The zygote starts its journey to develop into a complete organism
The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. The single celled or fertilized egg is transformed into a baby with trillions of differentiated cells organized into specialized tissues and organs. Human reproduction starts with union of gametes, which further develops into embryo.
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus. Embryogenesis always starts with the fertilization of the ovum (or egg) by sperm. The fertilized ovum is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. The single celled or fertilized egg is transformed into a baby with trillions of differentiated cells organized into specialized tissues and organs. An organism's development is determined by the genome of the zygote and also by differences in early embryonic cells, which set the stage for the expression of different genes in different cells. As cell division continues and the embryo develops, mechanisms such as cell signaling selectively control gene expression, which leads to cell differentiation and the specialization of cells in their structure and function. Development also involves morphogenesis, the process by which an animal takes shape and the differentiated cells end up in the appropriate locations.
Stages of fetal development
Prenatal development is the process in which a human embryo or fetus gestates during pregnancy, from fertilization until birth. After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age, the precursors of all the major organs of the body have been created. This phenomenon is known as organogenesis. This continues to develop int a complete embryo with all embryonic layers. Embryonic development proceeds through cleavage, gastrulation and organogenesis (development of organs). During the first stage, called cleavage, cell division creates a hollow ball of cells, the blastula, from the zygote. The second stage, gastrulation, produces a three–layered embryo, the gastrula. The third stage, organogenesis, generates rudimentary organs from which adult structures grow.
Human gestation can be divided for convenience into three trimesters of about three months each. The first trimester is the time of most radical change for both the mother and the embryo. The endometrium responds to implantation (embryo adheres to the wall of the uterus) by growing over the blastocyst. Differentiation of the embryo's body structures now begins. During its first 2 to 4 weeks of development, the embryo obtains nutrients directly from the endometrium. Meanwhile, the outer layer of the blastocyst, called the trophoblast, grows out and mingles with the endometrium to form the placenta, a combination of maternal and embryonic tissues. Placenta transports nutrients, respiratory gases and wastes between the embryo or fetus and the mother. Maternal blood enters the placenta in arteries, flows through blood pools in the endometrium, and leaves via veins. Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein. Materials are exchanged by diffusion, active transport, and selective absorption between the fetal capillary bed and the maternal blood pools.
Birth, or parturition, is brought about by a series of strong, rhythmic uterine contractions. After birth, decreasing levels of progesterone free the anterior pituitary from negative feedback and allow prolactin secretion. Prolactin stimulates milk production after a delay of 2 or 3 days. The release of milk from the mammary glands is controlled by oxytocin.