thyroid gland-dr. darwish badran

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Thyroid gland

Anatomy, Histology andEmbryology

1DR. DARWISH H. BADRAN



Anatomy of

the thyroidgland




Lies in front and sides of the neck.

Consists of two lobe connected anterior to thetrachea by an isthmus, weighs 10-20 gm.

A small pyramidal lobe projects upwards from theleft lobe in 40% of cases.

The pyramidal lobe is connected to the hyoid boneby fibrous band, that may contain few smoothmuscle fibres; levator glandulae thyroidae.

Each lobe is conical in shape having: • Apex: rests on thyroid cartilage, and reaches its oblique

line. • Base: reaches 5-6 tracheal ring. • Isthmus: lies on tracheal rings 2-4.




Rounded and coveredby: • Skin, superficial fascia

including the platysmamuscle.

• Pretracheal fascia • Infrahyoid muscles except

thyrohyoid.

• Anterior border of sternocleidomastoid.

Surfaces and relations.. 1

Anterolateral (superficial) surface

5

Thyrohoid

Sternothyroid

Sternohyoid

Omohyoid

DR. DARWISH H. BADRAN



Slightly concave andrelated to: • Thyroid and cricoidcartilages. • Cricothyroid and inferior

pharyngeal constrictormuscles.

• Trachea and esophagus. • External laryngeal and

recurrent laryngealnerves.


Medial surface




Related to: • Common carotidartery in front of the

longus coli muscle. • Parathyroid glands. • Inferior thyroid

artery.


posterior surface




Superior thyroidartery: • The 1st branch from the

anterior aspect of theexternal carotid artery.

• Supplies the upper 1/3 of the thyroid lobe and upper

½ of the isthmus.

• Runs with the externallaryngeal nerve for part of its course and then divergesaway close to the gland.

Arterial supply of the thyroid gland..1




Inferior thyroid artery: • Branch from the

thyrocervical trunk from the

1st part of the subclavianartery.

• Supplies the lower 2/3 of the thyroid lobe and lower½ of the isthmus.

• Forms a loop before itreaches the gland, itsbranches are related to therecurrent laryngeal nerve.





Thyroidea ima artery: • An occasional branch that

is rarely seen. • If present it supplies the

isthmus. • It originates directly from

the aortic arch or the

brachiocephalic artery. • If accidently cut during

surgery it retracts to thethorax.





Superior thyroid vein: • Drains the apex of each lobe →

jugular or common facial vein. Middle thyroid vein: • Drains the lateral aspect of the lobes

→ internal jugular vein. Inferior thyroid vein (or veins):

• Drains the basal part of the gland →brachiocephalic vein.

Venous drainage of the thyroid gland




Prelaryngeal nodes: in front of thecricothyroid muscle. Pretracheal nodes: in front of the

trachea. Paratracheal nodes: alongside thetrachea. Upper and lower deep cervical nodes:alongside the internal jugular vein. Brachiocephalic nodes: in the superiormediastinum.

Lymphatic drainage of the thyroid gland




Histology of

the thyroidgland




Thyroid follicle: • The structural and functional unit of the thyroid

gland. • Consists of a group of cells resting on the same

basal lamina surrounding a lumen filled withcolloid.

• The follicles are roughly spherical and variablein size.

• Hormones are stored in the follicles. • Each follicle is surrounded by variable amount

of connective tissue, which is highly vascular. 17DR. DARWISH H. BADRAN



Follicular epitheliumcontains two types of

cells:

Follicular cells(principal cells) Parafollicularcells (C cells)




Follicular cells (principal cells):

• Squamous-columnar cells according to activity. • Basophilic cytoplasm. • Nucleus: round-ovoid with 2 nucleoli.

• Supra-nuclear Golgi, many rER. • Supranuclear Golgi complex. • Apical microvilli. • Numerous vesicles, mitochondria and

lysosomes are seen in in the apical cytoplasm. 21DR. DARWISH H. BADRAN



Parafollicular cells (Clear cells, C cells): • Pale staining, larger than follicular cells. • Occur singly or in and included in the same basal

lamina of the follicle. • Overlapped by follicular cells.

• E.M: • Moderate rER. • Well-developed Golgi. • small, dense, basal secretory granules.

• Secrete calcitonin: • Inhibits bone resorption by osteoclasts. • Stimulated when Ca2 is high.




C

C

C




Thyroid gland function isessential to normalgrowth and development.




T3 and T4 are synthesized and

secreted by follicular cells.

Both hormones:

• Regulate cell and tissue basal metabolism

• Regulate heat production• Influence body growth and development




Parafollicular cells synthesize and secretecalcitonin, which

• Physiologic antagonist to parathormone• Has an important role in regulating serum calcium

• It lowers calcium level by suppressing the resorptive actionof osteoclasts and promotes calcium deposition in bones.

High calcium blood level stimulate calcitoninsecretion




Calcitonin is secreted by several endocrine tumors;that is why it is considered as a tumor marker tomonitor the progress of certain tumors.

Calcitonin is used to treat certain bone diseases(osteoporosis, Paget’s disease)

No disease has been associated with its deficiency oreven its absence after thyroidectomy.




Synthesis of T3 and T4 Regulated by:

• Iodide level in the follicular cells. • Binding of TSH to its receptors on follicular cells.

Thyroglobulin is synthesized on rER. Glycosylation of thyroglobulin occurs on rER and Golgi. Vesicles are transported to apical plasmalemma. Vesicular content is released into the colloid and stored in the lumen.




Steps of T3 and T4 formation and release

Release of T3 and T4. Resorption of colloid.

Formation of T3 and T4. Iodination of thyroglobulin.

Resorption, diffusion, and oxidation of iodide. Synthesis of thyroglobulin.




Iodine is reduced to iodide in the alimentary canal.

Iodide is transported to the thyroid gland.

Iodide is actively absorbed at the basal part of the cell.

In the cytoplasm iodide is oxidized in the presence of H

2O

2.

Activated iodideenters colloidiodination of tyrosineresidues of thyroglobulin.




Release of T3 and T4

Binding of TSH on the basalplasmalemma of follicular cells

formation of apical filopodia endocytosis of colloidcleavageof thyroglobulin by proteases

transfer to cytoplasm as T1, T2,T3, T4.




Thyroid

disorders




Embryology

of thethyroid gland




The thyroid gland is the first of the body'sendocrine glands to develop, on

approximately the 24th day of gestation.

It originates as a proliferation of endodermalepithelial cells on the median surface of the

developing pharyngeal floor which liesbetween 2 key structures, the tuberculumimpar and the copula, and is known as theforamen cecum.

It initially develops inferior to the

tuberculum impar, which is also known asthe median tongue bud. This embryonicswelling arises from the first pharyngealarch and occurs in the midline on the floor of the developing pharynx, eventually helpingform the tongue as the 2 lateral lingualswellings overgrow it.




The foramen cecum begins rostral tothe copula, also known as the

hypobranchial eminence. This medianembryologic swelling consists of mesoderm that arises from the secondpharyngeal pouch (although the thirdand fourth pouches are also involved).The thyroid gland, therefore,originates from between the first andsecond pouches.

The initial thyroid precursor, thethyroid primordium, starts as a simplemidline thickening and develops to

form the thyroid diverticulum. Thisstructure is initially hollow, although itlater solidifies and becomes bilobed.




The initial descent of the thyroid gland occurs anterior to the pharynx while the thyroidis still connected to the tongue via the thyroglossal duct.

The tubular duct later solidifies and subsequently obliterates entirely (during gestationalweeks 7-10).

The foramen cecum represents the opening of the thyroglossal duct into the tongue.

A pyramidal lobe of the thyroid may be observed in ~ 60% of cases. This lobe representsa persistence of the inferior end of the thyroglossal duct that has failed to obliterate.

Further descent of the thyroid gland carries it anterior (or ventral) to the hyoid bone and,subsequently, anterior (or ventral) to the laryngeal cartilages.

As the thyroid gland descends, it forms its mature shape, with a median isthmusconnecting 2 lateral lobes.

The thyroid completes its descent in the seventh gestational week, coming to rest in itsfinal location immediately anterior to the trachea.




Parafollicular (C cells) are a special subset of cells within the thyroidgland that secrete calcitonin.

The parafollicular cells arise from the ultimobranchial body. This bodyrepresents the last structure derived from the branchial pouches, hence itsname.

The ultimobranchial body arises from the fifth pharyngeal pouch, which isalternately described as the ventral portion of the fourth pharyngeal pouch.(Whether fifth pharyngeal pouches actually exist is debatable.)

Migrating cells from the neural crest region infiltrate the ultimobranchialbody. This structure is then incorporated into the thyroid gland, as theultimobranchial body fuses with the thyroid gland and disseminates itscells into it. The C cells of the thyroid, therefore, are of neural crest origin




The superior parathyroid glands are also known as parathyroid IVs becausethey arise from the dorsal wing of the fourth pharyngeal pouch, differentiatingat gestational weeks 5-6.

At gestational week 7, the glands lose connections with the pharynx and attachthemselves to the thyroid gland, which is migrating caudally, albeit far less a

migration than the thymus (with parathyroid IIIs as described above).

Because of the lesser length of migration, the superior parathyroid glands (IV)are in a more constant location than the inferior parathyroids (III).

The superior parathyroids are generally located more posterior and medial thanthe inferior parathyroids, and their final resting point is usually on the dorsalsurface of the thyroid gland, outside the fibrous capsule of the thyroid gland.




The inferior parathyroid glands are also known as parathyroid IIIs becausethey arise from the dorsal wing of the third pharyngeal pouch.

The third brachial pouch differentiates at gestational weeks 5-6, with theventral wing becoming the thymus. The thymus and parathyroids both losetheir connections to the pharynx at gestational week 7.

The thymus then migrates caudally and medially, pulling the parathyroidswith it; therefore, parathyroid IIIs are further inferior than are parathyroidIVs. The parathyroid in turn loses its connection with the thymus.

The inferior parathyroid glands usually stop at the dorsal surface of thethyroid gland, outside of the fibrous capsule of the gland itself


thyroid gland-dr. darwish badran

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