5.6: connective tissue (2023)

squamous epithelium

squamous epitheliumCells are generally round, flat, and have a small nucleus in the middle. The cell outline is slightly irregular and the cells fit together to form a cover or lining. When cells are arranged in a single layer (simple epithelium), they facilitate diffusion into tissues such as B. Gas exchange areas in the lungs and the exchange of nutrients and waste products in blood capillaries.

Figure \(\PageIndex{1}\) shows a layer of squamous epithelial cells whose membranes are connected to form an epithelium. The figure \(\PageIndex{1}\) shows squamous epithelial cells arranged in stratified layers where protection is needed in the body from external abrasion and damage. This is called stratified squamous epithelium and occurs in the skin and tissues lining the mouth and vagina.

cuboid epithelium

cuboid epitheliumThe cells shown in figure \(\PageIndex{2}\) are cuboidal with a single central nucleus. They are most commonly found in a single layer, which is a simple epithelium, in glandular tissues throughout the body, where they prepare and secrete glandular material. They are also found in the walls of the tubules and in the ducts of the kidneys and liver.

Columnar epithelium

columnar epitheliumCells are taller than they are wide: they resemble a stack of columns in a layer of epithelium and are most commonly found in a monolayer arrangement. The nuclei of the columnar epithelial cells of the digestive tract appear to be aligned at the base of the cells, as shown in figure \(\PageIndex{3}\). These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems.

The columnar epithelial cells lining the airways appear to be stratified. However, each cell is attached to the basement membrane of the tissue and hence they are simple tissues. The cell nuclei are arranged at different levels in the cell layer, which makes it appear that there is more than one layer, as can be seen in figure \(\PageIndex{4}\). Calledpseudostratified, columnar epithelium. This cell covering has cilia on the apical or free surface of the cells. Cilia improve the movement of trapped particles and mucus from the airways and help protect the system from invading microorganisms and harmful materials that have been inhaled into the body. Goblet cells are scattered in some tissues (eg, the lining of the trachea). The goblet cells contain mucus that traps irritants, which in the case of the trachea prevents those irritants from reaching the lungs.

transitional epithelium

crossingor uroepithelial cells are found only in the urinary system, mainly in the bladder and ureter. These cells are arranged in a stratified layer, but can stack on top of each other in an empty, relaxed bubble, as shown in figure \(\PageIndex{5}\). As the bladder fills, the epithelial layer unfolds and expands to accommodate the volume of urine that enters it. As the bladder fills, it expands and the lining thins. In other words, the fabric goes from thick to thin.

loose/areolar connective tissue

Loose connective tissue, also called areolar connective tissue, contains a sample of all components of a connective tissue. As shown in figure \(\PageIndex{6}\), the loose connective tissue shows some fibroblasts; Macrophages are also present. Collagen fibers are relatively wide and stain light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed tissue elements is filled with the matrix. The material in the connective tissue gives it a fluffy consistency similar to a stretched cotton ball. Loose connective tissue surrounds each blood vessel and helps hold the vessel in place. Tissue is also found around and between most organs in the body. In summary, areolar tissue is tough yet flexible and comprises membranes.

Fibrous connective tissue

fibrous connective tissuethey contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with strands aligned in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs in all directions, such as in the dermis of the skin. Normal fibrous connective tissue, shown in figure \(\PageIndex{7}\), is found in tendons (connecting muscles to bone) and ligaments (connecting bone to bone).

cartilage

cartilageIt is a connective tissue with a large amount of matrix and a variable amount of fibers. The cells, calledchondrocytes, form the matrix and fibers of the tissue. Chondrocytes reside in spaces within tissues called chondrocytesGaps.

A cartilage with few collagen and elastic fibers is hyaline cartilage, shown in figure \(\PageIndex{8}\). The gaps are randomly scattered throughout the tissue, and the matrix appears milky or rubbed in routine histological stains. Sharks have cartilaginous skeletons, as do almost all human skeletons at some stage of prenatal development. A remnant of this cartilage persists outside of the human nose. Hyaline cartilage is also found at the ends of long bones, which reduces friction and cushions the joints of those bones.

Elastic cartilage has a large number of elastic fibers, giving it great flexibility. The ears of most vertebrates contain this cartilage, as do parts of the larynx, or larynx. Fibrous cartilage contains a large number of collagen fibers that give the tissue enormous strength. Fibrous cartilage comprises the intervertebral discs in vertebrates. The hyaline cartilage in mobile joints such as the knee and shoulder is damaged by age or trauma. Damaged hyaline cartilage is replaced with fibrocartilage and causes the joints to become "stiff".

Osso

Bone, or bone tissue, is connective tissue that contains a large amount of two different types of matrix material. The organic matrix resembles matrix material found in other connective tissues and contains some collagen and elastic fibers. This gives the fabric strength and flexibility. The inorganic matrix consists of mineral salts, mainly calcium salts, which impart hardness to the tissue. Without sufficient organic material in the matrix, tissue degrades; without suitable inorganic material in the matrix, the tissue bends.

There are three types of cells in bone: osteoblasts, osteocytes and osteoclasts. Osteoblasts are active in producing bone for growth and remodeling. Osteoblasts deposit bone material in the matrix and continue to live after the matrix surrounds them, but in a reduced metabolic state as osteocytes. Osteocytes are located in the gaps in the bone. Osteoclasts are active in bone remodeling for bone remodeling and provide access to calcium stored in tissues. Osteoclasts are usually found on the tissue surface.

Bones can be divided into two types: compact and spongy. Compact bone is found at the shaft (or shaft) of a long bone and on the surface of flat bones, while cancellous bone is found at the end (or epiphysis) of a long bone. Compact bone is organized into subunits calledOsteonasen, as shown in figure \(\PageIndex{9}\). A blood vessel and nerve are at the center of the structure within Haversian canal, surrounded by radiating circles of gaps known as lamellae. The wavy lines between the gaps are called microchannelscanals; They plug the gaps to help spread between cells. Cancellous bone consists of small plates calledbunchThese plates serve as supports to give strength to the cancellous bone. Over time, these plates can break, causing the bone to become weaker. Bone tissue forms the internal skeleton of vertebrates and provides the animal with structure and attachment points for tendons.

adipose tissue

Adipose tissue, or adipose tissue, is considered connective tissue even though it has no fibroblasts or matrix proper and few fibers. Adipose tissue is made up of cells called adipocytes, which collect and store fat in the form of triglycerides for energy metabolism. Adipose tissue also serves as insulation to maintain body temperature, allowing animals to be endothermic and acting as a buffer against damage to body organs. Under the microscope, adipose tissue cells appear empty due to the removal of fat during processing of the material for visualization, as shown in figure \(\PageIndex{10}\). The thin lines in the image are the cell membranes, and the nuclei are the small black dots at the edges of the cells.

Sangre

Blood is considered connective tissue because it has a matrix as shown in figure \(\PageIndex{11}\). The types of living cells are red blood cells (erythrocytes), also called erythrocytes, and white blood cells (leukocytes), also called leukocytes. The liquid part of whole blood, its matrix, is commonly referred to as plasma.

The most common cell in the blood is the erythrocyte. The number of red blood cells in a blood sample is in the millions: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Red blood cells are consistently the same size within a species but vary in size between species. For example, the average diameter of the red blood cells in a primate is 7.5 µl, in a dog almost 7.0 µl, but the diameter of the red blood cells in a cat is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow, where they are produced. Red blood cells in fish, amphibians, and birds maintain their nuclei and mitochondria throughout the cell's lifespan. The main task of an erythrocyte is to transport and deliver oxygen to the tissue.

Leukocytes are the predominant white blood cells in the peripheral blood. The number of white blood cells in the blood is in the thousands, with measurements expressed as ranges: the number in primates ranges from 4,800 to 10,800 cells per µl, in dogs from 5,600 to 19,200 cells per µl, in cats from 8,000 to 25,000 cells per µl, in cattle from 4,000 to 12,000 cells per µl and in pigs from 11,000 to 22,000 cells per µl.

Lymphocytes act primarily in the immune response to foreign antigens or materials. Different types of lymphocytes produce antibodies adapted to foreign antigens and control the production of these antibodies. Neutrophils are phagocytes and participate in one of the first lines of defense against microbial invaders by helping to eliminate bacteria that have entered the body. Another leukocyte found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages, which remove dead and damaged cells from the body, whether foreign or from the host animal. Two other leukocytes in the blood are eosinophils and basophils that help facilitate the inflammatory response.

The slightly granular material between the cells is a cytoplasmic fragment of a bone marrow cell. This is called a platelet or thrombocyte. Platelets are involved in the steps that lead to blood clotting to stop bleeding from damaged blood vessels. Blood has multiple functions, but primarily it carries material around the body, to carry nutrients to cells and to remove waste from cells.

smooth muscle cells

Smooth muscle has no stripes in its cells. It has a single core located in the middle as shown in figure \(\PageIndex{12}\). Smooth muscle constriction occurs under involuntary autonomic nerve control and in response to local tissue conditions. Smooth muscle tissue is also referred to as non-striated because it does not have the banded appearance of cardiac and skeletal muscle. The walls of blood vessels, the ducts of the digestive system, and the ducts of the reproductive system are composed primarily of smooth muscle.

skeletal muscles

Skeletal muscle has streaks along its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei at the edge of the cell. Skeletal muscle is under the voluntary control of the somatic nervous system and resides in the muscles that move bones. Figure \(\PageIndex{12}\) illustrates the histology of skeletal muscle.

Herzmuskel

The heart muscle, shown in figure \(\PageIndex{12}\), only occurs in the heart. Like skeletal muscle, it has transverse stripes in its cells, but cardiac muscle has a single, centrally located nucleus. The heart muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An additional feature of heart muscle cells is a line that runs along the edge of the cell when it rests on the next heart cell in the line. This line is called the intercalated disc: it helps conduct the electrical impulse efficiently from one cell to another and maintains the strong connection between adjacent heart cells.