Lecture 2: Gametogenesis Flashcards
Gametogenesis is the process of
making sex cells.
- meiosis (converting diploid cell into haploid cell)
- spermatogenesis and oogenesis
Mitosis:
-homologous chromosomes
-DNA Replication = sister chromatids
- chromosome segregation (mitosis) sisters separate
= 2 IDENTICAL DAUGHTER CELLS, CLONES OF PARENT AND EACH OTHER
Meiosis:
- homologous chromosomes
-DNA Replication, RECOMBINATION = sister chromatids
-chromosome segregation (meiosis 1) homologues separate, sisters remain attached
-Chromosome segregation (meiosis 2) sisters separate
=== GAMETES (eggs or sperm)
HAPLOID and NOT identical
fertilisation is when
haploid gametes (egg & sperm) –> Diploid zygote (fertilised egg)
what produce gametes i most species?
highly specialised organs (gonads)
Spermatogenesis:
process of making sperm
sperm are produced from
spermatogonial stem cells. Spermatogonial cells divide by MITOSIS initially, producing the diploid primary spermatocytes THEN MEIOSIS
each primary spermatocyte gives rise to
four sperm cells
Primary spermatocyte (diploid) –> sperm (haploid)
Meiosis: primary spermatocyte (diploid) - meiosis -> secondary spermatocytes (diploid) -> spermatids (haploid) -> develop into sperm (haploid)
is spermatogenesis exactly the same throughout all taxa?
NO
Spermatogenesis in insects (e.g. fruit fly)
- occurs in cysts within testes
- spermatogenesis from distal to proximal (long tube) -starts at larval stage
- sperm produced in a few days
- ~<10,000 sperm / day
- semen from accessory glands
Spermatogenesis in mammals (e.g humans)
- Occurs in seminiferous tubules within testes
- Spermatogenesis from periphery to lumen
- starts in puberty
- sperm produced in approx 74 days
- ~300 million sperm per day
- stored in epididymis for <4 weeks or reabsorbed
- semen from seminal vesicles & prostate gland
is there much diversity own sperm morphology?
YES
sperm giantism:
type of fly??
body length = 3mm
sperm length = 60mm
sperm heteromorphism:
organism has multiple sperm morphologies used for blocking female tracts etc
oogenesis
production of the ovum (egg cell)
1 primary oocyte gives rise to _ egg cells
JUST ONE.
more precious than sperm
oogenesis stages:
meiosis 1:
-primary oocyte (diploid) –> secondary oocyte (diploid) and first polar body
(meiosis 2)
–> -ovum (haploid) and polar bodies –> just ONE egg (haploid)
- ovum (haploid)
oogenesis in insects e.g. fruit fly
- oogenesis from distal to proximal
- start in larval stage
- eggs produces in a few days
- <100 per day
- released into uterus for fertilisation
- egg laid whether fertilised or not
- oviparous (no pregnancy)
oviparous:
producing young by means of eggs, which are hatched after being played by the parent.
oogenesis in mammals e.g. humans
- oogenesis within a follicle (one ova with cells to nourish)
- 400,000 follicles at birth but ovulation starts at puberty
- starts in puberty
- 1 per month
- follicle ruptures, oocyte enter Fallopian tube for fertilisation
- If not fertilised, degenerates and prepared uterus lining (thick, vascular endometrium lining) sloughs away
Control of gametogenesis hormones
- At puberty, hypothalamus secretes gonadotrophin-realising hormone (GnRH)
- GnRH stimulates FSH (follicle stimulating hormone) and LH (lutenizing hormone) from the anterior pituitary cells
In spermatogenesis LH stimulates
LH – stimulates Leydig cells to secrete testosterone which stimulates spermatogenesis
In spermatogenesis FSH stimulates
FSH –stimulates development of seminiferous tubules (where sperm form) and spermatogenesis acting together with testosterone by stimulating Sertoli cells
In oogenesis LH -
LH – stimulates maturation of follicles, estrogen and progesterone, ovulation
In oogenesis FSH =
FSH – stimulates development of follicles and secretion of estrogens
role of estrogen:
enhances growth of follicles, grow endometrium
progesterone:
endometrium for pregnancy, secretes nutrients for embryo
oestrogen and progesterone fluctuate in
-28 day cycle in humans
the ovary and uterus need to be…
coordinated. achieved by hormonal feedback
if fertilised, embryonic placental cells secrete
chorionic gonadotropin to rescue the corpus luteum and maintains its function
birth control pill works by
negative feedback to FSH and LH
