Paper - Primordial germ cells in a 4.5 mm human embryo

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Hamlett GWD. Primordial germ cells in a 4.5 mm human embryo. (1935) Anat. Rec, 61, 273-279.

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Note this paper was published in 1935 and our understanding of germ cell development has improved since this historic human study.


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Primordial Germ Cells in a 4.5 mm Human Embryo

G. W. D. Hamlett

Department Of Anatomy, Harvard Medical School, Boston

One Text Figure And One Plate (Eight Figures)

Introduction

The existence of primordial germ cells which migrate into the anlage of the gonad during early embryonic development has been recognized in all groups of vertebrates, although it is still a matter of dispute as to whether or not the definitive genial cells are derived Wholly or in part from migratory elements. Among mammals these cells have been described in the guinea-pig‘, rabbit, mole, armadillo, pig, cat, and man (Felix, Fuss, Rubaschkin, Vanneman). The work on the human is quite fragmentary, Fuss’ account being the nearest complete. The present description is based on a single embryo, but I am presenting, 1 believe for the first time, photomicrographs of the cells and a. diagram of their distribution.


The embryo is 4.5-mm. crown-rump length, obtained by laparotomy from a ruptured tubal pregnancy. I received the unopened ovum, still in the injured tube, shortly after the operation and less than 3 hours after the tube’s rupture. Fixation was with Bouin’s, the sections were stained with iron haematoxylin and eosin. The circumstances, together with the appearance of the sections, warrant the statement that the embryo is entirely normal in development and that post mortem changes are absent. I express here my sincere thanks to Dr. Raymond M. Borland, of Bloomington, Indiana, for his kindness in donating this most valuable specimen.

Description

The primordial germ cells at this stage are found clustered ventral to the mid gut, and as isolated cells in the dorsal mesentery between gut and mesonephroi and in the genital ridges. Almost all of them are in the mesoderm, possibly a half dozen are wedged among the entoderm cells. Altogether there are about 330 of the germ cells, their distribution is plotted on a lateral View of the reconstructed embryo in figure 1. The gut, with its diverticula, and the position of the mesonephroi are indicated by dotted outlines. The migrating germ cells are indicated by dots, those already in the genital ridges by circles. Each dot or circle represents a single cell. In the cluster Ventral to the gut the germ cells are in close groups of three, four or more, while elsewhere they usually occur singly.


Fig. 1 Lateral View reconstruction of the embryo to show the distribution of the primordial germ cells. The gut, its divertieula, and the mesonephroi are indicated in dotted outlines. Germ cells within the genital ridges are shown by circles, those in the mesentery or around the gut by dots.



The appearance of the cells is shown in the photomicrographs. It will be seen that they are somewhat larger than the somatic cells, their cytoplasm is colorless, and the nucleus shows one or two conspicuous nucleoli; while the cell membrane is usually distinct so that the cells are definitely outlined. In many cells there is a zone of cytoplasm to one side of the nucleus which stains faintly with eosin; this is shown in photographs 6 and 7 as a dark sector of cytoplasm. This region seems to be more conspicuous in those cells in or approaching the genital ridge.


It has been sometimes objected, in discussions of the germ plasm theory, that blood cells or cells in mitosis have been misinterpreted as primordial germ cells. Figures of these types of cells are included iii the plate to show the unmistakable differences between them and the germ cells in the present human embryo (figs. 2, 3, 4, 7 and 9). Moreover, the definite localization of the germ cells is another criterion for discriminating them from other types of cells. Blood cells are abundant everywhere in the mesenchyme, and dividing cells are ubiquitous in the embryo; whereas the primordial germ cells are absolutely restricted to the neighborhood of a short part of the mid gut, to the genital ridges of the mesonephroi, and to the mescntery between these points. No cells resembling them are found anywhere else, although I have searched carefully through the whole embryo with this point in mind.

Discussion

The identification of these cells in the human embryo as primordial germ cells is based on the following points. In the first place, their appearance is quite similar to the early germ cells described in detail for other mammals and for lower Vertebrates, so much so, in fact, that their nature was suspected when they were first noted. This tentative diagnosis was later confirmed by the study of their distribution. Their origin near the gut and migration into the germinal ridges allow no interpretation other than that they are early germ cells, while their complete absence from all other regions of the embryo rules out any possibility of their being dividing cells, blood cells, or other tissue elements. 276 G. W. D. IIAMLETT


I have used the word migration in speaking of their passage from near the gut into the germinal ridge. I am undecided as to the means by which this is accomplished. I have seen nothing to indicate an active ameboid movement of the germ cells themselves, such has been described for some forms. Nevertheless, it seems doubtful that there could be enough shifting of the mesenehymal cells to carry the germ cells into position. Ameboid activity of the cells would seem to be the more plausible explanation, even though evidence for it is lacking.


An additional noteworthy feature of this embryo is the clustering of the germ cells ventral to the gut. This may indicate a multiplication in situ, and it is a fact i11 keeping with the observation that they tend to occur here in groups instead of being scattered as they are in the dorsal mesentery. Their segregation here, however, may simply be the result of their initial formation in a localized region of the early ovum. If the germ cells differentiated ina pre—somite stage, either from the entoderm or from the overlying mesoderm, in a restricted posterior area of the yolk sac ventral to the allantoic diverticulum, then the folding off of the hind gut from the yolk sac would bring this area into the mid line and ventral to the gut, where we actually find the germ cells aggregated. According to this interpretation, many of the germ cells have not yet left their site of origin, and only a few have reached the genital ridges; the majority of them at this stage being in the process of migration through the mesentery.


Literature Cited

FELIX, W. 1912 The development of the urinogenital organs. Kc-ibel and Mall, Human Embryology, vol. 2, pp. 752-979.

FUSS, A. 1912 Uber die Gc-sclileclitszelleii dos Menschen und (ler Siingetiere.

Arch. f. mikr. Anat., Bd. 81, S. 1-23.

RUBASCIIKIN, W. 1909 Uber die Urgeschlechtszellen bei Slsiugetieren. Anat. Ilefte, Bd. 39, S. 603-652.

VANNEMAN, AIMEE S. 1917 The early history of the germ cells in the armadillo, Tatusia novemcincta. Am. J. Anat., vol. 22, pp. 341-363.


Plates

Plate 1

All figures are unretouchecl photomicrographs, reproduced at a magnification of 1000 diameters.

2 Group of germ cells from the cluster ventral to the gut. Three cells are in sharp focus, the nucleus of a fourth was out of focus, but is indicated in the upper right hand corner of the group. An erythrocyte lies just below and to the left.

3 Germ cell in prophase stage, located in the mesentcry of the gut. Ooelom and mesenterial epithelium at left, two blood cells at bottom.

4 A germ cell wedged in the epithelium of the mesentery.

5 Two germ cells passing from the mesentcry into the genital ridge. Mescntery at right, genital ridge at left. c., coelom. .

6 Germ cell in the genital ridge. Mesentcry at right, part of a mesonephric glomerulus at left. Note the eosinophilic region, which photographs as black, to the right of the nucleus.

7 Germ cell in genital ridges. Note the eosinophilic region above the nucleus. At left, in the mesentery, a somatic cell in the usual type of mitosis.

8 Germ cell in the gut entoclerm. g., lumen of gut. Only a few germ cells are in thc entodcrm at this stage.

9 Aberrant type of mitosis, probably in a somatic cell. Such figures are not common, but may occur in various parts of the embryo. These cells should not be confused with the germ cells, however, as their cytoplasm does not become colorless until the onset of the metaphase stage. Compare with figure 3, which shows the clear cytoplasm of a germ cell in prophase.

Reference

Hamlett GWD. Primordial germ cells in a 4.5 mm human embryo. (1935) Anat. Rec, 61, 273-279.


Cite this page: Hill, M.A. (2019, August 20) Embryology Paper - Primordial germ cells in a 4.5 mm human embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Primordial_germ_cells_in_a_4.5_mm_human_embryo

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