• The genetic similarity which exists contributes to phenotypes expressed to be uniform.
• Small number of animals is needed to be used in order to gain a good statistical power.
• It is sufficient to just genotype one single animal from that strain which can represent the whole strain.
• Strains do not change or in other words stay constant for many generations as long as there is no contamination due to new mutations being passed on which is rare.
• Experiments can be done by using small numbers of the animal model.

CAUTION!

• To produce offsprings with minimal genetic variation
• Offsprings share the same characteristics
• Considered inbred after more than 20 generations of inbreeding

REPRODUCTIVE INFORMATION

• Type of Sexual cycle : Polyestrus and Nonseasonal
• Duration of Sexual cycle : 4-5 days
• Estrus duration : 12 hours
• Gestation period : 21 to 23 days
• Postpartum Estrus : Fertile
• Number of offspring : 8 to 14 pups
• Weaning age : 20 to 21 days

AXIAL

APPENDICULAR

• Lacrimal gland and Harderian gland secrete porphyrin-pigmented and lipid-waden tears. 
• Little to no aqueous component in tear film.
• Ocular surface covered by glycocalyx to resist desiccation.
• Rats are susceptible to light-induced damage (should be kept in levels  of 130-325 lux).
• Albino rats can develop retinal damage in light levels as little as 80 lux light.
• Rats have 2 types of retinal cones.
• One type has ultraviolet sensitive photopigment at 358nm.
• One type has pigment which is sensitive at middle wavelengths of visible spectrum at 510nm.
• Rats have low visual acuity compared to humans.
• Albino rats have 20/1200 vision.

Lacrimal gland

• Extraorbital= located ventral and rostral to ear
• Intraorbital= open into conjunctiva in dorsolateral region of the eye
• Fucntions to secrete lacrimal fluid. 

Harderian gland

• Horseshoe shaped.
• Occupies a huge portion of the orbit and extends medially and caudally circling the optic nerve.
• Functions to secrete secretions containing porphyrins which fluoresce.
• Overexpression during stress leads to red porphyrin to be secreted and cause a condition known as chromodacyorrhea.

• Type 1 = glial-like supporting cells responsible for detecting salt
• Type II = sweet, bitter, umami
• Type III = sour, sweet, bitter, umami

HEARING

TOUCH

• Main tactile organs = vibrissae which is used for the rats to navigate location, recognise objects, as well as it plays an essential role in social interactions, body balance and orientation.
• Mystacial vibrissae present in supraorbital, postero-orbital, lateral cervical, median cervical, submental and carpal forelimbs.
• Each vibrissae is sunken in a follicle sealed within blood sinus.

Mouth and Buccal Cavity

• Dental formula = 2(I 1/1, C 0/0, Pm 0/0, M 3/3) = 16

Oesophagus

• The oesophagus enters stomach at the inner curvature through fold if the limiting ridge that separates forestomach from glandular stomach.
• The way the oesophagus enters the stomach and together with the  lack of neurologic component to induce vomiting prevents rats from vomiting.

Stomach

• The stomach is located in left proximal portion of abdomen and comes into contact with liver.
• 2 regions – forestomach and glandular stomach, glandular is further divided into 3 -small cardia, fundus and antrum.
• The stomach secretes only pepsinogen C and cathepsins D and E, there is no secretion of pepsinogen A.

Small intestine

• Major site of digestion and absorption of nutrients.
• Longest portion of the Gastrointestinal tract.
• Comprises of the duodenum, jejunum, ileum.
• Mucosal surface is covered by villi with intestinal crypts, crypts of Lieberkühn located at the base.

Caecum

• It is a llarge-thin walled, blind pouch consisting of apical and basal portion.
• The base of caecum include ileocecal and colocecal junctions.
• The size of cecum reflects the omnivorous nature of rats, it is smaller than herbivores but larger than carnivores.
• The presence of bacteria supports its function as a fermentation vat to breakdown plant materials.

Large intestines

• It is shaped like an inverted V and is divided into 2 segments the ascending and descending colon.

• Pancreas is not a singular mass but is diffuse and extends from duodenal loop end.
• There are 3 regions = duodenal, gastric and splenic OR left, middle and right.
• Pancreas has a role in digestion by synthesizing and secreting enzymes.

LIVER

• The Liver functions to process nutrients, maintain energy homeostasis and clearing toxins.
• It also functions to support and regulate the immune system.
• The liver is divided into 4 lobes = right, median, left and caudate lobes.
• Bile ducts from each lobe come together and form a common bile duct and empty into descending duodenum.
• No bile concentration occurs as there is no gallbladder.

• Testes is located in scrotal sacs between anus and prepuce.
• Anogenital distance is longer in males than in females.
• The male rats possesses 5 types of accessory sex glands which are ampullary glands, prostate gland, vesicular gland, coagulating gland, Cowper’s gland.
• Penis lies within a loose prepuce with a single bony process.

Central nervous system

• Composed of brain and spinal cord
• The Meninges which has 3 layers covers these structures
• Dura mater(outermost)
• Arachnoid mater (middle)
• Pia mater (innermost)
• Spinal cord terminates at the slender filum terminale. (Cauda equina = caudal spinal nerves+filum terminale)

Peripheral nervous system.

• 7 Cervical
• 13 Thoracic
• 6 Lumbar
• 4 Sacral
• 27-30 Caudal

• Each nerve roots are formed by the dorsal and ventral roots. Dorsal root carries sensory impulses to the spinal cord. Ventral root carries nerve impulses away from the spinal cord.
• The First cervical nerve lacks a ganglion.
• Brachial plexus = give rise to ulnar nerve, axillary nerve, musculocutaneous nerve, radial nerve.
• Lumbar plexus = femoral nerve and saphenous nerve.
• Pelvic plexus = connects to the genitourinary tract.

• Rheumatoid arthritis (RA)
• Adjuvant-induced arthritis
• Autoimmune encephalitis
• Autoimmune thyroiditis

• Male-40 to 60 days
• Female-40-49 days

• Charles River Laboratories (Crl)
• Taconic (Tac)
• Janvier laboratories (OrlRj)

• Ovaries are located in the lateral border of the psoas muscle while embedded in fatty tissues near the kidneys.
• Anogenital distance is shorter in females than in males.
• Mature ovaries appear as mass of follicles.
• Only the clitoris is connected to the urinary system.
• Estrus cycle is about 4-5 days.