Embryo Development: From Fertilization to Gastrulation

1. Development of embryo & fetus PRESENTER - DR NAJAH MODERATOR – DR NIHAL

2. Embryology: Study of formation and development of an embryo from the moment of its inception up to the time when it is born as an infant • Embryo • Fetus

3. Gonads : Male – testis    Female – ovary • Gametes: Male – spermatozoa      Female – ova • Fertilization • Zygote

4. Spermatogensis: formation of spermatozoa in testis

6. Oogensis: formation of an ova in ovary

8. Stages of development • Fertilization • Cleavage • Morula • Blastocyst

9. Fertilization • Occurs in the ampulla of the fallopian tube

10. STAGES IN PENETRATION OF OVA INTO SPERM

11. As soon as spermatozoa enters ovum; 2nd meiotic division is completed & 2nd polar body is extruded • The definitive oocyte; arranges themselves in a vesicular nucleus known as the female pronucleus

12. The spermatozoa; moves forward & lie close to the female pronucleus. Its nucleus becomes swollen & becomes the male pronucleus • Both the pronucleus; loses their nuclear membrane & 23 chromosemes of each pronucleus get mixed up & form 23 pairs

13. Following this; these 46 chromosomes undergo mitotic division leading to the formation of a zygote having 2 cells

14. Cleavage • Subdivision of 2 cell stage of zygote into smaller cells • Blastomeres – cells which become smaller with each cleavage division

15. Morula • 3 days after fertilization; cells of the compacted embryo divide to form 16 cell stage – morula. It is still surrounded by zonapellucida

16. The cells of the outer layer of morula give rise to trophoblast • Inner cell mass – embryoblast ( embryo proper)

17. Blastocyst • Cells of trophoblast flatten & embroblast gets attached to trophoblast only on one side - blastocyst

18. Blastocele – cavity of blastocyst • The zonapellucida disappears & allows implantation to begin • 6th day – trophoblastic cells over the embryoblast pole penetrate between the epithelial cells of the uterine mucosa

19. Development Bilaminar germ disc 8th day • Cells of the embyoblastdiffrentiate into 2 layers • A outer layer of flattened cuboidal cells – hypoblast • Inner layer of high columnar cells – epiblast

20. At this time , a small cavity appear within the epiblast. This cavity enlarges & form amniotic cavity • In the area over the embryoblast ; trophoblastdiffrentiates into 2 layers • Inner layer of mononucleated cells – cytotrophoblast • Outer layer of multinucleated zone without distinct cell boundaries - syncytiotrophoblast

21. Day 9 • Blastocyst is more deeply embedded into endometrium • Flattened cells arising from hypoblast spread & line the inside of blastocystic cavity( Heusers membrane) • A cavity is formed lined on all sides by cells – primary yolk sac

22. Day 11 & 12 • Cells of trophoblast give rise to a mass of cells – extraembryonic mesoderm • Small cavity appear in the extraembryonic mesoderm. These join together & a large cavity is formed – extraembryonic coelom

23. This space surrounds the primitive yolk sac & amniotic cavity expect where the germ disc is connected to the trophoblast by connecting stalk

24. Day 13 • Hypoblast produce additional cells that migrate along the inside of the exoceolomic membrane • These cells proliferate & gradually form a new cavity within the exocoelomic cavity – secondary yolksac

25. During its formation large portion of exocoelomic cavity is pinched off – exocoelomic cyst • Extraembryonic coelom expands & form a large cavity – chorionic cavity • Extraembryonic mesoderm lining inside of cytotrophoblast is called chorionic plate

26. Connecting stalk – only place where extraembryonic mesoderm traverses the chorionic cavity • With development of blood vessels , stalk becomes the umbilical cord

27. Development Trilaminar germ disc GASTRULATION • Begins with formation of primitive streaks on the surface of epiblast • Initially it’s a rounded or oval swelling but with elongation of embryonic disc; it becomes a linear structure lying in the central axis of the disc

28. The cells that proliferate in the region of primitive streak passes sideways pushing themselves between epiblast & hypoblast • These cells form intraembryonic mesoderm • Some arise from the primitive streak displace the hypoblast & form endoderm • Remaining cells of epiblast form ectoderm

29. At one circular area near margin of the disc; the cubical cells of hypoblast becomes columnar – prochordal plate • Its appearance determines the central axis of embryo & enables to distinguish its future head & tail end • The intraembryonic mesoderm spreads throughout the disc except in the region of the prochordal plate

30. Ectoderm • Epidermal Ectoderm • Epidermis - skin, hair follicles & hair, nails • Terminal Gastro Intestinal system • Neural Plate Ectoderm • Central Nervous System - brain & spinal cord • Neural Crest Ectoderm • Peripheral nervous system • Ganglia of nervous system (cranial, spinal, autonomic) • Dentine of teeth • Head skeleton • Pigment cells • Covering of the brain (meninges)

31. Mesoderm • Three divisions: • Epimere • Mesomere • Hypomere Epimere • Epimere forms Somites(balls of tissue): • Dermatome - Dermis of skin • Myotome - Axial, limb, and body wall musculature • Sclerotome - Vertebral column & ribs

32. Mesomere • Also called Intermediate Mesoderm • Forms urogenital system: • Kidneys and urogenital ducts Hypomere • Somatic (body) - Somatic Lateral Plate Mesoderm • Limb skeletons • Splanchnic (gut) - Splanchnic Lateral Plate Mesoderm • Heart, blood vessels • Smooth muscle of the digestive system

33. Endoderm • Divisions of the digestive tube • Mouth & pharynx • Abdominal Foregut: Stomach, liver, pancreas, beginning of small intestine. • Abdominal Midgut: most of small intestine, beginning of large intestine. • Abdominal Hindgut: Terminal intestines, urinary bladder

34. Notochord • Cranial end of primitive streak thickens – primitive knot / hensons node • Depression in centre of primitive knot – blastosphere • Cells in primitive knot passes cranially in the midline between ectoderm & endoderm reaching upto caudal margin of prochordal plate - notochord process • Cells during this stage undergo several rearrangement & forms a solid rod - notochord

36. With the formation of head & tail folds parts of yolk sac becomes enclosed within the embryo; a tube lined by endoderm is formed in the embryo – primitive gut • Initially the gut is in wide communication with the yolk sac • Part of gut cranial to this communication – foregut • Caudal – hindgut • Intervening part - midgut

37. The communication with the yolk sac becomes narrower • As a result; the yolk sac becomes small & is termed definitive yolk sac • The narrow channel connecting it to the gut is called vitellointestinal duct. This duct elongates & gradually disappears • Following formation of all four folds; embryo comes to be enclosed all around by ectoderm except in the region through which vitellointestinal duct passes – umbilical opening

38. Connecting stalk • As embryo grows area of attachment of connecting stalk becomes smaller • Gradually this attachment is seen only near the caudal end of embryonic disc • With formation of tail fold the attachment moves to the ventral aspect of the embryo; it is now attached in the region of umbilical opening

39. By now; blood vessels develop in the embryo & placenta; these arteries are in communication by means of arteries & veins passing through the connecting stalk • Amnion has a circular attachement to the margin of umbilical opening & form a wide tube

40. CONTENTS • Vitellointestinal duct & its remnants of yolk sac • Whartons jelly • Blood vessels that pass from placenta to fetus • A small part of extraembryonic coelom

41. TERATOGENECITY • Teratogens causes permanent damage in structure & function of an organ ; acting during embryonic or fetal life • Timing of teratogen exposure & hazard • Before D31 – teratogens produce all or none effect. Conceptus either survives without anomalies or does not survive. Any damage in this phase is lethal or irrepairable

42. DAY 31 -71 – critical period for organogenesis. Effect depends on 1) amount of drug reaching the fetus 2) gestational age at the time of exposure 3)duration of exposure AFTER DAY 71 – developement of organ continues. Diethyl stilboesterol related uterine anomalies occur with exposure around 20 weeks

43. Physical events in the development of embryo 5thweek • Day 21- heart has began beating • Complete closure of neural tube  • Placenta begins to function • Spine and spinal cord grows faster then rest of the body and appearance of tail • Crown rump length: 0.05inches

44. 6thweek • Limb buds appear • formation of face

45. 7 weeks – • Facial features are visible, including a mouth and tongue. • The eyes have a retina and lens. • The major muscle system is developed • The child has its own blood type, distinct from the mother's. • These blood cells are produced by the liver now instead of the yolk sac.

46. 10 weeks • The heart is almost completely developed and very much resembles that of a newborn baby. • An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child's blood is oxygenated through the placenta. • Twenty tiny baby teeth are forming in the gums; some babies are even born with teeth emerging from the gums.

47. 12 weeks • The baby at 12 weeks notice the webbing on the fingers, with the digits still fused • external genetilia develops • Vocal cords are complete • The brain is fully formed • The eyelids now cover the eyes, and will remain shut until the seventh month to protect the delicate optical nerve fibers.

48. 14 weeks – • Muscles lengthen and becomes organized.

49. 16 weeks • Face is fully developed  • A downy hair covers the skin. • Face is fully formed. • Eyes are fully formed but not yet functional • Eyebrows, eyelashes and fine hair appear. • The child can grasp with his hands, suck her thumb, kicks

50. 20 weeks • Though still small and fragile, the baby is growing rapidly and could possibly survive if born at this stage. • Fingernails and fingerprints appear. • Sex organs are clearly visible.