Hormonal changes

Ovulation and the ocstrous cycle arc associated with a cycle of interrelated Iiormoniil changes in the ovary and anterior pituitary gland (see Ch. 6. Eig. 6.4):

1. External stimuli such as increasing daylenglh or a rise in environmental temperature Stimuliitc the hypothalamus in the forebrain.

2. The anterior pituitary secretesJollidt^, stiιnιιl(∣tiιi(∣ hormone (FSl 11, which stimulates a few primary follicles in Iheovary to ripen IntoGraaiian follicles.

L The ripening follicles secrete increasing levels of oestrogen, which:

- Produces the behaviour of the female seen during proestrus

- Prepares the reproductive tract for mating

- Stimulates the secretion of Iuteinising hormone (LH) from the anterior pituitary gland

- Inhibits further secretion of FSH.

4. As a result of FSH inhibition, oestrogen levels begin to fall. LH acts on the ripe follicles and brings about ovulation. After the release of the ovum, the remaining follicular tissue becomes Iuteinised. i.e. converted into the corpus Iuteum.

5. The corpus Iuteum begins to secrete progesterone. The increasing levels of progesterone and the decreasing levels of oestrogen stimulate mating behaviour in the bitch she will allow the male to mount her.

6. Progesterone is the dominant hormone during pregnancy. It:

- Prepares the reproductive tract to receive the fertilised ova

- Causcseiilargcmcnt of (he mammary glands

- Inhibits the secretion of GRH from the hypothalamus, which inhibits FSH output and prevents the development of any more follicles.

If the animal has not conceived, the corpus Iutcum regresses and the cycle begins again.

Pseudopregnancy (false pregnancy or pseudocyesis) is linked to the fact that the corpus Iuteum of the bitch remains in the ovary for about 6-7 weeks, whether she is pregnant or not Progesterone levels remain high and produce symptoms such as maternal behaviour, enlargement of mammary glands and lactation. Progesterone levels are normally high in all bitches, but clinical signs only occur in certain bitches.

Fig. 11.10 Phases of the oestrus eye'x? of the t>tch

The hormonal cycle of the queen is slightly different. As the queen is an induced ovulator, progesterone secreted by the corpus Iuteum will only be present if she is mated.