1 - Development of the Head, Mouth, and Pharynx Flashcards
Development of head involves:
-connective tissue and skeletal muscle structures that are of neural crest origin
Head region of embryo is associated with developing:
-nervous system
-digestive system
-respiratory system
Key distinguishing feature of head development is:
-formation of pharyngeal arches
Pharyngeal arches:
-begin developing around 3rd week of gestation in domestic animals
*neural crest-derived cells migrate into head and neck regions=begin to AGGREGATE
-6 pairs (5th and 6th are rudimentary)
First pharyngeal arch:
-divided into maxillary and mandibular process
-immediately caudal to oro-pharyngeal membrane that now seals off the foregut
Development of the face:
-involves formation, fusion, patterning of facial primordia that surround stomodeal depression
Primordia form from:
-proliferation of neural crest-derived mesenchyme
Development of face involves:
-single fronto-nasal prominence
-paired nasomedial and nasolateral processes
-paired maxillary and paired mandibular prominences
Fronto-nasal prominence:
-form 2 pairs of ectodermal thickenings
>nasal and lens placodes
Lens placodes:
-form before nasal placodes
-positioned laterally
Maxillary prominences:
-grow in size and extend medially to fuse with nasomedial processes
>*results in formation of maxillary process, primary palate, tissue of upper lip
Maxillary process is the future:
-maxilla
Upper lip:
-influenced by form and amount of fusion of the prominences
Mandibular prominences:
-fuse forming the lower jaw
Cleft lip: clinical relevance
-when fusion of nasiomedial processes and maxillary prominences does not occur properly
>philtrum is not formed
-congenital malformation
-rare in domestic animals
Nasal-lacrimal groove:
-early stages of development
-separates maxillary from nasal prominences
Nasolacrimal duct:
-when the nasal-lacrimal groove extends towards developing eye
>floor of ectodermal groove will lose contact with surface
Nasal pits:
-deepen and form sacs
-initially L and R nasal sacs are separated by a septum
Nasal sacs:
-separated from oral cavity by oro-nasal membrane which forms the primary palate
Maxillary process:
-common nasal cavity forms at caudal end of nasal sacs
-caudal plate atrophies
*leaves the maxillary process
Choana:
-opening
>direct communication between caudal portion of nasal cavity and oral cavity
Palatine processes:
-begin to grow from lateral walls of nasal cavity
Developing tongue:
-can initially fill the oral cavity and push into nasal cavity
Secondary palate:
-with growth of oral cavity there will be expansion of the palatine processes and fusion at midline
*partition
Where palate is fusing with maxillary process:
-small spaces in the palate remain
>incisive foramina
Incisive foramina:
-lead to incisive ducts
-allow small amounts of fluid from oral cavity to paired vomeronasal organ and to olfactory epithelium of nasal cavity
Conchae:
-projections of laminae that arise from lateral walls of nasal cavity
-made of mesoderm core and covered by ectodermal lining
>then became ossified
Turbinate bones:
-bony projections resemble scrolls
Meatus:
-divides the nasal cavity into 3 passages
What determines the communication between pharynx and nasal cavities?
-amount of fusion between nasal septum and secondary palate
Horses nasal septum:
-fuses with secondary palate along entire length
>each nasal cavity communicates separately with pharynx
Other domestic species septum:
*fusion is not so extensive
*nasal cavities have a common opening into nasopharynx
Secondary palate becomes:
- Hard Palate
- Soft Palate
Hard palate:
-first 2/3 of the secondary palate that becomes ossified
Soft palate:
-portion of palate that projects into pharynx
-remains membranous
-divides pharynx into oral and nasal parts
Cleft palate types: clinical relevance
-primary
-secondary
Primary cleft palate:
-due to incomplete fusion of nasomedial processes with maxillary prominences
Secondary cleft palate:
-due to incomplete fusion of lateral palatine processes
-leaves opening between oral and nasal cavities
Pharyngeal grooves/clefts:
-invaginations of ectoderm that form between adjacent arches
Pharyngeal pouches produced from:
-endoderm of expanding foregut evaginating between the arches
Pharyngeal membranes:
-contact of cleft and pouches
What is associated with each arch?
-aortic arch artery
-muscle component
-branch of cranial nerve
Second or hyoid arch:
-reichert’s cartilage
>gives rise to Stapes and bones of hyoid apparatus
Third arch:
-other bones of hyoid apparatus, stylopharyngeus muscles (dilates pharynx)
Fourth-six arches:
-laryngeal cartilages
Endodermal epithelium of pouches:
*differentiates into components of lymphatic and endocrine system
2nd pouch in late fetal period:
-contains aggregate of tissue
>palatine tonsil
Parathyroid glands develop from:
-dorsal parts of 3rd and 4th pouches
*PTH: increases blood calcium levels
Thyroid develops:
-as ventral midline endodermal diverticulum (downgrowth) of endodermal floor of pharynx
-becomes bilobar and moves caudally to be ventral to developing trachea
*isthmus
Isthmus:
-connects the thyroid
-not constant in all species
Pigs/primates isthmus:
-large and forms medial lobe
Cattle isthmus:
-smaller band between lobes
Horses isthmus:
-poorly defined
small ruminants isthmus:
-band of connective tissue
Cats/dogs isthmus:
-absent
With caudal migration of thyroid:
-parathyroid IV glands becomes embedded into thyroid gland
-parathyroid III glands caudal to thyroid
Thymus:
-from ventral parts of L and R 3rd pouches (4th in some species)
*initially 2 tubular structures, but become solid masses
-thymic hormones for T-lymphocytes
Thymus with extension caudally:
-eventually fuse in midline and become associated with pericardium
First pouch:
*auditory (eustachian) tubes of ear and tympanic cavities
Guttural pouches in horses:
-from a diverticula of auditory tubes
-mucus secreting sacs that communicate with pharynx
>controls blood pressure in internal carotids?
>cerebral blood cooling system?
First cleft:
-ectoderm of cleft gives rise to epithelial lining of external auditory meatus
Cervical sinus in mammals:
-second arch extends caudally over 2nd and 3rd clefts
>*forms a transient structure lined by ectoderm
Arch 1 (mandibular): bone derivatives
-mandible
-maxilla
-auricle of ear
-malleus
-incus
Arch 1: pouch derivatives
-auditory tube
-guttural pouch
Arch 1: cleft derivatives
-external auditory meatus
Arch 2 (hyoid): bone derivatives
-auricle of ear
-stapes
Arch 2: pouch derivatives
-palatine tonsils
Arch 3: pouch derivatives
-parathyroids III
-stroma of thymus
Arch IV-VI: pouch derivatives
-parathyroids IV
-stroma of thymus
Tongue develops:
-on floor of oral cavity from portion of 1st pharyngeal arch
3 elevations (tongue) at region of 1st arch:
-due to underlying mesoderm growth
>2 lateral lingual swellings
>1 tuberculum impar
Tongue 2nd pharyngeal arch:
-copula develops
Tongue 3rd and 4th arches:
-hypopharyngeal eminence forms
*become the root of the tongue
