REPRODUCTION
Reproductive
Hormone Assays
In Dogs and Cats
Dr Kate May , BVSc ( Pret ), MMed- Vet ( Gyn )( Pret ), DipACT Department of Production Animals , Theriogenology , Faculty of Veterinary Science kate . may @ up . ac . za
In order to fully understand the application of certain hormone assays currently available in small animals , we need to briefly recap the physiology behind it all .
The overall controlling structure in all reproductive pathways is the hypothalamus . The hypothalamus is a specialized ventral portion of the brain made up of various nerve cell bodies , such as the tonic centre , the surge centre and the pre-ventricular centre . The tonic centre is responsible for episodic ( basal ) release of the hormone , Gonadotrophin Releasing Hormone ( GnRH ). The hypothalamus is connected to the anterior lobe of the pituitary gland via the hypothalamo-hypophyseal portal system . Secretion of GnRH by the hypothalamus results in release of Follicle Stimulating Hormone ( FSH ) and Luteinizing Hormone ( LH ) from the anterior pituitary ( adenohypophysis ).
In the female , FSH stimulates recruitment and selection of a cohort of follicles on the ovary . In monotoccus species , once a single follicle becomes dominant , it starts to secrete inhibin and estradiol , which have negative feedback on the release of FSH from the adenohypohysis and also result in atresia of the non-dominant follicles .
Once the increasing amount of oestradiol from the dominant follicle reaches a threshold , it stimulates the surge centre in the hypothalamus to secrete a pre-ovulatory GnRH surge , which in turn causes an LH surge which results in final maturation and ovulation of the dominant follicle . Once the follicle has ovulated , the cells within the newly formed corpus luteum ( CL ) switch from oestradiol to progesterone secretion . The progesterone has negative feedback on both the hypothalamus and the adenohypophysis , therefore preventing further follicular development during dioestrus . The episodic secretion of FSH during dioestrus is enough to result in the recruitment of small follicles during dioestrus , however , these fail to produce enough oestradiol to cause the pre-ovulatory surge in GnRH and LH required for ovulation and these follicles soon regress .
The female animal then enters a period of dioestrus , the length of which depends on the species . In polyoestrous species like cows and horses , dioestrus will last for roughly 17 days , thereafter , if there has been no recognition of vet360
Issue 04 | AUGUST 2017 | 8