The Ovary (Figures 13.1 and 13.2)
The ovaries are the paired female gonads, homologous to the testes of the male. They include the ovarian follicles, each comprising a spherical aggregation of hormone- secreting cells together with a single ovum.
The ova are the female germ cells that undergo maturation in the ovaries and are finally released at ovulation. This process begins with a phase of enlargement forming the primary oocytes. The first of the two reduction divisions occurs before ovulation, forming the secondary oocyte and a first polar body. The second reduction division occurs after ovulation, in the uterine tube, and is triggered by the penetration of a spermatozoon; the ovum and second polar body result. Fertilisation occurs when the male and female pronuclei fuse, forming the zygote. The ovary is also an endocrine organ. In all mammals the ovaries are situated in the abdominal cavity, either in the sublumbar region as in the horse and dog or near the pelvic inlet as in the ruminants.In mammalian females the activity of the ovaries and, to a certain extent the reproductive tract, is controlled by endocrine release; this regular activity is termed the oes- trous cycle. In human females and in other primates the changes in the reproductive tract are characterised by limited haemorrhage due to shedding of the endometrium, and this activity is called the menstrual cycle. However, all mammals lose some endometrium and blood if the ova are not fertilized, but it is not usually a significant amount and, in any case, it is not always a monthly event as implied by the term menstrual (from the Latin mensis, meaning month). In non-primates any bleeding is more likely to originate from ovulation and the development of a corpus haemorrhagicum. This is followed by the development of a corpus luteum (from the Latin meaning ‘yellow body') This is a temporary endocrine structure comprising theca cells that produce mainly progesterone and oestradiol.
Luteinising hormone (LH) and follicle-stimulating hormone (FSH), both secreted by the pituitary gland, control the development and function of the corpus luteum cells. If the egg is not fertilised, the corpus luteum ceases production of progesterone and degenerates. If the egg is fertilised, the corpus luteum continues production of progesterone until the placenta can take over this function; the corpus luteum then degenerates.
Figure 13.1 The ovarian bursa - ventral views of the right ovary. 1 = mesosalpinx; 2 = uterine tube;
3 = ovarian bursa; 4 = mesometrium; 5 = proper ligament of ovary; 6 = ovary; 7 = mesovarium;
8 = infundibulum; 9 = follicles.
Figure 13.2 Diagrammatic transverse section of the basic ovarian bursa. 1 = mesosalpinx;
2 = uterine tube; 3 = ovarian bursa; 4 = mesometrium; 5 = proper ligament of ovary;
6 = ovary; 7 = mesovarium; 8 = infundibulum; 9 = follicles.
13.2