somatic cells.
When the primordial germ cells become established in the gonad, they become stem cells
that divide by mitosis to produce the supply of gametes that the organism requires for
reproduction. When they enter the gonads, the germ cells may associate with specific
somatic cells that support, nurture and protect them. In the female, these somatic cells
are called follicle cells. Various names are applied to the comparable somatic
cells in the male gonad. In mammals, they are called Sertoli cells.
During the proliferative phase, the germ cells are called gonia (spermatogonia
in the testis and oogonia in the ovary) and act as a stem cell population
that will divide by mitosis to produce the lifetime supply of gametes that the organism
requires for reproduction. The gonial cell divisions may be incomplete, so that the
daughter cells remain in communication with one another via intercellular bridges.
Successive incomplete divisions produce very large clones of interconnected cells. This
intercellular communication may serve to synchronize the development of the conjoined
When the organism reaches maturity, germ cells acquire the ability to differentiate
into functional gametes and undergo meiosis to reduce the chromosome number from 2n to 1n.
Gametes are produced through the process of gametogenesis.
This process differs in the two sexes in many ways, but sperm and eggs have two
fundamental similarities: They have a common origin during development, and they must
undergo meiosis to reduce the diploid genome (2n) to the haploid (1n) condition.
Before we discuss gametogenesis, let's review meiosis using some or all of the
following resources: (1) a tutorial
developed at The University of California, Santa Barbara and (2) a cool animation from Yale.
Beware: the 'Net can eat up your time!
Now you should be up to speed on meiosis.
A major difference between spermatogenesis and oogenesis is that spermatogenesis occurs
after meiosis, whereas differentiation of the female gamete may occur early in meiosis.
(See Browder et al., 1991, Figs. 2.1 and 2.2; Gilbert, 1997, Figs. 22.16 and
4.5; Kalthoff, 1996, Fig. 3.5; Shostak, 1991, Fig. 6.27; Wolpert et al., 1998, Fig.
Gametogenesis in the two sexes is tailored to the roles of the gametes in reproduction.
The eggs and sperm provide both the blueprint and the raw material
from which the embryo is formed. Both classes of gametes make an equal contribution to the
nucleus of the zygote, each providing a haploid genome. However, the male gamete makes a
minimal contribution to the cytoplasm; the female gamete provides the zygote with
virtually all of the cytoplasm, which contains the constituents from which the embryo is
formed. This role of the female gametes makes their differentiation a complex process,
during which the foundations of embryonic development are formed.
In the male, meiosis precedes sex cell differentiation. A single
spermatogonium enters the first meiotic division as a primary spermatocyte. This
division produces two secondary spermatocytes, each of which divides to form two
haploid spermatids. Each spermatid then differentiates (by a process called spermiogenesis)
into a spermatozoon by the elaboration of structural and functional specializations
that enable the sperm to fertilize the egg. Consequently, four haploid sperm result from
each diploid spermatogonium.
The utilization of all four haploid cells in the male is
significant because the testis must produce millions of sperm simultaneously. The loss of
a portion of these cells during meiosis would make this task monumental. As we discussed
previously, the spermatogonial mitotic divisions may be incomplete, leaving daughter cells
in continuity with one another via cytoplasmic bridges. Meiotic divisions may also be
incomplete, thus enlarging the clones of interconnected cells, which are then composed of
numerous haploid spermatids. The intercellular bridges that connect members of a clone are
lost in the final stages of spermiogenesis when excess cytoplasm is sloughed from the
In contrast to the situation in the male, germ cell
differentiation in the female may occur early in meiosis. It is also important to keep in
mind the difference in the number of sex cells that result from meiosis in the male and
female. Each of the meiotic divisions in the female is uneven, producing only one
full-sized cell. During the first meiotic division, the primary oocyte divides to
produce one small polar body and one secondary oocyte. The latter enters the second
meiotic division to produce the second polar body and the haploid ovum, which is
the only functional sex cell to result from meiotic reduction of an oogonium. In most
species of animals, differentiation of the oocyte occurs during a protracted prophase of
the first meiotic division, which can often last for a very long time. Resumption of
meiosis, which occurs after the oocyte is fully grown, is called oocyte maturation.
Manipulating the Germ Line
Each generation of sexually-reproducing organisms is dependent
upon the germ line of its predecessors for its existence. Likewise, the production of
succeeding generations is dependent upon the current generation's germ line. Thus, the
germ line is the link that ensures the very survival of these species. The germ line is
also the vehicle of evolution. Transmission of mutations and recombination of existing
genes in germ line cells generates the diversity that, in turn, generates species
diversity. Scientists have developed tools that enable them to manipulate the germ line in
a variety of ways, such as freezing germ line cells to store them for later utilization in
fertilization and altering their genetic composition.
The storage of frozen sperm has been used by veterinarians and
medical researchers for many years to propagate both domestic animals and human beings.
However, the use of frozen sperm is inefficient in a number of respects. Sperm lose their
fertilizing capacity if stored frozen for long periods of time, and thawing sperm also
reduces their ability to fertilize eggs. It has now been demonstrated that spermatogonia
of mice can be frozen in liquid nitrogen at -196ĚC, perhaps indefinitely (Avarbock et
al., 1996). Furthermore, the thawed spermatogonia have been transplanted into testes,
where they could differentiate into morphologically normal, mature sperm. This procedure
has the potential for perpetuating the germ line indefinitely. The techniques employed
were routine and are likely applicable to a wide variety of species.
Freezing and reimplanting the germ line has important practical
applications and is also valuable for basic developmental biology research. For example,
the germ lines of endangered species or valuable domestic animals of any age could be
maintained indefinitely by freezing them and implanting them into surrogate testes so that
they could differentiate into sperm. Likewise, men who are to undergo chemotherapy or
radiation treatment for cancer treatment could bank their spermatogonia before treatment
so that they would be reimplanted to enable the men to father children at a later date.
Genetic manipulation of germ line cells is a potentially powerful
means for controlling the genotypes of embryos. Genetic "knock-outs", in which
the expression of a specific gene is eliminated, is a technique for determining the roles
of genes during development. For many years, gene targeting was only practical for mice,
and it required the use of embryonic stem cells. The use of spermatogonia instead of
embryonic stem cells would simplify this procedure significantly and may be applicable to
other species besides mice.
A technique for genetically alterating mature sperm has been
developed for the South African Clawed Frog, Xenopus laevis. This species is one of
the most widely-used organisms for the study of development. The ability to produce
embryos with a "genome to order" extends the utility of this species
substantially (Kroll and Amaya, 1966).
Human eggs have also been frozen successfully and later
fertilized. When eggs are collected for in vitro fertilization, several eggs are
normally collected and fertilized. Those embryos that are not implanted can be frozen. At
a later date, the embryos can be thawed and implanted. However, the disposition of
unwanted frozen embryos is a moral dilemma. This dilemma can be bypassed if the eggs are
frozen, fertilized and implanted. This procedure has been performed successfully by Dr.
Carlo Flamingi of Bologna, Italy. Thus, the possibility now exists that in vitro fertilization
could utilize frozen germ line cells of both sexes
- Why do certain cells become germ cells (and why don't all cells become germ cells)?
- You had better know what a stem cell is.
- ...and you had better know meiosis inside and out
- How many differences between spermatogenesis and oogenesis can you think of?
- What are the practical advantages of freezing mammalian spermatogonia?
Hawkins, N.C., Thorpe, J. and Schupbach ,T. 1996. Encore, a gene required for
the regulation of germ line mitosis and oocyte differentiation during Drosophila
oogenesis. Development 122: 281-290.
McKearin, D. 1997. The Drosophila fusome, organelle biogenesis and germ cell
differentiation: if you build it... BioEssays 19: 147-152.
McLaren, A. and Southee, D. 1997. Entry of mouse embryonic germ cells into meiosis.
Developmental Biology 187: 107-113
Avarbock, M.R., C.J. Brinster and R.L. Brinster. 1996.
Reconstitution of spermatogenesis from frozen spermatogonial stem cells. Nature Med.2:
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology.
Third edition. Saunders College Pub. Philadelphia.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer. Sunderland,
Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New York.
Kroll, K.L. and E. Amaya. 1996. Transgenic Xenopus embryos
from sperm nuclear transplantations reveal FGF signaling requirements during gastrulation.
Development, 122: 3173-3183.
Shostak, S. 1991. Embryology. An Introduction to Developmental Biology.
HarperCollins. New York.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E.
1998. Principles of Development. Current Biology. London.