BSCI 1511 - Gametogenesis and fertilization

Features of Asexual reproduction

Offspring are clones

No fusion of gametes

Sexual reproduction

Non-motile egg fuses with motile sperm

Offspring are usually genetically distinct

Fertilization can be either internal or external

creates genetic variation

Spermatogenesis

The formation and development of sperm

Occurs in the seminiferous tubules contained within the testes

Occurs throughout adolescence and adulthood

Describe the initial stages of spermatogenesis before meiosis begins.

Journey of the Sperm steps

1. Testes produce sperm in highly coiled seminiferous tubules

2. Sperm traverse epididymis (~6 m) and complete maturation

3. During ejaculation, sperm are propelled via a muscular vas deferens

4. Vas deferens joins duct from the seminal vesicle – becomes ejaculatory duct

5. Ejaculatory duct joins the urinary duct – becomes the urethra

Seminal vesicles

They produce 60% of the semen: it is yellowish mucus, protein, and fructose

Prostate gland

The prostate gland produces a thin, milky fluid: anticoagulant enzymes and citrate (nutrient)

Bulbourethral glands

secretions neutralize the acidity in the urethra

They are positioned along the urethra, located just below the prostate gland

Urethra

Conduit for semen and urine

What is oogenesis?

Formation and development of oocytes

Occurs in the ovaries

immature oocytes form in the embryo and undergo developmental arrest for years or decades

The production of mature oocytes stops at around the age of 50 - menopause

Which cell types and developmental milestones are reached during oogenesis while the female is still an embryo?

1. Primordial Germ Cells: These cells undergo mitotic divisions to form oogonia.

2. Oogonia: These diploid cells also divide by mitosis and develop into primary oocytes.

3. Primary Oocytes: These cells begin meiosis but are arrested in prophase I before birth. They reside within follicles in the ovaries.

4. Follicle Formation: Each primary oocyte is surrounded by a small follicle, which consists of protective support cells.

Steps of Oogenesis - In adult

1. FSH stimulates a few follicles to resume growth and development

2. One follicle matures each month which allows primary oocyte to complete meiosis I becoming a 2ndary oocyte

3. The secondary oocyte, arrested at metaphase of meiosis II, is released during ovulation

4. Fusion of sperm and secondary oocyte triggers completion of meiosis II

Ovulation

The release of an oocyte from an ovary

After ovulation, the ruptured follicle develops into the corpus luteum

How does the presence or absence of fertilization affect the corpus luteum and the uterine lining?

If YES, the corpus luteum secretes estradiol and progesterone, maintaining the uterine lining during pregnancy

If NO, the corpus luteum degenerates, and a new follicle matures during the next cycle

Outlines the steps of human fertilization and early embryonic development

1. Ovulation: The ovaries release a secondary oocyte into the body cavity. This oocyte is surrounded by support cells within follicles.

2. Oocyte Collection: Cilia on the oviduct (fallopian tube) collect the oocyte and move it toward the uterus.

3. Fertilization: Sperm meets the oocyte in the oviduct. Fusion of the sperm and oocyte results in fertilization, forming a zygote.

4. Embryo Development: The fertilized egg undergoes cleavage (cell division) as it moves toward the uterus, becoming a blastocyst.

5. Implantation: The blastocyst implants in the endometrium (uterine lining) about seven days after conception.

Cervix

The neck of the uterus

Vagina

Elastic chamber where the male penis is inserted and sperm is deposited

Vestibular glands

Secrete lubricating mucus that facilitaes intecourse

What are the three major developmental differences between spermatogenesis and oogenesis?

1. 4 mature gametes are produced v. only 1 mature gamete is produced

2. Develops throughout adulthood and adolescence v. dveloped before birth

What are the primary hormones and cell types involved in regulating the male reproductive system, and how is this system controlled through feedback?

• Gonadotropin-releasing hormone (GnRH): Produced by the hypothalamus, it stimulates the anterior pituitary to release FSH and LH.

• Follicle-stimulating hormone (FSH): Stimulates Sertoli cells in the testes to nourish developing sperm.

• Luteinizing hormone (LH): Stimulates Leydig cells to produce testosterone.

• Testosterone: Promotes spermatogenesis and regulates GnRH, FSH, and LH levels through negative feedback.

• Inhibin: Produced by Sertoli cells (seminiferous tubules) it inhibits FSH secretion from the anterior pituitary.

Uterine cycle

cyclic changes in the endometrium that occur in the absence of pregnancy, including menstruation

What are the simple steps of the follicular phase in the ovarian cycle? (days 0-14)

1. Hypothalamus releases GnRH

2. Anterior pituitary releases FSH and LH

3. FSH stimulates growth of the follicle

4. Follicle produces estradiol

5. Estradiol level is low, inhibiting the hypothalamus and keeping FSH and LH levels low

6. As follicle develops, estradiol level rises steeply, stimulating a sharp increase in GnRH, FSH and LH

7. One day after the FSH and LH surge, the follicle ruptures and releases the secondary oocyte – ovulation

Ovarian cycle – Luteal phase (days 15-28)

1. LH stimulates the remaining follicular tissue to form the corpus luteum

2. Stimulated by LH, the corpus luteum secretes progesterone and estradiol

3. High concentration of progesterone and estradiol inhibit hypothalamus (GnRH) and pituitary (FSH and LH)

4. Very low GnRH, FSH and LH prevent another follicle from maturing

5. If no pregnancy, low gonadotropin concentration causes corpus luteum to disintegrate

6. Progesterone and estradiol concentration declines

7. GnRH, FSH and LH levels rise, initiating the next ovarian cycle

Describe the phases of the uterine cycle (menstrual cycle)

• Menstrual Flow Phase (Days 0-5): The cycle begins with menstruation, where the uterine lining (endometrium) is shed if pregnancy does not occur. This phase corresponds to low levels of estradiol.

• Proliferative Phase (Days 5-14): The endometrium thickens as estradiol levels rise, preparing for possible embryo implantation.

• Ovulation (Day 14): Triggered by a surge in LH and FSH, ovulation marks the release of an oocyte.

• Secretory Phase (Days 15-28): The endometrium further thickens and becomes more vascularized, supported by progesterone and estradiol from the corpus luteum.

If no fertilization occurs, the cycle repeats starting with menstruation

Steps for fertilization (pregnancy)

1. Sperm fuses with egg

2. Egg completes meiosis II

3. Nuclei (sperm and egg) fuse, yielding the zygote

4. Zygote divides to become blastocyst

5. Blastocyst implants in the endometrium

6. Pregnancy

Steps of blastocyst implantation

1. Blastocyst implants in the endometrium

2. Estradiol + progesterone from the corpus luteum signals the endometrium to nourish the embryo

3. Blastocyst secretes hCG

4. hCG maintains corpus luteum, which continues to secrete estradiol and progesterone

5. Endometrium is maintained AND the ovarian cycle is turned off

6. In the 2nd trimester, the corpus luteum disintegrates; placenta maintains pregnancy

Fertilization process (detailed steps)

1. Sperm penetrates any protective layer surrounding the egg and reaches the plasma membrane

2. Molecules on the sperm surface bind to receptors on the egg surface – ensures species compatibility

3. Sperm enters the egg

4. The surface of the egg is changed to prevent polyspermy

What is an acrosomal reaction?

• The sperm release specialized enzymes to digest the eggs’ outer layers

• It is necessary to allow sperm to reach and bind with the eggs’ plasma membrane, ensuring compatible sperm of the same species

Cortical reaction - slow block

1. Binding of sperm to the egg activates a signal transduction pathway that triggers the release of Ca2+ into the cytosol from the ER

2. Calcium diffuses throughout the cell

3. Cortical granules in the egg fuse with the plasma membrane

4. The contents of the cortical granules clip off spermbinding receptors and the fertilization envelope forms

Egg activation and cleavage (steps)

1. The influx of Ca2+ into the cytoplasm also activates the egg

2. Nuclei of sperm and egg fuse

3. Cell division begins

4. Cleavage results in blastomeres that form a blastula that surrounds a blastocoel

Birth control pill

synthetic estrogens and progestins that prevent pregnancy by inhibiting the production of GnRH, LH and FSH.

Shuts down the ovarian cycle

Plan B

a type of progestin that prevents ovulation

How does The abortion pill work?

Blocks progesterone receptors in the uterus, which prevents progesterone from maintaining the pregnancy.

Prostaglandins are taken to induce uterine contractions finishing the process

Menopause

Ovarian cycle stops

Ovaries lose responsiveness to LH and FSH, thus reducing estradiol production.

Menstruation

shedding of the endometrial lining (uterine lining) that occurs in the absence of pregnancy

It is induced when ovarian hormone levels drop (specifically progesterone and estradiol) because the corpus luteum has disintegrated

Ectopic pregnancy

When the fertilized egg implants in an oviduct

Endometriosis

Uterine cells migrate to an ectopic location and respond to hormones, causing pain and discomfort.

Difference Between Primary and Secondary Oocyte

Primary Oocyte:

• Formed during fetal development from oogonia.

• Arrested in prophase I of meiosis until puberty.

• Resides within a follicle in the ovary

Secondary Oocyte:

• Formed from the primary oocyte after completion of meiosis I.

• Haploid and arrested at metaphase of meiosis II.

• Released during ovulation.

• Completes meiosis II only if fertilized by a sperm.

What is cleavage?

rapid cell divisions that partition the cytoplasm of the fertilized egg into smaller cells called blastomeres

The process results in the formation of a blastula, a hollow ball of cells surrounding a fluid-filled cavity known as the blastocoel.

What is the role of FSH during the luteal phase?

It is suppressed during the luteal phase due to the negative feedback from the corpus luteum producing high concentrations of estradiol and progesterone

This prevents another follicle from maturing while preparing for a potential pregnancy

In humans, where does cleavage begin

In the oviduct

begins while the embryo is traveling through the oviduct (Fallopian tube) towards the uterus

What does LH do during the luteal phase?

Stimulates the ruptured follicle to transform into the corpus luteum

What does meiosis yield in oogenesis?

One gamete and three polar bodies (cytokinesis is unequal in oogenesis)

When does the sperm become capable of moving?

Epididymis, Sperm mature and become motile