You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
plug date was considered e0.5 and the age of individual embryos confirmed by measuring the crown - rump length and checking for developmental landmarks such as digits and eye formation .
3 ) knockout ( / ) , heterozygotes ( + / ) and wild type ( + /+ ) littermate embryos were generated by crossing pet-1 males and females .
the pet-1 and sert knockout lines have been backcrossed on the c57bl/6j background for > 10 generations .
all procedures using mice were approved by the institutional animal care and use committee at university of southern california and conformed to nih guidelines . unless otherwise noted , all reagents and antibodies were purchased from sigma ( usa ) .
protocols for hplc measure , tissue staining , in vitro 5-ht synthesis assays , maoa enzymatic activity assays , in vivo synthesis inhibition assays and ex vivo placental perfusions are described in supplementary methods .
normal human placental villi samples were obtained from women undergoing elective pregnancy terminations at the reproductive options clinic at the los angeles county ( lac ) + university of southern california ( usc ) medical center . for the purposes of this study ,
the tissue was carefully dissected by the pathologist and immediately flash - frozen in liquid nitrogen .
plug date was considered e0.5 and the age of individual embryos confirmed by measuring the crown - rump length and checking for developmental landmarks such as digits and eye formation .
3 ) knockout ( / ) , heterozygotes ( + / ) and wild type ( + /+ ) littermate embryos were generated by crossing pet-1 males and females .
the pet-1 and sert knockout lines have been backcrossed on the c57bl/6j background for > 10 generations .
all procedures using mice were approved by the institutional animal care and use committee at university of southern california and conformed to nih guidelines . unless otherwise noted , all reagents and antibodies were purchased from sigma ( usa ) .
protocols for hplc measure , tissue staining , in vitro 5-ht synthesis assays , maoa enzymatic activity assays , in vivo synthesis inhibition assays and ex vivo placental perfusions are described in supplementary methods .
normal human placental villi samples were obtained from women undergoing elective pregnancy terminations at the reproductive options clinic at the los angeles county ( lac ) + university of southern california ( usc ) medical center . for the purposes of this study ,
the tissue was carefully dissected by the pathologist and immediately flash - frozen in liquid nitrogen .
serotonin ( 5-hydroxytryptamine ; 5-ht ) is thought to regulate neurodevelopmental processes through maternal - fetal interactions that have long - term mental health implications . 
 dogma states that beyond fetal 5-ht neurons , there are significant maternal contributions to fetal 5-ht during pregnancy1,2 , but this has not been tested empirically . to examine putative central and peripheral sources of embryonic brain 5-ht 
 , we used the pet-1/ mice in which most dorsal raphe ( dr ) neurons lack 5-ht3 
 . measures of 5-ht revealed previously unknown differences in accumulation between the fore- and hindbrain during early and late fetal stages , through an exogenous source of 5-ht . we show that this source is not of maternal origin . using additional genetic strategies , a new technology for studying placental biology ex vivo , and direct manipulation of placental neosynthesis 
 , we investigated the nature of this exogenous source and uncovered a placental 5-ht synthetic pathway from a maternal tryptophan precursor , in both mice and humans . 
 this study reveals a new , direct role for placental metabolic pathways in modulating fetal brain development and implicates novel maternal - placental - fetal interactions that could underlie the pronounced impact of 5-ht on long - lasting mental health outcomes .