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== Embryological Origin ==

== Embryological Origin ==

The thyroid gland is a downgrowth from the pharyngeal endoderm of the developing [[Tongue - Anatomy & Physiology|tongue]]. The calcitonin producing cells are different and are derived from the fourth branchial pouch.  

The thyroid gland is a downgrowth from the pharyngeal endoderm of the developing [[Tongue - Anatomy & Physiology|tongue]].

==Anatomy==

==Anatomy==

===Ultrastructure and Histology===

===Ultrastructure and Histology===

The gland consists of varying sized follicles, which are bounded by a single layer of cuboidal epithelial cells ('''follicular cells'''} and a basement membrane, surrounding a central lumen filled with a homogenous protein rich colloid (thyrogloblin). The apical surface of the cell membranes is covered with numerous micovilli to increase surface area. The follicular cells are connected by tight junctions, and have a dense capillary network. The colloid is a store of thyroid hormones prior to secretion.  The thyroid gland is the only endocrine gland to store its hormone in large quantities. In the active gland colloid is diminished and epithelial cells are tall and columnar. 

The gland consists of varying sized follicles. These are a single layer of cuboidal epithelial cells: '''follicular cells''' surrounding a central lumen filled with a protein rich colloid (thyrogloblin). The apical surface of the cell membranes is covered with numerous micovilli to increase surface area. The follicular cells are connected by tight junctions, and have a dense capillary network.

Within the connective tissue close to the follicles are '''C-cells''' alternatively known as '''parafollicular cells'''. These secrete ''[[calcitonin]]'', a hormone which acts to lower plasma Ca2+ levels.

Within the connective tissue close to the follicles are '''C-cells''' alternatively known as '''parafollicular cells'''. They are found in clusters in the interfollicular space and are also known as '''clear''' cells as their cytoplasm doesn't stain with H and E. They secrete ''[[calcitonin]]'', a hormone which acts to lower plasma Ca2+ levels.

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Image:Thyroid Gland Low Power.jpg|<p>''' Thyroid Gland Low Power 1'''</P>^{©RVC 2008}

Image:Thyroid Gland Low Power.jpg|<p>''' Thyroid Gland Low Power 1'''</P>^{©RVC 2008}

:[[Reproductive System Overview - Anatomy & Physiology|'''Reproductive system''']] is affected by  reduced levels of thyroid hormone causing irregular cycling and decreased libido.  

:[[Reproductive System Overview - Anatomy & Physiology|'''Reproductive system''']] is affected by  reduced levels of thyroid hormone causing irregular cycling and decreased libido.  

:Finally, in the [[Alimentary System Overview - Anatomy & Physiology|'''alimentary system''']], thyroid hormone increases appetite and feed intake, increases secretion of [[Pancreas - Anatomy & Physiology|pancreatic]] enzymes and increases motility.

:Finally, in the [[Alimentary System Overview - Anatomy & Physiology|'''alimentary system''']], thyroid hormone increases appetite and feed intake, increases secretion of [[Pancreas - Anatomy & Physiology|pancreatic]] enzymes and increases motility.

==Problems associated with the Thyroid Gland==

==Problems associated with the Thyroid Gland==

Problems with the thyroid gland include enlargement, or [[Goitre|Goitre]] and also the effects of increased level of hormones in [[Hyperthyroidism|Hyperthyroidism]] or decreased levels of hormones in [[Hypothyroidism|Hypothyroidism]].

Problems with the thyroid gland include enlargement, or [[Goitre|Goitre]] and also the effects of increased level of hormones in [[Hyperthyroidism|Hyperthyroidism]] or decreased levels of hormones in [[Hypothyroidism|Hypothyroidism]].

==Functional Anatomy (summary)==

The thyroid gland lies in the neck, in front of the upper part of the trachea.  Two types of hormones are produced, which are the iodine containing hormones; '''Tri-iodothyronine'''(T3) and '''Thyroxine''' (T4).  Thyroid hormones regulate the basal metabolic rate and are important in the regulation of growth of tissues, particularly nervous tissue.  Release stimulated by TSH from the pituitary. The second type of hormone produced from the thyroid gland is '''Calcitonin''', which regulates blood calcium levels along with parathyroid hormone and acts to reduce blood calcium by inhibiting its removal from bone. 

|flashcards = [[Thyroid_Gland_Flash_Cards_- Anatomy & Physiology|Thyroid Gland]]<br>[[Equine Internal Medicine Q&A 21]]

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|powerpoints = [[Endocrine Histology resource|Histology of the Endocrine system]]

The majority of the gland is derived from a downgrowth of the foetal tongue.  The calcitonin producing cells are different and are derived from the fourth branchial pouch. 

|OVAM = [http://www.onlineveterinaryanatomy.net/content/feline-thyroid-gland-histology Feline Thyroid Gland Histology]

[[Image:Normal thyroid.jpg|right|thumb|125px|<small><center>'''Normal Thyroid'''. Courtesy of A. Jefferies</center></small>]]

|Vetstream = [https://www.vetstream.com/canis/Content/Factsheets/FactSheet020.asp Hypothyroidism]

The throid gland is divided into follicles which are bounded by a single layer of cuboidal epithelial cells and a basement membrane.  Follicles contain a homogenous colloid material called '''thyroglobulin'''.  This is a store of thyroid hormones prior to secretion.  The thyroid gland is the only endocrine gland to store its hormone in large quantities. 

In the active gland colloid is diminished and epithelial cells are tall and columnar.

'''Parafollicular''' cells are found in clusters in the interfollicular space and are also known as '''clear''' cells as their cytoplasm doesn't stain with H and E.

These cells synthesise and secrete calcitonin in response to raised plasma calcium.

 

[[Category:Endocrine System - Anatomy & Physiology]]

[[Category:Endocrine System - Anatomy & Physiology]]

[[Category:To Do - Review]]

[[Category:A&P Done]]