Lesson 7 Lesson Outline: Embryonic Origins of the Dermis Specializations of the Dermis o Scales in Fish o Dermal Armour in Tetrapods Epidermal/Dermal Interactions o Feathers o Hair o Teeth Objectives: At the end of this lesson you should be able to: Describe the basic structure and function of the dermis Describe the embryonic origin of the dermis Describe the phylogenetic trends seen in the development of the dermis. Describe the phylogenetic and ontogenetic origins of feathers, hair and teeth References: Chapter 6: 115-127 Chapter 12: 273-279 Reading for Next Lesson: Chapter 6: 127-132
Dermis The dermis arises from the dermatome (derived from the segmental epimeres) from mesenchyme, and from "other sources". (Remember it also contains elements from the neural crest (sensory cells such as chromatophores) and from other parts of the neural ectoderm and mesoderm (nerves and blood vessels, etc.)). It produces an outer layer called the reticular lamina. The basal lamina (of the epidermis) and the reticular lamina (of the dermis) together form the basement membrane. The dermatome primarily gives rise to connective tissue. This is usually diffuse and irregular but can be arranged into distinct ordered layers. The connective tissue of the dermis can also become ossified giving rise to dermal bone. Bone is a constituent of the dermis in some members of every vertebrate group except birds. Between the integument and deep body musculature is a transitional zone made up of very loose connective tissue and adipose tissue the Hypodermis or superficial fascia. Below this is the musculature of the body Fishes The collagen in fishes within the dermis is regularly organized into plies that spiral around the body of the fish allowing the skin to bend without wrinkling. This is not the case in tetrapods. A classic example of this is found in the integument of the dogfish with plies of layers being oriented on a bias to give their skin stretch that accommodates lateral bending but resists distortion of the body shape. As such it allows the skin to stretch without wrinkling. This prevents turbulence during swimming as water flows over the body. This not only occurs in the dogfish but is common in aquatic reptiles and marine mammals such as the cetaceans. This is not seen in terrestrial vertebrates. In early (ancestral) fish, the dermis often gave rise to dermal bone. The generalized pattern of dermal bone consisted of lamellar bone, spongy bone, dentin and enamel or an enamel like substance. Lamellar bone is compact bone that has been deposited in successive layers or lamellae. Spongy bone is penetrated by large blood vessels giving it a spongy appearance. Dentin is another variety of bone. Enamel is an epidermal secretion. Dermal bone in turn gives rise to dermal scales. The surface of these scales is sometimes covered with a hard, acellular enamel of epidermal origin and a deeper dentin layer of dermal origin.
In primitive (extinct) fishes, the dermal armour was prominent encasing their bodies in an exoskeleton. In the agnathans, dermal bone is lost and the integument is smooth and without scales. In the cartilaginous fishes, dermal bone is also absent. Surface denticles (made of dentin and enamel) in the form of placoid scales are present and appear to reduce friction and drag during swimming. They develop in the dermis but project through the epidermis. In bony fishes, dermal scales do not actually penetrate the epidermis. There is an epidermal layer, with mucus secreting cells on the outside. There are a number of types of scales, the (cosmoid, now extinct), ganoid (gar and polypterus), cycloid and ctenoid scales (teleosts). You will see the various types of scales in lab. The trend is towards scales that are lacking in enamel, dentin and the vascular bone layer leaving only lamellar bone which is acellular and mostly non-calcified. Tetrapods Dermal Bone Within the tetrapods the occurrence of dermal bone is greatly reduced. Dermal bone contributes to osteoderms (minute bony scales) in caecilians, toads, crocodilians, some lizards gastralia in reptiles. (gastralia form in the abdominal area of many reptiles). turtle shell scales of the armadillo in most mammals it only contributes to the skull and pectoral girdle. It does not form scales. Chromatophores are cells containing pigment granules. They are occasionally found in the epidermis but are usually found in the dermis. They are responsible for skin colouration. In some cells they can be aggregated into the vicinity of the nucleus or dispersed throughout the cell by hormones or neurotransmitters. Thus, some pigmented cells are a constant colour and some can undergo physiological colour change. In birds and mammals, chromatophores that arise from neural crest cells may be found anywhere in the body but their pigments are very prevalent in the epidermis. The chromatophores secrete their pigments (primarily melanin) to the epithelial cells which move up to become part of the stratum corneum (dead outer layer) of feathers and hair. Teeth, Feathers, Hair and Dermal Papillae There are three important characteristics that form as a result of an interaction between the dermis and the epidermis, rather than as a feature of either the epidermis or the dermis alone. These are teeth, feathers and hair. It is an embryonic interaction between the two layers that induces their formation.
Teeth If we return to the placoid scales of elasmobranchs, note that the scale is formed primarily of dermal origin but does have an epidermal component - the enamel. Also note how these scales originate as dermal denticles. They are composed of both dermal and epidermal cells and have blood vessels and nerves It is an interaction between the epidermal cells and the dermal cells that induces their formation. In the earliest fishes, these dermal scales extended into the mouth and oral cavity. Find differences in - location. In some animals they are found throughout the oral cavity in some only along the ridges of the jaws in some they are confined to only parts of the jaw - how they attach to the jaw. They may be attached to the medial side of the bone, they may sit in shallow sockets on the crest of the bone or they may be sunken deeply into sockets within the bone. - whether/how they are replaced. In some they are replaced continuously, in some they are only replaced once. In some this is a slow process so that they erupt continuously throughout life. In some mammals they continue to grow throughout life and are worn down at the tips. - shape. They may have abrasive surfaces, they may be flat for crushing, sharp for catching holding and penetration, serrated for shearing. In some animals there is only one type of tooth (isodont/homodont = although they may vary in size but not shape) while in others there are several types of teeth in the same animal (heterodont) - some teeth can become highly specialized. Examples are the hollow fang of the snake, the tusk of the narwahl (left incisor), elephant tusks (both incisors), walrus tusk (canines) canines of carnivores Feathers There are several types of feathers. Contour feathers aerodynamic shape Down feathers insulation Filoplumes for display Flight feathers for locomotion Feather development is initiated by the formation of a dermal papilla, a mound of mesodermal cells from the dermis that indent out into the epidermis and induce the epidermis to produce the feather. The dermal contribution is to provide the blood supply for nutrients and waste removal while the feather follicle develops from the epidermis. When the feather is fully grown, the dermal papilla in the shaft of the feather dies and becomes the pulp. The base of the dermal papilla usually begins to induce the production of another feather which ultimately passively pushes out the previous feather during moulting.
Most biologists believe that feathers have evolved from epidermal scales. Like the epidermis of reptiles, feathers are also sloughed and replaced by moulting. New feathers begin to develop in the same follicles and erupt upwards pushing out the old feather. They contain no nerves or blood vessels. There are arguments that feathers first evolved from epidermal scales to 1) insulate to keep heat out in hot environments, 2) insulate to keep heat in, in endothermic reptiles, 3) for aerodynamic streamlining in gliding reptiles. Hair There are several types of hair Guard hair - course outer hair Underfur - fine short hair Both serve for insulation. In marine mammals, the underfur is lost and the guard hair layer is also reduced. Vibrissae - whiskers are specialized hair for mechanoreception Quills in porcupine are specialized hairs for defense. Hair is largely composed of epidermal cells. The epidermis invaginates and where it makes contact with the dermis a small dermal papilla forms. (similar but slightly different from what we saw in feathers). The papilla appears to be involved in stimulating the epidermis to differentiate to produce hairs. There are several theories for the evolution of hair. The two main one are that it evolved first for 1) insulation and 2) as a mechanosensory structure in the folds of epidermal scales which secondarily took on an insulative role.