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Microvasculature of the dorsal mucosa of human fetal tongue: A SEM study of corrosion casts
= = = = = = = = = ANNALS
Of ANATOm'
=========
Microvasculature of the dorsal mucosa of human fetal tongue: A SEM study of corrosion casts Pawel Strfk*, Jan A. Litwin**, Maria Nowogrodzka-Zagorska*** and Adam J. Miodonski***
* Department of Otolaryngology, ** Department of Histology and *** Laboratory of Scanning Electron Microscopy, Jagiellonian University School of Medicine, Kopernika 23 a, PL-31-501 Krakow, Poland
Summary. The vasculature of the dorsal mucosa of the tongue was investigated in 18 - 21 week human fetuses by corrosion casting and scanning electron microscopy. Microvascular systems of the fungiform, foliate and circumvallate papillae, albeit less complex, showed similarity to those described in children, while the capillary networks of the filiform papillae were not yet fully developed, having either a knot-like or cone-like form instead of the corollalike pattern typical of the postnatal period. Morphological features suggesting angiogenesis included both capillary outgrowths characteristic of vascular sprouting and tiny holes in the vascular walls regarded as evidence of intussusceptive capillary growth. The subpapillary vascular network supplying and draining the papillary vessels was composed of the more superficial capillary bed and the deeper plexus of larger arterioles and venules.
Key words: Blood vessels - Tongue - Human fetus - Corrosion casting
Introduction The unique organization of the dorsal mucosa of the tongue reflects its specific function as both a masticatory and gustatory organ. When breathing through the mouth the mucosa of the tongue provides a certain air-conditioning function. These functions require an efficient blood supply and indeed the tongue receives particularly large amounts of blood as compared with other muscular organs (Hellekant 1972). The vasculature of the tongue has been studied by dye injection methods under the light microscope and, more recently, by the corrosion casting technique and scanning electron microscopy, which offer much higher resolution Correspondence to: A. J. Miodonski
)~I2 SEMPER
Ann Anat (1995) 177: 361- 366 Gustav Fischer Verlag Jena
and quasi-3-dimensional images, and are especially suitable for investigation of the microvascular architecture (Lametschwandtner et al. 1990). The latter studies have been performed on the cat (Jasinski and Miodonski 1979), dog (Kishi et al. 1990), rat Ohshima et al. 1990), monkey (Nakamura and Okada 1992) and in humans (Yu et al. 1992). This paper describes the microvasculature of the dorsal surface of the tongue in human fetuses, the aim of the study being to examine the complex vascular systems of the lingual papillae at a prenatal developmental stage.
Material and methods Four human fetuses, two male and two female, aged 18 - 21 gestational weeks, with a crown-rump length ranging from 170 mm to 200 mm, were obtained after spontaneous abortions from the Obstetric Clinic of the Jagiellonian University School of Medicine in Cracow. The abortions were due to maternal disorders and no developmental malformations or vascular anomalies were found in the fetuses on macroscopic inspection. After abortion, the thorax of each fetus was opened to expose the heart and large vessels. The heart apex was cut off and a cannula inserted via the left ventricle into the aorta and fixed by ligation at the level of its ascending part. The vascular system of the fetus was subsequently perfused manually with a sequence of solutions, outflow occurring via the umbilical vessels and additionally incised posterior tibial veins. The human fetuses were perfused with 800 ml of prewarmed (37°C) heparinized saline (12.5 I. U.lml) containing 3070 Dextran, M. W. 70000, and 0.025070 Iidocain (Lignocain, Pol fa). Perfusion fixation was then carried out with 200 ml of 0.08070 glutaraldehyde in 0.15 M cacodylate buffer, pH 7.4, at 37°C. Finally, 60 ml of a mixture consisting of 8 ml Mercox CL-2B (V ilene Comp., Tokyo, Japan) and 2 ml methyl methacrylate (Fluka), and containing 0.2 g MA initiator per 10 ml of the casting medium was injected. Following the injection, the fetuses were kept overnight in water at 60°C in order to accelerate and complete resin polymerization (Miodonski et at. 1981).
Fig. 1. An overview of the dorsal surface of the human fetal tongue, showing vascular systems of the papillae and the subpapillary vascular network composed of more superficially located capillaries and a deeper layer of larger vessels arranged in parallel. Arrowheads point to the fungiform papillae. Bar = 100 ~m. Fig. 2. Knot-like vascular systems of the filiform papillae and of two fungiform papillae (arrowheads) from the apical region of the tongue. Note small avascular areas at the top of the fungiform papillae. Bar = 100 ~m. Fig. 3. Vascular systems of the filiform papillae from the posterior region of the tongue. Note nodular protrusions (arrows) and sprouts (arrowheads) projecting from the capillary casts. Bar = 100 ~m.
362
After polymerization of the resin, the tongue was removed and macerated at 37 °C in 8 -10070 potassium hydroxide for several days, with daily changes of the solution and alternating washes in running hot (approx. 60 0c) tap water. The resulting vascular casts were carefully and thoroughly cleaned in 5070 trichloroacetic acid, followed by washing in distilled water for a few days. The casts were then freeze-dried, mounted onto specimen stubs, using colloidal silver and "conductive bridges" (Lametschwandtner et al. 1980) coated with gold and examined in a Jeol JSM 35-CF scanning electron microscope at 20 - 25 kV.
Results The mucosa lining the dorsal surface of the human fetal tongue receives its blood supply from the arterial branches emerging from the underlying layer of skeletal muscle and from the superficially located anterior and posterior dorsal branches of the lingual artery. The mucosal vasculature at the examined stage of prenatal development consists of three well defined layers: (1) the vascular systems of the lingual papillae, mostly composed of capillaries, (2) the subpapillary capillary network, also containing small arterioles and venules, and (3) a deeper layer of larger vessels, 50 - 70 ~m in diameter, forming a dense plexus (Figs. 1, 2). In the body of the tongue, the vessels of the deep layer were arranged in parallel (Fig. 1), forming a herring-bone pattern with the acute angles located in the midline and directed
Fig. 4. The vasculature of the foliate papillae. Bar
=
posteriorly, while on the lingual apex and root the arrangement of the deep vessels was irregular (Fig. 2). The vascular systems of the filiform papillae could be divided into two types. Papillae covering the major part of the lingual surface contained a simple system composed of an afferent and an efferent vessel, 7 - 15 ~m in diameter, interconnected by a small apical plexus having the form of an irregular knot (Fig. 2). In the posterior areas of the tongue, the microvasculature of the filiform papillae was more complex, with capillaries forming primary, secondary and tertiary loops supplied by small arterioles and drained by venules which were often centrally located and distended (Fig. 3). The vessels of the fungiform papillae stood out among the surrounding filiform papillae, being characterized by their larger size and the circular shape of the subepithelial plexus. The surface of that plexus often seemed to be concave due to the presence of an avascular area at the top of the papilla (Figs. 1, 2). The foliate papillae, located on the posterior lateral margins of the tongue, took the form of short folds separated by parallel grooves (Fig. 4). In such papillae, the supplying arteriole and the draining venule ran along its longitudinal axis and were covered by an irregular network of subepithelial capillaries (Fig. 5). The circumvallate papillae contained a very complex vascular arrangement. The deeper vessels of the mucosa, approx. 50 ~m in diameter, acquired an annular course and
100 Ilm.
Fig. 5. A close-up view of three adjacent foliate papillae, showing the larger vessels oriented along their longitudinal axis and covered by the subepithelial capillary plexus. Arrows and arrowheads indicate nodular capillary outgrowths and sprouts respectively. Bar = 100 Ilm.
363
gave off capillary branches which formed a narrow plexus covering the larger vessels and delineating the circular furrow surrounding the papilla (Fig. 6). The bush-shaped vascular system of the papilla itself was composed of several ascending and descending vessels located near its base, bifurcating into secondary and tertiary branches and interconnected by a very dense superficial plexus of capillaries. These capillaries often showed dilated, sinusoidal segments, occasional bulbous or appendiform sprouts and small holes, 1 - 5 J!m in diameter (Fig. 7). Bi- or tripartite vascular systems divided by deep clefts, as well as small avascular
areas on the papillary surface, were observed in the majority of circumvallate papillae (Figs. 6, 8). The area of the lingual root occupied by the lingual tonsil was characterized by the presence of short, irregular folds made up of very dense capillary plexuses in which regular openings, up to l00!lm in diameter, could be seen (Fig. 9). Larger vessels, with a diameter of 50 - 80 !lm were located beneath the capillaries (Fig. 10). Occasional nodular protrusions and short sprouts were observed in the capillary network of all types of papilla (Figs. 3, 5, 7).
Fig. 6. Vascular systems of two circumvallate papillae. Note an avascular area at the top of the smaller papilla and a deep cleft splitting the vascular system of the larger one. Bar = 100 11m. Fig. 7. Higher magnification of the circumvallate papilla microvasculature showing the presence of capillary outgrowths (arrows) and tiny holes (arrowheads) typical of intussusceptive angiogenesis. Bar = 100 11m. Fig. 8. A circumvallate papilla with its vascular system separated into three parts by deep groves. Bar = 100 11m.
364
Fig. 9. The vascular system of the lingual tonsil. Note regular openings representing the epithelial invaginations. Bar
=
100 ~mm.
Fig. 10. Fragment of the microvasculature of the lingual tonsil, showing a dense subepithelial capillary plexus and the underlying larger vessels. Bar = 100 ~m.
Discussion The results of the present study show that the general arrangement of the dorsal mucosal vasculature in the tongues of human fetuses from the second trimester is similar to that described for the tongues of children aged 0.5 - 2 years (Yu et al. 1992). The vascular systems of the individual papillae are already developed, and the underlying vascular bed is differentiated into two levels: superficial and deep. This bed has been commonly named in the literature the "subpapillary capillary network" (SPCN). In our material, only the superficial layers consisted mainly of capillaries, while the deep one was built up of considerably larger vessels ranging from 30 to 70 ~m in diameter. Similar observations have also been reported by other authors who studied the vasculature of the tongue in animals and man (Hellekant 1976; Kishi et al. 1988, 1990; Nakamura and Okada 1992). Therefore, the term "capillary network" seems to be inadequate, and we propose to call that bed the "subpapillary vascular network" (SPVN). The shape of the papillary vascular systems in the fetal tongue resembles that described in the postnatal period (Yu et al. 1992) in the case of the fungiform, foliate and circumvallate papillae, although the fetal systems consist of a smaller number of capillary loops. It strikingly differs, however, in the case of the filiform papillae, which have been shown to contain characteristic corolla-shaped
capillary networks composed of primary, secondary and tertiary loops in both humans and other primates (Nakamura and Okada 1992; Yu et al. 1992). In our material, the microvasculature of most filiform papillae seemed to be underdeveloped, having the form of irregular knots. Only in the posterior regions of the tongue were the vessels of the filiform papillae more complex, but even there they did not acquire the typical corolla-like pattern, resembling instead the arrangement described for the simple conical papillae of the rat tongue (Ohshima et al. 1990; Selliseth and Selvig 1993). This discrepancy in the microvascular development between the mechanical (filiform) and sensory papillae may result from differences in functional maturation: blood vessels have to supply taste buds which are already present at 8 -9 weeks of gestation (Mistretta 1991), whereas the mechanical function of the filiform papillae starts in the postnatal period. Our previous studies on the fetal vascular systems revealed that they are more similar to the postnatal patterns in organs that are more or less functional in the fetal period than in those that are not (Miodonski et al. 1993; Pitynski et al. 1994; Strfk et al. 1994). The avascular areas observed in the fungiform and circumvallate papillae, as well as deep clefts dividing the vascular systems of the latters into two or three parts, have been described in the tongues of children (Yu et al. 1992), and result from invaginations of the epithelial lining. The presence of regular openings in the capillary network of
365
the lingual tonsils also reflects the epithelial invaginations forming the crypts. The defined globular or cone-shaped capillary networks of the lymphoid follicles reported to occur in the lingual tonsil of children (Yu et al. 1992) were not observed in our material. Further development of the microvasculature of the lingual papillae in the fetus requires angiogenesis. In the corrosion casts of the fetal tongue, the morphological features suggesting that process included capillary outgrowths in the form of nodules or sprouts. We also encountered tiny holes in the already formed capillaries, reported to represent intussusceptive capillary growth: a special form of angiogenesis, different from the commonly observed sprouting. It proceeds by the formation of slender pillars of the extravascular tissue penetrating the vascular walls and then expanding to create new meshes in the capillary network (Burri and Tarek 1990; Patan et al. 1993).
References Burri PH, Threk MR (1990) A novel mechanism of capillary growth in the rat pulmonary microcirculation. Anat Rec 228: 35 -45 Hellenkant G (1976) The blood circulation of the tongue. In: Kawamura Y (ed) Frontiers of oral physiology, vol 2. Karger, Basel, pp 130 - 145 Jasinski A, Miodonski A (1979) Blood vessels in the tongue of the kitten: scanning electron microscopy of microcorrosion casts. Anat Embryol 155: 347 - 353 Kishi Y, So S, Harada Y, Takahashi K (1988) Three-dimensional SEM study of arteriovenous anastomoses in the dog's tongue using corrosive resin casts. Acta Anat 132: 17 - 27 Kishi Y, Thkahashi K, Trowbridge H (1990) Vascular network in papillae of dog oral mucosa using corrosive resin casts with scanning electron microscopy. Anat Rec 226: 447 -459 Lametschwandtner A, Miodonski A, Simonsberger P (1980) On the prevention of specimen charging in scanning electron microscopy of vascular corrosion casts by attaching conductive bridges. Mikroskopie 36: 270 - 273
Lametschwandtner A, Lametschwandtner U, Weiger T (1990) Scanning electron microscopy of vascular corrosion casts - technique and application: Updated review. Scanning Microsc 4: 889-941 Miodonski A, Kus J, Tyrankiewicz R (1981) SEM blood vessel casts analysis. In: Allen DJ, Motta PM, Didio LJA (eds) Three-dimensional microanatomy of cells and tissue surfaces. Elsevier, Amsterdam, pp 71-87 Miodonski AJ, Gorczyca J, Nowogrodzka-Zagorska M, Litwin JA, Skladzien J (1993) Microvascular system of the human fetal inner ear: a scanning electron microscopic study of corrosion casts. Scanning Microsc 7: 585 - 595 Misretta CM (1991) Developmental neurobiology of the taste system. In: Getchell TV (ed) Smell and taste in health and disease. Raven Press, New York, pp 35 - 64 Nakamura M, Okada S (1992) Microvascular architecture of the lingual papillae in the Japanese monkey (Macaca fuscata fuscata). Okajimas Folia Anat Jpn 69: 183-198 Ohshima H, Yoshida S, Kobayashi S (1990) Blood vascular architecture of the rat lingual papillae with special reference to their relations to the connective tissue papillae and surface structures: A light and scanning electron microscope study. Acta Anat 137: 213 - 221 Patan S, Haenni B, Burri PH (1993) Evidence for intussusceptive capillary growth in the chicken chorio-allantoic membrane. Anat Embryol 187: 121-130 Pitynski K, Litwin JA, Nowogrodzka-Zagorska M, Gorczyca J, Miodonski AJ (1994) The vascular architecture of human fetal oviduct: a scanning electron microscopic study of corrosion casts. Human Reprod 9 (10): 1958 -1963 Selliseth NJ, Selvig KA (1993) Microvasculature of the dorsum of the rat tongue: a scanning electron microscopic study using corrosion casts. Scand J Dent Res 101: 391 - 397 Strfk P, Nowogrodzka-Zagorska M, Litwin JA, Miodonski AJ (1994) The lung in closeview: a corrosion casting study on the vascular system of human foetal trachea. Eur Resp J 7: 1669-1672 Yu QX, Ran W, Pang KM, Philipsen HP, Theilade J, Chen XH, Mok YC (1992) The microvasculature of human oral mucosa using vascular corrosion casts and India ink injection. I. Tongue papillae. Scanning Microsc 6: 255 - 262 Accepted October 10, 1994
366
Of ANATOm'
=========
Microvasculature of the dorsal mucosa of human fetal tongue: A SEM study of corrosion casts Pawel Strfk*, Jan A. Litwin**, Maria Nowogrodzka-Zagorska*** and Adam J. Miodonski***
* Department of Otolaryngology, ** Department of Histology and *** Laboratory of Scanning Electron Microscopy, Jagiellonian University School of Medicine, Kopernika 23 a, PL-31-501 Krakow, Poland
Summary. The vasculature of the dorsal mucosa of the tongue was investigated in 18 - 21 week human fetuses by corrosion casting and scanning electron microscopy. Microvascular systems of the fungiform, foliate and circumvallate papillae, albeit less complex, showed similarity to those described in children, while the capillary networks of the filiform papillae were not yet fully developed, having either a knot-like or cone-like form instead of the corollalike pattern typical of the postnatal period. Morphological features suggesting angiogenesis included both capillary outgrowths characteristic of vascular sprouting and tiny holes in the vascular walls regarded as evidence of intussusceptive capillary growth. The subpapillary vascular network supplying and draining the papillary vessels was composed of the more superficial capillary bed and the deeper plexus of larger arterioles and venules.
Key words: Blood vessels - Tongue - Human fetus - Corrosion casting
Introduction The unique organization of the dorsal mucosa of the tongue reflects its specific function as both a masticatory and gustatory organ. When breathing through the mouth the mucosa of the tongue provides a certain air-conditioning function. These functions require an efficient blood supply and indeed the tongue receives particularly large amounts of blood as compared with other muscular organs (Hellekant 1972). The vasculature of the tongue has been studied by dye injection methods under the light microscope and, more recently, by the corrosion casting technique and scanning electron microscopy, which offer much higher resolution Correspondence to: A. J. Miodonski
)~I2 SEMPER
Ann Anat (1995) 177: 361- 366 Gustav Fischer Verlag Jena
and quasi-3-dimensional images, and are especially suitable for investigation of the microvascular architecture (Lametschwandtner et al. 1990). The latter studies have been performed on the cat (Jasinski and Miodonski 1979), dog (Kishi et al. 1990), rat Ohshima et al. 1990), monkey (Nakamura and Okada 1992) and in humans (Yu et al. 1992). This paper describes the microvasculature of the dorsal surface of the tongue in human fetuses, the aim of the study being to examine the complex vascular systems of the lingual papillae at a prenatal developmental stage.
Material and methods Four human fetuses, two male and two female, aged 18 - 21 gestational weeks, with a crown-rump length ranging from 170 mm to 200 mm, were obtained after spontaneous abortions from the Obstetric Clinic of the Jagiellonian University School of Medicine in Cracow. The abortions were due to maternal disorders and no developmental malformations or vascular anomalies were found in the fetuses on macroscopic inspection. After abortion, the thorax of each fetus was opened to expose the heart and large vessels. The heart apex was cut off and a cannula inserted via the left ventricle into the aorta and fixed by ligation at the level of its ascending part. The vascular system of the fetus was subsequently perfused manually with a sequence of solutions, outflow occurring via the umbilical vessels and additionally incised posterior tibial veins. The human fetuses were perfused with 800 ml of prewarmed (37°C) heparinized saline (12.5 I. U.lml) containing 3070 Dextran, M. W. 70000, and 0.025070 Iidocain (Lignocain, Pol fa). Perfusion fixation was then carried out with 200 ml of 0.08070 glutaraldehyde in 0.15 M cacodylate buffer, pH 7.4, at 37°C. Finally, 60 ml of a mixture consisting of 8 ml Mercox CL-2B (V ilene Comp., Tokyo, Japan) and 2 ml methyl methacrylate (Fluka), and containing 0.2 g MA initiator per 10 ml of the casting medium was injected. Following the injection, the fetuses were kept overnight in water at 60°C in order to accelerate and complete resin polymerization (Miodonski et at. 1981).
Fig. 1. An overview of the dorsal surface of the human fetal tongue, showing vascular systems of the papillae and the subpapillary vascular network composed of more superficially located capillaries and a deeper layer of larger vessels arranged in parallel. Arrowheads point to the fungiform papillae. Bar = 100 ~m. Fig. 2. Knot-like vascular systems of the filiform papillae and of two fungiform papillae (arrowheads) from the apical region of the tongue. Note small avascular areas at the top of the fungiform papillae. Bar = 100 ~m. Fig. 3. Vascular systems of the filiform papillae from the posterior region of the tongue. Note nodular protrusions (arrows) and sprouts (arrowheads) projecting from the capillary casts. Bar = 100 ~m.
362
After polymerization of the resin, the tongue was removed and macerated at 37 °C in 8 -10070 potassium hydroxide for several days, with daily changes of the solution and alternating washes in running hot (approx. 60 0c) tap water. The resulting vascular casts were carefully and thoroughly cleaned in 5070 trichloroacetic acid, followed by washing in distilled water for a few days. The casts were then freeze-dried, mounted onto specimen stubs, using colloidal silver and "conductive bridges" (Lametschwandtner et al. 1980) coated with gold and examined in a Jeol JSM 35-CF scanning electron microscope at 20 - 25 kV.
Results The mucosa lining the dorsal surface of the human fetal tongue receives its blood supply from the arterial branches emerging from the underlying layer of skeletal muscle and from the superficially located anterior and posterior dorsal branches of the lingual artery. The mucosal vasculature at the examined stage of prenatal development consists of three well defined layers: (1) the vascular systems of the lingual papillae, mostly composed of capillaries, (2) the subpapillary capillary network, also containing small arterioles and venules, and (3) a deeper layer of larger vessels, 50 - 70 ~m in diameter, forming a dense plexus (Figs. 1, 2). In the body of the tongue, the vessels of the deep layer were arranged in parallel (Fig. 1), forming a herring-bone pattern with the acute angles located in the midline and directed
Fig. 4. The vasculature of the foliate papillae. Bar
=
posteriorly, while on the lingual apex and root the arrangement of the deep vessels was irregular (Fig. 2). The vascular systems of the filiform papillae could be divided into two types. Papillae covering the major part of the lingual surface contained a simple system composed of an afferent and an efferent vessel, 7 - 15 ~m in diameter, interconnected by a small apical plexus having the form of an irregular knot (Fig. 2). In the posterior areas of the tongue, the microvasculature of the filiform papillae was more complex, with capillaries forming primary, secondary and tertiary loops supplied by small arterioles and drained by venules which were often centrally located and distended (Fig. 3). The vessels of the fungiform papillae stood out among the surrounding filiform papillae, being characterized by their larger size and the circular shape of the subepithelial plexus. The surface of that plexus often seemed to be concave due to the presence of an avascular area at the top of the papilla (Figs. 1, 2). The foliate papillae, located on the posterior lateral margins of the tongue, took the form of short folds separated by parallel grooves (Fig. 4). In such papillae, the supplying arteriole and the draining venule ran along its longitudinal axis and were covered by an irregular network of subepithelial capillaries (Fig. 5). The circumvallate papillae contained a very complex vascular arrangement. The deeper vessels of the mucosa, approx. 50 ~m in diameter, acquired an annular course and
100 Ilm.
Fig. 5. A close-up view of three adjacent foliate papillae, showing the larger vessels oriented along their longitudinal axis and covered by the subepithelial capillary plexus. Arrows and arrowheads indicate nodular capillary outgrowths and sprouts respectively. Bar = 100 Ilm.
363
gave off capillary branches which formed a narrow plexus covering the larger vessels and delineating the circular furrow surrounding the papilla (Fig. 6). The bush-shaped vascular system of the papilla itself was composed of several ascending and descending vessels located near its base, bifurcating into secondary and tertiary branches and interconnected by a very dense superficial plexus of capillaries. These capillaries often showed dilated, sinusoidal segments, occasional bulbous or appendiform sprouts and small holes, 1 - 5 J!m in diameter (Fig. 7). Bi- or tripartite vascular systems divided by deep clefts, as well as small avascular
areas on the papillary surface, were observed in the majority of circumvallate papillae (Figs. 6, 8). The area of the lingual root occupied by the lingual tonsil was characterized by the presence of short, irregular folds made up of very dense capillary plexuses in which regular openings, up to l00!lm in diameter, could be seen (Fig. 9). Larger vessels, with a diameter of 50 - 80 !lm were located beneath the capillaries (Fig. 10). Occasional nodular protrusions and short sprouts were observed in the capillary network of all types of papilla (Figs. 3, 5, 7).
Fig. 6. Vascular systems of two circumvallate papillae. Note an avascular area at the top of the smaller papilla and a deep cleft splitting the vascular system of the larger one. Bar = 100 11m. Fig. 7. Higher magnification of the circumvallate papilla microvasculature showing the presence of capillary outgrowths (arrows) and tiny holes (arrowheads) typical of intussusceptive angiogenesis. Bar = 100 11m. Fig. 8. A circumvallate papilla with its vascular system separated into three parts by deep groves. Bar = 100 11m.
364
Fig. 9. The vascular system of the lingual tonsil. Note regular openings representing the epithelial invaginations. Bar
=
100 ~mm.
Fig. 10. Fragment of the microvasculature of the lingual tonsil, showing a dense subepithelial capillary plexus and the underlying larger vessels. Bar = 100 ~m.
Discussion The results of the present study show that the general arrangement of the dorsal mucosal vasculature in the tongues of human fetuses from the second trimester is similar to that described for the tongues of children aged 0.5 - 2 years (Yu et al. 1992). The vascular systems of the individual papillae are already developed, and the underlying vascular bed is differentiated into two levels: superficial and deep. This bed has been commonly named in the literature the "subpapillary capillary network" (SPCN). In our material, only the superficial layers consisted mainly of capillaries, while the deep one was built up of considerably larger vessels ranging from 30 to 70 ~m in diameter. Similar observations have also been reported by other authors who studied the vasculature of the tongue in animals and man (Hellekant 1976; Kishi et al. 1988, 1990; Nakamura and Okada 1992). Therefore, the term "capillary network" seems to be inadequate, and we propose to call that bed the "subpapillary vascular network" (SPVN). The shape of the papillary vascular systems in the fetal tongue resembles that described in the postnatal period (Yu et al. 1992) in the case of the fungiform, foliate and circumvallate papillae, although the fetal systems consist of a smaller number of capillary loops. It strikingly differs, however, in the case of the filiform papillae, which have been shown to contain characteristic corolla-shaped
capillary networks composed of primary, secondary and tertiary loops in both humans and other primates (Nakamura and Okada 1992; Yu et al. 1992). In our material, the microvasculature of most filiform papillae seemed to be underdeveloped, having the form of irregular knots. Only in the posterior regions of the tongue were the vessels of the filiform papillae more complex, but even there they did not acquire the typical corolla-like pattern, resembling instead the arrangement described for the simple conical papillae of the rat tongue (Ohshima et al. 1990; Selliseth and Selvig 1993). This discrepancy in the microvascular development between the mechanical (filiform) and sensory papillae may result from differences in functional maturation: blood vessels have to supply taste buds which are already present at 8 -9 weeks of gestation (Mistretta 1991), whereas the mechanical function of the filiform papillae starts in the postnatal period. Our previous studies on the fetal vascular systems revealed that they are more similar to the postnatal patterns in organs that are more or less functional in the fetal period than in those that are not (Miodonski et al. 1993; Pitynski et al. 1994; Strfk et al. 1994). The avascular areas observed in the fungiform and circumvallate papillae, as well as deep clefts dividing the vascular systems of the latters into two or three parts, have been described in the tongues of children (Yu et al. 1992), and result from invaginations of the epithelial lining. The presence of regular openings in the capillary network of
365
the lingual tonsils also reflects the epithelial invaginations forming the crypts. The defined globular or cone-shaped capillary networks of the lymphoid follicles reported to occur in the lingual tonsil of children (Yu et al. 1992) were not observed in our material. Further development of the microvasculature of the lingual papillae in the fetus requires angiogenesis. In the corrosion casts of the fetal tongue, the morphological features suggesting that process included capillary outgrowths in the form of nodules or sprouts. We also encountered tiny holes in the already formed capillaries, reported to represent intussusceptive capillary growth: a special form of angiogenesis, different from the commonly observed sprouting. It proceeds by the formation of slender pillars of the extravascular tissue penetrating the vascular walls and then expanding to create new meshes in the capillary network (Burri and Tarek 1990; Patan et al. 1993).
References Burri PH, Threk MR (1990) A novel mechanism of capillary growth in the rat pulmonary microcirculation. Anat Rec 228: 35 -45 Hellenkant G (1976) The blood circulation of the tongue. In: Kawamura Y (ed) Frontiers of oral physiology, vol 2. Karger, Basel, pp 130 - 145 Jasinski A, Miodonski A (1979) Blood vessels in the tongue of the kitten: scanning electron microscopy of microcorrosion casts. Anat Embryol 155: 347 - 353 Kishi Y, So S, Harada Y, Takahashi K (1988) Three-dimensional SEM study of arteriovenous anastomoses in the dog's tongue using corrosive resin casts. Acta Anat 132: 17 - 27 Kishi Y, Thkahashi K, Trowbridge H (1990) Vascular network in papillae of dog oral mucosa using corrosive resin casts with scanning electron microscopy. Anat Rec 226: 447 -459 Lametschwandtner A, Miodonski A, Simonsberger P (1980) On the prevention of specimen charging in scanning electron microscopy of vascular corrosion casts by attaching conductive bridges. Mikroskopie 36: 270 - 273
Lametschwandtner A, Lametschwandtner U, Weiger T (1990) Scanning electron microscopy of vascular corrosion casts - technique and application: Updated review. Scanning Microsc 4: 889-941 Miodonski A, Kus J, Tyrankiewicz R (1981) SEM blood vessel casts analysis. In: Allen DJ, Motta PM, Didio LJA (eds) Three-dimensional microanatomy of cells and tissue surfaces. Elsevier, Amsterdam, pp 71-87 Miodonski AJ, Gorczyca J, Nowogrodzka-Zagorska M, Litwin JA, Skladzien J (1993) Microvascular system of the human fetal inner ear: a scanning electron microscopic study of corrosion casts. Scanning Microsc 7: 585 - 595 Misretta CM (1991) Developmental neurobiology of the taste system. In: Getchell TV (ed) Smell and taste in health and disease. Raven Press, New York, pp 35 - 64 Nakamura M, Okada S (1992) Microvascular architecture of the lingual papillae in the Japanese monkey (Macaca fuscata fuscata). Okajimas Folia Anat Jpn 69: 183-198 Ohshima H, Yoshida S, Kobayashi S (1990) Blood vascular architecture of the rat lingual papillae with special reference to their relations to the connective tissue papillae and surface structures: A light and scanning electron microscope study. Acta Anat 137: 213 - 221 Patan S, Haenni B, Burri PH (1993) Evidence for intussusceptive capillary growth in the chicken chorio-allantoic membrane. Anat Embryol 187: 121-130 Pitynski K, Litwin JA, Nowogrodzka-Zagorska M, Gorczyca J, Miodonski AJ (1994) The vascular architecture of human fetal oviduct: a scanning electron microscopic study of corrosion casts. Human Reprod 9 (10): 1958 -1963 Selliseth NJ, Selvig KA (1993) Microvasculature of the dorsum of the rat tongue: a scanning electron microscopic study using corrosion casts. Scand J Dent Res 101: 391 - 397 Strfk P, Nowogrodzka-Zagorska M, Litwin JA, Miodonski AJ (1994) The lung in closeview: a corrosion casting study on the vascular system of human foetal trachea. Eur Resp J 7: 1669-1672 Yu QX, Ran W, Pang KM, Philipsen HP, Theilade J, Chen XH, Mok YC (1992) The microvasculature of human oral mucosa using vascular corrosion casts and India ink injection. I. Tongue papillae. Scanning Microsc 6: 255 - 262 Accepted October 10, 1994
366