Mitosis vs. Meiosis

Mitosis vs. Meiosis

• Mitosis:
  • Starts with one parent cell.
  • Results in two offspring cells.
  • Offspring cells are identical and diploid (2n).
• Meiosis:
  • Starts with one parent cell.
  • Results in four haploid (n) offspring cells.
  • Offspring cells are non-identical.
  • These cells have half the number of chromosomes as the parent cell, with different traits due to different alleles.

Gametogenesis Differences in Males and Females

• Compared to spermatogenesis and oogenesis involving more detailed information:
  • When it starts and finishes.
  • How many are produced.
  • Non-equal cytokinesis.
  • Polar bodies.
  • Female meiosis completion dependent on sperm penetration.

Key Differences in Gamete Production

• Male Gametes (Sperm):
  • Produced in vast numbers (hundreds of millions per day, potentially trillions).
  • Small and minimalist: 23 chromosomes and a tail, primarily nucleus.
• Female Gametes (Ova):
  • Limited number (around 400 per lifetime).
• Both are gametes (23 chromosomes).
• Sperm consists of mostly nucleus.

Hormone Requirement for Female Development

• Female development proceeds as default in the absence of a signal to develop as male.
• The absence of testosterone leads to female development; estrogen is not required.
  • Developing the SRY gene.

Ovary and Oogenesis

• Oogenesis starts pre-birth, pauses in meiosis then resumes at puberty.
• Ovulation releases an uncompleted ovum, finishing only after sperm penetration.
• Results in one ovum and three polar bodies due to unequal cytokinesis.

Male Reproductive System

• Components:
  • External genitalia.
  • Ducts (including the urethra which is part of both reproductive and urinary system).
  • Accessory glands (seminal vesicles, prostate, bulbourethral glands) contribute to the seminal fluid but not gametes.
  • Gametes originate from the testes.

Sperm Production and the Blood-Testis Barrier

• Sperm production occurs within the seminiferous tubules.
• Sertoli cells form tight junctions, creating the blood-testis barrier.
• Importance of the Barrier:
  • Sperm are not identical to the body's own cells and would be recognized as foreign by the immune system, triggering an immune response where general phagocytes recognize anything foreign and specialized B cells and T cells trigger an immune response
  • Prevents immune cells from attacking sperm.
• Clinical Relevance:
  • After vasectomy reversal, sperm may break through the barrier, leading to the production of antibodies against sperm, reducing fertility. A vasectomy is a surgical sterilization that cuts out a chunk of the vas.
  • The body actually produces antibodies against our own sperm.

Sperm Characteristics

• Minimalist structure:
  • 23 chromosomes.
  • The nucleus occupies most of the cell.

Testes Function and Hormonal Control

• Testes produce sperm and testosterone (androgen).

• Interstitial/Leydig cells produce testosterone.

• Hormonal regulation:

  • Hypothalamus releases gonadotropin-releasing hormone (GnRH).
  • GnRH stimulates the anterior pituitary to release gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
    • FSH stimulates spermatogenesis.
    • LH stimulates testosterone production in interstitial cells.

Semen Composition

• Typical ejaculate: 2-5 ml with several hundred million sperm.
• Accessory glands contribute over 90% of ejaculate volume.
• After vasectomy, only sperm are missing, with no significant volume or visual difference.

Male Erection

• Hydraulic system to achieve an erection.
• Erotic thoughts, visual images, and tactile stimulation.
• Alternative systems in other mammals involve a bone (os penis) or cartilage.
• Eyes and imagination as accessory organs.

Female Reproductive System: Ovary and Follicle

• Ovary contains the follicle, where most of oogenesis occurs.
• Follicle maturation parallels oocyte maturation.
• Follicle development stages: primordial, primary, secondary, dominant follicle.

Ovulation and Corpus Luteum

• Ovulation is the release of the secondary oocyte.
• After ovulation, follicle cells remain in the ovary and accumulate lipids, becoming the corpus luteum (yellow body).
• Corpus luteum is an endocrine tissue producing hormones influencing the uterine cycle.

Significance of Color

• Yellow color indicates lipids, fats and cholesterol.

The 28-Day Cycle: Ovarian and Uterine Events

• Cycle duration: 28 days from primordial follicle to corpus luteum degeneration.
• Corpus luteum becomes corpus albicans (white body) as scar tissue after degeneration.

Ovarian Cycle

• Primordial follicle to mature follicle to ovulation, then corpus luteum formation, producing progesterone in the second half.

Uterine Cycle

• Begins with menstruation: functional layer of endometrium dies, sloughs off, and passes through the vagina for about five days.
• Followed by proliferative phase: endometrium grows and thickens leading up to ovulation.
• Secretory phase after ovulation: endometrium maintains thickness, less active growth.
• Link between ovarian hormones and uterine cycle:
  • Estrogen: the slope in the first half where it is dominant before ovulation comes from the follicle and stimulates the growth of the endometrium.
    • Progesterone from the corpus luteum and is responsible for the maintenance of the endometrium.

Spiral Arteries and Progesterone

• Spiral arteries grow into the endometrium providing nutrients.
• Progesterone causes vasodilation, maintaining blood flow to the endometrium.

Pregnancy vs. No Pregnancy

• If no pregnancy occurs:
  • Corpus luteum degenerates after 10-12 days (corpus albicans).
  • Progesterone levels decline->spiral arteries constrict -> blood flow decreases -> endometrium dies and is shed (menstruation).
• If pregnancy occurs:
  • Placenta produces a hormone that maintains the corpus luteum for three to four weeks.
  • Elevated progesterone levels maintain the endometrium.

Clinical Application: Emergency Contraception

• Emergency contraception (morning-after pill) utilizes the progesterone mechanism.
  * Taking a large dose of progesterone, then stopping, causes a sudden progesterone drop -> spiral arteries constrict -> endometrium sheds even with any embedding, causing a loss.
• Ethical Considerations

Ovulation Trigger

• Luteinizing hormone (LH) peak triggers ovulation.
• Low-dose contraceptive pills prevent the LH surge to prevent ovulation.

Summary Points

• Estrogen stimulates endometrium growth.
• Progesterone maintains the endometrium.
• A sudden decrease in progesterone is responsible for menstrual period.

Steroid Hormone Research

• Dr. Julian=the father of birth control pills.