Chapter 23 Development of the face, neck and limbs

1. Development of branchial apparatus * branchial apparatus: including branchial arch, branchial groove, branchial membrane and branchial pouch

---by the 4th week, ---frontonasal prominence: mesenchymal proliferation of ventral surface of brain vesicle ---heart prominence: development of heart

---primitive pharynx: cranial part of the foregut

---branchial or pharyngeal arch: /proliferation of mesenchyma of primitive pharynx /6 pairs ---branchial groove: the depressions between branchial arch

---branchial pouch: /outpocketings of endoderm of primitive pharynx /opposed to branchial groove ---branchial membrane: the thin membrane between branchial groove and branchial pouch

2. Formation of face ---five elevations /frontonasal prominence /maxillary prominence: paired, upper branches of first branchial arch /mandibular prominence: paired, lower branches of first branchial arch

---stomodeum /the depression between five elevations /opposed to primitive pharynx by buccopharyngeal membrane, will break down, then stomodeum communicates with the foregut

---nasal placode: by the 5th week, thickening of ectoderm of lower part of frontonasal prominence ---nasal pit: depressions in nasal placode ---median nasal prominence and lateral nasal prominence: proliferation of mesenchyme at the margins of the nasal placodes

---by the 5th week, right and left mandibular prominences fuse to form the lower jaws and lips ---by the 6th week, medial nasal prominence merge to give rise to median portion of the nose, middle portion of upper lip, the philtrum

---by the 6-7th week, maxillary prominence fuse with median and lateral nasal prominences to give rise to upper jaw, lateral portion of upper lip, lateral portion of nose ---by the end of 6th week, nasal pits deepen and fuse to form nasal sac, separate with oral cavity by oronasal membrane, then this membrane ruptures by the 7th week, primitive nasal cavity opens into the oral cavity

---development of eye: appear on each side of frontonasal prominence, grow in a medial direction ---development of ear: appear on lower portion of lower jaw, grow in a posterior and upper direction

3. development of palate ---median palatine process: or primary palate /by the 6th week /mesenchymal proliferation of inner portion of intermaxillary segment /to give rise to small portion of palate

---lateral palatine process: /by 8-9th week /mesenchymal proliferation of inner portion of maxillary prominence /fuse with each other, then fuse with the triangular primary palate to form secondary palate

---incisive foramen: the midline landmark between the primary and secondary palates ---hard palate: ossification of anterior portion of secondary palate ---soft palate: posterior portion of secondary palate

---nasal septum: formed by ectoderm and mesoderm proliferation of frontonasal prominence and maxillary prominence, then grows down and joins with the cephalic aspect of palate

4. development of neck * developed by 2nd ,3rd ,4th ,6th pairs of branchial arches ---by 4-5th week, the 2nd branchial arch grow to caudal direction, cover the 3,4,6th branchial arches, fuse with epicardial ridge

---epicardial ridge: upper ridge of heart prominence ---cervical sinus: the space between 2nd and 3,4,6th branchial arches

---the 2nd branchial arches fuse with 3, 4,6th branchial arches, cervical sinus disappear ---become longer: differentiate of branchial arches, become longer of esophagus and trachea, descent of heart will help the formation of neck

5. development of limbs ---upper and lower limb buds: /by 4th week, formed by the proliferation of somatic or parietal mesoderm /mesodermal core /ectodermal cap

---circular constriction: 2, divided the each limb bud into 3 segments ---hand and foot plates: /terminal portion of the buds becomes flattened /digital ray: thickening portion in plates /7-8th week, the tissue between digital ray apoptosis

---somatic or parietal mesoderm of limb buds differentiate into cartilage, then through endochondral ossification to form bone ---somitic mesoderm differentiate into muscle of limb buds

6. Congenital malformations 1) cleft lip: a. unilateral cleft lip: results from failure of the maxillary prominence to merge with medial nasal prominence on the affected side

b. bilateral cleft lip : results from failure the maxillary prominences to merge with the medial nasal prominence on both sides c. median cleft lip: results from failure of the medial nasal prominences to merge and form the intermaxillary segments

2) oblique facial cleft: results from failure of the maxillary prominence to fuse with the lateral nasal prominence

3) cleft palate a.  cleft of the primary palate b.  cleft of secondary palate c.  cleft of the primary and secondary palates

4) cervical cyst and cervical fistula: cervical cyst results from failure of atresia of cervical sinus, if cervical cyst open to cavity of pharynx or body surface, called cervical fistula

5) abnormalities of the extremities: a.  reduction defect: meromelia, phocomelia and amelia b.  duplication defect: polydactyly c.  dysplasia: sirenomelus and syndactyly