MadSci Network: Development

Re: Can you explain the proccess/development of human life?

Date: Thu Jun 22 18:23:20 2000
Posted By: Irene Yan, Post-doc/Fellow, Vertebrate Embryology, Rockefeller University
Area of science: Development
ID: 959721319.Dv

	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: (has good images of early fetuses) 
(has more detailed description of the stages described here)

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