Ovarian asymmetry in the shrew, Crocidura russula monacha

Abstract

Summary. The total number of corpora lutea developed in the left and right ovaries of 170 white-toothed shrews (Crocidura russula monacha) differed significantly, showing a functional dominance of the left ovary. Blastocysts were equally distributed in both uterine horns.In most birds, only the left ovary is functional, although rare instances of individuals with both or only right ovaries have been reported (see Sadleir, 1973). In other vertebrate groups ovarian asym¬ metry is an unusual phenomenon and is rare among mammals, although it has been found in the platypus, in some marsupials and in eutherians (see Mossman & Duke, 1973;Wimsatt, 1975). The asymmetry varies from minor differences in the average weight of the two ovaries, to the occurrence of a nonfunctional ovary which can be appreciably smaller than the functional ovary.The data in the present paper were obtained during a wider study of reproduction in the whitetoothed shrew, Crocidura russula monacha, which is common in the Eastern Mediterranean region. Ovulation in this shrew occurs after mating or can be induced by exogenous hormones (see Hellwing, 1971 ;Funkenstein, 1974). In the course of a broader experiment on ovulation, sufficient data have emerged showing differences in the number of ova shed and of corpora lutea (CL) produced in the two ovaries. The animals used were 26 pregnant shrews trapped in the wild and 144 mature nulliparous females, born and raised in the breeding colony maintained at the Research Zoo of the Tel-Aviv University. Ovulation in captive shrews was artificially induced using PMSG (Gestyl: Organon) and FSH (NIH-FSH-S9) and the details will be reported elsewhere. Shrews were injected s.c. with doses of 0-5,1-0,2-5, 5-0,10, 25, 50 or 100 i.u. PMSG. The percentage of animals ovulating and the number of ovulated ova were dose dependent up to 5 i.u. PMSG. The FSH doses were 0-5,1-0,2-5,5-0,10,50,100 and 200 µg: doses of 0-5 and 1-0 µg FSH yielded a nearly maximal ovula¬ tory response. Only shrews that ovulated were scored. The mean number of ova shed after injection of PMSG or FSH, 3-14 and 3-17, respectively, was about the same as the natural litter size (3-0).Tissues for histological examination (ovaries, oviducts and uteri) were fixed in Bouin's fluid, embedded in paraffin wax, serially sectioned and stained with haematoxylin and eosin. In Crocidura russula monacha the number of CL corresponds to the number of ova shed (Funkenstein, 1974). Consequently, ovarian activity in this paper is expressed as the total number of corpora lutea counted in each ovary.The results presented in Table 1 show that in wild Crocidura russula monacha there was a numerical dominance of CL in the left ovary (67 %) but not in the right (52 %). The difference in number of corpora lutea in left and right ovaries in the same individual was significant : < 005 (t test for paired comparison, Sokal & Rohlf, 1969). Notwithstanding this degree of functional ovarian asymmetry in wild shrews the number of blastocysts was found to be distributed equally betwe...

Greater Loss of Female Embryos During Human Pregnancy: A Novel Mechanism

“…Asymmetric PITX2 expression is maintained and plays a role in subsequent organ development, and in birds, underlies asymmetric development of the gonads, reaching its most extreme manifestation in the single (left) ovary and oviduct of many species . Among mammals, functional asymmetry of left and right ovaries has been reported in many species, including mice, shrews, gerbils, viscachia, bats, and waterbuck . Although human gonadal asymmetry is perhaps most apparent in males, where the right testis is larger, the inherent directional asymmetry of vertebrate embryos demonstrates that the human left and right ovary are not equal from the earliest stages of development.…”