Morphometric analysis and function in vivo and in vitro of corpora lutea from ewes treated with LHRH during seasonal anoestrus.

Ovulation was induced by 3 X 30 micrograms LHRH i.v. at 90-min intervals in anoestrous Corriedale ewes. Plasma LH surges occurred in all of 31 ewes given LHRH, but ovulation occurred in only 16 of these ewes. Luteal weight and plasma progesterone concentration were lower in ewes in which ovulation was induced during anoestrus than in cyclic control ewes in the breeding season, and when data from induced and control ewes were pooled luteal weight was strongly correlated with plasma progesterone concentration (r = +0.612, P less than 0.01). Five mature corpora lutea (CL) resulting from ovulation induced during anoestrus were compared by morphometric methods with 5 CL from cyclic control ewes. When data from induced and control CL were pooled, luteal volume was positively correlated with total number of cells per CL (r = +0.869, P less than 0.01) but negatively correlated with number of cells per mm3 luteal tissue (r = -0.676, P less than 0.05), i.e. smaller CL contained fewer cells, but more cells per unit volume. Relative numbers of large to small luteal cells, at approximately equal to 1:6, were similar in LHRH-induced and cyclic control CL. Large and small luteal cells were smaller in induced CL than in control CL, but cytoplasmic:nuclear ratio did not differ between induced and control CL. Basal and LH-stimulated progesterone production by dispersed luteal cells in vitro were lower for CL from LHRH-treated ewes than from controls. However, percentage increase in progesterone production in response to LH was not different between LHRH-treated and control ewes at any dose rate of LH used. It is concluded that the small size of CL induced by LHRH is due primarily to the low numbers and small volumes of the luteal cells in these CL, and that subnormal luteal weight and subnormal progesterone production per luteal cell contribute to the low plasma progesterone concentrations in ewes treated with LHRH during seasonal anoestrus.