Embryology
From Conception through Death
- Immune system – 18 months after birth - Lungs – 3-8 years after birth
- Nervous – 12 years after birth - Reproductive – Puberty
- Skeletal – 20-25 years after birth - Hematopoiesis – 3rd week to death
- Remodeling / Repair – birth to death
Prenatal Development & Birth
Stages of Prenatal Development: 3 stages:
Fertilization to 2 weeks (not susceptible to teratogens, rather death of embryo and spontaneous abortion are common)
Embryonic period = 3-8 weeks (major teratogenic effects in all organ systems)
Fetal Period = 9 weeks to term (birth) (minor teratogenic effects in most organ systems),
Partuition = birth Post-Partum
0-18 months – immune system susceptible to minor congenital malformation 0- 3 yrs – respiratory system susceptible to minor congenital malformations 0-12 yrs – nervous system susceptible to MAJOR congenital malformations
0-puberty – reproductive systems susceptible to minor congenital malformations 0-25 yrs – skeletal system susceptible to minor congenital malformations
Stages of Postnatal Development: 6 stages Neonatal – birth to one month of age
Infancy - one month to 1 year of age
Childhood - - 13 months to puberty
Puberty - - 12-15 in girls, 13-16 in boys
Adolescence - - 11-21 yrs
Prenatal Development: Directly responsible for events and time periods listed: Timetable of Human Prenatal Development, 1-6 weeks, Fig. 1-1, pg. 4-5 Moore Timetable of Human Prenatal Development, 7-38 weeks, Fig. 1-1 & 1-2, pg. 6-7 Moore
Fertilization to 2 wks
Fertilization occurs in ampulla, day 1
Sperm have 48 hr viability window after ejaculation for fertilization Ovum has 24 hr viability window after ovulation for fertilization
day 2: Zygote divides day 3: Morula formed day 4: Early blastocyst
day 5: Late blastocyst (inner cell mass & trophoblast), day 6: Zona Pellucida shed & implantation begins,
day 7: Invasion of syncytiotrophoblast into endometrium,
day 8: Amniotic cavity appears, Bilaminar disc (epiblast & hypoblast) forms,
Day 9: Lacunae appear in syncytiotrophoblast, extra-embryonic endoderm; Primary yolk sac appears,
day 10: Closing plug, extra-embryonic mesoderm, amnion,
day 11: Maternal blood within cytotrophoblast lacunar network, cavitation within extra-embryonic mesoderm,
day 12: Extra-Embryonic Coelom coalesces within extra-embryonic mesoderm forming extra-embryonic splanchnic mesoderm & extra-embryonic somatic mesoderm,
day 13: Primary chorionic villi, secondary yolk sac forms,
day 14: Connecting stalk, formation of pro(pre)chordal plate within hypoblast at anterior end of embryonic disc, denotes position with respect to future embryo, i.e., anterior, posterior, dorsal, ventral, right, and left.
Embryonic Period - weeks 3 to 8
Week 3:- extra-embryonic hematopoiesis in yolk sac and allantois, Hofbauer cells (the “Arnold Schwartzenager, Terminator” of all macrophages) migrates to placenta and takes up residence in tertiary chorionic villi, function: protection of embryo/fetus until 20 weeks of age. Actually, Hofbauer cells provide protection against “living” entities (i.e., viruses, bacteria, parasites, etc.) through 20 weeks. After 20 weeks, Hofbauer cells provide protection from these entities only if they cross paths within the extra-embryonic somatic mesoderm of the tertiary chorionic villi.
- formation of primitive streak - Formation tri-laminar embryo - neural plate forms
- formation of heart tubes
Week 4:- heart begins to beat - closure of neural tube begins - primordia of eye and ear
- rostral neuropore closes
- first 3 pairs of pharyngeal arches and aortic arches - upper limb buds form
- forebrain develops - lower limb buds form
Week 5:- intra-embryonic hematopoiesis in para-aortic LP (lateral plate) splanchnic mesoderm,
- blood cells for embryo proper
- lens pits, optic cups, nasal pits forming - primitive mouth
- hand plate forms
- cerebral vesicles become distinct - foot plate forms
Week 6: - hematopoiesis begins in liver
- oral and nasal cavities become confluent (they connect with each other) - large head
- upper lip and nose formed
- external auditory meatus (external opening of ear) - digital rays in hand and foot plate
Week 7:
Week 8: Basic body plan in place by this time
Weeks 3-8: Major congenital malformations initiated by teratogens.
Any (non-living) teratogen can have major effects on the body plan during this time frame. Major congenital malformations cease at week 8 and only minor alterations occur after that time with respect to ALL organ systems but one. The only outlier is the nervous system which is susceptible to major
teratogenic activities through 12 years after birth.
Fetal period - 9 wks to birth
Week 9:
Week 10: - hematopoiesis in spleen begins, still occurs in liver
Week 11:
Week 12: - hematopoiesis in bone marrow begins, still occurs in spleen
Important intraembryonic hematopoietic beginning time points! Weeks: 5 (para-aortic LP splanchnic mesoderm), wk 6 (liver), wk 10 (spleen), wk 12 (bone
marrow)
Please be cognizant that hematopoiesis is so important we will revisit this issue in the phases: Host Defense, Hematology, and Cardiology.
Postnatal Development - depends on particular organ system, for example: - immune system is not fully developed until 18 months after birth the infant’s
ability to fight infection during this time period is dependent on antibodies received from Mom.
- Pre-birth antibodies from Mom work until ~ 18 months, even if Mom has to feed formula. Depending on how long Mom breast feeds, her antibodies can remain in the child past the 18 month time point when the child’s immune system is fully operational.
[On a side note, this is the reason for testing a child for HIV after 18 months of age if the child is born from a Mother who is positive. The child will have HIV-positive antibodies from their Mother through 18 months of age. Hence, any HIV testing prior to 18 months will be detecting the presence of their Mother’s
antibodies as well as those potentially generated by the infant/child. If the Mother is HIV-positive, the child prior to 18 months will usually score HIV-positive as
- wiring for “potty training” – 3 yr (females) & 5 yr (males) after birth
- respiratory system: Alveolar development is largely completed by 3 years of age, but new alveoli can be added until approximately 8 years of age.
- nervous system not fully developed (myelinated) until 12 yrs after birth - Nervous system can be affected with major congenital malformations up until 12 years of age by outside stimuli (e.g., excess phenylalanine).
- reproductive system not fully developed until - males – puberty throughout lifespan
- females - puberty + 3-6 years to menopause - optimum time to have children is 3-6 years after puberty because the reproductive system is not fully developed until that time.
- skeletal system not fully developed until - males - 20 yrs of age
- Hematopoiesis throughout life of individual
Blood cells for use outside the embryo:
Extraembryonic LP splanchnic mesoderm of yolk sac and allantois – 3 weeks: Hofbauer cells – protection of fetus (3-20 weeks)
Blood cells for use inside the embryo:
Para-aortic LP splanchnic mesoderm – 5 weeks to 20 weeks
(hematopoiesis shuts down completely and permanently)
Liver – 6 weeks to birth (hematopoiesis can be reinduced under extreme
conditions)
Spleen – 10 weeks to birth (hematopoiesis can be reinduced under extreme
conditions)
