In human the time it takes for the initial diploid cell to develop into a fully grown fetus ready to be delivered from the mother s uterus at parturition is normally The Horomones of Pregnancy Parturition and Lactation Introduction approximately 260 270 days cid 127 Generally divided into 3 trimesters Formation of a placenta which will act as a vital intermediary between the mother and growing fetus Provide hormones which are essential for the maintenance of normal growth and development of the fetus itself Fertilization in their containment fluid In order to successfully fertilize an ovum the spermatozoa need to have reached a certain stage of development which includes the ability to swim effectively A contraceptive method for males is tying off the vasa deferentia preventing the passage of spermatozoa into the urethra Stored spermatozoa are propelled via the ejaculatory duct into the urethra along with the fluid released from the seminal vesicles which usually represents two thirds of the total ejaculate volume cid 127 More fluid and useful molecules such as nutrients is provided by other accessory sex glands including the prostate which provides most of the remainder Semen is released into the female vagina at copulation by the process of ejaculation Contains fibrinogen and fibrinogens as well as fibrinolytic enzymes so that when it is deposited within the vagina it rapidly clots and then subsequently returns to the fluid stage as spermatozoa are released through the cervix into the uterus itself cid 127 While in the vas deferens the spermatozoa develop a limited capability to move by means of whiplash like activity of their tails activation but they are Full capability to move effectively and to fertilize an ovum are properties which are attained only within the female reproductive tract in the presence of cid 127 Of the many spermatozoa released in the ejaculate very few actually enter the uterus and survive the journey up to the Fallopian tube where the ovum is unlikely to be able to fertilize an ovum estrogens capacitation usually to be found after ovulation In the presence of the ovum the first spermatozoon to reach it binds to a glycoprotein called ZP3 on the zona pellucida membrane around the ovum In the presence of progesterone calcium ions enter the spermatozoon head and the inner surface of the membrane and the large enzyme containing vesicle called the acrosome become exposed to the exterior Another binding size on the inner membrane of the spermatozoon head binds to ZP2 on the zona pellucida surface and the spermatozoon penetrates through the membrane by the simultaneous release of hyaluronase and other proteolytic enzymes from the ruptured acrosome which digest their way through the zone allowing it to reach the ovum surface cid 127 Ovum immediately undergoes a zonal reaction during which the second polar body is expelled The genetic material from the spermatozoon now enters the ovum and a new diploid cell is produced Implantation and creation of the placenta Fertilization usually takes place in a Fallopian tube Initially the cells divide however when the number of cells has reached approximately 16 they compact to form a spherical morula Begin to specialize into an inner cell mass and an outer layer of trophoblast cells surrounding a fluid filled blastocoel From initial formation to the formation of the blastocyst the group of developing cells is called the conceptus and it remains surrounded by the zona pellucida As the morula becomes a blastocyst it moves from the Fallopian tube into the uterus The inner cell mass develops into the embryo The trophoblast cells become extra embryonic tissue ultimately forming part of the placenta In humans implantation is invasive meaning that it will penetrate into the endometrium where subsequent development takes place Attachment phase when the trophoblast cells attach to the endometrial surface Zona pellucida is removed by digestive enzymes secreted by the trophoblast cells and or the endometrial glands Attachment requires a particular endocrine environment of estrogens superimposed upon the dominant progestogen background cid 127 One key molecule involved in bringing about the apposition of the outer trophoblast cells with the epithelial cells lining the endometrium is the cytokine glycoprotein Leukemia Inhibitory Factor LIF Promotes general receptivity of the endometrial lining to the approaching blastocyst and without it implantation does not occur cid 127 More localized attachment process is brought about by various molecules including the estrogen stimulated release of heparin bound epidermal growth factor HB EGF the cytokine IL 11 and leptin cid 127 Within hours of the outer trophoblast cells touching the endometrial lining they begin to infiltrate the epithelial cells and penetrate the stroma Invading trophoblast cells lose their membranes and interconnect forming a syncytiotrophoblast The decidua becomes a source of nutrients for the now implanting blastocyst in a process called decidualization The epithelial cell layer gradually re forms over the implanting blastocyst as it invades the endometrium Hormones of pregnancy Human chorionic gonadotrophin hCG Produced by the implanting syncytiotrophoblast cells cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Can be detected by the 10th day of pregnancy despite the now diminished maternal LH Estrogens precursor molecules Binds to the LH receptors on the luteal cells in the corpus luteum and thereby stimulates these cells to continue synthesizing estradiol and progesterone Concentration in the maternal blood increases to reach maximum levels around the 10th to 12th week of pregnancy after which levels decline From the sixth week of pregnancy the placenta is the source of most of the estrogens being produced but both mother and fetus provide the necessary The largest quantity of estrogen produced is the less potent estriol cid 127 Measurement of urinary estrogen excretion levels can be a useful indicator of fetal well being Estriol precursors are provided by the fetus Vital for the normal growth and development of the fetus throughout the pregnancy and necessary for preparing the mammary glands by stimulating their growth and development and the smooth muscle of the myometrial lining