**2. Histology of reptile's skin**

An integumental challenge for reptilian's terrestrial life was developing mechanisms in order to prevent water loss and to protect against ultraviolet irradiation, mechanical shields which offered protection and enabled evolution of different types of reptilian scales and scutes [3, 7].

Some skin histology features are similar between mammals and reptiles; on the other hand, they also have numerous differences. Reptiles have a reputation that they are "slimy" when we touch and hold them; however, they have dry skin, which has even fewer glands than mammals or amphibians. The main special feature of their skin is that the **epidermis** is heavily keratinized with a layer, which also prevents water loss. This feature reflects their greater commitment to a terrestrial existence. Scales are present but are fundamentally different from the dermal scales of fish. In reptiles, scales cannot be scraped off as in fish because they are an integral part of the skin. The reptilian scale usually lacks the bony under support of any significant structural contribution from dermis. It is a fold in the surface epidermis, an epidermal scale. The junction between adjacent epidermal scales provides a flexible hinge. If the epidermal scale is large and plate-like, it is also termed scutes. Epidermal scales in different species can be overgrown and skin protrusions can be formed in different regions, such as microornamentation, pits, sensory receptors, spines, horn-like processes, crests, scutes, plastron, carapace, and some others [7, 8]. These protrusions are essentially only of epidermal origin, without dermal participation [1]. In the perfect resting phase, the epidermis generally consists of four layers of dead but fully differentiated keratinocytes and basal live keratinocyte layer that form three main layers: *stratum basale (germinativum*), *stratum granulosum,* and *stratum corneum*.

The inner layer, *stratum germinativum*, consists of cuboidal dividing cells that produce the protein keratin. The intermediate layer (*stratum granulosum*) has a lipid-rich film that plays a major role in providing water-permeable barrier in the skin. The outer *stratum corneum* is heavily keratinized in scales. Two forms of keratin are produced in reptiles: α-keratin, which is flexible, and β-keratin, which provides strength and hardness and is unique to reptiles. β-keratin is found on the chelonian shells, whereas α-keratin is found in the hinges or between the scutes [9–11]. It is in these weaker links that mites or infections can be present. The thick, keratinized skin of reptiles is at the expense of the cutaneous sensation. Reptiles have far less sensory feeling in their skin than birds or mammals, which is why they are more at risk from thermal burns in captivity (e.g., lizards and hot stones). In

**137**

**scutes**

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

however, they are not associated with scales.

with the vertebrae to form a shell.

many reptiles, dermal bones (*gastralia*) are present especially in the abdominal area;

The main layers of epidermis change prior to molting in the reptiles that slough large pieces of the cornified skin layer. In the turtles and crocodiles, sloughing of skin is modest, comparable to birds and mammals, in whom small flakes fall off at irregular intervals. But in lizards, and especially in snakes, shedding of the cornified layer, termed molting or ecdysis, results in removal of extensive sections of superficial epidermis. As molting begins, the *stratum basale*, which has given rise to the *strata granulosum* (inner) and *corneum* (outer), duplicates the deeper layers of granulosum and corneum, pushing up under the old layers. White blood cells invade the *stratum intermedium*, a temporary layer between old and new skin. These white blood cells are thought to promote the separation and loss of the old superficial layer of the skin [8]. Molting of different reptile species will be discussed in more detail later in the chapter. The **dermis** in reptiles consists of fibrous connective tissue, blood and lymphatic vessels, nerves, and pigmentary cells. At the areas where dermal bones support the epidermis, bony plates called osteoderms, plates of dermal bone, are located under epidermal scales. They are present in crocodilians, some lizards, and some extinct species. Some bones of the turtle shell are modified osteoderms which have fused

**Subcutaneous layer** (*hypodermis*, hypoderm, and *subcutis*) is the layer of tissue, which lies beneath dermis and mainly consists of fibroblasts, adipose cells, and macrophages. Subcutaneous fat is mostly poorly developed in reptiles in comparison to mammals. Some species are known for substantial subcutaneous fat pads, such as some species of geckos like the Mediterranean house gecko (*Hemidactylus turcicus*), and some of the snakes and lizards have paired abdominal "fat bodies" (*corpora adiposa*) that serve as the primary location for fat storage in adipose tissue. Also, the tail, especially in geckos, can be a large deposit of the subcutaneous tissue, however not in the snakes. These tail deposits include "inner fat" surrounding the caudal vertebrae, as well as subcutaneous caudal fat and are most concentrated near the base of a tail. These fat tissues could be for some animals the major adipose store in the body [12, 13].

**Integumental glands** of reptiles are usually restricted to certain areas of the body. In different species, there are some glandular-type tissues in different parts of the body, such as rows of femoral glands under femoral pores alongside the inner part of the thigh region of the hindlimb, which are observed especially in males. In some species of Crocodilians and turtles, scent glands are present. In male and female alligators, one pair of scent glands open in cloaca and another pair on the margins of the lower jaw. In some turtles, scent glands can produce quite pungent odors, especially when the animal is alarmed by handling. Precloacal pores are observed in some lizards and crocodiles. Most integumental glands in reptiles are thought to play a role in reproductive behavior or when predators are close, and

**3. Reptile groups and their special skin features, especially their scales/**

The skin of reptiles reflects their greater commitment to a terrestrial existence as mentioned earlier in the chapter. Keratinization is extensive and skin glands are fewer than in amphibians. Scales are present, but these are fundamentally different from the dermal scales of fish. The reptilian scale usually lacks the bony under support or any significant structural contribution from the dermis. Instead, it is a fold in the surface epidermis, hence, an epidermal scale. The junction between adjacent epidermal scales is the flexible hinge (**Figure 1**). If the epidermal scale is large and

their social role has not yet been well studied [8, 9, 11].

*Veterinary Anatomy and Physiology*

the driest deserts on the earth [3, 4].

**2. Histology of reptile's skin**

types of reptilian scales and scutes [3, 7].

of other and they only have one functional lung. Some species have venom, used primarily to kill prey. Their skin is covered in scales and snakes are not slimy [2]. Lizards are quadrupedal squamates, except some legless, snake-like-bodied species. Often, they are territorial and have many antipredator strategies, such as camouflage, venom, reflex bleeding, and the ability to destroy and then regenerate their tails after destruction. They are covered in overlapping keratin scales, enabling them to live in

Crocodilians are the largest reptiles, and include the alligators, crocodiles, gharials, and caimans. They have elongated, structurally reinforced skulls, powerful jaw muscles, teeth in sockets, and a complete secondary palate; they are oviparous and,

Turtles are among the most ancient of the reptiles alive today and have changed little since they first appeared 200 million years ago. They have a protective shell that encloses their body and provides protection and camouflage. They have keratinized plates instead of teeth and a shell that consists of a carapace and plastron [5, 6].

An integumental challenge for reptilian's terrestrial life was developing mechanisms in order to prevent water loss and to protect against ultraviolet irradiation, mechanical shields which offered protection and enabled evolution of different

Some skin histology features are similar between mammals and reptiles; on the other hand, they also have numerous differences. Reptiles have a reputation that they are "slimy" when we touch and hold them; however, they have dry skin, which has even fewer glands than mammals or amphibians. The main special feature of their skin is that the **epidermis** is heavily keratinized with a layer, which also prevents water loss. This feature reflects their greater commitment to a terrestrial existence. Scales are present but are fundamentally different from the dermal scales of fish. In reptiles, scales cannot be scraped off as in fish because they are an integral part of the skin. The reptilian scale usually lacks the bony under support of any significant structural contribution from dermis. It is a fold in the surface epidermis, an epidermal scale. The junction between adjacent epidermal scales provides a flexible hinge. If the epidermal scale is large and plate-like, it is also termed scutes. Epidermal scales in different species can be overgrown and skin protrusions can be formed in different regions, such as microornamentation, pits, sensory receptors, spines, horn-like processes, crests, scutes, plastron, carapace, and some others [7, 8]. These protrusions are essentially only of epidermal origin, without dermal participation [1]. In the perfect resting phase, the epidermis generally consists of four layers of dead but fully differentiated keratinocytes and basal live keratinocyte layer that form three main layers: *stratum basale (germinativum*), *stratum granulosum,* and *stratum corneum*. The inner layer, *stratum germinativum*, consists of cuboidal dividing cells that produce the protein keratin. The intermediate layer (*stratum granulosum*) has a lipid-rich film that plays a major role in providing water-permeable barrier in the skin. The outer *stratum corneum* is heavily keratinized in scales. Two forms of keratin are produced in reptiles: α-keratin, which is flexible, and β-keratin, which provides strength and hardness and is unique to reptiles. β-keratin is found on the chelonian shells, whereas α-keratin is found in the hinges or between the scutes [9–11]. It is in these weaker links that mites or infections can be present. The thick, keratinized skin of reptiles is at the expense of the cutaneous sensation. Reptiles have far less sensory feeling in their skin than birds or mammals, which is why they are more at risk from thermal burns in captivity (e.g., lizards and hot stones). In

interestingly, adults provide extensive parental care to young.

**136**

many reptiles, dermal bones (*gastralia*) are present especially in the abdominal area; however, they are not associated with scales.

The main layers of epidermis change prior to molting in the reptiles that slough large pieces of the cornified skin layer. In the turtles and crocodiles, sloughing of skin is modest, comparable to birds and mammals, in whom small flakes fall off at irregular intervals. But in lizards, and especially in snakes, shedding of the cornified layer, termed molting or ecdysis, results in removal of extensive sections of superficial epidermis. As molting begins, the *stratum basale*, which has given rise to the *strata granulosum* (inner) and *corneum* (outer), duplicates the deeper layers of granulosum and corneum, pushing up under the old layers. White blood cells invade the *stratum intermedium*, a temporary layer between old and new skin. These white blood cells are thought to promote the separation and loss of the old superficial layer of the skin [8]. Molting of different reptile species will be discussed in more detail later in the chapter.

The **dermis** in reptiles consists of fibrous connective tissue, blood and lymphatic vessels, nerves, and pigmentary cells. At the areas where dermal bones support the epidermis, bony plates called osteoderms, plates of dermal bone, are located under epidermal scales. They are present in crocodilians, some lizards, and some extinct species. Some bones of the turtle shell are modified osteoderms which have fused with the vertebrae to form a shell.

**Subcutaneous layer** (*hypodermis*, hypoderm, and *subcutis*) is the layer of tissue, which lies beneath dermis and mainly consists of fibroblasts, adipose cells, and macrophages. Subcutaneous fat is mostly poorly developed in reptiles in comparison to mammals. Some species are known for substantial subcutaneous fat pads, such as some species of geckos like the Mediterranean house gecko (*Hemidactylus turcicus*), and some of the snakes and lizards have paired abdominal "fat bodies" (*corpora adiposa*) that serve as the primary location for fat storage in adipose tissue. Also, the tail, especially in geckos, can be a large deposit of the subcutaneous tissue, however not in the snakes. These tail deposits include "inner fat" surrounding the caudal vertebrae, as well as subcutaneous caudal fat and are most concentrated near the base of a tail. These fat tissues could be for some animals the major adipose store in the body [12, 13].

**Integumental glands** of reptiles are usually restricted to certain areas of the body. In different species, there are some glandular-type tissues in different parts of the body, such as rows of femoral glands under femoral pores alongside the inner part of the thigh region of the hindlimb, which are observed especially in males. In some species of Crocodilians and turtles, scent glands are present. In male and female alligators, one pair of scent glands open in cloaca and another pair on the margins of the lower jaw. In some turtles, scent glands can produce quite pungent odors, especially when the animal is alarmed by handling. Precloacal pores are observed in some lizards and crocodiles. Most integumental glands in reptiles are thought to play a role in reproductive behavior or when predators are close, and their social role has not yet been well studied [8, 9, 11].
