Unit 6: Animal Tissues

Introduction

Animal tissues are groups of cells that share a similar structure and work together to perform specific functions. They represent the basic structural and functional units of multicellular animals, enabling specialization and complexity. The four major tissue categories—epithelial, connective, muscular, and nervous—each arise from distinct embryonic layers and exhibit unique characteristics that suit their roles in the body.

Epithelial Tissues

Overview

Epithelial tissues form continuous sheets of cells that cover body surfaces, line cavities, and create glands. Their primary functions include protection, secretion, absorption, sensation, and selective permeability. Epithelial cells are tightly packed with minimal extracellular material, exhibit polarity (apical and basal surfaces), and are anchored to underlying connective tissue by a basement membrane.

Simple Epithelia

Simple epithelia consist of a single layer of cells, allowing efficient diffusion, filtration, or secretion. They are classified according to cell shape:

• Squamous epithelium: Flattened, scale-like cells that provide a low-resistance surface for rapid exchange. Found in the alveoli of lungs, endothelium of blood vessels, and lining of serous cavities.
• Cuboidal epithelium: Cube‑shaped cells with central nuclei, specialized for secretion and absorption. Located in kidney tubules, ducts of glands, and the surface of ovaries.
• Columnar epithelium: Tall, pillar‑like cells often bearing microvilli or cilia. Predominant in the digestive tract (stomach, intestines) where they facilitate absorption and secretion.
• Ciliated epithelium: Columnar or cuboidal cells equipped with motile cilia on the apical surface. Found in the trachea, bronchi, and fallopian tubes, where they move mucus or ova.

Compound (Stratified) Epithelia

Compound epithelia contain multiple layers of cells, offering enhanced protection against abrasion, dehydration, and pathogens. The apical layer determines the functional name.

• Stratified squamous epithelium: The most widespread stratified type; superficial cells are flattened. Keratinized versions (epidermis of skin) provide a waterproof barrier; non‑keratinized forms line the oral cavity, esophagus, and vagina.
• Transitional epithelium (urothelium): A specialized stratified tissue capable of distension and recoil. Cells change shape from cuboidal to squamous as the organ stretches. Found in the urinary bladder, ureters, and part of the urethra.

Glandular Epithelia

Glandular epithelium is modified for secretion. Glands may be unicellular (e.g., goblet cells) or multicellular (exocrine and endocrine glands). Secretion modes include merocrine (exocytosis), apocrine (budding of apical cytoplasm), and holocrine (cell disintegration).

• Exocrine glands: Release products onto epithelial surfaces via ducts (e.g., sweat glands, salivary glands, pancreas).
• Endocrine glands: Ductless; secrete hormones directly into the bloodstream (e.g., thyroid, adrenal medulla).

Specializations of Epithelial Cells

• Microvilli: Microscopic projections that increase surface area for absorption (intestinal epithelium).
• Cilia: Motile structures that move mucus or gametes (respiratory tract, fallopian tubes).
• Keratin: Tough fibrous protein that provides mechanical strength and waterproofing (epidermis).
• Junctional complexes: Tight junctions, adherens junctions, desmosomes, and gap junctions regulate permeability and cell‑cell communication.

Summary Table – Epithelial Tissues

Connective Tissues

Overview

Connective tissues are the most abundant and varied group, characterized by cells dispersed within an extensive extracellular matrix (ECM). The ECM comprises protein fibers (collagen, elastic, reticular) and ground substance (glycosaminoglycans, proteoglycans, water). Functions include structural support, connection of tissues, protection, transport, storage, and immune defense.

Types of Connective Tissues

Loose (Areolar) Connective Tissue

Consists of a loose arrangement of collagen and elastic fibers within a gel‑like ground substance, containing fibroblasts, macrophages, mast cells, and plasma cells. It provides flexibility, cushioning, and a medium for nutrient and waste exchange.

Location: Underneath epithelia (lamina propria), around blood vessels, nerves, and organs; fills spaces between muscle fibers.

Adipose Tissue

Specialized for lipid storage; cells (adipocytes) contain a large central lipid droplet that pushes the nucleus to the periphery. Besides energy storage, adipose tissue provides thermal insulation and acts as an endocrine organ (secreting leptin, adiponectin).

Location: Subcutaneous layer (hypodermis), around kidneys and heart, within bone marrow (yellow marrow), and in mesenteries.

Dense Regular Connective Tissue

Collagen fibers are densely packed and aligned parallel to the direction of tensile stress, providing great strength in one plane. Fibroblasts are arranged in rows between the fibers.

Location: Tendons (muscle‑to‑bone), ligaments (bone‑to‑bone), aponeuroses.

Dense Irregular Connective Tissue

Collagen fibers are thick but arranged randomly, offering resistance to stress from multiple directions. Found in areas where tension is variable.

Location: Dermis of skin, fibrous capsules of organs (liver, kidneys), submucosa of the digestive tract.

Cartilage

A firm yet flexible avascular tissue composed of chondrocytes residing in lacunae within a matrix rich in collagen and proteoglycans. Three main types:

• Hyaline cartilage: Most abundant; glassy appearance due to fine collagen fibers. Provides smooth surfaces for joint movement and supports fetal skeleton.
• Elastic cartilage: Contains abundant elastic fibers, granting flexibility and resilience.
• Fibrocartilage: Dense collagen fibers make it tough and able to withstand heavy compressive forces.

Locations: Hyaline – tracheal rings, nasal septum, articular surfaces, fetal skeleton; Elastic – external ear (pinna), epiglottis; Fibrocartilage – intervertebral discs, pubic symphysis, menisci of knee.

Bone (Osseous Tissue)

The hardest connective tissue; its matrix is calcified with hydroxyapatite crystals, providing compressive strength. Bone is organized into microscopic units called osteons (Haversian systems) containing concentric lamellae around a central canal housing blood vessels and nerves.

Functions: Structural support, protection of vital organs, lever for movement, mineral storage (Ca²⁺, PO₄³⁻), hematopoiesis (red marrow).

Location: Forms the entire skeleton.

Blood

A fluid connective tissue composed of plasma (water, ions, proteins, hormones, nutrients) and formed elements: erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets). It transports gases, nutrients, waste, hormones, and mediates immunity and clotting.

Lymph

Similar to plasma but with lower protein concentration and lacking erythrocytes. Contains lymphocytes and macrophages; transports interstitial fluid back to the bloodstream and participates in immune surveillance.

Summary Table – Connective Tissues

Muscular Tissues

Overview

Muscular tissues are specialized for contraction, enabling movement, maintenance of posture, and generation of heat. They consist of elongated cells (muscle fibers) containing contractile proteins actin and myosin organized into sarcomeres. Based on control, appearance, and location, three types are recognized: skeletal, cardiac, and smooth.

Skeletal Muscle

Skeletal muscle fibers are long, cylindrical, multinucleated cells with prominent striations due to regular arrangement of sarcomeres. Contractions are voluntary and rapid, powered by somatic motor neurons.

Key Features: Multiple peripheral nuclei, abundant mitochondria, sarcoplasmic reticulum for Ca²⁺ storage, tightly packed myofibrils.

Location: Attached to bones via tendons; also found in the tongue, upper esophagus, and diaphragm (voluntary control of breathing).

Function: Locomotion, facial expression, posture, breathing, heat production.

Cardiac Muscle

Cardiac muscle cells (cardiomyocytes) are shorter, branched, and usually contain a single central nucleus. They are striated like skeletal muscle but are involuntary. Intercalated discs—specialized junctions containing gap junctions and desmosomes—allow rapid electrical coupling and mechanical adhesion.

Key Features: Single nucleus, abundant mitochondria, numerous intercalated discs, well‑developed T‑tubules.

Location: Exclusively in the walls of the heart (atria and ventricles).

Function: Rhythmic, coordinated pumping of blood.

Smooth Muscle

Smooth muscle cells are spindle‑shaped, with a single central nucleus, and lack visible striations because actin and myosin are not organized into regular sarcomeres. Contractions are involuntary, slow, and can be sustained for long periods.

Key Features: Single nucleus, dense bodies anchoring actin filaments, caveolae for Ca²⁺ entry.

Location: Walls of hollow organs (gastrointestinal tract, urinary bladder, uterus, blood vessels), arrector pili muscles of hair follicles, iris, and ciliary body of the eye.

Function: Propulsion of substances (peristalsis), regulation of vessel diameter (vasoconstriction/dilation), pupillary size, piloerection.

Summary Table – Muscular Tissues

Nervous Tissues

Overview

Nervous tissue is responsible for receiving stimuli, generating electrical impulses, conducting signals, and integrating information to coordinate bodily activities. It consists of two principal cell types: neurons (excitable cells that transmit impulses) and neuroglia (supporting cells that maintain homeostasis, provide insulation, and protect neurons).

Neuron Structure

A typical neuron comprises:

• Cell body (soma): Contains the nucleus, Nissl bodies (rough ER), mitochondria, and Golgi apparatus; site of metabolic activity.
• Dendrites: Branched extensions that receive incoming signals from other neurons or sensory receptors; increase surface area for synaptic contacts.
• Axon: A single, usually long projection that conducts action potentials away from the soma toward target cells; may be myelinated.
• Myelin sheath: Lipid‑rich insulation formed by oligodendrocytes (CNS) or Schwann cells (PNS); increases conduction speed.
• Nodes of Ranvier: Gaps between myelin segments where voltage‑gated Na⁺ channels are concentrated; enable saltatory conduction.
• Axon terminal (synaptic bouton): Contains vesicles filled with neurotransmitters; releases chemicals into the synaptic cleft.
• Synapse: Junction between the axon terminal of a presynaptic neuron and the dendrite or soma of a postsynaptic cell; transmission can be electrical or chemical.

Types of Neurons (by Function)

• Sensory (afferent) neurons: Detect stimuli (mechanical, thermal, chemical) and transmit impulses toward the central nervous system (CNS). Their dendrites are often specialized as receptors.
• Motor (efferent) neurons: Carry commands from the CNS to effector organs (muscles or glands). Their axons form motor nerves.
• Interneurons (association neurons): Located entirely within the CNS; connect sensory and motor neurons, facilitating integration and reflexes.

Neuroglia (Supporting Cells)

Neuroglia outnumber neurons in the CNS and perform vital supportive roles:

• Astrocytes: Star‑shaped cells that regulate extracellular ion concentration, uptake neurotransmitters, provide metabolic support to neurons, and contribute to the blood‑brain barrier.
• Oligodendrocytes: Produce myelin sheaths around axons in the CNS.
• Microglia: Phagocytic cells that act as the CNS immune response, removing debris and pathogens.
• Ependymal cells: Line the ventricles of the brain and central canal of the spinal cord; assist in cerebrospinal fluid circulation.
• Schwann cells (PNS): Myelinate peripheral axons; also involved in nerve regeneration.
• Satellite cells (PNS): Surround neuronal cell bodies in ganglia, providing structural support and regulating the microenvironment.

Signal Transmission – Action Potential

The resting membrane potential of a neuron is about –70 mV. Upon stimulation, voltage‑gated Na⁺ channels open, causing rapid depolarization to approximately +40 mV. Subsequently, K⁺ channels open, leading to repolarization and a brief after‑hyperpolarization. The impulse propagates along the axon; in myelinated fibers, the jump from node to node (saltatory conduction) increases speed to up to 120 m/s.

Simplified equation for the Nernst potential of an ion:

Eion = (RT / zF) · ln([ion]out / [ion]in)

where R = universal gas constant (8.314 J·mol⁻¹·K⁻¹), T = absolute temperature (K), z = ion valence, F = Faraday constant (96 485 C·mol⁻¹).

Summary Table – Nervous Tissue Components

Integrated Perspective

Although each tissue type has distinct structural and functional characteristics, they constantly interact to maintain homeostasis. For example:

• Epithelial linings of the respiratory tract are supported by underlying loose connective tissue, which supplies nutrients and immune cells.
• Skeletal muscle contractions are coordinated by motor neurons; the connective tissue tendons transmit the generated force to bones.
• Cardiac muscle relies on a rich capillary network (connective tissue) for oxygen delivery, while cardiac neurons modulate heart rate.
• Glandular epithelia secrete hormones or enzymes that are transported via blood (connective tissue) to target organs.

Understanding these interrelationships provides a foundation for studying physiology, pathology, and medical disciplines.

Diagrams are indicated as placeholders; in a complete textbook they would be replaced with detailed illustrations of each tissue type.