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PreClinEazy X Cardiff Medsoc Embryology YR 2 BACK TO SCHOOL SERIES Nermeen Belblidia Email: BelblidiaN@cardiff.ac.ukEvent Topics Fertilization Cleavage Implantation Gastrulation 3 germ layer derivativesTopic 1: Fertilization • What is fertilisation? • Sperm structure + components of semen • 3 stages of fertilisation • Sites of fertilisation vs ectopic pregnancy Fertilization → egg + sperm fuse to create a zygote Where→ uterine tube Why→ 3 functions 5 day window Ejaculated sperm enters through vagina. 3 days before ovulation→ sperm 1. Establish diploid number of remain viable 48-72 hours in the Ovary releases egg during ovulation. chromosomes in zygote female reproductive tract 2. Sex determination +1 day of ovulation 3. Induces cleavage + 1 day post ovulation→ oocyte can be fertilised~ 24 hours after ovulation Sperm structure + semen components Head Acrosome → vesicle containing hydrolytic enzymes. Involved in acrosome reaction Middle Haploid nucleus→ containing 23 chromosomes piece Mitochondria→ provides energy for locomotion Tail Flagellum→ allows for sperm movement Plasma membrane→ outer layer of sperm where cholesterol + glycoproteins are removed during capacitation Semen= 10% sperm + 90% alkaline fluid • Fructose→ energy • Fibrinolysin→ liquifies semen for ejaculation • Ascorbic acid (Vit C) → protects sperm from oxidative damage • Spermine → base, stabilizes sperm pH 7.2- 7.6, protecting against vagina’s acid environment • Zinc→ essential in spermatogenesis • Prostaglandins → encourages contraction of uterine tubes to facilitate sperm movement Question 1 Ampulla then isthmus A The fertilization process is made up of 3 stages→ capacitation, then the acrosome reaction then the cortical B Isthmus then ampulla reaction. During the acrosome reaction, the sperm’s acrosome membrane releases hydrolytic enzymes to digest Corona radiata then zona pellucida C through the layers of the egg surrounding the nucleus. Identify the 2 layers which are broken down, specifying the D Zona pellucida then corona radiata order in which they are broken down. Answer 1 During the acrosome reaction, the sperm digests through the A Ampulla then isthmus layers surrounding the nucleus. Firstly, through the corona radiata since it’s the outermost layer, then through the inner B Isthmus then ampulla zona pellucida (ZP). Zona pellucidNucleuscytoplasm C Corona radiata then zona pellucida Corona radiata Zona pellucida then corona radiata D 3 stages of Fertilization 1. Capacitation 2. Acrosome reaction 3. Cortical reaction Sperm removes glycoproteins + Acrosome membrane is exposed which Sperm head + tail enter secondary oocyte→ it cholesterol from its plasma membrane completes metaphase II, forming diploid ovum → increasing fluidity + exposes the activates its surface antigens→ Release acrosome membrane of hydrolytic enzymes → digests By products → female pronucleus + polar body HCO3- ions released from female through corona radiata→ then fuses (Surplus DNA generated by the completion of reproductive tract meiosis). Sperm separates from its tail then swells → Allows sperm to undergo with ZP→ releases acrosin causing ZP to form the male pronucleus hyperactivation to increase motility to rupture when reaching uterine tube → Ovum releases cortical granules via exocytosis → flagellar motion changes from they fuse with the ovum membrane to chemically modify the ZP into a fertilisation membrane, wavelike to whiplike preventing polyspermy → Neutralizes acid environment in vagina Polyspermy→ multiple sperm fertilize the egg leading to aneuploidy (more than 2 sets of → Prostaglandins in semen also chromosomes) which makes zygote unviable. facilitates sperm movement Question 2 Ampulla You are a Junior doctor working in A+E, and a 25 yr old A female patient presents with severe RLQ pain. After taking her history, you discover that she has been sexually active for the last 6 weeks and she has not been using B Isthmus contraception. Her pregnancy test shows elevated hCG, leading you to suspect a ruptured ectopic pregnancy. Infundibulum C What is the most common site of an ectopic pregnancy in the uterine tube? D Fimbriae Bonus Q in chat→ What is the most common site of rupture of an ectopic pregnancy in the uterine tube? E Pouch of Douglas Answer 2 Ampulla Ectopic pregnancy→ the fertilized egg implants outside of the A endometrium, usually in the uterine tube or abdominopelvic cavity. This still gives a positive pregnancy result showing raised hCG. B Isthmus Ampulla→ most common site of an ectopic pregnancy in the uterine tube. Infundibulum Isthmus→ most likely site of rupture of an ectopic pregnancy in C the uterine tube. Pouch of Douglas/ rectouterine pouch→ most likely site of ectopic pregnancy outside of the uterine tube, since it is the D Fimbriae most inferior fold of abdomino- pelvic cavity, E Pouch of Douglas Ectopic pregnancy sites in uterine tube Ampulla – Isthmus widest part of the uterine tube, making it the most -narrowest part of uterine tube likely site of fertilisation + ectopic pregnancy - acts as sperm reservoir, allowing gradual entry of sperm into ampulla for fertilisation, slow entry prevents polyspermy Infundibulum – - most likely site of rupture of ectopic pregnancy located between fimbriae + ampulla Interstitial/ intramural- Fimbriae – captures the secondary oocyte which is connects uterine tube to the released from the ovary during ovulation uterus, allowing the fertilised egg to travel toward the (day 14 menstrual cycle) endometrium for implantation Ovary- site of oogenesis→ development of the primary oocyte into graafian follicle (mature secondary oocyte)Topic 2: Cleavage Process of cleavage After fertilization, the newly formed zygote undergoes rapid mitotic divisions. Overall size of embryo remains same→ cells become smaller with each division by skipping G1 and G2 phases of interphase. male + female pronuclei replicate their DNA→ paternal + maternal chromosomes intermingle, split longitudinally, undergo mitosis 2-cell zygote 4-cell zygote 8-cell zygote Morula (hollow ball of 16 cells Blastulation→ Morula forms blastocyst by day 7 surrounded by blastomeres) ZP (day 3) which breaks away when blastocyst fully formed (day 7) by day 3 outer mass cells (become trophoblasts) inner mass cell (becomes embryoblast) blastocoel (inner fluid filled space) → formed when morula enters uterus, Na/K pump pumps Na+ into morula, water follows by osmosis - Provides space for trophoblasts and other blastocyst cells to migrate and divideTopic 3: Implantation • Early implantation (days 8-9) → differentiation of blastocyst into bilaminar embryo → decidual reaction • Late implantation (days 10-13) OUTER syncytiotrophoblast → Differentiation of blastocyst INNER cytotrophoblast Inner mass cells mass cells amniotic cavity Blastocoel primary yolk sac embryoblast epiblast Outer mass cells→ trophoblastic cells → INNER cytotrophoblast and OUTER hypoblast syncytiotrophoblast Cytotrophoblasts are visible individual cells that migrate to + differentiate Inner mass cells→ embryoblast → into syncytiotrophoblasts bilaminar disc= OUTER epiblast +INNER hypoblast Syncytiotrophoblasts form “syncytium” and invades the endometrium • forms trophoblastic lacunae → later connects to the chorion to form the • Embryoblast maintained by LH, then placenta as part of the uteroplacental circulation hCG) • secretes b hCG which maintains the corpus luteum and embryoblast • epiblast forms amniotic cavity • hypoblast forms primary yolk sac Question 3 A menstruation The decidual reaction is the process in which the endometrium prepares itself for the implantation of the developing blastocyst. B Proliferation State the phase of the uterine cycle in which the decidual C Secretory reaction occurs in. D Follicular Luteal E Answer 3 A menstruation Menstrual cycle→ uterine and ovarian cycle Follicular Luteal B Proliferation Menstruation Proliferative Secretory Day 0 Day 7 Day 14 Day 28 (ovulation) C Secretory Secretory phase of uterine cycle Follicular • endometrial cells become polyhedral/ tortuous (saw D toothed, contain more glycogen and lipids) E Luteal Also in decidual reaction: • extracellular spaces of endometrium fill with fluid to limit invasiveness of trophoblast and forms maternal part of placenta Late implantation (days 10-13) LATE bilaminar embryo: secondary Extraembryonic cavity yolk sac amniotic cavity Extraembryonic cavity EARLY bilaminar embryo: amniotic cavity OUTER syncytiotrophoblast INNER primary yolk cytotrophoblast sac Primary villus Chorion(shaded in black)Connectingstalk epiblast embryoblast hypoblast Bilaminar disc Rule of 2 • inner cytotrophoblast + outer synctiotrophoblasts form primary villus • Epiblast + hypoblast form bilaminar disc • epiblast layer forms amniotic cavity and hypoblast forms → primary yolk sac→ secondary yolk sac Extraembryonic cavity • surrounds chorionic cavity + amniotic cavity + secondary yolk sac • extraembryonic mesoderm derived from epi+ hypoblast cells • Connecting stalk (later becomes umbilical chord) suspends bilaminar disk in chorionic cavity • Chorion later becomes placentaTopic 4: Gastrulation • between days 13-17 • Formation of trilaminar embryo • Cranial- caudal axis formation Trilaminar embryo + cranial- caudal axis Blastula/ bilaminar embryo becomes gastrula/trilaminar embryo when the epiblast + hypoblast differentiate into 3 germ layers→ ectoderm, mesoderm and endoderm Endoderm→ epiblast cells invaginate inwards, into the hypoblast Epiblast Ectoderm cells and displace them Mesoderm→ epiblast cells migrate down primitive streak (located between epiblast + hypoblast) Mesoderm Ectoderm→ after endoderm + ectoderm have formed, remaining Hypoblast Endoderm epiblast cells differentiate into ectoderm Epiblast gives rise to all 3 germ layers, hypoblast only endoderm!! Primitive node forms cranial axis. cranial axis caudal axis cranial axis Primitive streak forms caudal axis and extends toward primitive node. Endoderm begins to develop into rudimentary gut → extends from mouth (cranial) to anus (caudal end) forms at ~ day 15 + is FIRST Primitive/Hensen’s node signal of gastrulation caudal axis Primitive streak Formed by epiblast thickeningTopic 5: Germ layer derivatives We’ll be covering the structures formed from the 3 germ layers→ endoderm, mesoderm, ectoderm Ectoderm→ neural plate→ 3 derivatives Outer ectoderm Neural crest cells Neural tube LAME → Lens, Anterior PNS structures: CNS structures: pituitary, Mammary glands, • Autonomic nerves • Brain (Anterior pit. Outer Epidermis • Cranial nerves ectoderm!) • Schwann cells (myelination) • spinal cord Adrenal medulla (adrenal • Melanocytes (skin) • Retina cortex from mesoderm!) • Facial and skull bones • CNS nerves→ oligodendrocytes Teeth (myelination), astrocytes (maintain BBB), ependymal cells (CSF production) Neural tube forms CNS (brain + spinal cord) Alphabetically ordered: meSencephalon, then meTencephalon, then mYelencephalon (Screenshot from OSCEAZY case 7 slides, 2022). Mesoderm derivatives Endoderm derivatives • Adrenal cortex (adrenal medulla outer ectoderm) • Think GI and INNER lining of organs • → GI epithelial lining • Think bones (skull and facial bones from neural crest cell) • → all GI organs apart from spleen (mesoderm) • Spleen • Think endocrine→ thyroid, parathyroid, thymus • Think genitourinary organs→ kidneys, ureters, gonads • Bladder (other genitourinary mesoderm) (bladder from endoderm) • Think serous lining of body cavities, surrounding organs→ parietal + visceral layers, connective tissue, muscle • Think M for myocardium The notochord is also derived from mesoderm: • Induces neurulation, next process after gastrulation • Neurulation→ notochord induces ectoderm differentiation into the neural plate • Derived from axial mesoderm, becomes nucleus pulposus (inner jelly-like substance in IVD) Ectoderm: → Outer ectoderm think LAME, teeth, adrenal MEDULLA → NCC forms PNS → Neural tube forms CNS (brain + spinal cord) Germ layer Mesoderm: derivatives → Think serous layers SURROUNDING organs → Think genitourinary structures (exception bladder) simplified → Adrenal CORTEX → Notochord from AXIAL mesoderm→ signals neurulation (step after gastrulation, induces ectoderm to form neural plate) Endoderm: → Think inner lining of organs → Think GI organs (exception spleen) → Think endocrine glands: thyroid, parathyroid, thymus Question 4 Both derived from outer ectoderm A Which germ layers give rise to the adrenal cortex B Both derived from mesoderm and adrenal medulla. Both derived from endoderm C Adrenal medulla outer ectoderm, adrenal D cortex mesoderm Adrenal cortex outer ectoderm, adrenal E medulla mesoderm Question 4 Both derived from outer ectoderm A Which germ layers give rise to the adrenal cortex B Both derived from mesoderm and adrenal medulla. Both derived from endoderm C Adrenal medulla outer ectoderm, adrenal D cortex mesoderm Adrenal cortex outer ectoderm, adrenal E medulla mesoderm Question 5 A Derived from endoderm Where is the notochord derived from? Derived from mesoderm B Derived from ectoderm C Derived from outer ectoderm D Derived from axial mesoderm E Question 5 A Derived from endoderm Notochord is derived from the axial mesoderm. During neurulation, it induces the ectoderm to form Derived from mesoderm B the neural plate which then becomes 3 structures: 1. Outer ectoderm C Derived from ectoderm 2. Neural crest cells (NCC) 3. Neural tube D Derived from outer ectoderm Derived from axial mesoderm E Fertilization: egg + sperm fuse in the uterine tube (ampulla) to form diploid zygote (46 chromosomes). 3 stages: capacitation, acrosome reaction, cortical reaction. Cleavage: zygote undergoes rapid mitotic divisions, by day 3 forms morula (hollow ball of 16 cells). Forms blastocyst which is made of 3 structures→ inner mass cells (embryoblast), outer mass cells (trophoblasts), blastocoel (fluid filled space). Embryology summary Implantation- 2 important events: → decidual reaction: during secretory phase of menstrual cycle, endometrial cells become tortuous + extracellular spaces become fluid filled to limit invasiveness of trophoblasts. → blastocyst becomes bilaminar embryo Gastrulation- 2 important events: → bilaminar embryo (epiblast + hypoblast) develops into trilaminar embryo (endoderm, mesoderm, ectoderm). Epiblast gives rise to all 3 germ layers, hypoblast only endodermm. st → Cranial – caudal axis, primitive node is 1 signal of gastrulation REFERENCES • https://www.khanacademy.org/test-prep/mcat/cells/embryology/a/egg-meets-sperm • https://www.istockphoto.com/vector/human-sperm-cell-anatomy-structure-gm1062907750- 284162036 • https://www.nagwa.com/en/explainers/407162686539/ • https://study.com/academy/practice/quiz-worksheet-the-zona-pellucida.html • https://teachmeanatomy.info/abdomen/areas/peritoneal-cavity/ • https://www.shutterstock.com/image-illustration/bilaminar-blastocyst-preembryonic-stage- development-labeled-1958409115 • https://www.shutterstock.com/search/chorionic-cavity • https://basicmedicalkey.com/development-of-the-spine-and-spinal-cord-2/ • https://en.wikipedia.org/wiki/Intervertebral_disc PLEASE FILL OUT THE FEEDBACK FORM PLEASE TUNE IN TO OUR REMAINING SESSIONS THIS WEEK osceazyofficial OSCEazy osceazy@gmail.com OSCEazy osceazyofficial