THE NECK: Trachea and Thyroid Gland

What are the dimensions of the Trachea?

5 inches long, 1 inch in diameter

What shape and material form the structural bars of the Trachea?

U-shaped bars of hyaline cartilage

What is the structural composition of the Trachea?

Mobile cartilaginous, membranous tube

Where does the Trachea begin anatomically?

Begins at the neck at the lower border of the cricoid cartilage (level of 6th cervical vertebra / C6)

Where does the Trachea terminate in the thorax and how does it divide?

Ends at the carina by dividing into principal (main) bronchi at the level of the sternal angle (T4-T5 vertebrae)

What structures lie ANTERIOR to the Trachea in the neck?

Isthmus of thyroid gland, inferior thyroid veins, jugular arch, thyroidea ima artery, sternothyroids, and sternohyoids

What structures lie POSTERIOR to the Trachea in the neck?

Right and left recurrent laryngeal nerve, esophagus, and vertebral column

What structures lie LATERAL to the Trachea in the neck?

Lobes of the thyroid gland

Which arteries supply blood to the upper two thirds of the Trachea?

Inferior thyroid arteries

Which arteries supply blood to the lower third of the Trachea?

Bronchial arteries

Which lymph nodes receive drainage from the Trachea?

Pretracheal, paratracheal, and deep cervical lymph nodes

Which nerves supply innervation to the Trachea?

Vagus and Recurrent Laryngeal nerve

What type of epithelium lines the Trachea histologically?

Typical respiratory epithelium

Where are the C-shaped hyaline cartilages of the Trachea located histologically?

Between the submucosa and adventitia

What tissue bridges the posterior opening of the C-shaped tracheal cartilages?

Smooth muscle (trachealis muscle) and fibroelastic tissue

What is a Tracheoesophageal Fistula (TEF)?

Abnormal communication between the trachea and esophagus

With what congenital anomaly is a Tracheoesophageal Fistula frequently associated?

Esophageal atresia

What embryological error results in a Tracheoesophageal Fistula?

Improper division of the foregut by the tracheoesophageal septum

What is the most common variant of Tracheoesophageal Fistula (accounting for 85 to 90% of cases)?

Esophageal atresia with a TEF at the distal end

When should Esophageal Atresia be considered in a fetus?

If mother presents with polyhydramnios or if stomach gas is absent on prenatal ultrasound

What clinical signs of Esophageal Atresia are observed in neonates?

Frothy bubbles of mucus in mouth and nose; secretions persistent in spite of suctioning

What clinical signs of Esophageal Atresia present in infants?

Rattling respiration and episodes of coughing / choking; symptoms worsen during feeding

What structure connects the right and left lobes of the Thyroid Gland across the midline?

Narrow isthmus (located at second, third, and fourth tracheal rings)

What shape and apex/base orientations characterize each lobe of the Thyroid Gland?

Pear-shaped; Apex directed upward to the lamina of thyroid cartilage; Base at the level of the fourth or fifth tracheal rings

Which fascial layer surrounds the Thyroid Gland?

Pretracheal layer of deep fascia

What is the normal weight of the Thyroid Gland?

20 grams

Where is the Pyramidal Lobe of the Thyroid Gland located when present?

Projects upward from the isthmus, usually to the left of the midline

What structure frequently connects the Pyramidal Lobe of the thyroid gland to the hyoid bone?

Fibrous or muscular band called Levator Glandulae Thyroidea

What vertebral level corresponds to the Hyoid Bone?

C3

What vertebral levels correspond to the Thyroid Cartilage?

C4-C5

What vertebral level corresponds to the Cricoid Cartilage?

C6

What structure forms the laryngeal prominence (Adam's apple)?

Thyroid angle produced by the meeting of the thyroid cartilage laminae at the anterior midline

A surgeon encountering significant bleeding during a midline approach below the thyroid isthmus is most likely encountering which variant artery?

Thyroidea Ima artery

Which vessel may be encountered during a low tracheotomy performed below the isthmus of the thyroid?

Inferior thyroid vein

What is the arterial supply and origin for the Superior Thyroid region?

Superior thyroid artery from the external carotid artery

What is the arterial supply and origin for the Inferior Thyroid region?

Inferior thyroid artery from the thyrocervical trunk

What is the venous drainage path for the Superior Thyroid vein?

Superior thyroid vein to the Internal Jugular Vein (IJV)

What is the venous drainage path for the Middle Thyroid vein?

Middle thyroid vein to the Internal Jugular Vein (IJV)

What is the venous drainage path for the Inferior Thyroid vein?

Inferior thyroid vein to the Brachiocephalic vein

What is the prevalence and origin of the Thyroidea Ima Artery?

Present in up to 12% of individuals; arises from the aortic arch or brachiocephalic trunk and reaches the thyroid isthmus inferiorly

When does the Thyroid Gland begin to develop embryologically?

First endocrine gland to develop; 24 days after fertilization

What is the origin of the thyroid diverticulum during embryology?

Median endodermal thickening in the floor of the primordial pharynx forms a small outpouching called thyroid diverticulum

What duct connects the developing thyroid gland to the tongue during its descent?

Thyroglossal duct

What structure is the adult remnant of the thyroglossal duct on the tongue?

Foramen cecum

What is the most common site for an Ectopic Thyroid gland?

LINGUAL (lingual thyroid)

What embryological origin gives rise to the histology of the Thyroid Gland parenchyma?

Foregut endoderm near the base of tongue

What histological structure composes the parenchyma of the Thyroid Gland?

Thyroid follicles

What type of epithelium lines thyroid follicles around a central lumen?

Simple cuboidal epithelium

What tissue separates individual thyroid follicles?

Well-vascularized reticular stroma with fenestrated capillaries

What is Colloid in thyroid follicles?

Gelatinous acidophilic substance filling the follicle lumen containing thyroglobulin

What forms the outer capsule of the Thyroid Gland?

Pretracheal layer of the deep cervical fascia

What forms the true capsule of the Thyroid Gland and how does it organize the gland?

Extends septa into the gland to divide it into lobules

What are Thyrocytes and what is their primary function?

Follicular cells that synthesize and secrete thyroid hormones

What is the morphology of Thyrocytes when active vs inactive?

Low columnar when active, squamous when inactive; contains round central nucleus with abundant RER

What are Parafollicular Cells (C cells or clear cells)?

Larger, less intensely staining cells located in the basal lamina or as clusters between follicles with less RER and many granules

What is the primary endocrine function of Parafollicular Cells (C cells)?

Synthesize and secrete calcitonin

Which artery accompanies the External Laryngeal Nerve?

Superior thyroid artery

What muscles are supplied by the External Laryngeal Nerve?

Cricothyroid muscle and inferior pharyngeal constrictor

What clinical deficit occurs upon injury to the External Laryngeal Nerve?

Inability to register high notes

Which artery accompanies the Recurrent Laryngeal Nerve?

Inferior thyroid artery

Which muscles and sensory regions are supplied by the Recurrent Laryngeal Nerve?

Intrinsic muscles of the larynx except cricothyroid; sensory fibers to the larynx below the vocal cords

What clinical deficit occurs upon injury to the Recurrent Laryngeal Nerve?

Hoarseness

Which artery accompanies the Internal Laryngeal Nerve?

Superior laryngeal artery

What sensory fibers are supplied by the Internal Laryngeal Nerve?

Sensory fibers to the larynx above the vocal cord and taste fibers to the epiglottis

What clinical complication arises from injury to the Internal Laryngeal Nerve?

Recurrent aspiration

What physical finding is typical for a Thyroglossal Duct Cyst?

Painless, progressively enlarging movable mass (asymptomatic unless infected)

What physical examination maneuver is performed to confirm a Thyroglossal Duct Cyst?

Ask the patient to PROTRUDE the tongue - the mass will move

What forms following infection and perforation of a thyroglossal duct cyst?

Thyroglossal duct sinus that opens in the median plane of the neck

What is the anatomical location difference between a Thyroglossal Duct Cyst and a Branchial Cleft Cyst?

Thyroglossal duct cyst is midline (near hyoid bone); Branchial cleft cyst is lateral to midline (anterior to SCM)

How do a Thyroglossal Duct Cyst and a Branchial Cleft Cyst differ during swallowing or tongue protrusion?

Thyroglossal duct cyst moves up with swallowing or tongue protrusion; Branchial cleft cyst does not move with swallowing or tongue protrusion