Chapter 4: Tissues – Human Anatomy & Physiology

Tissues are groups of cells that are very similar to each other in how they are built and what they do together.

The study of tissues is called histology.

Your body has four main types of tissues:

• Epithelial Tissue: This tissue covers the body's surfaces and lines its cavities, like the skin and the lining of organs.

• Connective Tissue: This is the most common tissue type, providing support and connecting different parts of the body.

• Muscle Tissue: This tissue is responsible for all body movements, from walking to your heart beating.

• Nervous Tissue: This tissue sends messages throughout your body, controlling its functions.

Every cell in your body has a specific role, and they all work together to keep the body's internal environment stable, which is known as homeostasis.

Epithelial Tissue

Epithelial tissue is made of sheets of cells that cover your body's outer surfaces or line internal cavities. It also forms the secretory parts of glands.

There are two main types of epithelial tissue:

• Covering & Lining Epithelium: This type covers surfaces, such as the skin, or lines organs like the stomach.

• Glandular Epithelium: This type is specialized for making and releasing substances, for example, the cells in salivary glands that produce spit.

This tissue performs several important jobs: it protects you, absorbs nutrients, filters out unwanted substances, helps remove waste, secretes useful liquids, and plays a role in your senses.

Special Characteristics ("P.A.S.A.R.")

The unique features of epithelial tissue can be remembered using the acronym "P.A.S.A.R."

• Polarity: Epithelial cells have a distinct top and bottom. The top part, called the apical surface, often faces outwards or into a body space and might have tiny finger-like projections called microvilli or small hairs called cilia. The bottom part, called the basal surface, is attached to the tissues beneath it, usually connective tissue, by a sticky layer called the basal lamina.

• All cells have Specialized Contacts: These cells are packed very closely together to form continuous sheets, almost like a seal, except in glands. This tight packing is due to special cell connections like tight junctions, which prevent substances from leaking between cells, and desmosomes, which act like rivets to hold cells firmly together and prevent them from being pulled apart.

• Supported by Connective Tissue: Every sheet of epithelial tissue rests on a layer of connective tissue directly underneath it. Together, these two layers form the basement membrane, which provides a strong foundation, prevents stretching and tearing, and defines the boundary of the epithelial tissue. It is made up of the basal lamina (from epithelial cells) and the reticular lamina (from connective tissue).

• Avascular but Innervated: Epithelial tissue does not have any blood vessels (avascular). It receives all its nutrients and oxygen by diffusion from the blood vessels in the connective tissue below it. However, it is richly supplied with nerve endings, meaning it is innervated.

• Regeneration: Epithelial tissue has an excellent ability to repair itself. If cells are lost or damaged, they can quickly be replaced, especially if their cell connections are broken or they lose their "polarity," as long as there are enough nutrients available to create new cells.

Classification of Epithelia

Epithelial tissues are classified mainly by two features: the number of cell layers and the shape of the cells.

• By Layers:

  • Simple epithelia have only one layer of cells. They are very thin, which makes them ideal for processes like absorption or filtration, where a minimal barrier is needed.

  • Stratified epithelia have two or more layers of cells. These are designed for protection in areas subject to friction and wear, such as the skin.

• By Cell Shape:

  • Squamous: These cells are flat and scale-like.

  • Cuboidal: These cells are cube-shaped.

  • Columnar: These cells are tall and column-shaped.

Important note: When classifying stratified epithelia, the name comes from the shape of the cells in the very top layer!

Representative Types & Details

Let's look at some specific types of epithelial tissue:

• Simple Squamous Epithelium: This is a very thin tissue, consisting of a single layer of flat cells. Its main role is diffusion and filtration because it presents very little barrier. It also produces lubricating fluids in the serosae. You can find it in the filters of your kidneys, the air sacs of your lungs, the lining of your heart and blood vessels (called the endothelium), and the lining of your body cavities (called the mesothelium).

• Simple Cuboidal Epithelium: This tissue is a single layer of cube-shaped cells. It is specialized for secreting substances and absorbing nutrients. You will find it in kidney tubules, small gland ducts, and on the surface of your ovaries.

• Simple Columnar Epithelium: This durable tissue is a single layer of tall, column-like cells. It is highly effective at absorbing nutrients and secreting mucus and enzymes. There is a ciliated version with small hairs that move mucus or reproductive cells, and a non-ciliated version found in your digestive tract and gallbladder. The ciliated type lines your small airways (bronchioles) and uterine tubes.

• Pseudostratified Columnar Epithelium: This tissue appears to have multiple layers because its cells are of different heights and their nuclei are scattered, but it is actually just one layer of cells. It often features cilia and goblet cells, which produce mucus. Its function is to secrete and move mucus. The ciliated form is common in your windpipe (trachea) and upper respiratory tract, while the non-ciliated type is found in male sperm ducts and large gland ducts.

• Stratified Squamous Epithelium: This is a thick, protective membrane designed to withstand a lot of friction. It can be keratinized, meaning it contains the tough protein keratin (like your dry skin), or non-keratinized, which forms moist linings (like in your mouth, esophagus, and vagina).

• Transitional Epithelium: This unique stratified tissue has basal cells that are cuboidal or columnar, but the cells on the top layer change shape (from dome-like to flattened) depending on how much they are stretched. It is crucial because it allows your urinary organs to stretch without tearing. You will only find it in your ureters, bladder, and part of your urethra.

Glandular Epithelium

A gland is essentially one or more cells that produce and release a watery fluid called secretion.

Glands are classified based on two main factors:

• Site of Product Release:

  • Endocrine Glands: These are "ductless" glands that release their products, usually hormones, directly into your blood or lymph system to be distributed throughout the body. They come in various shapes and sizes and produce different substances.

  • Exocrine Glands: These glands use ducts (small tubes) to release their products onto a body surface or into a body cavity. Examples include sweat, oil, saliva, and mucus glands.

• Number of Cells:

  • Unicellular Glands: These are made of just one cell that performs the glandular function, such as goblet cells that produce mucus.

  • Multicellular Glands: These are made of many cells working together, like your salivary glands.

It is important to know that goblet cells are single cells that produce mucus, while multicellular mucus cells work with club cells to create the mucous layer that lines various tissues.

Extracellular Matrix (ECM)

The extracellular matrix (ECM) is the non-living material that fills the space between cells. Its composition varies greatly depending on the type of tissue. Generally, it consists of a jelly-like ground substance combined with protein fibers.

For example, blood has a thin, watery matrix with very few fibers (until blood clotting occurs, at which point fibers become very numerous and it gels). Epithelium has cells packed extremely tightly with hardly any ECM, whereas connective tissue has fewer cells and a much greater amount of matrix – this is a key characteristic of connective tissue.

Connective Tissue (CT)

Connective tissue is the most common and widespread tissue in your body. It is abundant in the skin but sparse in the brain.

Its main functions are:

• Binding and supporting other tissues

• Protecting organs and structures

• Insulating the body

• Storing energy (e.g., in fat tissue)

• Transporting substances (like blood)

Your body has four main types of connective tissue:

• CT Proper (Connective Tissue Proper)

• Cartilage

• Bone (also called Osseous Tissue)

• Blood

Common Characteristics

All connective tissues share two very important characteristics:

• ECM Dominance: Unlike other tissues, connective tissue is primarily composed of its non-living extracellular matrix. This dominant ECM is what allows CT to withstand weight, resist stretching, and endure significant stress.

• Common Origin: All connective tissues develop from the same embryonic tissue called mesenchyme. This is their shared origin.

Structural Elements

Connective tissue is made up of three key components: ground substance, fibers, and cells.

1. Ground Substance: This is the unstructured, gel-like material that fills the spaces between cells and holds the fibers. It contains interstitial fluid, cell adhesion proteins (which act like glue for cells and fibers), and proteoglycans (a protein core combined with GAGs like hyaluronic acid). These proteoglycans trap water, making the ground substance thick and ideal for allowing nutrients to spread through.

2. Fibers: These provide support and strength to the connective tissue.

   • Collagen fibers: These are the strongest and most common type, providing incredible tensile strength, meaning they are highly resistant to stretching.

   • Elastic fibers: These are long, thin fibers made of elastin, allowing tissues to stretch and then return to their original shape after being deformed.

   • Reticular fibers: These are short, thin, branched collagen fibers that form delicate, net-like structures. They are more flexible than the larger collagen fibers and help support soft organs.

3. Cells: Connective tissues contain various types of cells, each with a specific role:

   • "Blast" cells: These are young, active, dividing cells that produce the ground substance and fibers for their specific tissues:

     • Fibroblasts (found in CT proper)

     • Chondroblasts (found in cartilage)

     • Osteoblasts (found in bone)

     • Hematopoietic stem cells (found in blood)

   • "Cyte" cells: These are the mature, less active versions of the "blast" cells. Their main job is to maintain the health of the matrix:

     • Fibrocytes

     • Chondrocytes

     • Osteocytes

   • Other cells: You will also find other cells in connective tissues:

     • Adipocytes (fat cells) are for storing energy.

     • White Blood Cells (WBCs), such as neutrophils, eosinophils, and lymphocytes, are part of your body's defense system.

     • Mast cells act like tiny alarms, detecting foreign substances and initiating local inflammation by releasing chemicals like heparin (which prevents blood clotting), histamine (which increases blood vessel permeability), and proteases.

     • Macrophages are large cells that engulf (eat) foreign invaders and dead cells.

Connective Tissue Proper

Connective tissue proper is divided into two main categories: loose and dense.

Loose CT: These tissues have fewer fibers and more ground substance, making them softer and more flexible.

• Areolar CT: This is a very common tissue and is loosely arranged. It surrounds and cushions organs, plays a significant role in inflammation, and stores body fluids. It is considered the "classic" CT because it contains all three types of fibers: collagen, elastic, and reticular.

• Adipose CT: This is commonly known as fat tissue! It is primarily composed of fat cells (adipocytes). Its main functions are storing energy, keeping you warm (insulation), and protecting your organs. White fat is the most prevalent type in adults.

• Reticular CT: This tissue forms a delicate internal framework, or stroma, which supports various cell types, including white blood cells, mast cells, and macrophages, in organs such as lymph nodes, the spleen, and bone marrow.

Dense CT: These tissues are packed with fibers, especially collagen, which makes them very strong and resistant to tension.

• Dense Regular CT: In this tissue, collagen fibers are all aligned in parallel. This arrangement gives it incredible tensile strength in one direction. You will find it in places that need strong, unidirectional resistance to pulling forces, such as tendons (connecting muscles to bones or other muscles), ligaments (connecting bones to bones), and aponeuroses (sheet-like tendons).

• Dense Irregular CT: Unlike dense regular tissue, this tissue has collagen bundles arranged in various directions. This allows it to handle tension from multiple directions. It is found in the deep layer of your skin (dermis) and in fibrous joint capsules.

• Elastic CT: This dense connective tissue is rich in elastic fibers, allowing it to stretch and then return to its original shape. This feature is vital for your large arteries to manage blood flow and for your lungs to recoil when you breathe out.

Cartilage

General Features: Cartilage is tough yet flexible, able to withstand both pulling and compressing forces. A notable characteristic is that it is avascular (it has no blood vessels) and aneural (it has no nerves). Chondrocytes (cartilage cells) receive their nutrients by diffusion from the perichondrium, which is a sheath of dense irregular connective tissue surrounding most cartilage. Cartilage cells include chondroblasts (the young cells that create the matrix) and chondrocytes (the mature cells residing in small spaces called lacunae).

Types: There are three main types of cartilage:

• Hyaline Cartilage: This is the most common type. It provides support and acts as a cushion. You can find it at the ends of long bones (articular cartilage), in your ribs (costal cartilages), nose, windpipe (trachea), and voice box (larynx).

• Elastic Cartilage: Similar to hyaline cartilage, but with many more elastic fibers, making it extremely flexible while still maintaining its shape. Look for it in your external ear and epiglottis.

• Fibrocartilage: This type contains thick collagen fibers, giving it excellent tensile strength and shock-absorbing capabilities. It is found in your intervertebral discs, the pubic symphysis, and the menisci of your knee.

Bone (Osseous Tissue)

Bone is a highly vascular (rich in blood vessels) connective tissue. It contains more collagen than cartilage and, importantly, a large amount of inorganic calcium salts that make it very hard. Its functions include supporting your body, protecting organs, enabling movement by muscles, storing minerals (like calcium!) and fat, and producing blood cells (hematopoiesis) in the bone marrow. Bone cells are osteoblasts (cells that form the matrix) and osteocytes (mature maintenance cells located in lacunae). The fundamental structural units of compact bone are osteons.

Blood

Blood is an unusual connective tissue because it is a fluid! Its liquid matrix is called plasma, and it originates from mesenchyme. Its cells include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Unlike other connective tissues, blood's fibers are soluble proteins that only become visible and clump together when your blood clots. Blood's primary role is to transport substances throughout your body, such as oxygen (O2), carbon dioxide (CO2),

nutrients, waste products, and hormones.

Muscle Tissue

Muscle tissue has a rich blood supply and is responsible for all movement in your body. This movement occurs due to special contracting proteins called myofilaments (actin and myosin).

• Skeletal Muscle: This is your "voluntary" muscle, meaning you can consciously control its movements. It is mostly attached to your bones, allowing you to move your body. Skeletal muscle cells are long, cylindrical, contain multiple nuclei, and have a striped appearance (striated).

• Cardiac Muscle: This muscle is found only in the walls of your heart, and it is "involuntary," meaning you do not consciously control it. Its cells are also striated, usually branched, and contain one nucleus. A unique feature is intercalated discs, which are special junctions containing gap junctions (for rapid electrical signal transmission) and desmosomes (for strong cell-to-cell attachment).

• Smooth Muscle: This is another involuntary muscle, characterized by spindle-shaped cells that do not have striations. It forms the walls of most hollow organs in your body, such as your digestive and urinary tracts, blood vessels, and uterus, where it propels substances through internal pathways.

Nervous Tissue

Components: Nervous tissue is primarily composed of two cell types:

• Neurons: These are the main cells that generate and transmit electrical impulses, essentially sending information throughout your body.

• Neuroglia: These are the supporting cells that insulate, protect, and care for the neurons, ensuring they function optimally.

Locations: Nervous tissue is the primary component of your brain, spinal cord, and nerves. Its main function is to manage and control all your bodily activities.

Covering & Lining Membranes

• Cutaneous Membrane: This is what you know as your skin! It covers the outside of your body. It is made of a tough, keratinized stratified squamous epithelium (epidermis) resting on a thick layer of connective tissue (dermis). It is a dry membrane.

• Mucous Membranes (Mucosae): These membranes line all body cavities that are open to the outside world, such as your digestive, respiratory, and urinary tracts. They are typically moist and often produce mucus, which helps protect and lubricate these areas.

• Serous Membranes (Serosae): These are moist membranes found in closed body cavities (like your chest and abdomen). They are composed of a simple squamous epithelium called a mesothelium resting on a thin layer of areolar connective tissue. Serous membranes have two layers: a parietal layer (lining the cavity wall) and a visceral layer (covering the organs inside that cavity). A thin layer of serous fluid between these layers reduces friction.

Tissue Repair

When your body's protective layers are damaged, tissue repair begins, often starting with an inflammatory response. Your body has two main ways to fix injuries:

• Regeneration: This occurs when the damaged tissue is replaced by the exact same type of tissue, allowing it to regain its original function and structure – a perfect repair!

• Fibrosis: This happens when the damaged tissue is replaced by non-functional connective (scar) tissue, meaning it loses its original function – it's a necessary repair, but not ideal. Whether regeneration or fibrosis occurs depends on the type of tissue damaged and how severe the injury is.

Steps of Repair

Tissue repair usually involves three major steps:

1. Step 1: Inflammation: When an injury occurs, damaged cells and local mast cells release chemicals. These chemicals cause blood vessels to widen (vasodilation) and become leakier, allowing fluid, clotting proteins, and immune cells to rush into the injured area. Then, a blood clot forms to stop bleeding and wall off the injury to prevent the spread of harmful substances.

2. Step 2: Organization (Restores Blood Supply): In this phase, the blood clot is replaced by granulation tissue, which is delicate, pink tissue containing new capillaries (restoring blood supply!) and fibroblasts. The fibroblasts begin producing collagen fibers to bridge the gap caused by the injury. Macrophages actively consume debris and dead cells, clearing the area for repair. Simultaneously, the epithelium on top starts to regenerate.

3. Step 3: Regeneration/Fibrosis (Permanent Repair): As healing progresses, the scab falls off, and the regenerated epithelium underneath thickens, completely covering the wound. The fibrous tissue beneath continues to mature and contract. The final outcome is permanent repair: the epithelium is fully regenerated, but the underlying fibrous tissue forms a scar. The visibility of this scar depends on how much fibrosis has taken place.

Regenerative Capacity by Tissue

Different tissues have varying abilities to heal:

• High Regeneration: Epithelia, bone, areolar connective tissue, dense irregular connective tissue, and blood-forming tissue – these tissues are excellent at recovering.

• Moderate Regeneration: Smooth muscle and dense regular connective tissue – they can heal, but it requires more time and effort.

• Poor/None Regeneration: Cardiac muscle and nervous tissue (especially in the brain and spinal cord) – these tissues are very difficult to heal, and scientists are still researching how to improve regeneration in these crucial tissues.