Table of Contents
Fertilization .................................................................................................................................. 4
Cleavage ...................................................................................................................................... 5
Ootypes ............................................................................................................................................... 5
Patterns of cleavage ............................................................................................................................ 5
Blastulation.................................................................................................................................. 7
Gastrulation ................................................................................................................................. 8
Types of Cell Movement...................................................................................................................... 8
Body Axes .......................................................................................................................................... 11
Organogenesis ........................................................................................................................... 12
Pattern Formation ............................................................................................................................. 12
Morphogenesis .................................................................................................................................. 16
Positional information ....................................................................................................................... 19
Important Molecules .................................................................................................................. 19
Transcription factors ......................................................................................................................... 20
Signaling Molecules ........................................................................................................................... 20
Receptor molecules ........................................................................................................................... 22
Animal models ........................................................................................................................... 22
Drosophila ......................................................................................................................................... 22
Caenorhabditis elegans ..................................................................................................................... 25
Zebrafish ............................................................................................................................................ 26
Xenopus laevis ................................................................................................................................... 28
Gallus gallus domesticus ................................................................................................................... 33
Mus musculus .................................................................................................................................... 35
Other animals .................................................................................................................................... 36
Human Embryology .................................................................................................................... 37
Clinical Relevance .............................................................................................................................. 37
Stages of Human Embryology ........................................................................................................... 37
Gametogenesis .......................................................................................................................... 39
Primordial germ cells ......................................................................................................................... 39
Chromosomes ................................................................................................................................... 39
Mitosis ............................................................................................................................................... 40
Meiosis .............................................................................................................................................. 40
First Week of Development: Ovulation to Implantation ............................................................... 41
Fertilization........................................................................................................................................ 41
1
, Cleavage and blastulation ................................................................................................................. 41
Implantation ...................................................................................................................................... 42
The end of the first week .................................................................................................................. 43
Second Week of Development: Bilaminar Germ Disc ................................................................... 43
The Week of Twos ............................................................................................................................. 44
Extraembryonic membranes ............................................................................................................. 44
Third Week of Development: Trilaminar Germ Disc ..................................................................... 44
Gastrulation ....................................................................................................................................... 44
Formation of the Notochord ............................................................................................................. 45
Germ layer differentiation ................................................................................................................ 45
Third to Eighth Weeks: The Embryonic Period ............................................................................. 46
Derivatives of the Ectodermal Germ Layer ....................................................................................... 46
Derivatives of the Mesodermal Germ Layer ..................................................................................... 48
Derivatives of the Endodermal Germ Layer ...................................................................................... 49
Third Month to Birth: The Fetus and Placenta ............................................................................. 50
The Placenta ...................................................................................................................................... 50
Development of the Fetus ................................................................................................................. 50
Time of Birth ...................................................................................................................................... 52
Amniotic Fluid.................................................................................................................................... 53
Fetal Membranes in Twins ................................................................................................................ 54
Development of the Central Nervous System .............................................................................. 56
Flexures ............................................................................................................................................. 58
Vesicles and Ventricles ...................................................................................................................... 58
The Developing Brain: A 3D structure ............................................................................................... 59
Differentiation of the Brain: Cytodifferentiation .............................................................................. 60
Differentiation of the Brain: Organization of the Neural Tube ......................................................... 61
Development of the Prosencephalon: Telencephalon ...................................................................... 61
Development of the Prosencephalon: Diencephalon ....................................................................... 64
Development of the Mesencephalon................................................................................................ 67
Development of the Rhombencephalon ........................................................................................... 68
Development of the Cardiac System ........................................................................................... 70
Primary Heart Field............................................................................................................................ 70
Secondary Heart Field and Proepicardium........................................................................................ 70
Formation of the Primitive Heart Tube ............................................................................................. 71
Composition of the Heart Tissue ....................................................................................................... 72
Regions of the Cardiac Tube .............................................................................................................. 73
2
, Formation of the Cardiac Septa......................................................................................................... 74
Development of the Circulatory System ...................................................................................... 79
Early Vasculature ............................................................................................................................... 79
Embryonic Vasculature...................................................................................................................... 81
Development of the Arterial System ................................................................................................. 82
Development of Venous Systems...................................................................................................... 83
Development of Cardinal System ...................................................................................................... 84
Circulation Before and After Birth ..................................................................................................... 86
Development of the Urogenital System....................................................................................... 88
Urinary System .................................................................................................................................. 88
Genital System................................................................................................................................... 92
Genital Ducts and External Genitalia................................................................................................. 94
Development of the Digestive System ......................................................................................... 94
Divisions of the Gut Tube .................................................................................................................. 97
Mesentery ......................................................................................................................................... 98
Foregut .............................................................................................................................................. 99
Stomach ........................................................................................................................................... 100
Duodenum ....................................................................................................................................... 101
Liver and Gallbladder ...................................................................................................................... 101
Pancreas .......................................................................................................................................... 102
Midgut ............................................................................................................................................. 102
Hindgut ............................................................................................................................................ 103
Development of the Pulmonary System .....................................................................................104
Development of the Head and Neck ...........................................................................................105
Pharyngeal Arches ........................................................................................................................... 105
Pharyngeal Pouches ........................................................................................................................ 107
Pharyngeal Clefts ............................................................................................................................. 108
Face ................................................................................................................................................. 108
Development of The Axial Skeleton ...........................................................................................112
Skull ................................................................................................................................................. 112
Birth Defects: Teratology ...........................................................................................................114
Types of Abnormalities.................................................................................................................... 115
Environmental Factors .................................................................................................................... 117
Principles of Teratology ................................................................................................................... 118
Infectious Agents ............................................................................................................................. 118
Illicit Drugs, Alcohol, and Cigarettes................................................................................................ 119
3
, Maternal Disease............................................................................................................................. 120
Nutritional Deficiencies ................................................................................................................... 121
Obesity............................................................................................................................................. 121
Birth Defects: Chromosomal and Genetic Factors .......................................................................121
Numerical Abnormalities................................................................................................................. 122
Structural Abnormalities ................................................................................................................. 126
Gene Mutations............................................................................................................................... 126
Diagnostic Techniques ..................................................................................................................... 126
Prenatal Diagnosis .....................................................................................................................127
Ultrasonography .............................................................................................................................. 128
Amniocentesis ................................................................................................................................. 128
Chorionic Villus Sampling ................................................................................................................ 129
Cordocentesis or Percutaneous Umbilical Blood Sampling ............................................................ 129
Maternal Serum Screening .............................................................................................................. 129
Tips for the Exam ......................................................................................................................130
Developmental Biology
Fertilization
• A zygote refers to a newly fertilized egg
• After a sperm penetrates an egg, a fertilization envelop forms. This membrane is responsible
for the fast block to polyspermy.
When a sperm cell contacts the jelly coat surrounding an egg (the zona pellucida), the acrosome, a
vesicle at the tip of the sperm, discharges its contents. This acrosomal reaction releases hydrolytic
enzymes that enable an extending acrosomal process to penetrate the zona pellucida of the egg.
A “lock-and-key” recognition of molecules that will trigger further enzymatic digestion of
the vitelline envelope, located just above the plasma membrane of the egg.
The plasma membrane of the sperm and egg fuse. This results in an instant depolarization of the egg
(due to an influx of sodium), which prevents a phenomenon called polyspermy: multiple fertilizations
that would result in an aberrant chromosome count. Due to this depolarization, the intracellular
calcium levels also rise leading to the fusion of cortical granules (located in the gelatinous outer
zone of the cytoplasm) with the plasma membrane. These granules will release their contents by
exocytosis into the space between the plasma membrane and the vitelline membrane, which will
be filled with water via osmosis. Other enzymes polymerize molecules in the elevated vitelline layer
to form a hardened fertilization membrane, preventing additional sperm from entering.
The oocyte resumes the second meiotic division in which it was arrested until fertilization, to
become the definitive oocyte with its female pronucleus. The pronucleus of the sperm is fused with
4
Fertilization .................................................................................................................................. 4
Cleavage ...................................................................................................................................... 5
Ootypes ............................................................................................................................................... 5
Patterns of cleavage ............................................................................................................................ 5
Blastulation.................................................................................................................................. 7
Gastrulation ................................................................................................................................. 8
Types of Cell Movement...................................................................................................................... 8
Body Axes .......................................................................................................................................... 11
Organogenesis ........................................................................................................................... 12
Pattern Formation ............................................................................................................................. 12
Morphogenesis .................................................................................................................................. 16
Positional information ....................................................................................................................... 19
Important Molecules .................................................................................................................. 19
Transcription factors ......................................................................................................................... 20
Signaling Molecules ........................................................................................................................... 20
Receptor molecules ........................................................................................................................... 22
Animal models ........................................................................................................................... 22
Drosophila ......................................................................................................................................... 22
Caenorhabditis elegans ..................................................................................................................... 25
Zebrafish ............................................................................................................................................ 26
Xenopus laevis ................................................................................................................................... 28
Gallus gallus domesticus ................................................................................................................... 33
Mus musculus .................................................................................................................................... 35
Other animals .................................................................................................................................... 36
Human Embryology .................................................................................................................... 37
Clinical Relevance .............................................................................................................................. 37
Stages of Human Embryology ........................................................................................................... 37
Gametogenesis .......................................................................................................................... 39
Primordial germ cells ......................................................................................................................... 39
Chromosomes ................................................................................................................................... 39
Mitosis ............................................................................................................................................... 40
Meiosis .............................................................................................................................................. 40
First Week of Development: Ovulation to Implantation ............................................................... 41
Fertilization........................................................................................................................................ 41
1
, Cleavage and blastulation ................................................................................................................. 41
Implantation ...................................................................................................................................... 42
The end of the first week .................................................................................................................. 43
Second Week of Development: Bilaminar Germ Disc ................................................................... 43
The Week of Twos ............................................................................................................................. 44
Extraembryonic membranes ............................................................................................................. 44
Third Week of Development: Trilaminar Germ Disc ..................................................................... 44
Gastrulation ....................................................................................................................................... 44
Formation of the Notochord ............................................................................................................. 45
Germ layer differentiation ................................................................................................................ 45
Third to Eighth Weeks: The Embryonic Period ............................................................................. 46
Derivatives of the Ectodermal Germ Layer ....................................................................................... 46
Derivatives of the Mesodermal Germ Layer ..................................................................................... 48
Derivatives of the Endodermal Germ Layer ...................................................................................... 49
Third Month to Birth: The Fetus and Placenta ............................................................................. 50
The Placenta ...................................................................................................................................... 50
Development of the Fetus ................................................................................................................. 50
Time of Birth ...................................................................................................................................... 52
Amniotic Fluid.................................................................................................................................... 53
Fetal Membranes in Twins ................................................................................................................ 54
Development of the Central Nervous System .............................................................................. 56
Flexures ............................................................................................................................................. 58
Vesicles and Ventricles ...................................................................................................................... 58
The Developing Brain: A 3D structure ............................................................................................... 59
Differentiation of the Brain: Cytodifferentiation .............................................................................. 60
Differentiation of the Brain: Organization of the Neural Tube ......................................................... 61
Development of the Prosencephalon: Telencephalon ...................................................................... 61
Development of the Prosencephalon: Diencephalon ....................................................................... 64
Development of the Mesencephalon................................................................................................ 67
Development of the Rhombencephalon ........................................................................................... 68
Development of the Cardiac System ........................................................................................... 70
Primary Heart Field............................................................................................................................ 70
Secondary Heart Field and Proepicardium........................................................................................ 70
Formation of the Primitive Heart Tube ............................................................................................. 71
Composition of the Heart Tissue ....................................................................................................... 72
Regions of the Cardiac Tube .............................................................................................................. 73
2
, Formation of the Cardiac Septa......................................................................................................... 74
Development of the Circulatory System ...................................................................................... 79
Early Vasculature ............................................................................................................................... 79
Embryonic Vasculature...................................................................................................................... 81
Development of the Arterial System ................................................................................................. 82
Development of Venous Systems...................................................................................................... 83
Development of Cardinal System ...................................................................................................... 84
Circulation Before and After Birth ..................................................................................................... 86
Development of the Urogenital System....................................................................................... 88
Urinary System .................................................................................................................................. 88
Genital System................................................................................................................................... 92
Genital Ducts and External Genitalia................................................................................................. 94
Development of the Digestive System ......................................................................................... 94
Divisions of the Gut Tube .................................................................................................................. 97
Mesentery ......................................................................................................................................... 98
Foregut .............................................................................................................................................. 99
Stomach ........................................................................................................................................... 100
Duodenum ....................................................................................................................................... 101
Liver and Gallbladder ...................................................................................................................... 101
Pancreas .......................................................................................................................................... 102
Midgut ............................................................................................................................................. 102
Hindgut ............................................................................................................................................ 103
Development of the Pulmonary System .....................................................................................104
Development of the Head and Neck ...........................................................................................105
Pharyngeal Arches ........................................................................................................................... 105
Pharyngeal Pouches ........................................................................................................................ 107
Pharyngeal Clefts ............................................................................................................................. 108
Face ................................................................................................................................................. 108
Development of The Axial Skeleton ...........................................................................................112
Skull ................................................................................................................................................. 112
Birth Defects: Teratology ...........................................................................................................114
Types of Abnormalities.................................................................................................................... 115
Environmental Factors .................................................................................................................... 117
Principles of Teratology ................................................................................................................... 118
Infectious Agents ............................................................................................................................. 118
Illicit Drugs, Alcohol, and Cigarettes................................................................................................ 119
3
, Maternal Disease............................................................................................................................. 120
Nutritional Deficiencies ................................................................................................................... 121
Obesity............................................................................................................................................. 121
Birth Defects: Chromosomal and Genetic Factors .......................................................................121
Numerical Abnormalities................................................................................................................. 122
Structural Abnormalities ................................................................................................................. 126
Gene Mutations............................................................................................................................... 126
Diagnostic Techniques ..................................................................................................................... 126
Prenatal Diagnosis .....................................................................................................................127
Ultrasonography .............................................................................................................................. 128
Amniocentesis ................................................................................................................................. 128
Chorionic Villus Sampling ................................................................................................................ 129
Cordocentesis or Percutaneous Umbilical Blood Sampling ............................................................ 129
Maternal Serum Screening .............................................................................................................. 129
Tips for the Exam ......................................................................................................................130
Developmental Biology
Fertilization
• A zygote refers to a newly fertilized egg
• After a sperm penetrates an egg, a fertilization envelop forms. This membrane is responsible
for the fast block to polyspermy.
When a sperm cell contacts the jelly coat surrounding an egg (the zona pellucida), the acrosome, a
vesicle at the tip of the sperm, discharges its contents. This acrosomal reaction releases hydrolytic
enzymes that enable an extending acrosomal process to penetrate the zona pellucida of the egg.
A “lock-and-key” recognition of molecules that will trigger further enzymatic digestion of
the vitelline envelope, located just above the plasma membrane of the egg.
The plasma membrane of the sperm and egg fuse. This results in an instant depolarization of the egg
(due to an influx of sodium), which prevents a phenomenon called polyspermy: multiple fertilizations
that would result in an aberrant chromosome count. Due to this depolarization, the intracellular
calcium levels also rise leading to the fusion of cortical granules (located in the gelatinous outer
zone of the cytoplasm) with the plasma membrane. These granules will release their contents by
exocytosis into the space between the plasma membrane and the vitelline membrane, which will
be filled with water via osmosis. Other enzymes polymerize molecules in the elevated vitelline layer
to form a hardened fertilization membrane, preventing additional sperm from entering.
The oocyte resumes the second meiotic division in which it was arrested until fertilization, to
become the definitive oocyte with its female pronucleus. The pronucleus of the sperm is fused with
4
