Before copulation, physiological and psychological stimuli are needed to best prepare the body, especially the sex organs, for coitus. These involve various courtship rituals, like flirting and foreplay in human. All these stimuli leads to the excitement phase and the plateau phase, when vasoconstiction and myotonia occur, and finally to the orgasm such that the sperms enter the vagina.
An ovum starts its journey from the ovary, through the Fallopian tube, finally to the uterus. The epithelium of fimbriae at the end of Fallopian tube are lined with cilia. These, together with fimbriae muscle contraction, move the ovum into the Fallopian tube as it emerges from the ovarian surface. When it reaches the Fallopian tube, it is propelled rapidly by ciliary beat and muscle contraction. The muscular contraction soon diminishes and the ovum is only moved by cilia. The movement is so slow and it takes 4 days to reach the uterus.
After ejaculation, sperms struggle up to the uterus at a fast rate to fertilize the awaiting ovum. They are moved, by the fluid pressure of the ejaculate, out of the vagina, through the cervix and into the uterus. Their motion is also assisted by the beating action of cilia on the inner surface of the cervix. The cervix mucus becomes thinner to allow the passage of the swimming sperms. When the sperms are in the uterus, they mainly move by their tails. However, the beating of uterine cilia and uterine muscle contraction also play a part. During the journey of the sperms, many of them are killed by the acidic medium of the vagina before reaching the Fallopian tube. Only the best few hundreds out of the several hundred million sperms are able to reach the tube. However, a sperm is not able to fertilize an ovum before it undergoes capacitation which occurs a few hours after a sperm enters the female reproductive tract. This enables the sperm to propel more strongly with a whiplike movement and its plasma membrane undergo changes to fuse with the surface membrane of the ovum.
The union of sperm and ovum, known as fertilization, is the first step of embryo development. The sperm receptors on a region called zona pellucidea of an ovum response to a number of incoming sperms and bind with the proteins in the plasma membrane of the sperm heads. This binding is followed by acrosome reactions in the sperms when the acrosomal enzymes, originally being bound inside the plasma membrane, become exposed to the zona pellucida. The sperms penetrate the zona with the help of the digestive enzymes. The first sperm that moves through the entire zona and reaches the plasma membrane of the ovum fuses with this membrane. The contractile activities of the ovum help the sperm to penetrate into the cytoplasm of the ovum. This fertilized ovum is now termed a zygote. The fusion of the ovum and the sperm causes the secretory vesicles located around the periphery of the ovum to release their contents by exocytosis into the space between the zona pellucida and the plasma membrane. These consist of enzymes which inactivate the sperm-binding sites on and cause hardening of the entire zona. As a result, no more sperms are able to penetrate to the cytoplasm of the ovum.
The fertilized ovum then undergoes its second meiotic division within the next few hours. Each of the 23 chromosomes, both maternal and fraternal, are surrounded by the gametes' own membranes, which are known as pronuclei, then migrate to the center of the cell. The DNA of the chromosomes in the pronuclei then undergo replication and the pronuclear membranes are broken down. Meiosis then takes place in the cell and fertilization follows. The ovum enzymes are activated during fertilization which trigger off mitosis to ensure embryogenesis.
It is interesting to note that recent research has shown that sperm carrying an X chromosome can survive longer than one carrying a Y chromosome. Therefore, if intercourse is carried out a few days before ovulation, the chances of having a girl may be increased. At the same time, sperm carrying a Y chromosome is expected to swim faster than an X sperm. As a result, having intercourse exactly during ovulation can increase the chance of perceiving a boy. Sex can somehow be determined in this sense.
After fertilization, the tiny unicellular zygote starts to multiply in the Fallopian tube by mitotic division, a process known as cleavage. However, cell growth does not occur before each division. The cell conceptus reaching the uterus is thus having the same size as the original fertilized ovum. If during cleavage, the dividing cells are separately completely into two or even more growing masses, identical twins will result.
The conceptus finally reaches the uterus, where further mitosis takes place, and it develops into a blastocyst which consists of a trophoblast layer, an inner cell mass which later develops into the embryo with a fluid-filled cavity. At the same time, the corpus luteum secretes estrogen and progesterone , leading to thickening of uterine wall to receive the blastocyst. The sticky trophoblast not only is responsible for supplying nutrients and secreting certain hormones during the growth of the embryo, in the first two months after fertilization, and later the fetus, but also helps the blastocyst to adhere and embed into the endometrium, known as implantation.
The contact between blastocyst and endometrium induces the proliferation of the troploblast to secrete proteolytic enzymes to promote embedding of the blastocyst into the endometium. Under the effect of certain hormones, the endometrium also becomes more vascularized and permeable. The composition of the endometium also changes which further enhances implantation. When implantation completes, the nutrient-rich endometrium is responsible for providing energy for embryo development for a few weeks.
However, the endometrium nutritive system is insufficient to provide energy for further embryo growth and the placenta soon, after about five weeks of implantation when the fetal heart is able to pump blood, takes over. Placenta is a combination of fetal and maternal tissues serving as a medium of exchange between the mother and the fetus for the remaining pregnancy. Chorion, the outermost layer of trophoblast, where villi are present and projected into the endometrium, gives rise to the fetal part of placenta, whereas the maternal part is from the endometrium underlying the chorion. Inside the villi is a network of capillaries linking to the embryo's circulatory system and all villi are completely immersed in maternal blood. Blood flows from the mother to the placenta through the uterine artery and exits through the uterine veins. At the same time, fetal blood flows to the mother via the capillaries in the chorionic villi and then out to the umbilical arteries and back to the fetus through the umbilical veins and then the capillaries. The umbilical vessels are contained in a rope-like structure called umbilical cord which links the fetus to the placenta. Apart from nutrients, many other materials are exchanged between the mother and the fetus via the placenta. Excretion of the fetus takes place when wastes are being transported to the maternal blood via the chorionic capillaries, following just the reverse path of nutrients. For gaseous exchange, oxygen and carbon dioxide move by simple diffusion across the placenta lining. As there is such a huge connection between the mother and the fetus, it must be strongly emphasized that a pregnant woman should never take alcohol, cigarette or any drug without prescription since all toxic substances can be easily passed onto the fetus and seriously damage its health and growth.
As the fetus is growing, a space is developed between the inner cell mass and the trophoblast, which is better known as amniotic cavity. The epithelial lining of this cavity coming from the inner cell mass called the amnion will later fuses with inner chorion surface. Fluid present in the amniotic cavity, known as amniotic fluid, protects the fetus from mechanical disturbances and temperature variations. The fetus continues to develop during its next eight months in the amniotic cavity until it is ready to be born.
About 37 weeks following fertilization, the fetus is ready to arrive to the world. The delivery of infant and then the afterbirth is brought about by vigorous and rhythmical muscle contractions of the myometrium.
During the last few weeks of pregnancy, the cervix becomes soft and flexible due to the activities of estrogen and prostaglandins. The softening of cervix, also known as ripening, brings about cervix dilation during labor so that the infant can pass through more easily.
When the fetus is about to come out of the mother's womb, the amniotic membrane ruptures and the amniotic fluid leaks out through the vagina to mark the beginning of labor. The uterine contractions then become more intense and regular, occurring at 10-15 minute intervals. The contractions start in the upper part of the uterus and sweep downward.
The contractions gradually intensify and become more frequent, forcing open the cervical canal to a maximum diameter of 10cm. These waves of contractions move the fetus through the cervix and vagina. Finally, the fetus is forced out by the abdominal pressure. Within a few minutes of delivery, the umbilical cord and placenta lose their function and are expelled from the uterine wall by uterine contractions. The detached placenta is termed as afterbirth.
After the baby is born, it has to be nourished by its mother to sustain life. Milk, secreted by mammary glands of the mother, provides all the energy for her baby. Milk contains four major components, namely water, protein, lipid and lactose, a carbohydrate. These nutrients are extracted from the maternal blood in the simplest forms, monomers, by the alveolar cells inn the breasts. A hormone named prolactin, together with growth hormones, insulin and some other hormones, then help to synthesize those polymers.
Ducts present in the breasts branch all through the tissue and converge at the nipple. The ducts arise from breast alveoli, the sites of milk production. Alveoli and ducts are surrounded by myoepithelial cells which are contractile.
During pregnancy, increased concentrations of plasma estrogen, progesterone, prolactin and placental lactogen lead to marked breast enlargement. Under the influence of these hormones, the ductal and alveolar structures are fully developed. Although prolactin level increases under the stimulation of estrogen, no milk is secreted. It is because the high levels of estrogen and progesterone suppress the action of prolactin on milk production. Therefore, the overall effect of the above two hormones during pregnancy is just breast enlargement together with ductal and alveolar development. Delivery removes the source of these two hormones, the placenta, and thus induces milk secretion. Upon delivery, the basal prolactin level ceases together with estrogen level. However, despite the falling basal prolactin level after delivery, large secretory bursts of prolactin are induced by nursing to maintain milk production. The episodic pulses of prolactin become signals to the breasts for milk secretion.
The suckling of milk stimulates the nipple receptor. This leads to a neural input to the hypothalamus which stimulates the posterior pituitary neuron to secrete prolactin releasing factor. Suckling also triggers off release of oxytocin which is responsible for contraction of the myoepithelial cells surrounding the alveoli. This contraction is essential to drain milk from the alveoli into the ducts to make nursing possible. This reflex action is known as milk let-down.
Breast-feeding is advocated for infants between 3-6 month old. This is because breast milk is known to contain antibodies and all sorts of nutrients essential for infant growth and prevent infant from infection. Moreover, this practice is economical, reduces the risk of gastrointestinal infections and helps to boost the mother-infant relationship. However, one of the shortcomings of the practice is that it may transmit some pathogens.