Oogenesis: Definition, Process, Development, Maturation & Fertilization

What Is Oogenesis?

Oogenesis refers to the process of formation of egg cells in females. This process initiates before birth, continues in monthly cycles during the reproductive years of a woman's life, and concludes at menopause. Sexual reproduction requires this process; it consists of the growth and maturation of oocytes within the ovaries.

Oogenesis Process

This is an extremely intricate process, comprising successive stages of development. All these stages are under the control of hormones and ultimately lead to the formation of mature oocytes.

Definition and Process

• Initiates during foetal development.

• Involves the development of primary oocytes.

• Primary oocytes go into arrested development until puberty.

• At each menstrual cycle, some oocytes re-enter meiosis.

• Usually, only one oocyte completes the meiosis and is released during ovulation.

Stages in Oogenesis

• Primordial germ cells

• Development of primary oocytes

• Growth and maturation of the oocytes

• Ovulation - release of a mature oocyte

• Maybe fertilised and developed as an embryo

The Ovarian Cycle

The ovarian cycle refers to the monthly series of events associated with the maturation of an egg and its preparation for possible fertilization.

Overview Of The Ovarian Cycle

• Controlled by hormonal changes

• Required the development of ovarian follicles

• Prepares the endometrium for a potential pregnancy

Phases Of The Ovarian Cycle

The phases of the ovarian cylcle are:

Follicular Phase

• FSH stimulates follicle growth

• Dominant follicle secretes estrogen

• Ends with ovulation

Ovulation

• LH surge triggers the release of the mature egg

• Occurs around day 14 of the cycle

Luteal Phase

• Corpus luteum formation

• Progesterone and estrogen secretion

• Prepares the endometrium for implantation

Stages Of Oogenesis

Oogenesis consists of successive stages of development from the time primordial germ cells form until a mature ovum matures and is released.

Prenatal Phase

Formation of Primordial Germ Cells:

• Derivate from the yolk sac

• Migrate to the developing ovaries

Development into Primary Oocytes:

• Enter meiosis I and then arrest in prophase I

• Get surrounded by a layer of granulosa cells forming the primordial follicles.

Postnatal Phase

It includes:

Follicular Development

• Activation of primordial follicles

• Growth to become primary, secondary, and Graafian follicles

Growth And Maturation Of Oocytes

• Oocytes increase in size

• Granulosa cells grow and multiply into several layers

Puberty To Menopause

The details are given below:

Role Of Hormones

• FSH and LH promote the growth and development of the follicle.

• Estrogen and progesterone control the menstrual cycle.

Mechanism Of Ovulation

• The mature oocyte re-enters meiosis

• Ovulation liberates the oocyte from the ovary

• Elaborate drawing of stages of oogenesis

Hormonal Control Of Oogenesis

Hormones play a significant role in regulating oogenesis and the ovarian cycle.

Role of Gonadotropin-Releasing Hormone GnRH

• Produced by the hypothalamus

• Stimulates the release of FSH and LH from the pituitary gland

Follicle-stimulating hormone, FSH

• Stimulates follicular growth

• Promotes estrogen production

Luteinizing Hormone, LH

• Triggers ovulation

• Supports corpus luteum formation

Estrogen

• Produced by growing follicles

• Prepares the endometrium

Progesterone

• Secreted by the corpus luteum

• Maintains endometrial lining

Hormonal Feedback Mechanisms

• Negative and positive feedback loops

• Regulation of GnRH, FSH, and LH levels

Folliculogenesis

• Folliculogenesis is the process through which ovarian follicles mature and is important for successful oogenesis.

• Formation and development of follicles are necessary for the production of a viable oocyte.

Stages of Folliculogenesis

• Primordial Follicles: Resting stage

• Primary Follicles: Activation and growth begins

• Secondary Follicles: Further growth with the formation of the antrum

• Graafian Follicles: Mature follicle ready for ovulation

• Follicular Atresia: Degeneration of non-dominant follicles. The natural selection process for the healthiest oocyte.

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