|Year : 2022 | Volume
| Issue : 3 | Page : 148-153
Morphology and histogenesis of human fetal thyroid gland
Y Jalaja1, CP Anbarasi2
1 Assistant Professor, Department of Anatomy, Government Kilpauk Medical College, Chennai, Tamil Nadu, India
2 Assistant Professor, Department of Anatomy, Government Thiruvannamalai Medical College, Tiruvannamalai, Tamil Nadu, India
|Date of Submission||12-May-2022|
|Date of Decision||28-Jun-2022|
|Date of Acceptance||01-Jul-2022|
|Date of Web Publication||12-Aug-2022|
Department of Anatomy, Government Kilpauk Medical College, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Introduction: About 5% of the world population is affected from various thyroid disorders. The present study is undertaken to describe the morphological and morphometric variations in the fetal thyroid glands of thyroid at different stages of development in the intrauterine life. Methodology: Forty medically terminated fetuses of both sexes ranging from 15 to 36 weeks of gestation were included. The external parameters recorded were: Fetal weight; crown-rump length; foot Length; Location; Presence/absence of isthmus; Presence of pyramidal lobe; Presence of an accessory thyroid tissue were observed in situ. The length, width, and thickness of each lobe and the isthmus were noted. Results: The specimens were categorized into three gestational groups, i.e., 10–20 weeks, 21–30 weeks, and 31–40 weeks. The statistical analysis suggests that there is significant increase in fetal thyroid weight, lengths and widths of the right and left lobes, and length of the isthmus with increase in gestational age. There was no significant increase in width of isthmus with gestational age. There was no significant difference in all thyroid parameters by gender. Conclusion: The data regarding the various parameters collected and analyzed in the present study emphasized the significance of an insight into the morphological variations, morphometric dimensions, and histological findings of foetal thyroid glands in diagnosing various diseases.
Keywords: Foetal thyroid gland, histogenesis, levator glandulae thyroidea, pyramidal lobe
|How to cite this article:|
Jalaja Y, Anbarasi C P. Morphology and histogenesis of human fetal thyroid gland. Natl J Clin Anat 2022;11:148-53
| Introduction|| |
The thyroid gland is one of the largest and earliest differentiated endocrine glands in humans, and it is important for brain growth and development during pregnancy and the first few years after birth. Around the 3rd week of development, the thyroid gland appears as epithelial proliferation in the pharyngeal floor, which is later identified by the foramen cecum. In the 7th week, it continues to descend in front of the hyoid bone, laryngeal cartilages, and finally the trachea. Until the fetal thyroid develops into a follicular structure and becomes functionally active in about 12 weeks, normal maternal thyroid activity is necessary.
In foetus, thyroid hormones thyroxine (T4) and triiodothyronine (T3) are detectable early in gestation and have important developmental, metabolic and maturational effects, and deficiency of thyroid hormones during intrauterine life impairs growth and compromises its adaptation to extrauterine life. Conversely, fetal administration of thyroid hormones can promote tissue differentiation and activation of many physiological processes before birth that are essential for neonatal survival. Functional and anatomical abnormalities of the thyroid are common among the diseases of endocrine glands 10 affecting approximately one in 2000–4000 new born infants.
Limited literature is available on morphological and morphometric parameters of fetal thyroid gland. Hence, in light of the aforementioned facts keeping in mind, the applied importance and to add some additional information to the already existing studies, this study was undertaken to describe the morphological and morphometric variations in fetal thyroid glands, as well as to investigate thyroid histogenesis at various phases of development throughout intrauterine life. The implications of such studies not only elucidate and enhance the fundamental knowledge but furnish much insights on clinical application and significance of thyroid gland.
| Materials and Methods|| |
In a cross-sectional study, forty aborted fetuses were evaluated for morphology and morphometry of thyroid glands. Institutional ethical committee approval was obtained before starting the study (dated 15/10/2014). Fetuses were procured from the Government maternity hospital, Tirupati. Fetuses with gross congenital anomalies were excluded from the study. In the Department of Anatomy, SV Medical College, Tirupati, the following fetal external parameters were measured by using weighing machine, measuring scale, thread, and osteometric board. The external parameters recorded were: Fetal weight; Crown-rump length (CRL); Foot Length.
After recording the external parameters, the fetuses were preserved in 10% formalin. The peritoneal and orbital cavities were also injected with 10% formalin. One week after preservation, the front of the neck was opened by using routine dissection method and the following observations were made in situ; Location; Presence/absence of isthmus; Presence of pyramidal lobe; Presence of an accessory thyroid tissue.
Later, thyroid glands were removed by using routine dissection method and outer surface of fetal thyroids were dried with blotting paper and then weighed by using digital weighing machine. By using digimatic calipers (vernier– digital-MHUtoyo “mitutoyo”), thread, measuring scale and the digital weighing machine, the following parameters were recorded.
- Length of right and left lobes-measured between the poles
- Length of isthmus-measured between superior and inferior border
- Width of right and left lobes-measured between lateral margin to a perpendicular line drawn at the meeting point of isthmus and lobes
- Width of isthmus-measured between perpendicular lines drawn at the meeting point of isthmus and lobes
- Thickness of right, left lobes and isthmus-at maximum AP dimension.
Microanatomical development and evaluation of thyroid gland was undertaken at three gestational age groups, 10–20 weeks, 21–30 weeks, and >30 weeks of gestation. Routine histological processing for Hematoxylin and Eosin stains, as well as Periodic acid Schiff staining, were performed on representative samples-5 from each group. A total of 15 slide were examined in high power n low power-10 fields per slide. The photographs of representative fields were taken using photomicrography, with the results assessed.
| Results|| |
A total of 40 fetal thyroid glands from both sexes were studied for morphological alterations, morphometric parameters, and histological features between the ages of 15 and 36 weeks. The specimens were categorized into three gestational groups, i.e., 10–20 weeks, 21–30 weeks, and 31–40 weeks. [Graph 1] shows gestational group difference among male and female fetuses included.
Morphological features and variational anatomy of the specimens were noted. The location of the gland and relation to tracheal rings was normal. In 95% cases, thyroid glands were butterfly shaped, in 2.5%-W and H shaped, respectively [Table 1]. Isthmus was absent in one specimen, while the presence of a pyramidal lobe and levator glandulae thyroidea was noted in three cases. One specimen had an accessory thyroid nodule connected with the right lobe. The fetal thyroid weight ranged in size from 80.3 to 733 mg. The average weight was 431.3 mg.
In the present study, correlation between thyroid weight and fetal weight in males was 0.86 and females was 0.80 which is significant at 1% level. The correlation between thyroid weight and foot length in males was 0.93 and females was 0.81 which is significant at 1% level. This indicates that, as fetal weight and foot length increases thyroid weight also increases both in male and female fetuses. The length of right lobes ranged from 5.59 to 15.60 mm. The length of left lobes ranged from 5.34 to 15.84 mm. The length of isthmus ranged from 2.1 to 8.08 mm. These steadily increased with gestational age and the difference was highly significant (P < 0.05) [Table 2].
The increase in width of both lobes with increase in gestational age was statistically significant. The maximum width of the right lobe was 9.33 mm and maximum width of the left lobe was 9.78 mm. The average width was 6.3 mm in both right and left which suggests that there was no significant difference in width between lobes. There was no significant increase in width of isthmus with gestational age. The average width was 4.1 ± 1.3 mm.
In the present study of fetal thyroids, the thickness of both lobes was significantly increasing with age. The average thickness of right lobes was 2.7 ± 0.6 mm and of left lobes was 2.9 ± 0.6 mm. The increase in thickness of isthmus with age is not significant (P > 0.05) and average thickness was 3.3 ± 1 mm. There was no significant difference in all thyroid parameters by gender (P > 0.05) [Table 3]. However, the mean values of thyroid parameters are slightly higher in female fetuses except thickness of the left lobe and length of isthmus. Significant correlation was observed between thyroid weight versus fetal weight; Thyroid weight versus foot length; fetal weight versus length; width of both lobes and thickness of left lobe in females; fetal weight versus length; width, thickness of lobes and length of isthmus in males [Table 4].
|Table 4: Correlation of fetal weight with morphometric parameters in males|
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15th week stage: A capsule containing blood vessels encased the gland. The peripheral follicles are well developed with cuboidal epithelium and colloid, the central gland showed clusters of epithelial cells, the follicles were in the process of being arranged oval to round shaped, with rich peripheral vascularity, and most of the follicles were devoid of colloid at this stage [Figure 1].
20th week stage: The thick connective tissue septa along with blood vessel were seen invading the gland, but typical lobular appearance is not seen as septa are incomplete. Matured thyroid follicles filled with moderately eosinophilic colloid were present at this stage. The colloid found at the periphery was more intensely stained than in the central part of the gland. Vascular network was abundant among the follicles [Figure 2].
26th week stage: The image was similar to the preceding stage, with the exception that the size of the follicles had increased, as had vascularity.
30th week stage: This stage is marked by a rise in the number and size of follicles, as well as a darkly pigmented colloid. Colloid seemed uneven and vacuoles were visible in several follicles [Figure 3] and [Figure 4].
36th week stage: Thyroid follicles bordered by cuboidal epithelium with darkly eosinophilic stained colloid and enhanced vascularity were matured and expanded at this stage [Figure 5].
| Discussion|| |
Agenesis of the thyroid isthmus refers to the complete and congenital absence of the thyroid isthmus. The primary mechanism of isthmus agenesis can be connected to a developmental defect. The research suggests that chromosome 22 has a role in thyroid gland development. Monotremes, certain marsupials, cetaceans, carnivores, and rodents are among the mammals that lack the isthmus. The thyroid glands of rhesus monkeys (Macacus rhesus) are typical in that they are located without an isthmus.
The absence of isthmus was found to be in one case in this study due to the bifurcation of pyramidal processes. According to Marshall CF et al., the incidence of isthmus absence is 10%. The incidence of agenesis of isthmus was found to be 4% in a research on fetal thyroids.
In the present study, the incidence of presence of pyramidal lobe is 20% (8 specimens), out of which 3 were male and 5 were female. In study conducted on fetal thyroids by Marshall et al., the incidence is 43%, by Lokanadham et al., it is 4% and by Cicekcibasi et al. the incidence is 18.3%.
According to Gregory and Guse, the levator glandulae thyroidae is an auxiliary muscle that runs from the hyoid bone to insert partly on the thyroid cartilage and partly on the thyroid isthmus. In the current study, the weight of the thyroid gland increased in proportion to estimated gestational age, CRL, fetal weight, and foot length, with the mean weight of the gland being higher in male foetuses.
The lengths of both lobes and isthmus increased considerably with gestational age in this research of 40 fetal thyroids. The findings were consistent with those of Cicekcibasi et al., who studied 60 aborted babies. The width of both lobes increases significantly with gestational age in this study. Al-Azzawi and Takahashi did a study and found that the length and width of the right lobe of thyroid gland were found to be 4.61 and 2.38 cm, respectively.
The results of the present study showed that the thickness of both lobes is significantly increasing with gestational age There is no significant increase in the thickness of isthmus with age and average thickness is 3.3 ± 1 mm which is more than that of Cicekcibasi et al. This also implies that the lobe widths are not much different. In the present study, all the mean values are slightly higher than those of Cicekcibasi et al. In this study, there was no significant increase in isthmus width with gestational age.
There was no significant difference in the thyroid measurements by gender in this study (P > 0.05). Except for the thickness of the left lobe and the length of the isthmus, female fetuses had higher mean values for thyroid characteristics. According to Cicekcibasi et al., however, it is higher in male fetuses.
In an ultrasound investigation of 100 neonatal thyroids, Gregory JK et al. found no significant differences in right and left lobe dimensions, and no differences in thyroid characteristics by gender. However, to our knowledge, the analysis of thyroid morphometric parameters in aborted foetuses is very limited, and minor differences observed in them compared to previous studies can be attributed to ambient iodine, anthropometric differences between races, maternal dietary habits, antenatal care, and population socioeconomic status.,
The histological characteristics of different gestational ages identified in this study were consistent with Gaikwad et al. and Lokanadham et al., At 15 weeks, the peripheral follicles are highly formed, with cuboidal epithelium and colloid, and abundant vascularity. At 20 weeks, there were incomplete septa, interfollicular rich vascularity, and both central and peripheral follicles. Follicle size and vascularity increased between 24 and 26 weeks. At 30 weeks, an irregular colloid containing vacuoles near the epithelial cells was discovered. At 36 weeks, mature thyroid follicles loaded with eosinophilic colloid are visible. Tanriover et al. found that the number and size of follicles increased as gestational age increased. Follicles were lined by low cuboidal epithelium and appeared empty in sections.
| Conclusion|| |
The data regarding the various parameters collected and analyzed in the present study emphasized the significance of an insight into the morphological variations, morphometric dimensions, and histological findings of fetal thyroid glands in understanding and diagnosing various thyroid diseases.
The knowledge of position and developmental anomalies of the thyroid gland is of great importance to radiologists, gynecologists, and pediatricians to prevent both maternal and fetal complications in fetal and neonatal period. The parameters observed by our study regarding fetal thyroid provide useful information to sonologist to report thyroid disorders like congenital hypothyroidism and hypoplasia. The results of this study may be a useful reference for studies according to the age groups.
The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase humankind's overall knowledge that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude. We would like to thank our Department of Anatomy, Obstetrics and Gynecology for their support, Department of Community Medicine for the statistical analysis.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4]