novel approaches to expression and detection of oestrus in dairy cows

ecomes the main inhibitor of FSH and dependency switches to LH (Ginther
et al., 1997). Growth of the dominant follicle continues, and the increase in
oestradiol causes increased GnRH pulse frequencies, promoting LH
secretion. This in turn stimulates oestradiol production by the granulosa
cells (Fortune, 1994). This has a local effect on follicle development, but
also a systemic effect, acting upon the hypothalamic-pituitary axis to
increase LH production further, by positive feedback (Aerts and Bols,
2010). This ultimately results in oestrus, controlled by oestradiol, and the
LH surge resulting in ovulation.
1.2.1.2 Oestrus & Ovulation
It is this rise in oestradiol; enhanced by LH, stimulating production of
androgen in the theca cells (Garverick et al., 2002) and the subsequent
androgens being converted into oestradiol by aromatase enzyme from
granulosa cells, which causes oestrus (Fortune, 1994). The positive
feedback mechanism between oestradiol and LH, causes LH pulse
frequency to increase to about 1 pulse per hour (Roche, 2006). The
increase in LH concentration causes a cascade of events that induce the
release of the oocyte into the oviduct, by an inflammatory response;
involving prostaglandins, particularly prostaglandin E (PGE), produced by
the follicle (Aerts and Bols, 2010). Prostaglandins stimulate the
proliferation of cells and production of proteolytic enzymes to disrupt the
follicle wall, releasing the oocyte (Espey, 1980). This process is ultimately
under the control of the follicle itself, timing when it is appropriate to
trigger the LH surge for ovulation by production of oestradiol (Roelofs et
al., 2010), usually about 10-14 hours after oestrus (Forde et al., 2011).
1.2.1.3 Luteal Phase
LH is the key hormone stimulating luteinisation of the theca and granulosa
cells post ovulation, forming the CL from the cells of the ruptured cavity
(Alila and Hansel, 1984). The CL consists of small and large luteal cells,
which have steroidogenic properties (Smith et al., 1994), which secrete
progesterone, along with a range of other cell types. The function of the CL
is to produce progesterone, in order to maintain pregnancy if a conceptus
is present (Forde et al., 2011). Sustained production of progesterone
suppresses GnRH pulse frequency and hence LH secretion to prevent
ovulation, but does allow enough LH for the continuation of follicular waves
and dominant follicle growth (Savio et al., 1990).
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