Brian L Speer, DVM
There is some debate out there in avian medicine circles as to when reproductive medicine ends and pediatric medicine begins - and this author is intentionally including both areas for discussion, because the two are somewhat inter linked and interdependent. For the purposes of this paper, the two will be combined to provide a good review of pertinent reproductive anatomy, followed by embryology and normal and abnormal pediatrics.
The Female Reproductive Tract
The female reproductive tract consists of the left ovary and the left oviduct in most birds. The left and right ovary and oviducts develop embryologically as paired structures, but, after hatching, the right ovary and oviduct degenerate. If the left ovary and oviduct are removed from a chick before 30 days of age, the remnants of the right ovary will develop into an ovotestes. Two ovaries are noted in some birds of prey, but it is rare for the right ovary to be functional. The Brown Kiwi has two functional ovaries and one functional oviduct. In an exception to the general rule of one ovary, right and left ovaries have been observed in it least 16 orders of birds which are assumed to have only one ovary.
The left ovary is located in the coelomic cavity cranial to the left kidney and adjacent to the adrenal gland. The ovary is attached to the dorsal body wall by the mesovarian ligament, which can have considerably large blood vessels during an active breeding cycle. Surgical access as well as threat of hemorrhage makes a psittacine ovariectomy a particularly challenging procedure in an adult hen. The ovarian arterial blood supply is usually from the ovario-oviductal branch of the left cranial renal artery, which comes from the descending aorta between the cranial mesenteric artery and the external iliac artery. The venous drainage is via two ovarian veins directly into the vena cava. The ovary of the mature bird will have a "grape cluster" appearance, wheras the immature hen will have a ovarian surface frequently described as "brain-like" in appearance.
The ovary histologically consists of two major portions: the medulla and the cortex. The medulla contains connective tissue, nerves, smooth muscle and blood vessels. The cortex covers the medulla externally and contains the primary oocytes. These oocytes have developed from a set number of prenatal oogonia by the time the female bird has hatched. Within the ovarian cortex of the adult hen, several hundred to several thousand primary oocytoes may be visible to the naked eye. About twelve thousand are visible microscopically. Very few of these will enter the stage of rapid growth and actually develop beyond the primary oocyte stage. Primary oocytes visible on the surface of the ovary are termed follicles, which pertain to the primary oocyte and its membranous covering. After completion of the rapid growth period, the primary oocyte undergoes two maturation divisions. The first division occurs about two hours prior to ovulation and forms the secondary oocyte and the first polar body. This first maturation division is meiotic, resulting in a secondary oocyte which contains one half the adult bird's number of chromosomes. The second maturation division which forms ovum and second polar body occurs in the oviduct. Penetration by the spermatozoan is probably needed before this division can be completed. Fertilization of the ovum occurs within 15 minutes of ovulation, and presumably occurs in the infundibular oviduct.
Ovarian developmental activity stages noted in the mature hen include: 1) Prenuptial acceleration - the enlargement of the ovarian follicles, 2) Culmination - Ovulation and oviposition, and 3) Refractory period - ovarian follicles reduce in size.
The budgerigar and the crow are examples of determinant layers, meaning that they lay a fixed number of eggs. Many other birds, including chickens, ducks and most large psittacines, are indeterminant layers, meaning that they will be quick to replace eggs that are lost from their clutch or are removed. Aviculturists take advantage of this physiologic trait of indeterminant laying and remove eggs from the nest for artificial incubation, knowing that the parents will often "double clutch". The success of the propagation efforts of the California Condor were, in large part, due to wise use of the double clutch phenomenon.
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