Early Embryonic Development

Day/Week	Event
Week 1	💠 HCG secretion begins around the time of blastocyst implantation
💠 Blastocyst “sticks” on day six
Week 2	💠 Formation of bilaminar embryonic disc
💠 Two layers → epiblast and hypoblast.
Week 3	💠 Formation of trilaminar embryonic disc via gastrulation. (Three layers → endoderm, mesoderm, ectoderm)
💠 Gastrulation → epiblast cell invagination through primitive streak. 🔽
💠 Notochord arise from midline mesoderm and induces overlying ectoderm (via SHH) to become neural plate , which gives neural tube via neurulation.
Week 4	💠 Heart begins to beat (four chambers), cardiac activity visible by transvaginal ultrasound.
💠 Upper and lower Limb buds start to form. (four limbs)
Week 5	💠 Neural crest migration
Week 8	💠 Genitalia start to show male/female characteristics. (pronounce “genieightalia”)

Gastrulation 🔽

Embryologic derivatives

| Ectoderm | Surface ectoderm (CLEAN the surface with a SOAP) | 💠 Cornea, anal Canal below pectinate line 💠 Lens 💠 Epidermis 💠 Adenohypophysis (rathke’s pouch) 💠 Nipple (mammary glands), Nasal epithelium 💠 Sweat glands 💠 Olfactory epithelium , Oral epithelium 💠 Auditory organs (Inner ear sensory organs) 💠 Parotid gland (salivary gland) | | --- | --- | --- | | | Neural tube (CNS) | 💠 Brain & spinal cord 💠 Posterior pituitary, pineal gland 💠 Retina | | | Neural crest (PNS) | 💠 Neural ganglia (DRG), adrenal medulla 💠 Schwann cells; pia & arachnoid mater 💠 Aorticopulmonary septum & endocardial cushions 💠 Branchial arches (bones & cartilage) 💠 Skull bones 💠 Melanocytes | | Mesoderm | Muscloskeletal components | 💠 Muscles (skeletal, cardiac & smooth) 💠 Connective tissue, bone & cartilage 💠 Serosal linings (eg, peritoneum) 💠 Cardiovascular & lymphatic system 💠 Spleen & hemopoietic cells 💠 Kidney & ureters, internal genitalia 💠 Adrenal cortex | | Endoderm | Visceral organs | 💠 Gastrointestinal tract, liver, pancreas 💠 Lungs 💠 Thymus, parathyroids, thyroid follicular cells 💠 Middle ear epithelium ***💠 Bladder & urethra 💠 Parafollicular (C) cells | | *Formerly thought to be of neural crest origin. | | |

<aside> 💡

Mesodermal defect (VACTERL association)

Vertebral defects
Anal atresia
Cardiac defects
Tracheo-esophageal fistula
Renal defects
Limb defects </aside>

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Summary of embryologic derivatives

Neural crest vs Neural tube
- Neural tube → CNS
- Neural crest → PNS , melanocytes , Aorticopulmonary septum & endocardial cushions
Ear
- Inner ear → surface ectoderm
- Middle ear → endoderm
Adrenal gland
- Cortex → Mesoderm
- Medulla → neural crest
Pituitary gland
- Anterior → Surface ectoderm (rathke’s pouch)
- Posterior → Neural tube
Spleen , thymus
- Spleen → mesoderm
- Thymus → endoderm
Thyroid, parathyroid → Endoderm </aside>

Multiple pregnancy

Types of Twins

Dizygotic (“Fraternal”) Twins

Formation
- Arise from two separate eggs fertilized by two different sperm cells.
Characteristics:
- Number of Zygotes → Two
- Amniotic Sacs → Two separate
- Placentas → Two separate (two chorions)

<aside> 💡 **Twin of different genders (boy and a girl)**→ Dizygotic twins (can’t be monozygotic)

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Monozygotic (“Identical”) Twins

Formation
- Arise from one fertilized egg (1 egg + 1 sperm) that splits into two embryos.
Characteristics:
- Timing of Splitting (determines chorionoicity (number of chorions) and amnioninicity (number of amnions)
  - 0–4 Days → Separate chorion (di-chorionic) and amnion (di-amniotic) (Di-Di)
  - 4–8 Days → Shared chorion (mono-chorionic) and Separate amnion (di-amniotic) (Mo-Di)
  - 8–12 Days → Shared chorion (mono-chorionic) and amnion (mono-amniotic) (Mo-Mo)
  - 13+ Days → Shared chorion (mono-chorionic), amnion (mono-amniotic) and body (Mo-Mo , conjoint)

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Another Picture

Twin-Twin Transfusion Syndrome (TTTS)

Occurrence

Monochorionic twin gestations (shared placenta)