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Seasonal Variation in Thyroid Gland of the Female Bat, Taphozous
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Kachhensis (Dobson) During Pregnancy
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Pankaj Chavhan1, Amir Dhamani2
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1 Department of Zoology, N.H.College, Bramhapuri 441206
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2 Department of Zoology, S. S. S. Sci. College, Ashti 442707
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Pankaj Chavhan: [email protected], Amir Dhamani: [email protected]
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Abstract The aim of present study is to compare the changes in thyroid gland during the
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reproductive cycle of the female bat Taphozous kachhensis. The thyroid gland of the bat is
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studied morphometrically and histologically, it showed marked seasonal variation in weight,
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quantity of colloid, epithelial cell height and plasma concentration of thyroid hormones. The
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thyroid gland is a symmetrical, bilobed, and located on the lateral side of the trachea, between
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the first and the third tracheal rings, connected by an isthmus. The mean weight of the noted
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lobes was (2.240 ± 0.121) mg, (2.88 ± 0.05) mg and (2.704 ± 0.067) mg, during early pregnancy,
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mid pregnancy and late pregnancy respectively. The thyroid hormone shows marked seasonal
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variations. During early pregnancy there is rise in the level of TSH (1.7 ng/mL) but the slight
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decrease in the level of T3 (54 ng/mL) and T4 (2.9 ng/mL) was noticed. The level of TSH and
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T4 was decreases during mid pregnancy but the increase in the level of T3 was noticed and it is
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highest as compare to other stages and it is found to be 1.4 ng/mL, 1.5 ng/mL, 69 ng/mL
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respectively. During late pregnancy, TSH and T3 concentration decreases but the increase in the
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level of T4 was observed and it is found to be 0.9 ng/mL, 60 ng/mL, 2.9 ng/mL respectively. The
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diameter of colloid in small, medium and large follicle during early pregnancy and late
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pregnancy were 28 µm, 56 µm, 96 µm and 36 µm, 60 µm, 72 µm respectively. The epithelial cell
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heights of small, medium and large size follicles during early pregnancy and late pregnancy were
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3.6 µm, 4 µm, 8 µm and 4 µm, 8 µm, 8 µm respectively.
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Keywords Gross anatomy; Histology; Thyroid gland; Bat; T3; T4
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Introduction
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The thyroid is an endocrine gland which secretes thyroglobulin, triiodothyronine and thyroxin
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hormones (Banks, 1993). Thyroxin plays an important role in metabolism of the animal body.
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Thyroid gland consists of follicles lined by follicular cells. The interfollicular space has been
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reported to be filled with connective tissue (Leeson and Leeson, 1970). The most outstanding
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feature of the vertebrate thyroid gland is its ability to concentrate large amount of iodine for the
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synthesis of thyroxin (Turner, 1966). In addition, thyroid hormones appear to play a key role in
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the expression of events that underlie seasonal reproductive cycles. The thyroid gland has been
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studied histologically in many seasonally breeding microchiropteran bats (Kwiecinski et al.,
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1991; Damassa et al., 1995). In general, these morphological studies indicate that the thyroid
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increases activity late in the winter or early spring, is active throughout the summer, and
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regresses by autumn. Such an activity cycle is commonly found in most seasonally breeding
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mammals.
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A number of hormones secreted by various endocrine glands are directly or indirectly involved
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in normal functioning of the reproductive processes in various species and the thyroid hormone
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appear to be one of those that are intricately linked with reproduction. Thyroid hormones may
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influence reproductive activity either by direct action on the gonads or by effects mediated at the
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level of the hypothalamo-pituitary axis.
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Thyroid hormones are described as unique because they exert effects within almost every tissue
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of the body throughout the life of an individual. In humans, thyroid hormone deficiencies have
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resulted in a number of abnormalities with respect to growth, development, behavior,
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metabolism and reproduction. The thyroid gland is an important modulator for reproductive
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functions. The metabolic hormone, thyroxin, T4 is the physiological regulator of energy balance
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as well as in maintaining normal reproductive function in mammal (Schwartz et al., 1992; Shi
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and Berrel,1992). They reported that the T3 is essential for mammalian reproduction in cattle,
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the deficiency of which result into female infertility. Further it has been reported that a reduction
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in the secretion and plasma levels of gonadotrophins was associated with hypothyroidism
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(Hagino, 1971; LaRochelle and Freeman, 1974; Buchanan et al., 1977). Likewise, Dunn et al.
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(1976) have shown that the daily rhythmic release of luteinizing hormone (LH) and prolactin in
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rats was altered by thyroidectomy.
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Thyroid hormones have a significant mode of action on seasonal reproduction. In 1994, work by
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researchers in Michigan implicated thyroid hormones as having a role in decreasing LH secretion
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at the end of the breeding season, resulting in the normal cessation of follicular activity and
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ovulation. Along with follicle stimulating hormone (FSH), luteinizing hormone (LH) and
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indirectly, gonadotropin-releasing hormone (GnRH) are hormones responsible for follicle growth
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and ovulation.
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1 Results
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1.1 Gross Observations
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Gross studies of thyroid gland of female bat Taphozous kachhensis revealed that the gland was
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located near the first ring and third ring of trachea and consisted of two lobes. An isthmus
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connected these lobes to each other. The colour of the gland was reddish brown.
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1.2 Seasonal Changes in Body, Ovary, and Thyroid Weights
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The weights (mean ± SEM) of the body, ovary, and thyroid during reproductive phases are
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shown in Table 1 and Graph 1. There was not much difference between the highest and lowest
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mean thyroid weight of adult female bats caught during different phases of pregnancy. The
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thyroid weight showed two peaks coinciding with the peaks of ovary weight during early
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pregnancy and mid pregnancy. Both thyroid and ovary began to increase in weight from early
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pregnancy (January to February) and attained a peak during mid pregnancy (Late February to
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Middle April). Thyroid weights declined during late pregnancy similarly to ovary weight (Graph
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1).
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Table 1 The mean weights of the body, ovary, and thyroid during different phases of pregnancy
Early pregnancy (mg)
Mid pregnancy (mg)
Late pregnancy (mg)
Thyroid
2.24 ± 0.120
2.88 ± 0.051
2.70 ± 0.067
Ovary
2.38 ± 0.086
2.49 ± 0.075
1.49 ± 0.064
Body
50.00
45.00
39.00
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78
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Graph 1 The mean weights of the body, ovary, and thyroid during different phases of pregnancy
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1.3 Histology
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The thyroid gland of the female bat is studied histologically; it showed marked seasonal variation
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in quantity of colloid, epithelial cell height and plasma concentration of thyroid hormones.
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Diameter of colloid in large, medium and small size follicle of thyroid gland shows variations,
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with higher value during early pregnancy and lower value during mid pregnancy in large follicle,
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while in medium size follicle higher value of colloid during late pregnancy and lower during mid
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pregnancy. Small size follicle shows higher value of colloid during late pregnancy and lower
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during early pregnancy (Table 2). Thyroid follicular epithelial cells height also showed marked
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variations during various period of pregnancy. The large follicles were lined by low cuboidal
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epithelium having flattened nuclei and 3.6 µm, 4 µm and 4 µm in diameter during early, mid and
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late pregnancy respectively and were assumed to be inactive. The medium size follicles were
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lined by cuboidal epithelium having darkly stained nuclei and 4 µm, 8 µm and 8 µm in diameter
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during early, mid and late pregnancy respectively. The small size follicles were lined by high
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cuboidal epithelium with rounded nuclei having diameter 12 µm, 12 µm and 8 µm during early,
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mid and late pregnancy respectively(Figure1-Figure 9,Table 2, Geaph 2).
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Table 2 The mean diameter of colloid and epithelial cell height of thyroid during different phases of pregnancy
Early pregnancy
Middle pregnancy
Late pregnancy
Large
Medium
Small
Large
Medium
Small
Large
Medium
Small
follicle
follicle
follicle
follicle
follicle
follicle
follicle
follicle
follicle
96.0
56
28
64
40
28
72
60
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3.6
4
12
4
8
12
4
8
8
Diameter of
colloid
Epithelial
cell height
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97
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Figure 1
Figure 2
Figure 3
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100
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Figure 4
Figure 5
Figure 6
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Figure 7
Figure 8
Figure 9
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Figure 1 Large follicle during early pregnancy showing low cuboidal epithelium.
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Figure 2 Medium size follicle during early pregnancy showing cuboidal epithelium.
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Figure 3 Small size follicle during early pregnancy showing high cuboidal epithelium.
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Figure 4 Large follicle during mid pregnancy showing low cuboidal epithelium.
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Figure 5 Medium size follicle during mid pregnancy showing cuboidal epithelium.
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Figure 6 Small size follicle during mid pregnancy showing high cuboidal epithelium.
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Figure 7 Large follicle during late pregnancy showing low cuboidal epithelium.
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Figure 8 Medium size follicle during late pregnancy showing cuboidal epithelium.
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Figure 9 Small size follicle during late pregnancy showing high cuboidal epithelium.
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Graph 2 The mean diameter of colloid and epithelial cell height of thyroid during different phases of pregnancy
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1.4 Plasma Concentration of Thyroid Hormone
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The thyroid hormone examined during different stages of reproductive cycle of bat Taphozous
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kachhensis are described below.
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The thyroid hormone shows marked seasonal variations. During early pregnancy there is rise in
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the level of TSH (1.7 ng/mL) but the slight decrease in the level of T3 (54 ng/mL) and T4 (2.9
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ng/mL) was noticed. The level of TSH and T4 was decreases during mid pregnancy but the
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increase in the level of T3 was noticed and it is highest as compare to other stages and it is found
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to be 1.4 ng/mL, 1.5 ng/mL, 69 ng/mL respectively. During late pregnancy, TSH and T3
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concentration decreases but the increase in the level of T4 was observed and it is found to be 0.9
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ng/mL, 60 ng/mL, 2.9 ng/mL respectively (Table 3,Graph 3).
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Table 3 Hormonal concentration of Thyroid hormone and TSH during various phases pregnancy
Concentration of hormone (ng/mL)
Reproductive period
TSH
±SEM
T3
±SEM
T4
±SEM
Early pregnancy
1.7
±0.13
54
±1.80
2.9
±0.14
Mid-pregnancy
1.4
±0.12
69
±1.91
1.5
±0.05
Late-pregnancy
0.9
±0.09
60
±1.28
2.9
±0.11
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Graph 3 Hormonal concentration of Thyroid hormone and TSH during various phases pregnancy
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2 Discussion
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Thyroid is an endocrine gland that secretes hormones including thyroglobulin, triiodothyronine
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and thyroxin. Thyroxin hormone secreted by this gland plays an important role in metabolism of
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the body (Turner, 1966). It is a metabolically important gland, which is a suggested to be
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essential for the normal maintenance of reproductive function, impairment of thyroid activity
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may be inhibitory to reproduction (Peebles et al., 1984; Jannini et al., 1995).
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Pregnancy alters thyroid status in rodents. In the rat, pregnancy results in decreased total T4 and
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T3 concentrations and enlarged thyroid gland volume. However, unlike the case of humans,
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iodine up take is decreased in pregnant rats, and urinary iodide excretion remains unaltered the
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last days of gestation (Calvo et al., 1999, Feldman, 1958. Versloot et al., 1997).
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In Taphozous kachhensis, the location of thyroid gland in the body is similar to other large
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animals like cattle and buffaloes (Getty et al., 1986) and Camel (Kausar et al., 2006) i.e., with
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the first ring of trachea and consisted of two lobes on both side and an isthmus connecting these
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lobes. In Taphozous kachhensis the gland appeared reddish brown in colour which is in
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concordance with the findings of Schwartz and Dioli (1992). The present study also showed a
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close relationship between changes in the weight and morphological features of the thyroid and
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the ovarian cycle of Taphozous kachhensis.
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Histological studies revealed similar results as reported in camel (Abdel-Magied et al, 2000;
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Atoji et al., 1999) i.e. the gland consisted of follicles of variable sizes with small size follicle
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lined by high cuboidal epithelium to columnar epithelial cells, while the larger ones were lined
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by low cuboidal epithelial cells. Medium size follicles were lined by cuboidal epithelium.
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Similar observation were reported in Taphozous longimanus (Nerkar, 2007), Megaderma lyra
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lyra (Sonwane, 2010).
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In present study the thyroid hormone show marked seasonal variation. The concentrations of
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TSH, T3, and T4 show significant seasonal changes. The hormonal level of TSH, T3, T4 changes
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during early pregnancy and observed1.7 ng/mL, 54ng/mL and 2.9ng/mL respectively. During
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mid pregnancy the T3 level is found to be 69ng/mL and 60ng/mlL during late pregnancy. T4
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level is highest during early and late pregnancy (2.9 ng/mL) and lowest during mid-pregnancy
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(1.5ng/mL). The T3 level is higher than T4 level during reproductive cycle. The serum T3 and
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T4 concentration showed significant variation with changes in reproductive cycle of bat,
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Taphozous longimanus (Singh et al., 2002).
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The thyroid hormones are important for regulation of nutrient assimilation, metabolism,
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calorigenesis (Todini et al, 2007), reproduction (Blaszczyk et al, 2004). The report of thyroid
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hormone on pregnancy was reported in Macrotus californicus (Burn et al., 1972). Thyroid
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hormone plays a important role in the central regulation of body temperature, stimulating the
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thermogenesis and regulating cellular metabolism (Seitz et al., 1985). The metabolism hormone
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Thyroxin (T4) has been implicated in the physiological regulation of energy balance as well as
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maintaining normal reproductive function in mammals (Boswell et al., 1994).
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In ewes, plasma T4 levels is lower during the luteal phase; T3 concentrations were higher during
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the luteal phase, This observation supports the present study.
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During pregnancy, thyroid activity and circulating hormone levels are reported to increase in all
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the investigated mammalian species. Several mechanisms have been claimed to explain these
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observations: increased binding protein concentrations in plasma, secretion of thyrotropic factors
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by the placenta, enhanced responsiveness of pituitary TSH secretion to hypothalamic TRH and
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changes in maternal TH catabolism (De Leo et al., 1998; Glinoer, 2001).Towards the end of
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pregnancy, the goat foetus(es) should play a competitive role (higher thyroid activity, iodine
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affinity and uptake than maternal ones), so that a decrease in maternal plasma T4 concentrations
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has been observed (McDonald et al., 1988).
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Plasma T4 concentration was highest during early pregnancy and decreased gradually, reaching
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lowest values during late pregnancy and post partum (Assane and Sere, 1990; Okab et al., 1993;
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Yildiz et al., 2005). Like in goats, maternal T3 and T4 in twin pregnancy were lower compared
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with single-bearing sheep (Yildiz et al., 2005), especially at the end of pregnancy (Assane and
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Sere, 1990).
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3 Material and Methods
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All bats were trapped alive from Ambai-Nimbai adjacent to Kampa-Tempa . Body weight of
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each bat was recorded. Based on the reproductive cycle of Tapozous kachhensis, females were
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classified into the following three stages:
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Early pregnancy (January to February): Ovary shows well developed introvert corpus luteum
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occupied ¾ part of ovary.
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Mid pregnancy (Late February to Mid April): Ovary shows well developed introvert corpus
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luteum occupied entire ovary except small peripheral region.
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Late pregnancy (Late April to Late May): Ovary shows regress corpus luteum with shrunken
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luteal cells.
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1.1 Collection of Serum and Tissues Histology
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The female bats were sacrificed as soon as they arrived in the laboratory. Their blood serum was
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collected and assayed in laboratory. Ovary and thyroid were excised out from the body cavity
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and excess fat and connective tissue attached were separated out. All the tissues were fixed in
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Alcoholic Bouin’s fluid for 24 h, followed by preservation in 70% ethyl alcohol. Each tissue was
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weighed separately after placement in 70% alcohol. The tissues were dehydrated in ethanol,
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cleared in xylol, embedded in paraffin wax, serial sections at 6 μm were cut and stained with
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haematoxylin and eosin.
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1.2 Morphometry
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Thyroid follicular epithelial height and diameter of colloid was measured using ocular
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micrometer. Measurements were taken from at least 5 different randomly selected thyroid
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follicles. The areal fraction of the colloid within a particular section of the thyroid gland was
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estimated using standard point counting techniques (Weibel, 1979; Singh and Krishna, 1996).
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The sections used for morphometric analysis were selected by systematic random scheme (West,
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1993).
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