Lecture 3 - Gametogenesis Intro, Spermatogenesis Flashcards
gametogenesis - where does it occur, what does it result in?
occurs in gonads (ovaries and testes) - results in haploid gametes via meiosis.
occurs in a manner that is related to distinct functions of the two gametes.
what are gamete precursors? where are they initially?
primordial germ cells (PGCs) - do not form in gonads, must migrate to developing gonads during embryogenesis.
what is associated with somatic cells in gonads?
germ cells are associated w/ somatic cells in gonads. somatic portions are derived from mesoderm.
describe migratory pathway of PGCs in humans (and with what molecules this occurs)
PGCs migrate around hindgut + up mesentery of hindgut from an appendage of the embryo called the yolk sac.
involved molecules include ECM proteins which guide movement, integrins + actin/myosin which are the mechanical source of movement.
*note: in birds/reptiles, PGCs move thru bloodstream.
describe the general speciality of the sperm. what is its feature? how does it impact the sperm’s structure?
sperm is highly specialized - sole function is to fertilize an egg
as a result, its very “bare-bones” compared to a normal somatic cell. very small; strong flagellum, no organelles (except mitochondria), etc. its made exactly for its mission.
describe the specialized features of a sperm cell.
1) mobile; adapted to deliver male chromosomes to egg
2) strong flagellum, no ER, Golgi, ribosomes, etc.
3) many mitochondria to power motility
4) head + tail covered by 1 plasma mem.
5) head contains condensed haploid nucleus + acrosomal vesicle - secretory vesicle that will exocytose hydrolytic enzyme to penetrate egg coat + induce sperm binding
6) sperm tail - midpiece full of mitochondria to generate ATP, flagellum w/ central axoneme.
sperm has interesting variations - C. elegans: amoeboid movement, fruit flies: sperm is very long, composed mostly of tail, delivered in a dense packet.
relative to other cells, how large is an egg cell? what is its (initial) purpose?
largest cell made by a given species - 5-10X size of a somatic cell in humans, 50,000X in chicken
initial purpose is to sit and wait to be fertilized.
what is the egg’s purpose after fertilization?
egg contains resources to:
nourish new organism (glycoprotein, carbohydrate, lipids)
initiate cleavage (tubulin, actin, histones)
provide spatial information (e.g. tubulin, actin, mRNAs)
(except for mammals), egg appears to be pumped full of these molecules + yolk proteins during meiotic arrest - sits and waits for fertilization to occur, has nourishing molecules + determinants ready for embryogenesis.
what is the germ line, and what is the germ line concept?
germ line is the special lineage of cells that give rise to gametes and future progeny’s gametes, i.e. sperm, egg, zygote (and those of future progeny). in essence - it is continuous and intergenerational.
further along in germ line = PGCs (give rise exclusively to gametes).
how does determination of PGCs occur in most animals?
(not including mammals) - determination of PGCs brought about by cytoplasmic localization of specific determinants.
some functions of known deters. include prevention of mitosis + repress transcription of certain genes.
other species: cell-cell interactions play bigger role.
why does the germline concept conflict with Lamarckism?
Lamarckism = the theory of “inheritance of acquired characteristics” - through use or disuse, progeny will either inherit physical characteristics of the parent organism or will lose them over time.
germline concept conflicts with Lamarckism in that somatic cells are separate from the germ line - they are not passed on to progeny. it is known that inheritance of genetics occurs, but not the actual somatic cells themselves.
what would be the result of a somatic mutation in respect to progeny? could it affect them?
what is epigenetics?
a mutation itself is not always a heritable change in DNA (e.g. somatic mutations).
however, due to maternal effect (phenotype of an organism can be influenced by genotype of its mother; progeny could develop phenotype expected of mother’s genotype due to mRNA/protein signaling to the egg), somatic mutations could heavily interfere and wreck development.
epigenetics = modifications in phenotype not caused by alterations in DNA sequence (e.g. affect of smoking during pregnancy on infant’s phenotype)
general sequence from PGC to gamete - what occurs first?
PGCs in gonads divide mitotically, produce diploid oogonia + spermatogonia (latter derived from gonocytes; mammalian PGCs in embryos that produce sperm) - these undergo DNA rep at which time, they are primary oocytes or primary spermatocytes (and ready to undergo meiosis).
what occurs after DNA rep. in primary spermatocytes/oocytes?
meiosis is initiated. after meiosis I, products = secondary oocytes/spermatocytes.
describe the state of secondary oocyte/spermatocyte after meoisis II
after completion of meiosis, male germ cells = spermatids, female germ cells = eggs or ova.
spermatids must undergo spermiogenesis (a maturation process) to become sperm.
