Early Concepts and Terminology

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:

• The very old concepts generated between 1830 and 1900

• The newer understanding of cell phenotype generated in the 20th century from in vitro and in vivo experimentation and cell culture

• 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.