Development of Multicellular Organisms

what are germ cells

-precursors to eggs and sperm

process of formation of germ cells

-primordial germ cells (PGCs) are first identifiable in proximal epiblast, before gastrulation

what do PGC’s release

-Blimp1

-nanog

what does Blimp1 do and what is it

-transcriptional repressor

-distinguishes them from surrounding somatic cells

-allows cell to remain on germline track

-during gastrulation they migrate to posterior end of embryo

-enter hindgut endoderm

-migrate dorsally to genital ridges which are future gonads

what is nanog and what does it do

-transcription factor

-helps to prevent premature differentiation

-during long migration to gonads

why do PGCs migrate rather than forming gonads directly

-exclude signalling that patterns body plan

-migration acts as selection filter, only healthiest cells survive to colonise gonads

how is PGC migration guided

-chemical signals from surrounding environment

how does meiosis ensure gametes have the correct genetic material

-involves 2 sequential divisions but only 1 round of DNA rep

how do germ cells undergo meiosis

-before meiosis I; chromosomes rep (each sister chromosomes become 2 sister chromatids)

-during prophase I, homologous chromosomes pair up into structures called bivalents

-crossing over occurs here; DNA segments physically exchanges between homologs

-generates chromosomes with new combination of alleles; inc genetic diversity

-meiosis I separates homologs

-each daughter cells has 1 copy of each chromosome

-meiosis II separates sister chromatids like normal mitosis

what is the process of oogenesis

-in females PGC’s multiply by mitosis during migration and after entering ovary

-once meiosis begins they become primary oocytes

-immediately arrest in prophase I during embryonic dev

-meiosis I completed at ovulation in adult

-meiosis II completed upon fertilisation

-division is asymmetric; one division prod a large cell with most cyto and a tiny polar body

why is division asymmetrical

-so only 1 functional egg is produced per meiosis

-polar body is discarded

what is the process of spermatogenesis

-in males

-PGCs enter embryonic testis and arrest G1

-after birth they proliferate by mitosis; forming spermatogonia

-they give rise to spermatocytes at sexual maturity

-prod 4 equal spermatids per meiosis

-differentiate into mature sperm

-continuous throughout adult life

what is a spermatogonia

-self renewing stem cell population

what is fertilisation

-fusion of egg and sperm; initiates development

what is it important only 1 sperm enter egg

-too many chromosomes resulting in abnormal development

what is the function of acrosome

-contains enzymes to digest protective coat aroung egg

what is the function of plasma membrane in sperm

-proteins bind to egg and facilitate entry

what happens to sperm cell after it has been deposited in mammalian female repro tract

-undergoes capacitation

what is capacitation

-facilitates fertilisation

-membrane remodelling and removal of certain inhibitory factors

what is the process of fertilisation of mammalian egg

-sperm penetrates stick layer of hyaluronic acid and somatic follicle cells

-sperm binds to zona pellucida

-penetrates zona pellucida

-plasma membrane of sperm fuses with egg plasma membrane

-sperm nucleus enters egg cyto

what is zona pellucida

-layer of fibrous glycoproteins via acrosomal reaction

why does fertilising sperm trigger Ca wave

-ca needed for fusion of cortical granules with cell membrane

what does the high conc of Ca cause

-causes cortical granule to fuse with egg membrane

-contents of cortical granule release into space between cell membrane and vitelline envelope

-enzymes from cortical granules harden vitelline envelope

-release sperm bound to it

what is parthenogenesis

-dev of embryo form unfertilised egg cell