【正文】 ves a combination of passive （被動的）movements rounds of division, so that the population of cells colonizing the gonads is larger than the population that started the journey.,第七十七頁，共一百零九頁。,2. Germcell migration In Drosophila During gastrulation, the Drosophila pole cells are passively moved from their peripheral 外周的origin to the posterior后面(h242。u mian)的 midgut( 中腸） inside the embryo .The cells then actively migrate through the gut wall and into the adjacent mesoderm（中胚層）, where they split into two populations and continue their journey to the gonads. Genes such as nanos are required for germ cell migration cells to the correct destination（目的地）.,第七十八頁，共一百零九頁。,3.Germcell migration in vertebrates In Xenopus and mammals, PGCs move from their site of origin to the endodermal cells of the hindgut( 后腸） during gastrulation. They then migrate along the hindgut, across the dorsal mesentery（腸系膜） to the lateral plate mesoderm of the abdominal（腹的） body wall, where they split into two populations and move across to the paired gonadal ridges（生殖(shēngzh237。)嵴）. Migration may involve a polarized network of fibronectin（纖連蛋白） molecules in the extracellular matrix, and factors secreted by cells along the migratory route stimulate the PGCs to proliferate. In reptiles（爬行類） and birds, germ cells are transported to the gonadal ridges by a completely different route involving the vascular system, but migration from the capillarites（毛細血管） into the gonadal ridge may also involve chemoattraction（化學吸引）.,第七十九頁，共一百零九頁。,Spermatogenesis in mammals Diploid germ cells arriving at the embryonic testis are arrested in the mitotic cell cycle until the animal is born. Now known as spermatogonia(精原細胞）, the cells proliferate to form a population of stem cells that continuously generate primary spermatocytes（精母細胞） committed to enter meiosis. Meiosis produces four cells termed spermatocytes committed to enter meiosis. Meiosis produces four cells termed spermatids（精子細胞）. These undergo extensive differentiation, involving nuclear condensation, ejection of the cytoplasm, and formation of the acrosomal vesicle and flagellum, to form functional sperm. Mammalian specmatogenesis is representative of most of the animal kingdom.,第八十頁，共一百零九頁。,Oogenesis in mammals,Dipoid germ cells arriving at the embryonic ovary(oogonia卵原細胞) divide several times by mitosis and then many die, the remainder entering meiosis as primary oocytes（卵母細胞）. Meiosis arrests at prophase前期(qi225。nqī) I until sexal maturity, when periodic ovulation（排卵） begins. Groups of oocytes are then stimulated to continue meiosis and mature, and eventually one is ovulated as a secondary oocyte, arrested at prophase II. This block is removed at fertilization . oogenic生卵的 meiosis is asymmetrical, producing only one egg (the other products being discarded as polar bodies). oogenesis in mammals is relatively specialized, and different strategies are used in other species.,第八十一頁，共一百零九頁。,Comparison of oogenesis and spermatogenesis,Sperm and eggs are haploid gametes that unite at fertilization to generate a diploid zygote. Their development form diploid primordial germ cells has certain features in common indicating homology (e.g. the requirement for meiosis and the derivation of specialized secretory organelles（細胞器） from the Golgi apparatus).There are also major differences, including the asymmetry and interrupted nature of oogenic meiosis, and differences in the timing of maturation.,第八十二頁，共一百零九頁。,人類(r233。nl232。i)生命周期 Life cycle of humans,第八十三頁，共一百零九頁。,第八十四頁，共一百零九頁。,第八十五頁，共一百零九頁。,第八十六頁，共一百零九頁。,第八十七頁，共一百零九頁。,第八十八頁，共一百零九頁。,第八十九頁，共一百零九頁。,Spermatogenesis,Creating sperm from spermatogonia精原細胞 Consists of meiosis + spermiogenesis Meiosis begins with the replication of the DNA within a spermatogonium精原細胞. This gives us a primary spermatocyte. It now has 2 sets of 46 chromosomes. The 1? spermatocyte divides to yield 2 secondary spermatocytes. Each with 23 chromosomes. The 2? spermatocytes divide again to give us a total of 4 spermatids, each with 23 chromosomes (i.e., each is haploid) The process by which the spermatid matures and differentiates into a sperm is spermiogenesis,Spermatogonium,Primary Spermatocyte,Secondary Spermatocyte,Secondary Spermatocyte,Spermatid,Spermatid,Spermatid,Spermatid,第九十頁，共一百零九頁。,精子發(fā)生,精子形成,第九十一頁，共一百零九頁。,第九十二頁，共一百零九頁。,cohort股,一群(yī qn) synchronously同步地,第九十三頁，共一百零九頁。,第九十四頁，共一百零九頁。,第九十五頁，共一百零九頁。,第九十六頁，共一百零九頁。,第九十七頁，共一百零九頁。,第九十八頁，共一百零九頁。,第九十九頁，共一百零九頁。,第一百頁，共一百零九頁。,第一百零一頁，共一百零九頁。,altruism利他行為,第一百零二頁，共一百零九頁。,disentangle松開(sōnɡ kāi),第一百零三頁，共一百零九頁。,多態(tài)現(xiàn)象,神風(sh233。n fēnɡ),連續(xù)(li225。nx249。),忠誠(zhōngch233。ng),proportion 比例confidence置信度 fidelity保真度,第一百零四頁，共一百零九頁。,第一百零五頁，共一百零九頁。,第一百零六頁，共一百零九頁。,Sperm,Highly conserved and differentiated cell. Consists of: Head: Contains the nucleus and the acrosome – organelle filled with enzymes. Midpiece: Chock full of mitochondria Tail: Contains the flagellum,第一百零七頁，共一百零九頁。,第一百零八頁，共一百零九頁。,內(nèi)容(n232。ir243。ng)總結(jié),Chapter 2。納耶里納表示：“我 們測試用這種方式(fāngsh236。)制造出的男性和女性精子。Consists of:,第一百零九頁，共一百零