L2 Emb Lecture_1st week of pregnancy FOM Damietta University fekry PDF

Faculty of Medicine Medical Education- Damietta University Level 1 Semester 1 Module 1A  Level 1 Semester 1 Module 1A General Embryology FIRST WEEK By Dr. Fekry Elsayed Shata 3 2024  Name of instructor Contact: FEKRY SHATA Email: dr_fekry...

Faculty of Medicine Medical Education- Damietta University Level 1 Semester 1 Module 1A  Level 1 Semester 1 Module 1A General Embryology FIRST WEEK By Dr. Fekry Elsayed Shata 3 2024  Name of instructor Contact: FEKRY SHATA Email: [email protected] Mobile: 01097921617 Academic hours: Contents Contents 1. Capacitation 2. Fertilization 3. Blastocyst formation  Learning outcomes By the end of this lecture, the students should be able to: 1. Identify the steps of capacitation & acrosomal reaction 2. Identify the steps of fertilization 3. Define the normal & abnormal fertilization 4. Know the steps of blastocyst formation  Case Scenario or Clinical correlate A woman has had several bouts of pelvic inflammatory disease and now wants to have children. However, she has been having difficulty becoming pregnant.  Embryology The study of the developmental events that occur during the prenatal period Begins with Fertilization: A single fertilized cell divides by mitosis to produce all of the cells in the body. 9  The Prenatal Period The first 38 weeks of human development between fertilization and birth. The pre-embryonic period (germinal): first 2 weeks of development zygote becomes a spherical, multicellular structure. The embryonic period: 3rd to 8th weeks all major organ systems appear. The Fetal Period: Includes the remaining weeks of development prior to birth The fetus continues to grow Its organs increase in complexity 10  The Stages of Embryogenesis Cleavage: zygote divides by mitosis forms a multicellular structure called a blastocyst. Gastrulation: blastocyst cells form three primary germ layers basic cellular structures from which all body tissues develop. Organogenesis: three primary germ layers arrange themselves in ways that give rise to all the organs within the body. 11 Events during the first week of pregnancy 1. Fertilization. 2. Cleavage. Fertilization 13 Definition: meeting and union between male gamete (sperm) and female gamete (secondary oocyte). Site: ampulla of the uterine tube (widest part of the tube). Process of Fertilization: After ovulation, the Secondary Oocyte is picked up by the fimbria of the fallopian tube and passes through the tube towards the uterus. From about 400 millions of deposited sperms, only about 400 reach the site of fertilization (an example of real natural selection to select the best sperm for future generation). Sperms can live only for 24 hours after being deposited. The sperm to be capable of fertilizing the oocyte undergo: 1. Capacitation. 2. Acrosome reaction. 14  Sperm Transport Semen is ejaculated and deposited into the vagina. Number of sperms from 200 – 600 million. One sperm is only needed for fertilization. The rest of the sperms will die inside the female genital tract.  Inside the vagina, most of the sperms die from high vaginal acidity  The mucus inside the cervix makes another barrier  death of some sperms  The sperms ascend inside the uterine cavity to the fallopian tube Others lose their way  Steps of Fertilization: 1. Egg coat peneteration 2. Egg activation 3. Cortical reaction 4. Zona pellucida reaction 5. Union of the pronuclei 6. Results of fertilization 7. Abnormal fertilization  Egg Coats Penetration Cumulus oophorus & Corona radiata: The sperm penetrates them through secretion of hyaluronidase enzyme Zona pellucida: The sperm secretes zona lysin that digest zona pellucida layer Egg cell membrane: The sperm binds to the plasma membrane of the 2nd oocyte. The male nucleus enters inside the oocyte cytoplasm leaving the cell membrane behind. Sperm Zona Pellucida 22  Capacitation Sperms cannot fertilize oocytes when they are newly ejaculated. Capacitation = the sperm becomes ready for fertilization. The process of capacitation takes 5-7 hrs. It takes place at the female genital tract (cervix). The acrosomal reaction cannot occur until capacitation is completed.  Capacitation 1.Removal of adherent proteins from the seminal fluid. 2.Reorganization of the plasma membrane over the head region. 3.The sperm becomes more active and motile.  Acrosomal Reaction The sperm secrets enzymes from its acrosome to penetrate egg coats For example: 1. Hyaluronidase  disperse corona radiate cells. 2. Trypsin like  digest ZP 3. Zona lysin  softens the ZP  Egg Activation Before fertilization, the 2nd oocyte was arrested in 2nd meiotic division. Upon binding with the sperm: The 2nd oocyte completes the 2nd meiotic division  ova and 2nd polar body Each contains 23 chromosomes and 1n DNA  Cortical Reaction It is liberation of cortical granules from the oocyte after sperm penetration. Cortical granules contain a mixture of enzymes. These enzymes change the structure of the ZP, inducing what is known as the zona reaction.  Zona Reaction 1.To block polyspermy. 2.The zona pellucida hardens. 3.Sperm receptors in the zona pellucida are destroyed.  Results of Fertilization 1. The 2ry oocyte completes the second meiotic division. 2. Restores the normal diploid number of chromosomes (46) in the zygote (the newly formed cell). 3. Determines the sex of the embryo. 4. Initiation of cell division of zygote. 5. Determines the state of health and disease of the embryo. 29 Abnormalities of Normal Fertilization: 1. Parthenogesis: The unfertilized oocyte starts cleavage without sperm penetration. 2. Dispermy and triploidy: Ovum fertilized by two sperms, resulting in a 30  Abnormal fertilization Superfecundation: Fecundity: the state of being fertile; heteropaternal-superfecundation-twins- capable of producing offspring from-different-fathers-dr-casey Superfecundation: fertilization of two or more ova released during the same menstrual cycle by sperms from separate acts of coitus  Abnormal fertilization Parthenogenesis (virgin birth) Growth and development of an embryo without fertilization by a male ! It occurs naturally in some plants, invertebrates and some fish The offspring are all-female  Abnormal fertilization Triploidy: Occurs when there is double fertilization of an ovum (dispermy). This results in zygot with 69 chromosomes (3n). The extra set of chromosomes results in the formation of different anomalies, syndactyly Segmentation or Cleavage 34 Definition: repeated mitotic division of the zygote, resulting in rapid increase in the number of cells. These cells (blastomeres) became smaller with each cleavage division Site: occurs as the zygote passes along the uterine tube toward the uterus. Changes: First the zygote divides into two blastomeres. During cleavage, the zygote is within the thick zona pellucida. When there are 12 to 32 blastomeres, is called a morula. It is formed about 3 days after fertilization. 35  Cleavage Cleavage occurs with no cytoplasm synthesis so the divided cells get smaller (2  4  8  16……). Each is called a blastomere. Zona pellucid is still intact (division occurs within it) TO keeps the cells together during cleavage and prevents adhesion of these cells to the uterine tubes.  About 4 days after fertilization, a fluid - filled cavity called the blastocele appears inside the morula. The blastocele separates the blastomeres into two groups: 1. Trophoblast: gives rise to the embryonic part of the placenta. 2. Inner cell mass: gives rise to the embryo. The morula is, now, called a blastocyst (blastulla). 37 Blastocyst Blastocyst Formation 1. Cleavage 2. Morula 3. Blastocyst 4. Hatching 5. Implantation Morula Formed at day 3 It is a solid ball of cells up to 16- 32 cells against zona pellucida. Swept to the uterus by the cilia of the tube. Small cavities appear between the cells  leading to the next stage of development.  Blastocyst Formed at day 4. Reaches uterine cavity. It has: 2 identifiable cell types (trophoblasts and embryoblasts) A fluid-filled cavity (blastocoel) Blastocyst Trophoblast (outer cell layer) Peripheral flattened cells Forms the placenta and placental membranes Blastocyst Inner cell mass (Embryoblast) Rounded cells Located on one wall of the blastocoel Forms the entire embryo Blastocyst Hatching Zona pellucida degenerates from sperm entry site, and the entire zona pellucida breaks down by the uterine and the blastula secretions. Now the blastocyst is ready for implantation by the end of week 1. Case Scenario or clinical correlate (to be discussed here) The repeated pelvic inflammation will cause fibrosis and the fimbriae of the uterine tube will stick together or block the tube. Picking up of the ova and sperm transport might be difficult and this results into no fertilization or ectopic pregnancy Summary Fertilization The usual site of fertilization is the ampulla of the uterine tube The fertilization process takes approximately 24 hours It is a sequence of coordinated events which include the following stages: Passage of a sperm through the corona radiate. For sperms to pass through the corona radiata, they must have been capacitated (removal of the glycoprotein coat and seminal plasma proteins from the plasma membrane that overlies the acrosomal region of the spermatozoa) 1st week of Human Development (Days 1–7) The following events take place during the 1st week of development I. Fertilization II. Cleavage III. Blastocyst Formation IV. Implantation Question 1 Which of the following processes is responsible for preventing a second sperm to enter the fertilized ovum? A. Capacitation. B. Segmentation. C. Acrosomal reaction D. Cortical reaction. E. Gastrulation. Answer is: D Question 2 Each individual cell in the morula is referred to as a which of the following? A. Blastomere B. Amnion C. Chorion D. Zygote E. Blastocyte Answer is: A Question 3 Regarding capacitation of the sperm, choose the correct statement? A. Removal of the head of the sperm. B. Is essential for fertilization. C. Prevents polyspermy. D. Occurs inside the male genital system. E. Is triggered by the zona pellucida. Answer is: B Discussion 10 minutes  Brain storming question (preferred as Case Scenario to be discussed next lecture or in the logbook) What is the fluid chamber in the blastocyst called? Where does fertilization normally take place? What are the cells in the early dividing embryo called? What is the outer layer of the oocyte called? Where does implantation normally take place? Feedback References or further readings Sadler,T.W.:Langman's,MedicalEmbryologyWillliamsandWilkinsCo.,Bal timore,12thedition.Chapter2,P.21–24&P.29-30. Sadler,T.W.:Langman's,MedicalEmbryologyWillliamsandWilkinsCo.,Bal timore,12thedition.Chapter2,P.24–27. http://www.med.unc.edu/embryo_images/ http://cna.uc.edu/embryology/ http://embryology.med.unsw.edu.au/embryology/index.php?title=M ain_Page