1. Fundamentals of Laboratory Animal Anatomy and Physiology   

   Reading ASSIGNMENT:  Malocclusion  

   (Lecture I--  through & including Digestive System
    Lecture II--  from Endocrine System to end)

    

   1. Introduction

      There is an incredible variety of living organisms used in research, from bacteria, cell cultures and radishes to sea slugs, mice and chimpanzees.  

      When using animals for research, it is essential to know how the body is organized and how it functions.  This knowledge is required to provide proper husbandry, to recognize when an animal is sick, and to interpret the results of an experiment.

      The study of the structure and the relationships among structures is called anatomy.   The study of how these structures work is called physiology.  It is impossible to separate a structure from how it works--structure often determines function, and function influences structure.
      
      

   2. Levels of Structural Organization

      At its simplest level, an animal's body consists of chemicals, some of which are classed as organic chemicals, because they contain atoms of carbon and are associated with living things.  These chemicals form the fundamental living unit of an organism: the cell.  

      Cells are organized into tissues, aggregates of cells with similar structures and functions.  Examples of tissues include muscle and nervous tissues.

      Several types of cells may join together to form an organ--a structure which has specific functions and often has a distinct shape, such as the heart or kidney.  When several organs have a common function, such as heart, arteries and veins, they are called a system--in this case, the cardiovascular system. 

      The sum of the chemicals, cells, tissues, organs and systems form the highest level of organization, the organism.  All of the levels are interrelated and interdependent--they function together to create a healthy animal.
      
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   3. Anatomical Directions
      

      There are specific words that are used to describe different directions and locations on animals.  It is essential to know the technical terms for these directions in order to properly care for or treat an animal.

If you're told not to restrain a rat by the distal tail or you're instructed to treat the lesion on the dolphin's dorsal fin, the animal relies on you to know what these words mean.

Dorsal means toward the top or back of an animal, such as the dorsal fin of the dolphin or on the back of the rat illustrated above.  The opposite of dorsal is ventral, a term that describes the bottom or lower part of an animal.  The rat's abdomen is ventral.

Cranial or anterior is directed toward the head or the front part of the animal.  A similar term is rostral, which means something is on or near the nose.  Caudal  and posterior described something directed toward the rat's rear. 

The proximal part of the tail or leg is closest to an animal's body.  The distal tail is the tip and the distal leg ends with the toes, farthest away from the body.

If something is medial, it is on the inside or toward the middle of the animal.  The inside of the hind leg, for example, is the medial surface of that leg.  The lateral side of the leg is toward the outside, the visible surface of the left front leg of the rat above.

The palmar surface is the bottom of the front foot and the plantar surface is the bottom of the hind foot.  Volar is an alternate term for either of these foot bottoms.

A superficial injection is given near the surface of the skin.  A blood vessel that is located close to the skin, such as the marginal ear vein of the rabbit, is also called superficial.  A deep injection is given inside the animal, such as you would do if you gave an intramuscular injection in the neck muscles of a horse.

Most animals used in research are bilaterally symmetric:  a line drawn between their ears and down to their toes evenly divides them into right and left halves.

4. The Integumentary System

   The integumentary system consists of the exterior of an animal--the skin and structures derived from skin, such as hair, feathers, scales, claws and beaks.  This system has many important functions, including protecting from external damage by forming a physical barrier, regulating body temperature through sweat glands and insulating fur and feathers, preventing excessive water loss, sensing the environment with receptors for touch, hot and cold, pain, etc., and communicating through scent glands.

Skin consists of two primary layers.  The superficial layer is the epidermis and the deep layer is the dermis.  In most animals, there is a space between the skin and underlying structures:  this is the site where a subcutaneous injection is given.

Many wild animals, such as mice and rabbits, have fur color  called agouti (ah-GOO-tee).  This primitive color is a pattern of black and brown bands on each hair.  Many laboratory rodents and rabbits are albino--they lack pigment in their skin, hair and eyes.  Their fur and skin are white and their eyes appear pink (due to the blood vessels in the unpigmented eyes). 

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Albino rat

4. Musculoskeletal System

   The musculoskeletal system consists of the materials that hold an animal together and allows it to move.  Things like bones, joints, ligaments, tendons and muscle are components of this system.  Functions of the musculoskeletal system include supporting the body, protecting the internal organs, storing minerals, producing blood cells in the bone marrow, moving for locomotion, breathing and cardiac function and facilitation of heat production via muscle contraction.

   Ligaments connect bone to bone at joints (articulations where bones meet and can bend) and tendons connect muscle to bone.  Muscles produce motion by contracting and relaxing, putting force on tendons that pull on bones or other structures.

   When muscles move limbs toward the body (medially), such as when a bird holds its wings against its sides, it is called adduction.  When the wings are moved laterally, the movement is abduction.

   Adduction

   Abduction

   Animals have from one to five toes on their feet.  The number of toes may be the same or there may be a different number of toes on the front and back feet.  Toes can end in hooves, claws or nails, depending upon the species of animal.
   
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5. Respiratory System

   The respiratory and cardiovascular systems are located in the thoracic, or pleural, cavity of the chest.

   There are two types of respiration that are essential for an animal's survival:

   • Internal (cellular) respiration is a chemical process required by every cell of the body and that uses oxygen to produce energy

   • External (organismal) respiration results in the exchange of oxygen and carbon dioxide between blood (specifically the erythrocytes or red blood cells) and the cells of the body.  It is accomplished by breathing in (inspiration) and breathing out (expiration).

   Some animals, such as frogs and salamanders, are able to absorb oxygen through their skin.  Other animals, such as fish, larval amphibians and the mammalian fetus, utilize gills.

   Most animals, including birds and mammals, use the same type of respiratory system as humans.  Air enters and exits the body through the nares (nose) and mouth.  The air travels down the trachea (windpipe) into the lungs, where the trachea branches into bronchi and terminate as alveoli (air sacs).  The erythrocytes in the capillaries surrounding the alveoli exchange oxygen and carbon dioxide.  In mammals, the diaphragm, a muscle located between the abdomen and the thorax, produces a vacuum that helps move air in and out of the lung.
   
   

6. Cardiovascular System

   The cardiovascular (circulatory) system circulates substances around the body.  Its two primary functions are transport and temperature regulation.

   Some of the substances transported in blood include oxygen and carbon dioxide, nutrients, metabolic waste products and hormones.  An animal can cool its body temperature by moving blood to the extremities, such as a rabbit's ear; this blood near the surface is cooled before it returns to the core of the body.

   The pumping system consists of the heart  (chambers and valves that pump blood), arteries  (carry blood away from the heart), veins  (return blood to heart) and capillaries  (small vessels where oxygen exchange occurs)

   The substance located in the heart and vessels is blood.  Blood consists of plasma (water with dissolved chemicals and proteins), erythrocytes (red blood cells that transport oxygen), leukocytes (white blood cells that produce immunity) and platelets (cells that function with clotting factors to repair damaged blood vessels).
   
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7. Digestive System

   The abdominal, or peritoneal, cavity contains the digestive, urinary and genital systems.

   The digestive system is responsible for taking food into the body, using it and eliminating the waste products. The type of food eaten can be used to classify animals as herbivores (plants only), omnivores (plants and animals) or carnivores (animals only).

   Teeth are found in mammals and some other animals and are to used to grasp, tear and crush food.  There are four types of teeth:  incisors that cut food, canines that tear and shred food, and the premolars and molars that crush food.

   Each species of animal has a dental formula that describes the teeth on one side of the mouth.  A mouse, for example, has 16 teeth and its dental formula is:

2 ( I  1/1   C 0/0   P 0/0   M 3/3)

The "2" at the beginning of the formula indicates that the numbers must be doubled to determine the actual number of teeth.  Teeth are numbered from the center of the mouth.  The upper number represents the teeth on one side of an animal's upper jaw and the lower number is the teeth on one side of the lower jaw.  

Notice that in the mouse, there are only incisors and molars.  This creates a space in the mouth where the absent canines and premolars would be.  This space is called the "diastema." 

The first teeth that are present in young animals are deciduous teeth;  they fall out in a pattern based on the age of the animal.  Permanent teeth erupt in an age-related sequence as well, and can be used to determine the animal's age.

Rodents, rabbits, horses and some other gnawing animals have "open-rooted" teeth that continue to grow throughout the life of the animal.  Contact between the occlusal surfaces ensures that the teeth wear down evenly.  If these surfaces do not meet properly, the teeth may not wear correctly and malocclusion may result.

Other animals, such as dogs, cats and people, have "closed-rooted" teeth.  Their teeth reach a maximum size and stop growing. 

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9. Endocrine System

   The endocrine system is the chemical communication system for the body.  Hormones produced by endocrine glands enter the bloodstream and are carried to target sites throughout the body.  This system works cooperatively with other organ systems to maintain homeostasis, the stable internal environment needed to allow proper functioning of cells, tissues, organs, and organ systems.  

   Specific functions of the endocrine system include the regulation of growth and development, metabolism and energy balance, and reproduction.

   Examples of endocrine organs include the thyroid and adrenal glands, the insulin-producing pancreas, and the ovaries.
   

10. Nervous System

    The nervous system can be viewed as the electrical communication system for the body.  It is one of the primary control and integration centers of the body.  The nervous system has three principal functions:  sensing changes in the internal and external environments  (sensory), interpreting these changes (integration), and responding to these changes with some action  (motor)

    An animal's special senses allow it to interact with the external environment.  These senses include sight, hearing and equilibrium, smell and pheromone reception, taste, touch (including hot, cold, pain and pressure) and proprioception (awareness of the positions of the parts of the body...i.e. where the paw is located).

    Eyes:
    
    Birds, reptiles and some mammals, such as cats and rabbits, have a nictitating membrane, a protective plate of moveable cartilage on the surface of the eye.  The inner corner of the eye is called the "medial canthus."  Rodents and some other animals have a lubricating structure called the Harderian gland located behind the eye.  This gland produces a red fluid that constantly flows over the eye.  If an animal is stressed, increased amount of fluid is produced, and it may look like it's bleeding from the eyes or nose.  

    Albino animals lack pigment in their eyes.  Their eyes appear pink because of the blood vessel located in the eye.  Albino eyes are much more sensitive to bright lights and have much lower visual acuity than colored eyes.

    Ears:
    
    An animal's ear consists of the:

    • External ear (externa)--the pinna (ear flap) and outer ear canal

    • Middle ear (media)--separated from the external ear by the ear drum and containing the tympanum (ear drum).  This is where the auditory ossicles which receive sound are located

    • Inner ear (interna)

    • Eustachian tubes connect the middle ear to the pharynx (mouth)

    Odors and Pheromones:
    
    Smell is an important sense in most mammals.  Of particular importance are pheromones, hormone-like chemicals used for communication, particularly during reproduction or when an animal is frightened.  The vomeronasal (Jacobson's) organ is located in the nasal cavity and is responsible for detection of pheromones.
    

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11. Urinary System

The urinary system is another component in the homeostatic regulatory system.  It filters blood, selectively removes or retains some of the blood constituents and excretes wastes, toxins and excess substances as urine.

The kidney is the filter.  Functional units called nephrons allow water and dissolved materials from blood plasma to pass through the filter; cells and other large materials stay in the blood stream.  The nephrons return some of the water and chemicals such as glucose, sodium and potassium to the blood vessels surrounding them.  The ureters connect the kidneys to the urinary bladder, where urine is stored until it is released from the body as urine through the urethra.

12. Reproductive System

    The reproductive system is responsible for producing new individuals and perpetuating the species.  Genes located in the cell nuclei are duplicated and recombined, resulting in variation in offspring.

    The female reproductive tract consists of:

    • Ovary- site of egg production
    • Oviduct (Fallopian tube in people)- transports egg to the uterus
    • Uterus or uterine horns-  where the embryo develops
    • Cervix-  connects uterus to vagina
    • Vagina-  chamber between cervix and vulva
    • Vulva-  entrance / exit from the body

    Ovulation is the release of an egg from the ovary.  Fertilization typically occurs in the oviduct, and a zygote is formed from the union of the egg and sperm.  The zygote is implanted (joined) to the wall of the uterus (which may be a single chamber, as seen in humans, or may consist of two horns, which is typical of dogs and rabbits).  A placenta forms around the developing embryo, allowing blood and nutrients to flow from the mother to the embryo.  The period of time from fertilization to birth is called "gestation."  Parturition is the act of giving birth.  A characteristic of mammals is the possession of mammary tissues which produce milk for nutrition of the newborn animal.   When a young animal is able to subsist on solid food, it is weaned.

    The male reproductive tract consists of:

    • Testes (testicles)-  site of spermatozoa (sperm) production
    • Epididymis-  connects testes to vas deferens
    • Vas deferens-  propels sperm to penis
    • Penis-  exit from the body
    • Prepuce-  sheath surrounding penis
    • Scrotum-  tissue surrounding testes

    Male rodents have an open inguinal canal, the passageway from the abdomen into the scrotal sac.  When frightened, these animals can withdraw their testes into their abdomen, making it difficult to sex them.  When castrating these animals, care must be taken to close the inguinal canal, so that a loop of intestine doesn't herniate by extruding though the canal.

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