1987 Developmental Stages In Human Embryos - Introduction

Introduction

The norm should be established; embryos should be arranged in stages. Franklin P. Mall

The combined use of fixation, sectioning with a microtome, and reconstruction from the resultant sections first enabled Wilhelm His, Senior, to begin to elucidate thoroughly the anatomy of individual human embryos. Indeed, His may rightfully be called the “Vesalius of human embryology” (Müller and O'Rahilly, 1986a).

Although fixatives other than spirits were introduced early in the nineteenth century, formalin was not employed until the 1890s. His devised a microtome in about 1866. (A microtome had already been employed as early as 1770.) The wax plate reconstruction technique of Born (1883), introduced in 1876, has undergone numerous modifications over the years. These, as well as graphic reconstruction, have been discussed in a number of publications, e.g., by Gaunt and Gaunt (1978). Florian, who used graphic reconstruction of the human embryo to great advantage, elaborated the mathematical background in Czech in 1928. (See also Fetzer and Florian, 1930.)

It has been pointed out that “the idea of working out a complete account of the development of the human body was always before the mind of His,” and his collaborator, Franz Keibel, proposed to provide “an account of the development of the human body, based throughout on human material” (Keibel and Mall, 1910) rather than from the comparative standpoint. The result was the Manual of Human Embryology edited by Keibel and Mall (1910, 1912), which was an important step in the goal of seeking precision in human embryology. The hope was expressed that, subsequently, a second attempt, “whether made by us or by others, will come so much nearer the goal” (ibid.).

The Carnegie Collection

Mall's collection of human embryos, begun in 1887, later became the basis of the Carnegie Collection (Mall and Meyer, 1921). Mall (1913) stated his indebtedness to His in the following terms: “We must thank His for the first attempt to study carefully the anatomy of human embryos, but his work was planned on so large a scale that he never completed it…. Thus we may trace back to him the incentive for Keibel's Normentafeln, Minot's great collection of vertebrate embryos and mine of human embryos.”

In more recent years the Carnegie Collection has benefited enormously from the meticulous investigations of Bartelmez, the technical adroitness of Heuser, and the donation of, as well as research on, remarkably young specimens by Hertig and Rock. The microtomy of Charles H. Miller and William H. Duncan, the reconstructions by Osborne O. Heard, the artwork by James F. Didusch, and the photography of Chester F. Reather and Richard D. Grill, have each played a key role in the establishment of the superb embryological collection on which the present monograph is so largely based. In George W. Corner's apt comparison, the Collection serves “as a kind of Bureau of Standards.”

Embryological Seriation

His had made the first thorough arrangement of human embryos in the form of a series of selected individual embryos, numbered in the presumed order of their development. The same principle was followed in the published plates known as the Normentafeln, Page 2 edited by Franz Keibel from 1897 onward; the volume on the human (by Keibel and Elze) appeared in 1908. The limitations of the method are (l) that individual embryos cannot be arranged in a perfect series, because any given specimen may be advanced in one respect while being retarded in another, and (2) that it may prove impossible to match a new embryo exactly with any one of the illustrated norms. The need for a more flexible procedure than a mere Entwicklungsreihe soon became apparent in experimental embryology.

Embryonic Staging

In the words of Ross G. Harrison (Wilens, 1969), “the need for standardized stages in the embryonic development of various organisms for the purpose of accurate description of normal development and for utilization in experimental work has long been recognized.” Because “development is a continuous process with an indefinite number of stages” (ibid.),a certain number have to be chosen. Thus each stage “is merely an arbitrarily cut section through the time-axis of the life of an organism” (deBeer, 1958). It resembles, in Harrison's apt comparison, a frame taken from a cine-film. Stages are based on the apparent morphological state of development, and hence are not directly dependent on either chronological age or on size. Furthermore, comparison is made of a number of features of each specimen, so that individual differences are rendered less significant and a certain latitude of variation is taken into account.

Although embryonic staging had been introduced toward the end of the nineteenth century, it was first employed in human embryology by Franklin P. Mall (1914), founder of the Department of Embryology of the Carnegie Institution of Washington.

On the basis of photographs of their external form, Mall (1914) arranged 266 human embryos 2–25 mm in length in a series of fourteen stages, lettered from H to U. (A to G were to have been the earlier stages.)

Mall's successor, George L. Streeter, provided the definitive classification of human embryos into stages, which he termed “developmental horizons.” Attention was concentrated on embryos up to about 32 mm greatest length because it was believed that, during the fetal period, the rate of increment in size and weight might be large enough to provide an adequate index of relative development.

The original plan was “to cover as far as possible the earliest specimens up to fetuses between 32 and 38 mm. long, the stage at which the eyelids have come together,” and “twenty-five age groups” were envisioned (Streeter, 1942). Subsequently, Streeter (1951) decided that stage 23 “could be considered to mark the ending of the embryonic period” proper. The onset of marrow formation in the humerus was “arbitrarily adopted as the conclusion of the embryonic and the beginning of the fetal period of prenatal life. It occurs in specimens about 30 mm. in length” (Streeter, 1949). A scheme of the 23 stages, as modified and used in the present monograph, is provided in Table 0-1.

The term “horizon” was borrowed from geology and archaeology by Streeter (1942) in order “to emphasize the importance of thinking of the embryo as a living organism which in its time takes on many guises, always progressing from the smaller and simpler to the larger and more complex.” However, the somewhat infelicitous term “horizon” has now been replaced by “stage” because the latter is the simple term employed for all other vertebrate embryos. Not only was the term “stage” used decades ago by Harrison for Ambystoma and subsequently by Hamburger and Hamilton for the chick embryo, as well as by others for a variety of reptiles, birds, and mammals, but, even in the case of the human, the term “stage” was employed by Mall (1914) when he first staged the human embryo more than half a century ago. The term is simpler, clearer, of widespread usage, and can be employed as a verb (to stage an embryo) as well as a participial adjective (a staging system). Furthermore, it should be pointed out that such expressions as “at the 3-mm stage” should be replaced by “at 3 mm.” In other words, the length of an embryo is a single criterion that is not in itself sufficient to establish a stage. The term “stage” should be confined to its present-day usage in embryology (such as the 46 stages of Hamburger and Hamilton in the chick, and the 46 stages of Harrison in Ambystoma maculatum).

Additional alterations that have been made in the current work include the replacement of Roman by Arabic numerals and the elimination of the scientifically meaningless term “ovum.” Atlases based on the Carnegie system of staging have Page 3 been prepared by Blechschmidt (1973) and by Gasser (1975). Alternative systems of staging (discussed by O'Rahilly, 1973) are now obsolescent.

Stages 10–23 were published either by Streeter (1942, 1945, 1948, and 1951) or at least with the aid of his notes (Heuser and Corner, 1957). “The earliest age groups” were wisely “to be reserved to the last, so that advantage may be taken of any new material that becomes available” (Streeter, 1942). These groups, stages 1–9, which were to have been completed by the late Chester H. Heuser, became the task of O'Rahilly (1973).

Embryonic Length

Because most embryos are received already in fixative, it is more practicable for comparisons to use measurement after fixation as the standard (Streeter, 1945). The most useful single measurement is the greatest length (G.L.) of the embryo as measured in a straight line (i.e., caliper length) without any attempt to straighten the natural curvature of the specimen (Mall, 1907) and preferably (for purposes of standardization) after two weeks in 10 percent formalin (Streeter, 1920). Up to stage 10, measurements are frequently made on accurately scaled models, although the results (because of shrinkage in preparing the sections) are then smaller (by 25 percent, according to Streeter, 1942). A particularly interesting study has been made of the shrinkage of (pig) embryos in the procedures preparatory to sectioning (Patten and Philpott, 1921). Careful technique (see Heard, 1957) is naturally to be encouraged in order to keep artifactual changes to a minimum.