Embryonic Development Stages

Pre-embryonic Development (Weeks 1-3)

• Begins with fertilization and lasts until the 3rd week.

Embryonic Development (Weeks 4-8)

• Occurs from the 4th to the 8th week.

Fetal Development (Week 9 to Birth)

• Starts at the 9th week and continues until birth.

Day 1: Fertilization

• Initialization with Zygote, requires female gamete (secondary oocyte) and male gamete (spermatozoon).
• Occurs in the ampulla of the fallopian tube.
• Zygote's characteristics:
  • 46 chromosomes (23 from each parent, XX or XY).
  • Pluripotent.
  • Mitotic division.
• Takes approximately 24 hours.

Day 2: Segmentation & Compaction

• Formation of blastomeres occurs through mitotic divisions to increase cell number
  • 1st division: after 30 hours, resulting in two blastomeres.
  • 2nd division: after 40 hours, forming 4 cells.
  • 3rd division: forming 8 cells.
• Blastomeres move closer due to glycoproteins.

Day 3: Morula Stage

• The embryo's compacted cells divide to form a morula consisting of 16 cells.
• Cells inside the morula include:
  • Inner cell mass (trophoblast): Future placenta.
  • Outer cell mass (embryoblast): Future embryo.

Day 4: Morula Enters the Uterus

• The morula enters the uterus and absorbs fluid through the zona pellucida, forming the blastocele.
• The embryo is becomes a blastocyst; the embryoblast positions itself at the embryonic pole.

Day 5: Loss of Zona Pellucida

• The zona pellucida is eliminated to allow adhesion to the endometrium; without adhesion, abortion may occur.

Day 6: Implantation

• Blastocyst attaches to the endometrium via the embryonic pole.
• Upon attaching, the trophoblast divides into two layers.
  • Inner layer: Cytotrophoblast (mitotic division).
  • Outer layer: Syncytiotrophoblast (produces hCG).

Day 7: Implantation Process

• The syncytiotrophoblast erodes the endometrium, generating two layers.
  • Columnar cells (epiblast).
  • Cuboidal cells (hypoblast).
• Subsequently, they form the bilaminar germ disc.

Day 8: Complete Endometrial Implantation

• Trophoblast differentiates into:
  • Cytotrophoblast.
  • Syncytiotrophoblast.
• Embryoblast differentiates into
  • Epiblast: Forms the amniotic cavity.
  • Hypoblast.
• These give rise to the bilaminar germ disc.

Day 9: Fibrin Coagulum Forms

• The blastocyst is implanted deeper, and a fibrin coagulum forms in the epithelium.
• Vacuoles fuse to form trophoblastic lacunae.
• The exocoelomic membrane, or Heuser's membrane, forms with the hypoblast: Exocoelomic cavity or primary yolk sac.

Days 11-12: Lacunar Network Formation

• The blastocyst is fully immersed; trophoblastic lacunae of the syncytiotrophoblast form an interconnected network.
• Maternal blood enters the lacunar system.
• Maternal blood flows, establishing uteroplacental circulation.
• The extraembryonic mesoderm develops cavities, forming the extraembryonic coelom or chorionic cavity.
• Decidual reaction: The maternal endometrium becomes rich in glycogen, lipids, and edematous.

Day 13: Development of Primary Villi

• Cytotrophoblast cells penetrate the syncytiotrophoblast to form cellular columns called primary villi.
• Primary yolk sac: Hypoblast cells migrate around the primary yolk sac.
• Exocoelomic cysts are formed.
• The extraembryonic mesoderm transitions into the chorionic plate.

Third Week of Gestation - Embryonic Development

• Gastrulation: Establishes the 3 germ layers (trilaminar disc).

• Layers from outer to inner:

  • Ectoderm
  • Mesoderm
  • Endoderm

• Formation is Initiated by the primitive streak

  • Primitive node and pit

• Epiblast cells migrate towards hypoblast and invaginate.

• Cranial-lateral expansion and growth (broader cephalic part and narrower caudal part).

• There is also;

  • Bucopharyngeal membrane

• The prechordal plate = prosencephalon

  • Formation of the notochord

• The cephalic part of the embryonic disc is larger than the caudal part, with "Caudal-cephalic growth."

• Cell migration from the primitive streak continues until the end of the 4th week of gestation, then disappears.

• Cellular differentiation occurs between the middle of the 3rd and 4th weeks, cephalocaudally.

• Around the fourth week, the trophoblast subdivides.

  • Primary villi: Growth of the mesoderm towards the decidua.
  • Secondary villi: Mesoderm differentiates into blood vessels and blood cells.
  • Tertiary villi or definitive placenta."

End of the Third Week

• Cytotrophoblast mitotic division. penetrates the endometrium, forming the outer cytotrophoblastic shell, referred to as the "Chorionic Cavity" and increasing its size.
• Days 19-20: Formation of the connecting stalk/future umbilical cord.

Week 4

• Three pharyngeal arches appear between days 24-26:
  • 1st arch: Mandibular arch.
  • 2nd arch: Hyoid arch.
• The heart produces a large ventral prominence and pumps blood.
• The prosencephalon creates a prominent elevation of the head.
• Limb buds can be identified around days 26-27.
• The fourth pair of pharyngeal arches and the buds of the inferior limbs can be observed at the termination of the 4th week.

End of Week 4

• The fourth pair of pharyngeal arches and the buds of the inferior limbs can be observed at the termination of the 4th week.
• A fetocardia exam can be performed

Week 5

• The head grows larger than other regions due to rapid development of facial and brain prominences
• The face establishes contact with the cardiac prominence

Week 6

• The inferior limbs start to spread when the elbows appear to split from the hands
• Digital rays start to appear on the hands indicating future digit formation.

Week 7

• Notches appear on the hand plates identifying future digits appear on digital hand plates.

Week 8

• Hands and feet approach each other. - Hand digits are free and longer. - Foot digits differentiate although they are still webbed
• The head becomes more rounded and develops distinct facial characteristics at 56 days.
  • Eyelids and auricular pavilions are more developed.

Fetal Development

• Occurs from the 9th week until birth (40-42 weeks)
• Measurements can be taken:
  • Fetal Length
  • Crown-Rump Length
  • Crown-Heel Length
  • Weight

Estimating Fetal Age

• First Trimester ( up to 22 weeks): Margin of error is 2 days
• Use the following measurements:
  • BIPARIETAL DIAMETER
  • CRANIAL CIRCUMFERENCE
  • ABDOMINAL CIRCUMFERENCE
  • FEMUR LENGTH
  • FOOT LENGTH
• Late ultrasound: Not important for age estimation

Week 9

• Measures 45-52 mm/4 to 5 cm.
  • Auricular pavilions form at the level of the eyes. -The head makes up 50% of the anatomy
  • The Liver enters hematopoietic period
  • Nervous system has Mylenation
  • Further differentiated Genitals
  • Developed kidneys

Weeks 10-13

• Measures 49-112 mm/5 to 11 cm.
  • Head makes up 1/3 proportion
  • Central hematopoietic spleen
  • CNS completes coarse morphogenesis
  • The upper limbs are proportionately more mature
  • Shorter lower limbs and incipient nails appear
• Anomalies like Sd of down can be detected via:
  • Ultrasound
  • Chromosomal or Translucence

Weeks 14-16

• Accelerated body Growth
• Measures 100-150 mm/10-15cm
• Lanugo appears
• Definitive Inferior Limb
• Marked ossification (bones present)
• Well difined Genitals.

Weeks 17-20

• Measure 130-195 mm/13-19cm
• More Pronounced Lanugo
• Vernix Caseosa covers skin and Production of melanin
• accumulation of brown fat
• More of the finger nail covers the ungual plate
• Reach Limb Proportions
• Intervention by law

Weeks 21-25

• Fetal Viability is achieved
• Type 2 pneumocyte produces the tension producing Surfactant
• Dense capilaries and superior nail border on fingers

Weeks 26-30

• Greater likelihood Fetal viability
• Eyes voluntarily open and moves respiratory
• bone marrow and blood made

Weeks 31-38

• White Fat, Surfactant peaks 34 week, increases tenso actives and testicles in scrotum
• The testicles are in the scrotum

Birth

Term Product

• Occurs within 37 and 41 weeks

Premature Product

• Occurs before 37 weeks

Post-Term Product

• Occurs after 41+ weekds

Last Menstruation Date

• Last Period + 7 days
• Month -3 months
• • 1 Year

Legal Pregnancy Interruption

• Occurs by victim of violation
• 24-48 hours and performed in 22-26 weeks
• 800 Mg Misoprostol administered orally and continued with 800 mg