2. Oogenesis spermatogenesis, fertilisation and embryogenesis Flashcards
When does oogenesis begin?
During embryonic development - diploid primordial germ cells migrate to embryonic ovary and proliferate by mitosis
What are primary oocytes?
Cells that have progressed through first meiotic prophase, arrested at diplotene stage - arrested before birth and until puberty
Oocytes are in metaphase II at ovulation, meiosis II not completed until fertilisation
What occurs at telophase on meiosis I?
Unequal cytokinesis, creates first polar body and secondary oocyte
Describe the process of spermatogenesis
Spermatogonia generated during embryonic development by mitosis from primordial germ cells
Some daughter cells stop proliferating - differentiate into primary spermatocytes
Symmetrical meiosis –> haploid secondary spermatocytes –> spermatid –> differentiation and maturation to sperm
How does the number of mitotic divisions differ in oogenesis and spermatogenesis?
Twice as many in spermatogenesis = increased mutation rate
Describe how the sperm nucleus enters the oocyte cytoplasm at fertilisation
Takes place in ampulla of fallopian tube
Sperm attaches to outer layer of follicle cells of oocyte, reaches ZP
Cell surface receptor on sperm head binds to ZP glycoprotein –> acrosome releases enzymes to digest ZP
Membranes fuse
What is the cortical reaction?
Cortical granules in oocyte fuse with plasma membrane –> cross-linking of glycoproteins = impenetrable
What happens after the cortical reaction?
Oocyte undergoes second meiotic division - unequal cytokinesis = halploid mature ovum + second polar body
Haploid oocyte and sperm nucleus form pronucleus - migrates to centre of oocyte, mitotic spindles form –> first mitotic division = totipotent zygote
What happens at the cleavage stage of embryogenesis?
Zygote repeatedly divides –> blastomeres
Very slow divisions, controlled by embryonic genome
First cleavage plane is horizontal, then horizontal and vertical (rotational cleavage)
What happens at the compaction stage of embryogenesis?
Blastomeres of 8 cell embryo flatten against each other –> morula (introduces polarity)
Sphere is sealed and filled with fluid = blastocoel cavity
What happens at the blastocyst stage of embryogenesis?
16 cells stage - can differentiate between inner, non-polar cells and outer, polar cells
Outer cells (trophoblast) form chorion
Inner cells form off centre inner cell mass - give rise to all cells of organism
Splitting of ICM = monozygotic twins
What happens at the implantation stage of embryogenesis?
ZP degrades, blastocyst attaches to uterine epithelium
Trophoblast layer invades connective tissue of uterus
ICM cells differentiate into inner hypoplast and outer epiblast
Fluid-filled cavity formed in ICM = amniotic cavity, enclosed by amnio
What do the hypoblast and epiblast layers form?
What is the name of the structure that they form?
Epiblast (external) = all embryonic tissue, amnion, yolk sac, allantois
Hypoblast (internal) = extraembryonic mesoderm that lines yolk sac and blastocele
Bilaminar germ disc
What happens at the gastrulation stage of embryogenesis?
Bilaminar structure –> 3 layers (endoderm, ectoderm, mesoderm
Some epiblast cells proliferate and replace hypoblast cells –> endoderm
Some epiblast cells diverge into mesoderm between endoderm and epiderm
Residual epiderm cells form ectoderm
Formation of primitive streak (marks bilateral symmetry, establishes orientation)
What does the different germ layers form?
Ectoderm - skin and the nervous system
Mesoderm- bone, muscle, and connective tissue
Endoderm - linings of the digestive and respiratory system, liver and pancreas
