Reproductive-Embryology,Anatomy

*Sonic hedgehog gene is involved in proper development of CNS/in Patterning of ANTERIO- POSTERIOR axis, it is expressed at the base of the limbs in zone of polarizing activity....do NOT confuse this with Wnt-7 gene which is expressed on Most distal part of DISTAL Limbs(APICAL ECTODERMAL ridge), note that This gene is involved in patterning VENTRAL-DORSAL Axis at the DISTAL Limb..Once again fo Sonic hedehog gene it is HY to know that it is responisble for CHAPERONING of directional growth of CNS and defects have been implicated in HOLOPROSONCEPHALY in PATAU syndrome.

*Gene produced APICAL ECTOdermal ridge that stimulates MITOSIS of Underlying mesoderm is involved in LENGTHENING of the LIMBS and is known as FGF gene(Thus FGF promotes growth of Limbs trough stimulation of PROLIFERATION<Has been tested)...remember that FGF RECEPTOR3(FGFR3) is involved in development of ACHONDROPLASIA where trunk/head size is normal but growth of LONG TUBULAR BONES/LIMBS Is defective due to defective ENDOCHONDRIAL ossification..

*Knowing that Polysyndactyly/appendages in wrong places=mutation in HOMEBOX (HOX)genes is not enough, they often test the fact that this gene codes for DNA Binding Transcription regulators...they can also potentially ask its function so know that it is involved in SEGMENTAL organization of fetus in CRANIOCAUDAL direction....

*Sperm fertilizes SECONDARY oocyte(has 1 Chromosome and thus 2 DNA copies/Chromatids) when haploid genomes of sperm and secondary oocyte combine they form DIPLOID Zygote on day 0-1 in FALLOPIAN TUBES(Ampulla)...note that Secondary oocyte is halted in METAphase of MEIOSIS II, until it is fertilized by sperm,Acrosomal reaction is needed to degrade zona pellucida and to allow fusing of gametes to form the zygote..*Note that ACROSOME of sperm is (Derived from GOLGI ) and Flagellum is derived from CENTRIOLES...After fertilization oocyte becomes impermeable to other sperm(By releasing enzymes from cortical granules) and completes meiosis II releasing the Polar body, after this we get the Single-cell zygote after nucleus is reformed around DIPLOID genome(2chromosomes, with 4 chromatids/DNA copies)..

*MORULA...After Morula develops Blastocele(Central cavity) and divides further it produces BLASTOCYTS on Day 5. ...

*Blastocyts is formed on day 5 however it is Not implanted until 6-10 days(on average day 7) and till that it can't implant until it gets rid of Zona pellucida....*After Blastocyst implants on uterus ,it begins to synthesize hCG by SYNCITIOtrophoblasts(Which were cells that helped in implantation too), which is first detected in Blood/urine 8-14 days after fertilization(needs time to reach detectable levels)...

*Bilaminar disk is formed within 2 weeks after fertilization, this disk is formed by INNER cell mass and this is also time when Yolk sac(by hypoblast) and amniotic cavity(By Epiblast) form...Note that Bilaminar disk is composed of EPIblast layer located dorsally and Hypoblast layer located Ventrally...to note mess these 2 up remember: I love to BLAST my EPI pen DORSALLY on my bottom :)Note that prechordial plate(site of future mouth) is formed by fusion of epiblast and Hypoblast)You should know that EPIBLAST is precursor to ALL 3 layers(as it gives rise to Primitive streak during gastrulation-within 3 weeks.)

Notochord arises from Midline MESOderm....vs*Neural Plate is derived from ENDOderm that overlies the Notochord..and they are BOTH formed within 3 weeks from fertilization...vsNeural TUBE is formed from NEUROECTODERM after 3 weeks and is closed by week 4...

*THEY LOVE This question...answer is EMBRYONIC period(WEEKS 3-8)<BETWEEN week 3-8, most common WRONG answer is DURING week 3 and before, and that is incorrect because if teratogen/exposure happened BEFORE Embryonic period you would get "ALL or NONE" phenomenon, basically there would be spontaneous abortion or baby would be born normal....

*Even though heart First begins to beat in week 4(also time when Limb buds also begin to form)<<BOTH SUPER HYBUT we can't visualize fetal cardiac activity by transvaginal ultrasound till week 6.

*EMBRYOLOGICALLY week 10 is when genitalia first develop male/female characteristics.BUT we clinically we can only tell the difference by second trimester ultrasound(specifically weeks 16-19)

*WEEK 8.M O V E M E N T - 8 letters

1)FOLLICULAR cells of thyroid are derived from ENDOderm.vs2)PARAfollicular cells of thyroid are derived from NEURAL CREST cells....

*NOTOCHORD....Note that Notochord is itself derived from MESODERM and induces formation of Neural PLATE from overlying ECTODERM and then NEUROECTODERM cells of Neural plate give rise to Neural tube...

*Thymus is derived from ENDODERM while Lymphatics are derived from MESODERM.