Epithelial Tissues

Epithelial Tissues

Epithelial tissues cover the outer surfaces of the body and the lumen of internal organs; they are classified by shape and number of layers.

LEARNING OBJECTIVES

Differentiate among the types of epithelial tissues

KEY TAKEAWAYS

Key Points

• Epithelium composed of only a single layer of cells is called simple epithelium, while epithelium composed of more than one layer of cells is called stratified.
• Squamous epithelial cells are round, flat, and have an irregular border; their function is usually to diffuse or filter substances across tissues.
• Cuboidal epithelial cells, as wide as they are tall, are cube shaped; they are usually found lining glands where they secrete substances.
• Columnar epithelial cells are taller than they are wide and function mostly in absorption, such as in the digestive tract.
• Pseudostratified columnar epithelia appear to be stratified because there seems to be more than one row of nuclei, but, in fact, it is a single layer of cells with the nuclei at different levels.
• Transitional epithelium has the ability to stretch; it usually lines the interior of organs such as the bladder.

Key Terms

• goblet cell: glandular simple columnar epithelial cells whose function is to secrete mucin, which dissolves in water to form mucus
• lumen: The cavity or channel within a tube or tubular organ.

Epithelial Tissues

Epithelial tissues cover the outside of organs and structures in the body. They also line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia.

Types and Shapes of Epithelial Tissues

Squamous Epithelia

Squamous epithelial cells are generally round, flat, and have a small, centrally-located nucleus. The cell outline is slightly irregular; cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple squamous epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs or the exchange of nutrients and waste at blood capillaries.

Squamous epithelia: Squamous epithelia cells (a) have a slightly-irregular shape and a small, centrally-located nucleus. These cells can be stratified into layers, as in (b) this human cervix specimen.

Cuboidal Epithelia

Cuboidal epithelial cells are cube-shaped with a single, central nucleus. They are most-commonly found in a single layer, such as a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver.

Cuboidal epithelia: Simple cuboidal epithelial cells line tubules in the mammalian kidney where they are involved in filtering the blood.

Columnar Epithelia

Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer. They are most-commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells. 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.

Columnar epithelia: Simple columnar epithelial cells absorb material from the digestive tract. The nuclei line up at the base of the cells. Goblet cells secret mucous into the digestive tract lumen.

Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer. This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which, in the case of the trachea, keep these irritants from getting into the lungs.

Pseudostratified columnar epithelia: Pseudostratified columnar epithelia line the respiratory tract. They exist in one layer, but the arrangement of nuclei at different levels makes it appear that there is more than one layer.

Transitional Epithelia

Transitional (or uroepithelial) cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it; the lining becomes thinner. In other words, the tissue transitions from thick to thin.

Connective Tissues: Loose, Fibrous, and Cartilage

Connective tissue is found throughout the body, providing support and shock absorption for tissues and bones.

LEARNING OBJECTIVES

Distinguish between the different types of connective tissue

KEY TAKEAWAYS

Key Points

• Fibroblasts are cells that generate any connective tissue that the body needs, as they can move throughout the body and can undergo mitosis to create new tissues.
• Protein fibers run throughout connective tissue, providing stability and support; they can be either collagen, elastic, or reticular fibers.
• Loose connective tissue is not particularly tough, but surrounds blood vessels and provides support to internal organs.
• Fibrous connective tissue, which is composed of parallel bundles of collagen fibers, is found in the dermis, tendons, and ligaments.
• Hyaline cartilage forms the skeleton of the embryo before it is transformed into bone; it is found in the adult body at the tip of the nose and around the ends of the long bones, where it prevents friction at the joints.
• Fibrocartilage is the strongest of the connective tissues; it is found in regions of the body that experience large amounts of stress and require a high degree of shock absorption, such as between the vertebrae.

Key Terms

• chondrocyte: a cell that makes up the tissue of cartilage
• motile: having the power to move spontaneously
• fibroblast: a cell found in connective tissue that produces fibers, such as collagen

Connective Tissues

Connective tissues are composed of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is composed of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast, an immature connective tissue cell that has not yet differentiated. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues, while others may have specialized cells. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has a proportionally-less-dense matrix.

The organic portion, or protein fibers, found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length, returning to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers, the third type of protein fiber found in connective tissues, consist of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected.

Loose (Areolar) Connective Tissue

Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. Loose connective tissue has some fibroblasts, although macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel, helping to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes.

Loose connective tissue: Loose connective tissue is composed of loosely-woven collagen and elastic fibers. The fibers and other components of the connective tissue matrix are secreted by fibroblasts.

Fibrous Connective Tissue

Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly-arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).

Fibrous connective tissue: Fibrous connective tissue from the tendon has strands of collagen fibers lined up in parallel. This arrangement helps the tissue resist tension that occurs from all directions.

Cartilage

Cartilage is a connective tissue. The cells, called chondrocytes (mature cartilage cells), make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called “lacunae. ”

A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during some pre-birth developmental stages. 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.

Hyaline cartilage: Hyaline cartilage consists of a matrix with cells called chondrocytes (shown here) embedded in it. The chondrocytes exist in cavities in the matrix called lacunae.

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. In contrast, fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals, which must withstand a tremendous amount of stress. Cartilage can also transform from one type to another. For example, hyaline cartilage found in movable joints, such as the knee and shoulder, often becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage, resulting in “stiff” joints.

Connective Tissues: Bone, Adipose, and Blood

Bone, adipose (fat) tissue, and blood are different types of connective tissue that are composed of cells surrounded by a matrix.

LEARNING OBJECTIVES

Describe the structure and function of connective tissues made of bone, fat, and blood

KEY TAKEAWAYS

Key Points

• Bone contains three types of cells: osteoblasts, which deposit bone; osteocytes, which maintain the bone; and osteoclasts, which resorb bone.
• The functional unit of compact bone is the osteon, which is made up of concentric rings of bone called lamellae surrounding a central opening called a Haversian canal, through which nerves and blood vessels travel.
• Compact bone, made of inorganic material that gives it strength and stability, is located on the shaft of long bones, while spongy bone, made of organic material, is found inside the ends of the long bones.
• Adipose (fat) tissue contains cells called adipocytes that store fat in the form of triglyerides; these can be broken down for energy by the organism.
• Blood is composed of erythrocytes (red blood cells), which distribute oxygen throughout the body; leukocytes (white blood cells), which mount immune responses; and platelets, which are involved in blood clotting.

Key Terms

• osteon: any of the central canals and surrounding bony layers found in compact bone
• canaliculi: plural form of canaliculus; any of many small canals or ducts in bone or in some plants
• trabecula: a small mineralized spicule that forms a network in spongy bone
• osteoblast: a mononucleate cell from which bone develops
• osteoclast: a large multinuclear cell associated with the resorption of bone

Connective Tissues

Bone

Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is materially similar to 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, 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. They 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 and assist in maintenance of the bone. Osteoclasts are active in breaking down bone for bone remodeling, providing access to calcium stored in tissues in order to release it into the blood. 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. A blood vessel and a nerve are found in the center of the osteon within a long opening called the Haversian canal, with radiating circles of compact bone around it known as lamellae. Small spaces between these circles are called lacunae. 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, which serve as struts, giving the spongy bone strength.

Bone structure: (a) Compact bone is a dense matrix on the outer surface of bone. Spongy bone, inside the compact bone, is porous with web-like trabeculae. (b) Compact bone is organized into rings called osteons. Blood vessels, nerves, and lymphatic vessels are found in the central Haversian canal. Rings of lamellae surround the Haversian canal. Between the lamellae are cavities called lacunae. Canaliculi are microchannels connecting the lacunae together. (c) Osteoblasts surround the exterior of the bone. Osteoclasts bore tunnels into the bone and osteocytes are found in the lacunae.

Adipose (Fat) Tissue

Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix, and has only a few fibers. Adipose tissue is composed of cells called adipocytes that collect and store fat in the form of triglycerides for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic. They also function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing. The thin lines in the image are the cell membranes; the nuclei are the small, black dots at the edges of the cells.

Adipose tissue: Adipose (fat) is a connective tissue composed of cells called adipocytes. Adipocytes have small nuclei localized at the cell edge and store fat for energy usage.

Blood

Blood is considered a connective tissue because it has a matrix. The living cell types are red blood cells, also called erythrocytes, and white blood cells, also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.

Blood Tissue: Blood is a connective tissue that has a fluid matrix, called plasma, and no fibers. Erythrocytes (red blood cells), the predominant cell type, are involved in the transport of oxygen and carbon dioxide. Also present are various leukocytes (white blood cells) involved in immune response.

The cell found in greatest abundance in blood is the erythrocyte, responsible for transporting oxygen to body tissues. Erythrocytes are consistently the same size in a species, but vary in size between species. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues.

Leukocytes are white blood cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.

Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells that participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte, which give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils, both of which help to facilitate the inflammatory response.

The slightly-granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them.

Muscle Tissues and Nervous Tissues

The function of muscle tissue (smooth, skeletal, and cardiac) is to contract, while nervous tissue is responsible for communication.

LEARNING OBJECTIVES

Describe the structure and function of nervous tissue; differentiate among the types of muscle tissue

KEY TAKEAWAYS

Key Points

• Smooth muscle cells, spindle shaped with only one nucleus, contract involuntarily to push food through the digestive tract and blood through blood vessels.
• Skeletal muscle cells, long, striated, multinucleate cells under voluntary control, are responsible for the movement of skeletal muscles.
• Cardiac muscle cells, found only in the heart, are striated and branching (with one nucleus); they are joined by intercalacted discs which allow the cells to synchronize the beating of the heart.
• Nervous tissue is comprised of nerves, which are comprised of neurons, that send and receive signals, and glial cells, which support the neurons.

Key Terms

• intercalated disc: identifying features of cardiac muscle; these connect individual heart muscle cells to work as a single functional organ
• myosin: a large family of motor proteins found in eukaryotic tissues, allowing mobility in muscles
• oligodendrocyte: a cell that provides support and insulation to axons in the central nervous system of some vertebrates
• astrocyte: a neuroglial cell, in the shape of a star, in the brain
• actin: A globular structural protein that polymerizes in a helical fashion to form an actin filament (or microfilament).

Muscle Tissues

There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body.

Smooth Muscle

Smooth muscle cells have a single, centrally-located nucleus and are spindle shaped. Constriction of smooth muscle occurs under involuntary, autonomic nervous control in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed primarily of smooth muscle. Contractions of smooth muscle move food through the digestive tracts and push blood through the blood vessels.

Types of muscle fibers: Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell.

Skeletal Muscle

Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins, actin and myosin, that run throughout the muscle fiber. Skeletal muscle cells can contract by the attachment of myosin to actin filaments in the muscle, which then ratchets the actin filaments toward the center of the cells. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Stimulation of these cells by somatic motor neurons signals the cells to contract.

Cardiac Muscle

Cardiac muscle is found only in the heart. Similar to skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally-located nucleus; the muscle branches in many directions. Cardiac muscle is not under voluntary control, but is influenced by the autonomic nervous system to speed up or slow down the heart beat. An added feature to cardiac muscle cells is a line that extends along the end of the cell as it abuts the next cardiac cell in the row. This line, an intercalated disc, assists in passing electrical impulses efficiently from one cell to the next while maintaining the strong connection between neighboring cardiac cells, allowing the cardiac muscle cells to synchronize the beating of the heart.

Nervous Tissues

Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body organized into structures called nerves. A nerve consists of a neuron and glial cells. The main cell of the nervous system is the neuron. There is a large structure with a central nucleus: the cell body (or soma) of the neuron. Projections from the cell body are either dendrites, specialized in receiving input, or a single axon, specialized in transmitting impulses. Glial cells support the neurons. Astrocytes regulate the chemical environment of the nerve cell, while oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic, removing debris or damaged cells from the tissue.

Neuron: The neuron has projections called dendrites that receive signals and projections called axons that send signals. Also shown are two types of glial cells: astrocytes to regulate the chemical environment of the nerve cell, and oligodendrocytes to insulate the axon so the electrical nerve impulse is transferred more efficiently.