Epithelial Coverings: Invertebrate and Vertebrate

Epithelial Coverings

• Covers all external and internal animal body surfaces.
• Its structure and functions are adapted to the animal's environment and lifestyle.
• Contains receptors receiving sensory signals from the environment.
• It can aid in gas exchange, waste excretion, temperature regulation, and secretion.

Invertebrate Epithelium

• Secretes protective or supportive nonliving material layers.
• Insects secrete a cuticle.
• Corals and mollusks secrete a calcium carbonate shell.
• Epithelial cells are made sensitive to light, chemical stimuli, and mechanical stimuli.
• Specialized for absorption, gas exchange, or secretion.
• Earthworms use lubricants like mucus for gas exchange and friction reduction.
• Secretions for communication can be odorous.
• Poisons for defense/offense protection.

Vertebrate Skin

• Vertebrates possess a complex epithelial system referred to as the skin.
• Skin and derived structures create the integumentary system.
• Differs considerably among species.
• Skin is strong, protective, and elastic.
• May have armor-like scales, like fish and reptiles.
• Amphibians can be covered with protective mucus for gas exchange and lubrication.
• Mammals possess hair, and birds possess feathers to protect as well as maintain temperature and insulating structure.
• It is colored to aid communication.

Mammalian Skin

• Mammals have skin-derived structures such as claws, fingernails, and toenails.
• Includes hair, sweat glands, oil (sebaceous) glands, and horns.
• Sensory structures for pressure, temperature, and pain.
• Female mammals have mammary glands and secrete milk.

Human Skin

• Oil (sebaceous) glands go into the hair follicle, which holds hairs.
• Sebaceous glands release sebum, a mix of waxes and fats.
• Sebum keeps hair moist, pliable, and protects against water loss, as well as inhibits bacterial growth.
• Epidermis is the outer layer of vertebrate skin and functions as a waterproof protective barrier.
• The epidermis produces keratin, an elaborated coiled protein, for mechanical strength to help reduce water loss.
• The epidermis provides tough outer strata of cells, the deepest being the stratum basale.
• Pigment cells produce melanin that determines skin color; cells divide, mature, and are pushed outward.
• The most superficial layer is the stratum corneum where epidermal cells die as they move through and wear off.

The Dermis

• Beneath the epidermis lies the dermis made of dense fibrous connective tissue of collagen that provides strength and flexibility.
• The dermis contains blood vessels and sensory receptors and embeds hair follicles, sweat glands, and melanocytes.
• It rests on a subcutaneous adipose tissue layer which insulates the body.
• Exposure to ultraviolet radiation thickens the epidermis and stimulates melanin production.
• Skin becomes inflamed when melanin cannot absorb all UV rays.
• Dark-skinned people have more melanin.
• Excessive UV exposure can lead to malignant melanoma.

Skeletal Systems

• Skeletal systems give the body support and protect internal organs.
• These systems act on hard structures, like chitin or bone for locomotor activity.
• Calcium is stored with blood calcium homeostasis in vertebrates.
• Types include hydrostatic skeletons, exoskeletons, and endoskeletons.

The Hydrostatic Skeleton

• Body fluids transmit force.
• Many soft-bodied invertebrates have fluid-filled body compartments, such as cnidarians, flatworms, annelids, and roundworms.
• The system works like a water-filled balloon
• Muscles in the wall contract, causing shape change.
• Two antagonistic muscle groups: longitudinal epidermal muscles and circular gastrodermal muscles.
• Longitudinal epidermal muscles shorten, while circular gastrodermal muscles lengthening movement.
• Delicate movements are hard.

The Hydrostatic Skeleton of Earthworms

• Earthworms contain hydrostatic skeletons.
• It is made of more than 100 segments, separated by septa.
• Each septum isolates the portion of the body cavity and the coelomic fluid.
• Contractions of one region minimally affecting another allows for complex crawling motions.

Exoskeletons

• This is a lifeless shell deposited atop the outer epithelial covering.
• Mollusks form a calcium carbonate exoskeleton shell, secreted by the mantle.
• Mollusks can retreat into their shells for protection.
• Arthropods produce a tough exoskeleton: a nonliving cuticle with chitin.
• It is pliable, armorlike, and has a continuous one-piece covering varying in thickness with deformability in different locations.
• Thin, flexible joints arrange inflexible plates separated from one another.
• Serves as extensions of the muscle system and transmits force.
• It must molt, shed, and replace with a new larger one.

Endoskeletons

• Endoskeletons are alive in vertebrates and echinoderms, specifically sea stars and sea urchins.
• With calcium-impregnated tissue, the endoskeleton grows with the organism and is not shed.
• Chondrichthyes skeletons consisting of rays and sharks are alive, flexible, and cartilaginous.
• Vertabrates skeletons typically made of bones.

The Vertebrate Skeleton

• It contains the axial skeleton and the appendicular skeleton.
• The axial skeleton is along the body creating the skull, vertebral column, ribs, and sternum.
• The appendicular skeleton forms the limb bones consisting of arms, legs, shoulder girdle, and hip girdle.

Human Bony Structures

• Eight cranial bones cover the brain, some fusing during development after birth.
• Fourteen facial bones underlie the facial region.
• The vertebral spine or column contains 24 free vertebrae.
• There are seven cervical (neck) vertebrae.
• There are twelve thoracic (chest) vertebrae.
• There are five lumbar (lower back) vertebrae.
• Vertebrae include two fused types: sacrum with five fused vertebrae and coccyx with several fused vertebrae.

Human Bony Structures: Rib Cage, Pectoral & Pelvic Girdle

• The human bony structures are the rib cage, which protects internal chest organs.
• This includes the sternum and thoracic vertebrae.
• There are 12 pairs of ribs in mammals with each pair is dorsally attached to a separate vertebra.
• The first seven pairs are ventrally attached to the sternum.
• The next three ribs are attached indirectly by cartilage.
• The last two are "floating ribs" without attachments.
• The pectoral girdle contains two clavicles and two scapulas or shoulder blades.
• The pelvic girdle holds a pair of large bones forming with three fused hipbones that are securely fused to the vertebral column.
• Each human limb consists of 30 bones and five digits for fingers or toes.

A Typical Bone: The Radius

• The radius is a forearm bone in humans.
• It is covered with periosteum: a connective tissue membrane for muscle tendon and ligament attachment.
• It creates new layers of bone in diameter.
• The bone's main shaft is the diaphysis.
• The expanded ends are the epiphysis.
• In children, the metaphysis, a disc of cartilage, fits between the diaphysis and epiphysis.
• The metaphysis makes the individual to reach maturity and the epiphyseal lines of the bone contain bone marrow.
• Yellow marrow is fatty tissues.
• Red marrow produces blood cells.
• The outer layer is compact bone made up of osteons.
• Osteocytes exist in lacunae and the Haversian canals that are canals in bones, all of which are connected by canaliculi.
• The inside bone is spongy bone.
• The bone contains a network of thin bone strands.

Bone Development

• Bones develop in two ways in the fetus: endochondral bone development for long bone growth and development and intramembranous bone development for many other bone such as flat outer bones of the skull.
• Endochondral starts as bones existing as cartilaginous bone that ossify in the diaphysis with epiphysis ossification and bones within cartilage that eventually fuse.
• Intramembranous develops on connective tissue membranes that do not undergo cartilaginous ossification.

Bone Cells

• Osteoblasts build bone by secreting collagen, forming strong fibers of bone.
• Osteoblasts form hydroxyapatite, calcium phosphate, in the interstitial fluid that crystallizes around the collagen fibers, creating a matrix.
• The collagen serves as a nucleating site for hydroxyapatite deposition.
• Osteoblasts become isolated within lacunae and convert to osteocytes.
• Osteoclasts break down bone with multinucleated cells secreting acid that dissolves crystals and enzymes that digest collagen.
• Osteoblasts and osteoclasts work together to shape and remodel bone.
• Bones are entirely remodeled every 10 years or so.
• Osteoporosis occurs when bone resorption is greater than bone formation.

Joints

• Joints are articulations between bones.
• The outer bone surface consists of articular cartilage.
• Immovable joints include sutures between skull bones.
• These hold a thin layer of dense fibrous connective tissue.
• Slightly movable joints include vertebral discs made of cartilage.
• Most joints are freely movable with a connective tissue joint capsule, which surrounds the joint region.
• The joints are lined with a membrane holding and secreting synovial fluid which lubricates the joint and absorbs shock.
• Ligaments strengthen and are fibrous connective tissue bands that connect bones and limit its movements.
• Osteoarthritis is cartilage wear.
• Rheumatoid arthritis is synovial membrane inflamed.

Muscle Contraction

• The muscular system works with the skeletal system.
• Muscles are anchored to the skeleton and generate mechanical forces for movement.
• Bones are levers amplified by muscles.
• Internal movements include blood flow, digestive movements, and blood pressure regulation.

Eukaryotic Contractility

• Basic, simple animals do not have muscle tissue.
• Eukaryotic cells contain essential actin.
• Actin is a major microfilament component which helps cellular processes like amoeboid movement and cell attachment to surfaces.
• It is functionally associated with myosin.
• Myosin is cell motor with ATP.
• Myosin converts to chemical energy of mechanical energy and movement.

Muscle Pairs

• Tendons produce movements in skeletal musles.
• Muscles only actively contract and pull because muscles are antagonistic, not extend.
• The muscles move in pairs and will work against each other.
• The agonist will contract agonist muscle and help to extend relaxing antagonist muscle.
• Sets of these muscles must work against each other.

Vertebrate Muscle

• It is the most abundant tissue in the body.
• It is made of elongated cells called muscle fibers and wrapped in bundles via connective tissue.
• Each fiber is a long cylindrical cell.
• Muscle fibers are multinucleate with nuclei under sarcolemma plasma.
• Plasma membrane contains inward channels of transverse tubules.
• Cytoplasm is the sarcoplasm, and the endoplasmic reticulum is the sarcoplasmic reticulum.

Muscle Substructure

• Myofibrils extend the fiber bundle length.
• Myofibrils are made of myofilaments.
• Myosin forms thick filaments, and actin forms thin filaments.
• Thin filaments are made of polymerized actin.
• They are wrapped with tropomyosin and the troponin complex which regulate contraction and motility.
• Myofibrils consist of sarcomeres.
• Sarcomeres are the basic contraction units, connected end-to-end by Z lines, containing A band, H band and I band.

Sarcomere Structure

• Thick and thin filaments overlap.
• Dark A bands represent the width of the thick filaments with overlapping myosin and actin.
• Light H zones are the "light" regions of thick filaments.
• I bands have only actin filaments between 2 adjacent sarcomeres.
• Thick and thin filaments slide over each other during muscular contraction.

Muscle Contraction -1

• Sarcomeres shorten when muscles contract.
• The thin filaments slide past the thick filaments referred to as the sliding filament model.
• Skeletal muscles are excitable, similar to neurons.
• Motor neurons transmit messages from the brain to muscle fibers via the neuromuscular junction or synaptic cleft.
• Acetylcholine is released in synaptic vesicles and binds receptors on muscle fiber, causing cell depolarization.
• Depolarization creates electrical signal in sarcolemma.

Muscle Contraction -2

• Action potentials are a wave of depolarization, which can enter T-tubule membranes.
• The depolarization in T-tubules causes voltage-sensitive proteins' conformational change, which opens the sarcoplasmic reticulum of the SR.
• Stored calcium ions of troponin bind with it.
• Tropomyosin shifts, in turn providing an active site on the actin filament.
• Myosin molecules have ends folding into globular heads extending from myosin filament with tails that join other myosin.
• Each myosin molecule has one end folded into two globular structures called heads.

Muscle Contraction -3

• ATP binds to the myosin at rest, which is an adenosine triphosphatase or ATPase.
• Myosin converts ATP's chemical energy into movement.
• ATP is split forming ADP and phosphate, energizing the myosin head.
• The myosin head forms a cross-bridge with the exposed actin, binding site.
• Pi is released causing the myosin head to bend, flexing it with a force to pull the actin filament closer to the center of the sarcomere; ADP is released.
• Myosin head binds a new ATP detaching from actin.
• Calcium concentration restarts cycles for sliding thick filaments over the thin filaments.

The Resting State

• Impulses from the motor neuron cease.
• Acetylcholinesterase in the synaptic cleft inactivates acetylcholine.
• Muscle fibers return to their resting state.
• Calcium is pumped into sarcoplamic reticulum, relaxing muscles.
• While not moving, muscles remain in a partial contraction also called muscle tone.
• Rigor mortis is post-mortem muscular rigidity caused by ATP depletion following the end of cellular respiration.

The ATP Supply in Muscle

• ATP, which results from creatine-P + ADP, is the immediate source for muscle contraction allowing hydrolysis of the myosin for "cocking".
• Because there is limited ATP, stored creatine phosphate and muscle tissue intermediates help restore levels allowing Glycogen via the breakdown of glucose through cellular respiration.
• Lactic acid fermentation occurs which causes oxygen depletion.
• Buildup of lactic acid results in fatigue/pain slows down process from oxidative mechanisms.

How Muscles Work

• Contraction of a whole muscle depends on the number of muscle fibers contracting and the tension developed by each fiber.
• A motor neuron connects with about 150 fibers.
• Some motor neurons control more muscle fibers than others.
• Motor recruitment relies on messages in
• It becomes stronger when many motor units are recruited.

How do muscles Work: Frequency and Activation

• Increasing the tension of contracting muscle fibers depends on the frequency of stimulation.
• When activated with a brief electrical stimulus, skeletal muscle reacts with a simple twitch.
• Summation occurs when a second stimulus happens before the first contraction completes.
• Tetanus occurs when muscle is stimulated through many stimuli timed close sustained to contraction.

Skeletal Muscle Fibers

• The three types of skeletal muscle fibers are slow-oxidative fibers, fast-oxydative fibers, and fast-glycolytic fibers.
• Slow-oxidative fibers are specialized for endurance, sustained effort, and posture.
• They contract and fatigue slowly and need a steady oxygen supply.
• Slow-oxidative fibers are rich in mitochondria, capillaries, and myoglobin for oxygen storage.
• Fast-oxydative fibers are for rapid respnse, where a rapid rate of fatigue / intermediate rate of fatigue.
• They are rich in mitochondria, myoglobin, and are found in physically fit individuals
• Fast-glycolytic create large amounts of force briefly, and contain glycogen supplies.
• Not many mitochondria are availible, this helps obtain energy from glycogen.
• It contain Low myoglobin (white).
• Commonly observed in sedentary individuals, but can be converted with physical training to fast-oxydative fibers.

Smooth Muscle

• Well-adapted to tissues attaching to bones.
• Found in the lining of blood vessel walls and intestines.
• They contract slowly which constricts by contracting muscles.
• They are not striated but by having sarcometic organization, and they connect through gap junctions.
• It contracts cross bridges slower with less atp.

Cardiac Muscle

• Cardiac muscles contract by sarcomeric construction.
• Cardiac muscle contracts and relaxes through gap junctions independent nerve supply that are intercalated discs.
• The heartbeat is intiated by pacemaker and transfers signals throughout the entire heart.
• Heart rate is regulated by the medulla.