Embryology 1 PDF

Embryology 1 Emma Kemp-Gee Learning Objectives Describe the characteristics of gametes Describe fertilization Describe implantation List the major functions of the placenta  Embryology Embryo...

Embryology 1 Emma Kemp-Gee Learning Objectives Describe the characteristics of gametes Describe fertilization Describe implantation List the major functions of the placenta  Embryology Embryo Fetus Infant 8 Weeks ~30-34Weeks Embryological development begins with fertilization, the joining of a male and female gamete during sexual reproduction, to produce a single cell containing a unique set of genetic information. The combination of genes produced by this association helps to determine the growth, appearance, and function of an individual Gametes: Egg and sperm Produced in the Produced in the testis ovary before birth from puberty (young) Oocyte in Meiosis II 4 viable sperms 1 ‘arrested’ from each primary oocyte Every month  Gametes: Egg and sperm Oocyte Sperm Largest cell – 0.1mm diameter Non- Smallest cell – 65 µm (0.065mm) motile Highly motile Active metabolically Little metabolic activity Millions One per cycle Produced in the testis from puberty (young) Produced in the ovary before birth (old) During maturation decrease volume – cytoplasmic droplets Limited numbers Pronucleus content: 22+ X or Y chromosomes (define During maturation increase volume – gain yolk granules the sex of the embryo) (n) Pronucleus content: 22+ X chromosomes and Mit (n)  Structural features of the secondary oocyte Intracellular: Large haploid nucleus Dense cytoplasm Surrounded by plasma membrane and vitelline membrane Extracellular: Oocyte itself surrounded by granulosa cells that form the corona radiata These originate from the (Graafian) follicle in the ovary Granulosa cells sit in a matrix containing high levels of hyaluronan (hyaluronic acid) Below this is the zona pellucida (means transparent zone) Extracellular matrix that is secreted by oocyte and granulosa cells Consists of glycoproteins including ZP3 receptors  Structural features of spermatozoa Head: Contains the nucleus and acrosome, each are contained within their own membrane Acrosome contains powerful hydrolytic enzymes: 1. Acrosin – a protease enzyme that can digest proteins in the zona pellucida 2. Hyaluronidase – breaks down hyaluronan in the corona radiata Mid piece: Contains many mitochondria for synthesis of ATP Flagellum: Allows movement and is a long and whiplike structure Made up of long tubulin proteins that are surrounded by plasma membrane Cylindrical structure at the core called axoneme with 9 doublets of microtubules arranged radially 2 centrally located microtubules 9 + 2 arrangement in cross section – characteristic of eukaryote flagella (and cilia)  Seminal fluid Seminal fluid acts as a transport medium for sperm cells It also aids the processes that lead to successful fertilisation Seminal vesicles: Present in the male reproductive system Produce 60-70% seminal fluid Contains: 1. Fructose as source of energy (carbohydrate) for sperm cell – why not glucose? 2. Prostaglandin – sperm motility and induce uterine contractions 3. Coagulase – makes semen less liquid/ more gel-like Prostate gland: 30% seminal fluid Citrate energy source  Quiz 1 1. Where are the male and female gametes produced? Males gametes – testes female gametes – ovaries 2. What is the corona radiata and what purpose does it serve? Group of cells derived from the Graafian follicle that the oocyte matured in, in the ovary. Provides nourishment and proteins necessary for development 3. What is the acrosome and what does it contain? Membrane-bound structure in the head of the sperm containing hydrolytic enzymes that assist the sperm in digesting the corona radiata and zona pellucida 4. What is the characteristic arrangements of microtubules in the flagellum of a sperm cell? 9 + 2 (9 arranged radially and 2 centrally) 5. Where is the seminal fluid produced and what purpose does it serve? Seminal vesicles and prostate gland, transport medium and solvent for substances that assist in fertilisation and provision of energy for sperm cells 6. Why is fructose the energy source in seminal fluid, rather than glucose? Glucose is the preferred respiratory substrate for many bacterial species, so fructose reduces competition  Embryology: Basic concepts Embryological development occurs during the first eight weeks after fertilization (embryo), fetal development from week nine to birth of the infant. During prenatal development, limbs, organs, and systems need to be established in a time-dependent sequence to ensure normal growth. Gestation: 38-42 weeks- 3x13weeks periods 1st trimester, 2nd trimester, and 3rd trimester. Implantation Embryonic folding W2 W4 Fertilization W1 (d1) Gastrulation W3 O rg a n o  A week of wonders: Fertilization to Blastocyst Environment: Acidic environment Only 100s-1000s (from ~200Millions) Mobile tail and head Moves via chemotaxis 1-2 hours to get to the Oocyte Oocyte Retain their fertilizing power within the female reproductive tract for 24 to 48 Only one and viable hours after ejaculation for 12 to 24 hours after ovulation For fertilization to occur, sexual intercourse must occur no more than 2 days before ovulation and no later than 24 hours after Uterine tubes  A week of wonders: Fertilization 1 6.. 4. 5. Acrosome reaction Cortical reaction Fusion of membranes, Corona radiata & zona pellucida Enzymes released release of nucleus and digest the Single sperm contact centriole 2. the oocyte’s receptor membrane Oocyte completes meiosis II- The oocyte ‘reacts’ ovum and polar body Capacitation and no longer Female and male allows the entrance pronucleus Faster, head more to others (blocks (n+n) ‘fluid’ 7. polyspermy) Zygote (2n)  Capacitation Series of changes to the sperm that happen in the female reproductive tract to prepare them for fertilising the egg Involves biochemical changes that increase the sperms motility and allow it to traverse the zona pellucida Epididymal sperm cells – plasma membrane is covered in surface molecules such as glycoproteins, carbohydrates and cholesterol Capacitated sperm cells – efflux of cholesterol from the sperm cell and alkalinisation increases fluidity of the sperm membrane and hyperactivate motility  Acrosome reaction ZP3 receptors (glycoproteins) in the zona pellucida Sperm have specific recognition proteins Sperm bind to receptors Triggers the acrosome to swell and fuse with the ZP3 receptor Release of hydrolytic enzymes that digest through zona pellucida  Cortical reaction – prevents polyspermy (fast and slow) Sperm cell binds to docking site on Binding of sperm cell causes fusion of oocyte membrane sperm cell membrane with oocyte Causes protein channels to open membrane Influx of Ca2+ into oocyte Sperm pronucleus delivered into oocyte Smooth endoplasmic reticulum Changes potential difference across releases Ca2+ membrane Causes lysosomes within oocyte to Prevents further sperm binding to migrate to membrane and fuse with it membrane Releases hydrolytic enzymes break Fast block to polyspermy down zona pellucida and ZP3 receptors Slow block to polyspermy  Quiz 2 1. How does capacitation aid fertilisation? It involves biochemical changes to the sperm such as efflux of cholesterol which contributes to increased fluidity of the sperm cell membrane and alkalinisation which is associated with hyperactivated motility 2. Summarise the events in the acrosome reaction Sperm cells bind to ZP3 receptors, special glycoproteins on the zona pellucida triggering release of acrosomal enzymes and digestion of the zona pellucida, exposing sperm docking sites on oocyte membrane 3. What happens during the fast block to polyspermy? Ca2+ influx to the oocyte causes a change in p.d. across the oocyte membrane, causing a block to further sperm binding to membrane docking sites 4. What happens during the slow block to polyspermy? Sperm pronucleus entering the oocyte triggers Ca2+ release from the smooth ER and lysosomes to release hydrolytic enzymes by exocytosis to digest the zona pellucida and any further ZP3 receptors 5. What is the name of the single cell resulting from fertilisation? Zygote 6. What is its nuclear contents? 2n - diploid  A week of wonders: Fertilization to Blastocyst Cleavage same size, many more cells, compaction occurs as cells become smaller and more closely associated Morula (d3) Blastulation Blastocyst Zygote 2n Little mulberry Secrets hCG 16 cells Nutrition from (human chorionic gonadotropin) Made from uterine secretions blastomeres called uterine milk Stops endometrium shedding Floats through oviduct Week 1 | Embryology - A Web until approx 100 cells Site At d4 enter the uterine cavity  Blastulation sees the blastomeres becoming more specific in their function and location Still within the zona pellucida but this breaks down imminently Outer Inner cell mass evident on one side cell – becomes embryoblast mass Fluid filled cavity Outer cell mass – becomes trophoblast A week of wonders: Fertilization to Blastocyst Cleavage same size, many more cells D3: Morula D4: Uterine cavity- Fills with fluid D4-D7: Blastocyst (inner cell mass/blastocyst cavity/ outer cell mass) D7: Implantation  Quiz 3 1. What is the name of the stage where the cells undergo rapid mitotic divisions? Cleavage 2. Where in the female reproductive system is the ball of cells at this time? Oviduct 3. How do the cells at this stage get nutrients? Uterine secretions (uterine milk) 4. Why and how do they move? Movement due to the wafting motion of the cilia and fimbriae, necessary to reach the uterus for implantation 5. What is the name of the process that see the ball of cells turn into a blastocyst? Blastulation 6. What are the 2 cells masses in the blastocyst and what do they each develop into? Outer cell mass – trophoblast, inner cell mass - embryoblast Week 2: Implantation Implantation of the blastocyst usually occurs 6-8 days after fertilization By day 8, the blastocyst has 'burrowed' into the uterine wall (endometrium) The uterine wall envelopes the blastocyst. Blastocyst gains nourishment from the uterus cells Trophoblast secretes human chorionic gonadotrophin (hCG) telling the corpus luteum in the ovary to survive and continue secreting progesterone to suppress the next menstruation A few days after implantation hCG levels in the mother's bloodstream are detectable by pregnancy test  Early placental development Trophoblast cells (once the outer cell mass) begin to proliferate outside of the blastocyst Cell membranes break down making a region of multinucleated cytoplasm Penetrates the cells of the uterus in finger-like projections Interact with maternal blood vessels in the uterine cells  Ectopic pregnancy Ectopic pregnancy is the presence of a developing embryo in a site other than the endometrium of the uterus. It is a relatively common condition with a frequency of approximately 20 per 1,000 pregnancies. The implantation of the developing embryo outside the uterine cavity can result in symptoms of early pregnancy with associated abdominal pain. Prevalence: Ampulla implantation – approx. 70% Complications resulting from the pregnancy and Isthmic implantation – approx. 12% appropriate management depend on the site of Fimbiae implantation – approx. 11% implantation. Ovarian implantation – approx. 0.5%  Week 3: Gastrulation Cells from the embryoblast (once the inner cell mass) proliferate and migrate to form a bilaminar disc and then a trilaminar disc - the ectoderm, mesoderm and endoderm. These layers are the building blocks of tissue and organ formation. This process, called gastrulation, forms what is known as a gastrula.  Week 3 to 8: Organogenesis The period of embryonic development from the third to eighth week, the majority of the major organ systems begin to become established. For this reason, the embryonic period is also known as the period of organogenesis. The 3 layers form specific organs, but there are 3 broad categories of ways that organs are thought to form: Folds – folds in the layers of cells forming an enclosed tube, e.g. neural tube formation Splits – splits or pockets form in the layer of cells forming vesicles, e.g. lungs and glands Condensation – previously dispersed tissue within the layer condenses into a tight cell mass, e.g. cartilage and bone Organogenesis By day (x) the following organs are developed Day 15 CNS and spinal cord Day 18 Heart Day 22 Eye and ear Day 28 Digestive system Day 35 Respiratory tract Day 35 Skeletal system Day 38 Muscular system  The placenta The placenta is a flat, circular structure, which connects the embryo/fetus to the uterine wall of the mother. When fully developed, the placenta weighs 300/400 g and measures 15-25 cm in diameter. By week 12 of pregnancy, it is formed and ready to take over nourishment for the foetus It develops from the yolk sac of the embryo (from the hypoblast) Until the placenta is formed, the yolk sac nourishes the developing embryo Maternal and embryonic tissue The placenta is made of both maternal and embryonic tissue The trophoblast cells continued their journey into the endometrium and formed chorionic villi The umbilical cord forms from the yolk sac and routes the embryos blood into the chorionic villi and in close proximity to the mother's blood Mothers blood and baby's blood do not mix –why? Functions of the placenta The umbilical cord contains umbilical blood vessels and embryonic connective tissue, and is surrounded by the amniotic membrane. Functions: - Gas exchange - Nutrient storage/exchange - Barrier - Anchorage - Secrets hormones (Human chorionic gonadotropin/Human placental lactogen /estrogen/progesterone) - Passive immunity – passing of mother's antibodies to baby Placenta previa Placenta previa is the presence of the placenta overlying, or close to, the internal cervical opening. It occurs in around 4 per 1,000 births and is caused by implantation in the lower part of the uterine cavity. It causes vaginal bleeding in the second half of pregnancy, which can be severe. It requires close monitoring and if it does not resolve, then the baby must be delivered early via C-section in order to avoid severe blood loss during labor. Foetal alcohol syndrome Fetal alcohol syndrome (FAS) is the most severe form of fetal alcohol spectrum disorders. It is caused by maternal ingestion of alcohol during pregnancy and is characterized by poor growth, abnormal brain growth, and specific abnormalities of facial features.  Umbilical cord structure Contains embryonic blood vessels and carries blood to and from the placenta One umbilical vein – carries oxygenated blood from the placenta to the embryo Two umbilical arteries – carry deoxygenated blood from the embryo to the placenta Early cardiovascular system One umbilical vein – carries oxygenated nutrient-rich blood from the placenta to the embryonic inferior vena cava to enter embryonic circulation Two umbilical arteries – carry deoxygenated, waste-rich blood from the embryonic aorta to the placenta  Quiz 4 1. Which layer of the gastrula forms the muscles of the body? Mesoderm 2. What is the role of hCG in development so far? Maintained the corpus luteum in the ovary to prolong secretion of progesterone to maintain the uterus lining and prevent next menstruation 3. Which cells of the blastocyst penetrate the endometrium and eventually form chorionic villi? Trophoblast 4. What are the main processes to form organs from the trilaminar disc? Folding, splitting, condensation 5. What nourishes the embryo before the placenta? Yolk sac 6. What are the functions of the placenta? Gas exchange, excretion, passive immunity, anchorage Embryology