Endocrinologic Control of Normal Canine Ovarian Function

Abstract

In dogs, the termination of the 3-10-month obligate anoestrus involves selection of a cohort of LH-sensitive follicles, presumably from a wave of dominant small antral follicles that would otherwise undergo atresia. The number and size of such follicles appears to increase, especially during the last 50 days of anoestrus when the already elevated concentrations of FSH become further elevated. The final selection and eventual terminal development of these follicles is caused by an increased frequency of high-amplitude LH pulses at the end of anoestrus. Concomitant increases in FSH are typically small or negligible. High concentrations of FSH in anoestrus are likely to be important in maintaining, if not stimulating, overlapping waves of dominant follicles throughout anoestrus, their expression of aromatase activity and basal oestradiol secretion sufficient to suppress LH by negative feedback. An attractive hypothesis is that late anoestrus increases in LH-stimulate synthesis of precursor androgen for already available FSH-dependent aromatase. After 7 or more days of elevated LH, and perhaps 2-5 days of semi-autonomous growth, with maximal oestradiol production reached, follicle capacity to further increase oestradiol becomes limited and excess progesterone becomes increasingly secreted. The pre-ovulatory LH surge and oestrus onset are then triggered - often synchronously and in concert with the terminal maturation of the follicles - by central effects of the large decrease in the oestrogen to progestin ratio. Follicular endocrine and paracrine events during and following the LH surge are likely similar to those reported for other species. The prolonged luteal phase lengths of 55-75 days in non-pregnant bitches bracket the 64 +/- 1 day in pregnancy and represent a genetically programmed luteal cell lifespan approximating gestation length as occurs in the luteal phase of hysterectomized animals of most polyoestrous artiodactyls and rodents. The 30-40-day slow regression after day 20 to 30 involves periodic cell death, diminution in cell size, low levels of apoptosis and minimal or modest involvement of endogenous prostaglandin F (PGF) production. The canine corpus luteum (CL) is dependent on both LH and prolactin as stimulating luteotrophins by day 15, and as required luteotrophins by days 20-25, if not earlier. Thereafter, both luteotrophins likely have cellular mechanisms of action similar to those reported for other species. Progesterone secretion during pregnancy is greatly enhanced by characteristic, and probably relaxin-stimulated, increases in prolactin concentration starting at or after day 25, and persisting to term. Near term, foetoplacental maturation results in the placental release of large, luteolytic amounts of PGF for 1-2 days pre-partum. Pre-partum luteolysis, like that induced by exogenous prostaglandin, likely involves a cascade enhanced by the removal of progesterone inhibition of PGF release and some degree of intra-luteal PGF synthesis. That a likely twofold or greater increase in progesterone ...

Evaluation of cell proliferation and endometrial thickness of bitches in different periods of diestrus

“…In the more developed secondary follicles with greater cell proliferation, the granulosa cells presented generally small NORs, dispersed throughout the nucleus and in significantly higher numbers, thus demonstrating that NORs with larger areas and low quantities indicate less cellular proliferation, while smaller NORs in greater quantities indicate increased cell proliferation. As E2 is responsible for cell proliferation in the endometrial cells (Van Cruchten et al 2004), a positive correlation can be established between E2 levels and cell proliferation; the periods in which most proliferation occurs, in the 40 days after ovulation, is also the period with the greatest production of E2 (Concannon 2009, Fonseca et al 2012, Papa and Hoffmann 2011. In the period in which the lowest proliferation occurs, at 60 days after ovulation, the lowest production of E2 throughout the diestrus period also occurs.…”

“…During the pre-ovulatory phase, the E2 reaches peaks of very high concentrations, but after ovulation a rapid fall occurs in this concentration which then turns, rising from the 25th day post ovulation, peaking at 45 days (Fonseca et al 2012). E2 presents varying concentrations and intermediate values from 15 to 30 pg/mL, being found at concentrations of approximately 12 pg/ mL 10 days after ovulation and rising to 40 days, after which it begins to show a decline, returning to the concentration of the 10th day 60 days post ovulation (Concannon 2009, Fonseca et al 2012, Papa and Hoffmann 2011.…”

Perfil de proteínas plasmáticas em cadelas gestantes e não gestantes

“…O estímulo para síntese proteica pode ser decorrente de ação hormonal, principalmente do estrógeno e corticosteroides (Harvey & West 1987). De fato, sabe-se que as concentrações de estrógeno e cortisol elevam-se ao final da gestação em cadelas (Concannon 2009). Desta maneira, é possível inferir que o aumento dos níveis de proteína total seja reflexo indireto da ação hormonal (estrógeno e cortisol), por estímulo ao anabolismo hepático.…”

Correlation of Oxytocin (OTR) and Estrogen Receptor (ER) mRNA in the Canine Placenta with the Detected Circulating Levels of Oxytocin and Estrogen during Pregnancy and Parturition

“…However, the canine placenta has a unique characteristic, as it lacks steroidogenesis to support pregnancy, leaving to the corpus luteum the main function to produce progesterone and estrogen (Hoffmann et al, 1994;Concannon, 2009). In fact, Nishiyama et al (1999) could not identify immunohistochemically the enzyme aromatase (estrogen convertase) and other steroidogenic enzymes in the canine placenta.…”

“…Moreover, the prepartum release of placental prostaglandin F2alpha is the main event responsible for canine luteolysis (Concannon, 2009). Nevertheless, it has been addressed in several species that a narrow relationship exists between estrogen (E), oxytocin (OT) and prostaglandins concentrations, especially towards the transition from pregnancy to parturition.…”