EmbryogenesisOverview1Learning aims2Overview3Gametogenesis4Fertilization5Preimplantation6Implantation7Embryonic disk8Embryonic phase9Fetal phase10Fetal membranes and placenta11Chromosomal and gene aberrationsClick on the desiredmodule to get to itand its chapters.You can see a smallselection of themodules's illustrationsby clicking on images.Presentation of the learning aims of the individual modules of this courseor, inother words, the knowledge that the student should acquire while working through thiscourse material.Introduction to the course using the individual module summaries.Gametesand their predecessor cells (the primordial germ cells) are differentiatedfrom other somatic cells very early, and emigrate from the ectoderm (third week) viathe extraembryonic endoderm (fifth week) into the primordium of the future gonads,the gonadal ridge. There, through an interaction with the coelomic epithelial cells, theprimordium for the testis evolves in the seventh week, if a Y chromosome is present, orthe primordium for the ovary in the eighth week, if it is absent .The development of the testisoccurs under the influence of testosteroneamongother factors. This is produced by the Leydig's interstitial cells that stem from themesenchyma of the gonadal ridge, in an initial stage of activity (beginning of the 7thweek). A second surge of secretory activity of the same cells starts at puberty. Thisleads to the maturation of the gonadal epithelium and to the growth and lumenformation in the tubuli seminiferi contorti. Spermatogenesis, which takes place from
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puberty onwardsleads to a 64 day-long cyclein which the spermatogonia developinto sperm cells. At the beginning of spermatogenesis three steps of mitosisup toprimary spermatocytes type I occur before meiosiscommences. The first meiosis lasts24 days, of which the prophase, with its four typical histological phases, takes thelongest time. The secondary spermatocytes are engendered in the first meiosis andthey immediately continue with the second meiosis, which is very brief because neithera synthesis of DNA nor a new grouping of the chromosomes takes place. The resultsof the second meiosis are the haploid spermatids. Within 24 days they differentiatethemselves to become sperm cells that are then released into the lumen of the tubuli.Sperm cell production happens within innumerable temporally and spatially separatedspermatogenesis wavesthat are spread throughout the whole lengths of the tubuli,that are wound up in each other in a spiral fashion. The sperm cell production issubject to large variations with an average value of around 100 million / day. Oogenesisbegins in roughly the 7th week(stage 20). The secondary germinalcordsthat have grown into the ovarian cortex decompose into individual groups ofcells. A lively proliferationresult, whereby the oogonia, similar to the spermatogonia,remain connected with each other via cellular bridges, permitting a synchronizationofthe mitosis and the subsequent meiosis steps (prophase). As soon as these oogonia
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