16 



GESTATION 



M. X. Zarrow, Ph.D. 



PROFESSOR OF ZOOLOGY, PURDUE UNIVERSITY, LAFAYETTE, INDIANA 



I. Introduction 958 



II. Length of Gestation 958 



III. Normal Reproductive Potential... 959 



IV. Environment 962 



A. Crowding 962 



B. Body Temperature and Hypoxia. . . 962 

 V. Maternal Hormone Levels during 



Gestation 963 



A. Estrogens 964 



B. Gestagens 964 



C. Sources of Gestagens 972 



D. Relaxin 973 



E. Sources of Relaxin 976 



F. Adrenal Cortex 976 



L Hydrocortisone 976 



2. Aldosterone 979 



G. Thyroid Gland 979 



H. Growth Hormone 980 



I. Prolactin 981 



J. Placental Gonadotrophins 982 



1. Human chorionic gonadotrophin 



(HCG) 983 



2. Equine gonadotrophin (PMS). . . 985 

 VI. Pregnancy Tests 986 



VII. Water and Electrolyte Balance.. 988 



VIII. Plasma Proteins 993 



IX. Renal Function 994 



X. Enzymes 995 



A. Histaminase 995 



B. Carbonic Anhydrase 996 



XI. Factors in the Maintenance of Ges- 

 tation 997 



A. Thyroid Gland 997 



B. Adrenal Cortex 1000 



C. Pancreas 1003 



D. Ovarv: Progesterone, Estradiol, 



and Relaxin 1003 



E. Pituitarv Gland 1005 



F. Placenta 1006 



G. Pelvic Adaptation 1008 



H. Dilation of the Uterine Cervix .... 1011 



XII. Uterine Myometrial Activity 1013 



XIII. Parturition 1015 



A. Progesterone 1015 



B. Oxvtocin 1015 



C. Relaxin 1016 



D. Labor 1017 



^ Aided by grants from the Purdue Research 

 Foundation. 



XIV. Conclusion. 

 XV. References 



1018 

 1018 



I. Introduction 



Reproduction in the animal kingdom is 

 accomiilished by a wide variety of methods, 

 from simple budding and binary fission in 

 the invertebrates to gestation in the mam- 

 mal and the development of a new organ, 

 the placenta. The development of vivipar- 

 ity, which covers millions of years of evo- 

 lution, brought with it many new prob- 

 lems, and with each problem new factors 

 came into play so that reproduction in the 

 mammal is a highly co-ordinated series of 

 events — a co-ordination that is both tem- 

 poral and spatial, that requires certain 

 events to occur in a proper sequential ar- 

 rangement, and, above all, is dependent on 

 the endocrine system. 



It is obvious that the maintance of ges- 

 tation in the mammal is a complex phenom- 

 enon. It involves directly or indirectly a 

 major portion of the endocrine system with 

 concomitant changes in the general meta- 

 bolic state of the organism and in many 

 of the enzymes present in the blood and the 

 tissues. Finally, a new endocrine organ, the 

 placenta, comes into being also to play its 

 specific role in gestation. 



II. Length of Gestation 



The duration of gestation is highly vari- 

 able and depends primarily on the species 

 involved. In general, the longer the gesta- 

 tion, the more self-sufficient and mature 

 are the young at the time of birth. It is 

 obvious, however, that this is not true 

 under all conditions. The young of the 

 guinea pig are highly advanced at birth, 

 although the length of gestation is approxi- 



958 



