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.