What is oogenesis

• The formation of gametes in the ovaries
• Begins in females before they are born

Primordial germ cells

• Migrate from the yolk sac to the ovaries during early fetal development
• Differentiate into the oogonia

What are oogonia

Diploid (2n) stem cells that divide mitotically to produce millions of germ cells

What happens to oogonia before birth

• Most degenerate
• Few develop into larger cells called 1 ^o oocytes

What happens to the 1 ^o oocytes during fetal development

Enter prophase of meiosis I but do not complete it until a few are called upon to do so during the reproductive years

What occurs to the 1^o oocytes during the arrested stage of development

Each 1^o oocyte is surrounded by follicular cells in a primordial follicle

What happens to the primordial follicles during puberty

• Under the influence of LH and FSH, several will be stimulated each month
• Only one will typically reach the maturity needed for ovulation

What happens to maturing oocytes within maturing follicles

Undergo a series of developmental stages which ultimately brings a 2 ^o oocyte within a mature follicle to ovulation

• the ovulated 2 o oocyte will have completed meiosis I, and so have a haploid number of chromosomes (1n)

What is produced when the 1 ^o oocyte receives the signal to complete meiosis I

Two haploid (1n) cells of unequal size, each with 23 chromosomes

• Smaller cell is called the first polar body
• Larger 2 ^o oocyte inherits most of the cytoplasm

The ovarian cortex contains many follicles in different stages of development (T/F)

True

The ovarian cycle - picture

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Oocyte maturation and follicle maturation - picture

The follicle has important functions beyond hosting an oocyte!

1st step of ovulation

High levels of estrogen from almost mature follicle stimulate release of more GnRH and RH

2nd step of ovulation

GnRH promotes release of FSH and more LH

3rd step of ovulation

LH surge brings about ovulation

What happens to the mature follicle at ovulation

• It ruptures to expel the 2^o oocyte into the pelvic cavity, normally to be swept into the uterine tube

What happens to the 2^o oocyte after it is swept into the uterine tube

• If not fertilized, it degenerates
• If sperm are present and one penetrates the 2^o oocyte , meiosis II resumes

What is the more common name for the 2^o oocyte?

The egg

Oogenesis - picture

Oogenesis - Meiosis I summary

• Begins during fetal development
• After puberty, primary oocytes complete meiosis I, which produces a secondary oocyte and a first polar body that may or may not divide again

Oogenesis - Meiosis II summary

• The secondary oocyte and first polar body are ovulated
• After fertilization meiosis II resumes and the oocyte splits into an ovum and a second polar body.

Ovarian cycle - picture

What is the corpus luteum

Created from the mature follicle

A temporary structure in the ovary essential for establishing and maintaining pregnancy

What does the corpus luteum secrete

estrogens - responsible for the thickening of the endometrium

progesterone - responsible for the development and maintenance of the endometrium

When happens to the corpus luteum if the 2^o oocyte is not fertilized

• After about 14 days it stops secreting progesterone
• Degenerates into a corpus albicans (a mass of fibrous scar tissue)

What occurs in the uterus if the 2^o oocyte is not fertilized

Without estrogen, and then with abrupt progesterone withdrawal, the uterine lining cannot be maintained and it sloughs (menses)

What happens to the corpus luteum if pregnancy occurs

• It is “rescued” from degeneration by a hormone called human chorionic gonadotrophin (hCG).
• produces hormones well into the 1st trimester until the placenta can take over

When is the window of opportunity for fertilization to occur

Approximately 2 days before ovulation to 1 day after ovulation.

• Sperm can survive 48-72 hours in the uterine tube

When is meiosis II completed

• At the moment of conception, when the successful sperm penetrates the plasma membrane of the 2 ^o oocyte
• The nuclear material of the two cells unite to reconstitute the normal number of chromosomes (2n)

What is a zygote

The new diploid cell created at conception / completion of meiosis II

Oogenesis and follicular development - summary - picture