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AN43.1-9 | Head & neck Joints, Histology, Development, Radiography & Surface marking — Self-Directed Learning

Part 1: Atlantooccipital and Atlantoaxial Joints (AN43.1)

Atlantooccipital Joint

• Type: Synovial, ellipsoid (condyloid) joint — bilateral between occipital condyles and superior articular facets of atlas
• Movements: Nodding (yes movement) — flexion and extension; small amount of lateral flexion (but NO rotation)
• Muscles producing flexion: Prevertebral muscles (longus capitis, rectus capitis anterior)
• Muscles producing extension: Suboccipital muscles (rectus capitis posterior major and minor, obliquus capitis superior); trapezius; splenius capitis
• Ligaments: Anterior and posterior atlanto-occipital membranes; capsular ligaments

Atlantoaxial Joint

• Type: Three joints: Two lateral synovial joints (atlas lateral masses on axis superior articular facets) + One median atlantoaxial joint (dens of axis articulating with anterior arch of atlas + held by transverse ligament of atlas)
• Movements: Rotation (no movement) — head rotation (shaking "no"); approximately 40–45° each side; contributes 50% of total cervical rotation
• Key ligament: Transverse ligament of atlas — the most important stabilising structure; keeps the dens against the anterior arch of atlas; prevents dens from compressing the spinal cord

The alar ligaments (from dens to occipital condyles) limit excessive rotation.

Atlantoaxial interval (ADI) on lateral cervical X-ray:
• Adults: ≤ 3 mm
• Children: ≤ 5 mm
• >3 mm in adults = transverse ligament rupture or laxity; immediate neurosurgical concern

Clinical conditions causing atlantoaxial instability:
• Down syndrome — lax transverse ligament due to connective tissue hypotonia; all Down syndrome patients should have cervical X-ray before contact sports
• Rheumatoid arthritis — pannus formation erodes the transverse ligament
• Odontoid fractures (Type II most common) — dens fracture at the base; high-velocity trauma
• Grisel syndrome — non-traumatic atlantoaxial subluxation from ligamentous laxity due to head and neck infection (retropharyngeal abscess, tonsillitis) — seen in children

Part 2: Head & Neck Histology (AN43.2–43.3)

Key H&N Structures in Histology Practicals (AN43.2)

Thyroid Gland:
• Follicles — spherical units surrounded by simple cuboidal-to-columnar follicular cells
• Colloid — eosinophilic material in the follicle lumen (stored thyroglobulin)
• Parafollicular (C) cells — pale, larger cells between follicles; secrete calcitonin
• Key ID feature: Pink colloid within follicles + surrounding epithelium

Parathyroid Gland:
• Chief (principal) cells — small, dark-staining; secrete PTH
• Oxyphil cells — larger, acidophilic (pink), appear after puberty; function uncertain
• Fat cells — increase with age
• Key ID feature: Densely packed chief cells with scattered pink oxyphil cells; very little stroma

Pituitary Gland (Hypophysis):
• Anterior lobe (adenohypophysis): Acidophils (somatotropes, lactotropes) + Basophils (corticotropes, thyrotropes, gonadotropes) + Chromophobes
• Posterior lobe (neurohypophysis): Pituicytes (glial cells) + Herring bodies (axon terminals storing ADH and oxytocin)
• Key ID feature: Anterior = cellular with colour variation; posterior = pale, paucicellular with Herring bodies

Salivary Glands:
• Parotid — purely serous acini (basophilic, zymogen granules); striated ducts
• Submandibular — mixed (predominantly serous with some mucous); serous demilunes capping mucous acini
• Sublingual — predominantly mucous; few serous demilunes

Palatine Tonsil:
• Stratified squamous non-keratinised epithelium covering + invaginated to form crypts
• Germinal centres (lymphoid follicles with active B-cell proliferation) in connective tissue
• Capsule of fibrous tissue partially enclosing it
• Key ID feature: Crypts lined by squamous epithelium infiltrated by lymphocytes + germinal centres

Cornea:
• 5 layers (anterior to posterior): Stratified squamous non-keratinised epithelium → Bowman's membrane (acellular collagen) → Stroma (regularly arranged collagen lamellae = transparency) → Descemet's membrane (thick basement membrane) → Endothelium (simple squamous = active fluid pump)
• Key ID feature: Regular collagen lamellae in stroma; no blood vessels; regular stratified squamous epithelium anteriorly

Retina:
• 10 layers (from outermost → innermost): RPE → Photoreceptors → Outer limiting membrane → Outer nuclear → Outer plexiform → Inner nuclear → Inner plexiform → Ganglion cell layer → Nerve fibre layer → Inner limiting membrane
• Simplified: RPE (outermost) → Rods/Cones → Bipolar cells → Ganglion cells → Nerve fibres (innermost)

Additional structures for AN43.3:
• Epiglottis: Elastic cartilage core + lingual surface = stratified squamous; laryngeal surface = pseudostratified ciliated columnar (respiratory) with transition zone
• Olfactory epithelium: Pseudostratified columnar with Olfactory receptor cells (bipolar neurons, CN I), Supporting (sustentacular) cells, and Basal cells; Bowman's olfactory glands in lamina propria
• Cochlea — Organ of Corti: Sits on basilar membrane in scala media; Hair cells (inner and outer) with stereocilia; Tectorial membrane above; Tunnel of Corti between inner and outer pillar cells; hair cell deflection → CN VIII (cochlear) signal

Part 3: Development of H&N Structures (AN43.4)

Pharyngeal (Branchial) Arches — Overview

Arch	Skeletal derivatives	Muscles	Nerve	Artery
1st (mandibular)	Malleus, incus, mandible, maxilla, zygomatic, squamous temporal	Muscles of mastication, tensor tympani, tensor palati, mylohyoid, ant. digastric	CN V (trigeminal)	Maxillary artery
2nd (hyoid)	Stapes, styloid process, lesser horn of hyoid, upper body of hyoid	Muscles of facial expression, stapedius, stylohyoid, post. digastric	CN VII (facial)	Stapedial artery (regresses)
3rd	Greater horn + lower body of hyoid	Stylopharyngeus	CN IX (glossopharyngeal)	Common carotid, proximal internal carotid
4th	Thyroid cartilage, epiglottis	Pharyngeal constrictors, cricothyroid	CN X (vagus) — superior laryngeal	Aortic arch (left), right subclavian (right)
6th	Cricoid + arytenoid cartilages	Intrinsic laryngeal muscles (except cricothyroid)	CN X (vagus) — recurrent laryngeal	Ductus arteriosus (left), right pulmonary (right)

Development of the Face

The face develops from 5 processes surrounding the stomodeum (primitive mouth):
• Frontonasal process (1 midline) — forms forehead, bridge of nose, medial nose, philtrum of upper lip, primary palate
• Maxillary processes (2 lateral) — form cheeks, lateral upper lip, secondary palate
• Mandibular processes (2 inferior) — form lower jaw, lower lip, lower face

Cleft lip: Failure of fusion of the maxillary process with the medial nasal process of the frontonasal process. Usually paramedian (lateral to philtrum). Can be unilateral or bilateral. Associated with maternal folic acid deficiency, phenytoin use, and chromosomal anomalies.

Cleft palate: Failure of fusion of the two palatine shelves (from maxillary processes) with each other and/or with the primary palate anteriorly and the nasal septum superiorly. Can be isolated or associated with cleft lip.

Development of the Tongue

• Anterior 2/3: Two lingual swellings + tuberculum impar from the 1st pharyngeal arch → hence general sensory = CN V3, taste = CN VII (chorda tympani accompanies)
• Posterior 1/3: Copula (from 2nd and 3rd arches) → the 3rd arch overgrows the 2nd → hence CN IX supplies posterior 1/3
• Epiglottis: 4th arch + hypobranchial eminence → CN X (superior laryngeal nerve)

Development of the Thyroid Gland

The thyroid develops from the floor of the pharynx at the foramen caecum (between anterior 2/3 and posterior 1/3 of tongue — at the apex of the sulcus terminalis) and descends in front of the pharynx as the thyroglossal duct. It reaches its final position in the neck by week 7. The thyroglossal duct normally obliterates.

Thyroglossal duct cyst: Failure of obliteration → cyst along the thyroglossal duct path; most commonly at or below the hyoid bone; moves upward on swallowing AND on tongue protrusion (connected to tongue via the duct remnant through the hyoid bone — Sistrunk's procedure excises the cyst + middle 1/3 of hyoid bone).

Development of the Pituitary Gland:
• Adenohypophysis (anterior lobe) — from Rathke's pouch (ectodermal upward evagination of the roof of stomodeum)
• Neurohypophysis (posterior lobe) — from infundibulum (neuroectodermal downward outgrowth of hypothalamus)

Craniopharyngioma — tumour arising from Rathke's pouch remnants in the sella turcica → bitemporal hemianopia (compresses optic chiasm) + hypopituitarism

Development of the Eye:
• Optic vesicle (evagination from prosencephalon) → invaginates to form optic cup (double-layered: outer = retinal pigment epithelium, inner = neural retina)
• Lens vesicle — from surface ectoderm induced by the optic cup
• Cornea — from surface ectoderm + neural crest cells
• Congenital anomalies: Coloboma (failure of fetal fissure closure) → notch in iris/retina/choroid; Congenital cataract (rubella, galactosaemia, chromosomal); Cyclopia (failure of forebrain cleavage)

Part 4: Surface Anatomy and Radiography of the H&N (AN43.5–43.9)

Clinical Examination and Surface Projections (AN43.5–43.6)

Hyoid bone: C3 vertebral level; palpable in the midline just above the thyroid cartilage; moves with swallowing; fracture is rare but seen in strangulation/hanging — classic forensic finding

Thyroid cartilage (laryngeal prominence/Adam's apple): C4-C5 level; the thyroid notch is palpable superiorly; in males, the notch is angular; in females, more rounded; the inferior border is at C5

Cricoid cartilage: C6 vertebral level; the cricothyroid membrane is palpable between the thyroid and cricoid cartilages — site of emergency cricothyrotomy (when intubation is impossible)

Carotid arteries:
• Common carotid artery — palpate by compressing it against the carotid tubercle (C6) — the anterior tubercle of the transverse process of C6 (Chassaignac's tubercle); lies medial to SCM
• External carotid — has branches above the hyoid (facial, occipital, etc.)
• Internal carotid — no branches in the neck

Facial artery: Crosses the inferior border of the mandible at the anterior border of masseter — palpable here as a pulse; curves to the angle of mouth and nose

Superficial temporal artery: Palpable in the pre-auricular region (anterior to the tragus); branches on the temporal scalp; temporal artery biopsy site for giant cell (temporal) arteritis — the most common cause of vasculitis in adults over 50

External jugular vein: Visible on surface from angle of mandible, crossing SCM obliquely downward, to the midpoint of the clavicle; used for emergency venous access when peripheral veins are collapsed

Pterion: Junction of frontal, parietal, temporal, and greater wing of sphenoid — the thinnest part of the skull; overlies the anterior division of the middle meningeal artery; a blow to the temple → epidural haematoma

H&N Radiography (AN43.7)

Skull X-ray — lateral view, key structures:
• Pituitary fossa (sella turcica) — in the body of sphenoid; look for enlargement (pituitary tumour) or double floor (asymmetric)
• Calcified pineal gland — midline; displacement indicates space-occupying lesion
• Vascular markings — middle meningeal artery grooves on inner table
• Frontal sinus, maxillary sinus (not well seen), mastoid air cells

Skull X-ray — PA (frontal) view:
• Crista galli — midline bony projection of ethmoid
• Frontal and ethmoidal air sinuses
• Petrous ridge — dense horizontal line crossing the orbits; the dense petrous temporal bone
• Superior orbital fissure — oval lucency within each orbit
• Foramen rotundum — round lucency below each superior orbital fissure (CN V2)

Cervical spine X-ray — lateral view:
• Atlantoaxial interval (ADI) — distance between anterior arch of atlas and dens: ≤3 mm adults, ≤5 mm children
• Prevertebral soft tissue — anterior to C2-C3 = ≤7 mm (widening = retropharyngeal abscess or haematoma)
• Anterior and posterior vertebral lines, spinolaminar line, posterior spinal line (spinous processes) — step = fracture-dislocation
• C7 body is usually the most prominent on lateral view (cervicothoracic junction)

Carotid and Vertebral Angiogram (AN43.8–43.9)

Carotid angiogram anatomical route:
• Femoral artery (most common access) → Seldinger technique → aorta → common carotid artery → internal carotid artery → carotid siphon → intracranial circulation (MCA, ACA)
• Alternatively: radial or brachial artery access
• Structures visualised: Internal carotid artery, carotid bifurcation, common carotid

Vertebral angiogram:
• Femoral artery → subclavian artery → vertebral artery (first branch of subclavian, runs in transverse foramina of C6-C1) → basilar artery → posterior cerebral arteries (PCA)
• Normal vertebral artery enters the transverse foramen at C6 (occasionally C5 or C7 — important variant)

Indications in Indian practice: stroke, carotid stenosis, cerebral aneurysm, AVM, vertebrobasilar TIA