Animal Development
In sexually reproducing animals the first step in animal development is fertilization ( the union of male and female gametes). The protective layers of the egg include the jelly layer and the zona pellucida in mammals. Sperm’s acrosome contains a digestive enzyme that enables the sperm to break through the eggs’ cell membrane. Membrane fusion permist the sperm nucleus to enter directly into the egg’s cytoplasm. During egg activation there is an increase in the levels of free intracellular Ca2+ ions in the egg shortly after the sperm makes contact with the egg’s plasma membrane. When the sperm enters the egg it triggers the egg to complete meiosis, a cytoplasmic rearrangement, and causes a sharp increase in protein synthesis and metabolic activity in general. During the final stage of fertilization, the fusion of nuclei, the haploid sperm and the haploid egg cell nuclei fuse to form a diploid nucleus of a zygote. Mammalian eggs contain very little yolk. The blastocyst is made up of three layers the trophoblast: which is the outer later which contributes to the placenta, the blastocoel: which is the central fluid filled cavity and the inner cell mass: located at one pole and forms the developing embryo. Gastrulation is a process involving a complex series of cell shape changes and cell movements that occurs in the blastula. It establishes the basic body plan and creates the three primary germ layers. Extoderm ( exterior), Mesoderm (middle), Endoderm ( inner). Cells move during gastrulation using a variety of cell shape changes. Cells that are tightly attached will move as cell sheets. Invagination ( cells sheet dent inward), involution (cell sheets rolls inwards) delamination ( cell sheets split in two) ingressions ( cells break away and immigrate as individuals). Certain animals had to adapt to living on dry land, this is when the development of amniotic species occurred. The fluid protects and nourished the developing organism. Organogensis is the formation of organs in their proper locations. In vertebrates, this begins with the formation of the notochord and the dorsal nerve cord. The notochord forms from the mesoderm, this will become the brain and the spinal cord. Neurulation occurs in all chordates, in vertebrate it is accompanied by an additional step. Just before the neural groove closes to form the neural tube it pinches off forming a small cluster of cells called the neural crest cells.
Human Development
Human development takes about nine months, and is divided into trimesters. The first month the zygote undergoes its first cleavage about 30 hours after fertilization, by the time the embryo reaches the uterus 6-7 days after fertilization. Gastrulation occurs in the second week, neurulation occurs in the third week and organogenesis occurs in the fourth week, and the embryo is 5mm in length. By the second month the embryo is about 25mm and weighs 1gram. In the ninth week the transition is made from embryo to fetus. In the second trimester the basic body plan develops further, bones actively enlarge in fourth month. The fetus is about 300mm long and weighs 600g. Finally in the third trimester the organs mature and fetus doubles in weight. The brain continues to develop and produce neurons for months after birth. After birth prolactin stimulate the mammary alveoli to produce milk. The first milk produced after birth colostrum, is rich in nutrients and maternal antibodies. Growth of the infant continues rapidly after birth, the weight typically doubles within the first two months.