The Occurrence and Distribution of the Parafollicular Cells in the Thyroid, Parathyroid IV and Thymus IV in Some Mammals

Arch. histol. jap., Vol. 33, No. 4 (1971) p. 283-299

Department of Anatomy (Prof. H. OUTI), Okayama University Medical School, Okayama, Japan

Yoko KAMEDA

Received August 6, 1971

Summary. The thyroid glands of eight rabbits, fifteen cats and eight mice of various ages and both sexes were made into serial sections, one lobe from each individual, and investigated by means of silver impregnation, pseudoisocyanin and lead-hematoxylin stainings. It was recognized that the parafollicular cells stained by these techniques were not restricted to the thyroid gland but were also distributed in the parathyroid IV and thymus IV. 1. In rabbits the parafollicular cells occurred in the parathyroid IV in six out of seven cases. Although the proportion of parafollicular cells to parathyroid cells differed from individual to individual, it reached 10% in the most numerous case. 2. In cats the parafollicular cells constantly occurred in the parathyroid IV. They were found also in the thymus IV of nine out of eleven cases. From forty to fifty parafollicular cells were generally counted in cat parathyroid IV. In two cases with numerous parafollicular cells, they were estimated as 1-3% of the parathyroid cells. In the thymus tissue their population was lower than in the parathyroid gland. 3. No parafollicular cells were found in the parathyroid III and the thymus III. 4. The distribution of parafollicular cells in the thyroid of rabbits and mice was confined in a more or less restricted region of the gland around the parathyroid IV and thymus IV, or around ciliated cysts (vide infra). 5. In all three species a few ciliated cysts containing a foamy colloid-like substance constantly occurred in the thyroid grand. 6. These observations were discussed in relation to the ultimobranchial origin of the parafollicular cells. Experimental and immunohistochemical studies have supported that the parafollicular cells are the source of calcitonin (MATSUZAWA, 1966; BUSSOLATI and PEARSE, 1967; ERICSON, 1968; KAMEDA, 1970). The existence of the parafollicular cells in the thyroid gland has been established in a variety of mammalian species since the dis- covery of the cells by BABER(1876) in the dog thyroid. The possibility that the parafollicular cells might occur also in other branchiogenic organs such as the parathyroid gland and thymus, however, seems to deserve systematic studies as the source organs of the hormone calcitonin have been much disputed (see Discussion). Some authors (DUBOISand DUMONT, 1966; WELSCH and PEARSE, 1969; KAMEDA,1971) noticed the occurrence of the parafollicular cells in the parathyroid gland, but our knowledge concerning this is only fragmental. The thymus has generally been thought free of parafollicular cells except for the report of the present author showing their occurrence in the dog thymus (KAMEDA,1971). It should be further pointed out that even in the thyroid gland itself little has been investigated in detail on the distribution of the parafollicular cells. 283 284 Y. KAMEDA:

In the present study, the thyroid gland, parathyroid gland and thymus of cats, rabbits and mice were prepared in serial sections, and the occurrence and distribution of the parafollicular cells in these organs were systematically investigated by light microscopy. Materials and Methods In the present study, 15 cats weighing from 0.5 to 3.7Kg (3 adults and 12 young animals from 1 to 7 months old), 8 rabbits from 0.5 to 3.5Kg (2 adults, 4 puberty animals and 2 juvenile animals) and 8 adult mice (R3) of both sexes were used. The thyroid glands were removed under pentobarbital anesthesia and fixed in GPA solution (25% glutaraldehyde, 1 vol., saturated aqueous solution of picric acid, 3 vol. and acetic acid to give 1%, SOLCIAet al., 1968). One thyroid lobe from each animal was cut in longitudinal total serial sections 10μ thick and the other lobe was transversely (not serially) cut at 5-6μ. Besides the hematoxylin-eosin staining for general purposes, the silver impregnation (KAMEDA, 1968), pseudoisocyanin (SOLCIA et al., 1968) and lead-hematoxylin (SOLCIA et al., 1969) stainings were employed for the specific demonstration of the parafollicular cells. A female lion six years old was kindly provided by the Ritsurin Zoo, Takamatsu, and the thyroid and parathyroid glands were examined though not in serial sections.

Results Thyroid parafollicular cells In hematoxylin-eosin staining, the parafollicular cells of rabbits, cats and mice may be distinguished from the follicular cells only by their relatively large and

Fig. 1. Parafollicular cells in the rabbit thyroid gland stained dark by lead-hematoxylin. The cells partly surround the follicles. Fixed in GPA. ×1,100 Parafollicular Cells in Parathyroid and Thymus 285 chromatin sparse nuclei and by the more or less weak stainability of the cytoplasm. By means of silver impregnation, pseudoisocyanin and lead-hematoxylin stainings,

Fig. 2. Parafollicular cells in the cat thyroid stained dark by lead-hematoxylin. The cells are grouped in the parafollicular position. Fixed in GPA. ×600 Fig. 3. Parafollicular cells in the cat thyroid stained by pseudoisocyanin. The granules of the cells and the content of the ciliated cyst shown in the upper left of the picture react metachromatically. Fixed in GPA. ×1,100 286 Y. KAMEDA:

however, the parafollicular cells of these animals are clearly demonstrated as cells containing numerous stained granules (Fig. 1-3, 12) The parafollicular cells of the rabbit were located either in the perifollicular position as a surrounding element of the follicles, or in parafollicular position in small cell groups (Fig. 1). The shape of rabbit parafollicular cells varied from oval to slender. There seemed to be an individual difference in the population of these forms: in six animals slender parafollicular cells exceeded oval ones in number, where- as in two cases oval parafollicular cells were more numerous. Those cells located in perifollicular positions, however, usually showed elongated forms. On the other hand, the shape of the parafollicular cells seemed unrelated to the age and sex. The cells measured 20-25μ in long diameter and rarely reached 30μ with elongated cytoplasmic processes. Their nuclei were round and measured about 6.5μ in diameter. The parafollicular cells of the cat were found in parafollicular position and were mostly grouped in cell clusters (Fig. 2, 3). Although the boundary between the parafollicular cells was not very clear because of abundant stained granules, oval or round cells measuring about 13×9μ predominated. They contained a round nucleus of 6.5-7μ diameter. The parafollicular cell groups of the cat showed an oval or round outline and generally measured 20-30μ in long diameter, sometimes reaching 50μ. The parafollicular cells of the mouse were characterized by their solitary occurrence in perifollicular position or between the follicular cells (Fig. 12). The cell and nucleus were oval or round and measured around 13×9μ and 6.5μ, respectively.

The distribution of the parafollicular cells within the thyroid gland In rabbits, mice and cats, the parafollicular cells were not evenly distributed in the thyroid gland. Certain parts of the thyroid parenchyme did not contain them at all, while they were conspicuously concentrated around the parathyroid IV and thymus IV (vide infra). Their distribution in the thyroid of a rabbit is diagrammatically shown in Figure 13. In seven of eight rabbits the distribution of the cells was limited to the middle of the thyroid gland covering one third to one half of it, which also included the parathyroid IV. In the remaining one case in which the parathyroid IV was not situated in or beside the thyroid lobe, the parafollicular cells were more diffusely distributed in the thyroid parenchyme. The parafollicular cells of cats were more widely distributed within the thyroid lobes than in rabbits. The parathyroid IV and thymus IV usually were in the upper part of the thyroid lobes and the parafollicular cells were mostly restricted to the upper two thirds of the thyroid, although in three cases they were scattered through- out the gland. The parathyroid III of cats was usually located lower than the parathyroid IV in the thyroid. In all eight mice the distribution of the parafollicular cells was limited to the middle one third of the thyroid parenchyme along the long axis of the organ and around the ciliated cyst which generally occupied the central region of the thyroid lobes (Fig. 12, 13). The distribution of the cells in one case is diagrammatically shown in Figure 13. Fig. 4. Parathyroid IV (internal parathyroid gland) in the rabbit thyroid stained by lead-hematoxylin. Note the numerous stained cells distributed among the parathyroid cells. The same cells also are shown in the cyst epithelium. Fixed in GPA. C cyst. ×150 Parafollicular Cells in Parathyroid and Thymus 287

Fig. 5. Higher magnification of the parathyroid IV in a neighbouring section of Figure 4. Oval or fusiform cells are stained dark by lead-hematoxylin. The round small profiles correspond to the cytoplasmic processes crossed in the section. ×600

Fig. 6. A neighbouring section to that of Figures 4 and 5 stained by silver impregnation. The cells corresponding to the lead-hematoxylin positive cells are filled with the argyrophil granules and provided with the elongated cytoplasmic processes. ×1,100 288 Y. KAMEDA:

The parafollicular cells in the parathyroid IV Cells selectively stained by silver impregnation, pseudoisocyanin and lead- hematoxylin were found in the parathyroid IV (internal parathyroid gland) of rabbits and cats (Fig 4-8). In the mouse the parathyroid IV was not present as has been confirmed by previous authors (See, BARGMANN, 1939). In rabbits, the parathyroid IV was not always included within the thyroid gland. It was detected in seven out of eight thyroid lobes cut in serial sections. The parafollicular cells were found in six out of these seven parathyroids. They were oval or elongated cells containing a round nucleus and distributed among the parathyroid cells solitarily or in small groups (Fig. 4-6). Their size was the same as that of the thyroid parafollicular cells. With hematoxylin-eosin staining it was impossible to distinguish them from the parathyroid cells, because of the similar staining characteristics and cell size. In two cases the parathyroid gland contained numerous parafollicular cells and a rough count reached 10% of the parathyroid cells, while in the other four parathyroids only several to fifty cells were found in the whole organ. Age difference in the population of the stained cells was not clear, because of the large individual differences. Although they were usually concentrated in the peri- phery of the parathyroid, a diffuse distribution was observed in the cases of their numerous occurrence. In cat parathyroid IV, the elements stained by silver impregnation, pseudoisocyanin and lead-hematoxylin were round or oval cells with a round nucleus, and they were distributed among the parenchymal cells of the organ, forming small cell groups (Fig. 7, 8). Their size corresponded to that of the thyroid parafollicular cells. In hematoxylin-eosin staining, they were characterized by their more reddish cytoplasm than found in the clear parathyroid cells, but a clear discrimination could not be made between them. The cell groups occurred rather diffusely in the parathyroid gland, though they tended to gather along larger blood vessels supplying parathyroid parenchyme. In all the fifteen cats examined, the parathyroid IV contained parafollicular cells (Table 1). The proportion of these cells to the parathyroid cells was low and only forty to fifty cells were counted in an entire gland. In two specimens containing relatively numerous stained cells, their ratio was roughly counted 1-3%. Although no clearcut result of age difference in population was obtained owing to individual differences, two cats with relatively numerous stained cells were both 6 months old.

The parathyroid III In four out of eight rabbits, the parathyroid III occurred adjacent to the thyroid lobes. The occurrence of the parathyroid III in or beside cat thyroids is indicated in Table 1. In mice, the parathyroid III was encountered in the caudal portion of five thyroid lobes. No stained cells were found in the parathyroid III in any species examined.

The parafollicular cells in the thymus IV The thymus IV of rabbits was found in none of the thyroid lobes cut in serial sections. Parafollicular Cells in Parathyroid and Thymus 289

In eleven out of fifteen thyroid lobes of cats, a thymus gland was recognized in connection with the parathyroid IV. Since it has been described that in the cat the

Fig. 7. Parathyroid IV of the cat stained by lead-hematoxylin. Note the darkely stained cell groups corresponding to the parafollicular cells. Fixed in GPA. T thymus, C cyst. ×150

Fig. 8. Parafollicular cell groups in the parathyroid IV of the cat stained by pseudoisocyanin, Fixed in GPA.×1,100 290 Y. KAMEDA:

Table 1. Relationship between the parafollicular cells and the parathyroid gland and thymus in the cat

+: Occurrence -: Non-occurrence Pfc: Parafollicular cells PT: Parathyroid gland

Fig. 9. Thymus IV of the cat stained by lead-hematoxylin. Darkly stained cells are shown among the thymic cells. Fixed in GPA. ×600 Parafollicular Cells in Parathyroid and Thymus 291

thymus IV develops from the pharyngeal pouch IV and becomes intimately connected with the parathyroid IV to enter into the thyroid gland together with it (see, BARGMANN, 1939), the thymus gland found adjacent to the parathyroid IV was con- sidered as the thymus IV. Cells stained by silver impregnation, pseudoisocyanin and lead-hematoxylin were found in nine of the eleven cat thymuses IV examined (Fig. 9, 10). The cells were round or oval with a round nucleus and corresponded to the thyroid parafollicular cells in size. They were diffusely distributed in medullary portions and rarely in the cortex of the thymic lobules, forming small cell groups. They could not be identified by hematoxylin-eosin. A few cysts which contained a colloid-like substance and cell debris and were covered with stratified epithelium sometimes occurred in the medullary portion of the thymus. The stained cells were often intercalated between the epithelial cells of the cysts or in contact with the basal side of the cyst epithelium. The population of these cells in the thymic parenchyme was lower than in the parathyroid. In the present study the thymus IV consisted of one to four lobules. The stained cells were distributed in two lobules in two out of the nine thymus glands, while they were restricted to one lobule in the other seven cases. In two mice a thymus gland occurred beside the ciliated cysts (vide infra) but no stained cells were found in them.

The thymus III In two rabbits and in five cats, the thymus III was found beside the parathyroid III (Table 1). No cell stained by silver impregnation, pseudoisocyanin and lead- hematoxylin was found in the thymus III of rabbits and cats.

The parafollicular cell complex found in cats Large parafollicular cell complexes, similar to those found in dogs (KAMEDA,1971), were found in three out of fifteen cats (Table 1). They consisted of numerous cells with granules stained by lead-hematoxylin, pseudoisocyanin and silver impregnation, another type of epithelial cell unstained by these staining methods, microfollicles with or without colloid droplets which are similar to the thyroid follicles, and small cysts lined by a simple or stratified epithelium. The proportions of these elements differed from complex to complex. In cat No. 12 a parafollicular cell complex occurred in the parathyroid IV (Fig. 10) and measured 220×180×120μin size. It contained a number of microfollicles in spite of the absence of connection with the thyroid parenchyme. In cat No. 14 there were found in the interlobular space of the thymus IV two large parafollicular cell complexes in which unstained and apparently immature epithelial cells were numerous (Fig.11). They measured 1300×220×700μ and 270×50×300μ in size, respectively. In animal No.4 a cell complex measuring 490×220×420μ in size was found beside the parathyroid IV-thymus IV complex. A portion of this complex was connected with the thyroid parenchyme. In rabbits and mice, no parafollicular cell complex was found.

The cysts in the thyroid, parathyroid and thymus Ciliated cysts were encountered in all the thyroid glands of rabbits, cats and mice 292 Y. KAMEDA:

Fig. 10. A special parafollicular cell complex consisting of numerous parafollicular cells and microfollicles found in the parathyroid IV of the cat. Note the darkly stained parafollicular cells distributed in the parathyroid, thyroid and thymus tissue. Fixed in GPA and stained by lead-hematoxylin. T thymus IV, C cyst, PT parathyroid IV, Th thyroid. ×150

Fig. 11. A special parafollicular cell complex found among the thymic lobules of the cat. It consists of darkly stained parafollicular cells and unstained epithelial cells. Fixation in GPA and staining in lead-hematoxylin. ×150 Parafollicular Cells in Parathyroid and Thymus 293 cut in serial sections. In the thyroid parenchyme of seven rabbits a few cysts were revealed around the parathyroid IV. Also in one case in which the thyroid lobe did not contain the parathyroid IV cysts were found in the thyroid. As shown in Figure 4, the cysts of rabbits were lined by a mixed single and stratified epithelium mingled with some ciliated cells. In four of these eight cases the pseudoisocyanin and lead-hematoxylin positive and silver-impregnated cells were encountered among the epithelial cells of the cysts. The cysts contained a foamy colloid-like substance weakly stained in eosin and sometimes cell debris, too. In all the thyroid lobes of fifteen cats, many ciliated cysts occurred arround the parathyroid IV and thymus IV (Table 1). In three cases a few cysts related neither to the parathyroid IV nor to the thymus IV were also recognized in the thyroid lobes. They were lined by a single cuboidal or squamous epithelium, partly furnished with cilia (Fig. 3, 7, 10). Rarely the lumen was divided by incomplete septa. The cysts contained a finely foamy granular substance and sometimes cell debris, both of which were sometimes stained by pseudoisocyanin (Fig. 3). Similar cysts were encountered in the parathyroid IV, parathyroid III and thymus IV (Table 1). The cysts found in the thymic lobes, on the other hand, were usually lined by a squamous stratified epithelium and had much cell debris in the lumen. These cysts often were continuous to the cyst covered by a single ciliated epithelium in the thyroid parenchyme. In spite of the absence of the parathyroid IV in the mouse, a few cysts were found in the central region of the thyroid lobes in all of the eight mice. They were lined

Fig. 12. Parafollicular cells of the mouse thyroid gland distributing around the ciliated cysts and thymus, T thymus, CE ciliated epithelium. ×375 294 Y. KAMEDA: by single cuboidal or squamous epithelial cells and some of the cells possessed cilia (Fig. 12). Stained cells were often located among the cyst epithelial cells. The lumen was filled with a foamy colloid-like substance.

The parafollicular cells in the lion The parafollicular cells in the lion thyroid gland were similar to those of cats in their form, size and filled with quantities of stained granules. They were mostly located in parafollicular position and were grouped in cell clusters. A few parafollicular cells were also found in the parathyroid IV but the precise population of the cells is unknown as the tissue was not investigated in serial sections.

Discussion The parafollicular cells in the parathyroid IV and thymus IV The cells found in the parathyroid IV (internal parathyroid gland) of rabbits or in the parathyroid IV and thymus IV of cats are believed identical with the parafollicular cells of the thyroid gland, because firstly they are stained by silver impregnation, pseudoisocyanin and lead-hematoxylin known as methods for the positive demonstration of parafollicular cells and secondly, their form, size and the nature of the granules are similar to the thyroid parafollicular cells of the same animals. In the case of the dog which also possesses these cells in the parathyroid IV and thymus IV, the cells have been shown to undergo degranulation and to show an increased mitotic activity after experimentally induced hypercalcemia, just as the thyroid parafollicular cells do (KAMEDA, 1971). It seems now fully evidenced that the thyroid parafollicular cells are responsible for the secretion of the serum calcium lowering hormone, calcitonin (for literature see KAMEDA,1970). At the time calcitonin was originally suggested by COPP et al. (1962), they con- sidered the parathyroid gland as its origin. A confirmatory view soon was published by KUMARet al. (1963). "The perfusion of the parathyroid" with calcium-rich blood which caused a rapid fall in the calcium level of the systemic blood in the studies of those researchers, however, could not distinguish between thyroid and parathyroid glands as the source of calcitonin, because both glands in dogs cannot be perfused separately. Later, HIRSCHet al. insisted that a hypocalcemic element, named anew as thyrocalcitonin by them, was extracted not from the parathyroid but from the thyroid gland in rats (1963) and in many mammalian species (1964). In goats (FOSTER et al., 1964), and pigs (CARE,1965) in which the external parathyroid glands have a blood supply separated from the thyroid gland, it was confirmed that the hypocalcemic substance was secreted not from the parathyroid but from the thyroid gland during hypercalcemic perfusion. Although additional evidence in favor of a parathyroid origin was given by COPP and HENZE(1964) and COPP (1965), it now seems generally accepted that calcitonin and thyrocalcitonin are identical substances (HIRSCHand MUNSON,1969) and that they are secreted from the thyroid gland in mammals. The parathyroid gland as the possible source of this hormone has become more and more ignored by most authors. Neither has attention been paid to the fact that the thymus tissue may occur in or adjacent to the thyroid gland nor to the possibility that the hormone in question may possibly be produced from there. Parafollicular Cells in Parathyroid and Thymus 295

The present study indicates that the parafollicular cells are distributed not only in the thyroid gland but also very frequently in the parathyroid IV of rabbits and cats and in the thymus IV of cats. Their occurrence in the parathyroid IV and thymus IV of dogs has been reported elsewhere (KAMEDA,1971). The author proposes that the origin of calcitonin is not restricted to the thyroid gland, although it is generally the most significant source. Recently, DUBOIS and DUMONT (1966) by electron microscopy and SOLCIA and SAMPIETRO (1968) by light microscopy reported that cells with the same characteristics as the parafollicular cells of the thyroid gland existed in the internal parathyroid gland of the rabbit. CARVALHEIRA and PEARSE (1967) revealed that the thyroid parafollicular cells of the rabbit and dog showed a strong cholinesterase activity and that cells with the same enzyme activity occurred in the internal parathyroid gland. (As far as the photomicrograph by these authors indicates, the internal parathyroid of the dog described by them is not such but probably corresponds to the large parafollicular cell complex containing microfollicles described in this paper.) By electron microscopy, WELSCH and PEARSE (1969) revealed the cholinesterase-positive cells in the internal parathyroid gland of the rabbit. Their simply qualitative observations on the existence of the parafollicular cells in the internal parathyroid gland of the rabbit are confirmed in the present study by a more quantitative methodology. The cell population and its individual variability is thus first elucidated. The occurrence of the parafollicular cells in the parathyroid IV of the cat has not been reported by previous investigators. It is distinctly indicated in the present study that the parathyroid IV of the cat constantly contains parafollicular cells. The occurrence of parafollicular cells in the thymus tissue has not been demonstrated in any species (except for the dog, KAMEDA,1971). In the cat in which the thymus IV in connection with the parathyroid IV is often encountered in the thyroid glands, the parafollicular cells occur in the thymus parenchyme. The finding by KOHN (1895) that the thymus IV is also found in the thyroid of the rabbit could not be confirmed in this study. It was previously generally accepted that the mammalian ultimobranchial body undergoes degeneration during postnatal life. GODWIN(1937), however, described that the ultimobranchial body of the dog, in the early embryonic development, becomes incorporated within the thyroid and gives origin to parafollicular cells. Recently, this opinion of GODWIN was confirmed in the rat by PEARSE and CARVALHEIRA (1967) using fluorescent amine technique after injection of 5-HTP and L-DOPA, and by ISHIKAWA (1965) and STOECKEL and Porte (1970) by electron microscopy. The distribution of the parafollicular cells in the parathyroid IV and thymus IV which are known to be in an intimate connection with the ultimobranchial body during embryonic development and not in the parathyroid III and thymus III favors the hypothesis that the parafollicular cells derive from the ultimobranchial body. The parafollicular cell complexes found in the cat without doubt correspond to those in the dog previously described by the author (KAMEDA,1971) which also were intimately related to the parathyroid IV and thymus IV. The occurrence, in cats and dogs, of the large parafollicular cell complexes seems to support the view that at least some parts of the ultimobranchial body may remain as cell masses of this type even in postnatal life. A detailed discussion has been made in this author's previous 296 Y. KAMEDA: paper on the nature and significance of the parafollicular cell complex (KAMEDA, 1971). GALANTE et al. (1968) extracted in man a calcitonin-like substance from the parathyroid tissue with hyperparathyroidism and from the thymus tissue, although they did not discriminate between the parathyroid III and IV. Although the existence of the parafollicular cells in man was long disputed, SOLCIA et al. (1970) demonstrated that they occur concentrated in the posterior portions of the lateral thyroid lobes. The cells were not found in the parathyroid IV and thymus.

The distribution of the parafollicular cells in the thyroid In the present study, the parafollicular cells in the thyroid were concentrated around the parathyroid IV and the thymus IV (rabbits and cats) or around the ciliated cysts (mice). In rabbits and mice, the portion of the thyroid tissue containing none of those cells at all was conspicuous. This distinct distribution of the parafollicular elements near the parathyroid IV and thymus IV may eliminate the hypothesis that the parafollicular cells derive from the follicular cells suggested by some authors (SUGIYAMA, 1954; STUX et al., 1961). Similar distribution of the parafollicular cells in the thyroid was also reported by SOLCIA and SAMPIETRO (1968) in rabbits and by KAMEDA (1971) in dogs. In the papers of some other authors (SUGIYAMA, 1954, in guinea pigs; DUMONT, 1956, in rabbits; STUX et al., 1961, in rats) their relation to the parathyroid IV or to the thymus IV seems to have been overlooked.

The cysts in the thyroid, parathyroid and thymus In the present study a few ciliated cysts were constantly found in the thyroid gland of the rabbit, cat and mouse. The author supposed in the previous study (1971) that the ciliated cysts found in the dog thyroid may originated not only from the

Rabbit No. 1 Mouse No. 25

Fig. 13. Diagram indicating the distribution of the parafollicular cells in the thyroid lobes. The parafollicular cells are dotted. PT III parathyroid III, PT IV parathyroid IV, C cyst, T thymus. Parafollicular Cells in Parathyroid and Thymus 297 ultimobranchial body but also from the residual pharyngeal pouch IV and pharyngeal pouch III. This conclusion may applicable to the ciliated cysts of rabbits, cats and mice. The ciliated cysts have been recognized in the thyroid of a large variety of the mammalian species. They should be regarded as a regular constituent of the thyroid on the basis of their constant occurrence in rabbits, cats and mice as shown in the present paper. However, little is known about the function of these cysts. In the dog no changes were found in the frequency of their occurrence, quantity of colloid- like substances and cellular appearence after induced hypercalcemia or injection of thyroid inhibitors (KAMEDA, unpublished data).

Acknowledgment. The author wishes to express her cordial thanks to Prof. Hiromu OUTIof her department, and also to Prof. Tsuneo FUJITA, Department of Anatomy at Niigata University, for their kind advice and critical reading of the manuscript.

二三の哺乳動物の甲状腺, 上皮小体IV, 胸腺IVにおける旁濾胞細胞の存在と分布について

亀田芙子

ウサギ (8匹), ネコ (15匹) およびマウス (8匹) の甲状腺を1個体より1葉をとり, 連続切片にして, 渡銀, プソイドイソチアニンおよび鉛ヘマトキシリンによる染色法によってしらべた. これらの方法で染まる旁濾胞細胞は甲状腺だけに限らず, 上皮小体IVと胸腺IVにも分布することが明らかになった. 1. ウサギで旁濾胞細胞は7例中6例の上皮小体IVに存在した. 上皮小体内の旁濾胞細胞の数は個体差が著しいが, 最も多い例では全上皮細胞の10%に達した. 2. ネコでは上皮小体IVの中に旁濾胞細胞は常に存在した. それらは11例中9例の胸腺IVの中にも見いだされた. 一般にネコの上皮小体IVには40～50個の旁濾胞細胞が数えられたが, 細胞数の多い2例では1～3%の割合であった. 胸腺IVにおける旁濾胞細胞の数は上皮小体IVにおけるより少なかった. 3. 上皮小体IIIと胸腺IIIには旁濾胞細胞はみとめられなかった. 4. ウサギとマウスの甲状腺内の旁濾胞細胞の分布は上皮小体IVと胸腺IV, あるいは繊毛をもつ嚢胞の周囲の領域に局在していた. 5. これら3種の動物の全例の甲状腺内に, 泡状のコロイド様物質を含み繊毛をもつ嚢胞が常に存在した. 6. これらの観察から旁濾胞細胞の鰓後体起源が考察された. 298 Y. KAMEDA:

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亀田芙子 Miss Yoko KAMEDA 〒700岡山市鹿田町 Department of Anatomy 岡山大学医学部 Okayama University Medical School 第二解剖学教室 700 Okayama, Japan