FishFace

By 48 hpf, the hyosymplectic (hs) consists of a condensation of chondrogenic cells that is difficult to distinguish along its dorsal aspect from the anterior portion of the otic capsule (ot). Note the facial nerve (VII) foramen (ff) in the dorsal portion of the hyosymplectic. Ventrally, cells of the developing hyosymplectic are difficult to discern from those of the developing interhyal (ih) and ceratohyal (ch), whereas there is clear separation from cells of the more anterior developing palatoquadrate (pq). No Alcian blue staining of matrix is observed in these developing cartilages at this stage (data not shown). The hyosymplectic can be separated into dorsal (i.e., hyomandibula) and ventral (i.e., symplectic) regions, which at this timepoint are divided by a distinctive area of decreased fluorescence (*).

 

* See image at http://fishstix.uoregon.edu/kimmel-atlas/Hyosymplectic.html

By 55 hpf, cellular boundaries of chondrocytes of the hyosymplectic (hs) are more easy to distinguish, although cells of the developing hyosymplectic remain difficult to discern from those of the developing otic capsule (ot), interhyal (ih) and ceratohyal (ch). The more dorsal hyomandibula and the more ventral symplectic regions of the hyosymplectic are still separated by a distinctive area of decreased fluorescence (*). Alcian blue staining of matrix is observed in these developing cartilages at this stage (data not shown).


*  See image at http://fishstix.uoregon.edu/kimmel-atlas/Hyosymplectic.html
 

By 72 hpf, the hyosymplectic (hs) has increased in size and cell number from that seen at 48 hpf. The hyomandibula region of the dorsal hyosymplectic is still difficult to distinguish from the anterior portion of the otic capsule (ot) dorsally, but the symplectic region of the ventral hyosymplectic can be discerned easily from the ceratohyal (ch) ventrally. The interhyal (ih), which articulates where the two hyosymplectic regions meet, is still not easy to distinguish. The symplectic region overlaps considerably with the palatoquadrate (pq) anteriorly. Note the facial nerve (VII) foramen (ff) in the hyomandibula region. Alcian blue staining of matrix is observed in these developing cartilages at this stage (data not shown). The opercle (op) projects ventrally from the posterodorsal aspect of the hyomandibula region in a ball-and-socket joint.

By 96 hpf, the hyosymplectic (hs) has grown considerably from that seen at 72 hpf and now can be discerned easily from the interhyal (ih) ventrally. The hyomandibula region of the hyosymplectic articulates with the anterior portion of the otic capsule (ot) in a synchondrosis, which makes it difficult to distinguish the separate elements with fluorescent labeling of chondrocytes (darker area in hyomandibula region is an overlying melanocyte). The opercle (op) projects ventrally from its articulation at the posterodorsal aspect of the hyomandibula region. The symplectic region of the hyosymplectic overlaps considerably with the palatoquadrate (pq) anteriorly.

By 120 hpf, ossification of the hyomandibula (hm) bone can be detected in the perichondrium of the hyomandibular region of the hyosymplectic (hs) along the ventral surface of the facial nerve (VII) foramen (ff).

By 144 hpf, considerable growth of the entire hyosymplectic has occurred compared to its size at 96 hpf. Also, ossification of the hyomandibula (hm) bone has increased in the perichondrium of the hyomandibula region of hyosymplectic (hs) along the ventral surface of the facial nerve (VII) foramen (ff) compared to that seen at 120 hpf. The relative positioning of the hyosymplectic with the articulating opercle (op), otic capsule (ot), interhyal (ih), and palatoquadrate (pq) has not changed significantly from that seen at 96 hpf.

By 192 hpf, a morphological change is becoming obvious, whereby the ventral symplectic region of the hyosymplectic (hs), which consists of mostly a single layer of chondrocytes, increasingly shifts to a right angle with respect to the dorsal hyomandibular region of the hyosymplectic, with the articulation of the interhyal (ih) as the fulcrum of this angle. The hyomandibula (hm) bone has extended both dorsally and ventrally from the facial nerve (VII) foramen (ff) in the perichondrium of the hyomandibular region. While overlapped by the developing quadrate (q), initiation of the symplectic (sy) bone can be observed in the perichondrium of the symplectic region.

By 240 hpf, the hyosymplectic (hs) has grown considerably compared to its size at 192 hpf. In addition, the angle between the dorsal hyomandibula and ventral symplectic regions of the hyosymplectic, with the articulation of the interhyal (ih) as the fulcrum of this angle, appears to have decreased further compared to that seen at 192 hpf. The hyomandibula (hm) bone has grown both dorsally and ventrally from the facial nerve (VII) foramen (ff) in the perichondrium of the hyomandibula region, and dermal extensions of the hyomandibula bone from the perichondrium can begin to be seen anteriorly. The symplectic (sy) bone has extended anteriorly and posteriorly in the perichondrium of the symplectic region, abutting both the quadrate (q) anteriorly and the preopercle (pop) posteriorly.

By 288 hpf, the hyosymplectic (hs) has grown considerably from that seen at 240 hpf. In addition, the angle between the dorsal hyomandibula and ventral symplectic regions of the hyosymplectic, with the articulation of the interhyal (ih) as the fulcrum of this angle, appears to have decreased compared to that seen at 240 hpf. The hyomandibula (hm) bone has almost completely covered the hyomandibula region, and dermal extensions of the hyomandibula from the perichondrium are prominent both anteriorly and posteriorly. The symplectic (sy) bone has extended in the perichondrium of the symplectic region further anteriorly, medial to the quadrate (q), and further posteriorly, medial to the preopercle (pop).

Click on movie link on left to see a movie through the z-sections of this image starting with the most lateral, going through the stack and ending with the most media optical slice.

By 384hpf, the hyosymplectic has grown considerably compared to its size at 288hpf. In addition, the angle between the dorsal and ventral portions of the hyosymplectic (hs), with the articulation of the interhyal (ih) as the fulcrum of this angle, appears to have increased compared to that seen at 288hpf. The hyomandibula (hm) has almost completely covered the dorsal portion of the hyosymplectic, with prominent dermal extensions from the perichondrium. The symplectic (sy) has encompassed almost all of the ventral portion of the hyosymplectic, so the only portion of the body of the hyosymplectic that remains unossified is that adjacent to the interhyal, which even persists in the adult (data not shown).