Oogenesis: Definition, Process, Stages, Hormonal Control & MCQs

Oogenesis definition: It is the process of gametogenesis in females, in which ova are formed. The female gametes are also referred to as eggs. These eggs, when fertilised by the sperm, give rise to the zygote, which is diploid in nature. In placental animals, the eggs are fully formed, are ready for fertilisation. They are fertilised in the fallopian tube and then proceed to the uterus for implantation. The eggs are generally haploid in nature and contain a single copy of the chromosome set. Oogenesis is an important topic in biology subject.

This Story also Contains

1. What is Oogenesis?
2. Oogenesis Process
3. Stages of Oogenesis
4. Hormonal Control of Oogenesis
5. Folliculogenesis
6. The Menstrual Cycle
7. Recommended Video On 'Oogenesis'
8. Oogenesis MCQs

What is Oogenesis?

Oogenesis refers to the process of the 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 stages of meiosis up to Metaphase II and is released during ovulation.

Stages in Oogenesis

• The Primordial germ cells are released from the germinal epithelium

• Development of primary oocytes occurs

• Growth and maturation of the oocytes occur

• Ovulation leads to the release of a mature oocyte

• The ovum can be fertilised and developed as an embryo

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:

• Derived 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 releases the oocyte from the ovary

Hormonal Control of Oogenesis

Hormones play a significant role in regulating oogenesis and the ovarian cycle. The regulation and feedback mechanism of these hormones is very much essential for the process of oogenesis.

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 the endometrial lining

Hormonal Feedback Mechanisms

• Negative and positive feedback loops control the hormonal levels.

• Regulation of GnRH, FSH, and LH levels happens.

Folliculogenesis

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

• 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.

The Menstrual Cycle

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

Overview of the Menstrual Cycle

• Controlled by hormonal changes

• Required the development of ovarian follicles

• Prepares the endometrium for a potential pregnancy

Phases of the Menstrual Cycle

The phases of the menstrual cycle are:

Follicular Phase

• FSH stimulates follicle growth

• The 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

Recommended Video On 'Oogenesis'

Oogenesis MCQs

Q1. The second maturation division of the mammalian ovum occurs

Option 1: In the Graafian follicle following the first maturation division

Option 2: Shortly after ovulation before the ovum makes entry into the Fallopian tube

Option 3: Until after the ovum has been penetrated by a sperm

Option 4: Until the nucleus of the sperm has fused with that of the ovum

Correct answer: (3) Until after the ovum has been penetrated by a sperm.

Explanation:

The secondary oocyte continues to go through its second maturational division (meiosis II), however it is stopped at the metaphase stage. As the oocyte reaches this stage, ovulation occurs, and the ovum is shed. Once it enters the oviduct, the cell cycle is suspended until the sperm are present. The process through which female gametes are produced is called oogenesis. In contrast, once an egg has been fully developed, fertilised, and has begun to divide, it is no longer considered to be an egg by placental mammals.

Hence, the correct answer is Option (3) “Until after the ovum has been penetrated by a sperm”.

Q2. Identify the correct statement on 'inhibin':

Option 1: Inhibits the secretion of LH, FSH and Prolactin.

Option 2: Is produced by granulosa cells in ovary and inhibits the secretion of FSH.

Option 3: Is produced by granulose cells in ovary and inhibits the secretion of LH.

Option 4: Is produced by nurse cells in testes and inhibits the secretion of LH.

Correct answer: (2) Is produced by granulosa cells in the ovary and inhibits the secretion of FSH.

Explanation:

Inhibin is a protein secreted by granulosa (female) and Sertoli (male) cells in response to FSH, and its major action is the negative feedback control of pituitary FSH secretion by the pituitary gland. Inhibin helps regulate the reproductive system by maintaining appropriate levels of FSH, preventing overstimulation of the gonads. This feedback mechanism ensures proper gamete production and overall reproductive balance in both males and females.

Hence, the correct answer is option 2) Is produced by granulosa cells in the ovary and inhibits the secretion of FSH.

Q3. Read the statement carefully and pick up the correct option

The cells start division and enter into prophase-1 of meiotic division and get temporarily arrested at the stage called

Option 1: Zygote

Option 2: secondary spermatocyte

Option 3: primary oocyte

Option 4: secondary oocyte

Correct answer: 3) primary oocyte

Explanation:

The cells in meiosis advance to prophase I, where they pause at a phase known as diplotene. This pause can be extended, lasting until the organism's sexual maturation. In detail:

Prophase I involves several sub-stages:

1. Leptotene: chromosomes start to condense.
2. Zygotene: homologous chromosomes pair up.
3. Pachytene: homologous chromosomes synapse and crossing over occurs.
4. Diplotene: homologous chromosomes begin separating but remain linked at chiasmata.
5. Diakinesis: chromosomes fully mature and are visible.

In females, primary oocytes often stop in diplotene during fetal development and stay there until ovulation, which can be years later. For males, spermatocytes usually don't have this extended arrest and complete meiosis more quickly.

Hence, the correct answer is option 3) primary oocyte