A Practical Approach, Second Edition=Ronald D. Ho.pdf

462 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONresulting in erroneous at-birth sex ratio observations. Offspring from sex steroid–treated rodentfemales commonly have malpositioned vaginal and urethral ducts. Agents with estrogenic activity(such as DES) can accelerate the occurrence of vaginal patency while other agents (such asphytoestrogens and TCDD) can delay vaginal opening. Infantile or malformed vagina or vulva(including masculinized vulva or increased anogenital distance) and/or altered timing of vaginalopening should be considered adverse developmental effects.The estrous cycle can be monitored in rodents by observing changes in the vaginal smearcytology. 139,140 Repetitive occurrence of the estrous cycle stages at regular normal intervals suggeststhat neuroendocrine control of the cycle and ovarian responses to that control are normal. A morecomplete description of the vaginal cytology associated with the estrous cycle is presented below.In reproduction and fertility studies, vaginal smear cytology should be examined daily for atleast three normal estrous cycles (approximately 2 weeks) prior to treatment, after onset of treatment,and before necropsy. 141 In addition to the determination of estrous cycle length and the occurrenceor persistence of estrus or diestrus, daily vaginal smear data provide the incidence of spontaneouspseudopregnancy, an altered endocrine state reflected by persistent diestrus. The daily vaginal smeardata can be used to distinguish pseudopregnancy from pregnancy based on the number of days thesmear remains leukocytic. The evaluation of vaginal cytology also can detect onset of reproductivesenescence in rodents. 142 Generally, the estrous cycle will be lengthened or the females will becomeacyclic. Lengthening of the cycle may be a result of increased duration of estrus or diestrus. Asignificant alteration in the interval between occurrences of estrus for a treatment group whencompared with the control group should be considered an adverse female reproductive effect.5. PituitaryIn histological evaluations of the rodent anterior pituitary, the relative sizes of the acidophils andbasophils have been reported to vary during the reproductive cycle and pregnancy. 143 Significanthistopathological damage involving cells in the anterior pituitary that control gonadotropin orprolactin production should be considered an adverse reproductive effect.6. Mammary Gland and LactationThe development and growth, histology, and pathology of the rat mammary glands have been welldocumented. 144 Mammary tissue is highly endocrine dependent for development and function. 145–147Development of the six pairs of mammary glands in rats occurs between birth and puberty.Toxic agents can adversely affect mammary gland size, histological appearance, and milkproduction and release, and many exogenous chemicals and drugs or their metabolites can betransferred into milk. 46,148,149 Milk may contain lipophilic agents (mobilized from the adipose tissue)at concentrations greater than those present in the blood or organs of the dam, thereby exposingthe suckling offspring to elevated levels of such agents.During lactation, the mammary glands can be dissected, weighed, and processed for histologicalexamination. Techniques have been developed for measuring mammary tissue development, milkproduction, and milk composition in rodents. 147 Milk production may be estimated by weighingmilk-deprived litters before and after nursing or by measuring the milk collected from the stomachsof pups at necropsy. The DNA, RNA, and lipid content of the mammary gland and the compositionof the milk can be measured as indicators of toxicity.Reduced pup growth may be caused by altered milk palatability, reduced milk availability, orby ingestion of a toxic agent secreted into the milk. However, other factors unrelated to lactationcan cause reduced pup growth (e.g., altered maternal behavior, stress, litter size, or poor suckingability in the pup). Significant milk production decrements or alterations in milk quality, whethermeasured directly or reflected in altered development of the pup, should be considered adversereproductive effects. 38© 2006 by Taylor & Francis Group, LLC