Report - Carnegie Year Book 38

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George Linius Streeter (1873-1948)

Carnegie Institution of Washington - Year Book No. 38

July 1, 1938 - June So, 1939

Published By Carnegie Institution Of Washington

Washington, D. C. 1939

Department Of Embryology

George L. Streeter, Director (1939)

Address: Wolfe and Madison Streets, Baltimore, Maryland.

Contributions to Embryology

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Early Anthropoid and Human Embryos

The years 1938 and 1939 were eventful ones in this laboratory. They brought us specimens which revealed the character of the early ovum and the manner of its implantation in the chimpanzee, this being the first time such data have been available for any of the great anthropoid apes. Along with that major acquisition we have also obtained two human ova of a very early period of development and preserved in a manner that permitted complete photographic records and ideal histological technique in their preparation for microscopic study. These new human specimens together with the Miller ovum constitute a normal- group of the earliest known stage of human development, a stage preceding the appearance of the yolk-sac and the differentiation of villi. They replace the diagrammatic concept of this period of development, which up to the present time had been almost entirely hypothetical, and whose inaccuracies had led to confusion in the understanding of the initial organization of the embryo.

The early ova of the chimpanzee were obtained through cooperation with the Yale Laboratory at Orange Park, Florida. One of the two specimens is a pathological ovum just in the process of making its way through the uterine epithelium. The other is a normal specimen, of a stage a little younger than any known human specimen, and has an estimated age of 10^ days. It portrays the ovum just after it has penetrated the maternal epithelium, the point of entry being still open. The success in this carefully planned project is due, on the part of the Yale Laboratory, to Dr. Yerkes and Dr. Elder, and on the part of our laboratory, to Dr. Hartman and Dr. Heuser. The significance of the observations made in connection with this material, and of the material itself in its histological perfection, is far reaching because it constitutes our first view of the phenomenon of the mechanism of implantation in the great apes and because of the light it throws on human implantation, with which that in the chimpanzee possesses a close affinity. If there is any real gap between the macaque material, which we now have in such abundance, and the less well understood human implantation, this chimpanzee material will assist in bridging it. In the macaque the invasion of the maternal tissue is superficial and only sufficient to accommodate a partial submergence of the ovum; whereas in the human the whole ovum sinks beneath the uterine surface, with the epithelium healing over the denuded surface. The chimpanzee is like the human in this respect, and it also approximates the human in the precocity of the primitive coelomic reticulum (mesoblast) . A preliminary account of the "Yerkes A" ovum has already been published, but a more complete account is now in course of preparation by Dr. Heuser, and will include observations on the second chimpanzee ovum, which we regard as abnormal.

Regarding the two new human ova, which were secured through the cooperation of Dr. A. T. Hertig and Dr. J. C. Rock, only a brief note will be made at this time. They were obtained surgically on the 11th day, and after careful study ment, differing principally in its more of their gross relations they were cut in precocious and more abundant primitive a chosen plane of section in faultless fibroblastic reticulum, the so-called series. A preliminary examination with primitive or extraembryonic mesoblast. the microscope reveals that the two ova It is concluded that in none of these ova correspond closely to each other in size has the yolk-sac yet appeared. What was and character, and to the Miller ovum, first regarded as possibly a yolk-sac mass giving assurance that all three are in the Miller ovum is now shown to be a normal. This means that we now for part of the combined primordium of the the first time have a sure point of de- primitive mesoblast and gut endoderm parture for analysis of the subsequent (fig. 1), and with attention being called events in the organization of the ovum to it by the new specimens one can recog and its further development into embry- nize in the Miller specimen a small exo onic and extraembryonic or auxiliary coelomic cavity closed in by a meso structures. With the kind of evidence thelial membrane, known in this labora which such specimens place at our dis- tory as the Heuser membrane, posal, we shall no longer be willing to tolerate the poor histological material Studies of Early Stages in the and the wild speculations that have so Macaque Embryo largely dominated our past interpreta tions of the early stages of the human J he mesothehal membrane enclosing ovum. We have arrived at a day of the exocoe omic space in early macaque better embryological quality. The spe- ova > *? *h"* ° T u T r attention was first cialist is replacing the amateur in all called by Dr. C. H. Heuser, has recently steps of the work. It now remains for ^en studied by him in its entirety He us to provide a complete account of this has traced the origin of its first cells from important material and in a form that £e c f S rou P underlying the germ disc, will make it conveniently available to p then shows how they form a mem embryologists in general. Such an ac- brane tha * becomes continuous laterally count is now in course of preparation by W1 ^ h 1 B " ml ? r membranes formed from Dr Hertiff delaminated in situ from the trophoblast on all sides. In this way there is formed a thin-walled closed cavity or New Reconstruction of the Miller vesicle which occupies a large portion of the chorionic cavity. The functional The two new human ova obtained significance of this space and the memthrough Dr. Hertig and Dr. Rock made brane enclosing it has not yet been deit necessary to re-study the Miller ovum, termined. However, they appear to be which is of about the same stage of characteristic for primates, and that their development. By replotting its five utility is of short duration is made apavailable sections in the form of a profile parent by the fact that the membrane reconstruction, the Miller ovum can be remains intact only from the 12th to the brought into complete conformity with 20th day, which is when the chorionic the more complete Hertig specimens, villi become definitely formed. At that Both in size and in detailed form they time there occurs an increase in the approach one another closely, and all extraembryonic mesoblast in the space three are unquestionably normal. We between the exocoelomic membrane and now know that the human ovum is much the trophoblastic wall. This increase of like that of the macaque in its develop- mesoblast is partly at the expense of the former. As the membrane disintegrates the mesoblast takes the form of a fibrous reticulum whose strands extend from the chorionic wall to the surface of the yolksac and the amnion, and through whose meshes fluids can freely pass. It is these persisting delicate strands that are known as "magma reticulare" in human specimens.

A study of the macaque egg as it exists free in the uterine cavity during the two therewith grow larger, but they also fulfill the requirements of living organisms. In order that embryos may maintain themselves at their respective biological levels, it is necessary that their structure be so designed for each developmental stage as to ensure an adequate physiological performance. It follows that their design should enable them to continue living indefinitely at any respective stage so long as no days preceding implantation has been made by the writer. It was found that already in the blastocyst stage one can recognize ephemeral structures which are concerned with the immediate physiological requirements of the egg rather than being stages in the building of future organs. Embryos, in fact, are engaged in two quite different activities. They not only develop from the simpler forms into more complicated ones and

Embryonic Ectoderm Primitive Mesoderm Combined Primordium Of Cut-Endoderm And Yolk-Sac

Fig. 1. There are only five sections through the implantation area of the Miller ovum. The remainder had to be supplied by deduction and interpretation. A profile reconstruction of that character was made and published in 1926. Since two new normal human embryos of about this age are now available, it has been possible to improve on the original interpretations of the Miller specimen, and the latter is thus brought into close conformity with the two Hertig ova, as this schematic sketch reveals. The presence of an exocoelomic membrane (Heuser's membrane) had been overlooked, and what was thought to be a possible yolk-sac is now interpreted as a layer of multiplying cells which antecedes both gut endoderm and yolk-sac.

change in themselves or their environment renders that design inadequate. In short, the embryo not only develops but also exists. To a large extent it is possible to discern the respective cell groups engaged in these two functions. In tracing the maintenance group it is seen that as the requirements for existence progressively change there are developed a series of temporary devices with which those particular needs are met. When a particular requirement is no longer present, the respective device is altered or permanently discarded. What have been termed vestigial structures appear in many cases to fall in this series of evanescent organs. It is suggested that the thin trophoblastic wall of the free blastocyst is such an organ. It constitutes a functional membrane adapted to the concurrent needs of the young organism. It is to be distinguished from the undifferentiated remainder of the blastocyst wall, which is interset at the embryonic pole. This pole consists of the embryo-forming cells and, overlying them, certain auxiliary cells which will form the attachment mechanism and various extraembryonic membranes. At this place it may be pointed out that with better material and improved histological techniques, it is becoming possible to observe in greater detail the functional activities of the structural elements that constitute . the egg and early embryo. To have this information in the primates is of utmost importance.

Functional Maturation of Skeletal Muscle

Using living rat embryos, Dr. W. L. Straus, Jr., has studied the developmental changes that occur in skeletal muscle at the time of its first visible contraction. He found that on the 16th day of gestation electrical stimulation through the skin induced visible contractions of the arm musculature. On the 15th day stimulation gave no visible response. The point therefore was to learn how the fibers of the 16th day differed from those of the 15th. In general they proved to be larger, and the better-developed ones contained more myofibrils. These differences, however, are regarded by Dr. Straus as insufficient to account for the marked difference in their response to stimulation. He concludes that there must be important supplementary extramuscular factors, and suggests as possibilities the more advanced organization of the connective tissue and the greater degree of articular cavitation which characterize the 16-day specimens. He also finds that muscular contraction can be produced by direct stimulation of the nerves on the 16th day, though the nerve endings at this time are epilemmal and of relatively immature form. These studies of Dr. Straus are still in progress but are sufficiently advanced to warrant this preliminary report.

Development of the Adrenal Cortex of the Alligator

To provide a better structural basis for his endocrine studies in the reptile, Dr. T. R. Forbes has studied the embryology of the adrenal gland in the alligator, and especially the adrenal cortex. He traces the cortex from a primordium of coelomic epithelium which spreads laterally to merge with the germinal epithelium of the sex gland and which can be distinguished from the latter only by the absence of germ cells. The primordia of both organs thicken in a similar way and project cellular cords into the underlying mesenchyme, and the adrenal throughout development retains its contact with the sex gland. The cortical cords rapidly penetrate between the mesonephric glomeruli and the aorta. Branching and becoming tortuous, they become detached from their seat of origin, the coelomic epithelium. Before hatching, the adrenal chromaffin medullary tissue migrates forward to form basophilic islets in among the cortical cords. In the adrenal of the alligator, Dr. Forbes finds, the cortical and medullary tissues are never segregated into discrete peripheral and central zones. Ovum of the Mink Dr. R. K. Enders of Swarthmore College, who has been aiding in a cooperative study of the physiology of reproduction in mink, has obtained and studied in the living condition three tubal ova. They were estimated by him to be about 16 hours after fertilization. They were free of follicle cells, the vitellus had shrunk, and sperm heads were observed in each of them. The zona pellucida and general appearance of the ova are typically mammalian. The vitellus is very dark and opaque throughout, owing to the refractive properties of the globules of lipoid material contained in the cytoplasm. This is a characteristic common to carnivores. When focused properly, the peripheral granules can be measured and are 3 micra in diameter. The total diameter of the ova varied individually between 135 and 150 micra. The vitellus itself varied from 103 to 110 micra. These eggs are therefore larger than the mouse egg and about the size of that of the macaque.