Embryogenesis Explained

The following sections are included:

• Dedication
• Preface
• Contents

We define embryogenesis as the process of producing the right kind of new cell, in the right place at the right time. This phrase “the right cell, in the right place, at the right time” is an expression attributed to Hans Driesch around 1890. All other aspects of embryology like morphogenesis, differentiation, and regeneration arise from embryogenesis of a first simplest proto-life form. Where and how did this first life originate?…

In order to understand how embryogenesis works, you must understand what embryogenesis is. This obvious statement evades many people who begin and end their studies with molecular developmental biology. It is as if understanding architecture required only a deep appreciation of bricks and mortar, i.e., in our case, molecules and their interactions. Learning the basic anatomy of the embryo is no longer required in many university-level biology curricula. This was partly the fault of the classical embryologists who often required students to regurgitate letter perfect details written in arcane vocabularies. Today, there is an almost exclusive emphasis on the latest new techniques in molecular biology. Biochemical tools are not seen as valuable adjuncts to classical embryology: they have replaced them. Finding detailed lists of genes and proteins and their presumed interactions for this book was easy. In contrast, we had difficulty finding good anatomical figures of developing embryos…

Since the discovery of the double helix structure of DNA (Figure 3.1), our understanding of genetics has exploded like a supervolcano, blasting in multiple directions, and covering the scientific world with a broad layer of new understanding. Yet, it seems the more we learn, the more there is to learn about the way genes work. The scientific community is spending more time slowly digging out of the proverbial volcanic ash heaps than looking up to see the new landscape as a whole. A graduate student can still spend his or her entire thesis studying a single protein's effect on gene expression. And so scientists dig, often unaware of the fellow in the next block, also digging just as hard, in another little corner trying to uncover the same stretch of buried road…

We know that there is a hierarchy of genes. We know that some genes are able to regulate and control many other genes. In genetics such genes are referred to as “master genes”. If a specific master gene is turned on, it makes some of the genes below it within that hierarchy exposed (available for transcription) in a given cell. At the same time it will make other genes below it in the hierarchy sequestered (not available for transcription) in that same cell. If we imagine the genome is organized into multiple subsets, each subset controlled by a master gene, or key master genes, organized into regulons and differons, then a possible explanation of higher order organization becomes clearer. When the hierarchy is disrupted normal development can be affected…

If one consults any of the standard developmental biology textbooks, the cytoskeleton will be briefly presented as the support structure of the cell and about as interesting to most biologists as the floor struts in a ballet school would be to an artistic director seeking the next prima ballerina. However, to think of cytoskeleton only as the structure on which cells do their remarkable things, leaves out a good part of the story of how cells work…

As soon as the fertilized egg completes the first cleavage, it is no longer a single cell. For simple organisms like the nematode, determination begins with that first cleavage. The two daughters of the divided fertilized nematode egg are each determined during division and they become different cell types. The division is uneven, one large cell and one small cell. The major difference between yeast budding and the embryology of the nematode is that instead of the bud moving off to form an entirely separate organism, the determined cells of the nematode remain attached and as cell division progresses, determination occurs with each asymmetric division to eventually form a complete multicellular organism. The nematode C. elegans has 959 cells in the adult hermaphrodite, 1031 in adult males. All are unique except for 50 pairs of like cells, with each pair different from all the other pairs (Figure 2.15). In the nematode, determination is absolutely linked to cell division. In embryology the nematode is referred to as a mosaic organism. The term mosaic in this context is entirely unrelated to the mosaicism of females due to X inactivation (Chapter 4). If an embryologist cuts an early mosaic embryo in half, each half fails to develop properly as a whole organism, and that defines mosaicism…

In Chapter 2 we presented the anatomy of early embryogenesis in the salamander, Ambystoma mexicanum, whose common name is the axolotl (Figure 2.1). In Chapter 5 we explained the role of the cytoskeleton in a variety of cell functions. We introduced the cell state splitter, a specialized cytoskeletal organelle located at the apical end of the cells in a cell sheet (Figure 6.3), and we showed the types of expansion and contraction waves that the cell state splitter is capable of generating (Figures 6.28, 6.34, 6.35, 6.37, 6.38 and 6.39). We will now give a detailed case study of our own observations of differentiation waves on the axolotl. The bifurcating sequence of waves on an embryo is what we refer to as its differentiation tree (Figure 6.43). Here we present the empirical differentiation tree of the axolotl (Figure 7.1)…

In May of 1990 we got our first hints of the presence of a physical wave in the ectoderm. By the end of 1991 we had the entire trajectory of that first wave well documented. There were also long stretches of time between waiting for spawnings. We had classes and teaching duties and grants to write at the laboratory. At home, we had children who needed things from us like school lunches, stories before bed, hugs and clean clothing. Still, the differentiation waves occupied our thoughts in every spare moment. A wave of contraction at the surface of the cell results in a signal to the nucleus. The nucleus then uses the signal to turn off one set of genes and turn on another. What exactly was that signal? We referred to the cytoskeletal apparatus on the top of the cell as the cell state splitter. It was inevitable that in our own discussions we would begin referring to the process of turning the signal from the cell state splitter into a specific subset of gene expression patterns as a “nuclear state splitter”. Corresponding to the two outcomes for a cell's cell state splitter, contraction or expansion, we hypothesized two corresponding, readied states of the nucleus, only one of which would happen. In this conception, the nucleus too was in a specific bifurcation point, where it would go one way or the other: two choices, no more, no less. One choice would correspond to contraction of the cell state splitter, the other to expansion of the cell state splitter…

In the previous chapter we introduced the biochemistry of signal transduction. We showed our model for the signal transduction pathway from the cell state splitter to the nucleus. In Chapters 3 and 4 we looked at the control of individual genes or portions of the genome. After the passage of a wave caused by the cytoskeletal rearrangements, the signal transduction to the nucleus has taken place. The nucleus ends up in one of two new states, expressing one of two new possible differons. We call what happens inside the nucleus the “nuclear state splitter”. In order to explain the nuclear state splitter, we first to need to look at what is known about the control of gene expression on the level of the entire genome…

We live in the era of the ascendancy of molecular biology. In this book we are turning the idea that molecules control development on its head, and proposing that development controls the molecules of which we are made. Of course, both mechanics and molecules are involved, but we eschew the idea that molecules do it all by themselves. The idea that waves are involved in embryogenesis has a reasonably long history, which we shall now review to show that differentiation waves fit into a more general framework of embryo research…

One of the outstanding recent scandals of biology has been the notion that evolution is not progressive, a concept that flaunts the evidence of our eyes. Any reading of the fossil record shows simpler starts followed by increasing sophistication, along many lines. Even where morphology doesn't appear to change, competition between organisms can lead to an arms race, known as the Red Queen hypothesis from the children's book Through the Looking-Glass, and What Alice Found There: “Now, here, you see, it takes all the running you can do, to keep in the same place”. Overall, there is indeed an increase in biodiversity over evolutionary time (Figure 11.1), which we could deliberately maintain. To be sure, there is some apparent backsliding, as when an independent organism, even a bacterium, becomes a parasite, or parts become vestigial or discarded, or features disappear and recur in closely related species, or a comet or an asteroid wipes out 95% of living species (Figure 1.24). But on the whole, some species in each branch of the tree of life tend to march on with richer diversity of structure and perhaps behavior. One caveat is that the total number of extant species, though perhaps not their complexity, may have peaked 530 million years ago. A drop in the total number of extant species since that time may be due to subsequent extinctions…

A remarkable book was published in 1926 by statesman Jan Christiaan Smuts under the title Holism and Evolution. For such a busy and world-involved person, a founder of the League of Nations, the predecessor to the United Nations, and prime minister governing the large country of pre-Apartheid South Africa, to master the fundamentals of the then new fields of quantum mechanics and relativity well enough to write about them, is perhaps unprecedented. The word holism, which he coined, has since been hijacked for all sorts of wooly thinking. Just search the Internet for “12 holistic beauty tips to get beautiful skin”, “holistic camping”, and “holistic spirituality”. On Amazon.com you can buy books on Holistic Herbal Healing Medicine, Holistic Tarot, The Holistic Orchard, Holistic Aromatherapy for Animals, Career Counseling: A Holistic Approach, with more than 5500 other such titles available. Our canine companions Fred and Trusty eat “Holistic Dog Food”. Google Books lists over 200 volumes on holism, starting with Holism and Ecology and Holism: A Wonderful Faith. The database Web of Science lists over 12,000 publications with the words “holistic” or “holism” in the title. Going by Google Scholar, about 1700 academic publications cite Smuts' book. Smuts has had quite an impact on our popular and learned culture…

The following sections is included:

• Index