Important biological features

introduction

Taxonomy of the laboratory rat: Kingdom: Anlmalla Phylum: Chordata Class: Mammalia Order: Rodentla Suborder: Myomorpha Family: Muridae Genus: Rattus Species: norvegicus

Rats are thought to have originated in the area of Asia currently occupied by southern Russia and northern China. Rattus rattus (black or ship rat, 2n = 38) was well established in Europe by 1100 A.D. (following the Crusades), with Rattus norvegcus (brown rat, 2n = 42) commonly found in Europe in the 1700s. This recent reappearance followed thousands of years absence. Fossilized rat remains dating to the Pliocene and Pleistocene periods were found In Europe. Until the writings of Glraldus Cambrensls (1147-1223), there was no distinction between the R. rattus and mice. The late arrival of R. norvegicus to Europe is offset by its ferocious nature, essentially eradicating the black rat from its former strong holds. Today, the black rat is restricted to areas near water, and the brown rat has conquered the planet because of its climatic adaptability and ability to parasitize human refuse.

Today's laboratory rats are the domesticated descendants of Rattus norvegicus. Albino animals were held and used for rat shows, and frequent handling is thought to have tamed these animals. By the 1800s these animals were used for breeding and neuroanatomy studies in the United States and Europe. It was in the late 1800s and early 1900s that individual stocks and strains had their beginnings.

The laboratory rat has been, and continues to be a mainstay of biomedical research. Both albino and pigmented animals are available. There are recognized differences between wild and laboratory rodents. For example, laboratory rats have smaller adrenals and preputial glands, earlier sexual maturity, no reproductive cycle seasonability, better fecundity, and a shorter life span than their free-ranging wild counterparts.

nomenclature

Rats fall Into two basic groups depending on whether they are inbred or outbred. One generally refers to inbred animals as strains, and outbred animals as stocks. One develops inbred strains through at least 20 generations of brother-sister mat-ings, whereas outbred stocks have less than 1% inbreeding per generation and have been maintained in a closed colony for at least four generations.

Different stocks and strains show variability in many biological parameters including hematology, clinical chemistry, and anesthesia response. This variability is also observed in stocks and strains from different suppliers, so one should exercise caution in changing suppliers during mid-study.

There are specific nomenclature guidelines, with assistance available in using these guidelines. One may wish to consult the following:

ILAR, National Research Council, 2101 Constitution Avenue, Washington, D.C. 20418, U.S.A. Telephone: (202) 334-2590, Fax: (202) 334-1687;

PALM Institute, N29 W4 2-1-215 Sapporo 001, Japan. Telephone: 81-11-746-3988, Fax: 81-11-746-6722;

Registry of Inbred Strains, Dr. Michael F.W. Festing, IRC for Human Toxicology, Leicester University, University Road, Leicester LE2 7RH, UK;

Rat News Letter, 2542 Harlo Dr., Allison Park, Pittsburgh, PA 15101, U.S.A., Telephone: (412) 487-4289;

Transgenic Animal Database, TABD Coordinator, Oak Ridge National Laboratory, PO Box 2008, MS 6050, Oak Ridge, TN 37831-6050, USA, Telephone: (615) 574-7776, Fax: (615) 574-9888;

The Jackson Laboratory, Bar Harbor, ME 04609, U.S.A., Telephone: (207) 288-3371, Fax: (207) 288-8982.

behavior

Rats are nocturnal animals with most activity occurring at night and in the early morning. Changing the light cycle permits rats and investigators to share peak activity periods. This 12-hour shift will require a 2-week accommodation period for the rat. Although there are strain differences, rats are typically non-aggressive, inquisitive, and easily trainable. Frequent handling encourages their non-aggressive nature as they adapt to new surroundings or experimental situations. Improper handling, nutritional deficiencies, and vocalizations from other rats can result in undesired behavior. Males are usually more aggressive than females and when striking, bite once. Rats feel most comfortable in small, dark, confined spaces; a behavior investigators may use as a reward. When designing experiments, it is important to understand the rat's coprophagic behavior and its potential impact on metabolic, drug, and other studies. Male rats, unlike mice, are unlikely to fight when housed together. Rats also differ from mice in their willingness and acceptance of single housing.

anatomic and physiologic features

This section briefly summarizes the anatomic and physiologic characteristics of the laboratory rat. Special emphasis is placed upon those characteristics which are unique to the rat. Table 1 summarizes the basic biological parameters of the rat.

Table 1. Basic Biologic Parameters

Parameter Value

Lifespan (years) 2.5-3.5

Mammary glands 6 pr

Male body weight (g)* 450-520

Female body weight (g)* 250-300

Body temperature (rectal) 35.9-37.5°C

96.6-99.5°F O2 consumption (ml/m2/g body weight)f 0.84

Water intake (ml/100 g bw/day) 10-12

Gi transit time (hours) 12-24

Urine volume (ml/100 g bw/day) 5.5

Urine specific gravity 1.04-1.07

Total body water (ml)* 167

Intracellular fluid (ml)* 92.8

Extracellular fluid (ml)* 74.2

* Body weights will vary with stock or strain. f Based on a 250 g rat.

Oropharynx

• Dental formula: 2 (incisors 1/1 canines 0/0 premolars 0/0 molars 3/3) = 16.

• The incisors grow continuously.

• Lack water taste receptors found in other animals.

Salivary glands

• Three pair of salivary glands: parotid, submaxillary (submandibular), and sublingual.

• Parotid — The parotid salivary gland secretes a serous product, and consists of 3-4 lobes. The parotid gland extends ventrodorsally from behind the ear to the clavicle. The parotid duct opens opposite the molar teeth. The protein concentration (2%) of the saliva is unique.

• Submaxillary — The submaxillary (submandibu-lar) glands are mixed glands secreting a serous and mucous product. They are found in a ventral region between the mandibles and thoracic inlet. There are two types of secretory granules found in the submaxillary glands, one in the acinar cells and the other in the granulated portion of the secretory ducts. Secretory duct granules of immature animals contain substructures not found in adult animals.

• Sublingual — The sublingual glands are the smallest of the salivary glands and secrete a mucous product. The rounded glands may be found at the rostral aspect of the submaxillary glands, and may be found embedded in them.

Note: Brown fat is found in the ventral cervical region, and one should not confuse this structure with salivary glands or lymph nodes.

Esophagus

• The esophagus enters the lesser curvature of the stomach through a fold in the limiting ridge of the stomach. The fold prevents rats from vomiting.

• The esophageal lining is entirely keratinized epithelium.

Stomach

• The rat's stomach has nonglandular and glandular portions separated by the "limiting ridge." The nonglandular forestomach has a lining similar to the esophagus.

Small Intestine

Small intestine lengths and transit times vary with the age of the rat. The length values listed below are adult averages. The authors wish to direct the reader to the discussion by Varga concerning the interaction among age, intestine length, and transit time.

Large Intestine

• Cecum — The cecum is a thin-walled, comma-shaped pouch with a prominent lymphoid area found on the lateral aspect of the apex. Although the rat cecum does not possess an inner septa as seen in other rodents it has an inner constriction which divides the structure into apical and basilar sections. The lymphoid tissue is thought to be analogous to the vermiform appendix found in human beings.

• Colon — The colon has three divisions: ascending, transverse, and descending. The ascending portion has oblique mucosal ridges, whereas the mucosal folds of the transverse and descending regions have longitudinal mucosal folds.

• Rectum — The rectum is that region of the gastrointestinal tract found in the pelvic canal.

Liver

• Consists of four lobes: median, right lateral, left, and caudate.

• The bile from each lobe leaves via ducts. These ducts then form the common bile duct, which enters the duodenum approximately 25 mm distal to the pyloric sphincter.

Pancreas

• Consists of a lobulated, diffuse organ, extending from the duodenal loop to the gastrosplenic omentum. The pancreas has a darker color and firmer texture than the surrounding adipose tissue.

• The diffuse nature of the organ results in a network of ducts which coalesce into 2-8 larger ducts emptying into the common bile duct.

Urinary System

• Rats, like other rodents, possess a unipapillate kidney, which consists of one papilla and one calyx and enters the ureter directly.

• Long and short nephrons are present.

• Only animal whose kidneys contain significant amounts of L-amino acid oxidase.

• Female urethral orifice is at the base of the clitoris.

Reproductive System

To distinguish males and females, note that males have a greater anogenital distance than females, and larger genital papillae (Fig. 1).

female

• Six pair of mammary glands — 3 thoracic, 1 abdominal, 2 inguinal.

Female Rat Clitorid Gland
Fig. 1. Note the greater anogenital distance in the preweanling male rat on the right compared to the female rat on the left.

• Uterus is bicornuate and duplex consisting of two uterine horns, two cervices, and one vagina.

• Hemochorial discoid placentation.

• A copulatory plug forms from semen coagulation following copulation. Specifically it forms from secretions of the vesicular and coagulating glands, filling the reproductive tract from the vulva to the cervix. It will remain for a few hours following copulation, and then will decrease in size and fall out.

male

• Inguinal canal remains open throughout the animal's life.

• Accessory sex glands consist of:

• seminal vesicle

• bulbourethral glands

• coagulating glands

• preputial glands

Respiratory

• Normal respiratory function values are found in Table 2.

• Right lung consists of 4 lobes: cranial, middle, accessory, and caudal.

• Pulmonary veins contain striated heart muscle fibers continuous with those found in the heart.

• Bronchial constriction is under the control of the vagus nerve, not the adrenergic nerve supply.

• The lung is immature at birth and consequently devoid of alveoli, alveolar ducts, and respiratory bronchioles. Air exchange occurs through the smooth walled channels and saccules until 4-7 days following birth when remodeling occurs. Respiratory bronchioles are present 10 days following birth.

• There have been at least ten morphologically distinct cell types identified in the intrapulmonary airways. The epithelial serous cell is thought to be unique to the rat. It secretes a product which has a viscosity less than the mucous cell, and is thought to be responsible for the low-viscosity pericilliary liquid layer found at all levels of the rat's respiratory tract.

• Regulation of the respiratory system occurs through tissue CO2 exchange in the medullary respiratory center, with the carotid bodies playing a role. The carotid bodies, however, respond to low blood oxygen tension.

• The rat has the thinnest pulmonary artery and the thickest pulmonary vein studied. The thickness of the pulmonary vein is due to the cardiac striated fibers, which are mainly observed in the intrapulmonary branches. Unfortunately, this arrangement permits infectious agents to spread from the heart, through the pulmonary veins, into the lungs.

• As in man, precapillary anastomoses occur in the lungs, and are limited to the hilar region. Pulmonary vasculature will vasoconstrict in response to acetylcholine (0.2-0.5 pg).

• Rats have a high neuronal density, high serotonin activity, and low histamine activity in the lungs.

Note: Papers by Stahl and Leith present methods for comparing respiratory variables between mammals.

Table 2. Values for Respiratory Function

Parameter

Value

Tidal volume (ml)

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