2. IN THIS LECTURE WE WILL LEARN ABOUT
• Microscopic organization of the thyroid gland
• Synthesis and function of thyroid hormones
• Microscopic organization of the parathyroid gland
• Function of parathyroid hormone
3. The thyroid gland is found in the visceral compartment of the anterior neck and is invested by the
pre-tracheal fascia. It is formed from two bilateral lobes, the intervening isthmus and a pyramidal
lobe that arises from the isthmus. It develops from the thyroglossal duct and ultimobranchial body
and is involved in energy and calcium homeostasis.
4. The parenchyma of the thyroid gland is divided by septa/trabeculae of the fibrous capsule into lobules of irregular size and
shape. Septa/trabeculae of the capsule transmit blood vessels and sympathetic nerve supply from the superior, middle and
inferior cervical ganglia. Within each lobule are found spherical/cyst-like follicles composed of a core of proteinaceous colloid
surrounded by a simple epithelium resting on its basement membrane. The stroma around each follicle is formed from delicate
fibers containing a dense network of fenestrated capillaries with rich lymphatic and sympathetic nerve supply.
5. The parenchyma of the thyroid contains
follicular/principal cells and parafollicular/Clear/C-cells.
• Follicular cells function under the
influence of TSH
• Sustained ↑ TSH results in hypertrophy of
follicular cells and increased vascularity
which may lead to thyroid enlargement
• Follicular cells vary in structure depending
on activity level
• Squamous to low cuboidal cells surround
large masses of colloid during low activity
periods
• Columnar cells surround reduced masses of
colloid during periods of high activity
• Follicular cells exhibit features of protein
synthetic cells
• Large, spherical nuclei with one/two
nucleoli
• Abundant supranuclear golgi apparatus
• Abundant basal rER leading to basal
cytoplasmic basophilia
• Active follicular cells present with
functional polarity related to absorption
and secretion
• Apical junctional complexes
• Elongated and branched apical villi
• Colloid resorption droplets in the apical
cytoplasm following endocytosis
• Lysosomes in the apical cytoplasm
6. The follicular colloid is made up of an iodinated glycoprotein, iodothyroglobulin, which is a storage form of the
thyroid hormones T3 and T4. It is synthesised by iodination of tyrosine residues in thyroglobulin (by TPO),
followed by oxidative coupling of MIT and DIT residues to form T3 and T4 residues within the iodothyroglobulin.
Colloid is then resorbed and the T3 and T4 residues are converted to free forms by lysosomes before release into
blood. Thyroid follicles contain sufficient iodothyroglobulin to sustain metabolic activity for 3 months.
• <1% of released T3 and T4 is found in the
free state in blood
• 70% is bound to thyroid binding globulin
• 20% is bound to transthyretin (a serum protein)
• 10% is bound to albumin
• T4 is converted to T3 in peripheral tissues
(e.g heart, kidney, liver) by iodothyronine
deiodinase
• T3 is 5x more potent than T4 at their receptor
• Thyroid hormone transporters deliver T3
and T4 into the cell where they can act on
• Thyroid nuclear receptors
• Mitochondria to increase ATP production
7. Parafollicular/Clear/C-cells are a small population found singly, interspersed among
follicular cells within the basement membrane of thyroid follicles (with no exposure
to colloid), or in clusters within the interfollicular connective tissue. Their cytoplasm
is pale staining on routine preparation.
• Highest density is within the middle
third of each lateral lobe
• Contain numerous secretory vesicles
on LM that appear as dense granules
on EM
• Secreted the peptide
calcitonin/thyrocalcitonin
• Members of Amine Precursor Uptake
Decarboxylation system that also
includes
• Neuroendocrine cells
• Adenohypophysis
• Neurons of Hypothalamus
• C-cells of Thyroid
• Chief Cells of Parathyroid
• Adrenal Medullary Cells
• Glomus cells in Carotid Body
• Melanocytes of Skin
• Cells of Pineal Gland
• Renin producing cells in Kidney
• Enteroendocrine cells
• Kultchisky cells of the Lung
8. The parathyroid glands are small, flattened, lentiform, yellow-brown glands found
on the posterior surface of the thyroid within the pre-tracheal fascia. They possess
a thin capsule that penetrates the gland as septa/trabeculae carrying rich vascular
supply, lymphatics and vasomotor sympathetic innervation. Septa/trabecular DO
NOT divide the gland into lobules. The parenchyma consists of Chief/Principal cells
and Oxyphil cells. Accumulation of adipose begins in puberty and increases with
age.
• Principal/Chief cells are more numerous and are organised into cords
surrounded by networks of fenestrated capillaries and synthesise, store
and secrete the polypeptide parathyroid hormone
• Small, polygonal cells with slightly acidophilic cytoplasm and central nucleus
• Active cells present with protein synthetic machinery and numerous small
vesicles that appear as dense granules on EM
• Inactive cells contain abundant glycogen and lipids and ‘’clear’’ cytoplasm
• Replicate following chronic stimulation
• Oxyphil cells appear just before the onset of puberty, lack a clear secretory
role and their function is poorly understood
• Round cells larger than Principal cells and organized singly or in clusters
• Small darker staining nuclei
• Strong acidophilic reaction due to abundant mitochondria
• Little rER with no secretory vesicle
