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Aortic Arches
- Ventral and dorsal aorta are connected by 6 aortic arches.
- 1st, 2nd and 5th disappears.
- 3rd, 4th and 6th persist.
- 3rd- Carotid Arch with 3 outgrowths:-
- Common carotid
- External carotid
- Internal carotid
- Left 4th Arch-
- Arch of Aorta
- Right 4th Arch-
- Right subclavian artery
- 6th Arch-
- Divided into dorsal and ventral (or proximal and distal) part by the formation of branch to lung bud.
- Ventral part on either side forms the pulmonary arteries.
- Dorsal part on right side disappears but the left side persists to form Ductus Arteriosus.
- As the 6th arch disappears the recurrent laryngeal nerve ascends upwards and hooks around the right subclavian artery on right side and ductus arteriosus on left side(arch of aorta in adult life at the same level).
- Remnant of Ductus Arteriosus = Ligamentum Arteriosum
- NB: Pulmonary trunk is derived from truncus arteriosus
Heart Tube & Derivatives
- Various parts of fused endothelial heart tube:-
- Bulbus cordis
- Conus
- Truncus arteriosus
- Primitive ventricle
- Primitive atrium
- Unfused parts at venous end- horns of sinus venosus
- Bulbus cordis
- Structures derived from truncus arteriosus:-
- Ascending aorta
- Pulmonary trunk
- Fate of Bulbus Cordis:-
- Conus part gets absorbed into left and right ventricle.
- Conus after getting absorbed gives rise to:-
- Outflowing part of Right Ventricle called: Infundibulum
- Outflowing Part of Left Ventricle called: Aorta Vestibule
- Right horn of Sinus Venosus is enlarged and is absorbed into right atrium forming the posterior smooth part of Right atrium called Sinus Venorum.
- The anterior Rough part of right atrium derived from right half of primitive atrial chamber.
- Ventricle getting divided into 2 halves due to formation of Interventricular Septum.
- The left horn undergoes degeneration and forms coronary sinus.
- Margin of Pericardium absent in model.
- During formation of Cardiac Loop:-
- Bulbus cordis and ventricular part descends downwards anterior and to the right. OR Ventral, Caudal and to the right (VCR)
- Atrium and Sinus Venosus go up- Dorso Cranially.(DC)
- Now it looks like:-
- So, Ventricles are ventral and Atria are dorsal.
- Now entire heart gets accommodated into pericardial cavity.
Defect in loop formation: Bulbus Cordis descends to left: Dextrocardia
Interatrial & Interventricular Septum
- Formed by Septum Primum and Septum Secundum arising from roof of common atrial chamber
- Septum Intermedium = AV Cushion, in the atrio ventricular canal.
- Septum Primum descends downwards.
- Osteum Primum formed between Septum primum and septum intermedium.
- Then septum primum descends further and gets fused with septum intermedium
- Osteum primum is closed.
- Another communication develops- Osteum Secundum.
- Septum secundum starts descending
- Lower part of inter atrial septum is formed by Spetum primum only
- Septum secundum keeps descending.
- Septum primum approximates against septum secundum after delivery leading to formation of inter atrial septum and closure of osteum secondum.
- Remnants of Septum Primum in adult life:-
- Fossa Ovalis
- Remnant of Septum Secundum in adult life:-
- Limbus Fossa Ovalis
- Defect in formation of Osteum Secundum- Osteum Secundum Type of Atrial Septal Defect.
- Defect in fusion of Septum primum with septum intermedium = Osteum Primum Type of Atrial Septal Defect.
- Failure of fusion of Septum secundum and primum = Patent Foramen Ovale
- Muscular Part of Inter Ventricular Septum:-
- Formed from floor of common ventricular chamber
- Primary Interventricular Opening present which closed by the formation of the membranous part of the interventricular septum.
- Membranous part is formed by:-
- Formed by the upward ascending up of muscular part of interventricular septum and simultaneous proliferation of septum intermedium.
- Bulbar septum of right ventricle descends downward and joins with proliferated septum intermedium
Pharyngeal Pouch
- Remaining clefts get obliterated by downward growth of 2 arch.
- 1st arch– External acoustic meatus
- Cervical sinus-
- Also gets obliterated.
- May persist to form branchial cyst.
- Cyst present along anterior border of SCM muscle.
- 1st pouch-
- Elongates to form tubo-tympanic recess
- Tubal part forms auditory tube
- Cavity part forms tympanic cavity/middle ear cavity
- In between formation of tympanic membrane.
- Hence tympanic membrane origin is ectodermal, endodermal as well as intervening mesoderm.
- Pars densa part is derived from mesoderm
- 2nd pouch-
- Accumulation of lymphatic nodule around 2nd pouch to form palatine tonsil.
- Tonsillar crypt present in palatine tonsil is remnant of 2nd pouch
- 3rd pouch-
- Ventral part forms thymus
- Dorsal part forms inferior parathyroid
- 4th pouch-
- Ventral part forms ultimo-bronchial body which gives rise to parafollicular cells of thyroid gland
- Dorsal part forms superior parathyroid.
- Sometimes ventral part of 4th pouch fuses with rudimentary 5th pouch to form caudal-pharyngeal complex with three parts:-
- Lateral thyroid element along with developing thyroid from thyroglossal duct.
- Thymic rudiment- gets incorporated along with developing thymus from 3rd pouch.
- Ultimobranchial body
CNS
- Cranial end of neural tube forms brain
- Caudal end form spinal cord
- 3 parts-
- Forebrain/prosencephalon-
- Central part is called diencephalon
- Gives rise to thalamus, hypothalamus
- Cavity is 3rd ventricle
- Outgrowth called telencephalon–
- Cerebrum is derived from telencephalon
- Cavity is lateral ventricle
- Central part is called diencephalon
- Midbrain/mesencephalon-
- Gives rise to midbrain
- Cavity is Cerebral aqueduct / Aqueduct of Sylvius
- Connects 3rd and 4th ventricle
- Hindbrain/rhombencephalon-
- Cavity is 4th ventricle
- Metencephalon–
- Gives rise to pons and cerebellum
- Myelencephalon
- Gives rise to medulla
- Forebrain/prosencephalon-
- Flexures–
- 1st flexure-At junction of rhombencephalon and spinal cord
- 2nd flexure– Mesencephalic
- 3rd flexure– Telencephalic
- Interventricular foramen / Foramen of Monroe
Tongue
- At ventral end of first arch there is formation of 2 lingual swellings and 1 tuberculum impar
- Above 3 form mucus membrane of the anterior 2/3rd of the tongue,
- Hence general sensation is by lingual nerve, branch of mandibular nerve.
- Taste is by chorda tympani nerve, pre-tramatic(?) nerve of 1st arch.
- Hypobranchial eminence-
- Formed mainly by 3rd and 4th arch.
- Due to overgrowth of eminence, 2nd arch is submerged but hypobranchial eminence is NOT derived from 2nd arch.
- Cranial part of the eminence-
- Forms mucus membrane of posterior 1/3rd of tongue.
- Hence supplied by glossopharyngeal nerve.
- Caudal part of eminence-
- Forms epiglottis
- Hence taste receptors on ventral aspect of epiglottis is supplied by internal laryngeal nerve, which is branch of superior laryngeal nerve, which is a nerve of 4th arch.
- Endodermal swellings-
- Lingual swelling
- Tuberculum impar
- Cranial part of hypobranchial eminence
- Form only mucus membrane of tongue
- Tongue muscles are derived from occipital myotome/somites.
- Remaining fibrous tissue derived from mesoderm.
- Foramen Caecum- thyroglossal duct.
Pharyngeal Arches
- First arch
- Skeletal derivatives-
- Malleus and Incus
- Meckel’s cartilage
- Perichondrium covering the Meckel’s Cartilage persists to form two ligaments-
- Sphenomandibular ligament
- Anterior ligament of malleus
- Remaining part of cartilage forms mandible of the particular side.
- Other Derivative/Muscles-
- Mastication muscles
- Anterior belly of digastric
- Mylohyoid
- Tensor tympani
- Tensor palati
- Skeletal derivatives-
- 2nd arch:-
- Skeletal derivatives-
- Stapes
- Styloid process
- Upper part of body of hyoid bone
- Lesser horn of hyoid bone
- Stylohyoid ligament- perichondrium covering 2nd arch cartilage.
- Other Derivatives-
- Facial nerve
- Muscles of facial expression.
- Posterior belly of digastric
- Stylohyoid
- Stapedius muscle(smallest skeletal muscle, attached to stapedius)
- Skeletal derivatives-
- 3rd Arch-
- Derivatives are from ventral part of arch
- Dorsal part of arch entirely disappears.
- Skeletal-
- Greater horn of hyoid bone
- Lower part of body of hyoid bone
- Other-
- Glossopharyngeal nerves
- Stylopharyngeus muscle
- 4th arch –
- Derivatives are from ventral part of arch
- Dorsal part of arch entirely disappears.
- Skeletal derivatives-
- Upper part of lamina of thyroid cartilage
- Superior horn of thyroid cartilage
- Others-
- Cricothyroid muscle
- Superior laryngeal nerve- divides into external and internal laryngeal nerve
- 5th arch-
- Disappears
- 6th Arch-
- Skeletal derivatives-
- Lower part of lamina of thyroid cartilage
- Inferior horn of thyroid cartilage
- Cricoid cartilage
- Other derivatives-
- Recurrent laryngeal nerve
- Intrinsic muscles of larynx except cricothyroid.
- Skeletal derivatives-
Face
- The 2 medial nasal processes are getting compressed due to the proliferation of maxillary process to form philtrum and nasal septum.
- Failure of fusion: Central cleft/ hare Lip/Cleft in median part of upper lip
- Philtrum– Central part of upper lip with a depression
- Lateral nasal process forms the ala of the nose.
- Medial nasal process fuses with the maxillary process to form the lateral part of the upper lip.-
- Failure to fuse: cleft in lateral part of upper lip.
- Failure of fusion of lateral nasal process with maxillary process- oblique facial cleft.
- Maxillary process gets fused with the mandibular process to form cheeks.
- This reduces the size of mouth/stomodeum.
- Does not fuse in case of dogs- hence no cheeks in dogs
- Mandibular process fuse together to form lower lip and lower jaw
- Structures derived from frontonasal process are supplied by Ophthalmic division of maxillary nerve
- Structures derived from maxillary process are supplied by Maxillary division of maxillary nerve
- Structures derived from mandibular process are supplied by Mandibular division of maxillary nerve
- Source of frontonasal process– mesenchymal cells covering the forebrain.
- Source of maxillary and mandibular process– first pharyngeal arch.