3.2 Tissues in the Human Body
While the cell is the fundamental unit of life, tissues are fundamental units of function and structure for the human body. The term tissue is used to describe a group of cells that are similar in structure and perform a specific function (Biga et al. 2023). For example, muscle tissue is made up of special muscle fiber cells that collectively have the ability to shorten in length and form the basis of contractile.
Tissues are organized into four broad categories based on structural and functional similarities. These categories are epithelial, connective, muscle, and nervous. The primary tissue types work together to contribute to the overall health and maintenance of the human body. Thus, any disruption in the structure of a tissue can lead to injury or disease.
3.2.1. Epithelial Tissues
Epithelial tissues are located all over the body, forming a continuous external layer covering the entire body (skin), lining most of the body’s inner cavities, and composing various glands. On the surface, these tissues shield underlying cells from bacterial infiltration, harmful chemicals, and dehydration. Internally, they function as absorptive and secretory tissues, such as those found in the digestive system glands. Epithelial tissues can be classified into four groups based on the shape of the cells they consist of, showcasing the diversity and adaptability of these vital tissues.
The four groups of epithelial tissues are as follows:
- Squamous epithelium consists of a single layer of flat cells found in areas such as the linings of the mouth, esophagus, and blood and lymphatic vessels. This layer of cells allows for easy diffusion of substances.
- Cuboidal epithelium is composed of cube-shaped cells, which can be observed in structures like the lining of kidney tubules.
- Columnar epithelium features cells with a column or pillar-like shape. These cells are prevalent throughout the body, forming linings in the digestive and respiratory tracts. They serve as secretory or absorptive cells; some possess small hairs called cilia. These cilia beat rhythmically and help move materials out of a passage, such as in the respiratory tract, where they work to clear out foreign matter that could enter the lungs.
Glandular epithelial cells are responsible for secreting mucus and hormones, like those found in the salivary and thymus glands.
3.2.2. Connective Tissues
Connective tissues play a crucial role in the body by connecting structures, providing support, and offering protection. They are found throughout the body, connecting other tissues, attaching muscles to bones, and forming the skeleton that supports the body’s positions. Connective tissues consist of cells embedded in a non-living matrix, with the matrix determining the function of each specific type of connective tissue. The composition of connective tissues varies, but they typically contain cells, protein fibers, and a large amount of amorphous ground substance.
Blood is considered a connective tissue due to its fluid matrix with suspended cells. Connective tissues can range from firm yet flexible soft gels to hard, tough, and rigid structures. The matrix of a particular connective tissue grants it its unique characteristics.
Three broad categories of connective tissue exist: proper, supportive connective tissue, and fluid connective tissue.
Each person has a unique blood type, identified by the presence or absence of specific proteins on the surface of their red blood cells. However, the prevalence of different blood types can vary, with some types being more common than others in the general population
Connective tissue proper includes loose connective tissue and dense connective tissue, both of which have various cell types and protein fibers suspended in a viscous ground substance. Supportive connective tissue, such as bone and cartilage, provides structure and strength to the body, protecting soft tissues. Fluid connective tissue includes blood and lymph, with specialized cells circulating in a watery fluid containing salts, nutrients, and dissolved proteins.
From an athletic perspective, the most important connective tissues are cartilage, bones, tendons, and ligaments. Cartilage forms the foundation of bone tissue and is found at bone ends, in spinal disks, and in the soft “bone” of the nose. Bones form the skeleton, providing support and protection for the body. Tendons and ligaments are strong and flexible, connecting muscles to bones or other structures and joining bones to form joints.
Due to their composition, connective tissues have a limited capacity for regeneration, making damage to these structures a serious issue. Tendon and ligament injuries may require surgery for repair. Sound nutrition and strength training can help build strong connective tissues, increasing resistance to injury.
Other types of connective tissues include reticular tissue, found in the spleen, lymph nodes, and bone marrow, which functions as a filtering media for blood and lymph; areolar tissue, occurring between organs and tissues, connecting them; and adipose tissue, containing fat and found under the skin, functioning to protect, insulate, support, and serve as a food reserve. Blood, myeloid (red bone marrow), and lymph are also considered connective tissues.
Connective tissues encompass a wide range of tissues in the body, providing support and connection between structures. They comprise various components, including cells, fibers, and ground substance.
Key fibers found in connective tissues are:
- Collagen Fibers: These tough, strong fibers form the major fibrous component of the skin, tendons, cartilage, ligaments, and teeth. Composed of amino acids such as glycine, proline, lysine, hydroxyproline, and hydroxylysine, collagen fibers give connective tissue its versatility due to their ability to interconnect with other molecules and minerals. This results in a higher tensile strength than their separate parts. Collagen fibers occur in bundles, providing great tensile strength.
- Reticular Fibers: Delicate, supportive fibers of connective tissue form networks and support intricate structures such as capillaries and nerve fibers.
- Elastic Fibers: These fibers are elastic and extendible, providing flexibility to connective tissues.
Collagen fibers are the most abundant protein in the human body, accounting for about one-third of the body's total protein
Maintaining the health of connective tissues is vital for all humans, as injuries to tendons and ligaments can be serious and may require surgical intervention. Since we now know that our role as personal trainers is crucial in guiding our clients through the training process, usually involving strength training, our job becomes very clear.
avascular=does not contain blood vessels
Cartilage
Cartilage forms the foundation of bone tissue. It is found at the bone ends, in spinal discs, and makes up the soft “bone” of the nose. Cartilaginous tissue is avascular (does not contain blood vessels); 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.
avascular = does not contain blood vessels
The hardness of cartilage depends on the number of collagen fibers. Hyaline cartilage is the most common type of cartilage in the body. It is hard as it consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. It is found in the bone ends. Elastic cartilage contains elastic fibers, collagen, and proteoglycans and is therefore elastic. Elastic cartilage is, for example, ear lobes. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. It is found between the bones of the spine.
Bone
The adult skeleton is mainly bone, with just a little cartilage in some places. Most of the human skeleton develops first as cartilage, which is later replaced by bone (Roberts, 2016).
Bone is the hardest connective tissue. It protects internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both organic and inorganic matrix components contribute to bone’s unusual properties.
Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteoblasts are active bone-forming cells producing the organic part of the extracellular matrix. The mature bone cells, osteocytes, are located within the lacunae. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries relatively quickly.
Bones are not static and are continuously remodeling themselves throughout a person's life
Bones are not as lifeless as they seem. Within this hard, calcified, intercellular matrix exist many living cells that continually receive food and oxygen and excrete their wastes through the numerous blood vessels in bone tissue and bone marrow.
Tendons and Ligaments
Tendons are connective tissue that attaches muscle to the bone.
Ligaments are connective tissue that connects bone to bone or bone to cartilage.
Tendons and ligaments are the strongest connective tissues in the body
Ligaments and tendons are mostly formed from dense regular connective tissue. They are flexible but strong. In fact, they are the strongest connective tissues in the body. The intercellular matrix consists of a collagen and reticular fiber network, which originates from the cells they surround. Tendons can be thick, like the Achilles tendon, or thin, like aponeurosis – a thin layer of connective tissue that covers the skull (Hatfield et al., 2011).
Due to the nature of connective tissue, damage to these structures is a serious occurrence. Connective tissues consist of only a few cells and mostly of a non-living matrix, so they have a very limited capacity to regenerate themselves.
Disorders of the Connective Tissue: Tendinitis
Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. The condition often results from repetitive motions over time that strain the tendons needed to perform the tasks.
hile older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators; anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken to lessen the probability of developing tendinitis. Stretches before exercising and cross-training or changing exercises are recommended for active individuals.
Women are up to six times more likely to sustain connective tissue injuries during physical activity than men. The results from studies on this topic suggest that circulating estrogen may lower the extent to which exercise can stimulate collagen synthesis (Mackey et al., 2008).
3.2.3. Muscle Tissues
Muscle tissue is composed of cells that are specialized for contractions. These cells are organized into bundles called fibers, which are grouped together to form larger muscles. There are approximately 640 different muscles in the human body. These muscles are responsible for movement, posture, and breathing, among other functions.
The percentage of muscle tissue in the human body varies depending on age, gender, and body type. However, on average, men have around 40% muscle mass, while women have around 30%.
We will delve into more detail regarding muscle tissue in Chapter 3.4 – The Musculoskeletal system.
3.2.4. Nervous Tissues
Nervous tissue, an essential component of the body’s control system, is made up of several cell types and is characterized by its excitable nature and ability to send and receive electrochemical signals. These signals provide the body with critical information, facilitating the control of various bodily functions. The nervous tissue is primarily found in the brain, spinal cord, and nerves extending to all body parts.
The main types of cells constituting nervous tissue are neurons, neuroglia, and neurosecretory cells.
Each of these cells plays a distinct role in the functioning of the nervous system:
- Neurons: These cells conduct nerve impulses, register sensory inputs, and transmit motor impulses. Neurons consist of a central body containing a nucleus surrounded by cytoplasm and two projections at either end – axons and dendrites. Axons generally conduct impulses away from the neuron’s body, while dendrites transmit impulses from adjacent cells toward the cell body. Neurons propagate information through electrochemical impulses called action potentials, which are biochemically linked to the release of chemical signals.
- Neuroglia: This delicate network of branched cells and fibers provides essential support to the central nervous system’s tissue. Neuroglia plays a crucial role in maintaining neurons’ proper functioning and overall health.
- Neurosecretory cells: As their name implies, these cells secrete substances that can have effects elsewhere in the body, contributing to the overall communication and coordination of various physiological processes.