Animal Epithelial Coverings

Epithelial Coverings

• Cover external and internal animal body surfaces
• Structure and functions are specifically adapted to the animal's environment and way of life.
• Contain receptors that receive sensory input from the environment
• Aid in functions, including gas exchange, waste excretion, temperature regulation and secretion

Invertebrate Epithelium

• Secretes protective or supportive layers of nonliving material
• Insects feature a cuticle
• Corals and mollusks construct a shell of calcium carbonate
• May be specialized as sensory structures sensitive to light, chemical stimuli, and mechanical stimuli
• Specialized for absorption, gas exchange or secretion
• This includes lubricants or adhesives, mucus in earthworms that reduces friction and helps gas exchange.
• Also includes odorous secretions for communication
• Also includes poisons for offense and defense

Vertebrate Skin

• Vertebrates have a complex epithelial system referred to as skin, which is the integumentary system
• Varies considerably between species
• Provides strength and protection
• Is elastic
• May have scales for armor, as seen in fishes and reptiles
• May be covered with protective mucus, such as on amphibians, helping gas exchange and lubrication
• May produce hair in mammals and feathers in birds, to protect them and provide insulation
• May be colored, and derivatives may be colored, to aid in behavioral display and communication

Mammalian Skin

• Structures in mammals derived from skin: claws, fingernails, toenails in primates, hair, sweat glands, oil glands, horns, and sensory structures
• Sensory structures include pressure, temperature, and pain receptors
• Female mammals have mammary glands that secrete milk

Human Skin

• Oil glands empty into the hair follicle, which generates and holds hairs
• Sebaceous glands release sebum (mix of waxes and fats)
• Sebum keeps hair moist and pliable, protects skin from water loss, and inhibits bacterial growth,
• Epidermis is the outer, waterproof, protective barrier of skin in all vertebrates
• Epidermis produces keratin, an elaborated coiled protein that is insoluble and provides mechanical strength and water loss reduction
• Deepest layer is stratum basale
• Stratum basale pigment cells known as melanocytes produce melanin giving color to the skin
• New cells divide, mature, and are pushed outward
• Most superficial layer is stratum corneum; epidermal cells die while moving through, and then wear off

Dermis

• Beneath the epidermis is the dermis
• Dermis is a dense fibrous connective tissue mainly made of collagen, providing strength and flexibility
• Contains blood vessels and sensory receptors
• Embeds hair follicles and sweat glands (and melanocytes)
• Rests on a layer of subcutaneous tissue mainly made of adipose tissue, insulating the body
• In humans, exposure to ultraviolet radiation causes the epidermis to thicken and stimulates melanin production
• Skin becomes inflamed or sunburned when melanin can't absorb all of the ultraviolet rays
• Dark-skinned people have more melanin
• Excessive ultraviolet exposure can lead to malignant melanoma

Skeletal Systems

• Provide body support
• Protect internal organs
• Involved in locomotory activity, acting on hard structures, such as chitin or bone
• Store calcium and maintain blood calcium homeostasis in vertebrates
• Three main skeletal system types: hydrostatic skeleton, exoskeleton, and endoskeleton

The Hydrostatic Skeleton

• Body fluids transmit force; found in soft-bodied invertebrates
• Fluid-filled body compartments are found in cnidarians, flatworms, annelids, roundworms
• Operates like a balloon full of water
• Muscles in the wall contract, causing shape changes
• Two antagonistic groups of muscles
• Longitudinal epidermal muscles cause shortening while circular gastrodermal muscles cause lengthening
• Delicate movements are hard

Hydrostatic Skeleton - Earthworms

• Earthworms also use hydrostatic skeletons
• The earthworm body has over 100 segments separated by septa
• Each septum isolates a portion of the body cavity and the coelomic fluid they contain
• Contractions of one region have little effect on another
• Waves of muscular contraction cause complex crawling motions

Exoskeletons

• Exoskeleton is a lifeless shell deposited atop the outer epithelial covering
• Mollusks form a shell (calcium carbonate exoskeleton) secreted by the mantle
• Many can retreat into the shell for protection
• Arthropods produce a tough exoskeleton that is a non-living cuticle containing chitin
• Armor-like, continuous one-piece covering that varies in thickness and deformability in different locations
• Inflexible plates are separated from one another by thin flexible joints arranged segmentally
• Serve as muscle system extensions, transmitting force
• The exoskeleton gets shed or molted and then replaced with a new, larger one

Endoskeletons

• Unlike arthropods and mollusks, the endoskeleton of vertebrates and echinoderms are alive
• The endoskeleton, calcium-impregnated tissue, can therefore grow with the organism, and does not get shed
• Chondrichthyes skeletons like rays and sharks, are also alive and flexible, and made of cartilage
• Skeleton consists mainly of bones in most vertebrates

The Vertebrate Skeleton

• Consists of two distinct divisions: axial skeleton and appendicular skeleton
• Axial skeleton is located along the axis of the body -Includes the skull, vertebral column, ribs, and sternum
• Appendicular skeleton includes limb bones (arms, legs)
• Includes the shoulder (pectoral) girdle, and hip (pelvic) girdle

Human Bony Structures

• The skull includes 8 cranial bones that cover the brain which can fuse after birth -14 facial bones underlie the facial region
• Vertebrate spine/vertebral column contains 24 free vertebrae
• 7 cervical (neck), 12 thoracic (chest), and 5 lumbar (lower back)
• Consists of 2 fused vertebrae: the sacrum which has 5 fused sacral region vertebrae, and the coccyx with several fused coccygeal region vertebrae

Human Bony Structures cont.

• The rib cage protects the internal organs of the chest including the sternum, the thoracic vertebrae, and then twelve pairs of ribs
• Each pair of ribs is attached dorsally to a separate vertebra
• The first 7 are attached ventrally to the sternum
• The next 3 are attached indirectly by cartilage
• The last 2 floating ribs have no attachments.
• Pectoral girdle has two collarbones (clavicles), and two shoulder blades (scapulas)
• Pelvic girdle has a pair of large bones, each composed of three fused hipbones. securely fused to the vertebral column.
• Each human limb consists of 30 bones and five digits (fingers or toes)

A Typical Bone - Radius

• The radius is a forearm bone of a human.
• Covered with periosteum to attach muscle tendons and ligaments
• This makes new layers of bone in diameter
• The main shaft is the diaphysis.
• The expanded ends are epiphysis.
• In children, the metaphysis is a disc of cartilage between the diaphysis and epiphysis; disappears at maturity leaving epiphyseal lines
• Contains bone marrow that stores yellow fatty connective tissue and red blood cell producing matter
• Outer layer is compact bone made up of osteons and osteocytes (in lacunae) connected by canaliculi
• Inside the bone is spongy bone with a network of thin strands of bone

Bone Development

• In the fetus bones develop in two ways
• Endochondral bone development is related to long bone and development; radius growth
• Bones are first cartilaginous and start ossification in the diaphysis followed by epiphysis ossification making bones within cartilage model. Eventually ossified regions fuse
• Intramembranous bone development related to many other types of bones including flat outer bones of the skull
• Refers to making bones within connective tissue membranes from a non cartilage connective tissue model which becomes ossified.

Bone Cells

• Osteoblasts (build bone) secrete collagen which are strong fibers and hydroxyapatite (calcium phosphate)
• Hydroxyapatite is present in the interstitial fluid and crystallizes around the collagen fibers forming the hard matrix of bone
• Collagen serves as a nucleating site for the deposition of hydroxyapatite
• Osteoblasts become isolated within the lacunae and become trapped osteocytes
• Osteoclasts break down bone by secreting acid that dissolves the crystals - and enzymes that digest collagen
• Osteoblasts and osteoclasts are synergistic and work together to shape and remodel bone
• Bones are entirely remodeled every 10 years or so
• Bone resorption is greater than bone formation during osteoporosis

Joints

• Articulations or joints are junctions between bones
• The outer surface of each bone consists of articular cartilage at the joint
• Immovable joints exist between skull bones: Sutures are held in place with a thin layer of dense fibrous connective tissue
• Slightly movable joints are vertebral discs made of cartilage
• Most joints are freely movable joints with a joint capsule of connective tissue that surrounds the joint region
• Lined with a membrane that holds and secretes synovial fluid which lubricates the joint and absorbs shock
• Reinforced by ligaments (bands of fibrous connective tissue) that connect bones and limit movement at the joint
• Osteoarthritis is a common joint disorder where articular cartilage wears out
• Rheumatoid arthritis happens when the synovial membrane becomes inflamed

Muscle Contraction

• Works with the skeletal system
• Muscles are anchored to the skeleton
• Generates mechanical forces and motion
• Bones which are levers powered by muscles amplify the muscular movement causing internal movement
• Includes blood flow, Digestive movements (peristalsis) and blood pressure regulation

Eukaryotic Contractility

• No muscle tissue is present in very simple animals
• All eukaryotic cells contain contractile protein actin, which is a major component of microfilaments, and plays a critical role in cell processes -Includes amoeboid movement and attachment of cells to surfaces
• Functionally associated with the contractile protein known as myosin
• Myosin is an essential actin-binding cellular motor molecule
• Uses ATP and converts chemical energy to mechanical energy and movement

Muscles Work in Pairs

• Skeletal muscles produce movements by pulling on tendons
• Muscles do not actively extend but act antagonistically to one another in pairs
• Agonist muscles contracts and actively extends the opposing relaxed antagonist muscle

Vertebrate Muscle

• Most abundant tissue in the body
• Composed of elongated cells known are muscle fibers
• Wrapped in bundles by connective tissue
• Each fiber is long cylindrical cell
• Multinucleate, with nuclei just under the plasma membrane (sarcolemma)
• The plasma membrane contains inward channels called T tubules
• Cytoplasm is the sarcoplasm
• Endoplasmic reticulum is called sarcoplasmic reticulum

Muscle Substructure

• Myofibrils are even smaller units that extend through the length of the fiber bundle
• Includes myofilaments (or filaments)
• Myosin form thicker filaments
• Actin forms thinner filaments
• Thin filaments are made of polymerized actin and wrapped with tropomyosin and the troponin complex (regulates contraction and motility)
• Myofibrils are built of repeating structures of thin and thick filaments known as sarcomeres
• These are the basic units of contraction and are connected end-to-end by Z lines
• Includes A band, H band and I band

Sarcomere Structure

• Thick and thin filaments overlap
• A band refers to the width of the region of thick filaments
• Which is dark and made of overlapping myosin and actin filaments
• H zone refers to the "light" region made up only of thick filaments
• I band is made up of only actin filaments of two adjacent sarcomeres
• During muscular contraction, thick and thin filaments slide over each other

Muscle Contraction Pt. 1

• Muscle contraction occurs because sarcomeres shorten
• This happens when thin filaments slide past the thick filaments, known as the sliding filament model
• Skeletal muscles are excitable, much like neurons
• Motor neurons transmit messages from the brain or spinal cord to muscle fibers
• The site of innervation of the muscle by the neuron is called the synaptic cleft/neuromuscular junction
• Synaptic vesicles filled with acetylcholine release contents into the synaptic cleft
• Acetylcholine binds with receptors on the muscle fiber which causes depolarization of the muscle cell
• A change in the distribution of electric charge across its sarcolemma, or plasma membrane
• Depolarization travels along the sarcolemma and generates an electrical signal, or action potential, in the muscle fiber

Muscle Contraction Pt. 2

• Action potential/wave of depolarization causes proteins in T-tublule membranes to change shape and causes channels to open in the sarcoplasmic reticulum
• Stored calcium ions rush out
• Troponin binds calcium, undergoes conformational change
• Pushes the tropomyosin away from the active sites on the actin filament, exposing myosin-binding sites
• Each myosin molecule has an end folded into two bulbous heads and a long tail joining other myosin tails

Muscle Contraction Pt. 3

• At rest, ATP is bound to the myosin which is an adenosine triphosphatase (ATPase)
• Myosin converts the chemical energy of ATP to the mechanical energy through sliding filaments
• ATP is split to form ADP and inorganic phosphate Pi
• Myosin head is therefore in an energized state as it is "cocked"
• Myosin head binds to an exposed site on the actin filament forming and creating a cross bridge
• Pi release causes the myosin head to bend about 45° in a flexing motion (called power stroke)
• Pulling the actin filament closer to the center of the sarcomere releases ADP
• Myosin head binds the new ATP and detaches from the actin
• If the concentration of Ca2+ is high enough, the cycle restarts
• Progressive actions of the myosins cause the steady sliding of the thick filaments over the thin filaments

Resting State

• Impulses from the motor neuron cease
• Acetylcholinesterase in the synaptic cleft inactivates the Acetylcholine
• Muscle fibers return to a resting state
• Calcium is pumped back into the sarcoplasmic reticulum for muscle relaxation
• When not moving, the muscle is in a state of partial contraction or muscle tone
• Rigor mortis is temporary muscular rigidity that appears shortly after death
• Results from ATP depletion

The ATP Supply in Muscle

• ATP is the immediate source of energy for muscle contraction.
• ATP hydrolysis provides the energy to "cock" the myosin
• There is a limited amount of ATP in the muscle
• Muscle tissue has an intermediate creatine phosphate energy storage compound
• Creatine phosphate & ADP will produce ATP and creatine
• Glycogen is also broken down within the muscle, yielding glucose, which is degraded in cellular respiration
• When there is not enough oxygen to provide ATP via the mitochondria, lactic acid fermentation occurs causing an oxygen debt
• Buildup of lactic acid results in fatigue and pain
• Muscles are forced to slow down and relax to replenish ATP by oxidative mechanisms

How Muscles Work

• Muscle contraction depends on
• The number of fibers contracting and the tension from each fiber
• In a motor unit, a motor neuron is connected with an average of about 150 muscle fibers
• Some motor neurons control more
• Motor recruitment depends on brain messages that activate motor neurons
• The more motor units recruited, the greater the contraction

How Muscles Work - Stimulation

• The frequency of stimulation affects the force of fiber contraction
• Activated by brief electrical stimulus, skeletal muscle responds with a simple twitch
• Two twitches add (summate) when a second stimulus arrives before the first contraction fully completes
• Skeletal muscle stimulated with series of separate stimuli timed close together gives sustained contraction

Skeletal Muscle Fibers

• Three types: slow-oxidative, fast-oxydative, and fast-glycolytic
• Slow-oxidative fibers are specialized for endurance activities such as marathon running
• Contract and fatigue slowly
• Require steady oxygen supply
• Rich in mitochondria and capillaries for aerobic respiration
• Rich in myoglobin for oxygen storage (red)
• Fast-oxydative fibers give rapid response/intermediate rate of fatigue
• Rich in mitochondria and myoglobin (red)
• Individuals in physically fit condition have these
• Fast-glycolytic fibers offer great deal of power for brief period (sprinting)
• Contract rapidly and deplete Glycogen supply
• Less mitochondria and obtaining energy from glycolysis
• Less myoglobin producing white fibers
• Those in sedentary people could change to fast-oxydative fibers with training

Smooth Muscle

• Well-adapted to moving tissues not attached to bones - blood vessel walls, intestinal linings, soft tissues
• Smooth muscles contract slowly in response to simple stretching, but create impressive squeezes.
• Less sarcomeric organization; not striated
• Fibers function as a unit and are connected by gap junctions for electrical signals to pass rapidly from fiber to fiber
• Cross bridges remain in the attached state longer in smooth muscle than they do in skeletal muscle, requiring less ATP

Cardiac Muscle

• Contracts and relaxes in alternating rhythm independent of a nerve supply but has cardiac muscles with sarcomeric construction
• Like smooth muscle, connected via gap junctions, in this case known as intercalated discs
• Heartbeat is initiated by a pacemaker, a specialized cardiac muscle mass
• A conduction system transmits the signal throughout the cardiac muscle
• Heart rate is regulated by neurons in the cardiac centers (medulla of the brain)