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612 SectioN iii RepRoductive ` REPRODUCTIVE—EmbRyOlOgy RepRoductive ` REPRODUCTIVE—EmbRyOlOgy
` REPRODUCTIVE—EmbRyOlOgy
Important genes of embryogenesis
gENE lOCATION FUNCTION NOTES
Sonic hedgehog (SHH) Zone of polarizing activity at Anterior-posterior axis Mutations holoprosencephaly
gene base of limb buds patterning, CNS development
Wnt-7 gene Apical ectodermal ridge at Dorsal-ventral axis patterning,
distal end of each limb limb development
Fibroblast growth Apical ectodermal ridge Limb lengthening (via mitosis of “Look at that Fetus, Growing
factor (FGF) gene mesoderm) Fingers”
Homeobox (Hox) Multiple Segmental organization in Mutations appendages in
genes cranial-caudal direction, wrong locations. Isotretinoin
transcription factor coding Hox gene expression
Early fetal development
Early embryonic
N = # of chromosomes
development C = # of chromatids/DNA copies
DAY 1 DAYS 2-3
Zygote
(2N4C)
DAY 4
Morula
Fertilization
(2N2C) Corpus albicans DAY 5
DAY 0 Developing Blastocyst
follicle
DAYS 6–10
Ovary
Implantation
Myometrium
Ovulation
2° oocyte Early corpus
(1N2C) luteum Endometrium
Within week 1 hCG secretion begins around the time of Blastocyst “sticks” at day 6.
implantation of blastocyst.
Within week 2 Bilaminar disc (epiblast, hypoblast). 2 weeks = 2 layers.
Within week 3 Gastrulation forms trilaminar embryonic disc. 3 weeks = 3 layers.
Cells from epiblast invaginate primitive
streak endoderm, mesoderm, ectoderm.
Notochord arises from midline mesoderm;
overlying ectoderm becomes neural plate.
Weeks 3–8 Neural tube formed by neuroectoderm and Extremely susceptible to teratogens.
(embryonic period) closes by week 4.
Organogenesis.
Week 4 Heart begins to beat. 4 weeks = 4 limbs and 4 heart chambers.
Upper and lower limb buds begin to form.
Week 6 Fetal cardiac activity visible by transvaginal
ultrasound.
Week 8 Fetal movements start. Gait at week 8.
Week 10 Genitalia have male/female characteristics. Tenitalia.
FAS1_2019_15-Repro.indd 612 11/7/19 5:52 PM
