**5. Skin and scute features in crocodiles**

In crocodiles and turtles, the dermal armor is formed from the deeper dermis rather than the epidermis and does not form the same sort of overlapping structure as snake scales. These dermal scales are more properly called scutes. Similar dermal scutes are found in the feet of birds and tails of some mammals and are believed to be the primitive form of dermal armor in reptiles [8].

The crocodile skin has horny plates, named scutes, in which shape, number, and position are important for the identification of the species. They can also become similar to bones and form an outer bone armor. The horny plates on the back are referred as the back shield, and below are dermal plates (4–10 longitudinal plates whose number varies depending on the animal species). On the abdominal side beneath the horny scutes, there are no bone plates. On the tail, scutes form rings with two rows continuing in one row of scutes by the end of the tail. The position of the horny scutes on the head is characteristic for each animal species. On the head beneath the horny scutes, bone plates are located. Unlike other reptiles, crocodiles do not shed their scutes, and they are renewed by scrubbing against different outer surfaces [28].

#### **Figure 13.**

*Skin histology of the lip of Cuvier's Dwarf Caiman (Paleosuchus palpebrosus), stained with H&E. (A) The epidermis and dermis are observed. Thick stratified epidermis consists of several layers with recognizable stratum basale and a few more layers with enucleated keratinocytes. Stratum corneum is thinner and more compact, especially above the ISO region. ISOs are in the dermis, between thick fibers of connective tissue, vessels, and nerves. Melanophores are not observed. Around ISO, multiple vessels and nerves are observed (B). ISO bodies are concentrically shaped, similar to mammals. (A) 100× magnification, (B) 400× magnification.*

In the skin of the crocodile, pigmented cells are located that give a color that varies from green to light brown to gray. In most animals, the belly is lighter than the rest of the body. Scent glands in crocodiles open in the cloaca. Alligators of both sexes have one pair of scented glands.

Crocodiles can recognize the prey on land even when they are under water because their eyes are located dorsally on their heads. They have very well-developed hearing and vision. Their upper eyelids are more mobile than the bottom ones and there is a tarsal bone plate located in the lower eyelid, which can develop into bony structure in some years. The upper eyelids are used to close the eye. The crocodile has also developed a third eyelid containing a cartilage, covering the eye when the animal is under water. They have an external hole on the head that looks like a rasp to collect sounds from the environment and is closed with a fibrous moveable lid that closes the aperture when the animal dives [14].

A very interesting feature in the crocodilian skin is the higher density of "integumentary sensory organs" (ISOs) in their dermis, which are particularly dense in the mouth area and the facial part of the head. They contain multiple mechanoreceptors, which are innervated by the vast network of the peripheral nerves [29]. They are important for the detection of surface waves generated by the moving prey and important for regulating jaws closing, depending on the size of the prey [29, 30]. ISOs are observed as a common feature in the skin, observed as a lamellar body (**Figure 13**).

### **6. Turtles and their special skin features**

In the turtle, there are free parts of the body, such as the head, legs, and tail, covered with scales. In a turtle, the skin's appearance varies from smooth skin, where we can hardly see scales, to thick and crusty skin, which depends on the adaptation and the way of life. Toward the neck, the skin is wrinkled. Because of the adaptation of the land-based lifestyle in the Testudinidae family, the thicker skin is visible, and the scales are more pronounced. Changing the scales in turtles is periodic and individual and is more pronounced in aquatic turtles [14].

The turtle skin consists of the superficial part (*epidermis*) and the inner layer (*dermis*). Between these two layers, there is a basal lamina (BL). The surface layer

**149**

**Figure 14.**

*Reptilian Skin and Its Special Histological Structures DOI: http://dx.doi.org/10.5772/intechopen.84212*

fatty and slightly connective tissue (**Figure 14**).

consists of three layers: *stratum basale*, *stratum granulosum,* and *stratum corneum*. In the *stratum basale,* new cells proliferate and replace old and dead cells and push them toward the surface of the skin. Epithelial cells, keratinocytes, which are found in the *stratum corneum*, produce the protein keratin, which plays a key role in reducing loss of water. On the parts of the body that are more exposed to mechanical pressure, the keratinized layer may be even thicker. There are no blood vessels in the skin epidermis, so the epidermis cells are fed by diffusion from the deeper layers of the skin through the BL. Apart from keratinocytes in the epidermis, melanocytes and Langerhans cells are also located there. Epidermis is developed from ectoderm, creates the BL, and has the function of retaining water in the body, as well as the protection against infections and harmful external influences. New cells created in *stratum basale* replace old and dead cells and suppress them at the surface of the skin. The skin dermis is derived from mesoderm and creates a reticular lamina (*lamina reticularis*). In this layer, there are many sensory nerves (nerve endings and mechanoreceptors) as well as glands, blood vessels, and lymph vessels. Subcutis is a

In the turtle skin, horny plates are formed together with osteoderms. Dermal bones are found below in the inner part of the skin (*dermis*) and they grow together to gain more strength. Corneal scales are made of water-insoluble keratin, which are laid in the arrangement allowing a thin layer of skin between them that makes it easier for the animal to move. In tortoises, the osteoderms are grown together with the spine and ribs, thus forming the back of the armor, carapace. The back and abdomen of the armor, depending on the type of the turtle, consists of several bones (shields). Above the bones (osteoderms), there is a layer of skin (epidermis) which is in the turtles with soft shell (the genus Apalone, the genus of the turtle *Dermochelys*) "skinned." In the other turtles, above the bony plates, there are also horny plates, which do not entirely match the shells' strength and ability to regenerate [31]. Carapace is constructed from at least 38 corneal scutes, depending on the species of the turtle. In the middle of the carapace, along the back, there are vertebral or neural corneal scutes (mostly five). On the left and the right sides, the neural scutes have either bony or costal plates, and, laterally, there are marginal scutes. A series of smaller plates, which on the border with carapace and plastron,

*Skin histology of the leg skin of the Red-ear slider (Trachemys scripta elegans), stained with H&E. On Panel A, epidermis and dermis are observed. Epidermis is thick and keratinized. It consists of several layers with recognizable stratum basale and a few more layers with enucleated keratinocytes. Stratum corneum is thick. In the dermis, vast melanophores are observed along with a thick layer of dense connective tissue, together with blood vessels and nerves. Subcutis consists of gentile connective tissue, and, in the tunica muscularis, skeletal* 

*muscles cells are observed (A) 100× magnification, (B) 400× magnification.*

### *Reptilian Skin and Its Special Histological Structures DOI: http://dx.doi.org/10.5772/intechopen.84212*

*Veterinary Anatomy and Physiology*

sexes have one pair of scented glands.

lid that closes the aperture when the animal dives [14].

**6. Turtles and their special skin features**

individual and is more pronounced in aquatic turtles [14].

In the skin of the crocodile, pigmented cells are located that give a color that varies from green to light brown to gray. In most animals, the belly is lighter than the rest of the body. Scent glands in crocodiles open in the cloaca. Alligators of both

*Skin histology of the lip of Cuvier's Dwarf Caiman (Paleosuchus palpebrosus), stained with H&E. (A) The epidermis and dermis are observed. Thick stratified epidermis consists of several layers with recognizable stratum basale and a few more layers with enucleated keratinocytes. Stratum corneum is thinner and more compact, especially above the ISO region. ISOs are in the dermis, between thick fibers of connective tissue, vessels, and nerves. Melanophores are not observed. Around ISO, multiple vessels and nerves are observed (B). ISO bodies are concentrically shaped, similar to mammals. (A) 100× magnification, (B) 400× magnification.*

Crocodiles can recognize the prey on land even when they are under water because their eyes are located dorsally on their heads. They have very well-developed hearing and vision. Their upper eyelids are more mobile than the bottom ones and there is a tarsal bone plate located in the lower eyelid, which can develop into bony structure in some years. The upper eyelids are used to close the eye. The crocodile has also developed a third eyelid containing a cartilage, covering the eye when the animal is under water. They have an external hole on the head that looks like a rasp to collect sounds from the environment and is closed with a fibrous moveable

A very interesting feature in the crocodilian skin is the higher density of "integumentary sensory organs" (ISOs) in their dermis, which are particularly dense in the mouth area and the facial part of the head. They contain multiple mechanoreceptors, which are innervated by the vast network of the peripheral nerves [29]. They are important for the detection of surface waves generated by the moving prey and important for regulating jaws closing, depending on the size of the prey [29, 30]. ISOs are observed as a common feature in the skin, observed as a lamellar body

In the turtle, there are free parts of the body, such as the head, legs, and tail, covered with scales. In a turtle, the skin's appearance varies from smooth skin, where we can hardly see scales, to thick and crusty skin, which depends on the adaptation and the way of life. Toward the neck, the skin is wrinkled. Because of the adaptation of the land-based lifestyle in the Testudinidae family, the thicker skin is visible, and the scales are more pronounced. Changing the scales in turtles is periodic and

The turtle skin consists of the superficial part (*epidermis*) and the inner layer (*dermis*). Between these two layers, there is a basal lamina (BL). The surface layer

**148**

(**Figure 13**).

**Figure 13.**

consists of three layers: *stratum basale*, *stratum granulosum,* and *stratum corneum*. In the *stratum basale,* new cells proliferate and replace old and dead cells and push them toward the surface of the skin. Epithelial cells, keratinocytes, which are found in the *stratum corneum*, produce the protein keratin, which plays a key role in reducing loss of water. On the parts of the body that are more exposed to mechanical pressure, the keratinized layer may be even thicker. There are no blood vessels in the skin epidermis, so the epidermis cells are fed by diffusion from the deeper layers of the skin through the BL. Apart from keratinocytes in the epidermis, melanocytes and Langerhans cells are also located there. Epidermis is developed from ectoderm, creates the BL, and has the function of retaining water in the body, as well as the protection against infections and harmful external influences. New cells created in *stratum basale* replace old and dead cells and suppress them at the surface of the skin. The skin dermis is derived from mesoderm and creates a reticular lamina (*lamina reticularis*). In this layer, there are many sensory nerves (nerve endings and mechanoreceptors) as well as glands, blood vessels, and lymph vessels. Subcutis is a fatty and slightly connective tissue (**Figure 14**).

In the turtle skin, horny plates are formed together with osteoderms. Dermal bones are found below in the inner part of the skin (*dermis*) and they grow together to gain more strength. Corneal scales are made of water-insoluble keratin, which are laid in the arrangement allowing a thin layer of skin between them that makes it easier for the animal to move. In tortoises, the osteoderms are grown together with the spine and ribs, thus forming the back of the armor, carapace. The back and abdomen of the armor, depending on the type of the turtle, consists of several bones (shields). Above the bones (osteoderms), there is a layer of skin (epidermis) which is in the turtles with soft shell (the genus Apalone, the genus of the turtle *Dermochelys*) "skinned." In the other turtles, above the bony plates, there are also horny plates, which do not entirely match the shells' strength and ability to regenerate [31].

Carapace is constructed from at least 38 corneal scutes, depending on the species of the turtle. In the middle of the carapace, along the back, there are vertebral or neural corneal scutes (mostly five). On the left and the right sides, the neural scutes have either bony or costal plates, and, laterally, there are marginal scutes. A series of smaller plates, which on the border with carapace and plastron,

#### **Figure 14.**

*Skin histology of the leg skin of the Red-ear slider (Trachemys scripta elegans), stained with H&E. On Panel A, epidermis and dermis are observed. Epidermis is thick and keratinized. It consists of several layers with recognizable stratum basale and a few more layers with enucleated keratinocytes. Stratum corneum is thick. In the dermis, vast melanophores are observed along with a thick layer of dense connective tissue, together with blood vessels and nerves. Subcutis consists of gentile connective tissue, and, in the tunica muscularis, skeletal muscles cells are observed (A) 100× magnification, (B) 400× magnification.*

are called inframarginal scutes. Cranial from the first neural scutes it is nuchal plate. Above the tail are two scutes named suprapygeal (supracaudal). In the intramarginal plates, Rathke's pores are visible in sea turtles and similar structures can be observed in freshwater turtles. Below Rathke's pores, Rathe's glands are located, covered with fat tissue [31, 32]. The plastron is the nearly flat part of the shell structure of a turtle, which is basically the ventral surface of the shell. It also includes within its structure the anterior and posterior bridge struts and the bridge of the shell [32]. The plastron is made up of nine bones and the two epiplastra. The plastron usually consists of 12 plastral scutes, six on each side, which come together in the central line and their number depends on the shape of the shell and the type of turtle. Plastral formula is consisted from intergular, gular, humeral, pectoral, abdominal, femoral, and anal plastral scutes. The shape and mutual relationship of these scutes are of great importance in determining the species. In addition to the armor, turtles may also have specifically deployed jaw shells that may also be important in identifying the species. For example, in the sea turtles between the eyes, there are two horned shells that are characteristic of the Green sea turtle (*Chelonia mydas*), while in the other species there are more or only one. In the turtle, the dormant scutes are shed individually [14, 32].

In some turtles, fragrant glands are open in the cloaca, and in some species, they produce an intensive smell, especially when they feel endangered. For most skin glands, it is considered to play a major role in reproduction or defense against predators. In the terrestrial turtles, glands are located only on the thighs, while in the water turtles, the mucous glands are found along the skin. During the hibernation of turtles, gas exchange occurs through the skin, while being buried in the ground or for example at the bottom of the lake. The turtles have developed lacrimal glands (*gll. lacrimalis*) and Harder's glands that, like lacrimal glands, produce tears and contain immunocompetent cells. In the sea turtles, the lacrimal gland has been altered and modified into the solitary gland. The turtles do not have nasolacrimal ducts (*ductus nasolacrimalis*) and tears are secreted by evaporation.
