--- module: 085-01 language: en chapter: 85 title: "Developmental Biology, Germ Layers, Folding, and Placentation" module_title: "Gametogenesis, fertilisation, cleavage, blastocyst formation, and implantation" source_sha256: 1635d42967dc6a2ab9c4390195a5a186dfb275dbca8ae2ec5feb96c714528229 --- # Gametogenesis to implantation ## Timeline and germ cells ### Developmental age counted from fertilisation #### Gestational age from last menstrual period #### Gestational age about two weeks greater ### Primordial germ cells specified outside gonad #### Migrate to genital ridges #### Survival depends on somatic signals #### Imprints erased and re-established #### Failure: infertility, dysgenesis, germ-cell tumours ## Spermatogenesis ### Begins at puberty in seminiferous tubules ### Basal spermatogonia maintain stem cells ### Meiosis one separates homologues ### Meiosis two separates sister chromatids ### Spermiogenesis remodels without division #### Nucleus condenses, acrosome from Golgi #### Flagellum with mitochondria proximally ### Sertoli cells maintain blood-testis barrier ### Epididymal transit matures sperm ### Testosterone and follicle-stimulating hormone ### Heat, toxins, obstruction impair sperm ## Oogenesis and follicle ### Fetal arrest in prophase one #### Finite pool declines by atresia, ovulation ### Dominant follicle completes meiosis one #### Secondary oocyte and first polar body ### Meiosis two arrests at metaphase #### Completes only after fertilisation ### Unequal cytokinesis preserves cytoplasm ### Years of arrest raise aneuploidy risk ### Granulosa processes regulate meiotic arrest ### Theca makes androgen, granulosa aromatises ### Luteinising-hormone surge: ovulation, corpus luteum ## Fertilisation ### Capacitation in female tract #### Stepwise changes, not a single switch ### Occurs in ampulla of uterine tube ### Cumulus, zona binding, membrane fusion ### Fusion triggers calcium oscillations #### Meiosis two completes, metabolism activates ### Cortical reaction modifies zona #### Blocks polyspermy and genome imbalance ### Sperm gives paternal genome and centriole ### Oocyte gives cytoplasm, mitochondria, messenger RNA ### Pronuclei align on first spindle ### Many conceptions lost before recognition ## Cleavage and blastocyst ### Divisions without growth inside zona ### Compaction at about eight cells #### Position and polarity bias lineage ### Morula by about day three ### Trophectoderm mainly forms placenta ### Inner cell mass forms embryo proper ### Hatching sheds zona pellucida ### Delayed tubal transport risks ectopic #### Rupture with life-threatening haemorrhage ## Implantation ### Progesterone-dependent receptive window ### Decidualisation stores glycogen and lipid ### Apposition, adhesion, embryonic pole orientation ### Syncytiotrophoblast invades and erodes ### Cytotrophoblast is the proliferative source ### Partly embedded week one, enclosed week two ### Human chorionic gonadotropin rescues corpus luteum #### Trends and ultrasound beat a single value ### Lacunae start uteroplacental circulation ### Invasion must be controlled #### Too little: insufficiency, hypertensive disorders #### Too much: abnormally deep invasion ## Bilaminar disc and cavities ### Epiblast forms all three germ layers ### Hypoblast lines yolk-sac structures ### Amnioblasts line amnion ### Primary then secondary yolk sac ### Extraembryonic mesoderm and chorionic cavity #### Somatic layer lines trophoblast and amnion #### Splanchnic layer covers yolk sac ### Connecting stalk later forms cord ### Prechordal plate marks cranial end ## Assisted reproduction and synthesis ### Stimulation, retrieval, insemination or injection ### Genetic testing samples trophectoderm #### No guarantee of implantation or health ### Each failed transition gives a distinct outcome