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Thyroid Glands REPTILES INTRODUCTION

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Thyroid Glands REPTILES INTRODUCTION Thyroid Glands REPTILES INTRODUCTION • The location and anatomy of the thyroid gland vary among reptiles species. In snakes the thyroid gland is located ventral to the trachea, cranial to base, and caudal to the thymus. It is a single unpaired structure. In lizards and crocodiles the thyroid gland is located within the ventral cranial region. ORIGIN • Since the thyroid gland in snake is located cranial to the heart base and ventral to the trachea, an enlarged Thyroid gland was considered a possible origin Of the coelomic mass in the present case. STRUCTURE AND FUNCTIONS OF THYROID GLAND • Reference to recent general reviews concerning thyroid morphology and physiology reveals a paucity of information of Reptilia as compared with any other vertebrate class ( Eggert, 1938; Goldsmith,1949; Fleischman,1951; Lyunand wachowski; 1951, Gorbman, 1955; kolloros 1959). This paper gives an account of the present state of knowledge of the reptilian Thyroid in the hope of stimulating further researches in this field. HORMONES • The main Thyroid hormones are triiodothyronine (TS)and thyroxine (T4), which is reptiles, participate in growth, development, reproduction, and skin shedding. They also play in reptilian metabolism and behaviour. In most reptile species T4 levels are higher than T3. GROWTH •Most reptiles are characterized by a period of rapid growth that shows upon reaching full adulthood. Growth then ceases altogether a few years after maturity. In contrast, some large bodied species likely have what is known as indeterminate or attenuated growth. AMPHIBIOUS Origin of thyroid gland It is endodermal in origin, develops in the early amphibians itself. Its functional maturation occurs at time of hatching, separated into lobes. Location of thyroid gland •It is located between the eyes. •2 lobed structures located at floor of pharynx. Structure of thyroid gland •Thyroid follicles, parafollicles and colloidal cells are present. •Thyroid follicles secrete thyroxine, stored form as thyroglobulin(large molecule) . FISH Introduction •The gland that secrete their products into the bloodstream and body tissues along with the central nervous system to control and regulate many kinds of body functions are known as endocrine gland. In fishes various endocrine gland has been found associated with different tasks and functions. Here we see about Thyroid gland. Thyroid gland •Thyroid gland that makes and stores hormones that help regulate the heart rate, blood pressure, body temperature and the rate at which food is converted into energy. Thyroid hormones are essential for the function of every cell in the body. •The thyroid hormones of fishes appear to be identical with those of mammals, including mono- and di- iodo-tyrosine and thyroxin. These hormones are kept Stored in the thyroid follicles and are released into blood stream on metabolic demands. Origin • The thyroid gland in fishes arises from the floor of the pharynx as a median evagination. Location The location of thyroid gland varies considerably in different fish species such as, * In cyclostomes, follicles of thyroid are dispersed around the ventral aorta and do not form compact capsulated structure. * In bony fishes, it may lie under the 1st branchial arch on each side. * In many teleosts it is found along the afferent branchial arteries of the gills. * In many teleosts the thyroid gland is situated in the pharyngeal region in between the dorsal basibranchial cartilages and ventral sternohyoid muscle. * In other teleosts, the follicles of thyroid migrate to distant unusual localities, such as the liver, kidney, brain, eye, gut, spleen, gonad etc as in platy fish. * In heteropneustes it occupies almost the entire length of the ventral aorta and afferent arteries. In Clarias batrachus the thyroid gland is concentrated around the ventral aorta, middle each ends of two pairs of afferent and the paired inferior jugular veins. Function SECRETION : Chiefly thyroxine •Thyroid hormones role is regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP and calcium homeostasis. •Thyroid hormones role is oxygen consumption in fishes have been pointed out though it lacks consistency. • Thyroid hormones influences osmoregulation in salmon and gastrosteus. • Thyroid along with other endocrine glands also influences migration in fishes. • It is also known to effect growth and nitrogen metabolism in gold fish. • Scale and bone formation in fishes is also influenced by thyroxine. BIRDS ➢Thyroid gland is one of the first endocrine organs to develop in chick embryo. ➢On day 2 of incubation, the thyroid gland appears as a ventral outpocketing of the pharynx. ➢On day 4 of incubation, the primordial thyroid gland pinches off from the developing digestive tract and remains attached by a thin, two cell layer structure, the thyroglossal duct. ➢The thyroglossalduct eventually regresses to a small isthmus of cells between the two lobes of the adult gland. ➢Which is completed embriologicallyby day 5 of incubation. ➢By day 5 the thyroid gland is bilobed and in the process of relocation to its final, adult position. ➢After day 5 mesenchymal tissue invades the gland and separates the primordial follicular cells into cords or nests. ➢With continuous development, the median diverticulum is displaced caudally, following the myocardial cells in their descent. ➢The primitive stalk connecting the primordiumwith the pharyngeal floor elongates into thyroglossalduct . ➢During its caudal displacement the primordiumassumes a bilobate shape, coming into contact and fusing with the ventral aspect of the fourth pharyngeal pouch. ➢Normally, the thyroglossal duct undergoes dissolution and fragmentation ➢That leaving at its point of origin a small dimple at the junction of the middle and posterior thirds o the tongue ➢Cells of the lower portion of the duct differentiate into thyroid tissue, forming the pyramidal lobe of the gland. ➢At the same time, the lobes contact the ultimobranchial glands, leading to incorporation of C cells into the thyroid. ➢Complex interconnecting, cord-like arrangements of cells interspersed with vascular connective tissue replace the solid epithelial mass and become tubule like structures. ➢The thyroid gland of bird is a paired organ. ➢It is located on the ventral surface of the base of the neck within the thoracic inlet. ➢The left thyroid gland is placed more cranially than the right one . ➢Each thyroid gland is closely connected to the common carotid artery on the medial side from which it is supplied and to the jugular vein on the lateral side. ➢It is a reddish brown organ and of lenticular profile. ➢The gland measures on average 10mm in length, 6mm in width and 2mm in thickness and is covered by a thin connective tissue capsule which holds adipose cells. ➢It seems that each thyroid follicle is surrounded by a net of capillaries. ➢The investigation on scanning microscope proved that the follicles are oval with a pyramidal top on each end, ➢The cuboidal epithelium cells leave impressions in the colloid. ➢Epithelium cells carry microvilli on the follicle side surface. ➢Seasonal changes on thyroid gland in size and activity were able to be confirmed by the examination of the organ in July to December. ➢In winter the follicular cells were higher and the follicles had a greater volume. ➢The structural and functional features of avian thyroid glands are similar to those of mammals. ➢Thyroid hormones have been reported to affect reproduction, growth, metabolism, temperature regulation, moulting and various behaviours. 1. Development of thyroid function in birds ➢ The thyroid axis develops early in some birds, being functional in the chicken by day 13 development. ➢ The embryonic chick thyroid actually responds to TRH and TSH administration on the 6th day of incubation, indicating that although the thyroid glands secretory capacity is all ready to go the endogenous activity of the hypothalamic pituitary connection is still immature. ➢Plasma levels of T4 increase steadily after day 13 to maximal levels by day 20. ➢Levels of T3 remain low until day 19 when there is a surge of T3 production during hatching ➢Conversion of T4 to T3 increases as degradation of T3 decreases. ➢This relationship continues after hatching. Thyroid Gland In Mammals ULTIMOBRANCHIAL GLAND Ultimobranchial gland,in biology any of the small bodies in the pharynx that develop behind the fifth pair of gill pouches in the vertebrate embryo. In mammals the ultimobranchial tissus has become incorapted into the parafollicular cells of the thyroid gland. ultimobranchial glands produce the hormone calcitonin,which reduces the amount of calcium in the blood. STRUCTURE OF UBB The gland itself consists of two oblong lobes lying on either side of the trachea (windpipe)and connected by a narrow band of tissue called the isthmus. In normal adults the thyroid gland weighs 10 to 15 grams,though it has the capacity to grow much larger. In several recent papers descriptions of the positions of the left and right ultimobranchial bodies in the relation to the surrounding organs in the anterior thorax. the present description is based up on the dissections of about the 20 chickens,mainly adults hens but also including a few young cockerels.all the birds were of the white leghorn strain and its possible that birds of other strains might have slightly different anatomical relationship. In semidiagramatic form a ventral view of the anterior thoracic cavity of the young cockerel. The left ultimobranchial body lies in close opposition to the
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