fusion of a sperm and egg to form a zygote.
Cleavage: special, rapid mitotic cell divisions of the zygote to form a multicellular embryo.
Morphogenetic Movements: gastrulation and neurulation initially form primary germ lines establishing cell lineages in appropriate arrangements for further development.
Cell and Tissue Differentiation: cellular interactions regulate gene activity leading to specialization of cell types, tissues and organs.
Pattern and Polarity: humans have left-right, front-back (dorsal-ventral), top-bottom (anterior-posterior) symmetry that is reflected in the development of tissues and organs.
Growth and Maturation: the fetus grows as development continues while post-embryonic development is characterized by growth and sexual maturation (leading us back to gametogenesis and fertilization and the life cycle).
Senescence (aging) and Death: are the final developmental events of life leading to the end of the individual.
These key terms and concepts should be learned and understood before progressing in this book. Some of these topics are not covered (e.g., aspects of growth and maturation; senescence and death) in this volume because its primary focus is on embryonic development.
Chapter 2
Saga of the Sex Cells
Primordial germ cells (PGCs) are the source of sperm and eggs. They are specialized cells that are localized in our ovaries and testes. The primordial germ cells give rise to the male and female sex cells which in turn transmit the genome from generation to generation. The “Saga of the Sex Cells” is a true tale of cellular struggle, long journeys and tragic events. The future eggs and sperm are critical to the survival of each species. As a result, in some species, their fate is determined long before the egg begins to divide. Special factors (determinants) dictate that the cells that contain them will become sex cells. The cells that acquire these determinants then have a long haul ahead. First they must migrate through the embryonic tissues to the sites where the future gonads will form.
Along the way some will get lost and simply die. Other lost cells will not accept their inability to get to where they were supposed to and will begin to develop inappropriately forming dangerous malignant tumors called teratomas. Those that do arrive safely will have to wait many long years as the genital ridges first develop into gonads and then, at puberty, become functional producers of mature gametes. In the ovaries, a limited supply of eggs will be produced that will have to suffice for the life of the female while in the male sperm production will be a continuous process that can last until death. The way in which eggs and sperm form and the differences in their formation are equally interesting and reflect the important role each has in ensuring that successful fertilization will result so that the next generation will begin.
What are Primordial Germ Cells (PGCs)?
Primordial germ cells (PGCs) are the precursors of the sex cells. As such they are considered to be a type of stem cell. In lower animals, PGCs are determined by a specialized region of the cytoplasm called “germ plasm”. However there is no solid evidence that true germ plasm exist in humans or other mammals. In contrast, research has shown that similar genes to those found in lower animals are involved in the formation of human PGCs. Alkaline phosphatase is a classic germ cell "marker enzyme" for PGCs and can be used to determine when they first appear. Evidence indicates that human PGCs arise in a special region just prior to gastrulation appearing in the epiblast in the area that will become the extraembryonic mesoderm. Since they arise extra-gonadally (i.e., outside of the gonads), PGCs must migrate through different tissues to reach the genital ridges where the gonads will form.
What is Germ Plasm?
The germ plasm is morphologically similar in all lower animals. However a defined cytoplasmic localization of germ plasm material has not been detected in mammals. Germ plasm contains dense fibrillar and granular material. The particles have been shown to contain RNA and protein. Mitochondria and ribosomes are also localized within the germ plasm. This concentration of components segregates into specific cells during cleavage. Those cells that receive the germ plasm during cleavage are destined to become germ-line cells while those that don’t will become somatic cells of the body. Extensive research has been done on Drosophila and other organisms (e.g., Caenorhabditis) which has helped the study of germ-line determination in humans. These germ-line determinants have been found to be genetically controlled. A diversity of experiments has shown that specific mRNAs localized in the germ plasm are the cytoplasmic determinants that direct the formation of germ cells.
The Journey of the Germ Cells
Figure 2.1. The journey of the germ cells. The migration of PGCs and the events that transform them into sperm or eggs.
Staining with alkaline phosphatase in fixed embryo sections provided the first evidence that the PGCs migrate from their site of origin in the epiblast to the future genital ridges. The subsequent events involved in the journey of the germ cells were subsequently revealed a diversity of techniques (Figures 2.1, 2.2). During the early stages the number of PGCs increases by normal mitotic divisions. When the PGCs enter the genital ridges they will come under various influences (i.e., hormones, cellular interactions) that will dictate whether they will become oogonia (females) or spermatogonia (males).
In the female genital ridges, after a specific period of cell divisions, cell death of the PGCs will begin and continue. Mitosis will stop. In contrast, in the male genital ridges mitosis of the PGCs will continue throughout life. Cell death is comparatively minimal since it has not yet been detected to any significant degree in the testes. The hormonal events of puberty will cause the PGCs to complete their differentiation into functional eggs and sperm as detailed in the following chapters. Gametogenesis continues until fully formed gametes are produced and released (ovulation or ejaculation).
Germ Cell Markers
Current research still uses alkaline phosphatase staining to study primordial germ cell migration in various mammals including humans. The method is limited because it requires the embryo to be fixed, sectioned and stained. More recently, however, cellular labeling (e.g., GFP; green fluorescent protein linked to a germ cell specific protein) and use of staining with monoclonal antibodies directed against germ cells added further insight into this subject. For example, stromal cell derived factor-1 (SDF-1) and its chemokine receptor CXCR-4 are essential for germ cell migration. GFP-SDF1 (GFP coupled to SDF1) allow researchers to follow living primordial germ cells within the embryo.
As detailed below, transplantation studies have shown that the germ cell lineage begins in the posterior region of the epiblast in the mouse, a model for the study of human development. The pictures below (Figure 2.2) show that PGCs are subsequently detected in the yolk sac, a long distance—in cellular terms—from the future ovaries and testes. They migrate up the mesentery (i.e., splanchnopleure) ultimately exiting left and right to enter the just-forming genital ridges. Always remember, that as these events happen the embryo is continuously growing and changing its shape. So a structure or tissue relationship that was present early in a developmental process may not be present later or may have changed in its organization and appearance. While we should keep this in mind, our goal is to understand the events and how they occur. It is important not to get overly concerned about the complexity of these ongoing events in terms of the whole embryo or to let them overshadow your understanding of
