HL Biology Unit 6.7: Reproduction

What causes intersex phenotypes in reproduction?

Minor adjustments in the complex series of genes + hormones that control the development of sex organs

How do male sex organs develop?

The SRY gene on the Y chromosome codes for the Teste Determining Factor (TDF) protein, which causes embryonic gonads to develop into testes.

Testes develop testosterone secreting glands around 8 weeks, increased testosterone in puberty signals development of sperm

How do female sex organs develop?

If the SRY gene is not present, gonads develop into ovaries from maternal estrogen and progesterone. Estrogen and progesterone increase during puberty, signalling development of the menstrual cycle

Using a graph, describe the presence of different hormones involved in the menstrual cycle

• Estrogen spikes before ovulation

• Right before day 14, Luteinizing hormones (LH) and Follicle Stimulating Hormone (FSH) spike, which is where ovulation occurs.

• Near day 21, progesterone and estrogen spike

At which point of the menstrual cycle are females most fertile?

During ovulation when an egg is released into the oviduct (~days 10-17)

What are two examples of ovarian hormones? Pituitary hormones?

Ovarian: Estrogen, Progesterone

Pituitary Hormones: LH, FSH

Describe the follicular phase of the menstrual cycle

Occurs the first day right after menstruation

1. FSH signals estrogen increase → endometrium thickens

   FSH present → increased follicle growth → more estrogen is released from follicles

2. Increased FSH → increased estrogen → increase in LH release
   Increased LH → increased follicular wall size → even more estrogen release

3. OVULATION: LH spikes, the most developed follicle grows and ruptures, releasing its ovum into the oviduct

4. When estrogen is highest, it inhibits LH and FSH , allowing progression to the luteal phase

Describe the luteal phase of the menstrual cycle

1. The endometrium continues to thicken

2. Ruptured follicle differentiates, becoming a corpus luteum that releases estrogen and progesterone → progesterone maintains the endometrium

3. New hormones inhibit LH and FSH until the next cycle

4. If no fertilization, the corpus luteum breaks down → decreased progesterone and estrogen → endometrium breaks down

5. The endometrium sheds during menstruation

What are characteristics of both spermatogenesis and oogenesis?

Mitosis, cell growth, 2 meiotic divisions, differentiation

Describe the structure of an ovum

*note, condensed nucleus is not actually present because ovum development stops in metaphase II, only there to represent the presence of haploid genetic material

Describe the structure of a spermatozoa

Describe the structure of the seminiferous tubules

Describe the steps of spermatogenesis

1. Begins in the outer part of the seminiferous tubules and progresses inwards towards the lumen

2. Spermatogonia are diploid germinal epithelial cells beneath the basement membrane. These divide by mitosis into either more spermatogonia or primary spermatocytes

3. Formation of primary spermatocytes begins 1st meiosis, forming 2 haploid secondary spermatocytes

4. Secondary spermatocytes undergo 2nd meiosis, becoming 4 haploid spermatids

5. Spermatids are attached to sertoli cells which help them differentiate into mature spermatozoa

6. Once spermatozoa develop tails, they detach from sertoli cells and get carried down the lumen of the seminiferous tubules towards the epididymis

Describe the steps of oogenesis

1. Begins in utero by the developing female fetus. Diploid germinal cells in the fetal ovary divide by mitosis into oogonia, which are distributed throughout the ovaries

2. Oogonia develop into primary oocytes, beginning meiosis but stopping at prophase I. This cell is then surrounded by follicle cells, forming a primary follicle.

3. During menstrual cycle in puberty, a few follicles become oocytes as the first meiotic division produces 2 haploid cells by uneven cytoplasm division to create a secondary oocyte and a small polar body.

4. The polar body divides and degenerates, the secondary oocyte is protected by the follicle, starting the second meiotic division and stopping at prophase II. The follicle cells divide and produce a liquid that causes follicles to swell.

5. The follicle cell bursts during ovulation, releasing the secondary oocyte to the oviduct.

Describe the steps of fertilization

1. The sperm binds to the jelly coat of the ovum

2. Enzymes are released from the acrosome of the sperm to break down the jelly coat

3. The sperm nucleus enters the egg (end of the Acrosome Reaction)

4. The cortical granules in the ovum contain molecules that harden the zona pellucide, making it impermeable to prevent polyspermy (Cortical reaction)

What occurs in the embryo post-fertilization?

Human gonadotropin (hCG) is produced to continue the release of progesterone and estrogen

What hormone do pregnancy tests look for to confirm pregnancy?

Presence of human gonadotropin

What occurs in the placenta post-fertilization?

Allows the exchange of nutrients between mother and fetus, since maternal and fetal blood don’t mix. ALSO secretes progesterone and estrogen

Using a graph, describe the levels of the hormones involved in pregnancy post-fertilization

hCG promotes initial increases of progesterone and estrogen

hCG declines as placenta becomes main secretor of progesterone and estrogen

Describe the positive feedback loop between estrogen and oxytocin during birth

1. Head of fetus pushes against cervix, transmitting a nerve impulse to the brain that stimulates the pituitary gland to secrete oxytocin

2. Oxytocin enters the bloodstream and into the uterus

3. Oxytocin promotes uterine contractions that continues the cycle by pushing the fetus closer to the cervix

What does prolactin do?

A hormone that stimulates milk production

• Secreted during and after feed in order to produce the next feed

• More is secreted at night

• Suppresses ovulation (no more babies right now!)

• Suckling promotes prolactin secretion