Oogenesis and Follicular Development

Oogenesis and Follicular Development

Introduction

• Oogenesis: Development of oocytes (eggs) in females.
• Follicular Development: How follicles within the ovary develop.
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Ovaries as Gonads

• Gonads: Special organs, one pair in males (testes) and one pair in females (ovaries).
• Testes: Produce testosterone (hormones) and sperm.
• Ovaries: Produce estrogen and progesterone (hormones) and oocytes (eggs).
• Size and Shape: Ovaries resemble almonds in size and shape.
• Location: Situated on either side of the uterus in the upper pelvis, located internally.

Endocrine and Exocrine Functions of Ovaries

• Ovaries are both endocrine and exocrine organs.
• Endocrine: Produce hormones (estrogen and progesterone).
• Exocrine: Produce oocytes (eggs).
• This dual function is unusual for glands.

Appearance of the Ovaries

• Surrounded by a connective tissue capsule called the tunica albuginea.
• Tunica Albuginea: Dense connective tissue providing support and protection.

Microscopic Structure of the Ovary

• Two layers visible: cortex and medulla.

Cortex

• Outer, darker region.
• Contains oocytes (eggs) surrounded by follicular cells (supporting cells).
• Follicle: Oocyte plus surrounding follicular cells.
• Function: Site of folliculogenesis (follicle development) and oogenesis (oocyte development).

Medulla

• Inner, paler region.
• Contains mainly connective tissue, blood vessels, and nerves.
• Fewer follicles compared to the cortex.

Blood Supply to the Ovaries

• Ovaries are richly supplied with blood vessels, especially in the medulla.
• Follicles produce hormones (estrogen and progesterone) that need to enter systemic circulation.
• Hormones travel via blood vessels to target structures like the uterus.

Follicles and Folliculogenesis

• Follicle: Oocyte and its surrounding supporting follicular cells.
• Folliculogenesis: Development of follicles.
• Follicle development in females begins before birth, unlike sperm development in males which starts at puberty.
• As follicles mature, they increase in size and the number of follicular cells increases.

Primordial Follicle

• Earliest follicle stage.
• Consists of one oocyte surrounded by one layer of flat follicular cells.

Primary Follicle

• Develops from a primordial follicle.
• Primary oocyte surrounded by one or two layers of cuboidal follicular cells (no longer flat).

Secondary Follicle

• Oocyte becomes a secondary oocyte (more on this later).
• Dramatic increase in the number of follicular cell layers (4-10 layers).
• Spaces containing follicular fluid begin to form between follicular cells.

Tertiary Follicle

• Mature follicle, much larger than secondary follicle.
• Spaces with follicular fluid merge to form one large fluid-filled space.
• Oocyte is surrounded by follicular cells.
• Granulosa Cells: Follicular cells right around the oocyte.

Ovulation

• Rupturing of the follicle and release of the oocyte and granulosa cells.
• Oocyte and granulosa cells enter the oviduct to travel down.
• The remaining follicle stays behind in the ovary.

Corpus Luteum

• The ruptured follicle collapses and forms the corpus luteum.
• Corpus Luteum: Empty tertiary follicle that has collapsed.
• Remaining follicular cells multiply and fill the space.
• Corpus luteum cells produce estrogen and progesterone.
• Prior to ovulation, follicular cells produce only estrogen. After ovulation (corpus luteum), follicular cells produce both estrogen and progesterone.

Function and Fate of Corpus Luteum

• Corpus luteum sits in the ovary and releases estrogen and progesterone.
• These hormones prepare the uterus for fertilization or implantation.
• If no fertilization occurs: Corpus luteum degenerates after about 10 days.
• If fertilization and implantation occur: Corpus luteum is maintained longer.

Corpus Albicans

• If the corpus luteum degenerates, it becomes the corpus albicans.
• Corpus Albicans: A lump of scar tissue (dense connective tissue) in the ovary.
• Does not produce estrogen or progesterone.
• Follicular cells make oestrogen until ovulation and then makes oestrogen and progesterone
• Corpus Albicans makes no hormones at all

Oogenesis

• Oogenesis: Development of the oocyte (egg) within the follicle. Different from folliculogenesis (development of the entire follicle).
• In males, sperm development is called spermatogenesis.
• Processes involved: Mitosis (cell division), Meiosis (reduction of DNA).
• Oogenesis, unlike spermatogenesis, starts before birth.
• Spermatogenesis results in four sperm per stem cell; oogenesis results in one oocyte per stem cell.

Process of Oogenesis in Detail

• Oogenesis: Production of oocytes within the ovary.

Oogonium

• Most immature cell that will become an oocyte.
• Located in the embryonic ovary.
• Diploid (2N): Has two of every chromosome (46 chromosomes).
• Oogonia divide by mitosis in the embryo.
• By month 2-4 of development, each ovary has 2-3 million oogonia.
• Mitosis stops at about month 3-4 of embryonic development.
• Females have a finite number of potential eggs.

Primary Oocyte

• Oogonia become primary oocytes after mitosis stops.
• Still diploid (2N).
• Primary oocytes start to divide after birth, but the process is arrested at meiosis I.
• Puberty triggers the completion of meiosis I.

Meiosis I and Secondary Oocyte

• Puberty brings hormones like follicle-stimulating hormone (FSH), which trigger primary oocytes to finish meiosis I.
• Primary oocyte divides into two daughter cells: a secondary oocyte and a polar body.
• Secondary Oocyte: Haploid (N), with only one of each chromosome (23 chromosomes).
• Polar Body: Receives half the DNA and degenerates.

Meiosis II

• Secondary oocyte needs to undergo meiosis II, but this process is also blocked.
• The block is released when a sperm fertilizes the secondary oocyte.

Fertilization and Zygote Formation

• Sperm fertilizes the secondary oocyte, bringing in half the DNA (haploid).
• The block on meiosis II is released, and the secondary oocyte divides into another secondary oocyte and another polar body.
• The sperm enters the secondary oocyte, resulting in a diploid (2N) cell with 46 chromosomes - a zygote (fertilized egg).

Final Result of Oogenesis

• One stem cell gives rise to a secondary oocyte and three polar bodies.
• Oogenesis is more complex than spermatogenesis, with several stop-start processes.