Animal Tissue
Easy Overview
Your body has four basic types of tissue. That is it. Just four. But from those four, we get everything — skin, bones, blood, brain, muscles, and organs. This chapter is about those four tissue types and how each one is built for its specific job. A tissue is a group of similar cells that work together to perform a specific function. The four primary tissues are: epithelial tissue (covers surfaces and lines cavities), connective tissue (supports and connects), muscular tissue (enables movement), and nervous tissue (coordinates and controls). These combine to form organs, and organs work together in organ systems. Epithelial tissue covers the outside of the body and lines internal organs and cavities. It acts as a barrier, protects underlying tissues, absorbs substances (in the intestine), secretes products (glands), and filters (in the kidneys). Epithelial cells are tightly packed with little intercellular space and rest on a basement membrane. They have no blood supply of their own (avascular) — they receive nutrients by diffusion from underlying connective tissue. Epithelial tissue is classified by cell shape — squamous (flat, like floor tiles), cuboidal (cube-shaped), and columnar (tall, like columns) — and by the number of layers — simple (one layer, for absorption and filtration), stratified (multiple layers, for protection), and pseudostratified (appears layered but is actually one layer). Connective tissue is the most abundant and diverse tissue type. Its cells are scattered within an extracellular matrix (protein fibers plus ground substance) that they secrete. The matrix determines the tissue's properties. Loose connective tissue (areolar and adipose) fills spaces and stores fat. Dense connective tissue (tendons and ligaments) provides strong attachments. Cartilage provides flexible support. Bone provides rigid support. Blood is a fluid connective tissue that transports everything around the body. Muscular tissue is specialized for contraction. Skeletal muscle moves your skeleton (voluntary, striated). Smooth muscle moves your internal organs (involuntary, non-striated). Cardiac muscle pumps your heart (involuntary, striated, with intercalated discs). Nervous tissue consists of neurons (nerve cells that conduct electrical impulses) and neuroglial cells (supporting cells that outnumber neurons 10:1). A neuron has a cell body with dendrites that receive signals and an axon that sends signals. Neuroglial cells include astrocytes (support and blood-brain barrier), oligodendrocytes and Schwann cells (produce myelin insulation), microglia (immune defense), and ependymal cells (produce cerebrospinal fluid). Understanding these four tissue types is like understanding the basic building materials of the body — once you know what each tissue can do, you can understand how every organ works.
Epithelial Tissue — Cover and Line
Epithelial tissue covers the body surfaces, lines body cavities and hollow organs, and forms glands. It is the body's first line of defense. Characteristics: cells are tightly packed with very little extracellular matrix between them; they rest on a basement membrane (a thin, fibrous layer secreted by the epithelial cells and underlying connective tissue); they are avascular (no blood vessels — receive nutrients by diffusion from below); they have a high regenerative capacity (rapid cell division to replace damaged cells — this is why skin heals quickly). Functions include protection (skin epidermis protects against pathogens, chemicals, physical damage), absorption (intestinal epithelium absorbs nutrients), secretion (glands secrete mucus, enzymes, hormones, sweat), filtration (kidney epithelium filters blood), and diffusion (lung alveolar epithelium allows gas exchange). Epithelial tissue is classified by the number of cell layers and the shape of the cells at the free surface. Simple epithelium has one layer — functions in absorption, secretion, and filtration. Stratified epithelium has multiple layers — functions in protection. Pseudostratified epithelium appears to have multiple layers but is actually one layer with nuclei at different levels. Cell shapes: squamous (flat, scale-like), cuboidal (cube-shaped, as tall as wide), columnar (tall, taller than wide).
Simple Epithelium
Simple squamous epithelium: a single layer of flat, thin cells with oval or disc-shaped nuclei. The cells look like irregular floor tiles from above. Thinness allows rapid diffusion and filtration. Location: lining of blood vessels (endothelium — allows exchange between blood and tissues), lining of body cavities (mesothelium — secretes lubricating fluid), lung alveoli (gas exchange between air and blood). Simple cuboidal epithelium: a single layer of cube-shaped cells with central, spherical nuclei. Functions in secretion and absorption. Location: kidney tubules (reabsorption of water and ions), thyroid gland follicles (secrete thyroid hormone), salivary gland ducts, covering of ovaries. Simple columnar epithelium: a single layer of tall, column-like cells with oval nuclei near the base. Functions in absorption and secretion. Location: lining of the stomach and intestines (absorbs nutrients, secretes digestive enzymes and mucus). Many have microvilli on their free surface — tiny finger-like projections that increase surface area for absorption (called a brush border in the intestine). Some have cilia that beat to move substances along the surface — found in the female reproductive tract (moves egg toward uterus) and respiratory tract (moves mucus upward). Goblet cells are specialized columnar epithelial cells that secrete mucus — found scattered among columnar cells in the intestine and respiratory tract.
Stratified Epithelium
Stratified epithelium has multiple layers of cells, providing protection against wear and tear, pathogens, and chemical damage. The cell shape in the outermost layer is used for naming. Stratified squamous epithelium: multiple layers with flat, scale-like cells at the surface. The deeper layers are cuboidal or columnar, and cells become flatter as they reach the surface. Keratinized type: found on the skin (epidermis). The surface cells fill with keratin (a tough, waterproof protein), die, and form a protective, waterproof layer that sloughs off gradually. This is why you lose about 30,000-40,000 skin cells per minute. Non-keratinized type: found on moist surfaces — mouth, esophagus, vagina, tongue. The surface cells remain alive and moist (not waterproof). Functions: protection against abrasion (chewing, swallowing), pathogens, and water loss. Stratified cuboidal epithelium: rare, two or more layers of cube-shaped cells. Found in the ducts of sweat glands, salivary glands, and mammary glands. Functions: protection and secretion. Stratified columnar epithelium: rare, superficial cells are columnar, deeper cells are more cuboidal. Found in the conjunctiva of the eye, parts of the pharynx, and large ducts of some glands. Transitional epithelium: specialized stratified epithelium found only in the urinary tract (urinary bladder, ureters, urethra). The cells can change shape — when the bladder is empty, the surface cells are large, rounded (dome-shaped); when the bladder is full, the cells stretch and become flat, allowing the bladder to expand. This epithelium is impermeable to urine.
Glandular Epithelium
Glandular epithelium consists of epithelial cells specialized for secretion. Glands are classified based on where they release their products. Exocrine glands secrete their products through ducts onto a body surface or into a body cavity. Examples: sweat glands (secrete sweat onto skin), salivary glands (secrete saliva into mouth), mammary glands (secrete milk), gastric glands (secrete gastric juice into stomach), sebaceous glands (secrete oil onto skin and hair). Exocrine glands can be unicellular (goblet cells — single cells that secrete mucus) or multicellular (composed of many cells). Multicellular exocrine glands are classified by duct structure: simple (unbranched duct — gastric glands) or compound (branched duct — salivary glands, pancreas). They are classified by secretory portion shape: tubular (tube-shaped — intestinal glands), acinar/alveolar (flask-shaped — sebaceous glands), or tubulo-acinar (both tubes and sacs — salivary glands). By mode of secretion: merocrine (exocytosis — the cell releases product by vesicle fusion, cell remains intact — salivary glands, pancreatic acini), apocrine (the apical part of the cell pinches off with the product — mammary glands, some sweat glands), and holocrine (the entire cell disintegrates to release its product — sebaceous glands in the skin, where cells fill with sebum and burst). Endocrine glands are ductless — they secrete hormones directly into the blood or interstitial fluid. Examples: pituitary, thyroid, adrenal, parathyroid, pineal glands, and pancreatic islets. The hormones travel through the bloodstream to target organs.
Connective Tissue — Support and Connect
Connective tissue is the most abundant and widely distributed tissue type in the body. Its defining feature is that its cells are scattered within an abundant extracellular matrix (ECM) that they produce and secrete. The matrix consists of protein fibers embedded in a ground substance (a gel-like or fluid material). The nature of the matrix determines the tissue's properties — from liquid (blood) to semi-solid (cartilage) to solid (bone). Connective tissue functions: support (bone, cartilage), binding and connecting (tendons connect muscle to bone, ligaments connect bone to bone), transport (blood carries gases, nutrients, wastes, hormones), protection (bone protects brain, ribcage protects heart and lungs; fat pads cushion kidneys and eyes), insulation (adipose tissue retains heat and stores energy), and defense (macrophages and white blood cells fight infections). Cell types in connective tissue: fibroblasts (most common — produce fibers and ground substance), chondroblasts (in cartilage — produce cartilage matrix), osteoblasts (in bone — produce bone matrix), adipocytes (fat cells), mast cells (release histamine and heparin — involved in inflammation and allergic reactions), macrophages (phagocytic — engulf pathogens and debris), and various white blood cells. Fiber types: collagen fibers (strong, flexible, resistant to stretching — the most abundant protein in the body, made of collagen), elastic fibers (stretch and recoil — made of elastin, found in lungs, skin, blood vessel walls), and reticular fibers (thin, branching, form a supportive meshwork — in lymph nodes, spleen, bone marrow).
Loose Connective Tissue
Loose connective tissue has a loosely arranged network of fibers with abundant ground substance and many cells. It is the 'packing material' of the body — filling spaces between organs and supporting structures. Areolar tissue is the most widely distributed connective tissue. It contains all three fiber types (collagen, elastic, reticular) in a random, loose arrangement. The ground substance is semi-fluid and rich in hyaluronic acid and proteoglycans. Cell types: fibroblasts (most common), macrophages, mast cells, plasma cells (produce antibodies), and some white blood cells. Location: beneath the skin (subcutaneous layer), between muscles, around blood vessels and nerves, in the stroma of organs. Functions: filling spaces, providing a reservoir for water and salts (important in edema), and acting as a binding medium. Adipose tissue (fat) consists primarily of adipocytes — large cells filled with a single fat droplet (triglyceride) that pushes the nucleus to the edge (signet-ring appearance). Two types: white adipose tissue (most common — stores energy, provides insulation and cushioning) and brown adipose tissue (contains many mitochondria and smaller fat droplets — generates heat, especially in newborns and hibernating animals, has more blood vessels and gives a brown color). Location: under the skin (subcutaneous fat), around internal organs (visceral fat — cushions kidneys, heart, eyes), in bone marrow (yellow marrow). Functions: energy storage, thermal insulation, mechanical cushioning. Reticular tissue: a network of reticular fibers and fibroblasts. Location: lymph nodes, spleen, bone marrow, liver (forms the supporting framework — stroma — of these organs). Helps trap and filter foreign particles and provide a scaffold for developing blood cells.
Dense Connective Tissue
Dense connective tissue has densely packed collagen fibers with little ground substance, providing great strength and resistance to tearing. Dense regular connective tissue has collagen fibers arranged in parallel bundles, with fibroblasts aligned between them. This provides maximum strength in one direction (the direction of fiber alignment). Location: tendons (connect muscle to bone — resist tension along the axis), ligaments (connect bone to bone — slightly more elastic than tendons, contain more elastic fibers), and aponeuroses (flat, sheet-like tendons). Dense irregular connective tissue has collagen fibers arranged in a random, interwoven network. This allows it to resist tension from all directions. Location: dermis of the skin (the layer beneath the epidermis — gives skin strength and elasticity), joint capsules, periosteum (covering of bone), perichondrium (covering of cartilage), organ capsules (covering of liver, spleen, kidneys), and the fibrous layer of body membranes. In addition to fibroblasts, dense connective tissue contains fewer cells than loose connective tissue and has less ground substance. It has a limited blood supply, which is why tendon and ligament injuries heal slowly. Elastic connective tissue is a specialized dense connective tissue with a high proportion of elastic fibers. Location: walls of large arteries (aorta — allows them to stretch and recoil with each heartbeat), lungs, trachea, and vocal cords. Allows these organs to stretch and return to their original shape.
Specialized Connective Tissue — Cartilage
Cartilage is a semi-rigid, flexible connective tissue. Its cells (chondrocytes) are located in small cavities called lacunae, surrounded by a firm but pliable matrix made of proteoglycans, hyaluronic acid, and collagen fibers (and sometimes elastic fibers). Cartilage is avascular — no blood vessels — so it receives nutrients by diffusion from the surrounding perichondrium (a layer of dense connective tissue that covers most cartilage). This is why cartilage heals slowly when damaged. Three types. Hyaline cartilage: the most common type — its matrix appears glassy (hyaline = glassy) and contains fine collagen fibers that are not visible. Location: at the ends of long bones (articular cartilage — reduces friction in joints), nose, trachea, larynx, bronchi, costal cartilages (connecting ribs to the sternum), and the developing fetal skeleton (most of the fetal skeleton is hyaline cartilage that is later replaced by bone). Functions: provides smooth surfaces for joint movement, flexibility, and support. Fibrocartilage: contains thick, visible collagen fibers in a dense matrix. It is the toughest type of cartilage and can withstand heavy compression and tension. Location: intervertebral discs (shock absorbers between vertebrae), pubic symphysis (joint between the two pubic bones), knee menisci (wedge-like pads), and the temporomandibular joint (jaw joint). Functions: resists compression, absorbs shock, provides strength. Elastic cartilage: contains numerous elastic fibers in its matrix, giving it great flexibility and resilience. Location: external ear (pinna — allows bending without damage), epiglottis (the flap that covers the trachea when swallowing), and Eustachian tubes. Functions: provides strength with flexibility — can bend and spring back to shape.
Specialized Connective Tissue — Bone
Bone (osseous tissue) is the hardest connective tissue. Its matrix is impregnated with mineral salts — mainly calcium phosphate (hydroxyapatite crystals) — which give bone its hardness, and collagen fibers, which give it some flexibility (preventing brittleness). Bone cells (osteocytes) are located in lacunae and are arranged in concentric rings (lamellae) around central canals (Haversian canals) that contain blood vessels, nerves, and lymph. This basic structural unit is called an osteon (Haversian system). Tiny channels called canaliculi connect the lacunae, allowing nutrients, oxygen, and waste to be exchanged between the bone cells and the blood vessels. Two types of bone tissue: compact bone (cortical bone) — dense, solid outer layer that provides strength and support. It forms the shaft of long bones and the outer layer of all bones. Spongy bone (cancellous/trabecular bone) — porous, honeycomb-like inner layer with a network of trabeculae (thin bone struts). The spaces contain red bone marrow (where blood cells are produced). Spongy bone reduces the weight of the skeleton while still providing strength. The outer surface of bone is covered by the periosteum (a fibrous membrane with bone-forming cells — osteoblasts), and the inner surface is lined by the endosteum. Functions of bone: structural support (the skeleton holds the body upright), protection (skull protects brain, ribcage protects heart and lungs, vertebrae protect spinal cord), movement (bones act as levers for muscles to pull on), mineral storage (calcium and phosphorus — can be released into blood when needed), and blood cell production (hematopoiesis in red bone marrow).
Specialized Connective Tissue — Blood
Blood is a fluid connective tissue with a liquid extracellular matrix called plasma and formed elements (cells and cell fragments). Plasma (~55% of blood volume) is about 90% water and contains plasma proteins (albumin — maintains osmotic balance and blood pressure, globulins — antibodies and transport proteins, fibrinogen — blood clotting factor), nutrients (glucose, amino acids, lipids), gases (O₂, CO₂), hormones, electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻), and waste products (urea, creatinine, uric acid). Formed elements (~45% of blood volume): Red blood cells (erythrocytes) are biconcave discs (doughnut-shaped but without a hole) that lack a nucleus in mammals. They contain hemoglobin (the iron-containing protein that binds oxygen — gives blood its red color). Lifespan: about 120 days. Produced in red bone marrow. About 5 million per mm³ of blood (in men) and 4.5 million in women. Function: transport O₂ from lungs to tissues and CO₂ from tissues to lungs. White blood cells (leukocytes) are nucleated, fewer in number (4,000-11,000 per mm³), and function in immune defense. Types: neutrophils (most abundant, 60-70% — phagocytic, engulf bacteria), lymphocytes (20-25% — B cells produce antibodies, T cells kill infected cells), monocytes (3-8% — become macrophages that phagocytose debris and pathogens), eosinophils (2-4% — kill parasites, involved in allergic reactions), basophils (less than 1% — release histamine and heparin in inflammation). Platelets (thrombocytes) are small cell fragments (from megakaryocytes in bone marrow) involved in blood clotting. About 150,000-400,000 per mm³. When a blood vessel is damaged, platelets aggregate at the site and release chemicals that initiate the clotting cascade, ultimately forming a fibrin clot that stops bleeding.
Muscular Tissue
Muscular tissue is specialized for contraction, generating force and producing movement. It is excitable (responds to stimuli) and contractile (shortens when stimulated). The three types differ in structure, location, and control. Skeletal muscle fibers are long, cylindrical, unbranched, and multinucleated (nuclei are located at the periphery). They show alternating light and dark bands — this striation is due to the regular arrangement of actin and myosin filaments into repeating units called sarcomeres. Skeletal muscle is under voluntary control (conscious control) via the somatic nervous system. It is attached to bones by tendons. Functions: body movement (walking, running), maintaining posture, generating heat (shivering), and protecting internal organs. Skeletal muscle fibers can be very long — some are up to 30 cm long. They are fatigue-prone but can generate powerful contractions. Smooth muscle fibers are spindle-shaped (tapered at both ends), uninucleated, and non-striated (no visible bands because actin and myosin are not arranged in regular sarcomeres). They are under involuntary control (unconscious control) via the autonomic nervous system, hormones, and local factors. Location: walls of hollow organs — blood vessels (regulate blood pressure), stomach, intestines (peristalsis — rhythmic contractions that move food), bladder (urination), uterus (childbirth), airways (bronchioles — regulate airflow). Smooth muscle contracts slowly and can maintain contraction for long periods without fatigue. Cardiac muscle fibers are branched, uninucleated, striated, and connected by specialized junctions called intercalated discs. These discs contain gap junctions (allow electrical signals to pass rapidly from one cell to another, coordinating contraction) and desmosomes (hold cells together during forceful contractions). Cardiac muscle is under involuntary control — it is self-excitable (pacemaker cells generate rhythmic contractions without nerve stimulation, though nerves can modify the rate). Location: only in the heart. Function: pumps blood throughout the body. Cardiac muscle is highly resistant to fatigue — it contracts about 3 billion times in an average lifetime without stopping.
Nervous Tissue
Nervous tissue is specialized for rapid communication through electrical and chemical signaling. It consists of two main cell types. Neurons are the functional units — they conduct electrical impulses. A typical neuron has a cell body (soma) containing the nucleus and organelles; dendrites (short, highly branched processes) that receive signals from other neurons or sensory receptors and carry them toward the cell body; and an axon (a single, long process) that conducts signals away from the cell body to other neurons, muscles, or glands. The axon may be wrapped in a myelin sheath — a fatty, insulating layer produced by Schwann cells (in the peripheral nervous system, PNS) or oligodendrocytes (in the central nervous system, CNS). Myelin speeds up signal conduction through saltatory conduction (the impulse jumps from one node of Ranvier — a gap in the myelin — to the next). The axon ends in synaptic terminals (boutons) that release neurotransmitters (chemical messengers) to communicate with the next cell across a synapse (the gap between neurons). The synapse can be between a neuron and another neuron, a muscle cell (neuromuscular junction), or a gland. Neurons are classified by function: sensory neurons (afferent — carry signals from sensory receptors to the CNS), motor neurons (efferent — carry signals from the CNS to muscles or glands), and interneurons (connect sensory and motor neurons within the CNS — most numerous, about 99% of all neurons). Neuroglial cells (glial cells) are supporting cells that outnumber neurons by about 10:1. They do not conduct impulses but provide support, nourishment, insulation, and defense. Types: astrocytes (star-shaped — support neurons, maintain the blood-brain barrier, regulate the chemical environment), oligodendrocytes (produce myelin in the CNS — each oligodendrocyte can myelinate several axons), Schwann cells (produce myelin in the PNS — each Schwann cell myelinates one segment of one axon), microglia (immune cells — phagocytose pathogens and debris), and ependymal cells (line the ventricles of the brain and central canal of the spinal cord — produce and circulate cerebrospinal fluid). Gray matter in the CNS contains neuron cell bodies, dendrites, and unmyelinated axons. White matter contains myelinated axons (the myelin gives it a whitish color).
Key Points
- •Four basic tissues: epithelial, connective, muscular, nervous.
- •Epithelial: covers surfaces, protects, absorbs, secretes. Avascular, on basement membrane.
- •Simple epithelium: one layer (squamous, cuboidal, columnar) for diffusion, absorption, secretion.
- •Stratified epithelium: multiple layers for protection (skin epidermis, mouth lining).
- •Glandular epithelium: exocrine (ducts) or endocrine (ductless, hormones into blood).
- •Connective tissue: cells scattered in matrix (fibers + ground substance). Most abundant tissue.
- •Loose connective: areolar (filler, reservoir), adipose (fat storage, insulation, cushioning).
- •Dense connective: regular (tendons, ligaments) and irregular (dermis, capsules).
- •Cartilage: semi-rigid, avascular, chondrocytes in lacunae. Types: hyaline, fibrocartilage, elastic.
- •Bone: mineralized with calcium phosphate. Compact (osteons) and spongy (trabeculae with marrow).
- •Blood: fluid connective tissue with plasma and formed elements (RBCs, WBCs, platelets).
- •Skeletal muscle: voluntary, striated, multinucleated. For body movement.
- •Smooth muscle: involuntary, non-striated, uninucleated. In hollow organs.
- •Cardiac muscle: involuntary, striated, intercalated discs. Only in heart.
- •Neurons: cell body, dendrites (receive), axon (send). Myelination speeds impulses.
- •Neuroglial cells: support, nourish, protect neurons (astrocytes, oligodendrocytes, Schwann cells, microglia).
Practice Questions
- What are the four types of animal tissue? Give one function and one location for each.
- Classify epithelial tissue based on shape and number of layers with examples.
- Differentiate between skeletal, smooth, and cardiac muscle with at least six points.
- Explain the structure of a neuron. What is the function of the myelin sheath?
- Why is blood considered a connective tissue? Describe the components of blood and their functions.
- Differentiate between cartilage and bone. Describe the three types of cartilage with locations.
- Describe the structure and function of the three types of connective tissue fibers.
- Differentiate between exocrine and endocrine glands with examples.