Vertebrate Life 9th Edition PDF: Hox Genes, Neural Crest, and More

Hox Genes Hox genes regulate the expression of a hierarchical network of other genes that control the process of development from front to back along the body. Vertebrates have more Hox genes than other groups of animals: Jellyfishes (and possibly also sponges) have one or two Hox genes, the common a...

Hox Genes Hox genes regulate the expression of a hierarchical network of other genes that control the process of development from front to back along the body. Vertebrates have more Hox genes than other groups of animals: Jellyfishes (and possibly also sponges) have one or two Hox genes, the common ancestor of protostomes and deuterostomes probably had seven, and more derived metazoans have up to thirteen. However, vertebrates are unique in having undergone duplications of the entire Hox complex. The first duplication event seems to have occurred at the start of vertebrate evolution because amphioxus and tunicates have a single Hox cluster, whereas the living jawless vertebrates have two. A second duplication event had taken place by the evolution of gnathostomes, because all jawed vertebrates have at least four clusters. Finally, an additional duplication event occurred in both teleost fishes (derived bony fishes) and frogs. Interactions among genes modify the effects of those genes, and more genes allow more interactions that probably produce more complex structures. The doubling and redoubling of the Hox gene sequence during vertebrate evolution is believed to have made the structural complexity of vertebrates possible. Neural Crest Neural crest is a new tissue in embryological development that forms many novel structures in vertebrates, especially in the head region (a more detailed description is given in section 2.3). The evolution of the neural crest is the most important innovation in the origin of the vertebrate body plan. Neural-crest tissue is a fourth germ layer that is unique to vertebrates and is on a par with ectoderm, endoderm, and mesoderm. Neural-crest cells originate at the lateral boundary of the neural plate, the embryonic structure that makes the nerve cord, and migrate throughout the body to form a variety of structures, including pigment cells. A similar population of cells, with a similar genetic expression, can be found in amphioxus, but here the cells do not migrate and do not change into different cell types. Recently cells resembling migratory neuralcrest cells have been identified in the larval stage of one tunicate species, where they differentiate into pigment cells. These cells in tunicates may represent a precursor to the vertebrate neural crest. (This appears to be one morphological feature that tunicates share with vertebrates, but it is not seen in cephalochordates.) If the Cambrian chordate Haikouella has been correctly interpreted as having eyes and a muscular pharynx, these features would imply the presence of neural crest in this animal. Placodes Another new type of embryonic tissue in vertebrates, which is similar to neural crest but probably has a different origin, forms the epidermal thickenings (placodes) that give rise to the complex sensory organs of vertebrates, including the nose, eyes, and inner ear. Some placode cells migrate caudally to contribute, along with the neural-crest cells, to the lateral line system and to the cranial nerves that innervate it. MicroRNAs The appearance of many microRNAs is a genetic innovation in vertebrates that may contribute to their anatomical complexity. MicroRNAs are noncoding RNA sequences 22 bases long that have been added to the genomes of metazoans throughout their evolutionary history. MicroRNAs regulate the synthesis of proteins by binding to complementary base sequences of messenger RNAs. The phylum Chordata is characterized by the addition of two new microRNAs, another three are shared by vertebrates and tunicates, and all vertebrates possess an additional 41 unique microRNAs. (All vertebrate lineages have independently acquired yet more microRNAs of their own, and mammals in particular have a great number of novel microRNAs.) MicroRNAs are involved in regulating the development of some derived vertebrate structures, including the liver and the kidney. Brains The brains of vertebrates are larger than the brains of primitive chordates, and they have three parts—the forebrain, midbrain, and hindbrain. The brain of amphioxus appears simple, but genetic studies show that amphioxus has all the genes that code for the vertebrate brain with the exception of those directing formation of the front part of the vertebrate forebrain, the telencephalon (the portion of the brain that contains the cerebral cortex, the area of higher processing in vertebrates). The presence of the genes in amphioxus combined with the absence of a complex brain reinforces the growing belief that the differences in how genes are expressed in different animals are as important as differences in what genes are present. Other unique vertebrate features include a multilayered epidermis and blood vessels that are lined by endothelium. 2.3 Basic Vertebrate Structure This section serves as an introduction to vertebrate anatomical structure and function. The heart of this section is in Table 2–1 and Figure 2–4, which contrast the basic vertebrate condition with that of a nonvertebrate chordate such as amphioxus. These same systems will Basic Vertebrate Structure 25 Amphioxus-like nonvertebrate chordate Cerebral vesicle V-shaped segmental myomeres Notochord Nerve cord Pigment spot Simple caudal fin Cloaca Gut (unmuscularized) Endostyle Atrium Gill slit Cecum Pharynx Single-chambered (unmuscularized) "heart" (a) Atriopore Hypothetical primitive vertebrate Tripartite brain Lateral line Notochord Nerve cord Cranium Figure 2–4 Generalized chordate structure. (a) A generalized amphioxus nonvertebrate chordate; (b) a hypothetical ancestral vertebrate. (Note that the myomeres actually extend all the way down the body; see Figure 2–10.) 26 CHAPTER 2 Liver Cranial sense Thyroid organs: nose, eye, Gill slit Gill arch inner ear with gill (cartilage) tissue Pharynx (b) (muscularized) be further discussed for more derived vertebrates in later chapters; our aim here is to provide a general introduction to the basics of vertebrate anatomy. More detail can be found in books listed at the end of the chapter. At the whole-animal level, an increase in body size and increased activity distinguish vertebrates from nonvertebrate chordates. Early vertebrates generally had body lengths of 10 centimeters or more, which is about an order of magnitude larger than the bodies of nonvertebrate chordates. Because of their relatively large size, vertebrates need specialized systems to carry out processes that are accomplished by diffusion or ciliary action in smaller animals. Vertebrates are also more active animals than other chordates, so they need organ systems that can carry out physiological processes at a greater rate. The transition from nonvertebrate chordate to vertebrate was probably related to the adoption of a more actively predaceous mode of life, as evidenced by the features of the vertebrate head W-shaped segmental myomeres Caudal fin with dermal fin rays Kidney Archinephric duct Pancreatic tissue Three-chambered heart Gut (muscularized) (largely derived from neural-crest tissue) that would enable suction feeding with a muscular pharynx, and a bigger brain and more complex sensory organs for perceiving the environment. Vertebrates are characterized by mobility, and the ability to move requires muscles and a skeleton. Mobility brings vertebrates into contact with a wide range of environments and objects in those environments, and a vertebrate’s external protective covering must be tough but flexible. Bone and other mineralized tissues that we consider characteristic of vertebrates had their origins in this protective integument. Embryonic Development Studying embryos can show how systems develop and how the form of the adult is related to functional and historical constraints during development. Scientists no longer adhere to the biogenetic law that “ontogeny recapitulates phylogeny” (i.e., the idea that the embryo Vertebrate Relationships and Basic Structure Table 2–1 Comparison of features in nonvertebrate chordates and ancestral vertebrates Generalized Nonvertebrate Chordate (based on features of the living cephalochordate amphioxus) Ancestral Vertebrate (based on features of the living jawless vertebrates—hagfishes and lampreys) Brain and Head End Notochord extends to tip of head (may be derived condition). Head extends beyond tip of notochord. No cranium (skull). Cranium—skeletal supports around brain, consisting of capsules surrounding the main parts of the brain and their sensory components plus underlying supports. Simple brain (= cerebral vesicle), no specialized sense organs (except photoreceptive frontal organ, probably homologous with the vertebrate eye). Tripartite brain and multicellular sense organs (eye, nose, inner ear). Poor distance sensation (although the skin is sensitive). Improved distance sensation: in addition to the eyes and nose, also have a lateral line system along the head and body that can detect water movements (poorly developed lateral line system on the head is found only in hagfishes). No electroreception. Electroreception may be an ancestral vertebrate feature (but absent in hagfishes, possibly lost). Pharynx and Respiration Gill arches used for filter feeding (respiration is by diffusion over the body surface). Gill arches (= pharyngeal arches) support gills that are used primarily for respiration. Numerous gill slits (up to 100 on each side). Fewer gill slits (6 to 10 on each side), individual gills with highly complex internal structure (gill filaments). Pharynx not muscularized (except in wall of atrium, or external body cavity). Pharynx with specialized (branchiomeric) musculature. Water moved through pharynx and over gills by ciliary action. Water moved through pharynx and over gills by active muscular pumping. Gill arches made of collagen-like material Gill arches made of cartilage (allows for elastic recoil—aids in pumping). Feeding and Digestion Gut not muscularized: food passage by means of ciliary action. Gut muscularized: food passage by means of muscular peristalsis. Digestion of food is intracellular: individual food particles taken into cells lining gut. Digestion of food is extracellular: enzymes poured onto food in gut lumen, then breakdown products absorbed by cells lining gut. No discrete liver and pancreas: structure called the midgut cecum or diverticulum is probably homologous to both. Discrete liver and pancreatic tissue. Heart and Circulation Ventral pumping structure (no true heart, just contracting regions of vessels; = sinus venosus of vertebrates). Also accessory pumping regions elsewhere in the system. Ventral pumping heart only (but accessory pumping regions retained in hagfishes). Three-chambered heart (listed in order of blood flow): sinus venosus, atrium, and ventricle. No neural control of the heart to regulate pumping. Neural control of the heart (except in hagfishes). Circulatory system open: large blood sinuses; capillary system not extensive. Circulatory system closed: without blood sinuses (some remain in hagfishes and lampreys); extensive capillary system. Blood not specifically involved in the transport of respiratory gases (O2 and CO2 mainly transported via diffusion). No red blood cells or respiratory pigment. Blood specifically involved in the transport of respiratory gases. Red blood cells containing the respiratory pigment hemoglobin (binds with O2 and CO2 and aids in their transport). (continued) Basic Vertebrate Structure 27 Table 2–1 Comparison of features in nonvertebrate chordates and ancestral vertebrates (Continued) Generalized Nonvertebrate Chordate Excretion and Osmoregulation No specialized kidney. Coelom filtered by solenocytes (flame cells) that work by creating negative pressure within cell. Cells empty into the atrium (false body cavity) and then to the outside via the atriopore. Body fluids same concentration and ionic composition as seawater. No need for volume control or ionic regulation. Ancestral Vertebrate Specialized glomerular kidneys: segmental structures along dorsal body wall; works by ultrafiltration of blood. Empty to the outside via the archinephric ducts leading to the cloaca. Body fluids more dilute than seawater (except for hagfishes). Kidney important in volume regulation, especially in freshwater environment. Monovalent ions regulated by the gills (also the site of nitrogen excretion), divalent ions regulated by the kidney. Support and Locomotion Notochord provides main support for body muscles. Notochord provides main support for body muscles, vertebral elements around nerve cord at least in all vertebrates except hagfishes. Myomeres with simple V shape. Myomeres with more complex W shape. No lateral fins; no median fins besides tail fin. Initially no lateral fins. Caudal (tail) fin has dermal fin rays. Dorsal fins present in all except hagfishes. faithfully passes through its ancestral evolutionary stages in the course of its development) proposed by the nineteenth-century embryologist Haeckel. Nevertheless, embryology can provide clues about the ancestral condition and about homologies between structures in different animals. The development of vertebrates from a single fertilized cell (the zygote) to the adult condition will be summarized only briefly. This is important background information for many studies, but a detailed treatment is beyond the scope of this book. Note, however, that there is an important distinction in development between vertebrates and invertebrates: invertebrates develop from cell lineages whose fate is predetermined, but vertebrates are much more flexible in their development and use inductive interactions between developing structures to determine the formation of different cell types and tissues. We saw earlier that all animals with the exception of sponges are formed of distinct tissue layers, or germ layers. The fates of germ layers have been very conservative throughout vertebrate evolution. The outermost germ layer, the ectoderm, forms the adult superficial layers of skin (the epidermis); the linings of the most anterior and most posterior parts of the digestive tract; and the nervous system, including most of the sense organs (such as the eye and the ear). The innermost layer, the endoderm, forms the rest of the digestive tract’s lining as well as the lining of glands associated with the gut—including the liver 28 CHAPTER 2 and the pancreas—and most respiratory surfaces of vertebrate gills and lungs. Endoderm also forms the taste buds and the thyroid, parathyroid, and thymus glands. The middle layer, the mesoderm, is the last of the three layers to appear in development, perhaps reflecting the fact that it is the last layer to appear in animal evolution. It forms everything else: muscles, skeleton (including the notochord), connective tissues, and circulatory and urogenital systems. A little later in development, there is a split within the originally solid mesoderm layer, forming a coelom or body cavity. The coelom is the cavity containing the internal organs. In mammals it is divided into the pleural cavity (around the lungs), the peritoneal cavity (around the viscera), and the pericardial cavity (around the heart). In other animals, which either lack lungs entirely or lack a diaphragm separating the pleural cavity from the peritoneal cavity, these two cavities are united into the pleuroperitoneal cavity. These coelomic cavities are lined by thin sheets of mesoderm—the peritoneum (around the pleural or peritoneal cavity) and the pericardium (around the heart). The gut is suspended in the peritoneal cavity by sheets of peritoneum called mesenteries. Neural crest forms many of the structures in the anterior head region, including some bones and muscles that were previously thought to be formed by mesoderm. Neural crest also forms almost all of the peripheral nervous system (i.e., that part of the nervous Vertebrate Relationships and Basic Structure Ectoderm Hindbrain Auditory placode Somites Nephrotome Nerve cord Midbrain Notochord Dorsal mesentery Outer layer of lateral plate (forms outer peritoneum and appendicular skeleton) Gut Pharynx Gut endoderm Forebrain Coelom Body wall Inner layer of lateral plate (forms gut muscles, heart muscles, blood and blood vessels, connective tissue, and inner peritoneum) Optic cup Stomadeum Pharyngeal Heart clefts Nasal placode Unsegmented lateral plate mesoderm Ectoderm Ventral mesentery Figure 2–5 Three-dimensional view of a portion of a generalized vertebrate embryo at the developmental stage (called the pharyngula) when the developing gill pouches appear. The ectoderm is stripped off the left side, showing segmentation of the mesoderm in the trunk region and pharyngeal development. The stomadeum is the developing mouth. system outside of the brain and the spinal cord) and contributes to portions of the brain. Some structures in the body that are new features of vertebrates are also formed from neural crest. These include the adrenal glands, pigment cells in the skin, secretory cells of the gut, and smooth muscle tissue lining the aorta. Figure 2–5 shows a stage in early embryonic development in which the ancestral chordate feature of pharyngeal pouches in the head region makes at least a fleeting appearance in all vertebrate embryos. In fishes the grooves between the pouches (the pharyngeal clefts) perforate to become the gill slits, whereas in land vertebrates these clefts disappear in later development. The linings of the pharyngeal pouches give rise to half a dozen or more glandular structures often associated with the lymphatic system, including the thymus gland, parathyroid glands, carotid bodies, and tonsils. The dorsal hollow nerve cord typical of vertebrates and other chordates is formed by the infolding and subsequent pinching off and isolation of a long ridge of ectoderm running dorsal to the developing notochord. The notochord itself appears to contain the developmental instructions for this critical embryonic event, which is probably why the notochord is retained in the embryos of vertebrates (such as us) that no longer have the complete structure in the adult. The cells that will form the neural crest arise next to the developing nerve cord (the neural tube) at this stage. Slightly later in development, these neural-crest cells disperse laterally and ventrally, ultimately settling and differentiating throughout the embryo. Embryonic mesoderm becomes divided into three distinct portions, as shown in Figure 2–5, with the result that adult vertebrates are a strange mixture of segmented and unsegmented components. The dorsal (upper) part of the mesoderm, lying above the gut and next to the nerve cord, forms an epimere, a series of thick-walled segmental buds (somites), which extends from the head end to the tail end. The ventral (lower) part of the mesoderm, surrounding the gut and containing the coelom, is thin-walled and unsegmented and is called the lateral plate mesoderm (or hypomere). Small segmental buds linking the somites and the lateral plate are called nephrotomes (the mesomere or the intermediate mesoderm). The nervous system also follows this segmented versus unsegmented pattern, as will be discussed later. The segmental somites will eventually form the dermis of the skin, the striated muscles of the body that are used in locomotion, and portions of the skeleton (the vertebral column, ribs, and portions of the back of the skull). Some of these segmental muscles later Basic Vertebrate Structure 29 migrate ventrally from their originally dorsal (epaxial) position to form the layer of striated muscles on the underside of the body (the hypaxial muscles), and from there they form the muscles of the limbs in tetrapods (four-footed land vertebrates). The lateral plate forms all the internal, nonsegmented portions of the body, such as the connective tissue, the blood vascular system, the mesenteries, the peritoneal and pericardial linings of the coelomic cavities, and the reproductive system. It also forms the smooth muscle of the gut and the cardiac (heart) muscle. The nephrotomes form the kidneys (which are elongated segmental structures in the ancestral vertebrate condition), the kidney drainage ducts (the archinephric ducts), and the gonads. Some exceptions exist to this segmented versus nonsegmented division of the vertebrate body. The locomotory muscles, both axial (within the trunk region) and appendicular (within the limbs), and the axial skeleton are derived from the somites. Curiously, however, the limb bones are mostly derived from the lateral plate, as are the tendons and ligaments of the appendicular muscles, even though they essentially form part of the segmented portion of the animal. The explanation for this apparent anomaly may lie in the fact that limbs are add-ons to the basic limbless vertebrate body plan, as seen in the living jawless vertebrates. The boundary of the complex interaction between the somite-derived muscles and structures derived from the outer layer of the lateral plate in the embryo is known as the lateral somitic frontier. This area is involved in the switching on and off of regulatory genes and is thus of prime importance in evolutionary change. Other peculiarities are found in the expanded front end of the head of vertebrates, which has a complex pattern of development and does not follow the simple segmentation of the body. The head mesoderm contains only somites (no lateral plate), which give rise to the striated eye muscles and branchiomeric muscles powering the pharyngeal arches (gills and jaws). Within the brain, the anteriormost part of the forebrain (the front of the telencephalon) and the midbrain are not segmented, but the hindbrain shows segmental divisions during development (rhombomeres). Adult Tissue Types There are several kinds of tissue in vertebrates: epithelial, connective, vascular (i.e., blood), muscular, and nervous. These tissues are combined to form larger units called organs, which often contain most or all of the five basic tissue types. Connective Tissue A fundamental component of most animal tissues is the fibrous protein collagen. Collagen 30 CHAPTER 2 is primarily a mesodermal tissue: in addition to the softer tissues of organs, it forms the organic matrix of bone and the tough tissue of tendons and ligaments. Vertebrates have a unique type of fibrillar collagen that may be responsible for their ability to form an internal skeleton. Collagen is stiff and does not stretch easily. In some tissues, collagen is combined with the protein elastin, which can stretch and recoil. Another important fibrous protein, seen only in vertebrates, is keratin. While collagen forms structures within the mesoderm, keratin is primarily an ectodermal tissue. Keratin is mainly found in the epidermis (outer skin) of tetrapods, making structures such as hair, scales, feathers, claws, horns, and beaks; it also forms the horny toothlike structures of the living jawless vertebrates. The Integument The external covering of vertebrates, the integument, is a single organ, making up 15 to 20 percent of the body weight of many vertebrates and much more in armored forms. It includes the skin and its derivatives, such as glands, scales, dermal armor, and hair. The skin protects the body and receives information from the outside world. The major divisions of the vertebrate skin are the epidermis (the superficial cell layer derived from embryonic ectoderm) and the unique vertebrate dermis (the deeper cell layer of mesodermal and neural-crest origin). The dermis extends deeper into a subcutaneous tissue (hypodermis) that is derived from mesoderm and overlies the muscles and bones. The epidermis forms the boundary between a vertebrate and its environment and is of paramount importance in protection, exchange, and sensation. It often contains secretory glands and may play a significant role in osmotic and volume regulation. The dermis, the main structural layer of the skin, includes many collagen fibers that help to maintain its strength and shape. The dermis contains blood vessels, and blood flow within these vessels is under neural and hormonal control (e.g., as in human blushing, when the vessels are dilated and blood rushes to the skin). The dermis also houses melanocytes (melanincontaining pigment cells that are derived from the neural crest) and smooth muscle fibers, such as the ones in mammals that produce skin wrinkling around the nipples. In tetrapods, the dermis houses most of the sensory structures and nerves associated with sensations of temperature, pressure, and pain. The hypodermis, or subcutaneous tissue layer, lies between the dermis and the fascia overlying the muscles. This region contains collagenous and elastic fibers and is the area in which subcutaneous fat is stored by Vertebrate Relationships and Basic Structure birds and mammals. The subcutaneous striated muscles of mammals, such as those that enable them to flick the skin to get rid of a fly, are found in this area. Mineralized Tissues Vertebrates have a unique type of mineral called hydroxyapatite, a complex compound of calcium and phosphorus. Hydroxyapatite is more resistant to acid than is calcite (calcium carbonate), which forms the shells of mollusks. The evolution of this unique calcium compound in vertebrates may be related to the fact that vertebrates rely on anaerobic metabolism during activity, producing lactic acid that lowers blood pH. A skeleton made of hydroxyapatite may be more resistant to acidification of the blood during anaerobic metabolism than is the calcite that forms the shells of mollusks. Vertebrate mineralized tissues are composed of a complex matrix of collagenous fibers, cells that secrete a proteinaceous tissue matrix, and crystals of hydroxyapatite. The hydroxyapatite crystals are aligned on the matrix of collagenous fibers in layers with alternating directions, much like the structure of plywood. This combination of cells, fibers, and minerals gives bone its complex latticework appearance that combines strength with relative lightness and helps to prevent cracks from spreading. Six types of tissues can become mineralized in vertebrates, and each is formed from a different cell lineage in development. Mineralized cartilage. Cartilage is an important structural tissue in vertebrates and many invertebrates but is not usually mineralized. Mineralized cartilage occurs naturally only in jawed vertebrates, where it forms the main mineralized internal skeletal tissue of sharks. (Sharks and other cartilaginous fishes appear to have secondarily lost true bone.) Some fossil jawless vertebrates also had internal calcified cartilage, probably evolved independently from the condition in sharks. Bone. The internal skeleton of bony fishes and tetrapods is formed by bone. Bone may replace cartilage in development, as it does in our own skeletons, but bone is not simply cartilage to which minerals have been added. Rather, it is composed of different types of cells—osteocytes (Greek osteo = bone and cyte = cell), which are called osteoblasts (Greek blasto = a bud) while they are actually making the bone; in contrast, chondrocytes form cartilage. The cells that form bone and cartilage are derived from the mesoderm, except in the region in the front of the head, where they are derived from neural-crest tissue. Bone and mineralized cartilage are both about 70 percent mineralized. Enamel and dentine. The other types of mineralized tissues are found in the teeth and in the mineralized exoskeleton of ancestral vertebrates. The enamel and dentine that form our teeth are the most mineralized of the tissues—enamel is about 96 percent mineralized, and dentine is about 90 percent mineralized. This high degree of mineralization explains why teeth are more likely to be found as fossils than are bones. The cells that form dentine (odontoblasts) are derived from neural-crest tissue, and those that form enamel (amyloblasts) are derived from the ectoderm. Enameloid. Enameloid resembles enamel in its degree of hardness and its position on the outer layer of teeth or dermal scales, but it is produced by mesodermal cells. Enameloid is the enamel-like tissue that was present in ancestral vertebrates and is found today in cartilaginous fishes. Both enamel and enameloid may have evolved independently on a number of occasions. Cementum. Cementum is a bonelike substance that fastens the teeth in their sockets in some vertebrates, including mammals, and may grow to become part of the tooth structure itself. Bone Bone remains highly vascularized even when it is mineralized (ossified). This vascularization allows bone to remodel itself. Old bone is eaten away by specialized blood cells (osteoclasts, from the Greek clast = broken), which are derived from the same cell lines as the macrophage white blood cells that engulf foreign bacteria in the body. Osteoblasts enter behind the osteoclasts and deposit new bone. In this way, a broken bone can mend itself and bones can change their shape to suit the mechanical stresses imposed on an animal. This is why exercise builds up bone and why astronauts lose bone in the zero gravity of space. Mineralized cartilage is unable to remodel itself because it does not contain blood vessels. There are two main types of bone in vertebrates: dermal bone, which, as its name suggests, is formed in the skin without a cartilaginous precursor; and endochondral bone, which is formed in cartilage. Dermal bone (Figure 2–6) is the earliest type of vertebrate bone first seen in the fossil jawless vertebrates called ostracoderms, which are described in Chapter 3. Only in the bony fishes and tetrapods is the endoskeleton composed primarily of bone. In these vertebrates, the endoskeleton is initially laid down in cartilage and is replaced by bone later in development. Dermal bone originally was formed around the outside of the body, like a suit of armor (ostracoderm Basic Vertebrate Structure 31 Dentine tubercles Spongy acellular bone (a) Basal acellular bone Enamel The Skeletomuscular System Dentine Pulp cavity Cementum Periodontal ligament Enamel organ Dermal papilla (b) Replacement tooth © 1998 The McGraw-Hill Companies, Inc. Figure 2–6 Organization of vertebrate mineralized tissues. (a) Three-dimensional block diagram of dermal bone from an extinct jawless vertebrate (heterostracan ostracoderm). (b) Section through a developing tooth (shark scales are similar). means “shell-skinned”), forming a type of exoskeleton. We think of vertebrates as possessing only an endoskeleton, but most of our skull bones are dermal bones, and they form a shell around our brains. The endoskeletal structure of vertebrates initially consisted of only the braincase and was originally formed from cartilage. Thus, the condition in many early vertebrates was a bony exoskeleton and a cartilaginous endoskeleton (Figure 2–7). Teeth Teeth form from a type of structure called a dermal papilla, so they form only in the skin, usually over dermal bones. When the tooth is fully formed, it erupts through the gum line. Replacement teeth may start to develop to one side of the main tooth even before its eruption. The basic structure of the teeth of jawed vertebrates is like the structure of odontodes, which were the original toothlike components of the original 32 CHAPTER 2 vertebrate dermal armor, and odontodes are homologous with teeth and the dermal denticles of cartilaginous fishes. Teeth are composed of an inner layer of dentine and an outer layer of enamel or enameloid around a central pulp cavity (Figure 2–6). Shark scales (dermal denticles) have a similar structure. There has been considerable controversy about whether dental tissues are always derived from the ectoderm, or whether they can form from the endoderm, as seen in pharyngeal teeth in some fishes. Recent experimental studies have shown that the critical issue in tooth development is the neural-crest precursor that forms the dentine, and that either ectoderm or endoderm may be co-opted to form the outer layers. The basic endoskeletal structural features of chordates are the notochord, acting as a dorsal stiffening rod running along the length of the body, and some sort of gill skeleton that keeps the gill slits open. The cranium surrounding the brain was the first part of the vertebrate skeleton to evolve. Next the dermal skeleton of external plates and the axial skeleton (vertebrae, ribs, and median fin supports) were added, and still later the appendicular skeleton (bones of the limb skeleton and limb girdles) evolved. The Cranial Skeleton The skull, or cranium, is formed by three basic components: the chondrocranium (Greek chondr = cartilage [literally “gristle”] and cran = skull) surrounding the brain; the splanchnocranium (Greek splanchn = viscera) forming the gill supports; and the dermatocranium (Greek derm = skin) forming in the skin as an outer cover that was not present in the earliest vertebrates. The splanchnocranial components of the vertebrate skeleton are known by a confusing variety of names. In general they can be called gill arches because they support the gill tissue and muscles. The anterior elements of the splanchnocranium are specialized into nongill-bearing structures in all extant vertebrates, such as the jaws of gnathostomes. Other names for these structures are pharyngeal arches (because they form in the pharynx region) and branchial arches (which is just a fancy way of saying gill arches because the Greek word branchi means gill). Yet another name for these structures is visceral arches, because the splanchnocranium is also known as the visceral skeleton. (Still another name associated with the pharyngeal region, aortic arches, refers not to the gill skeleton but to the segmental arteries that supply the gill arches.) Vertebrate Relationships and Basic Structure The dermal skeleton or exoskeleton (a) The endodermal skeleton or endoskeleton Axial Cranial Figure 2–7 Vertebrate skeletons. (b) Appendicular We will call these structures pharyngeal arches when we are discussing the embryonic elements of their development, and gill arches in adults, especially for those arches that actually do bear gill tissue (i.e., arches 3–7). The vertebrate chondrocranium and splanchnocranium are formed primarily from neural-crest tissue, although a splanchnocranium-equivalent formed by endodermal tissue is present in cephalochordates and hemichordates. Thus, a structure with the same function as the vertebrate splanchnocranium preceded the origin of vertebrates and of neural-crest tissue, although only vertebrates have a true splanchnocranium (i.e., one that is derived from neural-crest tissue). The chondrocranium and splanchnocranium are formed from cartilage in the ancestral vertebrate condition, but they are made of endochondral bone in the adults of some bony fishes and most tetrapods. The dermatocranium is made from dermal bone, which is formed in a membrane rather than in a cartilaginous precursor. (Because it forms in a membrane it is sometimes called membrane bone.) The dermatocranium is cartilaginous only as a secondary condition in some fishes, such as sturgeons, where ossification of the dermatocranium has been lost. Figure 2–8 shows a diagrammatic representation (a) the originally dermal bone exoskeleton and (b) the originally cartilaginous bone endoskeleton. (The animal depicted is an extinct bony fish.) of the structure and early evolution of the vertebrate cranium, and Figure 2–9 (see page 36) illustrates three vertebrate crania in more detail. The Cranial Muscles There are two main types of stri- ated muscles in the head of vertebrates: the extrinsic eye muscles and the branchiomeric muscles. Six muscles in each eye rotate the eyeball in all vertebrates except hagfishes, in which their absence may represent secondary loss. Like the striated muscles of the body, these muscles are innervated by somatic motor nerves. The branchiomeric muscles are associated with the splanchnocranium and are used to suck water into the mouth during feeding and respiration. Branchiomeric muscles are innervated by cranial nerves that exit from the dorsal part of the spinal cord (unlike striated muscles, which are innervated by motor nerves that exit from the ventral part of the spinal cord). The reason for this difference is not clear, but it emphasizes the extent to which the vertebrate head differs in its structure and development from the rest of the body. The Axial Skeleton and Musculature The notochord is the original “backbone” of all chordates, although it is never actually made of bone. The notochord has a core Basic Vertebrate Structure 33