Premed · Premed · General Biology 2
Lecture 15: Animal Form and Function — Tissues
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the four major tissue types in animals and their functions
- Identify the major subtypes of epithelial, connective, muscle, and nervous tissue
- Explain how tissue structure relates to function
- Describe the concept of organ systems and how tissues are organized into organs
- Explain homeostasis and the role of negative and positive feedback mechanisms
- Describe how body size and shape affect metabolic rate and exchange with the environment
Lecture Content
I. Levels of Organization in Animals
The animal body is organized hierarchically: cells of similar structure and function group together to form tissues, tissues combine to form organs, and organs work together as organ systems. A tissue is defined as a group of cells sharing a common structure and function. An organ is a structure composed of two or more tissue types that together perform a specific function -- the stomach, for example, contains epithelial, connective, muscle, and nervous tissue working in concert. An organ system is a group of organs cooperating to perform a broad physiological function. Vertebrates possess 11 major organ systems: integumentary, skeletal, muscular, nervous, endocrine, circulatory, respiratory, digestive, excretory (urinary), reproductive, and immune (lymphatic).
II. Epithelial Tissue
Epithelial tissue covers body surfaces, lines body cavities and organs, and forms glands. It serves diverse functions including protection, absorption, secretion, filtration, excretion, and sensory reception. Epithelial cells are tightly packed with minimal extracellular matrix between them, forming continuous sheets. These sheets are avascular -- lacking their own blood supply -- and receive nutrients by diffusion from the underlying connective tissue. Epithelial tissue has a high regeneration rate, reflecting the wear and tear these exposed surfaces endure. Each epithelial sheet has an apical surface (the free surface exposed to the body exterior or a cavity lumen) and a basal surface (attached to the basement membrane, a thin layer of extracellular matrix). Cell junctions -- tight junctions that seal adjacent cells, desmosomes that anchor them, and gap junctions that allow communication -- hold the epithelial sheet together.
Epithelial tissues are classified by two criteria: the number of cell layers and the shape of the cells. Simple epithelia consist of a single cell layer and function primarily in diffusion, absorption, filtration, and secretion. Stratified epithelia have multiple layers and provide protection in areas subject to abrasion. Pseudostratified epithelium appears multilayered because nuclei sit at different heights, but is actually a single layer. Cell shapes include squamous (flat, scale-like), cuboidal (cube-shaped), and columnar (tall, column-shaped).
Common types include simple squamous epithelium in lung alveoli and blood vessel linings (endothelium), where thinness facilitates diffusion; simple cuboidal epithelium in kidney tubules and glands for secretion and absorption; simple columnar epithelium lining the intestine, often bearing microvilli and interspersed with mucus-secreting goblet cells; stratified squamous epithelium in the skin (keratinized) and mouth/esophagus (non-keratinized) for protection; pseudostratified columnar epithelium in the trachea and upper airways, ciliated and bearing goblet cells that together form the mucociliary escalator; and transitional epithelium in the urinary bladder, which can stretch to accommodate varying volumes. Glandular epithelium forms exocrine glands (which secrete through ducts onto surfaces, such as sweat and salivary glands) and endocrine glands (which are ductless and secrete hormones into the bloodstream, such as the thyroid and pituitary).
<image>A classification grid of epithelial tissue types. The grid has rows for simple, stratified, and pseudostratified layers, and columns for squamous, cuboidal, and columnar shapes. Each cell in the grid contains a labeled diagram of the tissue type in cross-section, showing cell shape, number of layers, nuclei, and the basement membrane. Next to each diagram, the typical location in the body is listed (e.g., simple squamous — lung alveoli; stratified squamous — skin epidermis). Additional insets show pseudostratified columnar epithelium (trachea) with cilia and goblet cells, and transitional epithelium (bladder) in relaxed and stretched states.</image>
III. Connective Tissue
Connective tissue is the most diverse and abundant tissue type in the body. It provides support, protection, binding, insulation, and transport. Unlike epithelial tissue, connective tissue cells are scattered within an extensive extracellular matrix (ECM) composed of a ground substance (which may be gel-like, liquid, or calcified) and protein fibers. Most connective tissues are well vascularized, with the notable exception of cartilage.
Three types of protein fibers contribute to connective tissue properties. Collagen fibers are strong and flexible, resisting stretching, and collagen is the most abundant protein in the human body. Elastic fibers, containing the protein elastin, can stretch and recoil. Reticular fibers are thin, branching collagen fibers that form supportive networks.
The major connective tissue types span a wide functional range. Loose connective tissue (areolar) is the most common type, cushioning and supporting organs and holding tissue fluid. Adipose tissue stores fat for energy, provides insulation, and functions as an endocrine organ. Dense regular connective tissue, with parallel collagen fibers, forms tendons (connecting muscle to bone) and ligaments (connecting bone to bone). Dense irregular connective tissue, with randomly oriented collagen, forms the dermis of the skin and organ capsules. Cartilage is a firm but flexible tissue in which chondrocytes sit in lacunae within the ECM. Hyaline cartilage (the most common type, found in the trachea, nose, and joint surfaces) has a smooth, glassy matrix; elastic cartilage (in the ear and epiglottis) contains abundant elastic fibers; and fibrocartilage (in intervertebral discs and menisci) is reinforced with dense collagen for added toughness. Bone (osseous tissue) has a mineralized matrix of calcium phosphate (hydroxyapatite), with osteocytes housed in lacunae connected by tiny channels called canaliculi. Compact bone is organized into Haversian systems (osteons) with central canals for blood vessels, while spongy bone has a lattice-like structure filled with marrow. Blood is a fluid connective tissue consisting of plasma (the liquid matrix) plus formed elements (red blood cells, white blood cells, and platelets).
IV. Muscle Tissue
Muscle tissue is specialized for contraction, powered by the interaction of actin and myosin filaments. Three types serve distinct roles. Skeletal muscle is striated, voluntary, and composed of long, cylindrical, multinucleated fibers (syncytia) attached to bones. It is responsible for locomotion and body movement, generates fast and powerful contractions, but fatigues with sustained use. Cardiac muscle is striated and involuntary, found exclusively in the heart. Its branching cells, each with a single nucleus, are connected by intercalated discs containing gap junctions that allow electrical impulses to pass rapidly between cells, enabling synchronized contraction. Cardiac muscle is autorhythmic, generating its own contractions through pacemaker cells. Smooth muscle is non-striated and involuntary, consisting of spindle-shaped cells with single nuclei. It lines the walls of hollow organs -- blood vessels, the digestive tract, the uterus, and airways -- producing slow, sustained contractions that resist fatigue. Peristalsis, the wave-like contractions that move food through the digestive tract, is generated by smooth muscle.
<image>A comparison of three muscle tissue types with microscopic views and functional context. Panel A (Skeletal muscle): A long, cylindrical, multinucleated fiber with visible striations (alternating light and dark bands). An inset shows the sarcomere structure with actin and myosin filaments. The tissue is shown attached to a bone via a tendon. Panel B (Cardiac muscle): Branching, striated cells with single central nuclei. Intercalated discs are highlighted at cell junctions, with a magnified inset showing gap junctions and desmosomes. The tissue is shown in context within the heart wall. Panel C (Smooth muscle): Spindle-shaped cells with single central nuclei, no visible striations. The tissue is shown in the wall of the intestine, arranged in circular and longitudinal layers for peristalsis.</image>
V. Nervous Tissue
Nervous tissue is specialized for receiving stimuli and transmitting electrical signals, enabling rapid communication and coordination throughout the body. It is found in the brain, spinal cord, and nerves. The two main cell types are neurons and glial cells. Neurons are the functional units of the nervous system, generating and transmitting electrical impulses called action potentials. Each neuron consists of a cell body (soma) containing the nucleus, dendrites that receive incoming signals, an axon that conducts impulses away from the cell body, and axon terminals (synaptic terminals) that release neurotransmitters at synapses. Many axons are wrapped in a myelin sheath -- an insulating layer formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system -- that dramatically speeds signal transmission.
Glial cells (neuroglia) support, protect, insulate, and nourish neurons. They outnumber neurons and, while they do not transmit impulses themselves, play essential roles in modulating neural activity. The major types include astrocytes (which maintain the blood-brain barrier and provide metabolic support), oligodendrocytes (which form myelin in the CNS), Schwann cells (which form myelin in the PNS), microglia (the immune cells of the CNS), and ependymal cells (which line the brain ventricles and produce cerebrospinal fluid). Nervous tissue is covered in greater depth in Lecture 21.
VI. Homeostasis and Feedback Mechanisms
Homeostasis is the maintenance of a relatively stable internal environment despite fluctuations in external conditions. It is essential for proper enzyme function, cell metabolism, and organ performance. The body regulates numerous variables within narrow ranges, including body temperature, blood pH, blood glucose concentration, blood pressure, and osmolarity.
Negative feedback is the most common regulatory mechanism. A change in a regulated variable is detected by a receptor (sensor), which signals a control center (integrator), which activates an effector to produce a response that opposes and reverses the original change, returning the variable toward its set point. Thermoregulation provides a classic example: when body temperature rises above the set point, the hypothalamus detects the change and activates cooling mechanisms -- sweating and vasodilation of skin blood vessels -- that bring the temperature back down. Blood glucose regulation works similarly: elevated blood glucose stimulates the pancreas to release insulin, which promotes cellular glucose uptake and returns blood glucose to normal levels.
Positive feedback amplifies a change rather than reversing it, pushing a variable further from its starting point until the process reaches completion. Positive feedback is less common than negative feedback and typically drives processes that need to proceed rapidly to completion. During childbirth, for example, oxytocin stimulates uterine contractions, which push the baby against the cervix, stimulating the release of more oxytocin and producing stronger contractions in a self-reinforcing cycle that continues until delivery. Blood clotting follows a similar amplifying cascade.
Animals employ two fundamentally different strategies for thermoregulation. Endotherms (birds and mammals) generate body heat metabolically and maintain a constant internal temperature, but at high energetic cost. Ectotherms (most fish, amphibians, reptiles, and invertebrates) depend on environmental heat sources and regulate their body temperature behaviorally, through basking, burrowing, and other strategies, but at far lower energetic cost.
<image>A diagram illustrating negative feedback in thermoregulation. The center shows a thermometer indicating the normal body temperature set point (37 degrees C). On the left, a pathway for when body temperature rises above set point: thermoreceptors detect increase → hypothalamus (control center) activates cooling effectors → vasodilation of skin blood vessels (blood flow to skin shown with arrows), sweat glands activated (evaporative cooling) → body temperature decreases back to set point. On the right, a pathway for when body temperature drops below set point: thermoreceptors detect decrease → hypothalamus activates warming effectors → vasoconstriction of skin blood vessels, skeletal muscle shivering, thyroid hormone increases metabolic heat → body temperature increases back to set point. Circular arrows show the negative feedback loop returning to the set point.</image>
VII. Body Size, Metabolic Rate, and Exchange Surfaces
The relationship between body size and physiology is governed by a fundamental geometric principle: as an object increases in size, its volume grows faster than its surface area. Small organisms, with their high surface area-to-volume ratio, can exchange gases, nutrients, and wastes by simple diffusion across their body surfaces. Large organisms, with their lower ratio, require specialized exchange surfaces (lungs, gills, intestinal villi) and internal transport systems (circulatory systems) to meet their metabolic demands.
Metabolic rate -- the total energy expenditure per unit time -- increases with body size, but mass-specific metabolic rate (energy per gram of body mass) decreases. A mouse burns far more energy per gram than an elephant. This relationship is formalized in Kleiber's law, which states that metabolic rate scales with body mass raised to the three-quarter power (B = B0 x M^0.75). Endotherms have substantially higher metabolic rates than ectotherms of the same body size, reflecting the energetic cost of maintaining a constant internal temperature.
