Ovaries

Ovaries are egg-producing reproductive organs found in female organisms.

Mammalian ovary
Ovaries are part of the vertebrate female reproductive system. Normally, a female will have two ovaries, each performing two major functions: producing eggs, or (exocrine function) and secreting hormones, or (endocrine function). Ovaries in females are homologous to testes in males. The term gonads refers to the ovaries in females and testes in males. Most birds have only one functioning ovary; snakes have two, one in front of the other. As female mammals develop within the womb, each ovary develops a number of immature eggs associated with groups of other cells called follicles. While mammals were thought to develop their entire supply of eggs prenatally and soon after birth, new evidence from laboratory mice has called this into question, showing that female mice in fact produce new eggs throughout their reproductive lifetime. However, there is no direct evidence showing that human females produce new eggs after birth. As the animal becomes reproductively mature (the process called puberty in humans), eggs will periodically mature and be released from the ovary (a process called ovulation) so that they will be available for fertilization by sperm. A fertilized egg resulting from union with a sperm becomes a zygote and then an embryo as it develops.



Animal and human ovaries also produce various steroid and peptide hormones. Estrogen and progesterone are the most important of these in mammals. These hormones induce and maintain the physical changes of puberty and the secondary sex characteristics. They support maturation of the uterine endometrium in preparation of implantation of a fertilized egg. They provide signals to the hypothalamus and pituitary that help maintain the menstrual cycle. Estrogen plays an important role in maintaining subcutaneous fat, bone strength, and some aspects of brain function.

In humans, an egg launched from an ovary has to traverse a slight space before entering the fallopian tube and moving gradually down to the uterus. If fertilized, it implants itself into the lining of the uterus and develops as the pregnancy continues. If the fertilized egg settles into the fallopian tube instead of the uterus an ectopic pregnancy will result. Ectopic pregnancy can also happen if a fertilized egg settles onto the cervix or onto the ovary itself, or if a fertilized egg passes through the gap between the ovary and the fallopian tube into the abdomen. If the egg fails to release from the follicle in the ovary an ovarian cyst may form. Small ovarian cysts are common in healthy women but large cysts can be an advanced manifestation of polycystic ovary syndrome.

Human anatomy
In the human the paired ovaries lie within the pelvic cavity, on either side of the uterus, to which they are attached via a fibrous cord called the ovarian ligament. This end of the ovary is called the uterine extremity. The uterine tube (aka oviduct or fallopian tube) attachs to the ovary at the is tubal extremity. The part of the broad ligament of the uterus that covers these structures is known as the mesovarium. The ovaries are uncovered in the peritoneal cavity but are tethered to the body wall via the suspensory ligament. Each ovary receives blood from the ovarian (or gonadal) artery - which arises directly from the abdominal aorta - and the ovarian branch of the uterine artery that enters the ovary by way of the broad ligament and thus the mesovarium. The ovary (for a given side) is located in the lateral wall of the pelvis in a region called the ovarian fossa. The fossa usually lies beneath the external iliac artery and behind the ureter.

Ovaries are oval shaped and, in the human, measure approximately 3 cm x 1.5 cm x 1.5 cm. Functionally ovaries are involved in a number of processes: production of the female sex hormones (i.e., oestrogens and progesterone), storage and ovulation of the female gametes (oocytes) and formation of the corpus luteum to ensure regulation of the menstrual cycle and survival of the early embryo.

The adult ovary is a heterogeneous organ consisting of many cell types that lie in a dense collagenous extracellular matrix (ECM). Histologically, the ovary is divided into two distinct regions: the inner highly vascular medulla, and the peripheral cortex, which is comparatively avascular and contains the all-important oocytes. The ovary is surrounded by an outer fibrous connective tissue (tunica albuginea), with a specialised mesothelium (ovarian surface epithelium) that encapsulates the entire organ (Fig. 1.1b).

Extremities of the ovary
The end to which the uterine tube attach is called the tubal extremity. Also extending from this extremity are the ovarian artery and the ovarian vein, which are covered by a fold of peritoneum known as the suspensory ligament of the ovary.

The other extremity is called the uterine extremity. It points downward, and it is attached to the uterus via a fibrous cord called the ovarian ligament. The part of the broad ligament of the uterus that covers these structures is known as the mesovarium.