Bio 205, Lecture 9
1. Bone - bone is cellular and most often vascular, meaning that it is supplied with blood vessels. The extracellular matrix (ECM)contains some water, proteins like collagen, and the mineral hydroxyapatite, which is a calcium salt. There are other calcium salts that are deposited, as well. We will talk about the functional importanct of these components later. Bone is a very active tissue, especially in homeotherms (birds and mammals). Cells called osteoclasts are continually removing matrix while cells called osteoblasts are continually depositing it. We call this remodelling. All this requires a good blood supply to bring in O2 and other nutrients and remove wastes.
2. Cartilage - cartilage is cellular but avascular, and so is not a very metabolically active tissue. The ECM has a large organic (large protein-carbohydrate complexes) content and a large water content but a small mineral content. Some cartilages in some animals calcify (note - this is not bone), for example the teeth and vertebrae of sharks.
3. Ligaments - a ligament is a rope-like or sheet-like dense connective tissue with a huge organic content (mostly the proteins collagen and elastin) but only a moderate water content and small mineral content. Occasionally ligaments (or the similarly constructed tendons) ossify (forming true bone), for example some vertebral ligaments in dinosaurs that stiffened the tail.
There are several ways that bone can develop:
1. Endochondral or cartilage replacement bone - The initial structure is cartilage and this cartilage is replaced by bone. Note the cartilage matrix does not ossify but is removed and then replaced by new matrix.
2. Dermal (or membrane) bone - The initial structure is a sheet of dense connective tissue, which then ossifies.
cartilages that form anterior to and surrounding neural tube and ectodermal placodes. The parts around the neural tube begin as cartilage struts or bars. The parts around the placodes are capsules. These all fuse together to form the solid chondrocranium (or neurocranium)
Also called the viscerocranium. Ossifies endochondrally. This is primitively the structural support for the pharyngeal arches, which could have function either (or both) in ventillation or feeding, but there are major evolutionary transformations in its structure and function.
Primitive splanchnocranium. The splanchnocranium of sharks is a good model for the ancestral gnathostome (chondrichthyans + osteichthyans) state.
- jaws, or mandibular arch: dorsal palatoquadrate cartilage and ventral Meckel's cartilage
- hyoid arch: ventral-median basihyal, intermediate ceratohyal, and large dorsal hyomandibular.
- five gill (branchial) arches: from dorsal to ventral: pharyngobranchial, epibranchials, ceratobranchials, hypobranchials, basibranchial. Note that the gills are lateral to the arches.
Membrane bone that covers the brain dorsally, the chondrocranium anteriorly, the jaw muscles laterally, the roof of the mouth ventrally, and the lower jaw. See the figure below.
Series:
- Facial
- palatal
- mandibular
- vault
- orbital
- temporal
- mandibular
Where did these jaws and hyoid arch come from?
Lampreys, and many fossil agnathans (jawless craniates) have gill pouches supported by a cartilage skeleton called a branchial basket. The branchial basket of lampreys consists of a series of 7 roughly vertical and sinuous bars. The gills are medial to the bars.The lamprey has a velar skeleton anterior and medial to the branchial basket. The velar skeleton supports the velum, which is a water pumping organ.
In Karl Gegenbauer's model (1872), which has become the traditional view, primitive craniates had 7 pharyngeal arches so this creature looked something like a lamprey. In gnathostomes, the jaws are a modified first pharygeal arch and the hyoid arch is a modification of the second arch, hence the reason for terms mandibular arch and hyoid arch. There is a dissenting view that the mandibular arch is actually the second pharyngeal arch, the first being a premandibular arch, which includes the trabeculae (part of the neurocranium anterior to the hypophyseal foramen and notochord). The trabeculae, unlike the neurocranial elements posterior to the hypophyseal foramen, derives from the neural crest. There are even some vertebrate morphologists that think there are primitively 9 or 10 arches.
evidence for the jaws from arch theory:
- development of shark shows that mandibular and hyoid arch develop in series with branchial arches
- mandibular, hyoid, and branchial arches derived from neural crest
- manidbular, hyoid, and branchial arch muscles derived from head mesoderm (somitomeres).
- rhombencephalic (hind brain) motor nerves innervate arches in series (arch 1 = CN V; arch 2 = CN VII, arch 3 = CN IX; arches 4-7 = CN X)
- pharyngeal slit between mandibular and hyoid arch retained as spiracle/auditory tube
Potential problems: the gills are medial to the branchial bars in lamprey but lateral in gnathostomes. Either the gills have moved or the arches of lampreys and those of gnathostomes are not homologous. Jon Mallatt (1984) argued that it is the gills that are homologous, not the arches, and proposed that the extrabranchial cartilages of sharks, which are lateral to the gills, are homologous to the branchial basket of lampreys and that lampreys, in turn, have no homolog to the internal arches of gnathostomes. The ancestral craniate had both internal and external arches. Lampreys lost the internal arches. Most gnathostomes (not sharks) have lost the external arches. The first internal arch evolved into the mandibular arch of gnathostomes (see below) and the velar skeleton of lampreys.
Janvier has proposed something similar. Again, the internal arches of gnathostomes are not homologous to the external arches of agnathans. The mandibular arch is a modification of the agnathan velar skeleton and the hyoid and branchial arches are neomorphic in gnathostomes (not primitive to craniates as argued by Mallat in 1996 but not 1984).
Potential problem #2: In the traditional view, the it is supposed that the selection acted on the anteriormost jaw as a biting structure, so the original function of the jaws was to bite. The problem with this scenario is that modern and fossil agnathans, as well as gnathostomes, have cheeks and lips. In agnathans, this creates a mouth well anterior to the first arch. Jon Mallatt (1996) has suggested that the original function of the jaws was to facilitate ventilation. Fish ventillate by sucking in water and closing off the pharynx from the mouth and squeezing the water back through the pharyngeal slits past the gills. Mallatt proposed that selection increased the size and strength of the first arch in order to close off the mouth (the "old mouth") from the pharynx and squeeze the water back. Sucking in water not only faciltates ventillation but also is a method of sucking in prey. The enlarged jaws could now be used to hold or bite prey and more selection would further increase the size of the jaws. To further increase prey biting performance, the jaws would move forward constricting the space of the old mouth into that small space between the jaws and lips. The pharynx behind the jaws became the "new mouth", or buccal cavity. This doesn't imply that the mandibular arch is not a modified pharyngeal arch, only that the original function was not for biting.
terminology: 7 pharyngeal or visceral arches but only 5 branchial arches. The 1st branchial arch is the 3rd pharyngeal arch, etc.
Since the jaws are primitively separate from the neurocranium within the head skeleton, they need some way to attach, or be suspended from, the chondrocranium. Otherwise, when the lower jaw muscles shorten, the entire jaw would elevate toward the skull instead of the lower jaw relative to the upper jaw.
From Kardong:
Paleostyly: (agnathans) no suspension
Euautostyly: (placoderms, acanthodians) mandibular arch suspended byitself
Amphistyly: (primitive sharks, some osteichthyes, crossopterygians) dual suspension by palatoquadrate and hyomandiba
Hyostyly: (most fish): suspension by hyomandibula - provides more mobile upper jaw. Here is an example of what a mobile upper jaw can do for you, courtesy of Lara Ferry-Graham.
Metautostyly: (most amphibians, reptiles) suspension by quadrate bone (ossification of posterior region of palatoquadrate cartilage)
Craniostyly: (mammals) entire upper jaw incorporated into braincase and lower jaw is suspended
