BASIC OF EMBRYOLOGY by Dr. Roshan Kr. Yadav

About basic of human embryology.

1. EMBRYOLOGY GUIDED BY: PROF. DR. HEMANT KUMAR HALWAI ASSO. PROF . DR.SANDEEP KUMAR GUPTA ASST. PROF. DR.RAJU SHRESTHA ASST. PROF. DR.RANJANA SHAH PRESENTED BY: DR.ROSHAN KUMAR YADAV 1 ST YEAR JUNIOR RESIDENT DEPARTMENT OF ORTHODONTICS AND DENTOFACIAL ORTHOPAEDICS

2. Cont ents Introduction Pre-implantation period Embryonic period Fetal period A nomalies References 2 UCMS CODS 55

3. Introduction Embryology is the study of the formation and development the embryo (or fetus) from the moment of its inception up to its birth . Father of embryology – Karl Earnst Von Baer UCMS CODS 55 3

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5. Stages of development 1. Fertilization 2. Cleavage 3. Gastrulation 4. Organogenesis 5. Maturation 5 UCMS CODS 55

6. Pre-implantation period UCMS CODS 55 6 Fertilization to 1week Cleavage of ovum, attachment of ovum to intrauterine wall

7. Day 1:Fertilization Fusion of male and female gametes , occurs in the ampullary region of the uterine tube. The phases of fertilization include : Phase1 - penetration of the corona radiata , Phase 2 - penetration of the zona pellucida , Phase 3 - fusion of the oocyte and sperm cell membranes. Formation of zygote. 7 UCMS CODS 55

8. Day 2:Two-cell stage The cleavage division last for 6days i.e. up to 7day after fertilization. After 30 hours of fertilization, cell divides into two. Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, increasing the numbers of cells. These cells, which become smaller with each cleavage division, are known as blastomeres . 8 UCMS CODS 55

9. Day 3: Morula After 15 hours the two cell becomes four cell. And at the end of 3 days, fertilized egg cell becomes a mulberry like structure made up of 16 cells known as MORULA. The inner cell mass gives rise to tissues of the embryo proper, and the outer cell mass forms the trophoblast . 9 UCMS CODS 55

10. Day 4: Blastocyst Differentiation creates an approximately 100-cell fluid filled blastocyst. Some fluid passes into morula from uterine cavity. This seperates the inner mass from trophoblast. Cavity formed is called blastocoele . O uter cell mass-trophoblast , flatten and form the epithelial wall of the blastocyst . UCMS CODS 55 10

11. Blastocyst implants into the uterine endometrium on the 7 th day of postconception UCMS CODS 55 11

12. Day 7 - Implantation Zona pellucida degenerates and decomposes before implantation. Trophoblasts cells have the capacity to invade and burrow into tissue. The trophoblast converts into chorion – the outermost fetal membrane around the embryo . Choroinic implantation establishes the placenta , the organ of fetomaternal exchange of nutrition and waste desposal . 12 UCMS CODS 55

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14. Embryonic period 8 th day to 8 weeks IUL Divided into three periods: 1.Presomite - 8 to 21 days 2.Somite - 21 to 31 days 3.Postsomite – 32 to 56 days 14 UCMS CODS 55

15. Pre-somite period 8 to 21 days Formation of fetal membrane , amnion , chorion and germ layers. 15 UCMS CODS 55

16. Day 8- Bilaminar disc At the eighth day of development, the blastocyst is partially embedded in the endometrial stroma. In the area over the embryoblast , t rophoblast has differentiated into two layers: (1) an inner layer of mononucleated cells, the cytotrophoblast , and (2) an outer multinucleated zone without distinct cell boundaries, syncytiotrophoblast 16 UCMS CODS 55

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18. Cells of the inner cell mass or embryoblast also differentiate into two layers: (a) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer, and (b) a layer of high columnar cells adjacent to the amniotic cavity, the epiblast layer At the same time, a small cavity appears within the epiblast. This cavity enlarges to become the amniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts 18 UCMS CODS 55

19. Day 9 – Trophoblast with lacunae F lattened cells originating from the hypoblast form a thin membrane, the exocoelomic ( Heuser’s ) membrane , that lines the inner surface of the cytotrophoblast . This membrane , together with the hypoblast, forms the lining of the exocoelomic cavity, or primitive yolk sac. 19 UCMS CODS 55

20. Day 12 Completely embedded in the endometrial stroma. Establishment of uteroplacental circulation . Formation of extraembryonic coelom, or chorionic cavity 20 UCMS CODS 55

21. UCMS CODS 55 21 The only place where extraembryonic mesoderm traverses the chorionic cavity is in the connecting stalk. With development of blood vessels, the stalk becomes the umbilical cord

22. Day 13 Formation of new cavity within the exocoelomic cavity, known as the secondary yolk sac or definitive yolk sac . The extraembryonic coelom expands and forms a large chorionic cavity. The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the chorionic plate. 22 UCMS CODS 55

23. Trilaminar germ disc formation – Third week of development Most characteristic event occurring during the third week is gastrulation , the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. B egins with formation of the primitive streak on the surface of the epiblast Vaguely defined, later on day 15-16, fine narrow groove with slightly bulging regions on either side 23 UCMS CODS 55

24. Day 15 – Primitive streak The cephalic end of the streak, the primitive node , consists of a slightly elevated area surrounding the small primitive pit. Epibast cells migrate towards primitive streak and inwards to result in invagination. This results in accumulation of cells between the epiblast(later ectoderm) and displaced hypoblast (endoderm) forming the mesoderm. UCMS CODS 55 24

25. Day -16 Gastrulation UCMS CODS 55 25

26. Fate of germ layers 26 UCMS CODS 55

27. Formation of notochord Formation of the notochordal plate Cells of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord , which is a signaling center for inducing the axial skeleton. UCMS CODS 55 27

28. Day 17-Neurulation Neurulation is the process whereby the neural plate forms the neural tube 28 UCMS CODS 55

29. Day 21- Neural crest and Neural fold 29 UCMS CODS 55

30. Day 22- Neural tube and pericardial bulge 30 UCMS CODS 55

31. Neural crest cell formation and their migration 31 UCMS CODS 55

32. Neural crest cells derivatives Connective tissue and bones of the face and skull Cranial nerve ganglia C cells of the thyroid gland Conotruncal septum in the heart Odontoblasts Dermis in face and neck Spinal [dorsal root] ganglia Sympathetic chain and preaortic ganglia Adrenal medulla Glial cells Arachnoid and piamater Melanocytes 32 UCMS CODS 55

33. Homeobox gene UCMS CODS 55 33 Under the control of regulatory homeobox genes (Hoxa-1, Hoxa-2, Hoxb-1, Hoxb-3, Hoxb-4 , sonic hedgehog [SHH], Krox-20, patched [ Ptc ], paired box, [Pax1.9]), the segmented tissues are integrated into morphologically identifiable structures.

34. Somite period 21 st to 31 st day Period of organogenesis Developmental anomalies occur in this period 34 UCMS CODS 55

35. Pharyngeal arches and pouches Forms the typical feature in development of the head and neck Appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo Pharyngeal arches- consists core of mesenchymal tissue covered outside by surface ectoderm and inside by endoderm 35 UCMS CODS 55

36. P haryngeal arch Each arch contains a cartilaginous component, a cranial nerve, an artery, and a muscular component 36 UCMS CODS 55

37. PHARYNGEAL ARCH SKELETON ARTERY MUSCLES NERVE 1 ST MANDIBULAR MECKEL’S CARTILAGE EXTERNAL CAROTID ARTERY MAXILLARY ARTERY MASSETER,TEMPORALIS,MEDIAL AND LATERAL PTERYGOID TENSOR TYMPANI,TENSOR PALATINI,ANTERIOR DIAGASTRIC MUSCLE MANDIBULAR DIVISION OF TRIGEMINAL NERVE,LINGUAL NERVE 2 nd HYOID STAPES,STYLOID PROCESS,SUPERIOR HYOID BODY STAPEDIAL ARTERY FACIAL,STAPEDIUS,HYOID,POSTERIOR DIGASTRIC FACIAL NERVE,CHORDA TYMPANI UCMS CODS 55 37

38. UCMS CODS 55 38 PHARYNGEAL ARCH SKELETON ARTERY MUSCLE NERVE (MOTOR AND SENSORY) 3 rd INFERIOR HYOID BONE, GREATER CORNU HYOID INTERNAL CAROTID ARTERY STYLOPHARYNGEUS MUSCLES GLOSSOPHARYNGEAL 4 th THYROID AND LARYNGEAL CARTILAGES AORTA (L) SUBCLAVIAN (R) PHARYNGEAL CONSTRICTORS, LEVATOR PALATINI, PALATOGLOSSUS, PALATOPHARYNGEUS SUPERIOR LARYNGEAL NERVE VAGUS AURICULAR NERVE TO EXTERNAL ACOUSTIC MEATUS 6 th CRICOID, ARYTENOID CORNICULATE CARTILAGES PULMONARY ARTERIES, DUCTUS ARTERIOSUS CRICOTHYROID, LARYNGEAL MUSCLES PHARYNGEAL CONSTRICTORS INFERIOR LARYNGEAL NERVE VAGUS

39. Pharyngeal pouches Human embryo has five pair of pharyngeal pouches The pharyngeal pouches, appear along the lateral walls of the pharyngeal gut Give rise to important organs 39 UCMS CODS 55

40. Development of the pharyngeal clefts and pouches. The second arch grows over the third and fourth arches, burying the second, third, and fourth pharyneal clefts 40 UCMS CODS 55

41. Post somite period 32 nd to 56 th days External body feature and further development 41 UCMS CODS 55

42. Fetal period 8 th week IUL till birth Accelerated growth of craniofacial structures occurs Change in the proportion between various structures occurs 42 UCMS CODS 55

43. Anomalies of development UCMS CODS 55 43 STAGE TIME RELATED SYNDROMES Germ layer formation and initial organization of structures Day 17 Fetal alcohol syndrome (FAS) Neural tube formation Days 18-23 Anencephaly Origin , migration and interaction of cell populations Days 19-28 Hemifacial microsomia Mandibulofacial dysostosis ( Treacher Collins syndrome)

44. Fetal alcohol syndrome UCMS CODS 55 44 Profit W.R.contemporary orthodontics.,6 th ed

45. Treacher Collins syndrome A utosomal dominant disorder M utations in the gene TCOF1 A bnormal development of the first and second branchial arch A bnormal neural crest cell migration - improper cellular differentiation during development and abnormal extracellular matrix Clinical features : -Cleft palate -Hypoplasia of mandible and zygomatic complex - S loping of palpebral fissures - Colobomas of lower eyelids UCMS CODS 55 45 Profit W.R.contemporary orthodontics.,6 th ed

46. Pierre Robin Syndrome Etiology : Deletions; duplications; translocations; and mutations in chromosomes 1 to 6, 10 to 13, and 16 to 18 ,mutation in (GAD67, PVRL1, and SOX9) Triad of – micrognathia - glossoptosis -cleft palate Also associated with abnormality of pharynx and respiratory distress UCMS CODS 55 46 Randall P, Krogman WM, Jahina S. Pierre Robin and the syndrome that bears his name. The Cleft Palate Journal. 1965 Jun 30;2(3):237-46 .

47. Cri du chat syndrome Etiology : Deletion of the telomerase reverse transcriptase ( hTERT ) gene, localised to 5p15.33 , Clinical features: - Low birth weight - Hypertelorism -Round face -Large nasal bridge - Micrognathia -Characteristic cat like cry UCMS CODS 55 47 P Cerruti Mainardi - Orphanet journal of rare diseases, 2006

48. References Sadler T.W. Langman’s medical embryology. 12th ed. Sperber G.H.craniofacial embryogenetics and development., 2001 Profit W.R.contemporary orthodontics.,6 th ed Human embryology , Inderbir Singh , Randall P, Krogman WM, Jahina S. Pierre Robin and the syndrome that bears his name. The Cleft Palate Journal. 1965 Jun 30;2(3):237-46 . P Cerruti Mainardi - Orphanet journal of rare diseases, 2006 48 UCMS CODS 55