Charles Molnar and Jane Gair
Multicellular, complex animals have four primary types of tissue: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and 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 are called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. Table 14.2 summarizes the different types of epithelial tissues.
| Cell shape | Description | Location |
|---|---|---|
| squamous | flat, irregular round shape | simple: lung alveoli, capillaries stratified: skin, mouth, vagina |
| cuboidal | cube shaped, central nucleus | glands, renal tubules |
| columnar | tall, narrow, nucleus toward base tall, narrow, nucleus along cell |
simple: digestive tract pseudostratified: respiratory tract |
| transitional | round, simple but appear stratified | urinary bladder |
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries.
Figure 14.7a illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure 14.7b illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure 14.8, are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing 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.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and 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, as illustrated in Figure 14.9. 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 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, as seen in Figure 14.10. 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.
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 illustrated in Figure 14.11. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin.
Which of the following statements about types of epithelial cells is false?
- Simple columnar epithelial cells line the tissue of the lung.
- Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
- Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
- Transitional epithelia change in thickness depending on how full the bladder is.
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made 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. 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. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a 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 and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 14.3.
| Tissue | Cells | Fibers | Location |
|---|---|---|---|
| loose/areolar | fibroblasts, macrophages, some lymphocytes, some neutrophils | few: collagen, elastic, reticular | around blood vessels; anchors epithelia |
| dense, fibrous connective tissue | fibroblasts, macrophages, | mostly collagen | irregular: skin regular: tendons, ligaments |
| cartilage | chondrocytes, chondroblasts | hyaline: few collagen fibrocartilage: large amount of collagen | shark skeleton, fetal bones, human ears, intervertebral discs |
| bone | osteoblasts, osteocytes, osteoclasts | some: collagen, elastic | vertebrate skeletons |
| adipose | adipocytes | few | adipose (fat) |
| blood | red blood cells, white blood cells | none | blood |
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure 14.12, loose connective tissue has some fibroblasts; 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 and helps 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.
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, shown in Figure 14.13, is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, 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, illustrated in Figure 14.14. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. 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. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.”
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 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 14.15. 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.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up 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, and they 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, as seen in Figure 14.16. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure 14.17. The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter 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 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 the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they 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. Monocytes 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 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
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. Table 14.4 summarizes these differences.
| Type of Muscle | Striations | Nuclei | Control | Location |
|---|---|---|---|---|
| smooth | no | single, in center | involuntary | visceral organs |
| skeletal | yes | many, at periphery | voluntary | skeletal muscles |
| cardiac | yes | single, in center | involuntary | heart |
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure 14.18. Constriction of smooth muscle occurs under involuntary, autonomic nervous control and 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 of mostly smooth muscle.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. 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. Figure 14.18 illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure 14.18, is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
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. The main cell of the nervous system is the neuron, illustrated in Figure 14.19. The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Concept in Action
Click through the interactive review to learn more about epithelial tissues.
Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease.
Exercises
- Which of the following statements about types of epithelial cells is false?
- Simple columnar epithelial cells line the tissue of the lung.
- Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
- Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present.
- Transitional epithelia change in thickness depending on how full the bladder is.
- State whether each of the following processes are regulated by a positive feedback loop or a negative feedback loop.
- A person feels satiated after eating a large meal.
- The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.
- When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?
- Which type of animal maintains a constant internal body temperature?
- endotherm
- ectotherm
- coelomate
- mesoderm
- The symmetry found in animals that move swiftly is ________.
- radial
- bilateral
- sequential
- interrupted
- What term describes the condition of a desert mouse that lowers its metabolic rate and “sleeps” during the hot day?
- turgid
- hibernation
- estivation
- normal sleep pattern
- A plane that divides an animal into equal right and left portions is ________.
- diagonal
- midsagittal
- coronal
- transverse
- A plane that divides an animal into dorsal and ventral portions is ________.
- sagittal
- midsagittal
- coronal
- transverse
- The pleural cavity is a part of which cavity?
- dorsal cavity
- thoracic cavity
- abdominal cavity
- pericardial cavity
- Which type of epithelial cell is best adapted to aid diffusion?
- squamous
- cuboidal
- columnar
- transitional
- Which type of epithelial cell is found in glands?
- squamous
- cuboidal
- columnar
- transitional
- Which type of epithelial cell is found in the urinary bladder?
- squamous
- cuboidal
- columnar
- transitional
- Which type of connective tissue has the most fibers?
- loose connective tissue
- fibrous connective tissue
- cartilage
- bone
- Which type of connective tissue has a mineralized different matrix?
- loose connective tissue
- fibrous connective tissue
- cartilage
- bone
- The cell found in bone that breaks it down is called an ________.
- osteoblast
- osteocyte
- osteoclast
- osteon
- The cell found in bone that makes the bone is called an ________.
- osteoblast
- osteocyte
- osteoclast
- osteon
- Plasma is the ________.
- fibers in blood
- matrix of blood
- cell that phagocytizes bacteria
- cell fragment found in the tissue
- The type of muscle cell under voluntary control is the ________.
- smooth muscle
- skeletal muscle
- cardiac muscle
- visceral muscle
- The part of a neuron that contains the nucleus is the
- cell body
- dendrite
- axon
- glial
- When faced with a sudden drop in environmental temperature, an endothermic animal will:
- experience a drop in its body temperature
- wait to see if it goes lower
- increase muscle activity to generate heat
- add fur or fat to increase insulation
- Which is an example of negative feedback?
- lowering of blood glucose after a meal
- blood clotting after an injury
- lactation during nursing
- uterine contractions during labor
- Which method of heat exchange occurs during direct contact between the source and animal?
- radiation
- evaporation
- convection
- conduction
- The body’s thermostat is located in the ________.
- homeostatic receptor
- hypothalamus
- medulla
- vasodilation center
- How does diffusion limit the size of an organism? How is this counteracted?
- What is the relationship between BMR and body size? Why?
- How can squamous epithelia both facilitate diffusion and prevent damage from abrasion?
- What are the similarities between cartilage and bone?
- Why are negative feedback loops used to control body homeostasis?
- Why is a fever a “good thing” during a bacterial infection?
- How is a condition such as diabetes a good example of the failure of a set point in humans?
Answers
- A
- Both processes are the result of negative feedback loops. Negative feedback loops, which tend to keep a system at equilibrium, are more common than positive feedback loops.
- Pyrogens increase body temperature by causing the blood vessels to constrict, inducing shivering, and stopping sweat glands from secreting fluid.
- A
- B
- C
- B
- D
- B
- C
- B
- D
- B
- D
- C
- A
- B
- B
- B
- C
- A
- D
- B
- Diffusion is effective over a very short distance. If a cell exceeds this distance in its size, the center of the cell cannot get adequate nutrients nor can it expel enough waste to survive. To compensate for this, cells can loosely adhere to each other in a liquid medium, or develop into multi-celled organisms that use circulatory and respiratory systems to deliver nutrients and remove wastes.
- Basal Metabolic Rate is an expression of the metabolic processes that occur to maintain an individual’s functioning and body temperature. Smaller bodied animals have a relatively large surface area compared to a much larger animal. The large animal’s large surface area leads to increased heat loss that the animal must compensate for, resulting in a higher BMR. A small animal, having less relative surface area, does not lose as much heat and has a correspondingly lower BMR.
- Squamous epithelia can be either simple or stratified. As a single layer of cells, it presents a very thin epithelia that minimally inhibits diffusion. As a stratified epithelia, the surface cells can be sloughed off and the cells in deeper layers protect the underlying tissues from damage.
- Both contain cells other than the traditional fibroblast. Both have cells that lodge in spaces within the tissue called lacunae. Both collagen and elastic fibers are found in bone and cartilage. Both tissues participate in vertebrate skeletal development and formation.
- An adjustment to a change in the internal or external environment requires a change in the direction of the stimulus. A negative feedback loop accomplishes this, while a positive feedback loop would continue the stimulus and result in harm to the animal.
- Mammalian enzymes increase activity to the point of denaturation, increasing the chemical activity of the cells involved. Bacterial enzymes have a specific temperature for their most efficient activity and are inhibited at either higher or lower temperatures. Fever results in an increase in the destruction of the invading bacteria by increasing the effectiveness of body defenses and an inhibiting bacterial metabolism.
- Diabetes is often associated with a lack in production of insulin. Without insulin, blood glucose levels go up after a meal, but never go back down to normal levels.
Glossary
cartilage: type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers
chondrocyte: cell found in cartilage
columnar epithelia: epithelia made of cells taller than they are wide, specialized in absorption
connective tissue: type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia: epithelia made of cube-shaped cells, specialized in glandular functions
dorsal cavity: body cavity on the posterior or back portion of an animal; includes the cranial and vertebral cavities
ectotherm: animal incapable of maintaining a relatively constant internal body temperature
endotherm: animal capable of maintaining a relatively constant internal body temperature
epithelial tissue: tissue that either lines or covers organs or other tissues
estivation: torpor in response to extremely high temperatures and low water availability
fibrous connective tissue: type of connective tissue with a high concentration of fibers
hibernation: torpor over a long period of time, such as a winter
homeostasis: dynamic equilibrium maintaining appropriate body functions
lacuna: space in cartilage and bone that contains living cells
matrix: component of connective tissue made of both living and non-living (ground substances) cells
negative feedback loop: feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it
osteon: subunit of compact bone
positive feedback loop: feedback to a control mechanism that continues the direction of a stimulus
pseudostratified: layer of epithelia that appears multilayered, but is a simple covering
set point: midpoint or target point in homeostasis
simple epithelia: single layer of epithelial cells
squamous epithelia: type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
stratified epithelia: multiple layers of epithelial cells
torpor: decrease in activity and metabolism that allows an animal to survive adverse conditions
trabecula: tiny plate that makes up spongy bone and gives it strength