The Endocrine System Anatomy Lectures 3 & 4 Thyroid .

The Endocrine SystemAnatomy Lectures 3 & 4Thyroid & parathyroid glandsByDr. Mohamed fathiAssistant professor Of Anatomy1

By the end of this lectureswe must know*Anatomical position, shape ,weight and capsuleof thyroid gland.*Relation of thyroid gland.*Blood supply, lymphatic drainage and nervesupply of thyroid gland.*Histological features of thyroid gland.*Development of thyroid gland.*Applied anatomy of thyroid gland.*Anatomy, histology and development ofparathyroid gland

The thyroid gland is thelargest endocrine glandin the body.Weight:the average is 25 gm.

Position: It liesin lower partof the front &sides of theneck.

Shape:Butterfly orH shaped having 2 lateral (coneshaped) lobesconnected by anarrow isthmus.LOBEISTHMUS

Each lobe has:-an apex above,-a base below,-3 surfaces and-2 borders

-The narrow medianisthmus may showa small pyramidallobe which may beconnected to thebody of hyoid boneby a fibrous orfibromuscular bandcalled "levatorglandulaethyroidae".The isthmusHyoid boneLevatorGlandulaeThyroidaeLPyramidalLobe

Extensions Of The GlandThe apex of each lat. lobereaches the oblique lineof thyroid cartilage.The base of each lat.Lobe reaches the levelof the 5th or 6thtracheal rings.The isthmus crosses thetrachea opposite therings 2,3,4.The Oblique LineApexBase

It has two capsules1-Inner true fibrouscapsule: condensationof connective tissue.2-Outer false fascialcapsule:Derived from thepretracheal layer ofdeep cervical fascia.N.B. The vessels of thegland run betw. the 2capsules.CapsulesOf The Gland

N.B. The attachment of thepretracheal fascia tothe larynx above isresponsible formovement of the glandup & down withswallowing.Hyoid boneThyroid gl.&cartilage

Relations of the LobesEach Lobe Has 3 Surfaces:Anterolateral, posterolateral, medialEach lobe has 2borders:Anterior & posterior

5Relations ofanterolateralsurfaceAnt. Jug. Vs.Anterolateral surface:1-Skin2-superficial fascia (containing the platysma ms. & AJVs)3- Deep investing fascia enclosing sternomastoid ms.

4- Three pretacheal ms (infrahyoid ms.)Superior belly of omohyoid, Sternohyoid, Sternothyroid .5- Pretacheal fascia (false capsule)Three PretachealmsCapsuleDeep f.investingst. mastoidms.Obliqueline

Posterolateralsurface:1- The carotid sheath(CCA, IJV, & vagusnerve).2- Along the roundedposterior border.Parathyroid glandsAnastomosisbetw. Sup. & inf. thyroidvessels.Symp.trunkIJVCCA

upper part related to2 tubes & nerve.b) Medial surface1-Larynx (thyroid, cricoidcartilages & cricothyroid ms.)2-Pharynx (inferiorconstrictor ms.)3-External laryngealnerve of SLN of vagus.PharynxLarynx

Lower partrelated to2 tubes & nerve.1-trachea.2-oesophagus.3-recurrent laryngealnerve in the groovebetw. the two tubes.

Each lobe has 2bordersAnterior thin border: related to ant. branch ofsuperior thyroid a.Posterior thick border: rounded related to parathyroidglands.Sup.Thyroid a.Ant. Br.Sup. &inf.ParathyroidGlands.

Relations of the isthmusIt has:2 surfaces (ant. post.)2 borders (upper& lower).

Skin & S. fasciaPosterior surface: related to1. trachea (2nd & 3rd & 4thrings).ISTHMUS,surfacesAnterior surface:5related to1. Skin2. Superficial fascia(with ant. Jugularveins).3- Deep investingfascia.4. 2 pretracheal ms.(Sternohyoid &Sternothyroid).4. Pretracheal fascia

ISTHMUS, bordersa.Upper border:related to1-Anastomosingbranches of bothsuperior thyroidarteries2-Pyramidal lobe.b. Lower border:related to1-inferior thyroidveins &2-thyroidea ima a.superior thyroid arteriesPyramidal lobeInferior thyroid veins & thyroidea ima a.

ARTERIAL SUPPLYThe gland ishighly vascularIt is suppliedby3 arteriesa. Superior thyroidb. Inferior thyroidc. Thyroidae ima

a. Superior thyroida.: From the E.C.A. Descends down &medially with ext. LN tillthe apex of the lobe. It supplies the upper 1/3of the lobe & the upper1/2 of the isthmus.E.C.A.

b. Inferiorthyroidartyery:Posteriorview From thethyrocervicaltrunk of the 1stpart of subclavianartery.Inferior thyroidA.Thyrocervical TrunkSubclavianA.

b.Inferiorthyroid a.:passes behindcarotid sheath &middle cervicalganglion.Near the glandit is closelyrelated to theRECURRENTLARYNGEAL N.It supplies thelower 2/3 of thelobe & lower ½of the isthmus.Anterolateralview

c.Thyroidaeima a.:present in 310 %, arisesform brachiocephalictrunk oraortic archsupplyinglower part ofthe isthmus.

By 3 veins1.Superior thyroid: ends in IJV.VENOUS 2.Middle thyroid : Short & Wide. It ends in IJV.DRAINAGE 3.Inferior thyroid: ends in brachiocephalic v.specially the left .

LYMPH DRAINAGELymph vesselsaccompany thearteries &drain intodeep cervicalL.N. & Pre ¶trachealL.N.

*Nerve supply: sympathetic fibersthrough the plexuses that accompany thethyroid arteries. They are vasomotor, notsecretomotor.Sup. sympatheticganglionMiddlesympatheticganglion

Applied Anatomy

Enlarged thyroid glandmay compress thetrachea dyspneaOr esophagus dysphagia.Enlarged thyroid is calledGOITER

Swellings of the Thyroid Glandand Movement on SwallowingThe thyroid gland is invested in a sheathderived from the PRETRACHEAL FASCIA.This tethers the gland to the larynx and thetrachea.so any pathologic neck swelling that is part ofthe thyroid gland will MOVE UPWARDwhen the patient is asked to swallow.

Why does a thyroidswelling growdownward & notupward?The attachment of theSternothyroid Muscles tothe thyroid cartilage bindsdown the thyroid gland tothe larynx and preventsupward expansion of thegland.An enlarged thyroid glandcan extend downwardbehind the sternum(retrosternal goiter).ThyroidcartilageSternothyroid ms.Sup. Thyroid a.CricoidcartilageCoronal sectionThyroidlobe

Thyroid Arteries And Laryngeal NervesTo avoid injury of theexternal laryngealnerve duringthyroidectomyThe superior thyroidartery should beligated as near aspossible to the glandor even within theapex of the gland.Sup.Thy. A&Ext.LNThy.Cart.Cr.thy.COTInf. Thy.A. &RLN

Thyroid Arteries And Laryngeal NervesTo avoid injury of therecurrent laryngealnerve duringthyroidectomy.Ligation of theinferior thyroidartery should be asfar as possible fromthe base of thegland.Sup.Thy. A&Ext.LNInf. Thy.A. &RLN

In partialthyroidectomy, theposterior part of thethyroid gland is leftundisturbed so thatthe parathyroidglands are notdamaged.Removal of theparathyroid glandsduring the surgerymay lead tohypocalcemia andtetany.Thyroidectomyandthe Parathyroid Glands

Histological structure of thyroid gland The gland is formed of stroma and parenchyma.A-Stroma:*The gland is covered by capsule.*Septa: divide the gland into ill-defined lobulesand carry bl.vs and ns.

B-Parynchema:*Formed of epithilial cells which form thethyroid follicle which is the functionalunit of thyroid gland.*The follicles is spherical shape filled withcolloid ( formed of thyroglobulinprotein)which is acidophilic and PASpositive.*The lining of follicles is : Follicular cells98% Parafollicular cells 2%

1-Follicular cells:In normal functionating gland :cuboidal cells (basophiliccytoplasm and central, prominentand rounded nucleus).Function: synthesis of thyroidhormone.2- parafollicular cells:Larger and paler than follicularcells.Large rounded cells with sphericalnucleus.Function: synthesis of calcitonin(antagonize the parathyroidhormone) as it decrease the Ca level if exceed than normal level.

Development of the Thyroid Gland* Time: during the 3rd week of development.* It appears as an epithelial thickening in the floor of thepharynx at a point later indicated by the foramen cecum(which lies between tuberculum impar & hypobranchialeminence or between anterior 2/3 & posterior 1/3 oftongue).

Development of the Thyroid Gland* This thickening becomes a diverticulum.* Subsequently the thyroid descends in front of thepharynx as a bilobed diverticulm.* During this migration the thyroid remains connectedto the tongue by a narrow canal, the thyroglossal duct.This duct later disappears.

Development of the Thyroid Gland* With further development, thethyroid gland descends in front of thehyoid bone and the laryngealcartilages.* It reaches its final position in front ofthe trachea in the 7th week ofdevelopment.* By then, it has acquired a smallmedian isthmus and two lateral lobes.* The thyroid gland begins to functionat approximately the end of the 3rdmonth, at which time the first folliclescontaining colloid become visible.

Development of the Thyroid Gland* Cells at the lower end of the thyroglossalduct proliferate into 1ry and 2rythyroid follicles which start to functionat the end of the 3rd month.* The ultimobranchial body (from the 5thpouch) invades the gland theparafollicular or C-cells which secretecalcitonin.* Fate of the rest of the thyroglossal duct :1. The infrahyoid portion the levatorglandulae thyroidae the pyramidallobe.2. The suprahyoid portion disappears.3. The site of origin of the duct is markedby the foramen caecum at the apex ofsulcus terminalis of the tongue.

Development of the Thyroid Gland* Thyroglossal cysts:* Represent the mostcommon congenitalanomaly of the neck.* They arise from apersistent epithelial tract,the thyroglossal duct,formed with the descentof the thyroid from theforamen caecum to itsfinal position in the frontof the neck.

* Anomalies of thyroid glanddevelopment:1. Congenital cretinism :due to congenital absence ofthyroid gland.2. Aberrant thyroid tissue :lingual thyroid, suprahyoid,retrohyoid, infrahyoid orretrosternal3. Thyroglossal cyst and fistula:The fistula is due to ruptureof the cyst. They differ frombranchial cyst and fistula inbeing close to the midline andin moving up with deglutition.

ParathyroidGlands*Two pairs of smallendocrine glands lying on theposterior border of thyroidgland withinits capsule.*Shape:isoval.*Size:- 6 x 4 x2 mm.

Parathyroid Glands* Site:* Superior one: lies at the middle ofposterior border of thyroid gland.* Inferior one: has variable sites.a. Below inferior thyroid artery nearto the lower pole of the thyroidlobe.b. Outside the capsule immediatelyabove the inf. thyroid artery.*Blood supply: Inferior thyroid A.* Veins & LNs.: as thyroid gland.

Histology of the Parathyroid Glands* The parenchyma of the gland ismade up of two identifiable celltypes: the predominant chief(principal) cells (source ofparathyroid hormone) andoccasional oxyphil cells.* The chief cells are arranged asinterconnecting cords or clusters,with blood vessels andconnective tissue forming thepartitions between the cellcords.

Histology of the Parathyroid Glands(contd) The chief cells, smallpolygonalcellswithbasophilic cytoplasm andcentral vesicular nuclei. Function : synthesis ofparathyroid hormone The oxyphil cells arepolygonal cells with darkand large nuclei withacidophilic cytoplasm.* The function of oxyphilcells is yet unknown.

Thyroid & parathyroid glands By Dr. Mohamed fathi Assistant professor Of Anatomy 1. By the end of this lectures we must know *Anatomical position, shape ,weight and capsule of thyroid gland. *Relation of thyroid gland. . Histology