Development of Brachial Region in animals
Another early feature that is distinct in the head region involves the mesenchymal structure around the cranial most part of the foregut. Around this area of the foregut, the mesenchyme aggregates and proliferates to form five pairs of arch like structures lateral and ventral to the foregut.
The part of the foregut that is bounded by these arches will be the pharynx of the embryo and adult and the arches are called branchial arches.
There are grooves between the consecutive branchial arches that are apposed to the pharyngeal pouches, called the branchial grooves.
In these grooves, the ectoderm is in close association with the endoderm of the pouches. Thus between each pair of arches, there is a thin membrane consisting of endoderm and ectoderm with only a negligible amount of mesoderm between them.
The mesenchyme in each of the branchial arches forms the specific components of the musculo skeletal system of head, especially of the face.
FATE OF BRANCHIAL ARCHES
The branchial arches of animals are bar like ridges separated by grooves which appear on the lateral surface of the embryonic head. Usually there are five pairs of branchial arches develop which are separated by four external ectodermal grooves, the branchial grooves. The forming branchial arches encroach on the wall of the pharynx.
- Since the endoderm of the pharynx pushes the mesenchyme subjacent to the branchial grooves, pharyngeal pouches are formed. The ectoderm of each groove and endoderm of the pouch meet and unite to form thin plates. Sometimes, these plates rupture forming temporary openings, reminiscent of gill slit condition in fishes. Failure of these openings to close constitutes branchial fistulae. The mesenchyme in each branchial arch forms components of musculo skeletal system of the head, especially of the face.
- The first branchial arch divides into an upper maxillary and lower mandibular process.
- The maxillary process undergoes degeneration and the surrounding mesoderm develops into maxilla and palatine bones.
- The mesenchymal core (Meckle’s cartilage) of the mandibular process becomes enveloped by surrounding mesenchyme and is transformed into spheno-mandibular ligament, malleus and incus.
- The mesoderm of II branchial arch (Reichert’s cartilage) contributes to the stapes, styloid process of the temporal bone and small cornua of the hyoid.
- The cartilage of the III arch forms the styloid cornua and the body of the hyoid.
- The IV arch differentiates into epiglottis and thyroid cartilages of the larynx.
- The V branchial arch furnishes the cricoid and arytenoids cartilages of larynx.
- The first branchial arch divides into an upper maxillary and lower mandibular process.