Updated by: Arabinda Naik
The ovaries are primary sex organs in females. The ovaries are paired structures. Each ovary is 3.5 cm (about 1.38 in) long, 2 cm (about 0.79 in) wide and 1 cm (about 0.39 in) thick. The ovaries are on each side of the lower abdomen. It is connected to the walls of the pelvis and uterus through Ovarian ligaments. Ovary is. suspended from the abdominal wall by a broad ligament of the uterus called mesovarium.
Each ovary is covered by a layer called germinal epithelium. The germinal epithelium is covered by visceral peritoneum. Underneath the germinal epithelium lies a layer of connective tissue called tunica albuginea. Underlying this layer is stroma. Stroma is further divided into dense outer cortex and less dense inner medulla.
Follicular development
Primordial follicle: During fetal development, the primordial germ cells divide mitotically to produce millions of germ cells. A few of these germ cells developed into large cells called primary oocytes surrounded by a single layer of follicular cells called primary follicles.
Primary follicle: At birth, the ovary contains around 400 000 primordial follicles which contain primary oocytes that are diploid having 2n number of chromosomes and are starting their first meiotic division. These primary oocytes do not undergo further mitotic division, and they remain arrested in the prophase I stage of meiotic division I, until sexual maturity.
At sexual maturity, two hormones, produced by the pituitary gland:
follicle stimulating hormone (FSH) and luteinizing hormone (LH) cause these primordial follicles to develop. Thus completed the first meiotic cell division and primary oocyte divided into two n chromosomes. First polar body and secondary oocyte. The secondary oocyte begins meiosis II but again is arrested at metaphase II. Meiosis II is completed only if it is fertilized with sperm.
Follicular atresia: There are around four lakh follicles in both the ovaries of an adult woman. Most of the follicles disappear by phagocytosis during reproductive years. This process is called follicular atresia. Such follicles are known as atretic follicles. Due to this a female produces only around 450 ova in her entire reproductive life which ends between 40 and 50 years of age.
Secondary follicle: Primary oocytes are surrounded by more layers of granulosa cells called secondary follicles.
Tertiary follicle: The secondary follicle changes into a tertiary follicle. A cavity which is filled up with a fluid called follicular antrum.
Graafian follicle: The tertiary follicle is further converted into a mature follicle called Graafian follicle. Secondary oocyte covered by zona pellucida. When surrounding cells form several layers, they are called granulosa cells. The follicle has a follicular antrum filled with fluid. A solid mass of the follicular cells that surrounds an oocyte is called cumulus oophoricus. The granulosa cells lying close to oocytes and zona pellucida become elongated to form corona radiata. The membrane granulosa cells are surrounded by the theca interna and theca externa.
Ovulation: The release of secondary oocytes from the ovary is called ovulation. It raptures the Graafian follicle, and the secondary oocyte is released. It occurs on the 14th day of the 28-day menstrual cycle.
Corpus luteum: At ovulation, the follicle expels the secondary oocyte into the fallopian tube. The remnants of the follicle are called corpus luteum. The antrum of the collapsed follicle fills with a clotted fluid. The follicular cells enlarge and fill with a yellow pigment called lutein. Corpus luteum is an endocrine structure that secretes two hormones such as
. If fertilization occurs and pregnancy is established the corpus luteum persists and there is no more menstruation. If there is no pregnancy the corpus luteum degenerates into a structure called corpus albicans.
Corpus albicans: When fertilization does not take place the corpus luteum forms a white fibrous tissue called the corpus albicans.
Functions of ovaries: They produce female gametes called ova by the process of oogenesis. The growing Graafian follicle secrete the female sex hormone called estrogen which is responsible for development of secondary sex characters in females.