What Is the Process by Which Eggs Are Made Called?

Egg production in females is a complex biological process essential for reproduction. It begins before birth and continues until menopause, involving intricate cellular changes and hormonal regulation to ensure the development of viable eggs.

Key Terminology

The process of egg formation is called oogenesis, derived from the Greek words “oon” (egg) and “genesis” (origin or creation). This process occurs in the ovaries and involves a series of cellular transformations that produce mature oocytes capable of fertilization. Unlike spermatogenesis, which continuously generates sperm, oogenesis begins during fetal development and pauses at various stages until reproductive maturity.

At the core of oogenesis is the oogonium, the earliest form of the egg cell. These primordial germ cells proliferate through mitosis before differentiating into primary oocytes, which enter a prolonged phase of meiotic arrest in prophase I. This arrest can last for decades, resuming in select oocytes during each menstrual cycle.

Folliculogenesis describes the development of ovarian follicles that house and support maturing oocytes. Each follicle consists of granulosa and theca cells, which produce hormones and provide structural support. A dominant follicle is selected for ovulation each cycle, while non-selected follicles undergo atresia, or degeneration.

Anatomical Sites of Egg Development

Egg development occurs in the ovaries, paired reproductive organs located on either side of the uterus. These structures house a finite pool of oocytes enclosed within follicles, which serve as the fundamental units of ovarian function. The ovarian cortex contains follicles at various stages of development, while the medulla is rich in blood vessels and connective tissue that support follicular growth.

Granulosa cells within the follicle facilitate nutrient exchange and secrete estrogen, essential for follicular expansion. Theca cells contribute to androgen production, which granulosa cells convert into estrogen. This interplay ensures oocytes receive necessary biochemical support. As the dominant follicle enlarges, it forms a fluid-filled cavity called the antrum, a hallmark of late-stage folliculogenesis.

Once an oocyte reaches full maturity, it is released from the ovary through ovulation. Surrounded by a protective layer of cumulus cells, it enters the fallopian tube, where fertilization may occur. The fimbriae, finger-like projections at the end of the fallopian tube, create currents that guide the oocyte toward the ampulla, the primary fertilization site. If fertilization does not occur, the oocyte degenerates.

Cellular Stages of Formation

Oogenesis unfolds in three phases: multiplication, growth, and maturation. Each stage ensures genetic stability and the oocyte’s ability to support embryonic development.

Multiplication Phase

This stage begins during fetal development when primordial germ cells migrate to the ovaries and differentiate into oogonia, which undergo rapid mitotic divisions. By the fifth month of gestation, the female fetus possesses approximately six to seven million oogonia, the peak of her lifetime egg supply. However, many undergo apoptosis before birth.

By birth, the remaining oogonia have become primary oocytes, which enter meiosis but pause in prophase I. This arrest persists until puberty, when hormonal signals periodically trigger further development. Unlike spermatogenesis, which continuously produces new gametes, oogenesis is finite, with no new oogonia forming after birth.

Growth Phase

During this phase, primary oocytes accumulate the molecular components necessary for embryonic development. Cytoplasmic volume increases, RNA and protein synthesis occurs, and cortical granules form to prevent polyspermy after fertilization.

Follicular cells also proliferate, forming multiple layers around the oocyte. Granulosa cells establish gap junctions with the oocyte for nutrient and signaling molecule exchange. The zona pellucida, a glycoprotein-rich extracellular matrix, develops, serving as a protective barrier and mediating sperm binding. The growth phase lasts years, as primary oocytes remain arrested in prophase I until hormonal cues initiate their progression.

Maturation Phase

The final stage of oogenesis involves completing meiosis and preparing the oocyte for fertilization. At the onset of each menstrual cycle, a subset of primary oocytes resumes meiosis, but only one typically reaches full maturation. Luteinizing hormone (LH) triggers the completion of the first meiotic division, producing a large secondary oocyte and a smaller polar body, which degenerates.

The secondary oocyte enters the second meiotic division but halts at metaphase II until fertilization occurs. If a sperm penetrates the oocyte, meiosis resumes, forming a mature ovum and a second polar body. If fertilization does not take place, the secondary oocyte degenerates.

Hormonal Regulation of Egg Production

Egg development and release are orchestrated by the hypothalamic-pituitary-ovarian (HPO) axis, which coordinates hormone production throughout the menstrual cycle. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), stimulating the anterior pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

FSH promotes follicular growth, prompting granulosa cells to produce estrogen. Rising estrogen levels initially suppress further FSH release but eventually trigger an LH surge. This surge induces ovulation, causing the dominant follicle to rupture and release its oocyte.

After ovulation, the follicle transforms into the corpus luteum, which secretes progesterone to prepare the uterine lining for implantation. If fertilization does not occur, the corpus luteum degenerates, progesterone levels decline, and menstruation begins, resetting the cycle.