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Dynamic Development
CONTENTS

Main Page Dynamic Development

The Foundations of Developmental Biology

Gametogenesis

From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity


Dynamic Development at a Glance


Learning Resources

Research Resources

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 cells.

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.

Meiosis

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.

Gamete Differentiation

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. 12.18)

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 sperm.

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


Learning Objectives

  • 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?

Digging Deeper:

Background Information

Additional Topics

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


References

Avarbock, M.R., C.J. Brinster and R.L. Brinster. 1996. Reconstitution of spermatogenesis from frozen spermatogonial stem cells. Nature Med.2: 693-696.

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, MA.

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.


Dynamic Development at a Glance
Main Page Dynamic Development

Dynamic Development is a Virtual Embryo learning resource

This material may be reproduced for educational purposes only provided credit is given to the original source.
Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Thursday, June 11, 1998