Ovary

OVARY

  • This histological picture depends on the plane of section and phase of sexual cycle.
  • Ovary can be divided into two zones - cortex (zona parenchymatosa) and medulla (zona vasculosa).
  • It is surrounded by a tunica albuginea which itself is covered by a germinal epithelium. In the young this germinal epithelium is a single layer of cuboidal or columnar cells, which become flattened or squamous in adult. It is continuous with peritoneal mesothelium.
  • Tunica albuginea is rich in collagenous fibres. The cortical stroma is made up of a special type of connective tissue, which is predominantly cellular.
  • The connective tissue cells are fusiform or spindle shaped with elongated nuclei and is placed in a network of delicate collagenous fibres. Elastic fibres are practically absent.
  • Scattered in this stroma are the glandular structure of ovary, the ovarian follicles. Each follicle consists of an ovum surrounded by follicular epithelial cells.
  • In the embryo, germinal epithelium contains primordial germ cells. It forms cord-like proliferation, which grows down into the cortex as egg tubes of Pfluger.
  • It contains primitive ova surrounded by a layer of small cuboidal cells. The cortical stroma grown in between these primitive ova or oocytes, thus forming primary follicles.
  • At birth the tunica albuginea separates the germinal epithelium from the cortex. Thus at birth, the ovary contains all the primary follicles and no new follicles are supposed to form after formation of Tunica albuginea.
  • In a mature or adult animal, the cortex will contain ovarian follicles, in various stages of development and regressions. Smaller follicles with the ovum surrounded by a layer of follicular cells are located near the periphery.
  • They pass deeper into the ovary as they become larger and develop further but the matured follicle again approaches the surface. The follicle are described as primary, secondary, Graafian and mature Graafian follicles.

Primary Follicle

  • Are of 30-35 microns in diameter. Each contains an Oocyte surrounded by a layer of follicular cells and a basement membrane.
  • The follicular cells are first later becoming cuboidal or columnar. The follicular cells proliferate and become two layered and then becomes multilayered surrounding the Ovum. These are the secondary follicles.
  • The Ovum increases size and may occupy an eccentric position and develops a homogenous envelop zona pellucida between it and follicular cells and the stroma becomes more distinct.
  • Further, growth of the follicle is characterized by proliferation of follicular cells which form a stratified layer around the ovum. Irregular spaces appear in the follicular mass which fuse to form a crescent shaped cavity the anturm folliculi.
  • The cavity contains a fluid called the Liquor folliculi. These follicles with the formation of cavity are called Graafian follicles. The stromal cells surrounding the follicles form a sheath called Theca folliculi.
  • As the follicle increases in size and matures, the follicular cavity with its contained fluid increases in size.
  • In a mature follicle, the Ovum is pressed to one side where it is surrounded by an accumulation of follicular cells, it forms a definite projection into the cavity known as the germ hill or discus proligerus or cumulus oophorus.
  • The Ovum with the nucleus (germinal vesicle) and nucleolus (germinal spot) is surrounded by a thick homogenous membrane the zona pellucida.
  • Surrounding this are tall columnar follicular cells arranged radially forming the corona radiata. The follicular cavity is lined by several layers of cuboidal or polyhedral follicular cells forming the membrana granulosa.
  • There is a homogenous basement membrane surrounding the membrana granulosa. The theca folliculi or the follicular sheath differentiates into a theca interna containing numerous epithelioid, connective tissue cells and extensive capillary network and a theca externa consisting predominantly of fibres circularly arranged and closely packed and spindle shaped cells.
  • The mature Graafian follicle extends through the whole thickness of cortex and bulges on the surface of the Ovary. At this place, the stigma and the oocyte become thin and the follicle distended with fluid.

Ovulation

  • It is the release of the fully grown oocyte from the follicle at the surface of the ovary. The projecting follicle ruptures at the stigma through the albuginea ova escapes with the corona radiata cells.

Oogenesis

  • The ovum also undergoes maturation or reduction division, before it is fertilized. The egg cells of primary follicles are often described as oogonia but are actually primary oocytes, which have entered a period of rest.
  • It is believed that proliferation of oogonia and differentiation into primary oocytes are completed before birth a period of rest.
  • The primary oocytes give rise by reduction division to a secondary oocyte (containing the haploid number of chromosomes) and the first polar body. The polar body receives very little cytoplasm and soon degenerates.
  • The secondary oocyte again divides giving rise to a ootid or mature ovum and a second polar body, which degenerates. Generally the first maturation division occurs in the developing follicle and the second division is completed after ovulation and at the time of or after fertilization.

Corpus luteum

  • After rupture, the follicular cavity closes over by healing and becomes filled with a sero-fibrinous fluid, usually containing blood. This develops into temporary glandular structure known as corpus luteum.
  • The granulosa cells enlarge, the nuclei become vesicular and stain lightly. The cytoplasm shows progressively greater accumulation of lipid droplets and yellowish pigment granules. These are known as granulosa lutein cells.
  • The epitehlioid cells of theca interna also proliferate and show fatty droplets in the cytoplasm and form theca lutein cells. These are located peripherally and are smaller than granulosa lutein cells and have more compact, darker staining nuclei.
  • Connective tissue fibres from the theca externa penetrates into the lutein mass and forms interlacing septa, in which are numerous capillaries present. The follicular cavity is greatly reduced and disappears as the corpus luteum enlarges further.
  • If the discharged ovum is not fertilized, the corpus luteum develops after an initial period of development. This is known as cyclical corpus luteum or corpus luteum spurium (menstruations). The cells gradually decrease in size become vacuolated and are resorbed. The connective tissue increases in amount and finally there is transformation into a small body of connective tissue – the corpus albicans.
  • It the ovum is fertilized and pregnancy occurs, the corpus luteum increases in size and becomes corpus luteum of pregnancy or corpus luteum verum. It persists until later months of pregnancy. After termination of pregnancy, it undergoes slow involution, becoming transformed into corpus albicans.

Atretic follicles

  • Only a few of the oocytes in the follicle reach maturity. Most of the occytes together with their follicles undergo artesia. This may set in at any stage of development. The process involves degeneration of oocyte and invasion of the follicle by the internal cells.
  • Medulla of the Ovary
    • There is network of fibro-elastic tissue with numerous blood and nerve fibres.
  • Hormones of the Ovary
    • The ovary is under the direct influence of Gonadotrophic hormones of anterior pituitary.
    • The Follicular Stimulating Hormone (FSH) controls the maturation of the follicle and the Leutinzing hormone (LH) controls the formation of corpus luteum.
    • The maturing Graafian follicles of the Ovary produce oestrogen and the corpus luteum produces progesterone.
    • Since there is regular cyclical development of the follicle its maturation and subsequent development of corpus luteum after ovulation, there is a corresponding cyclical fluctuation in the levels of the two hormones secreted by the ovary. These are reflected in cyclical variations in the structure of oviduct, uterus and vagina in addition to changes in the behaviour of the animal.
    • The cycle itself is referred to as Oestrus cycle and considerable variations exist in different animals with regard to the duration of the oestrus cycle, different stages of the cycle, its periodicity and the nature and extent of changes in the various parts of the reproductive tract.

Species difference

  • In the mare, there is no division of the ovary into the cortex and medulla. The peripheral part contains blood vessels (corresponding to the medulla of other animals) except at the ovulatory fossa, located at the free border, which is covered by germinal epithelium.
  • The number of follicle that ripens in one cycle is more or less fixed for each species. Uniparous animals usually produce only one ovum at a time. The diameter of mature follicles is as follows: Women –9-12 m.m., mare –up to –10 m.m., Cow – 20 m.m., Ewe,Goat, sow-5-8 m.m., Bitch and Cat – 2 m.m.In carnivores and the sow, some follicles may contain 2 to 6 oocytes. (Giant follicles).
  • The lutein cells of the corpus luteum in the bitch and cat contain a little or no pigment and the corpus luteum is pale in colour. In the ewe, goat and sow, it is grayish white or flesh coloured because of the absence of pigment in the lutein cells. In the cow, the colour varies from bright yellow to orange and brown as the corpus luteum develops and perists during pregnancy, finally turning brick-red in a regressing corpus luteum.
Last modified: Tuesday, 24 August 2010, 5:32 AM