IB Bio HL: Sexual Reproduction (6.6/11.4)

Application: William Harvey’s investigation of sexual reproduction in deer

• Debunked the soil and the seed theory proposed by Aristotle (menstrual blood and a male seed form an egg)

• Studied sexual organs in deer, found the embryo develops after 6/7 weeks - so menstrual blood cannot be involved in embryogenesis

Testosterone

• male reproductive hormone secreted by testes

• responsible for prenatal development of male genitalia

• involved in sperm production following onset of puberty

• Aids in development of secondary sex characteristics

  • body hair, muscles, deepening of voice

• maintains male libido

Male reproductive system composed of…

• all the organs responsible for the production of sperm and necessary for synthesizing of semen

Seminiferous tubules

• composition of the testes

• where male gametes are produced

• composed of germline epithelium, spermatagonia, basement membrane, sertoli cells

Ovary

• where oocytes mature before release

• secretes estrogen and progesterone

Uterus

where fertilized egg implants and develops

Endometrium

• mucous membrane lining of uterus

• thickens (becomes more vascular) to prepare for implantation or lost by menstruation

Vagina

passage to uterus

Cervix

muscular opening protecting the uterus

Fimbriae

fringe of tissue, sweeps oocytes into oviduct

Fallopian tubes/ Oviduct

• transports oocytes to the uterus

• generally where fertilization occurs

Oogenesis timing of events

• begins during fetal development when oogonia, formed by mitosis, grow to form primary oocytes, which begin meiosis but are halted in P1 when they are surrounded by granulosa cells

• meiosis continues at the start of the menstrual cycle (puberty) in the follicular phase when FSH, released by the anterior pituitary, stimulates ovarian follicle growth- divides to form secondary oocytes

• secondary oocyte is then halted in M2 until fertilization occurs (influx of Ca²⁺) and meiosis is finished

  • ovum → zygote

Oogenesis sequence

Oogonia →(mitosis + growth)→ primary oocyte

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Primary oocyte →(meiosis 1)→ secondary oocyte + polar body

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Polar body →(meiosis 2)→ 2 Polar bodies

Secondary oocyte →(meiosis 2) → Ovum (post-fertilization; fertilization occurs during M2)

Compare and contrast spermatogenesis and oogenesis

Spermatogenesis	Oogenesis
4 haploid gametes of the same size Produced millions daily Doesn’t pause Starts during puberty Occurs till death	1 haploid gamete, big boi One produced per menstrual cycle Pauses in P1 and M2 Starts in germline epithelium during fetus development Occurs till menopause

Germline epithelium in the seminiferous tubules

• divides by mitosis to form spermatogonia

• on outside of tubule

Leydig cells

• interstitial cells (between tubules)

• produce testosterone

Sertoli cells

• provide nutrients for developing spermatozoa

  • necessary for differentiation and growth

Spermatogenesis sequence

spermatogonia → (mitosis+growth) → primary spermatocyte

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Primary spermatocyte → (meiosis 1) → secondary spermatocyte

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Secondary spermatocyte → (meiosis 2) → spermatid

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Spermatid → (differentiation) → spermatozoa

Vas Deferens

• Long tube which conducts sperm from testes to prostate gland

  • which connects to the urethra for ejaculation

Prostate gland

• Secretes an alkaline fluid to neutralize vaginal acids

  • allows for sperm viability + motility

Urethra

conducts sperm from the prostate to outside the body

Epididymis

• Structure surrounding the testicle

• Where sperm mature and develop motility, stored until ejaculation

Seminal vesicle

• Secretes fluid into the vas deferens, containing…

  • fructose - nourish sperm

  • mucus - protect sperm

  • Prostaglandin - triggers contractions

Menstrual cycle

• reoccurring changes that occur within the female reproductive system

• Makes pregnancy possible

• ~28 days, beginning at puberty, ending at menopause

Hormones from pituitary for menstrual cycle

• FSH and LH both secreted by the anterior pituitary

• FSH - follicle-stimulating hormone, stimulates follicular growth in ovaries, stimulates estrogen secretion from developing follicles

• LH - luteinizing hormone, surge causes ovulation, resulting in the formation of the corpus luteum

Hormones from ovaries for menstrual cycle

• Estrogen and progesterone both act on the uterus to prepare for pregnancy

• both thicken the endometrium and inhibit FSH and LH; however, estrogen stimulates FSH and LH production pre-ovulation

Follicular phase

• FSH from the anterior pituitary stimulates the growth of ovarian follicles

• The dominant one will produce estrogen to inhibit FSH and hence the growth of other follicles

• Estrogen will begin to thicken the endometrium (make it more vascular)

Ovulation - Menstrual cycle

• Estrogen stimulates the anterior pituitary to secrete hormones, resulting in a surge of LH (as well as a lesser surge of FSH)

• LH causes the dominant follicle to rupture and release an egg (secondary oocyte)

Luteal phase

• Ruptured follicle forms a degenerating corpus luteum

• this structure secretes estrogen and progesterone to thicken the endometrial lining as well as inhibit LH, FSH and hence follicle development

Menstruation

• Case 1) Fertilization, embryo implants in endometrium nad releases hormones to sustain the corpus luteum

• Case 2) Fertilization does not occur, corpus luteum degenerates; estrogen and progesterone levels drop

  • endometrial layer is shed as menstrual blood, how estrogen and progesterone concentrations are too low to block the anterior pituitary and so the cycle restarts

IVF

in vitro fertilization, uses drugs to suspend normal ovulation before hormone treatment (HCG) to collect eggs

IVF - Down regulation

• Drugs used to stop normal FSH and LH secretion, which then inhibits the secretion of estrogen and progesterone

• doctors can then control the timing and quantity of egg production in the ovaries

• nasal spray, two weeks

IVF - superovulation

• Artificial doses of hormones to develop and collect lots of eggs

• Patient injected with FSH to stimulate follicle development

• follicles treated with human chorionic gonadotropin (hCG), which is a hormone produced by developing embryo

• hCG stimulates the maturing of the follicle, the egg is collected prior to follicle rupturing

IVF - Fertilization

• The extracted eggs are incubated in the presence of a sperm sample from a male donor

• The eggs also undergo microscopic analysis to make sure it will be succesful

IVF - Implantation

• Two weeks before implantation the women will take progesterone to develop the endometrium

• Healthy embryos will be selected and transported into the uterus

• Many embryos will be transferred to improve chances (multiple births possible)

• Two weeks after, a pregnancy test will be taken

Corona radiata

Follicular cells, external layer of cells which provides support and nourishment

Zona Pelludica

Jelly coat glycoprotein matrix, barrier to sperm entry

Cortical granules

release their contents upon fertilisation (single-sperm entry) to prevent polyspermy

Timing of the nucleus in egg cells

no nucleus will form within the egg until after fertilisation has occurred (the egg cell is arrested in metaphase II until it becomes fertilised by a sperm)

Composition of the sperm tail

The tail (flagellum) is composed of a microtubule structure called the axoneme, which bends to facilitate movement

Sperm midpiece

Contains many mitochondria for ATP production

Sperm head composition

• Acrosome cap - region at tip, contains hydrolytic enzymes to digest Zona Pelludica

• Haploid nucleus - paternal DNA

• Paired centrioles for zygotic division

External fertilization

• fusion of gametes outside the parent body

• common in aquatic animals (water acts as a medium for gamete travel)

• Susceptible to environmental influences (pH, predators, temperatures)

  • Compensates by releasing lots of gametes

• Spawning = releasing gametes into water

Internal fertilization

• Fusion of gametes inside the parent body

• requires method by which one parent gamete can be introduced inside of the body of another - copulation

• terrestrial animals use internal fertilization to prevent exposure and desiccation of gametes and embryos

• greater degree of protection

• potential survival cost to parent

Human fertilization; state the three processes occuring

• Capacitation

• Acrosome reaction

• Cortical reaction

Capacitation

• biochemical changes after ejaculation improves semen mobility

• Uterus releases chemicals to dissolve the sperm’s cholesterol coat

• Improves sperm mobility (hyperactivity) so sperm are more likely to reach the egg

• weakens the acrosome cap so the reaction carried out by hydrolytic enzymes can occur upon contact between the sperm and the egg

Acrosome reaction

• reaction occurring upon contact between the sperm and egg allowing for passage through the jelly coat

• Sperm pushes through the corona radiata

• Acrosome vesicles fuse with the jelly coat, releasing digestive enzymes to soften the glycoprotein matrix

• the sperm then pushes through the softened jelly coat and binds to exposed docking proteins on the egg membrane

• egg and sperm membrane fuse, sperm nucleus and centriole enter the egg

Cortical reaction

• prevents polyspermy after the egg successfully penetrates

• cortical granules release enzymes via exocytosis into jelly coat

• enzymes in the granules then destroy sperm binding sites, thicken and harden the glycoprotein matrix

• preventing polyspermy so the zygote is diploid (not 3n, etc.)

Completion of meiosis 2 in the ova is triggered by…

• an influx of Ca⁺ ions

• occurs after fertlization

Zygote formed from

fusion of egg and sperm nuclei to form a diploid nuclei

Blastocyst

• Zygote undergoes mitotic divisions to form a solid ball of cells - blastocyst

• inner cell mass → develops into embryo

• trophoblast = outer layer → placenta

• Blastocoele - fluid cavity

Outline blastocyst implantation

• Blastocyst breaches the jelly coat that was surrounding it and previously preventing its attachment into the endometrium

• Digestive enzymes released, degrades the endometrial lining

• autocrine hormones from blastocyst trigger its implantation into the uterine wall

• growing embryo will gain O₂ + nutrients from endometrial tissue fluid to ensure continued development

hCG secretion and function

• human chorionic gonadotropin, secreted by a blastocyst upon implantation into the endometrial lining

• promotes the maintenance of the corpus luteum in the ovary, which will then continue to produce both estrogen and progesterone

  • estrogen will continue to inhibit FSH and LH so no more eggs released

  • progesterone will sustain the endometrium

• hCG levels are kept for 8-10 weeks while placenta develops, then placenta is responsible for progesterone secretion and embryo nourishment

  • the corpus luteum then degrades as hCG levels drop

Placenta key functions

1. facilitates material exchange between the mother and the fetus

2. secretes hormones to maintain pregnancy after the corpus luteum degrades

Placenta structure

• Disc-shaped, forms from the trophoblast-developing upon implantation, eventually invading the uterine wall

• maternal blood pools from open ended arterioles into spaces between the villi in the placenta called lacunae

• chorionic villi extend into the blood pools and moderate material exchange between the fetus and mother

• umbilical cord transports exchanged material from the chorionic villi to the foetus

Placenta birth

• The placenta is expelled from the uterus after the birth of the infant

• it is then separated from the child by separating the umbilical cord

Material exchange in the placenta

• the chorionic villi extend into the lacunae and exchange materials between the mother and fetus

• chorionic villi are lined by microvilli (increased SA)

• fetal capillaries within chorionic villi are close to the surface, decreasing diffusion distance from the blood in the lacunae

• Materials exchanged lacunae → fetal capillaries: H₂O, O₂, nutrients, vitamins, antibodies

• Fetal waste (CO₂, urea, hormones) diffuse from the lacunae to the maternal blood vessels

Hormonal role of the placenta

• the placenta takes over the hormonal role of the ovaries at 12 weeks; secretes estrogen and progesterone (so hCG no longer needed)

• estrogen promotes uterine muscle (myometrium) growth and the development of mammary glands

• progesterone maintains the endometrium and reduces uterine contractions as well as potential maternal immune responses

• both hormone levels drop near birth

Parturition

• process of childbirth; occurs by positive feedback loops under hormonal control

Positive feedback loops in childbirth: oxytocin and stretch receptors

• fetal growth causes stretching of the uterine walls, detected by stretch receptors

• triggers the release of oxytocin from the posterior pituitary that induces uterine muscles to contract (reducing space in the womb)

• causes more stretching and contraction until the fetus is removed

Progesterone’s inhibition of uterine contractions

Progesterone inhibits oxytocin secretion from the posterior pituitary gland, hence inhibits muscular contractions of the myometrium (muscular outer wall of the uterus)

Role of estriol in parturition

• Stress of baby stretching the uterus → release of chemicals triggering estriol release

• estriol prepares the smooth muscle for hormonal stimulation by increasing its sensitivity to oxytocin

• estriol inhibits progesterone, which previously inhibited oxytocin, so contractions may begin

Prostaglandins in contraction

The foetus responds to uterine contraction triggered by oxytocin by releasing prostaglandins, which triggers further uterine contractions (positive feedback loop)