NEET MDS Lessons
Anatomy
Tongue
Appears at 4th week.
Musculature derived from mesoderm of occipital somites. Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue. Midline frenulum continues to anchor tongue to floor of mouth.
Thyroid Gland
Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum). It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development. Thyroid gland descends to a point just caudal to laryngeal cartilages.
Facial structures (general)
a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines
Nose
At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour. Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP. Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes). These nasal prominences form the nose.
Mouth
Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch. The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence. The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion. Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).
Nasolacrimal structures
Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove. Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm. The epithelial cord canalizes to form the nasolacrimal duct. The upper end of the duct widens to form the lacrimal sac. After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.
Secondary (hard) palate
Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences. These appear at the 6th week. They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate. At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate. The incisive foramen marks the midline between the primary and secondary palate.
External Ear
The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear). These fuse to form the definitive auricle. At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.
Middle Ear
The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).
The first arch cartilage forms the malleus and incus. The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes. The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.
Inner Ear
The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes). The placodes invaginate to form otic vesicles (otocytes). The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.
Cochlear Duct
Derived from an outgrowth of the saccule during the 6th week. The outgrowth penetrates the surrounding mesenchyme in a spiral fashion. The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct. They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.
Semicircular canals
The utricle is initially three flattened outpocketings, which lose the central core. From this three semicircular canals are forms, each at 90 degree angles from one another. Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule.
The Inferior Wall of the Orbit
- The thin inferior wall of the orbit or the floor is formed mainly by the orbital surface of the maxilla and partly by the zygomatic bone, and orbital process of the palatine bone.
- The floor of the orbit forms the roof of the maxillary sinus.
- The floor is partly separated from the lateral wall of the orbit by the inferior orbital fissure.
The Palate
- The palate forms the arched roof of the mouth and the floor of the nasal cavities.
- The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.
Eye
At week 4, two depressions are evident on each of the forebrain hemispheres. As the anterior neural fold closes, the optic pits elongate to form the optic vesicles. The optic vesicles remain connected to the forebrain by optic stalks.
The invagination of the optic vesicles forms a bilayered optic cup. The bilayered cup becomes the dual layered retina (neural and pigmented layer)
Surface ectoderm forms the lens placode, which invaginates with the optic cup.
The optic stalk is deficient ventrally to contain choroids fissure to allow blood vessels into the eye (hyaloid artery). The artery feeds the growing lens, but will its distal portion will eventually degenerate such that the adult lens receives no hyaloid vasculature.
At the 7th week, the choroids fissure closes and walls fuse as the retinal nerve get bigger.
The anterior rim of the optic vesicles forms the retina and iris. The iris is an outgrowth of the distal edge of the retina.
Optic vesicles induces/maintains the development of the lens vesicle, which forms the definitive lens. Following separation of the lens vesicle from the surface ectoderm, the cornea develops in the anterior 1/5th of the eye.
The lens and retina are surrounded by mesenchyme which forms a tough connective tissue, the sclera, that is continuous with the dura mater around the optic nerve.
Iridopupillary membrane forms to separate the anterior and posterior chambers of the eye. The membrane breaks down to allow for the pupil
Mesenchyme surrounding the forming eye forms musculature (ciliary muscles and pupillary muscles – from somitomeres 1 and 2; innervated by CN III), supportive connective tissue elements and vasculature.
Eyelids
Formed by an outgrowth of ectoderm that is fused at its midline in the 2nd trimester, but later reopen.
The Cheeks
- The cheeks (L. buccae) form the lateral wall of the vestibule of the oral cavity.
- They have essentially the same structure as the lips with which they are continuous.
- The principal muscular component of the cheeks is the buccinator muscle.
- Superficial to the fascia covering this muscle is the buccal fatpad that gives cheeks their rounded contour, especially in infants.
- The lips and cheeks act as a functional unit (e.g. during sucking, blowing, eating, etc.).
- They act as an oral sphincter in pushing food from the vestibule to the oral cavity proper.
- The tongue and buccinator muscle keep the food between the molar teeth during chewing.
Sensory Nerves of the Cheeks
- These are branches of the maxillary and mandibular nerves.
- They supply the skin of the cheeks and the mucous membrane lining the cheeks.
The Orbital Vessels
- The orbital contents are supplied chiefly by the ophthalmic artery.
- The infraorbital artery, the continuation of the maxillary, also contributes blood to this region.
- Venous drainage is through the superior orbital fissure to enter the cavernous sinus.
The Ophthalmic Artery
- This artery arises from the internal carotid artery as it emerges from the cavernous sinus.
- It passes through the optic foramen within the dural sheath of the optic nerve and runs anteriorly, close to the superomedial wall of the orbit.
The Central Artery of the Retina
- This is the one of the smallest but most important branches of the ophthalmic artery.
- It arises inferior to the optic nerve until it approaches the eyeball.
- It then pierces the optic nerve and runs within it to emerge through the optic disc.
- The central artery of the retina spreads over the internal surface of the retina and supplies it.
The Ophthalmic Veins
The Superior Ophthalmic Vein
- The superior ophthalmic vein anastomoses with the facial vein.
- It has no valves and blood can flow in either direction.
- It crosses superior to the optic nerve, passes through the superior orbital fissure and ends in the cavernous sinus.
The Inferior Ophthalmic Vein
- This begins as a plexus on the floor of the orbit.
- It communicates with the inferior orbital fissure with the pterygoid plexus, crosses inferior to the optic nerve, and ends in either the superior ophthalmic vein or the cavernous sinus.
-> This is a wedge-shaped bone (G. sphen, wedge) is located anteriorly to the temporal bones.
-> It is a key bone in the cranium because it articulates with eight bones (frontal, parietal, temporal, occipital, vomer, zygomatic, palatine, and ethmoid).
-> It main parts are the body and the greater and lesser wings, which spread laterally from the body.
-> The superior surface of its body is shaped like a Turkish saddle (L. sella, a saddle); hence its name sella turcica.
-> It forms the hypophyseal fossa which contains the hypophysis cerebri or pituitary gland.
-> The sella turcica is bounded posteriorly by the dorsum sellae, a square plate of bone that projects superiorly and has a posterior clinoid process on each side.
-> Inside the body of the sphenoid bone, there are right and left sphenoid sinuses. The floor of the sella turcica forms the roof of these paranasal sinuses.
-> Studies of the sella turcica and hypophyseal fossa in radiographs or by other imaging techniques are important because they may reflect pathological changes such as a pituitary tumour or an aneurysm of the internal carotid artery. Decalcification of the dorsum sellae is one of the signs of a generalised increase in intracranial pressure.