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It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life. Lewis Wolpert, 1986 Early Human Development This section will cover a series of lectures that will begin, in a serious way, our exploration of human development. We will consider the developmental stages critical to the first month of development as we follow the zygote along its developmental path to its arrival at the basic vertebrate body plan. After this lecture you should be able to: • Outline and describe the stages associated with the first week of development, the preimplantation period. • Outline and describe the stages associated with the second week of development, the implantation period. • Outline the important third week and describe the the primitive streak, the development of the notochord, and the differentiation of the three germ layers. • Describe the early differentiation of the germ layers and the folding of the embryonic trilaminar plate. 21 First Week of Development The result of cleavage Zygote Cleavage What is it? When does it begin? Results – Blastomeres Relation to zona pellucida Timeline Developmental stages 2-cell stage 30 hours 4-cell stage 40 hours 6 to 12-cell stage Morula - at 12-cell stage 3 days 12 to 32-cell stage Compaction - approximately around 19-cell stage 4 days E m b r y o l o g y 22 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y Blastocyst H u m a n D e v e l o p m e n t Timeline Formation of blastocyst cavity 4 to 6 days Shedding of the zona pellucida Outer cell mass – trophoblast Inner cell mass – embryoblast Uterine contact 6 days Embryonic pole . Actual size Implantation Differentiation of the trophoblast Cytotrophoblast Syncytiotrophoblast Formation of the hypoblast (primary endoderm) 7 days 23 Second Week of Development Timeline Emergence of the bilaminar embryo During this week the embryo forms two layers of important cells and two sacs associated with each layer of cells. It also implants into the uterine wall and initiates the development of the placenta. The focus of this section will be on the embryo. We will come back to the extraembryonic membranes and implantation in a later lecture. Overview of trophoblastic tissues 8 days Cytotrophoblast Syncytiotrophoblast Bilaminar embryo stages and features Epiblast Hypoblast 8.5 days Exocoelomic membrane Exocoelomic cavity Amnion Primary yolk sac 9 days Extraembryonic mesoderm E m b r y o l o g y 24 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Emergence of the bilaminar embryo Closing plug Timeline Epiblast Embryo proper Amnionic sac 10 days Actual size Amnionic cavity Coelomic spaces appear 12 days Actual size 25 Hypoblast Timeline Primary yolk sac Extraembryonic mesoderm 13 days Extraembryonic coelomic space Extraembryonic coelom Connecting stalk Secondary yolk sac Prechordal plate 14 days Actual size E m b r y o l o g y 26 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Bilaminar embryo View at the end of two weeks Superior view Cross section Longitudinal section 27 Third Week of Development Emergence of the three embryonic germ layers Gastrulation What is it? Timeline Stages of gastrulation Prechordal plate 15 days Primitive streak Primitive node Primitive pit Primitive groove 17 days E m b r y o l o g y 28 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y Results of gastrulation Formation of the mesoblast H u m a n D e v e l o p m e n t Timeline 15 days Replacement of the hypoblast 17 days Trilaminar disc 18 days Fate of primitive streak 29 Formation of the notochord Notochordal process Notochordal canal Prechordal plate – oropharyngeal membrane Timeline 16 days 17 days 18 days E m b r y o l o g y 30 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Formation of the notochord (cont.) Fusion with underlying endoderm Degeneration and formation of notochordal plate Timeline Neurenteric canal Reclosure of notochord 18 days 18 days 19 days 31 Additional spread of the mesoblast Lateral migrations Cranial migration Cardiogenic mesoblast Prechordal mesenchyme Caudal migration Blocks to migration Oropharyngeal membrane Cloacal membrane E m b r y o l o g y 32 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Fate map of epiblastic plate Notochord Medial halves of somites Lateral halves of somites Lateral mesoderm Extraembryonic mesoderm Neural ectoderm Body ectoderm Primordial germ cells Endoderm 33 Mesoderm – Differentiation of the Mesoblast Timeline Axial mesoderm Notochord 21 days Prechordal mesoderm Cardiogenic mesoderm and septum transversum 18 days Caudal eminence Paraxial mesoderm Formation of somitomeres Formation of somites 20 days Occipital somites Craniocaudal progression 21 days E m b r y o l o g y 34 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Paraxial mesoderm (cont.) Unsegmented paraxial head mesoderm Intermediate mesoderm Lateral mesoderm Head Postcranial 35 Ectoderm Timeline Neurulation Neural plate 17 days Neural groove 18 days Neural folds 19 days 18 days 20 days 21 days E m b r y o l o g y 36 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Neurulation (cont.) Neural tube Neural crest Placodes Placodal ring Body ectoderm Endoerm Allantois 37 Development of intraembryonic coelom Timeline Coelomic spaces 18 days Intraembryonic coelom Somatic mesoderm 20 days Somatopleure Splanchnic mesoderm Splanchnopleure 21 days Communication with extraembryonic coelom E m b r y o l o g y 38 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Fourth Week of Development Achieving the basic vertebrate body plan Timeline Folding the trilaminar embryonic plate Cranial–caudal folding Head fold 22 days 23 days Tail fold 26 days 28 days Actual size 39 Side-to-side folding Lateral folds Midgut region E m b r y o l o g y 40 L e c t u r e M a n u a l b y M a r k N i e l s e n E a r l y H u m a n D e v e l o p m e n t Basic vertebrate body plan 2 1 Actual size 3 Cross-section 1 4 5 6 One month embryo, lines depict levels of cross sections below, gray depicts ectodermal ring Cross-section 2 Cross-section 3 Cross-section 5 Cross-section 4 Cross-section 6 41 Germ Layer Derivatives Appendicular skeleton Dermis, subcutaneous tissue, tendons, ligaments, fascia, general connective tissue Parietal membranes Dentine of teeth Connective tissue of head (including bones, fascia, tendons, ligaments, dermis, etc.) Cranial somatic sensory neurons All postcranial sensory neurons All postganglionic neurons Enteric neurons Adrenal medulla Melanocytes Blood vessels of head Adrenal cortex Visceral membranes and mesenteries Epicardium Smooth muscle, connective tissue, and blood vessels of the gut tube Somatic mesoderm Splanchnic mesoderm Trapezius Brain and spinal Lateral mesoderm Sternocleidomastoid cord (all motor and interneurons), Seminiferous tubules Retina and optic Epididymis Basioccipital bone nerve Vertebral column and Ductus deferens Uterine tubes ribs Neural tube Neural crest Uterus Body wall muscles Upper vagina Limb muscles Lens Urinary tubes Dermis of back Inner ear Hairs Branchial (pharyngeal) Ureters Olfactory nerves Nails muscles Cranial visceral sensory Sweat glands neurons Intermediate Sebaceous glands mesoderm Extraocular Mammary glands Paraxial Placodes muscles Glands of eyes mesoderm Ceruminous glands Prechordal mesoderm Nasal cavity lining Cornea Oral cavity lining Conjunctiva Parotid gland Endocardium Tooth enamel Myocardium Epidermis Anterior pituitary Ectoderm Anal canal Cardiogenic mesoderm Mesoderm Fascia, tendons, ligaments, adipose tissue, general connective tissue Body ectoderm Endoderm Mesenchyme Yolk sac Epithelial lining of tympanic cavity and pharyngotympanic tube. Cells of thyroid gland, parathyroid glands, tonsillar recess, thymus, ultimobranchial bodies. Lining of submandibular and sublingual glands. Epithelial lining of respiratory system from larynx to alveoli E m b r y o l o g y 42 L e c t u r e Gametes (spermatocytes and oocytes) of testes and ovaries Epithelial lining of digestive tract from pharynx to rectum Epithelial lining of biliary tubes of the liver and the pancreatic ducts and tubules M a n u a l b y M a r k N i e l s e n Epithelial lining of urinary bladder, urethra, and lower part of vagina