Why is cartilage replaced by bones in a vertebrate adult?

Why is cartilage replaced by bones in a vertebrate adult?

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Why is cartilage present in a vertebrate embryo replaced by bones in an vertebrate adult even though cartilage can also provide me the structural framework like as in Chondrichthyes and they seem to do well with it as Sharks are members of Chondrichthyes which are some of the most fearsome creatures of the world.

Bones are complex organs, made of more than just a structural element. Cartilage is just a tissue, usually devoid of nerves and blood vessels.

Chondrichythes live in water (ἰχθύς ichthys is Greek for 'fish'); the difference in specific gravity and buoyancy is considerable, and cartilage is not robust enough to mechanically support your body out of the water.

Vertebrates have evolved to have a bone marrow, which is part of the bone. If the structural element of the bone (the dense mineral matrix) was replaced by cartilage only, then the hollow structure would make things even worse.

Additionally, cartilage has about the same density as water (Pan et al, 2016) whereas dense bone is 60-90% more dense, which means extra weight pulling aquatic animals down.

Short answer, cartilage can't provide the same support as a bone, bone is stronger and stiffer.

Sharks may be big but they have pathetically weak bites for their size and that even when they partially ossify their jaws. They just can't fully ossify the tissue. The main swimming action of "fish" works even with flexible connections because each muscle is linking to the next linearly, the cartilage and collagen is mostly subjected to tension. But things like jaws and limbs need to withstand shear forces and withstand flexion, which cartilage sucks at, but bone is quite good at. It is not coincidence that the first bones evolved in jaw.


Javad Parvizi MD, FRCS , . Associate Editor , in High Yield Orthopaedics , 2010


Cartilage is a tough, semitransparent, elastic, flexible connective tissue consisting of cartilage cells scattered through a glycoprotein material that is strengthened by collagen fibers. There are no nerves or blood vessels in cartilage, which is found in the joints, the rib cage, the ear, the nose, the throat, and between vertebral disks.


The main purpose of cartilage is to provide a framework on which bone deposition may begin. Another important purpose of cartilage is to cover the surfaces of joints, allowing bones to slide over one another, thus reducing friction and preventing damage it also acts as a shock absorber


Hyaline Cartilage : Hyaline cartilage is the most abundant type of cartilage. Hyaline cartilage is found lining bones in joints (articular cartilage). It is also present inside bones, serving as a center of ossification or bone growth. In addition, hyaline cartilage forms the embryonic skeleton.

Elastic Cartilage: Elastic cartilage (also called yellow cartilage) is found in the pinna of the ear and several tubes, such as the walls of the auditory and eustachian canals and larynx. Elastic cartilage is similar to hyaline cartilage but contains elastic bundles (elastin) scattered throughout the matrix. This provides a tissue that is stiff yet elastic.

(From Ovale WK, Nahirey PC: Netter’s Essential Histology. Philadelphia, Saunders, 2008.)

Fibrocartilage: Fibrocartilage (also called white cartilage) is a specialized type of cartilage found in areas requiring tough support or great tensile strength, such as between intervertebral disks, at the pubic and other symphyses, and at sites connecting tendons or ligaments to bones.


Cells: Chondrocytes and the precusor forms of chondrocytes known as chondroblasts are the only cells found in cartilage . Chondrocytes make up “cell nests,” groups of chondrocytes within lacunae. Chondroblasts are responsible for the secretion and maintenance of the matrix.

Fibers: Cartilage is composed of collagen and elastic fibers. In hyaline cartilage, type II collagen makes up 40% of its dry weight. Elastic cartilage also contains elastic fibers, and fibrocartilage contains more collagen than hyaline cartilage.

Matrix: The matrix is mainly composed of proteoglycans, which are large molecules with a protein backbone and glycosaminoglycan (GAG) side chains.


Chondrodystrophies are a group of diseases characterized by disturbance of growth and subsequent ossification of cartilage . Some common diseases affecting/involving the cartilage are arthritis, achondroplasia, costochondritis, and herniated disk.

Growth and Development of Bones

Early in the development of a human fetus, the skeleton is made entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. This is a process in which mineral deposits replace cartilage. As shown in Figure below, ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including growth plates at the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.

Long bones ossify and get longer as they grow and develop. These bones grow from their ends, known as the epiphysis, and the presence of a growth plate, or epiphyseal line, signifies that the bone is still growing.

In the late teens or early twenties, a person reaches skeletal maturity. By then, all of the cartilage has been replaced by bone, so no further growth in bone length is possible. However, bones can still increase in thickness. This may occur in response to increased muscle activity, such as weight training.

Researchers find method to regrow cartilage in the joints

In laboratory studies, Stanford School of Medicine researchers have found a way to regenerate the cartilage that eases movement between bones.

Researchers at the Stanford University School of Medicine have discovered a way to regenerate, in mice and human tissue, the cushion of cartilage found in joints.

Loss of this slippery and shock-absorbing tissue layer, called articular cartilage, is responsible for many cases of joint pain and arthritis, which afflicts more than 55 million Americans. Nearly 1 in 4 adult Americans suffer from arthritis, and far more are burdened by joint pain and inflammation generally.

The Stanford researchers figured out how to regrow articular cartilage by first causing slight injury to the joint tissue, then using chemical signals to steer the growth of skeletal stem cells as the injuries heal. The work was published Aug. 17 in the journal Nature Medicine.

“Cartilage has practically zero regenerative potential in adulthood, so once it’s injured or gone, what we can do for patients has been very limited,” said assistant professor of surgery Charles K.F. Chan, PhD. “It’s extremely gratifying to find a way to help the body regrow this important tissue.”

The work builds on previous research at Stanford that resulted in isolation of the skeletal stem cell, a self-renewing cell that is also responsible for the production of bone, cartilage and a special type of cell that helps blood cells develop in bone marrow. The new research, like previous discoveries of mouse and human skeletal stem cells, were mostly carried out in the laboratories of Chan and professor of surgery Michael Longaker, MD.

Articular cartilage is a complex and specialized tissue that provides a slick and bouncy cushion between bones at the joints. When this cartilage is damaged by trauma, disease or simply thins with age, bones can rub directly against each other, causing pain and inflammation, which can eventually result in arthritis.

Damaged cartilage can be treated through a technique called microfracture, in which tiny holes are drilled in the surface of a joint. The microfracture technique prompts the body to create new tissue in the joint, but the new tissue is not much like cartilage.

“Microfracture results in what is called fibrocartilage, which is really more like scar tissue than natural cartilage,” said Chan. “It covers the bone and is better than nothing, but it doesn’t have the bounce and elasticity of natural cartilage, and it tends to degrade relatively quickly.”

The most recent research arose, in part, through the work of surgeon Matthew Murphy, PhD, a visiting researcher at Stanford who is now at the University of Manchester. “I never felt anyone really understood how microfracture really worked,” Murphy said. “I realized the only way to understand the process was to look at what stem cells are doing after microfracture.” Murphy is the lead author on the paper. Chan and Longaker are co-senior authors.

For a long time, Chan said, people assumed that adult cartilage did not regenerate after injury because the tissue did not have many skeletal stem cells that could be activated. Working in a mouse model, the team documented that microfracture did activate skeletal stem cells. Left to their own devices, however, those activated skeletal stem cells regenerated fibrocartilage in the joint.

But what if the healing process after microfracture could be steered toward development of cartilage and away from fibrocartilage? The researchers knew that as bone develops, cells must first go through a cartilage stage before turning into bone. They had the idea that they might encourage the skeletal stem cells in the joint to start along a path toward becoming bone, but stop the process at the cartilage stage.

The researchers used a powerful molecule called bone morphogenetic protein 2 (BMP2) to initiate bone formation after microfracture, but then stopped the process midway with a molecule that blocked another signaling molecule important in bone formation, called vascular endothelial growth factor (VEGF).

“What we ended up with was cartilage that is made of the same sort of cells as natural cartilage with comparable mechanical properties, unlike the fibrocartilage that we usually get,” Chan said. “It also restored mobility to osteoarthritic mice and significantly reduced their pain.”

As a proof of principle that this might also work in humans, the researchers transferred human tissue into mice that were bred to not reject the tissue, and were able to show that human skeletal stem cells could be steered toward bone development but stopped at the cartilage stage.

The next stage of research is to conduct similar experiments in larger animals before starting human clinical trials. Murphy points out that because of the difficulty in working with very small mouse joints, there might be some improvements to the system they could make as they move into relatively larger joints.

The first human clinical trials might be for people who have arthritis in their fingers and toes. “We might start with small joints, and if that works we would move up to larger joints like knees,” Murphy says. “Right now, one of the most common surgeries for arthritis in the fingers is to have the bone at the base of the thumb taken out. In such cases we might try this to save the joint, and if it doesn’t work we just take out the bone as we would have anyway. There’s a big potential for improvement, and the downside is that we would be back to where we were before.”

Longaker points out that one advantage of their discovery is that the main components of a potential therapy are approved as safe and effective by the FDA. “BMP2 has already been approved for helping bone heal, and VEGF inhibitors are already used as anti-cancer therapies,” Longaker said. “This would help speed the approval of any therapy we develop.”

Joint replacement surgery has revolutionized how doctors treat arthritis and is very common: By age 80, 1 in 10 people will have a hip replacement and 1 in 20 will have a knee replaced. But such joint replacement is extremely invasive, has a limited lifespan and is performed only after arthritis hits and patients endure lasting pain. The researchers say they can envision a time when people are able to avoid getting arthritis in the first place by rejuvenating their cartilage in their joints before it is badly degraded.

“One idea is to follow a ‘Jiffy Lube’ model of cartilage replenishment,” Longaker said. “You don’t wait for damage to accumulate — you go in periodically and use this technique to boost your articular cartilage before you have a problem.”

Longaker is the Deane P. and Louise Mitchell Professor in the School of Medicine and co-director of the Institute for Stem Cell Biology and Regenerative Medicine. Chan is a member of the Institute for Stem Cell Biology and Regenerative Medicine and Stanford Immunology.

Other Stanford scientist taking part in the research were professor of pathology Irving Weissman, MD, the Virginia and D. K. Ludwig Professor in Clinical Investigation in Cancer Research professor of surgery Stuart B. Goodman, MD, the Robert L. and Mary Ellenburg Professor in Surgery associate professor of orthopaedic surgery Fan Yang, PhD professor of surgery Derrick C. Wan, MD instructor in orthopaedic surgery Xinming Tong, PhD postdoctoral research fellow Thomas H. Ambrosi, PhD visiting postdoctoral scholar Liming Zhao, MD life science research professionals Lauren S. Koepke and Holly Steininger MD/PhD student Gunsagar S. Gulati, PhD graduate student Malachia Y. Hoover former student Owen Marecic former medical student Yuting Wang, MD and scanning probe microscopy laboratory manager Marcin P. Walkiewicz, PhD.

The research was supported by the National Institutes of Health (grants R00AG049958, R01 DE027323, R56 DE025597, R01 DE026730, R01 DE021683, R21 DE024230, U01HL099776, U24DE026914, R21 DE019274, NIGMS K08GM109105, NIH R01GM123069 and NIH1R01AR071379), the California Institute for Regenerative Medicine, the Oak Foundation, the Pitch Johnson Fund, the Gunn/Olivier Research Fund, the Stinehart/Reed Foundation, The Siebel Foundation, the Howard Hughes Medical Institute, the German Research Foundation, the PSRF National Endowment, National Center for Research Resources, the Prostate Cancer Research Foundation, the American Federation of Aging Research and the Arthritis National Research Foundation.

Study identifies stem cell that gives rise to new bone, cartilage in humans

Identification of the human skeletal stem cell by Stanford scientists could pave the way for regenerative treatments for bone fractures, arthritis and joint injuries.

A small bone structure arising from the human skeletal stem cell contains cartilage (blue), bone marrow (brown) and bone (yellow).
Courtesy of the Longaker and Chan labs

A decade-long effort led by Stanford University School of Medicine scientists has been rewarded with the identification of the human skeletal stem cell.

The cell, which can be isolated from human bone or generated from specialized cells in fat, gives rise to progenitor cells that can make new bone, the spongy stroma of the bone’s interior and the cartilage that helps our knees and other joints function smoothly and painlessly.

The discovery allowed the researchers to create a kind of family tree of stem cells important to the development and maintenance of the human skeleton. It could also pave the way for treatments that regenerate bone and cartilage in people.

“Every day, children and adults need normal bone, cartilage and stromal tissue,” said Michael Longaker, MD, professor of plastic and reconstructive surgery. “There are 75 million Americans with arthritis, for example. Imagine if we could turn readily available fat cells from liposuction into stem cells that could be injected into their joints to make new cartilage, or if we could stimulate the formation of new bone to repair fractures in older people.”

A paper describing the finding was published online Sept. 20 in Cell.

Longaker, the Deane P. and Louise Mitchell Professor in the School of Medicine and the co-director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine, is the senior author. The lead authors are Charles K.F. Chan, PhD, assistant professor of surgery medical student Gunsagar Gulati, MD Rahul Sinha, PhD, instructor of stem cell biology and regenerative medicine and research assistant Justin Vincent Tompkins.

‘True, multipotential, self-renewing’

The skeletal stem cells are distinct from another cell type called the mesenchymal stem cell, which can generate skeletal tissues, fat and muscle. Mesenchymal stem cells, which can be isolated from blood, bone marrow or fat, are considered by some clinicians to function as all-purpose stem cells. They have been tested, with limited success, in clinical trials and as unproven experimental treatments for their ability to regenerate a variety of tissues. Recently, three elderly patients in Florida were blinded or lost most of their sight after mesenchymal stem cells from fat were injected into their eyes as an experimental treatment for macular degeneration.

“Mesenchymal stem cells are loosely characterized and likely to include many populations of cells, each of which may respond differently and unpredictably to differentiation signals,” Chan said. “In contrast, the skeletal stem cell we’ve identified possesses all of the hallmark qualities of true, multipotential, self-renewing, tissue-specific stem cells. They are restricted in terms of their fate potential to just skeletal tissues, which is likely to make them much more clinically useful.”

Skeletal regeneration is an important capability for any bony animal evolving in a rough-and-tumble world where only the most fit, or the fastest-healing, are likely to survive very long into adulthood. Some vertebrates, such as newts, are able to regenerate entire limbs if necessary, but the healing ability of other animals, such as mice and humans, is more modest. Although humans can usually heal a bone fracture fairly well, they begin to lose some of that ability with age. And they are completely unable to regenerate the cartilage that wears away with age or repetitive use. Researchers have wondered whether the skeletal stem cell could be used clinically to help replace damaged or missing bone or cartilage, but it’s been very difficult to identify.

Adult stem cells lineage-restricted

Unlike embryonic stem cells, which are present only in the earliest stages of development, adult stem cells are thought to be found in all major tissue types, where they bide their time until needed to repair damage or trauma. Each adult stem cell is lineage-restricted — that is, it makes progenitor cells that give rise only to the types of cells that naturally occur in that tissue. For our skeleton, that means cells that make bone, cartilage and stroma.

Chan, Longaker and their colleagues had hoped to use what they learned from identifying the mouse skeletal stem cell to quickly isolate its human counterpart. But the quest turned out to be more difficult than they had anticipated. Most cell isolation efforts focus on using a technology called fluorescence activated cell sorting to separate cells based on the expression of proteins on their surface. Often, similar cell types from different species share some key cell surface markers.

But the human skeletal stem cell turned out to share few markers with its mouse counterpart. Instead, the researchers had to compare the gene expression profiles of the mouse skeletal stem cell with those of several human cell types found at the growing ends of developing human bone. Doing so, they were able to identify a cell population that made many of the same proteins as the mouse skeletal stem cell. They then worked backward to identify markers on the surface of the human cells that could be used to isolate and study them as a pure population.

“This was quite a bioinformatics challenge, and it required a big team of interdisciplinary researchers, but eventually Chuck and his colleagues were able to identify a series of markers that we felt had great potential,” Longaker said. “Then they had to prove two things: Can these cells self-renew, or make more of themselves indefinitely, and can they make the three main lineages that comprise the human skeleton?”

The researchers showed that the human skeletal stem cell they identified is both self-renewing and capable of making bone, cartilage and stroma progenitors. It is found at the end of developing bone, as well as in increased numbers near the site of healing fractures. Not only can it be isolated from fracture sites, it can also be generated by reprogramming human fat cells or induced pluripotent stem cells to assume a skeletal fate.

‘The perfect niche’

Intriguingly, the skeletal stem cell also provided a nurturing environment for the growth of human hematopoietic stem cells — or the cells in our bone marrow that give rise to our blood and immune system — without the need for additional growth factors found in serum.

“Blood-forming stem cells love the interior of spongy bone,” Chan said. “It’s the perfect niche for them. We found that the stromal population that arises from the skeletal stem cell can keep hematopoietic stem cells alive for two weeks without serum.”

By studying the differentiation potential of the human skeletal stem cell, the researchers were able to construct a family tree of stem cells to serve as a foundation for further studies into potential clinical applications. Understanding the similarities and differences between the mouse and human skeletal stem cell may also unravel mysteries about skeletal formation and intrinsic properties that differentiate mouse and human skeletons.

“Now we can begin to understand why human bone is denser than that of mice, or why human bones grow to be so much larger,” Longaker said.

In particular, the researchers found that the human skeletal stem cell expresses genes active in the Wnt signaling pathway known to modulate bone formation, whereas the mouse skeletal stem cell does not.

The ultimate goal of the researchers, however, is to find a way to use the human skeletal stem cell in the clinic. Longaker envisions a future in which arthroscopy — a minimally invasive procedure in which a tiny camera or surgical instruments, or both, are inserted into a joint to visualize and treat damaged cartilage — could include the injection of a skeletal stem cell specifically restricted to generate new cartilage, for example.

“I would hope that, within the next decade or so, this cell source will be a game-changer in the field of arthroscopic and regenerative medicine,” Longaker said. “The United States has a rapidly aging population that undergoes almost 2 million joint replacements each year. If we can use this stem cell for relatively noninvasive therapies, it could be a dream come true.”

Other Stanford authors are CIRM scholars Michael Lopez, Rachel Brewer and Lauren Koepke former graduate students Ava Carter, PhD, and Ryan Ransom graduate students Anoop Manjunath, and Stephanie Conley former postdoctoral scholar Andreas Reinisch, MD, PhD research assistant Taylor Wearda postdoctoral scholar Matthew P. Murphy, MD medical student Owen Marecic former life sciences researcher Eun Young Seo former research assistant Tripp Leavitt, MD research assistants Allison Nguyen, Ankit Salhotra, Taylor Siebel, and Karen M Chan instructor of stem cell biology and regenerative medicine Wan-Jin Lu, PhD postdoctoral scholars Thomas Ambrosi, PhD, and Mimi Borrelli, MD orthopaedic surgery resident Henry Goodnough, MD, PhD assistant professor of orthopaedic surgery Julius Bishop, MD professor of orthopaedic surgery Michael Gardner, MD professor of medicine Ravindra Majeti, MD, PhD associate professor of surgery Derrick Wan, MD professor of surgery Stuart Goodman, MD, PhD professor of pathology and of developmental biology Irving Weissman, MD and professor of dermatology and of genetics Howard Chang, MD, PhD.

Researchers from the Medical University of Graz in Austria, RIKEN in Japan and the University of California-San Diego also contributed to the study.

The study was supported by the National Institutes of Health (grants R01DE027323, R56DE025597, R01DE026730, R01DE021683, R21DE024230, U01HL099776, U24DE026914, R21DE019274, U01HL099999, R01CA86065, R01HL058770, NIAK99AG049958, P50HG007735, R01 R055650, R01AR06371 and S10 RR02933801), the California Institute for Regenerative Medicine, the Howard Hughes Medical Institute, the Oak Foundation, the Hagey Laboratory, the Pitch Johnson Fund, the Gunn/Oliver Research Fund, a Siebel Fellowship, a PCFYI Award, Stinehart/Reed, the Deutsche Forschungsgemeinschaft and the Ellenburg Chair.

The researchers have a pending patent for the isolation, derivation and use of human skeletal stem cells and their downstream progenitors.


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Cartilage, connective tissue forming the skeleton of mammalian embryos before bone formation begins and persisting in parts of the human skeleton into adulthood. Cartilage is the only component of the skeletons of certain primitive vertebrates, including lampreys and sharks. It is composed of a dense network of collagen fibres embedded in a firm, gelatinous ground substance that has the consistency of plastic this structure gives the tissue tensile strength, enabling it to bear weight while retaining greater flexibility than bone. Cartilage cells, called chondrocytes, occur at scattered sites through the cartilage and receive nutrition by diffusion through the gel cartilage contains no blood vessels or nerves, unlike bone.

Three main types of cartilage can be distinguished. Hyaline cartilage is the most widespread and is the type that makes up the embryonic skeleton. It persists in human adults at the ends of bones in free-moving joints as articular cartilage, at the ends of the ribs, and in the nose, larynx, trachea, and bronchi. It is a glossy blue-white in appearance and very resilient. Fibrocartilage is the tough, very strong tissue found predominantly in the intervertebral disks and at the insertions of ligaments and tendons it is similar to other fibrous tissues but contains cartilage ground substance and chondrocytes. Elastic cartilage, which is yellow in appearance, is more pliable than the other two forms because it contains elastic fibres in addition to collagen. In humans it makes up the external ear, the auditory tube of the middle ear, and the epiglottis.

A major role of cartilage in humans is to form a model for later growth of the bony skeleton. The clavicle, or collarbone, and some parts of the skull are not preformed in cartilage. In the embryo, cartilage gradually calcifies, and chondrocytes are replaced by bone cells, or osteocytes. After birth a thin plate of cartilage, called the epiphyseal plate, persists at the ends of growing bones, finally becoming ossified itself only when the bone behind it has completed its growth. At the growing edge of the plate, chondrocytes continue to grow and divide, while on the trailing edge they are replaced by osteocytes as new bone is laid down. The cartilage plate thus remains of a constant thickness while the bone grows behind it. Once this plate disappears, no further longitudinal bone growth is possible.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.

Why is cartilage replaced by bones in a vertebrate adult? - Biology

W hat group of bones forms the bottom and sides of the box?

The front of the box is the perpendicular bone dividing the nostrils. What is it called?

On either side, in the nostrils, is a complex of thin bones looking like swirls of flaky pastry. What are they called?

What happens with the original top of the box?

Draw a stylized (rectangular) brain box and label the parts of the neurocranium.

The splanchnocranium consists of the gill arches and their derivatives. The gill arches serve to support the gills and offer a site for respiratory muscle attachment. The original branchial skeleton of cartilage came from neural crest cells.

Splanchnocranium of a cartilaginous dogfish and salamander

The ossifications of the splanchnocranium of the teleost fishes cannot be readily identified on the wolf skull since they are either the framework for subsequent dermal tooth-bearing bones, or have moved from their association with the jaws to form the ear bones: quadrate (incus), articular (malleus) and hyomandibular (columella or stapes). Look at the alligator and identify the quadrate and articular bones. In mammals, gill arches also form the elements of the larynx (hyoid bone, thyroid and cricoid cartilages). See page 116.

The original dermal scales (or armour) of Ostracoderms sink down, attach to the neurocranium and are ossified to form dermal bones . These are from dermatome (epimere mesoderm). The dermatocranium forms most of the skull and functions as a protective shield for the brain.

See the dermal armour of Amia , the bowfin, which sits on the cartilaginous neurocranium. In other animal groups the cartilage disappears and when you look down on the skull you are looking at the bones of the dermatocranium.

The dermatocranium contributions to the mammal skull that we wish to learn are all those bones that are labeled on the drawing of the wolf skull, plus the dentary bone of the lower jaw. Identify all of these dermal bones on the wolf skull and learn them. These bones are grouped as the facial (upper jaw, nose), v ault (front of skull), orbital (around eyes), temporal (side of skull), palatal (roof of mouth) and m andibular (lower jaw) series.

alligator and wolf on display. Using the drawings and the skulls provided, identify the dermal bones in each skull. Count the total number of dermal bones (paired (X2) and unpaired (X1) in each species. Please use only the drawings in this lab guide for the count. Note that one pair of bones on one of the drawings is from the splanchnocranium and should not be counted. If any bone is not listed as neurocranium or splanchnocranium on page 39, it is dermal in origin.

The following drawings should provide an understanding of the trend towards loss of dermal bone ( Williston’s Law ).

Number of dermal bones in Amia skull:

Number of dermal bones in the alligator skull:

Number of dermal bones in wolf skull:

Use the space below to draw the paired wolf dentary bones.

Bones of the skull and their origins

The bones we will concentrate on in this lab are listed here.

Neurocranium (Chondrocranium) is from neural crest cells and mesodermal mesenchyme. It can remain catrilage or become replacement bone. We will study three groups of bones the Occipitals, the Sphenoids and the Ethmoids.

Splanchnocranium comes from neural crest cells and is either cartilage or replacement bone. Arch 1 forms the jaws and is called the mandibular arch. We will particularily study the articular which becomes the malleus of the ear in mammals and the quadrate which forms the incus . Arch 2 is the hyoid arch and we will see the hyomandibular become the collumella and then the stapes of the ear. The hyoid bone also remains as part of the hyoid bone of the larynx. Arches 3-5 are gill arches in fishes and also involved in jaw suspension. They form the caudal portion of the hyoid bone, and the thyroid and cricoid catrilage of the larynx in mammals as seen in the respriation lab on page 116.

Dermal bone is from mesenchyme and ectomesenchyme of the dermis and it overlies the neurocranium and splancnocranium. Learn all the bones on the dorsal drawing of the wolf (pg. 38) plus the lower jaw of mammals ( dentary bone) and the new bones, the palatine and bulla .


Fossae (cavities, pits, or holes), are modifications of the skull that allow for more powerful jaws. They provide more space in the skull for the jaw muscles to expand during contraction and they offer a more secure area for the muscles to attach.

Fish skulls have no fossa and are therefore called anapsid .

Study the turtle skull on demonstration. See the otic (or temporal) notch in the dorsal posterior region on both sides of the midline. This is an adaptation for muscle attachment that is necessary because of increased jaw musculature, and to offset the interference of the dermal bone contributions. The turtle skull, like the fish skull, has no fossa and is anapsid .

In reptiles (excluding turtles), there evolved a pair of openings on either side of the skull in the temporal region, called the temporal fossa . Study the location of the supratemporal fossa, and the i nfratemporal fossa on the skull of the alligator. The presence of two temporal fossae is the diapsid condition and is found in some reptiles and birds.

Some fossil reptiles lost the lower (infratemporal) fossa this is the parapsid (or euryapsid) condition, which is now extinct.

The loss of the supratemporal fossa and the presence of only the infratemporal is the synapsid condition. It occurred in some extinct reptiles, and is represented now by the mammals. Note the eye orbit may be separate from the fossa (cat, horse, human) or confluent with it (wolf, rat).


Epimorphosis: Vertebrate limb regeneration involves cell dedifferentiation and growth.
In postamputation newts, epidermal cells cover the wound to form a blastema.
The cells of the blastema arise from beneath the wound epidermis, dedifferentiate and start to divide.
Over weeks, these cells become cartilage, muscle and connective tissue.
Transdetermination can be seen by labling (multinucleate) muscle cells with rhodamine-dextran (a large marker dye).
Labled mononucleate cells arise that give rise to cartilage as well as muscle.
Note that cell that regenerate limb (in axolotl) have restricted potential: transgenic GFP transplants.
Transplanted dermis yield new dermis & cartilage but not muscle muscle giverise to muscle.
In regenerating newt cells, the Rb protein is inactivated by phosphorylation.
Limb regeneration is also dependent upon the presence of nerves.

The blastema gives rise to structures with positional distal values.
Regeneration always proceeds in a direction distal to the cut surface.
An amputated limb will re-establish blood supply when fused to trunk.
If the humerus is then cut, then both surfaces will regenerate distal structures.
Grafting a distal blastema to a proximal stump will induce the stump (mostly) to generate a normal limb and the distal blastema forms the wrist and hand.
This is accomplished by re-establishing positional values by inducing intercalary growth.
A distal blastema, grafted to a proximal cut limb, moves to the appropriate location to develop due to cell adhesion properties.
While mammals cannot regenerate limbs, many (including young children) can regenerate the ends of their digits.

Retinoic acid can change proximo-distal values in regenerating limbs.
Retinoic acid is present in developing vertebrate limbs and can alter positional values in the chick's limb.
Exposure to retinoic acid changes the positional value of a blastema to more proximal ones, such that elements proximal to the cut as well as those distal will be generated.
Wounded epidermis is a strong source of retinoic acid.
In regenerating limbs, retinoic acid is present in a distinct pattern & is higher in concentration in more distal blastemas.
Retinoic acid can induce extra limbs in the regenerating tail of a frog tadpole.

Insect limbs intercalate positional values.
When tissues of vastly different positional value are placed in conjunction, then intercalary growth occurs to replace the missing values.
Grafting of amputated cockroach legs demonstrate intercalation.
A distal cut tibia grafted onto a proximal cut will grow to intercalate the missing pieces.
However, a proximally cut tibia, grafted onto a distally cut host will also grow by intercalation.
In the latter case, the regenerated portion is in the reverse orientation (by bristle direction).
Circumferential values can also be regenerated by intercalation.

Morphoallaxis: Hydra grows by loss of cells from its ends and by budding.
Hydra has a hollow tubular body (0.5 cm long), with tentacles surrounding the mouth (hypostome) and, at the other end, a basal disc (foot).
Hydra has only two germ layers, the ectoderm and the endoderm separated by the basement membrane.
Hydra undergo continuous growth and pattern formation and cells are lost at the tentacle tips and from the basal disc.
The cells continually change their position and form new structures as they move up and down the body column.
Budding occurs, 2/3 down body axis which develops a head then detaches as a small new Hydra.

Regeneration in Hydra is polarized and does not depend on growth.
When cut in two, the lower piece will develop a head & the upper will develop a foot.
A piece excised from the Hydra body will regenerate both a head and a basal disc in the same polarity.
A small fragment will produce a small Hydra that will grow after feeding.
Heavily irradiated Hydra, that cannot undergo cell division (grow) will regenerate.

The head region inhibits the formation of a nearby heads
The head region of Hydra acts as an organizing region and as an inhibitor of inappropriate head formation.
The hypostome and the basal discs act as organizing centres to give polarity and act to induce head and tail formation.
Grafts of the hypostome to the gastric region will induce a 2nd head (& eventually a new body).
Grafts of the region next to the head to the gastric region will not generate a new head unless the original head is removed but will generate a new head in the foot region.
The time required to become able to produce head-inducing properties increases with distance from the head.

How did vertebrates evolve?

Starting from radial organism , organism starts to possess bilateral symmetry (symmetrical to the right and left). This is where vertebrates and invertebrates evolve from. The easiest way to observe bilateral animal is by looking at their embryo. They generally form head-to-tail-axis where the eye is located in the front and the anus is located at the back.

But before vertebrates build vertebrates, their progenitor organism produce a lot more exoskeleton. This includes armored jawless fish. .

Vertebrate tend to use bone, cartilage and dentine as exoskeleton material. Meanwhile, invertebrate tend to rely on chitin to which they form cuticle. that makes the material for their exoskeleton. (unlike bone which can grow, cuticle have to be replaced for every molting cycle.)

Especially the formation of hard exoskeleton around the skull region would help create an ideal condition for the development of brain. The hard exoskeleton around the skull region is displayed in craniates . Read: cephalization

After cephalization, the next involves the development of notochord or spinal cord which can connect the nerve cells from the main "brain" to the muscles and other peripheral tissues.

Section Summary

The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and some species are parasitic on other fishes. Gnathostomes include the jawed fishes (cartilaginous and bony fishes) as well as all other tetrapods. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Bony fishes can be further divided into ray-finned and lobe-finned fishes.

As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians are limbless. Amphibians have a moist, permeable skin used for cutaneous respiration. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of amphibians consists of two distinct stages: the larval stage and metamorphosis to an adult stage.

The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. A key adaptation that permitted reptiles to live on land was the development of scaly skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).

Birds are endothermic amniotes. Feathers act as insulation and allow for flight. Birds have pneumatic bones that are hollow rather than tissue-filled. Airflow through bird lungs travels in one direction. Birds evolved from dinosaurs.

Mammals have hair and mammary glands. Mammalian skin includes various secretory glands. Mammals are endothermic, like birds. There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to live young. Eutherian mammals have a complex placenta.

There are 16 extant (living) orders of eutherian mammals. Humans are most closely related to Primates, all of which have adaptations for climbing trees, although not all species are arboreal. Other characteristics of primates are brains that are larger than those of other mammals, claws that have been modified into flattened nails, and typically one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are divided into two groups: prosimians and anthropoids.