Paper - Histogenesis of the human aorta
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Jackson JL. Histogenesis of the human aorta. (1935) Amer. J Anat. 56(2): 306-
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Histogenesis of the Human Aorta
Joseph Leonard Jackson
Manitoba Medical College, Winnipeg, Manitoba, Canada
THREE runs (SIXTEEN maunns)
Age changes in the human aorta are very marked and a knowledge of them is necessary for the interpretation of its histological form. The vessel at birth has already acquired a complex structure, so that it is essential to consider the histo genesis of the organ if We are to comprehend the origin and relations of its various parts.
Materials and Method
For this investigation twenty-ﬁve human embryos and fetuses were used with the following crown-rump measurements in millimeters: 6, 17, 20, 24, 25, 35, 50, 85, 88, 90, 98, 101, 103, 110, 115, 120, 125, 130, 145,150, 160, 167, 170, 175, 195.
In the case of the fetuses, the thorax and abdomen were opened by a mid-line incision and a portion of the thoracic and abdominal aorta Was removed and placed in 5 per cent formalin. The embryos were also placed in 5 per cent formalin and when ﬁxation was completed the specimens were divided by a transverse incision just below the fore limbs, and from the abdominal portion sections were made of the aorta by means of the ordinary paraffin procedure.
Three of the more common methods of staining elastic tissue were tried: i.e., Weigert’s, orcein, and phosphotungstic acid haematoxylin. A modiﬁcation of Weigert’s method was found to be the most satisfactory. The length of time the tissues were left in the Weigert’s staining solution was considerably increased. As a counterstain, Heidenhain’s ‘azan’ stain was used. This Weigert-azan method gives a very clear differentiation of the tissues. Mature elastic ﬁbers are colored a deep violet to black, collagenous ﬁbres stain a dark blue, and all nuclei a carmine red.
Observations and Discussion
Evans (’12) states that the aorta was present as a deﬁnite structure in the Eternod embryo which measured 1.3 mm. in length. He describes the vessel as an endothelial tube, extending from the region of the heart to the chorion. According to Bremer (’24), this simple condition of the wall persists for a relatively long time in embryonic life.
A section of the Wall of the aorta of a 6-mm. embryo is shown in ﬁgure 1. It consists of a single row of endothelial cells. The endothelial nuclei are oval in shape, rather closely packed, and project into the lumen of the vessel. They are similar in appearance and in their staining reaction to those of the surrounding mesenchyme. The cytoplasm, like a delicate membrane, forms an unbroken line along the outer surface of the cells. The mesenchyme cells in immediate contact with the endothelium are similar in appearance and in arrangement to those more remotely situated, and do not seem to be taking any part in the formation of the vessel wall.
Regarding the source of vascular endothelium, there is still some doubt. According to Evans (’12), the position of the vascular anlagen-in the yolk sac does not preclude the possibility of the endothelial cells arising from the entoderm. It would seem, however, more probable that it is a product of the mesoblast. The very close relationship existing between embryonic endothelium and ﬁbroblasts has been pointed out by many recent workers. Berge (’24), observing the development of endothelial and connective tissue cells in the chorion, came to the conclusion that they are similar, both physiologically and morphologically. He describes endothelial cells as circularly arranged ﬁbroblasts. Maximow (’25), Horning and Richardson (’31), Silberberg (’29, ’30), in their experiments with developing vascular endothelium in tissue culture, all agree that proliferating endothelial cells become‘ indistinguishable from ﬁbroblasts. Heringa (’23) states, “it is now Well known that endothelial cells are purely connective tissue cells united by the ﬁne protoplasmic ﬁbres.”
The next step in the histogenesis of the aorta may be designated as the mesenchymatous stage. It is illustrated in ﬁgure 2, which was drawn from a section of the abdominal aorta of a 17-mm. embryo. The endothelial layer is similar to that described above, but the behavior of the mesenchyme cells is striking. The cells in the neighborhood are no longer indiﬁ”erent to the presence of the vessel, for they have altered in shape and become orientated in a regular manner. The round or oval nuclei of the mesenchyme have lengthened and form 3 to 4 fairly annular rows about the lumen of the vessel. These cells will form the media. They constitute a primitive connective tissue (McGill, ’08), the cells of which are rapidly dividing and by diﬁerentiation produce ﬁbroblasts, elastoblasts, and myoblasts; all very similar in appearance, but different in many fundamental respects.
The intercellular substance contains ﬁne, short ﬁbrils coursing in an annular fashion about the vessel. They stain a grayish color with Weigert’s elastic tissue stain, and consist for the most part of the cytoplasmic processes of the cells. They are not true intercellular ﬁbres, and have neither the character nor the appearance of elastic or collagen tissue.
According to Bremer (’24), mesenchymal diﬁerentiation appears at widely diﬁerent periods in different types of embryos and in different arteries; and is usually accompanied by a relative or actual narrowing of the vessels as the circulation becomes more deﬁnite. This variation in the rate of growth in the same and in different embryos of similar age was stressed by Masloff (’14) and accounts to some extent for the lack of agreement which exists between different observers in respect to the ﬁrst appearance of various tissues and structures which are concerned with the formation of embryonic organs.
A further stage in the development of the aorta has been reached when the embryo has become 20 mm. in length. Figure 3 is a. camera lucida drawing of a transverse section of such a vessel stained with haematoxylin and eosin. The thickness of the wall has increased greatly. This is not due to changes in the intima, which has scarcely altered, but to differentiation of the media and to the appearance of the adventitia. The media is considerably thicker than the entire wall of the 17 -mm. stage, due to an increase in the number of cells, which have become arranged into an outer and an inner stratum. The outer layer consists of 5 to 6 annular rows with a rather well-marked network of poorly stained ﬁbres surrounding the cells. The inner is a loose layer, consisting of irregularly shaped cells, with round, polygonal or rod-shaped nuclei, very irregularly arranged. This is a narrow zone, not more than two or three cells in thickness. A comparison of this latter section with that from which ﬁgure 2 was drawn shows that the outer annular layer is identical with the media of the 17-mm. aorta, whereas the inner irregular layer is something new. That this is not an accidental appearance is quite clear, because it persists, though not always with equal clarity, throughout the later periods of embryonic life. The irregularity of shape and orientation of the nuclei indicates that they have been newly produced. This is a formative layer, and is comparable to the epiphyseal line in a developing bone. Its cells have not yet conformed to the forces of the circulating blood, which will cause them to be arranged in the transverse axis of the vessel wall. The prominence of this layer decreases with the reduction of muscle formation in the media.
Sections of the aorta at this period, stained with Weigertazan, exhibit developing elastic ﬁbres (ﬁg. 4). The elastic elements appear as very ﬁne ﬁbrils or membranes (Krampecher, ’31), and form a very delicate network. The inner subendothelial portion, where the internal elastic lamina will develop, is not more distinct than the outer part. Indeed, it seems better developed in the outer part, where the cells are more closely packed. This effect may be due either to the fact that in this position the ﬁne ﬁbrils are pressed together, and appear coarser, or, as pointed out by Masloff (’14), the ﬁrst elastic tissue to be elaborated is formed in the middle of the media—therefore more deve1oped—and not directly under the endothelium, as has been claimed by others.
The outer part of the vessel wall is formed by a deﬁnite adventitia, which is distinct from the surrounding mesenchyme. Its cells are elongated and arranged in a circular manner. They are similar in appearance to the media, but they are more scattered. The primitive collagenous network is more pronounced, and is continuous with that of the media. A clear line of demarcation between the media and the adventitia is absent. Certain round spaces which occur in the adventitia are not more than 20 u in diameter. They are surrounded by one or two ﬂattened cells and have the appearance of capillaries. They are interpreted as being primitive vasa vasorum.
The question of the origin of connective tissue ﬁbres is still unsettled. A good summary of the various points of view is to be found in Maximow and Bloom (’30). The common opinion that collagenous ﬁbres appear much earlier than elastic ﬁbres in embryonic development i not true of the aorta where the development of elastic tissue is decidedly precocious. .
The early opinion that connective tissue ﬁbres were produced directly by the cell has been shown by recent workers to be inexact. The studies of Wolbach (’33) indicate that the formation of collagenous ﬁbres is preceded by a precollagenous matrix, which he describes as “homogeneous, resembling lightly stained amyloid, and therefore not of great density.” This stage was of short duration and was followed by the appearance of ﬁne collagenous ﬁbres which rapidly increased in size and numbers. Further, the work of Kerwily (’24) demonstrated that the development of elastic ﬁbres was preceded by the production of a pre-elastic substance which did not contain elastin and therefore was stained by Weigert’s solution. This ‘pseudo-elastica.’ Wollf (’28), is stained by the silver method of Cajal and appears as a granular element in the cytoplasm of the cell. It is only after it has been elaborated on the surface of the elastoblast and the granules have united to form ﬁbres or membranes, that the tissue shows an affinity for elastic stains.
Sections of the aorta of 24- and 29—mm. fetuses do not exhibit the appearance of any new structures. The elastic tissue is becoming more deﬁnite in outline; the various tunics are undergoing a general increase in size, and it is not until one reaches the 35-mm. stage that any marked developments have taken place.
A camera lucida drawing of a section of the abdominal aorta of a 35-mm. fetus stained with haematoxylin and eosin is shown in ﬁgure 5. The lumen of the vessel is ﬁlled with developing blood cells. The endothelial cells of the tunica intima are not so closely packed as in the earlier stages of development. The inner layer of the media is noteworthy because of the irregularity of its cells, which have the appearance of myoblasts. It is not as clearly marked off from the deeper layers as in the 20-mm. embryo. The outer layer of the media has increased in thickness and consists of rather closely packed, elongated cells lying in the transverse axis of the vessel. The adventitia consists of primitive ﬁbroblasts and ﬁbres. In it occur some thin walled vasa. vasorum which do not contain blood cells.
An examination of sections of the same vessel stained with Weigert-azan shows that another noteworthy advance has been made. Figure 6 is a camera lucida drawing of the elastic network. The thickening of the subendothelial portion to form a primitive internal elastic lamina is striking. In some places the endothelial cells seem to rest directly upon the elastic membrane; in others, they are separated from it by ﬁne elastic ﬁbres, which form the innermost part of the network. In the inner part of the media the network consists of ﬁne broken ﬁbres which anastomose with each other and enclose rather large spaces which are occupied by the young muscle cells of the inner layer. In the outer part, where the cells are more compact, the elastic network is better formed; the outermost ﬁbers of which are lost in the adventitia.
Opinions regarding the source of the aortic musculature and the time of its appearance varies greatly. Muller (1888) studied the origin of the smooth muscle of the aorta in the chick and came to the conclusion that it arose from the neighboring mesothelium and migrated into the vessel wall. Dubreuil and Lacoste (’30) studied the diﬁerentiation of muscle in the tunica media of arteries and maintained that at the stage of 46 mm. the mesenchyme cells elongate and pass into the form of promyeloblasts. When the fetus has become 160 mm. in length, true muscle cells occur between the elastic lamellae. They claim for this muscle two sources of origin: in the ﬁrst place, from mesenchyme cells in situ—interstitial growth; and, in the second place, from undifferentiated mesenchyme cells situated outside of the vessel, which glide into the media. These produce an appositional growth.
Hewer (’27) observed in large blood vessels of a 10-mm. human embryo ‘elongated cells’ arranged in an annular manner about the lumen of the vessel which had the appearance of primitive muscle. In an embryo of 12 mm. this author claimed that these cells are present in all those situations where smooth muscle will develop.
That the smooth muscle of the aortic wall does arise from undifferentiated mesenchyme cells seems to be fairly universally accepted. Regarding the position of these cells and the place where differentiation takes place, there is no agreement. That smooth muscle may develop from undiﬁerentiated mesenchyme cells in any situation in the aortic wall must be conceded. My own observations have led me to the conclusion that the main source of such muscle is the inner cellular layer of the media, just beneath the internal elastic lamina. As new cells are produced they are displaced toward the outer layers, and subsequently become attached to the elastic lamellae. 312 JOSEPH LEONARD JACKSON
The stages of development which follow are marked by a rapid increase in the thickness and complexity of both the intima and media. The adventitia remains relatively thin and simple in its structure. In a section of the aorta of a 90—mm. fetus stained with Weigert-azan the media has the appearance of a compact tunic consisting of twenty to thirty deeply stained elastic lamellae. The interlamellar spaces are ﬁlled with smooth muscle cells, ﬁne elastic elements which branch and anastomose, and a delicate network of collagenous ﬁbres.
The intima is also much advanced from the 35-mm. stage. The internal elastic lamina is a very prominent structure, as is shown in ﬁgure 7, which is a camera lucida drawing of the intima and the inner elastic ﬁbres of the media. The endothelial layer consists of round or egg—shaped cells, which are now rather sparsely placed. They appear to be enveloped by a very delicate membrane which is united to the internal elastic lamina by ﬁne protoplasmic processes. Between these delicate pillars spaces occur, which appear empty in the section. Figure 8 represents the inner part of the aorta of a 101-mm. fetus which is cut obliquely, and shows the subendothelial structure as a lightly stained fenestrated membrane. As growth proceeds it colors more deeply, due presumably to an increase of elastin in its substance.
A notable feature in the further development of the intima is the increase in elastic tissue. Figure 9 is a camera lucida drawing of the intima of the thoracic aorta of a 150-mm. fetus. It shows the internal elastic membrane as a strongly waved lamella about 3 p in thickness. The endothelial layer has been lost from the section and an intercellular substance seems to have separated the endothelial layer from the internal elastic membrane. In this ground substance the cut ends of longitudinal elastic ﬁbres are clearly shown. These ﬁbres lie close together and some of them by fusing form delicate membranes. According to Dubreuil and Lacoste (’30 b), all elastic membranes are formed in this way. In the matrix of the intima white connective tissue ﬁbre and muscle msrrocnsnsrs on cells also appear. Figure 10 was drawn from a transverse section of the aorta of a 170-mm. fetus. In it the elastic ﬁbres of the intima are so arranged as to constitute a deﬁnite layer, usually known as the longitudinal elastic layer of the intima. A layer of collagenous ﬁbres, containing a few muscle cells, all interwoven by a ﬁne elastic network constitutes the socalled musculo-elastic layer. The endothelium has been destroyed in this portion so that the intima appears without this covering.
Opinions diifer regarding the origin and the time of appearance of the elastic elements of the intima. According to Stumpf (’14), the diﬁerentiation of the musculo-elastic layer begins at the end of the ﬁrst year of life. Wolﬁ (’28), working on the common iliac artery, claimed that in the ﬁrst month after birth the internal elastic lamina consists of a single layer of elastic tissue directly in contact with the endothelium. Jores, reported by Wolff (’28), was of the opinion that the newly formed elastic tissue of the intima arose by a splitting of already formed elastic tissue. According to the same author, Heubner maintained that the endothelium has the faculty of building elastic tissue. Wolff (’28) doubts that the newly formed elastic elements of the intima are formed at the expense of the internal elastic lamina, for, in the ﬁrst place, at the point of apparent division of the lamina, its thickness is not diminished; and, in the second place, the newly formed tissue stains differently from the old. To the newly formed element Wolff gave the name of pseudo-elastica.
It must be conceded that the internal elastic lamina does undergo considerable splitting even before birth. Figure 11 is a camera lucida drawing of a transverse section of the internal elastic membrane of the aorta of a 120-mm. fetus which has apparently split. These so-called split off portions become incorporated in the tunica intima, but they do not form any considerable amount of its elastic component, for ﬁgure 10 shows a Well formed longitudinal elastic layer present before any extensive changes in the internal elastic lamina have taken place. My observations lead me to the conclusion that a certain amount of pre—elastic substance is elaborated in the ground substance of the intima, from which the elastic ﬁbres are formed in situ, and by means of which they decrease in size.
The internal elastic lamina which has been described develops more" rapidly than the elastic elements of the media. By the time the fetus reached a length of 80 mm. the original elastic network has been reinforced by the differentiation in it of twenty to thirty strongly waved elastic lamellae. Figure 12 is a camera lucida drawing of an obliquely cut longitudinal section of one of the lamellae of the media, from the aorta of a 150-mm. fetus. It shows that the lamellae consist of anastomosing ﬁbres of different sizes, held together by a membranous elastic substance which does not stain so intensely as the ﬁbres. Numerous fenestra of various sizes appear between the ﬁbres. The total number of elastic lamellae which are met with in the adult artery are present in a fetus of 220 mm. (Dubreuil, ’30).
The muscle element is clearly shown in a fetus of 170 mm. in length (ﬁgs. 13 and 14). The muscle cells of arteries are usually referred to as being short and branching (Maximow and Bloom, ’30) as shown in ﬁgure 14, but that they may be fusiform is demonstrated in ﬁgure 13. Benninghoff (’27) has described two kinds of muscle in arteries; spann-muscle, which is characteristic of the aorta and ring-muscle, which is found in small and medium sized vessels. The muscle cells in the aorta lie obliquely between the elastic lamellae. Their mode of attachment is very difficult to determine. In some cases the myoﬁbrillae seem to attach to knob-like formations of the lamellae (ﬁg. 13) ; in others, they seem to pass into the substance of the membranes. In embryonic life, the muscle cells lie close together between the elastic lamellae, but in adult life they are relatively far apart, due to the increase of white ﬁbrous tissue.
The adventitia is relatively wider in the embryonic aorta than in the adult. At the 20-mm. stage it is mesenchymal in appearance, but in the 35-mm. fetus, the cells have the shape of ﬁbroblasts, and the white ﬁbres are more mature. The elastic element does not appear in the adventitia until the 90-mm. stage. It develops rapidly so that in the 101-mm. fetus it forms thin layers of longitudinal ﬁbres. Muscle cells appear later, but are never an important constituent of the adventitia.
In the 35-mm. fetus the vasa vasorum are thin walled endothelial tubes. In the 101-mm. stage (ﬁg. 15) they have walls of considerable thickness, and the arteries can be distinguished from the veins. This difference is much more pronounced in the 17 5-mm. fetus (ﬁg. 16).
According to Masloif (’14), elastic ﬁbres do not occur in the adventitia of the aorta until the fetus is 95 mm. in length and vasa vasorum are not found until the 125-mm. stage.
Further progress in development is characterized by an increase of the structures already laid down. The intima does not undergo any very extensive development until after birth, so that in the newborn it appears as a thin ﬁbro-elastic layer containing in its deeper portion a few scattered muscle cells. The internal elastic lamina in the ascending aorta of the newborn has already undergone a considerable amount of splitting, but in the thoracic and abdominal portions it appears as a thickened lamella.
The constituents of the media continue to increase in amount. At birth the elastic lamellae lying near the adventitia are placed close together, because the interlamellar tissue is less in the outer part of the media. This is especially true of the muscular element, which is proportionally greater in the inner layer of the media.
As the intima and the media continue to develop the adventitia seems to remain more or less stationary. In the ascending aorta of the newborn it forms an inconspicuous tunic, but in the thoracic and abdominal portions it is better developed. It consists for the most part of white ﬁbrous tissue, containing within its meshwork longitudinal elastic ﬁbres, which lie side by side, forming thin layers. These longitudinal ﬁbres are held together by ﬁne transverse strands and rarely coalesce to form membranes. The vasa vasorum course in the adventitia and ﬁnally penetrate the outer layer of the media, where they branch freely and form a vascular stratum. This condition persists throughout adult life.
- The materials used consisted of twenty-ﬁve embryos and fetuses between 6 mm. and 195 mm. in length.
- Five per cent formalin was used as a ﬁxative and Weigert-azan as a routine stain.
- The aorta of a 6-mm. embryo is an endothelial tube, but in the 17 mm. stage the surrounding mesenchyme takes part in the formation of the vessel wall.
- Recent work on the nature of vascular endothelium shows that it is very similar to connective tissue and probably has a mesoblastic origin.
- In the 20 mm. embryo the three tunics of the vessel are present. The intima consists of a layer of simple endothelium; the media, of an inner and an outer stratum of cells, and a delicate network of elastic ﬁbres; the adventitia, of primitive ﬁbroblasts orientated in the transverse axis of the vessel. Capillary-like vasa vasorum appear in the adventitia.
- The question of the origin of connective tissue ﬁbres is discussed. It is claimed that they are not formed directly from cells, but are elaborated out of a more primitive substance, secreted by the cells.
- In the 35-mm. stage the internal elastic lamina appears as a thickening of the inner part of the elastic network of the media. The inner cellular layer of the media consists of developing myoblasts.
- The problem of the source of the aortic musculature is discussed. It is claimed that the main source of such muscle is the inner layer of the media.
- In the 90-mm. stage twenty to thirty elastic lamellae are present in the media, separated by smooth muscle, white ﬁbrous tissue and a ﬁne elastic network. The intima is the site of the formation of a pseudo-elastica, which is formed from a pre-elastic substance, but has the appearance of a reduplication of the internal elastic lamina.
- In sections of the aorta of 150-mm. and 170-mm. fetuses may be observed the development of longitudinal elastic ﬁbres in the intima, which form the ‘longitudinal elastic layer.
- Opinions regarding the formation of the elastic element of the intima are discussed. The View is advanced that they are formed for the most part from a pre-elastic substance produced in the intima by cells, and that they are not developed exclusively at the expense of the internal elastic lamina.
- The laminae of the media are membranes, consisting of elastic ﬁbres which vary greatly in size, held together by a homogeneous elastic substance, and are separated by many fenestra.
- The muscle cells of the media are either ﬂattened platelike elements, or fusiform bodies similar to smooth muscle cells elsewhere. They lie obliquely between the lamellae and attach to them by means of their myoﬁbrillae.
- Development to birth is characterized by-a growth of the structures described.
We wish to acknowledge our indebtedness to the Banting Research Foundation for a generous grant which made possible this research; to Miss Wilma Service who carried out all of the technical procedures; and to Prof. R. G. Inkster for his suggestions and criticism.
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Explanation of Plates
All of the drawings were made with the aid of the camera lucida The scale which is reproduced with each ﬁgure was drawn from a micrometer slide with the camera lucida. All of the plates have been reduced 13 off.
adv., tunica adventitia long-el-1., longitudinal elastic lamina e-f., elastic ﬁbres mes., mesenchyme
end., endothelial layer n., normoblast
int.-el-l., internal elastic lamina o-m., outer layer of the media
i-m., inner layer of the media ps-e1., pseudo-elastic‘ membrane. long-el—f., longitudinal elastic ﬁbres r—b-c., red blood cells
1 Transverse section of the aortic wall of a 6-mm. embryo stained with haematoxylin and eosin.
2 Transverse section of the abdominal aorta of a 17-mm. embryo stained with haematoxylin and eosin. Note the modiﬁcation of the mesenchyme under the endothelium.
3 Transverse section of the wall of the aorta of a 20-mm. embryo stained with haematoxylin and eosin. Note the arrangement of the cells in the inner (i-m) and the outer (o-m) parts of the media.
4 Transverse section of the wall of the ascending aorta of a 20-mm. embryo stained with Weigert-azan. Primitive elastic ﬁbres (e-f) appear throughout the media.
5 Transverse section of the wall of the abdominal aorta of a 35-mm. fetus stained with haematoxylin and eosin. Note the well-marked inner layer of the media (i-m).
6 Similar to ﬁgure 5 only stained with Weigert-azan. Note the thickening of the inner part of the elastic network to form the internal elastic lamina.
7 The intima and inner part of the media of a transverse section of the aorta of a 90-mm. fetus stained with Weigert-azan. Note the endothelial layer rests upon a delicate elastic formation (ps-el) which is attached at intervals to the internal elastic lamina.
8 Similar to ﬁgure 7, but from a 101-mm. fetus, to show the thickening of the pseudo-elastice (ps-el) into a fenestrated membrane.
9 The tunica intima of the aorta of a 150-mm. fetus stained with Weigert-azan. Note the cut ends of the longitudinal elastic ﬁbres (1-el-f). The endothelial layer has been lost.
10 The tunica intima and inner part of the media of a transverse section of the aorta of a 170-mm. fetus stained with Weigert-azan. Note the formation of a deﬁnite longitudinal elastic layer (long-el-1) in the intima, separated from the internal elastic lamina (int-el-1) by the developing musculo-elastic layer.
11 A portion of the internal elastic lamina when a so-called splitting has occurred. The section is from the aorta of a 120-mm. fetus, stained with Weigert-azan.
12 Oblique longitudinal section of an elastic lamella in the media of the aorta of a 150-mm. fetus stained with Weigert—azan. Note the anastomosing elastic ﬁbres in a more lightly stained matrix and separated by fenestrae.
13 A muscle cell attached to an elastic lamella from the media. of the aorta of a 170-mm. fetus. The section was stained with Heidenh.-a.in’s iron-haematoxylin, to show the myoﬁbrillae. Note the knob-like formation of the lamella. into which the myoﬁbrillae are attached.
14 From the same as ﬁgure 13 only stained with Weigert-azan. The muscle cell is of the usual branching type found in arteries.
15 Vasa. vasorum in the adventitia of the aorta of a 101 mm. fetus. The vessel on the right had the appearance of an artery. '
16 Vasa vasorum from a 175-mm. fetus. The vessel on the left is the artery and that on the right is a vein.
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Cite this page: Hill, M.A. 2017 Embryology Paper - Histogenesis of the human aorta. Retrieved October 22, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Histogenesis_of_the_human_aorta
- © Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G[[Category:Draft]