Book - Contributions to Embryology Carnegie Institution No.71

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Finley EB. The development of the subcutaneous vascular plexus in the head of the human embryo. (1923) Contributions to Embryology Carnegie Institution No.71; 155-161.

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This historic 1923 paper by Finley describes vascular development in the head of the human embryo.

Scalp vascular plexus (subcutaneous vascular plexus) A vascular feature visible on the head surface during late embryonic development from Carnegie stage 20 (week 8, day 50). Its development and position was historically used to stage late Carnegie embryos.

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

The Development of the Subcutaneous Vascular Plexus in the Head of the Human Embryo

By Ellen B. Finley with 2 plates, 1 text-figure, pp 155-161 (1923).

Anatomical Laboratory of The Johns Hopkins University.


Links: Volume XIV | Contributions to Embryology | Cardiovascular Development | Head Development

Introduction

Text-figure 1. Diagrammatic sketch of the growing edge of the subcutaneous plexus in the head of the human embryo, showing the four zones of transition from undifferentiated mesenchyme into definite vessels. Processes from the angioblastic plexus can be seen encroaching upon the territory of the avascular zone; these are indicated by lighter shading.

When the work of the last ten to fifteen years is analyzed, it becomes clear that the fundamental problem of the vascular system is concerned with the origin and growth of endothelium, since the entire vascular system begins from specific cells which later develop into vessels. These essential cells of the vascular system are the angioblasts of His or the vasoformative cells of Ranvier. With the term "angioblast" was early associated the idea of His that complete differentiation of vasoformative cells takes place in the yolk-sac, these cells later invading the embryo itself. This idea had to be abandoned when it was proved that angioblasts differentiate within the body of the embryo. In a living preparation of a chick embryo the aorta has been observed by Dr. Sabin to differentiate in situ, and it now seems probable that many of the primary veins differentiate in the same way.


In a consideration of the origin and growth of endothelium, one of the most important points to be determined is the length of the period during which angioblasts continue to differentiate from undifferentiated mesenchyme. With such a consideration in mind, the study of the development of the vascular system of the head in the human embryo becomes significant. There are two main vascular plexuses to be observed in the head: (1) the meningeal, which first appears in embryos of about 4 mm., and (2) the subcutaneous plexus, which appears at about the 20-mm. stage. There is thus a marked difference in the extent of differentiation of the embryo itself. From the meningeal plexus develop the vessels of the central nervous system, the dorsal sinuses, and the vessels of the skull, while from the subcutaneous plexus develop the vessels of the skin and of the head-musculature. The two unite in common with the vessels of the neck, but on the sides and vault of the head the two systems are completely separated by the developing membranous skull. The subcutaneous plexus, being thus isolated and spread out as a thin sheet that can be examined in a total mount, constitutes a particularly valuable field for study of angioblastic differentiation in this relatively late period of embryonic life.


The material used for the study of the subcutaneous plexus of the head consisted of serial sections of human embryos in the Carnegie Collection and of total mounts of skin flaps from the area of the head plexus. Before making the dissection, the fixed embryo was studied in the gross, whenever possible, and the general plan of the plexus was determined. Several small skin flaps were then carefully dissected from the sides of the head and examined, first unstained and later after staining. In several instances tangential serial sections were cut. The principal stains used for the total mounts were alum cochineal, and hematoxylin, either in combination with eosin, aurantia, and orange G or with eosin alone. Wright's blood-stain was used for some of the tangential sections. Total mounts were found to have distinct advantages, since they afforded an opportunity to study a portion of the vascular spread with the vascular elements intact and with their normal relations preserved, in contrast with serial sections in which the fields were necessarily discontinuous and comparatively limited.


In the gross examination of human embryos ranging from 19 to 45 mm., there could be observed a delicate, fringe-like plexus pushing up toward the vertex of the head, first described by Hochstetter (1916). It was always visible, but in some embryos it had a much more brilliant appearance than in others, due, possibly, either to the stage of development of the plexus at that particular time or to the fixing fluid used. It was most striking in an embryo that had been fixed in Bouin's solution, which is probably much better for this purpose than formalin. At earlier stages the transition from vascular to avascular conditions is more gradual. In later stages it is more abrupt and the transitional margin is in the nature of a narrow, well-defined line. The early stage is particularly well shown in plate 1, figure 2, where there appear to be two prominent foci of growth or growth centers, one anterior to the ear in the temporo-frontal region and the other posterior to the ear in the occipital region. The growth of the vessels radiates up and out from these centers. On the borders of these two semicircular areas, small, finely granular tips can be seen, a few of which seem to have no connection with the larger vessels below. The growing edge, as it advances, tends to become more and more flattened, as may be seen in figures 3 and 4, a slight indentation, however, persisting at a point almost directly above the anterior portion of the external ear. At about 40 to 45 mm. the growing tips anastomose across the mid-line and circulation over the head is established.

Finley1923 fig02.jpg Finley1923 fig03.jpg Finley1923 fig04.jpg
Fig. 2. Photograph of a human embryo 23 mm. in length (No. 966), showing the vascular plexus in the subcutaneous tissue of the head in its earliest form. It is characterized by two distinct growth centers, the temporofrontal and the occipital, from which the vessels gradually spread over the apex of the head. A sharply defined area between the two growth centers constitutes an angle of retarded growth. Fig. 3. Photograph of a human embryo 27.5 mm. in length (No. 2561), showing a later stage of the plexus. The angle of retarded growth is not as prominent and the margin of the plexus appears as a narrower and more well-defined line than that in figure 2. Fig. 4. Photograph of a human embryo 36 mm. in length (No. 1591), showing a late stage in the closing in of the plexus.


On microscopic examination of total mounts from the head region, four stages in the development of the blood-vessels were observed. Figure 1 shows diagrammatically four definite zones. First, toward the vertex, is the uppermost zone, which is a predominantly avascular area composed of undifferentiated mesenchyme. The zone next below consists of a network of solid, darkly staining masses of nucleated cells filled with hemoglobin. Toward their upper borders these masses often have slender tips which penetrate the avascular area. Between and beyond the tips stretches indifferent mesenchyme. This second zone may be called the zone of the angioblastic net. The third zone is a capillary network and in it can be seen delicate, branching capillaries whose endothelial walls appear to be intact and to inclose a definite lumen. Within the lumina of these vessels are scattered clumps of well-formed blood-cells (nucleated and non-nucleated), whose outlines are clearly defined. Occasionally the lumen is practically empty (plate 2, fig. 10), the most probable explanation for which is that liquefaction of cellular elements has taken place within the blood-vessels themselves, assuming that this area has been transformed from the solid zone just described. Finally, in the last zone are encountered more mature vessels, with slightly thickened walls, through which blood has evidently circulated to some extent. Some of these vessels may be forerunners of vessels destined to persist. These zones are the expression of a developmental process, and in the growing state the characteristic elements of one zone must become quickly transformed into those of the more mature zone adjoining it. Thus, in any given preparation there is a consecutive picture of the life history of a blood-vessel, from the earliest stage to maturity, from undifferentiated mesenchyme, through angioblast and capillary, to a fully formed vessel.


The second zone (that of the angioblastic net) is particularly interesting, not only because it represents the area of actual new growth, but also because of its possible significance in connection with the relation of red blood-cells to endothelium. Plate 2 (figs. 7,8,9, and 11) shows a few of the varied forms which this plexus assumes. Some of the tips are club-shaped, some thick at the center with two side extensions, like tiny branches on a tree, some so vaguely outlined distally as to seem to merge directly into the mesenchyme of the avascular area, while others, slender and long, are drawn out into a fine filamentous point. The cells of this zone are all nucleated and, for the most part, contain a considerable amount of hemoglobin. Those at the extreme tips contain less, while in a few cells the cytoplasm is entirely colorless and translucent (fig. 9). The cell boundaries are not clear-cut, and the cells vary greatly in shape and size, due to their pressure against each other. In this area there are indications of a very massive transformation of mesenchyme into red blood-cells. In occasional instances the cellular masses are edged by long endothelial cells, but for the most part they are entirely composed of the earliest forms of red blood-cells, then rounded contours marking the boundaries between the angioblastic net and the avascular zone. It is obvious that this is not exactly the process by which it has been demonstrated that red blood-cells arise in the chick, because it can not be stated that these cells originated within the lumen of a vessel (Danchakoff, 1908; Sabin, 1920). On the other hand, it can not be said that these observations indicate a diffuse extravascular origin of red blood-cells that would subsequently have to migrate into preformed vessels, such as Maxim ow (1909) believes characterize the late origin of red blood-cells in the mammal. Rather, the process seems somewhat intermediate between these two positions, the cells clearly arising in a definite relation to the vascular system, not quite independently.


At the border between the first and second zones are occasionally small clumps of cells which have no visible connections with the main plexus. Plate 2 (figs. 12 and 13) shows some of these isolated clumps. They most frequently occur as single chains of nucleated cells containing a slight amount of hemoglobin and often he in direct fine with the advancing plexus, though not continuous with it. Sometimes they are seen as solid clumps of cells, with fine, thread-like processes extending out from them, strongly suggestive of those described by Dr. Sabin in the two-day chick. She found a marked tendency on the part of syncytial masses of angioblasts to put out delicate sprouts by which they joined similar masses, thus developing the vascular plexus. Since most of these isolated chains and clumps of cells contain hemoglobin, they might easily be regarded as indicating the origin of red blood-cells from mesenchyme outside the vascular system, but when their proximity to the main plexus is considered, together with the probability of their joining it to form solid cellular masses, as has been described, their position and their hemoglobincontent do not seem to militate against an angioblastic origin for red blood-cells and endothelium. It seems quite clear that this process is intermediate between the two extreme views.


There are, it seems, at least three possible explanations for the development of the vascular area in the subcutaneous tissue of the head of the human embryo. First, it is possible to conceive of the tips of the vessels forcing their way into and through the undifferentiated tissue, taking nothing from it, but pushing the mesenchymal cells aside as they advance by means of their own active cellular division and growth. One would expect, under such conditions, that when the sections of these areas are fixed, the vessels would shrink, leaving in their place a hollow space. This has never been noted, nor have the surrounding mesenchymal cells a compressed appearance. A second possibility is that the vessels lengthen by true endothelial division and sprouting. Figures 7 and 8 (plate 2) are suggestive of such a process, but they are the exception rather than the rule, since there appears to be a great enlargement of the vascular tips, due to a marked differentiation of mesenchyme into red blood-cells, before many endothelial cells are clearly differentiated. Another conception is that the tips of the growing plexus exert just the stimulus needed for the mesenchymal cells lying close to them to differentiate into angioblasts or primitive blood-cells and to become joined to the tips. From observation of many different specimens, the impression has been gained that this last is the principal method of growth. The cells may be added one by one, or they may form single strands before adding themselves to the main plexus. Either before or after becoming a part of the plexus, it is probable that they quickly divide and grow, taking on the appearance of solid masses of cells of varied size and shape. The fact that the mesenchymal cells differentiate in such a precipitous manner into hemoglobin-containing red cells is doubtless to be explained by the relatively late stage of embryonic development at which the differentiation occurs.


In closing, I should like to say that this problem was suggested to me by Dr. Sabin, and I am greatly indebted both to her and to Dr. Streeter for helpful advice and assistance throughout the course of the work.

  1. In this paper evidence is presented which shows that the growing edge of the subcutaneous vascular head plexus in human embryos at about the end of the second month is still in the angioblastic stage, and consists of a plexus of cells rather than a plexus of vessels.
  2. The particular area studied was an interesting one for observation of the relation of red blood-cells to endothelium. Such an area is obviously simpler than adult bone-marrow, and though no distinctly angioblastic phase was noted intermediate between mesenchymal cells and red blood-cells, the origin of the red bloodcells seemed in direct relation to an advancing vascular zone. These observations indicate the origin of red blood-cells by a process somewhat between an intravascular development and an extravascular development, with subsequent entry of the cells into preformed vessels.

References Cited

Danchakoff, V, 1908. Untersuchungen iiber die Entwicklung des Blutes und Bindegewebes bei den Vogeln. 1. Die erste Entstehung der Blutzellen beim Huhnerembryo und der Dottersack als Blutbildenes Organ. Anat. Hefte, vol. 1", p. 471.

Hochstetter, F., 1916. Uber die Vaskularisation der Haut des Schadeldaches menschlicher Embryonen.

K. Akad. d. Wiss., Wien, Math.-Naturwiss KL Bd. 93. Maximow, A., 1909. Untersuchungen iiber Blut und Bindegewebe. 1. Die friihesten Entwicklungsstadien der Blut- und Bindegewebszellen beim Saugetierembryo, bis zum Anfang der Blutbildung in der Leber. Arch. f. mikr. Anat., vol. 73, p. 444.

Sabin, F. R., 1920. Studies on the origin of blood-vessels and of red blood-corpuscles as seen in the living blastoderm of chicks during the second day of incubation. Contributions to Embryology, vol. 9, Carnegie Inst. Wash. Pub. 272.

Description of Plates

Plate 1

Fig. 2. Photograph of a human embryo 23 mm. in length (No. 966), showing the vascular plexus in the subcutaneous tissue of the head in its earliest form. It is characterized by two distinct growth centers, the temporofrontal and the occipital, from which the vessels gradually spread over the apex of the head. A sharplj defined area between the two growth centers constitutes an angle of retarded growth. X4.

Fig. 3. Photograph of a human embryo 27.5 mm. in length (No. 2561), showing a later stage of the plexus. The angle of retarded growth is not as prominent and the margin of the plexus appears as a narrower and more well-defined line than that in figure 2. X4.

Fig. 4. Photograph of a human embryo 36 mm. in length (No. 1591), showing a late stage in the closing in of the plexus. X2.

Fig. 5. Photograph from a total mount of a piece of the scalp from a human embryo 28 mm. in length (No. 1240a). The varied forms of the growing tips are well shown and the transition from the angioblastic net to the capillary net can easily be followed. X80.

Fig. 6. Photograph from another portion of the same section as above, showing, under higher magnification, the first and second zones. In the center a long tip from the angioblastic plexus is seen to penetrate the avascular zone. This represents the first step in the differentiation of the mesenchyme into angioblastic tissue. X 150.

Finley1923 Plate 1.jpg

Plate 2

Fig. 7. Drawing of a growing tip, showing red blood-cells as they first appear, seen at the edge of the head plexus in a human embryo 28 mm. in length (No. 1240a, total mount). The club-shaped cellular mass has an indefinite connection with the main angioblastic plexus. X930.

Fig. 8. Drawing of a growing tip at the edge of the head plexus in a human embryo 23 mm. in length (No. 966). Several well-defined endothelial cells can be made out at the edge of the angioblastic strand, and there is a fine filamentous strand at the extreme tip, which appears to be an endothelial process. X930.

Fig. 9. Drawing of a growing tip at the edge of the head plexus in a human embryo 28 mm. in length (No. 1240a, total mount). Two cells with clear, colorless cytoplasm may be observed. X930.

Fig. 10. Drawing of a capillary from the third zone of the head plexus in a human embryo 19 mm. in length (No. 431). The capillary is seen to be empty save for three nucleated red blood-cells. X930.

Fig. 11. Drawing of a typical growing tip at the edge of the head plexus in a human embryo 26.4 mm. in length (No. 1008). X930.

Fig. 12. Drawing of a strand of early red cells, containing a slight amount of hemoglobin, and having no apparent connection with the main angioblastic plexus. Taken from a total mount of the scalp of a human embryo 23 mm. in length (No. 1358/, total mount). X930.

Fig. 13. Drawing of a chain of early red cells, similar to that seen in figure 12, showing no connections with the main angioblastic plexus. Taken from a total mount of the scalp of a human embryo 23 mm. in length (No. 1358/, total mount). X930.

Finley1923 Plate 2.jpg


1923 Head Subcutaneous Plexus: Plate 1 | Plate 2 | Fig 1 | Fig 2 | Fig 3 | Fig 4 | Fig 5 | Fig 6 | Fig 7 | Fig 8 | Fig 9 | Fig 10 | Fig 11 | Fig 12 | Fig 13 | Carnegie No.71 | Historic Disclaimer
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Reference

Finley EB. The development of the subcutaneous vascular plexus in the head of the human embryo. (1923) Contributions to Embryology Carnegie Institution No.71; 155-161.


Cite this page: Hill, M.A. (2018, December 15) Embryology Book - Contributions to Embryology Carnegie Institution No.71. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Contributions_to_Embryology_Carnegie_Institution_No.71

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Cite this page: Hill, M.A. (2018, December 15) Embryology Book - Contributions to Embryology Carnegie Institution No.71. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Contributions_to_Embryology_Carnegie_Institution_No.71

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© Dr Mark Hill 2018, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G