The top of the head, for example, has many hair follicles, whereas the soles of the feet have none. Below the dermis lies a layer of fat that helps insulate the body from heat and cold, provides protective padding, and serves as an energy storage area.
The fat is contained in living cells, called fat cells, held together by fibrous tissue. The fat layer varies in thickness, from a fraction of an inch on the eyelids to several inches on the abdomen and buttocks in some people. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world.
The Merck Manual was first published in as a service to the community. Learn more about our commitment to Global Medical Knowledge. This site complies with the HONcode standard for trustworthy health information: verify here. Common Health Topics. Commonly searched drugs. Layers of the Skin.
Epidermis Dermis Fat layer. Biology of the Skin. Test your knowledge. Dermatitis herpetiformis is an autoimmune disease that causes clusters of intensely itchy small blisters and hive-like swellings on the skin of people who have celiac disease. Which of the following foods can trigger an attack of dermatitis herpetiformis? More Content. Click here for the Professional Version.
It serves many important functions, including. Protecting the body against trauma. Here are 16 of the best sunscreens on the market in spray, lotion, and solid forms, and for different needs. They're tried and true. Everyone can benefit from a good facial cleanser, and many products are specially formulated for dry, sensitive skin. Blue lips are most often caused when something is preventing you from getting enough oxygen into your body.
Read on to discover some of the causes of…. Health Conditions Discover Plan Connect. What Is Hyperelastic Skin? Medically reviewed by Owen Kramer, M. What causes hyperelastic skin? When should you see your healthcare provider? Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage.
A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones. Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength.
Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma.
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material.
The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes.
Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue. Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft or diaphysis of a long bone and the surface of the flat bones, while spongy bone is found in the end or epiphysis of a long bone.
Compact bone is organized into subunits called osteons , as illustrated in Figure 4. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi ; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength.
Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Figure 4. Spongy bone, inside the compact bone, is porous with web-like trabeculae. Blood vessels, nerves, and lymphatic vessels are found in the central Haversian canal. As a consequence, it displays greater resistance to stretching. There are three major categories of dense connective tissue: regular, irregular, and elastic. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations.
Ligaments and tendons are made of dense regular connective tissue. In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh.
In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle.
The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching Figure 4. The distinctive appearance of cartilage is due to the presence of polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage.
Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.
The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage Figure 4. Hyaline cartilage , the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints.
It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. The knee and jaw joints and the the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity.
Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.
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