Prenatal Development

Prenatal development is the process in which an embryo or fetus (or foetus) gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.

After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age, the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.

Changes by weeks of gestational age:

• Week 1 (day 1-7 of fertilization)
  • Fertilization of the ovum to form a zygote. (day 1 of fert.^[2])
  • The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3 of fert.^[2])
  • The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the “embryonic pole” which include embryonic stem cells.
  • The blastocyst hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother’s uterus. (day 5-6 of fert.^[2])
  • If separation into identical twins occurs, 1/3 of the time it will happen before day 5.^[3]

• Week 2 (1 week from fertilization)
  • Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert. ^[2]
  • Formation of the yolk sac.
  • The embryonic cells flatten into a disk, two-cells thick.
  • If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
  • Primitive streak develops. (day 13 of fert.^[2])
  • Primary stem villi appear. (day 13 of fert.^[2])

• Week 3 (2 weeks from fertilization – first missed menstrual period)
  • A notochord forms in the center of the embryonic disk. (day 16 of fert.^[2])
  • Gastrulation commences. (day 16of fert.^[2])
  • A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert.^[2])
  • Somites, the divisions of the future vertebra, form. (day 20 of fert.^[2])
  • Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.^[2])

• Week 4 (3 weeks from fertilization)
  • The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
  • The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear. ^[2]
  • Branchial arches, grooves which will form structures of the face and neck, form.
  • The neural tube closes.
  • The ears begin to form as otic pits.
  • Arm buds and a tail are visible.
  • Pulmonary primordium, the first traits of the lung appear. ^[2]
  • Hepatic plate, the first traits of the liver appear. ^[2]
  • Buccopharyngeal membrane ruptures. This is the future mouth. ^[2]
  • Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear. ^[2]
  • Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated. ^[2]
  • Anterior and posterior horns differentiate in the spinal cord ^[2]
  • Spleen appears. ^[2]
  • Ureteric buds appear. ^[2]

Embryo at 4 weeks after fertilization.^[4]

• Week 5 (4th week of development)
  • The embryo measures 8 mm (1/4 inch) in length.
  • Lens pits and optic cups form the start of the developing eye.
  • Nasal pits form.
  • The brain divides into 5 vesicles, including the early telencephalon.
  • Leg buds form and hands form as flat paddles on the arms.
  • Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.

• Week 6 (5th week of development)
  • The embryo measures 13 mm (1/2 inch) in length.
  • Lungs begin to form.
  • The brain continues to develop.
  • Arms and legs have lengthened with foot and hand areas distinguishable.
  • The hands and feet have digits, but may still be webbed.
  • The gonadal ridge begins to be perceptible.
  • The lymphatic system begins to develop.

• Week 7 (6th week of development)
  • The embryo measures 18 mm (3/4 inch) in length.
  • Nipples and hair follicles begin to form.
  • Location of the elbows and toes are visible.
  • Spontaneous limb movements may be detected by ultrasound.
  • All essential organs have at least begun formation.

~ by enrique yabut on July 22, 2008.