Introduction to Embryology Lecture Notes PDF

Introduction to Embryology With Dr. N. Jordan-Mahy 27-1 Introduction to Embryology Lecture Plan Fertilization Implantation Placental development Fetal development Gastrulation Neurulation Organogenesis...

Introduction to Embryology With Dr. N. Jordan-Mahy 27-1 Introduction to Embryology Lecture Plan Fertilization Implantation Placental development Fetal development Gastrulation Neurulation Organogenesis 27-2  Fertilization Is the fusion of haploid sperm nucleus & haploid secondary oocyte nucleus to form a single diploid nucleus = zygote Occurs in the fallopian tubes within 24 hrs after ovulation (oocyte dies at 24 hrs) Before fertilization the sperm & oocyte must move: The fimbriae of the fallopian tube brush over the ovary & cause the release of a secondary oocyte into the lumen of the fallopian tubes The oocyte movement by peristalsis & cilia transport towards the uterus Oocyte releases chemical attractants & sperm swim by means of flagella / tail towards oocyte Prostaglandins within the semen (sperm + seminal fluid) stimulate uterine contractions that help move sperm towards the oocyte 27-3  Fertilization take time! Capacitation: the final maturation of the sperm occurs within female The acrosomal membrane of the head of the sperm becomes fragile When zona pellucida protein 3 (ZP3) binds to sperm head, it triggers the Acrosomal Reaction & the removal of a glycoprotein coat & seminal protein from the acrosome region Enzyme are released from the acrosomal region of the sperm & they penetrates the corona radiata & digests its way through the zona pellucida 27-4  Stopping Polyspermy The first sperm fuses with oocyte membrane triggers Fast & then Slow Block to Polyspermy 1-3 seconds after a sperm hits the zona pellucida, the oocyte membrane depolarizes & no more sperms can bind to the oocyte = Fast Block to Polyspermy This depolarization triggers the intracellular release of Ca2+ causing the exocytosis of secretory vesicles (cortical granules), which block the sperm receptors, hardening the entire zona pellucida = Slow Block to Polyspermy 27-5  Making a zygote Sperm enters the oocyte & triggers oocyte to complete meiosis II & produce a second polar body Once inside the oocyte, the sperm loses its tail & becomes a male pronucleus Now there is the fusion of male & female haploid pronuclei, which is the true moment of fertilization Fertilized ovum (2n) is now called a zygote 27-6  Cleavage 30hrs post-fertilisation the zygote (sperm + ovum) undergoes mitotic division = Cleavage 2 identical daughter cells or Blastomeres, are formed, this is surround by a membrane = zona pellucida In the next 10-12 hrs these cells divide & ↑ in size. 2 cells become 4 cells, which ↑ in size, then divide into 8 cells By day 3 there are 16 cells & this is now called a Morula Once the blastomere has more than 16-cells it is called an Advanced Morula 27-7  Blastocyst Formation At day 5 & 6 uterine fluid passes into the centre of the advanced morula creating a blastocele The cell mass is now called a blastocyst The cells of the blastocyst are now no longer identical, in size or shape An outer case is formed around the blastocyst, this is called a Trophoblast The trophoblast differentiates into a structure called the chorion that later becomes a portion of the placenta While the inner cell mass will now from the embryo 27-8 From Fertilization to Implantation 27-9  Implantation At day 6 the blastocyst attaches to the endometrium Trophoblast develops into 2 distinct layers: a syncytiotrophoblast layer & cytotrophoblast layer 27-10  Implantation The Syncytiotrophoblast secretes enzymes that digest the endometrial cells Cytotrophoblast layer defines the shape of the embryo The two layers of the trophoblast secrete human chorionic gonadotropin (hCG) that helps to keep the corpus luteum & maintain the uterine lining & the pregnancy Human chorionic gonadotropin (hCG) in urine is measured in a pregnancy test 27-11  Implantation & Placentation The placenta is not fully formed until the 3rd month of pregnancy So the developing embryo needs to implant into the endometrium quickly so it can tap into maternal nutrients & blood supply Chorionic villi extend from the syncytiotrophoblast layer into endometrium & surround a maternal blood-filled lacuna Maternal & fetal blood vessels do not join & blood does not mix There is diffusion of O2, nutrients, & wastes This becomes more complex & will form the placenta chorionic villi 27-12 Placentation ~ Day 21 - 56 Day 21 Day 28 Day 35 Day 42 Day 49 Day 56  The umbilical cord links the fetus & mother It contents 2 umbilical arteries that carry blood from the fetus to the placenta 1 umbilical vein that carries oxygenated blood from the placenta to the fetus 27-14 Formation of the Amniotic Cavity & Yolk Sac - Day 8-12 ▪ The inner cell mass separates from the trophoblast creating a fluid filled amniotic cavity, leaving behind a layer of cells attached to the trophoblast = amnion ▪ On the distal side of the amniotic cavity are 2 layers of cells = Bilaminar Embryonic Disc ▪ The Superficial layer (or epiblast) of the embryonic disc is made of ectoderm which lines the amniotic cavity & Deep layer (or hypoblast) is made of endoderm which lines a second cavity within the blastocele, called the yolk sac ▪ A third germ layer mesoderm will now form between the endoderm & ectoderm ▪ This formation of these 3 germ layers is called Gastrulation Formation of a Yolk sac The yolk sac is formed within the blastocele, creating the extra embryonic coelom These cavities act as shock absorbers & protect the embryo The amniotic cavity eventually surrounds the embryo & the yolk sac supplies food during the 2nd & 3rd wks of gestation Eventually the extra embryonic coelom will becomes lined with mesoderm cells that form blood cells & blood vessels, that link the embryo to the placenta mesoderm cells Embryogenesis Begins - Day 12- 14 Embryonic disc is divided into ectoderm, mesoderm & endoderm The endoderm forms epithelial lining of the GI & respiratory tracts The mesoderm forms muscle, bone & connective tissues (including blood & bone) The ectoderm forms the skin epidermis & nervous system The body stalk (umbilical cord) is formed  Formation of the Primitive Streak - Day 12-14 At day 12 - cells in the middle of the ectoderm layer thicken to from the neural plate this pinches to form the primitive streak (or neural groove) at the caudal end of the embryological disc Cells move down & outwards between the ectoderm & endoderm & the primitive node is formed The migration of these ectoderm cells gives rise to the third germ layer mesoderm At day 13 to 14 there is an elongation & folding of the embryonic disc – ectoderm at the top, endoderm at the bottom & the mesoderm in the middle This process is called Gastrulation This now triggers Neurulation Neurulation: Formation of the Nervous System - Day 12-18 A rod like structure or Notochord extends along the primitive streak This stimulates a thickening of the cephalic end of the neural plate & causes its edges or neural folds, to rise upwards to form a neural tube This is formed along the length of the embryo, the cephalic end forms the brain & the spinal cord Neural crest cells from the neural folds migrate into the embryo = neuroectoderm This gives rise to the peripheral nervous system, adrenal medulla & skin melanocytes In the head region the neural crest cells contribute to the skull, teeth, blood vessels, muscles & connective tissue Blocks of mesoderm form either side of the neural tube = somites These give rise to the paraxial mesoderm At day 19 the mesoderm layer lines the amniotic cavity & yolk sacs - these are called the parietal (somatic) & visceral (splanchnic) layers respectively 27-19  Yolk stalk ▪ At day 18 – at the same time as neural tube develops from the ectoderm layer, the endoderm layer gives rise to a primitive gastrointestinal tract ▪ A tube is pinched off in the upper region of the yolk sac & passes along the length of the embryo from mouth to the anus ▪ The gut remains connected & open to the yolk sac = Yolk stalk ▪ A fore-, mid- & hind-gut is formed along the embryo ▪ The overlying ectoderm forms 2 membranes: the oropharyngeal membrane at the fore gut & the cloacal membrane at the hind gut ▪ These membranes form the mouth & urethra / anus respectively  l ▪ At day 28 - evaginations from the primitive gut give rise to the anterior pituitary, thyroid gland, lungs, liver, pancreas & bladder (allantois) ▪ At this time, bars of tissue form either side of the head & foregut = Branchial Arches, these form the pharynx & pharyngeal pouches (tonsils, auditory tubes, thymus & parathyroid gland) ▪ Next, the underlying mesoderm layer form cavities or coelom ▪ The most cranial cavities fuses & forms the pericardial cavity around the heart ▪ The low abdominal or colonic cavities extend to the tail end of the embryo & form the pleural & peritoneal cavities ▪ Block of mesoderm or somites or smaller somitomeres, either side of the neural tube give rise to the skull, vertebrate column & skeletal muscles. Gestation Period - 38 weeks Development Growth Brain Development - Day 33 Development of the brain (telepcephalon region)  Limb Development Day 26 - upper Day 28 – upper (ul) Day 41 onwards limbs (ul) appear & lower limbs (LL) hands have fingers ▪ Arms & legs appear as limb buds ▪ They are made of specialised thickenings along the apical ectodermal ridge ▪ The buds elongate in a proximal to distal sequence, arm pits first Facial Development - Day 52 The head takes on a more human shape Trimester 2 & 3 - Growth ▪ Most developmental changes occur in the first trimester (0-60days). ▪ The fetus will ↑from 2.5g at Day 60 to 3300g at term ▪ A 15-fold ↑ in length & 1300-fold ↑ in weight ▪ The fetus is covered in soft hair & a waxy coat of sloughed off epithelial cells, which are protective against waste products in the amniotic fluid ▪ In trimester 2 & 3 - growth is very fast, but growth is limited by placenta growth rate & space ▪ By week 38 survival is assured & the average birth weight is 3250g for females & 3300g for males Trimester 3 Trimester 2 Videos that might help: Fertilization: https://youtu.be/_5OvgQW6FG4?si=7aXSimQBboa74-T3 Blastocyst formation & Cleavage https://youtu.be/IaHnrpAo0x4?si=u0FqoypaLlX5-Zlb Implantation https://www.youtube.com/watch?v=PAVo16b_6j4 Gastrulation https://youtu.be/ADlYn0ImTNg Neurulation https://youtu.be/lGLexQR9xGs?si=9T188TJYO7mhQvyR Folding of the embryo & formation of the embryonic gastrointestinal tract: https://youtu.be/yXUv4MPuNTA?si=Var5xAqv-IE-PUMf 27-27 Let do a quiz Or https://echo360poll.eu/ Plus: Password a007 27-29