The Skin (Integumentary System)

The outer surface covering of the animal is the skin. The skin and its derivative or accessory structures are called the integ­umentary system. It is the largest organ of the animal’s body, about 16% of the body weight in humans.

Fig. 12.4 Morphology of the skin. [The cross section of the skin showing the epidermis, dermis and hypodermis with various glands]

and birds. Skin’s morphological features of all animals and birds are not the same.

12.5.1 Morphology of the Skin

The skin consists of three layers (exterior to inner side): epidermis, dermis and hypodermis (Fig. 12.4). Dermis appears from the word ‘Derma’, which means skin, ‘epi’ denotes over and ‘hypo’ signifies below.

12.5.1.1 Epidermis

The outermost epidermis layer contains 4-5 cell strata (Fig. 12.5). The innermost single-cell layer, stratum germinativum or basale, is present over the basement mem­brane of the dermis and includes precursors of keratinocytes. The major cells of the epidermis are keratinocytes, which are generated and increase persistently, forming the second layer, stratum spinosum. The spiny-like multiple (8-10 layers in humans) cells of stratum spinosum are transitory and transformed to the third layer, stratum granulosum. Keratin filaments (fibrils) and matrix with keratohyalin granules are generated in the keratinocytes within the grainy-appearing multi-cell stratum granulosum.

Fig. 12.5 Morphology of the skin. [Various layers of the epidermis are depicted]

appearance in the region. The dead keratinocytes reach the superficial layer, the fifth layer, named stratum corneum, which contains about usually 15-30 layers of cells in humans. It is horny due to the completion of keratinisation or cornification of the keratinocytes. The dead cells of this layer appear as a protein mass, the keratin. The intracellular insoluble fibrous protein-rich water-resistant keratins in the stratum corneum provide strength, rigidity, elasticity, pliabil­ity and physical protection. The stratum corneum protects the body against invasion of microorganisms as the cells of the layer are periodically (within about 4-6 weeks) shed and replaced by cells from the immediately lower layers. Keratin is the principal component of hair, nails, hoofs and other appendages.

The epidermis layer also contains melanocytes and Merkel cells at stratum basale and Langerhans cells at stratum spinosum. Melanocytes are responsible for skin colour, and ultraviolet (UV) radiation (100-400 nm) damages the living cells by producing melanin. Merkel cells act as sensory receptors to sense touch and pressure. The Langerhans cells are the antigen-presenting cells (APCs) with immunogenic functions.

12.5.1.2 Dermis

The middle layer of the skin is the dermis, composed of collagen fibres, ground substances, hair follicles, sweat and sebaceous glands. The major cells present in the dermis are fibroblasts, mast cells, histiocytes and dendrocytes.

12.5.1.3 Hypodermis

The lower-most hypodermis is the subcutaneous fat tissue layer. Differentiation between the dermis and hypodermis is very difficult; hence, hypodermis is not firmly considered as part of the skin. It contains adipose tissues, areolar connective tissue, blood and lymph vessels and nerves. The profound adipose or fat tissues involve insulation, storing various substances and energy reserve.

12.5.1.4 Glands of the Skin

The skin contains several exocrine glands. The sebaceous gland secretes sebum to moisten the skin, and the sweat gland produces sweat involved in thermoregulation. Sweat is odourless, but has hormones; the odour may appear in certain conditions, viz. kind of feed, infection, medications and diabetes-like diseases. The sweaty smell usually develops the products of reactions of secreted sweat with the surface bacteria. The preen gland, mostly available near the base of the bird’s tail, secretes oil to keep the feathers in sound condition. The wax-producing gland is present in the ears. Mammary glands present only in mammals are considered modified sweat glands.

12.5.1.5 Thickness of the Skin

The thickness of skin varies depending on the species, breed, sex, age, strata position and location on the body, ranging from 0.5 to 5 mm (average 1.03 mm) in the dog, 0.4-2 mm in the cat and 1.5-5 cm in rhinoceroses. The average skin thickness of Devon is 8.2 mm, 5.8 mm in Zebu and 5.5 mm in Jersey breeds of cattle. The thickest skin is present at the back and dorsal neck regions, thinner at the abdomen and thinnest at the inguinal and axillary areas.

1.5 mm). The dermis is thickest on the back, about 30-40 times thicker than the immediate over the epidermis. The skin becomes thinner in advanced age due to less mitotic cell division in the germinativum layer. Androgens influence the collagen formation in the skin; hence, females’ skin is softer than males. The thickness of various stratum also varies in different appendages. In full-thickness grafting, the entire epidermis and dermis with their appendages transfer, whereas only the epidermis and parts of the dermis replace in split­thickness grafting.

12.5.2 Major Functions of the Skin

12.5.2.1 Protection Mechanism of the Skin

Physical protection governs by the hair of the skin. The water-repellent outermost keratinised layer protects the body fluid, and regular sloughing of this layer regulates the debris and invasion of microorganisms. The skin performs antimicrobial and antifungal activities through immunoglobulins, interferons, various antimicrobial peptides (AMPs), lipids and organic acids present in sebum (seba­ceous gland product) and sweat. Phagocytosis of the histiocytes and immunogenic dendrocytes is also involved in the antibacterial activity of the skin. The Langerhans cells trap antigens, like microorganisms and foreign proteins, and present them to T-helper lymphocytes.

12.5.2.2 Sensation

Skin can sense the pain, temperature (heat or cold) and amplification of itching by the Merkel cells. The receptor­like Merkel cell senses the external stimuli attaching to the sensory nerves to interpret the stimulus in the brain. The Merkel cell acts as thermo-receptors and nociceptors related to irritation and itching. Merkel cells are densely found in the digits, lips, oral cavity and outer root sheath of the hair follicle.

12.5.2.3 Pigmentation

The pigments like melanin, produced by melanocytes, caro­tene and haemoglobin, are responsible for skin pigmentation. Melanin exists in eumelanin (black and brown) and pheomelanin (red) forms. The abundant eumelanin forms of melanin pigment are stored in the melanosomes of keratinocytes. The melanin formation (melanogenesis) is stimulated by sunshine, specific genes and various hormones, viz. melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH), and inhibited by car­bon monoxide, hydrogen sulphide and various organic sul­phur compounds. The lack of melanin occurs because of albinism, the consequence of a genetic disorder. The inability of melanocytes in certain regions to form melanin causes local colourless patches, the vitiligo. Excess melanin will interfere with vitamin D production and interfere in calcium absorption. Liver disease (jaundice) causes accumulation of yellow pigment bilirubin over the skin. Yellowish coloured skin may also occur due to excess ingestion of a yellow­orange pigment-containing carotene-rich feed. Carotene is deposited in the stratum corneum of the skin. Reddening in the skin may occur due to allergy and inflammation leading to the accumulation of more blood or haemoglobin in the der­mis. Pale-coloured skin appears in anaemia and low blood pressure conditions. Prolonged lack of oxygen develops dark red deoxyhaemoglobin in the blood leading to cyanosis, a blue-coloured skin.

12.5.2.4 Thermoregulation

Blood vessels in the dermis form a complex network, and superficial and deep plexus are involved in thermoregulation by vasodilation during hot and vasoconstriction in a cold environment.

It contains little salt and waste products, like urea; hence, it involves excretion. But the total excretory amount is negli­gible compared with the excretion process of the kidney. Thus, it is not considered a part of the excretory system.

12.5.2.5 SynthesisofVitamin Dand Biochemical Reactions

The cholecalciferol (vitamin D3), a form of vitamin D, is synthesised from steroid cholesterol (7-dehydrocholesterol) in the skin in the presence of sunlight. The cholecalciferol converts to calcidiol in the liver and further transforms into an active form of vitamin D, the calcitriol, in the kidney. The formation of cholecalciferol reduces during advanced age, leading to an increase in the risk of osteoporosis. The skin can exert various biochemical reactions over some steroid hormones, like oestrogens, progestogens and glucocorticoids, and vitamin A through its specific receptors.

12.5.3 Role of Hair and Sebaceous Gland

12.5.3.1 TypesofAnimal Hair

Animal hairs are generally of three types: guard hair, fur or wool and tactile hair. Guard hairs, and outer coat, protect from physical injury, trauma and UV radiation. It also has a role in thermoregulation. The fur or wool hairs are the inner coat and mainly involve insulation. The tactile hair or whiskers present on the face of some animals (feline) provide sensory perception. Other hairs are also present in some animals, like mane hair in horses and tail hair.

12.5.3.2 Structure of Hair

Hair has two structures: the follicle remains within the skin, and the shaft or scalp exists outside the body. The follicle extends into the dermis. The papilla resides at the follicle’s base, contains blood capillaries and engages in nourishing the cells. The living cells surround the papilla called the bulb. Compound hair follicles with clusters of primary hairs encircling smaller secondary hairs are found in dogs and cats. Two sheaths cover the follicle. The inner sheath continues to the hair shaft and terminates below the opening of the sebaceous gland, and the outer sheath (cuticle) goes on with the gland. The central core is called the medulla. The outer sheath attaches to the arrector pili muscle through a connective tissue at the upper dermis below the sebaceous gland. Contraction of pili muscle causes the erection of hair and secretion of the sebaceous gland, the sebum.

12.5.3.3 Distinguishable Features in Animal Hair

The cuticle of the hair configures with the combination of its three structures: coronal (crown-like), spinous (petal-like) and imbricate (flattened). Coronal appearance is most com­mon in small rodents and bats, with triangular shaped spinous or petal-like scales present at the upper part of the hair of mink and fur hairs of seals and cats. The coronal and spinous shaped scales are hardly found in human hairs. The hairs of the humans and some animals have an imbricate or flattened- scale type. The medulla of the human hair is unstructured, whereas very regular and well defined in the animal. Micro­scopically, the animal hair can be distinguished into three broad classes. The scale pattern is the identifying trait having a regular diameter with a wave or crimp pattern that exists throughout the hair in the deer family and antelopes. A wide variation in diameter (20-150 μm) throughout a single hair with a banded appearance is common in the hair of commer­cial fur animals. Diversified pigmentation, root and medul­lary structure with a unique character predominate in particular domestic animals.

12.5.3.4 Hair Cycle

Hair contains three vital components: keratin, melanin and a trace amount of metallic elements. It grows and sheds in four stages: anagen (growth), catagen (regression), telogen (rest­ing or relative quiescence) and exogen (shedding). A new hair shaft forms in every cycle, and activation of hair-specific epithelial stem cells at the bulb controls the cycle or genera­tion of the new hair shaft. Various factors, like genetics, hormones, neurotrophins, photoperiod, nutrition, cytokines and some intrinsic factors, regulate the cyclic activity of the hair. Alopecia or abnormal hair loss may occur due to auto­immune reaction to anagen follicles or destruction of hair follicles after inflammation and other causes.

12.5.3.5 Sebaceous Gland

Sebum is a lipid-rich compound secreted from the sebaceous gland. Major lipid constituent of sebum is species specific and provides a unique scent to a particular species. For example, the sebum of goats contains caproic acid, and sheep contain lanolin. The glands are densely present in the facial region. Usually, sebum has a role in softening the hair and skin surface and bacteriostatic property. The modified sebaceous glands can produce a specific odour called a scent gland. The scent acts as pheromones, like territory marking and communication, mainly during the breeding season. Tes­tosterone and progesterone influence the growth of the gland.

Learning Outcomes

• The sense of vision: The eyes are the sensory organs that receive visual information from the envi­ronment and transmit them to the visual sensory area of the brain for interpretation. The eyes, equipped with an adjustable pupil and a lens, capture the patterns of illumination in the environment as an ‘optical picture’ on a layer of light-sensitive photo­receptor cells in the retina. The retina facilitates feature analysis of the image and transmits the visual signals through the steps of visual processing to the various structures of the brain where it is finally perceived.

• The sense of hearing: The auditory system perceives the frequency of sounds as pitch and their amplitude as loudness. The external and the middle ear conduct the sound to the auditory receptors (organ of Corti) in the cochlea of the inner ear. The auditory receptors are the hair cells embedded in the basilar membrane and their apical surface and convert sound wave signals into nerve impulses. The vestibular apparatus of the ear helps to maintain the posture and equilibrium of the body. It is mainly controlled by a gravity sensor (vestibu­lar apparatus) in response to visual and propriocep­tive information.

• The sense of smell: Olfaction (smell) is an animal’s primary special sense perceived by the main olfac­tory system and the accessory olfactory system with receptors in the vomeronasal or Jacobson’s organ located near the external nares. The odorant molecules combine with odorant receptors to medi­ate signal transduction mechanism that leads to a series of electrical events to facilitate the sense of smell.

• The sense of taste: Gustation or taste is a kind of chemical sensation that is perceived through chemoreceptors in the taste buds. Binding of a taste-provoking chemical, a tastant, with a receptor cell ultimately alters the cell’s ionic channels to produce a depolarising receptor potential for taste sensation.

• The skin and integumentary system: The skin and its derivative or accessory structures are called integumentary system that provides physical barrier and antimicrobial protection and involves in thermo- and immuno-regulation, excretion, secre­tion, pigmentation, sensation and locomotion of the animal.

Exercise

Short Questions

1. The opaque lens condition is termed ______.

2. The ‘3’-dimensional picture occurs in which types of vision?

3. Which layer of the skin does not contain blood vessels?

4. Why Langerhans cells are called antigen-presenting cells?

5. The scala vestibuli and scala tympani are connected by a

narrow channel called________.

6. acts as a gravity sensor during the

equilibrium.

7. Animals that are very sensitive in detection of odour are

called as_____________.

8. Umami taste is elicited by________________.

9. The antigen-presenting cells (APCs) of the skin are

10. act as mechanoreceptors to detect

pressure and vibration.

Subjective Questions

1. How does a panoramic vision occur in horses?

2. How can a cat’s hair be distinguished from a human’s hair in the forensic investigation?

3. Write the role of vomeronasal organ in sexual behaviour.

4. Briefly describe the mechanism of olfaction.

5. How skin helps in thermoregulation?

Answers to the Short Questions

1. Cataract

2. Stereo vision

3. Epidermis

4. They trapped antigens and presented them to the T-helper lymphocytes

5. Helicotrema

6. Vestibular apparatus

7. Microsmatic

8. Glutamate

9. Langerhans cells

10. Pacinian corpuscles

Keywords for the Answer to Subjective Questions

1. Field of binocular vision, monocular vision, position of blind spots

2. Structure of cuticle and scale appearance, the structure of medulla and pigmentation, the structure of the follicle

3. Vomeronasal organ, perception of pheromone

4. Odorant molecules, ciliary G protein-coupled receptor, signal transduction, change in membrane potential

5. Vasodilation during hot and vasoconstriction of blood vessels, secretion and evaporation of sweat

Further Readings

Choi EH, Daruwalla A, Suh S, Leinonen H, Palczewski K (2021) Retinoids in the visual cycle: role of the retinal G protein-coupled receptor. J Lipid Res 62:100040. https://doi.org/10.1194/jlr. TR120000850

Heo S, Hwang T, Lee HC (2018) Ultrasonographic evaluation of skin thickness in small breed dogs with hyperadrenocorticism. J Vet Sci 19(6):840-845. https://doi.org/10.4142/jvs.2018.19.6.840

Kolarsick PAJ, Kolarsick MA, Goodwin C (2011) Anatomy and physi­ology of the skin. J Dermatol Nurses Assoc 3(4):203-213. https:// doi.org/10.1097/JDN.0b013e3182274a98

McKnight G, Shah J, Hargest R (2022) Physiology of the skin. Surgery (Oxford) 40(1):8-12. https://doi.org/10.1016Zj.mpsur.2021.11.005

Willough CE, Ponzin D, Ferrari S, Lobo A, Landau K, Omidi Y (2010) Anatomy and physiology of the human eye: effects of mucopolysaccharidoses disease on structure and function—a review. Clin Exp Ophthalmol 38:2-11. https://doi.org/10.1111/j.1442-9071. 2010.02363.x