Endoskeletons

The study of endoskeletons can be approached in three basic ways. First, endoskeletons can be approached as gross structures—that is, as primarily mechanical or architectural systems. Second, they can be viewed as types of tissue—bone and cartilage—which involves study at the microscopic level. Third, endoskeletons can be considered products of the evolution of the endoskeletal organism. This area examines such factors as the organisms’ development from protochordates and the intrinsic biological advantages of the endoskeleton over the exoskeleton.

The Development of Endoskeletons

The evolution of vertebrate animals, such as fishes, reptiles, and mammals, having articulating endoskeletons represents a biological quantum leap forward for the phylum Chordata. The development of endoskeletons permitted several significant structural advantages that allowed the higher chordates to compete successfully with invertebrates and eventually to become dominant in many varied ecosystems.

The evolution of endoskeletons allowed a greater degree of general body efficiency and organization. Coupled with a great increase in the rapidity, power, and control of the movement with which endoskeletons and their improved musculature endowed their possessors, endoskeletons allowed vertebrates to become the highest and fastest flyers, the swiftest runners and swimmers, and the widest ranging of animals. The first phylum to develop an endoskeleton, the echinoderms, gained a calcium carbonate covering about 530 million years ago. The organizational plan of the endoskeleton of the more highly evolved vertebrates is exemplified by that of mammals. The mammalian endoskeleton is articulated in many ways, giving it a great degree of flexibility and range of movement. Cartilaginous joints facilitate the articulations. The endoskeleton itself is typically made of bone material composed of calcium phosphate. Other than the endoskeleton proper, minor externalizations can take the form of fingernails and toenails, claws, hooves, antlers, and horn cores, as well as teeth. In these cases, materials other than bone can sometimes originate at or near the body surface or in the skin. These expressions can be present in the form of scales or plates, though this is the exception rather than the rule among this vertebrate class. All these external expressions can be considered, in a way, forms of a limited exoskeleton.

Axial and Appendicular Skeletons

Aside from such exceptions as pangolins and armadillos among mammals, and turtles and tortoises among reptiles, exoskeletal tissue does not usually constitute an architectural structure that the animal’s other organs or structures depend on for support or protection. The one exception among the various bones of the vertebrate endoskeleton is the bones of the cranium or skull. The cranial bones are a group of hard, thick bones, usually ovoid or spheroidal in general geometry, that offer extensive protection to the brain and primary sensory organs, such as the eyes and ears. The cranium is such a universal feature among vertebrates that the subphylum is sometimes referred to as the craniates. Evolutionarily, lower vertebrates tend to have larger numbers of skull bones. Some fish have as many as 180 skull bones. Higher taxonomic groups have inversely lower numbers of skull bones: Amphibians and reptiles possess between fifty and ninety-five, while mammals have thirty-five or fewer. The skull itself is a member of two fundamental divisions between which the entire endoskeleton is usually subdivided; it is part of the axial skeleton. The axial skeleton also includes the bones of the vertebral system, the ribs, and the sternum. Possession of all axial features is not universal. Some vertebrates, such as the leopard frog, do not possess ribs, while others, such as the snakes, do not have sternums.

The other endoskeletal subdivision is called the appendicular skeleton, and it is made up of the bones of the pelvic girdle, shoulders, and limbs. The components of the appendicular system exhibit a great degree of variation from vertebrate group to vertebrate group and even among species, as do those of the axial system and reflect the many different environments and lifestyles to which their respective possessors have adapted. A case in point is the numerous variations in form and length found among limb bones. The various lengths represent adaptations to such external environmental factors as the medium through which the animals move from place to place (air, water, ground surface, and subsurface) and speed. The lower and upper limb bones themselves are connected by joints. The jointing arrangements of limb bones are highly efficient mechanical developments. Two basic types of limb joints exist: the pulley joint and the ball-and-socket joint. Pulley joints are exemplified by finger and toe joints and represent great freedom of motion in one plane. Ball-and-socket joints are exemplified by shoulder or hip joints and represent freedom of universal motion. Still, another joint type is a cross between these two. Such a combination of pulley and ball joints is exemplified by the elbow joint in humans.

Histology

Another major approach to the study of the endoskeleton is its histology or the fine details of tissues and how these tissues develop. Bone material itself is active metabolically—that is to say, it is alive. It can be considered not only an architecture along which the vertebrate body is arranged but also a complex and specialized connective tissue. As an organic material, it possesses a number of unique properties that are derived from the fact that it has evolved to perform its various duties efficiently for the size, weight, and arrangement of the materials of which it is composed. It is engineered like structurally reinforced concrete, having fibers of collagen, a tough, fibrous binding protein that is analogous to the function of steel rods.

The mineral calcium is analogous to the concrete in a building. Bone is formed in two different ways, depending on type. One process involves a means of growth of two bone types, termed lamellar bone (compact bone) and cancellous bone (spongy bone). Lamellar bone, sometimes also called membrane bone, develops through the process of ossification when certain cells called osteoblasts become bone-secreting. The osteoblastic cells, in association with numerous fibers of connective tissue cells, form a network in which layers of calcium mineral salts, called lamellae, are deposited. This network slowly builds up a plate that expands along its margins. As the plate thickens, some osteoblasts remain alive and are incorporated into the bone growth. At this point, they begin to have irregular shapes and are termed osteocytes. Spaces in which the osteocytes are sited are termed lacunae (cavities) and develop long, omnidirectional, branching processes, termed canaliculi. Neighboring canaliculi eventually link up and create a network through which life-supporting blood containing oxygen and food can reach the growing bone tissue. The canaliculi system grows such that no bone cell is more than 0.1 millimeter from a blood-carrying capillary. This overall arrangement is termed a Haversian system.

Cartilage tissue is another endoskeletal material that forms the adult skeletons of higher vertebrates, such as mammals, when they are still in early developmental stages. In mammals, this type of tissue is not formed directly but rather by a replacement process. In mammalian embryos, most of the skeletal structure is initially laid down in the form of cartilage and then subsequently replaced by true bone. The process does not reach completion in the higher vertebrates until the animal is full-grown; in humans, this is as late as twenty-five or twenty-six years of age. Cartilage is not as hard or rigid as bone, but it is extremely tough and is resistant to forces of compression or extension. Under microscopic examination, it appears as a clear matrix which possesses numerous embedded cells termed chondroblasts. These chondroblasts lie in fluid-filled voids termed lacunae. Chondroblasts secrete the matrix called chondrin, which surrounds the lacunae. Both the chondrin and the fluids act in an elastic manner and are resistant to compression and external shocks. Various types of cartilage have collagen fibers. The amount of collagen fiber present determines the amount of extension that the cartilage can resist. The total effect of the cartilage’s unique composition is to render it a good skeletal material for young, rapidly developing, vulnerable animals, such as mammal embryos.

Echinoderms have a unique endoskeleton composed of hundreds of thousands of ossicles, which are plates made of calcium carbonate, covered by an epidermis layer. Some species, like sea urchins, have tightly bound, microscopic ossicles, while others, like the sea cucumber, may have larger, loosely bound plates. Attachment tissues connect all ossicles.

Vertebrate Evolution

The last basic approach to the study of endoskeletons is that of examining the evolutionary development of the phylum Chordata in general and the subphylum of vertebrates in particular. The earliest history of chordates fossils, like the Pikaia gracilens and Yunnanozoon lividum fossils, date back over 500 years to the Cambrian period. However, the remains of early, ancestral forms of this group made poor candidates for the fossilization process because they lacked hard body parts. A line of hypothesized evolution, therefore, has been drawn through surviving marine animals called protochordates, which presently are sessile or stationary for most of their life cycles, although before attaining this current form, they were capable of locomotion. These curious animals are considered invertebrate chordates as they possess notochords or flexible skeletal rods that run up the long axes of their bodies. Among this group are such animals as the amphioxus or lancelets, and the so-called sea squirts or tunicates.

Further evolved along the path that eventually led to the present diversity of endoskeleton-possessing vertebrates are animals possessing bone matter, such as the agnathan (jawless) fishes. Later fishes evolved jaws and eventually true teeth and progressed from having cartilaginous skeletons (the class Chondrichthyes) to having true, bony endoskeletons (the class Osteichthyes). These more advanced fishes eventually gave rise to the land-pioneering class Amphibia and ultimately engendered the vertebrate classes of Reptilia (reptiles), Aves (birds), and Mammalia. The reason that bone tissue evolved at all in the lower vertebrates is a subject that is still open to debate. Several rival theories exist; one holds that bone evolved simply as a more improved, harder material for exoskeletons superior to such material as calcium carbonate, the most common building material for invertebrate exoskeletons. Another suggests that bone evolved as a phosphate reserve as one component for energy storage and transfer for metabolic processes within the bodies of ancestral vertebrates. Still, another theory postulates that bone materials such as dentine and enamel evolved originally simply as effective insulation for the electrosensory organs found in primitive marine vertebrates. Other theories integrate versions of the theories above in complex, interactive arrangements.

Endoskeletal Research

Histological research of endoskeletal tissues has, in the past, been the most productive approach to obtaining the large body of data on bone tissues and processes that currently exist. The majority of the data accumulated with this approach has been gained in the laboratory and has involved specialized equipment and the use of techniques tailored to produce useful information on bone cells, their composition, related tissues, and the various organic processes involved. These techniques and laboratory tools, and appliances were developed laboriously over the centuries. Real progress in the field had to wait until the advent of the simple microscope. Believed to have been invented by the Dutch scientist Antoni van Leeuwenhoek or one of his contemporaries in the seventeenth century, the simple optical microscope—utilizing only one lens—allowed the first close-up look at living structures at or near the cellular level. Examination of Leeuwenhoek’s original equipment has revealed that he was able to obtain the respectable magnification of as much as 250 times. His lenses thus allowed humans entry into a world that had hitherto been barred to them: the world of the very small. Subsequent developments in microscopes produced compound optical microscopes with several lenses working in series and an eventual exponential increase in magnifying power. Biologists quickly recognized that the new tool underscored the relation between the structure and function of organic materials at the microscopic level. It is this critical concept that has been the key to unlocking the many secrets of the organic microstructures of living things, among them endoskeletons.

Hand in hand with the development of research using optical microscopes has been the preparation of histological sections. These are extremely thin, transparent shavings of organic tissues prepared in such a way as to facilitate microscopic examination. With the increased sophistication of the use of microscopy has come the perfected use of many different types of staining and dying. The use of stains and dyes has been selectively employed to highlight the different types of tissue being observed. A further refinement in microscopy has been the use of various lens filters, such as polarizing filters, that have added control of the light target to emphasize or de-emphasize various features.

Late Twentieth-Century Advances

After the advent of applied nuclear physics during World War II, a technique called autoradiography appeared; this enhanced the resources available to histological research. Autoradiography involves the introduction of radioactive substances into animals; consequently, these substances are incorporated into various tissue components. The great advantage of this technique is that it can provide direct information on how long it takes for the various tissue components to be synthesized and how long they last.

As the spectrum of isotopic labels expanded and became refined, it became possible to label and study in great detail almost every common tissue component found in animals. The study of the most intricate or delicate endoskeletal tissues thus became realistic, along with the added advantage of being able to determine the duration of the metabolic processes involved in development, decay, and replacement.

A solution to the magnification limitation of the light microscope was reached when the first electron microscope (EM) was built in 1931. Further improvements were made until the 1950s saw the widespread use of more technologically advanced devices called scanning electron microscopes (SEMs), which allow observation and SEM photography, termed electron micrography, of target objects considerably less than 1 micron in diameter.

Other modern technologies, such as the use of fiber-optic probes and optical fiber sensors inserted into the living bodies of animals and humans, allow benign observation of tissues in their natural state in the midst of normal processes. Fiber optics involves the transmission of light (and therefore images) through very fine, flexible glass rods by internal reflection. Fiber-optic instruments called fiberscopes allow the viewing of extremely small, and normally dark, internal structures such as skeletal tissues. These technologies have applications for human surgeries, including the musculoskeletal system.

Principal Terms

Appendicular Skeleton: One of two main divisions of vertebrate skeletal systems, composed of the bones of the pelvic girdle, the shoulders, and the limbs

Axial Skeleton: The other main division of vertebrate skeletal systems, made up of the bones of the skull, the vertebral column, the ribs, and the sternum

Cancellous Bone: Spongy bone that is composed of an open, interlacing framework of bony tissue oriented to provide maximum strength in response to normal strains and stresses

Cartilage: A soft, pliable, typically deep-lying tissue that constitutes the endoskeletons of primitive vertebrates, such as sharks, as well as the embryonic skeletons and jointing structures of adult higher vertebrates

Compact Bone: A dense type of bone, often termed lamellar bone, formed of a calcified bone matrix having a concentric ring organization

Haversian Systems: Narrow tubes surrounded by rings of bone, called lamellae, that are found within compact bones of animals having endoskeletons; the tubes contain blood vessels and bone

Osteoblast: A bone-secreting cell found in vertebrates that is instrumental in the process of ossification

Bibliography

Alexander, R. McNeill. Bones: The Unity of Form and Function. Foreword by Mark A. Norell. Macmillan, 1994.

Bilezikian, John P., et al. Principles of Bone Biology. 4th ed., Academic Press, 2020.

Carroll, Robert L. Vertebrate Paleontology and Evolution. W. H. Freeman, 1988.

Ham, Arthur W. Ham’s Histology. 9th ed., J. B. Lippincott, 1987.

Hickman, Cleveland P., and Cleveland P. Hickman, Jr. Integrated Principles of Zoology. 19th ed., McGraw Hill, 2023.

Jaschke, Nikolai, et al. “Skeletal Endocrinology: Where Evolutionary Advantage Meets Disease.” Bone Research, vol. 9, no. 28, May. 2021, doi:10.1038/s41413-021-00149-x.

Kardong, Kenneth V. Vertebrates: Comparative Anatomy, Function, Evolution. 6th ed., McGraw-Hill, 2012.

Kent, George C. Comparative Anatomy of the Vertebrates. 9th ed., McGraw-Hill, 2008.

Langley, Liz. "Why Animals Developed Four Types of Skeletons." National Geographic, 19 Oct. 2021, www.nationalgeographic.com/animals/article/why-animals-developed-four-types-of-skeletons. Accessed 10 Sept. 2024.

Savage, R. J. G., and M. R. Long. Mammal Evolution: An Illustrated Guide. Facts on File, 1986.

The Visual Dictionary of the Skeleton. Dorling Kindersley, 1995.