The Establishment and Maintenance of Pregnancy

Abstract

In many submammalian species considerable energy is expended by the female to produce liver and/or reproductive tract secretions which are incorporated into ova prior to ovulation. Fertilization of these ova may or may not occur. Females of these submammalian species, therefore, expend energy for the reproductive process regardless of whether offspring result. In mammalian species, such as the pig, restrictions are placed upon the extent to which energy is expended for support of endometrial secretory activity. In the absence of fertilized ova and blastocysts developing in synchrony with the uterine endometrium, the period of endometrial secretory activity is limited primarily to the mid to late luteal phase of the oestrous cycle. In the pig, endometrial secretory activity is dependent upon maintenance of functional corpora lutea (CL). The lifespan of the CL, in turn, is limited during the oestrous cycle by endometrial production of a uterine luteolytic factor which is assumed to be prostaglandin F2a (PGF2a). Release of PGF2c, from non-pregnant uterine endometrium results in morphological regression of CL, progesterone secretion ceases and, therefore, progesterone-dependent endometrial secretory activity is terminated in the late luteal phase of the oestrous cycle. In pigs having normally developing blastocysts, endometrial—blastocyst interactions begin to occur by day 11 of pregnancy. Blastocysts signal their presence, presumably through oestrogen production, which results in CL maintenance and continued endometrial development and secretory activity. Species, such as the pig, that have central or fusion implantation (Schlafke and Enders, 1975) appear to depend upon endonietrial histotroph (Amoroso, 1952) for a major portion of pregnancy. Endometrial secretions of the pig are assumed to contain both a luteolytic factor (PGF20,) and embryotrophic (histotroph) factors. Blastocysts produce oestrogen which is believed to initiate events which result in CL maintenance, i.e. oestrogen acts as a luteostatic factor. This chapter will discuss evidence for the theory that oestrogens, of blastocyst origin, allow for continued secretion of PGF2o and other components of endometrial histotroph into the uterine lumen, i.e. in an exocrine direction, which prevents their release toward the uterine vascular bed, i.e. in an endocrine direction. This mechanism appears to be essential for the establishment and maintenance of pregnancy in pigs.