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Organization of the Human Body

The organization of the Human body can be broken down into 6 levels of structural organization:

1. Chemical level: This is where atoms form molecules. The four molecules of life are proteins (catalyze chemical reactions), lipids (form cell membrane), carbohydrates (supply fuel) and DNA & RNA (responsible for the genetic code). Molecules form organelles, which are the basic components of cells.
2. Cellular level: Cells are the basis of all living things. Cells vary in function, shape & size, but have the same basic parts, such as the plasma membrane, cytoplasm, organelles, and the nucleus. DNA replication allows cells to divide which then allows for body growth and tissue repair.
3. Tissue level: Similar cells with a common function group together to form tissues. Each of the four basic tissue types has a specific role in the body. The epithelium tissue covers the body surface and lines its cavities, muscle tissue provides movement, connective tissue supports and protects organs and nervous tissue provides internal communication.
4. Organ level: When 2 or more tissue types combine to form a structure that has a specific function in the body.
5. Organ system: This is made up of organs and structures that work together to perform a specific function. Organ systems of the body include cardiovascular, integumentary, skeletal, muscular, nervous, endocrine, respiratory, digestive, lymphatic, urinary and reproductive.
6. Organism level: When the entire body is working together to promote life.

Fetal Development
The gestation period of a fetus is about 280 days. The following outlines the steps of fetal development.

Fertilization
1. Sperm must reach the ovulated secondary oocyte and must undergo capicitation. Hundreds of sperm must also undergo the acrosomal reaction to expose the oocyte membrane so that one sperm can make contact.
2. Polyspermy is prevented in humans by Na+ diffusing into the oocyte after one sperm enters. This causes depolarization of the membrane. The cortical reaction also stops more sperm from entering by destroying sperm receptors.
3. The sperm loses its tail and midpiece. The ovum and sperm pronuclei rupture and release their chromosomes near the mitotic spindle. Fertilization occurs as these chromosome combine and form a zygote (a fertilized egg).
Preembryonic Development
1. After fertilization, cleavage produces two identical cells called blastomeres. These continue to divide and produce more cells until it reaches about 100 cells. Now it is called a blastocyst.
2. The blastocyst has a single layer of flattened cells, called trophoblast cells, which eventually form the placenta. It also has a small cluster of rounded cells, called the inner cell mass, which becomes the embryonic disc and then later becomes the embryo.
Implantation
1. The blastocyst implants into the endometrium about 6 to 7 days after ovulation. It is covered over and sealed off from the uterine cavity.
Placentation
1. The placenta is formed from the trophoblastic and maternal endometrial tissues.
2. The trophoblast gives rise to a layer of extra embryonic mesoderm on its inner surface, this becomes the chorion. The chorion develops chorionic villi which become the umbilical arteries and vein.
3. The placenta is a temporary organ and it detaches and sloughs off after the infant is born.
Embryonic Development
1. The blastocyst is a 2 layered flat sheet of germ cells (also called stem cells).
2. The embryonic membranes formed during the first 2 to 3 weeks include the amnion (which becomes filled with amniotic fluid), the yolk sac (which forms part of the digestive tube, produces the earliest blood cells and blood vessels, and seeds the gonads), the allantios (which becomes the umbilical cord) and the chorion (which helps form the placenta).
Gastrulation
1. During week 3, the 2 layered embryonic disc becomes a 3-layered embryo. The 3 layers are the primary germ layers, the ectoderm, endoderm and mesoderm.
2. Formation of the primitive streak, a raised groove on the dorsal surface of the embryonic disc, starts gastrulation.
3. The notochord forms the first axial support of the embryo and later it becomes the vertebral column.
Organogenesis
Formation of the body organs and organ systems.
a. Specialization of the Ectoderm
1. Neurulation-formation of neural tube, which eventually becomes the brain and spinal cord.
2. The ectoderm will eventually form the structures of the nervous system and the skin epidermis.
b. Specialization of the Endoderm
1. Flat plate of cells folds to form a cylindrical body, with two tubes. The inner tube is the endoderm tube, which is the primitive gut. It will eventually form the lining of the gastrointestinal tract.
c. Specialization of Mesoderm
1. Somites are formed to produce vertebra, ribs, dermis of the skin on the dorsal side, and the skeletal muscles and muscles of the limbs.
2. Intermediate mesoderm forms the gonads and kidneys.
3. Lateral mesoderm forms the dermis of the skin on the ventral part of the body and the parietal serosa and visceral serosa. The celum is the part in between the parietal and visceral serosa. It produces bones, ligaments and dermis of limbs.
4. Sphlanchnic mesoderm forms the heart and blood vessels and connective tissues. It also forms the entire wall of the digestive and respiratory organs.
Fetal Circulation
1. The cardiovascular system is the groundwork for the fetal circulatory system.
2. Endothelial cells form vascular networks, which later become the heart, blood vessels, and lymphatics.
3. The umbilical arteries, ductus venosus, foramen ovale and ductus arteriosus are only present during fetal development and are occluded at birth.