1. Embryology From Fertilization to Gastrulation

2. Fertilization

3. Fertilization Steps of fertilization (Conception).
Several sperm penetrate corona radiata.
Several sperm attempt to penetrate zona pellucida.
One sperm enters egg and nuclei fuse, producing a zygote.
Egg’s plasma membrane and zona pellucida change to prevent polyspermy.

4. Fertilization

5. First Days of Development Day 0-5
Compaction
starts

6. First Days of Development Day 6

7. Occurrence of Pregnancy
When a zygote begins dividing, it is termed an embryo (pre-embryo).
Developing embryo travels down oviduct and eventually implants in endometrium. (Implantation / pregnancy)
Presence of human gonadotropic hormone (HCG) in the blood confirms pregnancy.
If implantation does not occur, a woman never knows fertilization took place.

8. Human Development before Implantation

9. Pre-Embryonic and Embryonic Development
Processes of development.
Cleavage - Cell division without growth.
Growth - Increase in size of cells.
Morphogenesis - Shaping of embryo.
Differentiation - Cells take on specific structure and function.

10. Extraembryonic Membranes
Membranes that extend out beyond the embryo.
Amnion - Provides fluid environment for developing embryo and fetus.
Yolk sac - First site of red blood cell formation.
Allantois - Contributes to cardiovascular system.
Chorion – outermost membrane, develops from the trophoblast, contributes to the placenta.

11. Extraembryonic Membranes

12. Pre-Embryonic and Embryonic Development
Stages of development.
Morula - Solid mass of cells resulting from cleavage.
Blastocyst (Blastula) - Ball of cells formed from morula.
Embryonic disk - Inner mass of cells of blastocyst.
Gastrula - Embryo composed of three tissues.
Ectoderm, mesoderm, endoderm.
These are the foundation of the body systems (Germ Layer Theory).

13. Early Developmental Stages

14. Pre-Embryonic and Embryonic Development
Stages of development.
Neurula - Nervous system develops from ectoderm located just above the notochord.
Involves induction as one tissue influences the development of another tissue.

15. Review Question. Which of the following is the
origin of the mitochondrial DNA
of all human Adult Cells?
A) Paternal only
B) Maternal only
C) A combination of paternal and maternal
D) Either paternal or maternal
E) Unknown origin

16. Review Question. Which of the following is the
origin of the mitochondrial DNA
of all human Adult Cells?
A) Paternal only
B) Maternal only
C) A combination of paternal and maternal
D) Either paternal or maternal
E) Unknown origin

17. First Days of Development Day 7

18. Week Two Days 8

19. Day 9

20. Day 10

21. Day 11

22. Day 12

23. Days 12-13
Chorion membrane
consists of?

24. Days 13

25. Day 14-15 Primary ectoderm: primordial germ cells,
endothelial cells, and
hematopoietic stem cells.
Extraembryonic mesoderm
of the yolk sac wall is a
major site of hematopoiesis.
Chorion membrane
consists of?

26. Origin of the germ line A. Migration of the primordial germ cells. B
Origin of the germ line A. Migration of the primordial germ cells. B. Migration into posterior body wall.

27. Clinical Application Human chorionic gonadotropin (hCG)
hCG is a 57,000 MW glycoprotein with two subunits
(alpha and beta) produced by the syncytiotrophoblast.
Enters the maternal blood circulation.
Prevents degeneration of the corpus luteum.
Stimulates production of progesterone in the corpus luteum
and chorion, which sustains the placenta.
Can be assayed in maternal blood at day 8 after fertilization
And in maternal urine at day 10. This is the basis of early diagnosis of pregnancy.

28. Clinical Application Complete hydatidiform mole
Persistent trophoblastic disease
Metastatic choriocarcinoma

29. IV. Clinical Applications. F. Genomic Imprinting.
1. Cytogenetic analysis of hydatidiform moles suggests
Paternal genetic complement->placental development.
Maternal genetic complement->embryo development.
2. Methylation of DNA is a mechanism that leads to independent
expression of maternal and paternal genomes during
early development. Female germ line highly methylated.
3. Example of pattern of inheritance:
Father->Prader-Willi Syndrome
Mother->Angelman Syndrome
4. Severity and age of onset of several genetic diseases also differ
on inheritance pattern.

30. Week 2 – The Two-Layered Embryo
Bilaminar embryonic disc – inner cell mass divided into two sheets
Epiblast and the hypoblast
Together they make up the bilaminar embryonic disc
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31. Week 2 – The Two-Layered Embryo
Amniotic sac – formed by an extension of epiblast
Outer membrane forms the amnion
Inner membrane forms the amniotic sac cavity
Filled with amniotic fluid
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32. Week 2 – The Two-Layered Embryo
Yolk sac – formed by an extension of hypoblast
Digestive tube forms from yolk sac
NOT a major source of nutrients for embryo
Tissues around yolk sac
Gives rise to earliest blood cells and blood vessels
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33. Figure 3.4 (1 of 3)
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34. Figure 3.4 (2 of 3)
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35. Figure 3.4 (3 of 3)
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36. Week 3 – The Three-Layered Embryo
Primitive streak – raised groove on the dorsal surface of the epiblast
Gastrulation – a process of invagination of epiblast cells
Begins at the primitive streak
Forms the three primary germ layers
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37. Week 3 – The Three-Layered Embryo
Three Germ Layers*
Endoderm – formed from migrating cells that replace the hypoblast
Mesoderm – formed between epiblast and endoderm
Ectoderm – formed from epiblast cells that stay on dorsal surface
*All layers derive from epiblast cells!
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38. The Primitive Streak Figure 3.5e–h
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39. The Notochord
Primitive node – a swelling at one end of primitive streak
Notochord forms from primitive node and endoderm
Notochord – defines body axis
Is the site of the future vertebral column
Appears on day 16
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40. Formation of the Mesoderm and Notochord
Figure 3.6
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41. Neurulation
Neurulation – ectoderm starts forming brain and spinal cord
Neural plate – ectoderm in the dorsal midline thickens
Neural groove – ectoderm folds inward
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42. Neurulation Neurulation (continued)
Neural tube – a hollow tube pinches off into the body
Cranial part of the neural tube becomes the brain
Maternal folic acid deficiency causes neural tube defects
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43. Neurulation Neural crest Induction
Cells originate from ectodermal cells
Forms sensory nerve cells
Induction
Ability of one group of cells to influence developmental direction of other cells
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44. The Mesoderm Begins to Differentiate
Somites – our first body segments
Paraxial mesoderm
Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm
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45. The Mesoderm Begins to Differentiate
Lateral plate – most lateral part of the mesoderm
Coelom – becomes serous body cavities
Somatic mesoderm – apposed to the ectoderm
Splanchnic mesoderm – apposed to the endoderm
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46. Third week of development
Begins 6 week period of rapid development and differentiation
Gastrulation
1st major event of 3rd week – about 15 days
Bilaminar embryonic disc transforms into trilaminar embryonic disc
Ectoderm (skin and nervous system), mesoderm (muscle, bones, connective tissues, peritoneum), and endoderm (epithelial lining of GI tract, respiratory tract, and several other organs)
Involves rearrangement and migration of epiblast cells
Primitive streak establishes head (primitive node) and tail ends
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47. Gastrulation
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48. Third week of development (cont.)
Gastrulation (cont.)
16 days after fertilization notochord forms – induces tissue to become vertebral bodies
2 depressions form
Oropharyngeal membrane will later break down to connect mouth to pharynx and GI tract
Cloacal membrane will later degenerate to form openings of anus, urinary and reproductive tracts
When cloacal membrane appears, wall of yolk sac forms allantois
Extends into connecting stalk
In most other mammals used for gas exchange and waste removal – human placenta does this instead
Does function in early formation of blood and blood vessels and urinary bladder
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49. Development of the notochordal process
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50. Third week of development (cont.)
Neurulation
Notochord also induces formation of neural plate
Edges of plate elevate to form neural fold
Neural folds fuse to form neural tube
Develop into brain and spinal cord
Neural crest cells give rise to spinal and cranial nerves and ganglia, autonomic nervous system ganglia, CNS meninges, adrenal medullae and several skeletal and muscular components of head
Head end of neural tube develops into 3 primary brain vesicles
Prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain)
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51. Third week of development (cont.)
Development of somites
Mesoderm adjacent to notochord and neural tube forms paired longitudinal columns of paraxial mesoderm
Segment into paired, cube-shaped somites
Number of somites can be correlated to age of embryo
Each somite has 3 regions
Myotome – develops into skeletal muscles of neck, trunk and limbs
Dermatome – develops into connective tissue
Sclerotome - develops into vertebrae and ribs
Development of intraembryonic coelom
Splits lateral plate mesoderm into
Splanchnic mesoderm – forms heart, blood vessels, smooth muscle and connective tissues of respiratory and digestive systems
Somatic mesoderm – gives rise to bones, ligaments, dermis of skin
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52. Neurulation and the development of somites
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53. Third week of development (cont.)
Development of cardiovascular system
Angiogenesis – formation of blood vessels
Spaces develop in blood islands to form lumens of blood vessels
Pluripotent stem cells form blood cells
By end of 3rd week, heart forms and begins to beat
Development of chorionic villi and placenta
Chorionic villi – fingerlike projections of chorion projecting into endometrium
Blood vessels in chorionic villi connect to embryonic heart through body stalk (becomes umbilical cord)
Maternal and fetal blood do not mix – diffusion only
Copyright 2009, John Wiley & Sons, Inc.

54. Development of chorionic villi
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55. Placentation Process of forming placenta
By beginning of 12th week has 2 parts
Fetal portion formed by chorionic villi of chorion
Maternal portion formed by decidua basalis of endometrium
Functionally allows oxygen and nutrients to diffuse from maternal to fetal blood while carbon dioxide and wastes diffuse from fetal to maternal blood
Not a protective barrier – allows microorganisms, drugs, alcohol to pass
Connection between embryo and placenta through umbilical cord
2 umbilical arteries carry deoxygenated fetal blood to placenta
1 umbilical vein carries oxygenated blood away from placenta
Afterbirth – placenta detaches from uterus
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56. Placenta and umbilical cord
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57. Fourth week of development
4th -8th week - all major organs develop
Organogenesis – formation of body organs and systems
Embryo triples in size this week
Converted from flat disc to 3D cylinder through embryonic folding
Main force is different rates of growth for different parts
Head fold brings heart and mouth into eventual adult position
Tail fold brings anus into eventual adult position
Lateral folds for primitive gut – forerunner of GI tract
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58. Embryonic folding
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62. I. somite/Nt development.

64. I Axial skel. Vol muscl. Dermis Splanchnopleuric Urinary system
Somatopleuric
mesoderm
Urinary system
Genital system

66. Week 3 – The Three-Layered Embryo
Primitive streak – raised groove on the dorsal surface of the epiblast
Gastrulation – a process of invagination of epiblast cells
Begins at the primitive streak
Forms the three primary germ layers
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

67. Week 3 – The Three-Layered Embryo
Three Germ Layers*
Endoderm – formed from migrating cells that replace the hypoblast
Mesoderm – formed between epiblast and endoderm
Ectoderm – formed from epiblast cells that stay on dorsal surface
*All layers derive from epiblast cells!
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

68. The Primitive Streak Figure 3.5e–h
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

69. The Notochord
Primitive node – a swelling at one end of primitive streak
Notochord forms from primitive node and endoderm
Notochord – defines body axis
Is the site of the future vertebral column
Appears on day 16
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

70. Figure 3.6
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71. Neurulation
Neurulation – ectoderm starts forming brain and spinal cord
Neural plate – ectoderm in the dorsal midline thickens
Neural groove – ectoderm folds inward
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

72. Neurulation Neurulation (continued)
Neural tube – a hollow tube pinches off into the body
Cranial part of the neural tube becomes the brain
Maternal folic acid deficiency causes neural tube defects
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

73. Neurulation Neural crest Induction
Cells originate from ectodermal cells
Forms sensory nerve cells
Induction
Ability of one group of cells to influence developmental direction of other cells
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

74. The Mesoderm Begins to Differentiate
Somites – our first body segments
Paraxial mesoderm
Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

75. The Mesoderm Begins to Differentiate
Lateral plate – most lateral part of the mesoderm
Coelom – becomes serous body cavities
Somatic mesoderm – apposed to the ectoderm
Splanchnic mesoderm – apposed to the endoderm
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

76. Germ Layer Theory of Organ –System Development