BIOL 1030 – TOPIC 19 LECTURE NOTES

Topic 19: Animal Reproduction (Ch. 46)

I.  function – perpetuation of the species (central focus of nearly all life activities)

II.  asexual vs. sexual reproduction

A.  asexual – no new genetic combination in offspring

1.  genetic copies produced usually by mitosis; sometimes meiosis (drone bees)

2.  common in invertebrates, rare in vertebrates

3.  includes

·  fission – “normal” mitosis; for unicellular organisms

·  budding – smaller cells or groups of cells bud off

·  parthenogenesis (virgin birth), where even if a sperm is involved the egg develops directly into the embryo (egg usually diploid for this); in vertebrates, usually a parthenogenic species arises via hybridization producing an all-female species (some lizards, some fish)

B.  sexual – new genetic combination produced

1.  usually involves fertilization, the union of haploid sex cells (gametes, often smaller sperm and larger egg) to form a diploid zygote that gives rise to the diploid organism [we will call this gametic sex]

2.  conjugation is a form of sex in bacteria and some protists where genetic material is introduced by another cell to an intact cell

III.  different approaches to gametic sex determination

A.  hermaphroditism – one individual has both types of gonads (gamete-producing organs), testes (sperm-producing organs) and ovaries (egg-producing organs)

1.  both at same time

·  some can self-fertilize (example: tapeworms)

·  some must have a mate (example: earthworms)

2.  sequential – individual can change its sex (most common in fish)

B.  environmental determinism – in most fish, amphibians, and reptiles, sex determined my environmental conditions (often temperature); female is the default sex, something must be added to make the animal male

C.  chromosomal determination – specific sex chromosome(s); different systems

1.  mammals – males XY, females XX; probably, SRY on Y chromosome drives gonads to become testes

2.  birds – males XX, females XY

3.  insects - many different systems

IV.  mammal reproduction

A.  mammalian males almost always ready for sex at any time

B.  most female mammals are only capable of sexual reproduction during certain times

1.  estrus – period of sexually receptivity (these females have an estrous cycle)

2.  changes in follicle stimulating hormone (FSH)and lutenizing hormone (LH) from the anterior pituitary cause egg cell development and hormone secretion in the ovaries

3.  menstrual cycle – similar to estrous cycle but found only in humans and apes

·  females shed inner lining of uterus (menstruation)

·  females may engage in sex at any time during cycle, but fertilization more likely at certain times

4.  induced ovulation – no cycle; ovulate after sex, making them very fertile; examples: rabbits and cats

V.  male reproductive system (mammalian, focus on human)

A.  testes

1.  seminiferous tubules - produce sperm

2.  interstitial tissue - produce testosterone (hormone driving production of male sexual characteristics) in Leydig cells

B.  scrotum – outpocketing of coelom into which testes descend; keeps the testes at 34°C vs. 37°C (cooler is better for sperm production in humans)

C.  epididymis - receives sperm and transfers them to vas deferans

D.  vas deferans (ejaculatory duct) - takes sperm out of body - in mammals it joins with the urethra at the prostate gland to form the urogenital canal

E.  semen – fluid released upon ejaculation

F.  seminal vesicles – glands that produce a fructose-rich fluid for part of semen

G.  prostate – contributes 60% of bulk of semen

H.  bulbourethral glands – secrete fluid that lines urethra and lubricates tip of penis prior to ejaculation

I.  penis

1.  corpora cavernosa - two columns of erectile tissue along dorsal side

2.  corpus spongiosum - one column of erectile tissue along ventral side

3.  erection – nitric oxide released by neurons caused arterioles in penis to dilate; blood flows into erectile tissue, which compresses the veins so that blood cannot leave; some mammals have a bone (baculum) that aids this, but humans do not

4.  orgasm – reflexive muscular contractions spaced about 1 second apart; two phases; nerves responsible also stimulate pleasure centers in brain

·  emission – peristaltic waves in vas deferens and prostate move sperm and seminal fluid into a collecting zone at base of penis

·  ejaculation – violent contractions of muscles at the base of penis send collected semen out of body; release of approximately 5 mL of semen containing 300 million sperm (1% of semen); males with 20 million sperm/mL or less are considered sterile

5.  refractory period – after orgasm, usually at least 20 minutes where erection is difficult to achieve and orgasm almost impossible

J.  spermatogenesis - formation of sperm

1.  wall of seminiferous tubules with germinal cells (or spermatogonia) that split via meiosis to form sperm, and Sertoli cells that support spermatogenesis

2.  germinal cells first split by mitosis, one daughter cell will split via meiosis (primary spermatocyte), but other remains diploid

·  primary spermatocyte undergoes first meiosis to form 2 secondary spermatocytes

·  secondary spermatocytes undergo second meiosis to form 4 spermatids

3.  Sertoli cells - secrete products required for spermatogenesis and help to convert spermatids to spermatozoa (functional sperm) by engulfing the spermatids' extra cytoplasm

4.  sperm

·  head – compact with nucleus and acrosome (derived from Golgi body) that contains enzymes that allow the sperm to penetrate the egg

·  body – many mitochondria for energy; centriole – basal body of flagellum (tail)

·  tail – flagellum for movement

VI.  female reproductive system and fertilization (still focused on human)

A.  clitoris – from same tissue as penis (homologous structure); contains corpora cavernosa and is erectile

B.  labia majora – from same tissue as scrotum

C.  ovaries, each with about 1 million ovarian follicles – each with egg cell and granulosa cells

D.  estrodiol (estrogen) – major female sex hormone produced by granulosa cells beginning at puberty; cause start of menstrual cycle and development of female secondary sexual characteristics

E.  Fallopian tubes (oviducts) - transport ova to uterus

F.  uterus – in primates, a single pear-shaped organ where growth of fetus occurs; in most mammals there are two uterine horns- derived from oviduct; in any case, must be flexible and stretch with growth of fetus, and have smooth muscle for contractions to expel the fetus

G.  cervix – narrow end of uterus, next to vagina

H.  vagina – specialized canal to serve as receptacle for penis, and be able to stretch for exit of fetus (marsupials actually have two, and marsupial penis is forked)

I.  oogenesis – formation of eggs (human)

1.  at birth - 2 million follicles each with an ovum arrested in prophase I of meiosis (primary oocyte)

2.  follicular phase of menstrual cycle

·  FSH stimulates a few follicles to develop, but only one reaches full maturity - tertiary or Graafian follicle

·  follicle forms a thin-walled blister on surface of ovary

·  primary oocyte completes first meiosis in follicle; cleavage is uneven producing a small first polar body and a large secondary oocyte

·  secondary oocyte begins meiosis II but arrested at metaphase II

·  estrodiol levels in the blood are rising during this phase

·  estrodiol stimulates growth of endometrium lining uterus

3.  ovulation

·  estrodiol level stimulates anterior pituitary gland to produce a spike of LH

·  follicle bursts and ejects secondary oocyte from ovary

·  oocyte enters abdominal cavity and is pulled into the Fallopian tube by the action of ciliated epithelial cells

4.  luteal phase

·  period of menstrual cycle after ovulation

·  LH stimulates empty Graafian follicle to become corpus luteum

·  corpus luteum secretes estrodiol and progesterone; this inhibits FSH and LH secretion

·  endometrium becomes more vascular, glandular, and glycogen-enriched

·  atrophy of corpus luteum triggered toward end of phase due to dropping LH level

§  levels of estrodiol and progesterone drop

§  endometrium is then sloughed off (menstruation)

J.  fertilization

1.  fertilization is in upper 1/3 of fallopian tube; sperm must get to fertilize it

2.  female orgasm likely assists movement of sperm to Fallopian tube

·  sufficient stimulation of nerves sends impulse to hypothalamus

·  hypothalamus stimulates posterior pituitary gland to release oxytocin

·  oxytocin stimulates muscular contractions in uterus, vaginal opening

·  no refractory period

3.  many sperm typically surround oocyte and try to digest their way in; once one succeeds, a chemical change occurs and the rest are completely blocked out

4.  if oocyte is fertilized in Fallopian tube, meiosis II continues and forms second polar body and ovum, which joins with sperm to from the zygote

5.  embryo secretes human chorionic gonadotropin (hGC), which mimics LH and keeps the corpus luteum alive, preventing menstruation

6.  hGC is tested for in pregnancy tests – only made by embryo

7.  it takes 2-3 days for the zygote to implant in the endometrium of the uterus

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