fertilisation and gastrulation

• steps of fertilisation to implantation

  fertilisation, cleavages, morula, blastocyst, implantation
• sperm are deposited and accumulate in the vaginal fornices (near the cervix)
• fertilisation competence

  oocytes can activate just prior to ovulation, related to LH surge, capacitation is also needed
• LH induced meiotic maturation means there is development of activation competence, therefore, the oocyte is able to go through granule exocytosis, and complete meiosis
• ovulation

  LH surge: maturation and follicular changes (thinning of follicular wall)
• the secondary oocyte is arrested at metaphase 2, and encased within the zona pellucida, the zona pellucida is surrounded by the corona radiata
• the corona radiata is composed of cumulus cells and extracellular matrix
• zona pellucida

  thick, translucent, extracellular glycoprotein coat, contains receptors for acrosome reaction
• acrosome reaction

  sperm undergoes exocytosis, and can enter the zona pellucida
• capacitation causes hyperactivity of sperm, and they can then undergo the acrosome reaction
• fertilisation

  sperm penetrates corona radiata, binding to zona pellucida, influx of calcium into sperm causes the acrosome reaction
• zona penetration

  enzymatic digestion, and forward physical thrust of hyperactivated sperm
• sperm oolemma binding

  sperm binds with oolema, causes egg activation and sperm tail motility to stop, the sperm becomes part of the ooplasm and its nuclear envelope breaks down, the pronuclear envelope forms around chromosomes, and the male pronucleus is formed
• egg activation

  depolarisation spread and calcium influx (causes cortical reaction) results in the resumption of meiosis
• cortical reaction

  exocytosis of granules that prevent polyspermy
• events in fertilised egg

  meiosis 2 completed, pronuclear envelope forms around female chromosomes, S phase, fusion of haploid pronuclei (true moment of fertilisation), formation of zygote
• blastocyte

  hollow ball of cells, enters the uterine cavity at day 5, has an outer covering of trophoblast, inner cell mass, and a blastocele
• trophoblast develops into the fetal portion of the placenta
• most of the inner cell mass will become the embryo
• implantation

  attachment of blastocyst to endometrium, occurs 6-8 days after fertilisation
• day 8

  differentiation of the trophoblast into the syncytiotrophoblast and cytotrophoblast, and the differentiation of the inner cell mass into the bilaminar embryonic disc (hypoblast and epiblast)
• amniotic cavity

  shock absorber, regulates fetal temperature, prevents dessication, prevents adhesion of skin to membranes
• embryonic cells are sloughed off into amniotic fluid
• day 9

  site of implantation scar is covered by fibrin, syncytiotrophoblast layer develops lacunae, and primitive yolk sac is formed
• yolk sac

  hypoblast and exocoelomic membrane
• day 12

  embryo completely in endometrium, extraembryonic mesoderm develops, chorionic cavity is formed
• chorion

  mesoderm and trophoblast, protects the embryo and fetus from immune reaction, also produces hCG (maintains corpus luteum)
• gastrulation

  about day 14, primitive streak develops, endoderm, mesoderm, and ectoderm are formed
• epithelium

  ectoderm, mesoderm, endoderm
• connective tissue

  ectoderm (in head region), and mesoderm
• muscle tissue

  mesoderm
• nervous tissue

  ectoderm