Ova / ovum - what is, definition and ovulation

What are Ova and Definition

The female cells of reproduction, also called eggs or gametes. An ovum, also called an oocyte (single egg cell), is a haploid cell; it contains one half of the genetic material necessary for human life. At birth the OVARIES contain about 400,000 follicles, each of which holds a single immature ovum. At PUBERTY the follicles begin to ripen, with usually one ovum coming to maturity with each MENSTRUAL CYCLE. Over the course of a woman’s reproductive years her ovaries produce 400 to 600 ripened ova. About 10 to 20 times as many ova begin but do not complete the maturation process. The ovaries eventually absorb ova that fail to reach maturity.

Ova and Ovulation

The sequence of hormonal and physiologic changes that bring an ovum to maturity is OVULATION, which takes place during the start of the menstrual cycle’s luteal phase around day 14 of the menstrual cycle (day 1 being the first day of MENSTRUATION). The PITUITARY GLAND releases first a surge of FOLLICLE-STIMULATING HORMONE (FSH), which activates an ovarian follicle. The follicle secretes ESTROGENS, which begin the maturation process for the ovum the follicle contains. The pituitary gland then secretes LUTEINIZING HORMONE (LH), which induces the ovarian follicle to produce PROGESTERONE. The progesterone brings the ovum to full maturity and the follicle ruptures, releasing the ovum for capture into the fallopian tube.

Fertilization, implantation, and conception

The smooth MUSCLE walls of the fallopian tube contract in a gentle, wavelike pattern that draws the ovum through the tube toward the UTERUS. When SPERM are also present in the fallopian tube, fertilization takes place. Typically, though many sperm attempt to penetrate the outer membrane of the ovum only one succeeds. The chemical composition of the ovum’s membrane alters once the sperm is within the ovum, preventing other sperm from following. The nuclei of the gametes (ovum and sperm) fuse to form a single diploid cell, called a ZYGOTE. As the zygote moves along the fallopian tube toward the uterus it continues to grow and divide. By the time the zygote reaches the uterus it has become a two-layered mass of cells called a blastocyst. The outer layer of the blastocyst attaches to the endometrium; as pregnancy continues this layer becomes the PLACENTA and the inner layer develops into the EMBRYO. The completion of fertilization and implantation is CONCEPTION.

For further discussion of the ova within the context of the structures and functions of reproduction and sexuality, please see the overview section “The Reproductive System.”


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The Reproductive System

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