|MadSci Network: Development|
By Human Development, I assume that you want to know how a human being is formed, from fertilization to birth. Human development is usually measured in relation to the day of fertilization, and in trimesters. The first trimester, as you might know, is the most crucial one in that is when the foundation for a healthy development is established. Any perturbations to the embryo during this period have major repercussions during later stages. Roughly, the first trimester of human development can be divided into four major events: Pre-implantation, Implantation, Organogenesis and Fetal period. Before implantation, during the first week, the zygote divides and becomes a solid mass of cells surrounding cavity, and is known as a Blastocyst. Even at this early stage, this mass of cells is organized! There are two distinct layers of cells: the Trophoblast and the Inner Cell Mass. The Trophoblast gives rise to the embryonic portion of the placenta and are necessary for the implantation of the embryo to the uterine wall. The Inner Cell Mass gives rise to the embryo and amniotic structures. During this first week, the embryo migrates along the uterine tube, and approximately 6 days after fertilization, the blastocyst begins to attach itself to the lining of the uterus (also knows as the endometrium) in a process known as Implantation. It is also during this first week that fraternal (=identical) twins can be formed. Implantation is basically the burrowing of the embryo into the wall of the uterus. The uterus, at this time, has been prepared by hormones (mainly Estradiol) and is at its softest and most receptive. The embryo “hatches” from the Zona Pellucida (a layer that protected the Blastocyst during its migration, and prevented its implantation at the wrong spot), and with the help of protein-digesting enzymes buries itself into maternal tissue. The Trophoblast is responsible for the continuous invasion of the embryo into the uterus. A primitive analogy of the implantation process would be of a ball sinking into a thick blanket, with the trophoblast “pulling” the ball into the blanket to completely cover itself. By 10-12 days after fertilization, the embryo is completely inside the endometrium. As implantation occurs, the embryo is undergoing profound changes. The Inner cell mass starts to differentiate into the three germ layers: Ectoderm (outer layer), Mesoderm (middle layer) and Endoderm (inner layer). These three layers will give rise to the different organs in the body. Organogenesis (literally, formation of organs) occurs between 3 and 4 weeks after fertilization. And is responsible for the appearance of several organs. The three germ layers interact and influence each other’s development. A primitive streak forms, which will give rise to the Notochord, a cellular rod that runs longitudinally along the embryo, below the future brain/spinal chord. Also, the heart starts to take shape. At 4 and half weeks, the embryo is about 5mm in length, the head is large and distinct, and limb buds have begun to appear (where the arms and legs will form). At 5 weeks the embryo is about 10mm in length and the face is taking shape. At the end of 8 weeks, the embryo is about 30mm in size, and already shows human features, with a neck, eyelids and long limbs with toes on the feet. During the remainder of the pregnancy (Fetal Period) the embryo continues to develop and grow bigger. Nails and hair form, breathing movement can be detected, some sucking movements as well. At the second trimester, the mother can usually feel fetal movement (Quickening), and during the 3rd trimester, there is an increase in fat and the skin is no longer wrinkled. This is obviously a very simple account of what happens during human development. If you wish to explore further any particular issue, try the following websites: http://embryo.soad.umich.edu/index.html (has good images of early fetuses) http://www.med.unc.edu/embryo_images/unit-welcome/welcome_htms/akgs.htm (has more detailed description of the stages described here)
Try the links in the MadSci Library for more information on Development.