Structure of skin:
The skin of farm animals (Fig. 29.12) has been described as the largest organ of the body. It covers the outer surfaces of all animals. The skin is made up of two layers – epidermis and dermis.
The epidermis is the outermost layer and it is made up of three layers: (i) cornified layer (ii) granular layer (iii) malpighian layer.
The cornified layer:
It is the outermost layer of the epidermis. It is made up of flat, hard and scaly cells. The cells of cornified layer are dead.
The granular layer:
This contains the living cells where active division of cells takes place. These cells replace the cells of the granular layer which in turn are replacing the cells of the cornified layer which are constantly being worn-out.
The Malpighian layer:
This contains granules of pigment called melanin. This hair pigment is responsible for skin colour. The melanin also prevents the penetration of a lot of violet rays of sunlight. The Malpighian layer also contains another pigment called keratin which is responsible for the toughness and flexibility of the skin.
Dermis:fibrous collective tissues. The dermis contains nerves which enable the skin to be sensitive to changes in its environment such as pain, heat, temperature, touch, cold, etc.. The blood and lymph vessels supply materials to the skin, remove wastes from the skin cells, and carry out temperature regulation. The sweat glands which consist of a coiled tubular gland opening into the epidermis excrete water and salts as sweat, sebaceous or oil gland which produces an oily substance called sebum. This helps to lubricate the hair and make it waterproof, while the erector muscle controls the erection of the hairs on the skin surface by its contraction and relaxation. Underneath the skin is a layer of fat called subcutaneous fatty tissue. This fat layer varies in thickness, depending on the part of the body.
FUNCTIONS OF THE SKIN
The functions of the skin include the following:
(1) Protection: The skin protects the inner skin or tissue from mechanical injuries, bacterial infection, and ultraviolet rays of the sunlight and against desiccation.
(2) Excretion: The skin excretes excess water, mineral salts and nitrogenous wastes through the sweat glands as sweat
(3) Sensitivity: The skin is sensitive to its environment through the specialized sensory nerve endings scattered in the dermis. The skin is sensitive to such stimuli as change in temperature, pressure, pain and cold.
(4) Production of vitamin D: The skin manufactures Vitamin D by using infra-red rays from sunlight
(5) Production of milk in females: The mammary gland which is a modification of the skin produces milk which is used for feeding the young one.
(6) Storage of preserved foods: Fats are stored under the dermis of mammals, especially those of pigs. Fats also from an insulating layer.
(7) Regulation of body temperature: when an animal is hot, blood vessels under the epidermis of the skin dilate (vasodilation) and more blood is brought to the surface to be cooled through the process of convection and radiation. On the other hand, the blood vessels of the skin constrict (vasoconstriction) in cold weather, thereby conserving heat.
Facts about the skin
The skin is the body’s largest organ, covering the entire body. In addition to serving as a protective shield against heat, light, injury, and infection, the skin also:
Regulates body temperature
Stores water and fat
Is a sensory organ
Prevents water loss
Prevents entry of bacteria
Throughout the body, the skin’s characteristics vary (for example, thickness, color, and texture). For instance, the head contains more hair follicles than anywhere else, while the soles of the feet contain none. In addition, the soles of the feet and the palms of the hands are much thicker than skin on other areas of the body.
The skin is made up of the following layers, with each layer performing specific functions:
Subcutaneous fat layer
The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue and guards the underlying muscles, bones, ligaments and internal organs. Human skin is similar to most of the other mammals skin, and human skin is very similar to pig skin. Though nearly all human skin is covered with hair follicles, it can appear hairless. There are two general types of skin, hairy and glabrous skin (hairless). The adjective cutaneous literally means “of the skin” (from Latin cutis, skin).
Because it interfaces with the environment, skin plays an important immunity role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.
In humans, skin pigmentation varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habitat for bacteria that number roughly 1000 species from 19 phyla, present on the human skin.
The epidermis is the thin outer layer of the skin that consists of three types of cells:
Squamous cells. The outermost layer is continuously shed.
Basal cells. Basal cells are found just under the squamous cells.
Melanocytes. Melanocytes are found in every layer of the epidermis and make melanin, which gives the skin its color.
The dermis is the middle layer of the skin. The dermis contains the following:
The dermis is held together by a protein called collagen, which is made by fibroblasts. This layer gives skin flexibility and strength. It also contains pain and touch receptors.
Subcutaneous fat layer
The subcutaneous fat layer is the deepest layer of skin and consists of a network of collagen and fat cells. It helps conserve the body’s heat and protects the body from injury by acting as a shock absorber.
Skin has mesodermal cells, pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight. It also contains DNA repair enzymes that help reverse UV damage, such that people lacking the genes for these enzymes suffer high rates of skin cancer. One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has led to the classification of people(s) on the basis of skin color.
In terms of surface area, the skin is the second largest organ in the human body (the inside of the small intestine is 15 to 20 times larger). For the average adult human, the skin has a surface area of between 1.5-2.0 square meters (16.1-21.5 sq ft.). The thickness of the skin varies considerably over all parts of the body, and between men and women and the young and the old. An example is the skin on the forearm which is on average 1.3 mm in the male and 1.26 mm in the female. The average square inch (6.5 cm²) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1,000 nerve endings.[better source needed] The average human skin cell is about 30 micrometers in diameter, but there are variants. A skin cell usually ranges from 25-40 micrometers (squared), depending on a variety of factors.
Skin is composed of three primary layers: the epidermis, the dermis and the hypodermis.
Layers, Receptors, and Appendages of Human Skin
Main article: Epidermis
Epidermis, “epi” coming from the Greek meaning “over” or “upon”, is the outermost layer of the skin. It forms the waterproof, protective wrap over the body’s surface which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina.
The epidermis contains no blood vessels, and cells in the deepest layers are nourished almost exclusively by diffused oxygen from the surrounding air and to a far lesser degree by blood capillaries extending to the outer layers of the dermis. The main type of cells which make up the epidermis are Merkel cells, keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through mitosis at the basale layer. The daughter cells (see cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein keratin is inserted. They eventually reach the corneum and slough off (desquamation). This process is called “keratinization”. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection.
2D projection of a 3D OCT-tomogram of the skin at the fingertip, depicting the stratum corneum (~500 µm thick) with the stratum disjunctum on top and the stratum lucidum in the middle. At the bottom are the superficial parts of the dermis. The sweatducts are clearly visible. (See also: Rotating 3D Version)
The epidermis contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, melanocytes, Langerhans cells and Merkel cells. The epidermis helps the skin to regulate body temperature.
Epidermis is divided into several layers where cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the top layer called stratum corneum and are sloughed off, or desquamated. This process is called keratinization and takes place within weeks. The outermost layer of the epidermis consists of 25 to 30 layers of dead cells.
Epidermis is divided into the following 5 sublayers or strata:
Stratum germinativum (also called “stratum basale”).
Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.
Genes and proteins expressed in the epidermis
About 70% of all human protein-coding genes are expressed in the skin. Almost 500 genes have an elevated pattern of expression in the skin. There are less than 100 genes that are specific for the skin and these are expressed in the epidermis. An analysis of the corresponding proteins show that these are mainly expressed in keratinocytes and have functions related to squamous differentiation and cornification.
Main article: Dermis
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the Stratum basale of the epidermis.
The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
The papillary region is composed of loose areolar connective tissue. It is named for its fingerlike projections called papillae, that extend toward the epidermis. The papillae provide the dermis with a “bumpy” surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin’s surface. These epidermal ridges occur in patterns (see: fingerprint) that are genetically and epigenetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification.
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility, and elasticity.
Also located within the reticular region are the roots of the hairs, sebaceous glands, sweat glands, receptors, nails, and blood vessels.
Tattoo ink is held in the dermis. Stretch marks often from pregnancy and obesity, are also located in the dermis.
The subcutaneous tissue (also hypodermis and subcutis) is not part of the skin, and lies below the dermis of the cutis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue, adipose tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body.
HERE YOU WILL FIND EVERY AVAILABLE TOPICS ABOUT AGRICULTURAL SCIENCE AND BIOLOGY. AND THE LINKS TO THEIR VARIOUS SOURCES.
1. DEVELOPMENT OF AGRICULTURE
2. IMPORTANCE OF AGRICULTURE
3. SUBSISTENCE AGRICULTURE
4. COMMERCIAL AGRICULTURE
5. PROBLEM OF AGRICULTURAL DEVELOPMENT
6. SOLUTIONS TO POOR AGRICULTURAL DEVELOPMENT
7. AGRICULTURAL LAWS AND REFORMS
8. ROLES OF GOVERNMENT IN AGRICULTURAL DEVELOPMENT
9. AGRICULTURAL POLICIES
10. PROGRAM PLANNING IN AGRICULTURE
35. WILDLIFE CONSERVATION
36. FACTORS AFFECTING LAND AVAILABILITY
39. BIOLOGICAL FACTORS
40. SOCIAL-ECONOMIC FACTORS
41. ENVIRONMENTAL FACTORS AFFECTING AGRICULTURAL PRODUCTION
42. CLIMATIC FACTORS AFFECTING AGRICULTURAL PRODUCTION
47. SOLAR RADIATION
48. BIOTIC FACTOR AND AGRICULTURAL PRODUCTION
52. SOIL MICRO-ORGANISMS
53. SOIL PH
54. ROCK FORMATION
55. IGNEOUS ROCK
56. SEDIMENTARY ROCKS
58. SOIL AND ITS FORMATION
59. FACTORS OF SOIL FORMATION
60. LIVING ORGANISM
61. PARENT MATERIALS
62. SOIL FORMATION TOPOGRAPHY
63. PROCESS OF SOIL FORMATION
65. PHYSICAL WEATHERING
66. CHEMICAL WEATHERING
73. BIOLOGICAL WEATHERING
74. CHEMICAL AND BIOLOGICAL COMPOSITION OF THE SOIL
75. SOIL WATER
76. MICRO AND MACRO NUTRIENTS
77. SOIL MICRO ORGANISM
78. PROPERTIES OF SOIL
79. SOIL STRUCTURE
80. SANDY SOIL
81. CLAY SOIL
82. LOAMY SOIL
83. SOIL TEXTURE
84. IDENTIFICATION OF SOIL TYPES THROUGH EXPERIMENTS
85. RETENTION OF WATER BY VARIOUS SOIL TYPES
86. DETERMINATION OF SOIL PH REACTION
87. COLORIMETRIC DETERMINATION OF SOIL PH LEVEL
88. PH SOIL TEST
89. PLANT NUTRIENTS
90. MACRO NUTRIENTS IN GENERAL
112. THE MAINTENANCE OF SOIL FERTILITY
113. CROP ROTATION
114. APPLICATION OF ORGANIC MANURES
115. FARM YARD MANURE
116. APPLICATION OF INORGANIC MANURE
118. FARMING PRACTICES
119. BUSH BURNING
121. FERTILIZER APPLICATION
122. ORGANIC MANURING
123. FARM YARD MANURE
126. CROP ROTATION
133. FARM POWER AND MACHINERY
134. SOURCES OF FARM POWER
135. HUMAN SOURCE
142. FIELD MACHINES
164. SIMPLE FARM TOOLS
165. AGRICULTURAL MECHANIZATION
166. THE CONCEPT OF MECHANIZATION