Fertilisation, implantation and gastulation Flashcards
How does sperm get to the correct location for fertilisation?
- At ovulation cervical mucus changes, increases in amount and becomes thinner (aids transport)
- Fallopian tube come into close contact with ovary during ovulation/ fimbria sweep ovum into tube
- Peristaltic wave of FT musculature bring ovum into ampulla of FT
Capacitacion- how is sperm matured and readied for fertilisation?
Final spermatozoan maturation occurs in female R.Tract, includes remodelling of sperm GP coat
In uterus and oviduct plasma proteins removed, exposig binding portions of head (zp) (takes 5-7 hours)
What does passage of sperm through cumulus layer and into ZP consist of?
- Hyaluronidase released from sperm acrosome
- Tubal mucosal enzymes
- Flagellar action
ZP facilitates/ maintains binding and induces acrosome reaction- binding triggers opening of Ca2+ channels in SZ membrane (exoctyosis) > A. reaction needed for sperm to fuse with oocyte PM
What is the acrosome reaction
Acrosomes are modified lysosomes around ant. SZ head
- perforations develop in acrosome >
- enzymes released (protineases, acrosin, collagenases, esterases)- facilitate binding/ penetration of ZP
- Rxn must be completed before SZ can fully fuse w/ secondary oocyte
Perforation of ZP
Cortical reaction- exocytosis of cortical enzymes into ZP
- destroy sperm receptors to ZP, and causes hardening of ZP (Zona rxn, slow block to polyspermy) as more sperm entering ovum»_space; abortion
What happens when fusion of oocyte and sperm plasma membranes?
Triggers completion of meiosis II (mature ovum + secondary polar body), head and tail of SZ enter cytoplasm of oocyte
How are male/female pronuclei formed?
Pronuclei = 23 chromosomes each
At this stage male and female PN indistinguishable, but membranes break down and chromosomes arranged for mitotic cell division ( 1st cleavage division)
Fertilisation complete»_space; zygote formed (fertilised egg)
What are the results of fertilisation?
- Stimulates secondary oocyte to complete meiosis -Restores diploid number of chromosomes (46)
- Variation of human species as maternal/paternal chromosomes intermingle
- determination of sex of new individual
- Initiation of cleavage
What is cleavage?
Series of mitotic divisions- results in blastomeres, cells which becomes smaller with each division
After 3 divisions they undergo compaction, and compact blastomeres divide to form a 16 cell morula
Blastocyst formation
Blastocysts are embryos between 5-7 days following fertilisation
- 2 distinct cell types, inner cell mass (embryoblast - foetus)
outer cell mass (trophoblast-placenta)
Has central cavity filled with fluid (blastocoel)
- hatches from ZP before implantation
Implantation
Trophoblast divides and differentiates as soon as implantation begins-
1. Outer syncytiotrophoblasts
2. Inner cytotrophoblasts
and embryoblast-
1. Epiblast (upper)
2. Hypoblast (lower)
Uterus is in secretory phase- blastocyst implants in endometrium, if fertilisation does not occur then menstrual cycle begins and EM layers shed- basal layers remain/ regenerate other layers
Development of amniotic cavity and bilaminar disc
Day 8
Syncitiotrophoblasts > HCG (human chorionic gonadotrophin)- keeps producting progesterone and oestrogen to maintain secretory phase, avoiding menses
Cavity appears in epiblast layer of amnion, bilaminar embryonic disc > amniotic cavity
Gastrulation- how are the 3 embryonic germ layers formed?
Point in embryogenesis where basic organisation of organism established:
Week 2- 2 flat layers ( epiblasts and hypoblasts/ bilaminar disc)
Gastrulation begins in week 3- bilaminar > trilaminar disc
Ectoderm
Mesoderm
Endoderm- these 3 layers are origin of all human tissue, cells specific to future organs/tissues moved into right positions (ext/int)
What is a primitive streak?
Formation of PS starts gastrulation (formation of 3 germ layers) (15-16 day embryo)
Formed of groove, node and pit
Anterior- cranial
Posterior- caudal
Right and left- lateral
Epiblasts cells roll over primitive ridge and involute groove, cells lose contact and migrate inwards (ingression)
First cells through streak integrate and displace hyopblast (endoderm)
middle- mesoderm
epiblast - ectoderm
What are the 2 main results of gastrulation?
- primary germ layers formed
2. organised in proper locations- body axes created e.g anterior-posterior, dorsal-ventral and left-right
