Key Points
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This chapter considers the language used within embryological research and how it is evolving.
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The terms used to describe embryos, cells and tissues derive from the social constructs of science during the time they were created.
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Newer terms have been added as scientific methods have increased, although there may not be a consensus on the definition of some terms.
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The application of computer sciences to development has produced its own terminology.
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The use of computer ontologies may impact development and the evolution of embryological concepts and terminology with which to explain the observed processes.
The Changing Concepts and Language of Embryology
Embryological terminology used today is a strange and diverse mixture of terms accrued over the course of two centuries and used as a vernacular language with different dialect depending on the topic and techniques of study. The accumulated terms include:
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The very old concepts generated between 1830 and 1900
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The newer understanding of cell phenotype generated in the 20th century from in vitro and in vivo experimentation and cell culture
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The very modern and rapidly changing 21st-century terms which describe gene expression and metabolism within embryonic tissues
The language of the latter group is driven by computer ontologies: hierarchies of embryological terms and key words programmed as algorithms, which are used to mine databases and publications. The spur for this interest is a future ability to unlock embryological pathways as a method for treating adult pathology and harnessing the regenerative potential of stem cells.
Origin of the Early Embryological Terms
Embryological terms are a product of their time and a reflection of how developmental science was explained. At a time when the theory of evolution was being formulated in the mid-1800s, Ernst Haeckel promoted a concept which stated that embryos would pass through all the previous evolutionary stages, resembling a series of extant or extinct adult animals as they recapitulated evolution during development. Thus Haeckel designated a blastula stage of development when a sphere or bilaminar layer of embryonic cells was present and a later gastrula stage achieved after the blastula cells had invaginated to produce more than one or two layers. He also inaugurated the term gastrulation to describe the process where cells initially on the embryonic surface move inside the embryo to produce intraembryonic cell populations.
At this time the instruments for examining embryos were rudimentary, and cell theory, being formulated also in the mid-1800s, was still relatively young. Embryologists of the day saw layers of tissue rather than the individual cells composing the layers and did not link the morphology of the earliest cells with differences in function. In studies in which it is clear from the publications that cells could be seen, distinctions among early embryonic cell types probably could not be made with the instruments available. The concepts thus generated by these early embryologists were products of their time, dependent on the methods of experiment and observation customary when they were formulated.
For most of the 20th century, textbooks supported the notion that the tissues of the developed body were derived from one of the ‘three germ layers’. Whilst this is not untrue in simplistic terms, the accent on three layers (ignoring the cell phenotypes) moved attention away from and limited interrogation of the dynamic differentiation processes occurring in embryos. Without a full range of words with which to think about developmental processes the reflections on, and explanations of, what is seen histologically becomes obfuscated.
A similar interpretive process driven by evolution theory occurred with the description of external embryonic form. In 1828, Von Baer noted that all vertebrate embryos pass through externally similar stages, and Haeckel published a series of drawings demonstrating remarkable similarity between embryos which go on to become very dissimilar adults. This latter concept remained unchallenged for more than a century. Recent examination of Haeckel’s pictures, together with a clear analysis of the developmental stages of various organs in each embryo, revealed a different story. Richardson noted that drawings by contemporaries of Haeckel show much more accurate interpretations of mammalian embryos of the same developmental stage with clear differences among them. He noted that Haeckel’s drawings had given a misleading view of embryonic development. Thus the idea of one stage of development, during which all vertebrate embryos are the same, promoted extensively at the turn of the 20th century and repeated unchallenged, hampered investigation into what really occurs in a number of vertebrate embryos. This again obfuscated the search for what is actually present in embryos by limiting the embryological concepts taught, the language used and consequently the expectations and explanations of the processes observed.
Embryology was advanced in the middle and later years of the 20th century by in vitro studies of developing reptile, avian and mammalian embryos, particularly using chimeric embryos in which specific cells lines could be followed. These experiments provided information about the similarities and differences among species. Also at this time, the genes expressed within developing tissues were studied, and the range of genes used in basic cell functioning and at particular points of development were elucidated.
The functions of many genes were studied by the experimental production of animals in which specific genes were knocked out or knocked in, and the effects of the homozygous were compared with the wild type. These experiments demonstrated the importance of some genes, with knock-out causing lethality; in other cases, the actual effect of taking one gene out within an embryo was confounded by the catch-up mechanisms in built into development: the change in one part of the system causing compensatory change in the remainder.
Early Embryonic Cell Interactions
Expansion of in vitro fertilisation techniques and the selection of healthy embryos for implantation have demonstrated that the secondary oocyte has a range of genes ready for expression to ensure cleavage, morula and blastocyst formation. When the dividing cells are an appropriate size, polarity is expressed, junctions are formed and embryonic cell–cell interaction commences. After hatching from the zona pellucida, the blastocyst is able to interact with maternal tissue. There is no time when the genome is not being read and epigenetic consequences, because of local environmental conditions, are not part of the next process.
Three cell lineages are now identified in the blastocyst, leading to extraembryonic and embryonic cell populations ( Table 1.1 ). All of these lineages express polarity genes and form epithelia.
| Term | Meaning |
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| Three cell lineages identified as zygote undergoes cleavage |
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| Polarity genes | Cells within an early embryo form epithelia and mesenchyme. The epithelial cells exhibit polarity genes which specify the apical, basal and lateral surfaces; the position of junctional complexes; and the direction of the mitotic spindle during cell division. Not observed before compaction in morula. |
| Germ layers | These were historically the ectoderm , endoderm and mesoderm . The terms are still widely used regardless of cell phenotype. They all derive from the epiblast of the early blastocyst. |
The process of gastrulation produces cells which do not have an epithelial phenotype (i.e., mesoblast and mesenchymal cells). Recovery of embryonic cells has led to the development of embryonic stem cells which can be immortalised in two-dimensional culture conditions.
Embryonic Cells in Culture
Historically, the definitions of the terms used to describe the putative abilities of embryonic cells were easily found. Today recent papers note the difficulty of accurately defining these terms ( Table 1.2 ). Adult cells can be induced to grow in culture and now so can embryonic cells.
| Term | Meaning |
|---|---|
| Totipotent | The ability of a single cell to develop into an adult organism and generate offspring. In humans, the zygote is totipotent. The loss of totipotency is now seen as a process. |
| Pluripotent | The ability of a single cell to develop into cells from one of the ‘three germ layers’ and ‘germ’ cells in vitro and in vivo . EpiSCs are postimplantation epiblast stem cells. |
| Embryonic stem cells | Human embryonic stem cells (hESCs). Origin not clear. Not quite the same as inner cell mass cells. Need specific culture conditions; grow in two dimensions. Now have been adapted to long-term in vitro culture. |
| Human-induced pluripotent stem cells (hiPSCs) | Cells derived from adult cells (e.g., fibroblasts) which have been cultured with specific transcription factors. They undergo transition to epithelial cells and express epithelial genes, becoming polarised. They also change their metabolism. |
| Human spheroids or organoids | When hiPSCs are grown in three-dimensional culture and encouraged along a particular developmental pathway, they form spheres of inner epithelial and outer mesenchymal phenotypes. Organoids have been created from hiPSCs specified as endoderm which differentiate into airway or gut phenotypes with appropriate epithelial and supporting mesenchymal cells. Self-organisation into layered tissues has also been seen in three-dimensional cultured brain cortical cells and retinal tissue. |
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