Transcript for:
Video-Overview of Human Prenatal Development

Human prenatal development can be  divided into 3 stages: pre-embryonic,   embryonic, and fetal development. Fertilization usually occurs in the   ampulla of fallopian tube. The fertilized egg,  referred to as the conceptus at this stage,   immediately travels toward the  uterus. During the journey,   it undergoes several mitotic cell divisions,  producing daughter cells, called blastomeres.   This process is known as cleavage, because  the cells divide without growing in volume.  After about 3 days, the  conceptus arrives in the uterus:   it now contains about 16 cells and is called  a morula. Cells of the morula are totipotent,   they are capable of differentiating into all  cell types, both embryonic and extra-embryonic.  The morula continues to divide while floating  freely in the uterus for several more days.   During this time, it consumes nutrients  stored in the egg cytoplasm, and “uterine   milk” secreted by the endometrium. When there are about 100 cells,   the cells start to arrange themselves around  a fluid-filled cavity, forming a blastocyst.  The blastocyst consists of a pluripotent  inner cell mass, called the embryoblast,   destined to be the embryo; and an outer shell,  called the trophoblast, which nourishes the   embryo. The trophoblast later becomes the  chorion - the fetal portion of the placenta.  The trophoblast secretes an enzyme that dissolves  the membrane surrounding the conceptus, enabling   it to “hatch”, ready to be implanted. At the end of the first week,   the blastocyst becomes attached to the  endometrium. At the site of contact,   superficial cells of the trophoblast fuse together  to form the syncytiotrophoblast, which grows into   the endometrium and derives nutrition from  it. The endometrium responds by growing over   and eventually enveloping the blastocyst. The syncytiotrophoblast secretes the hormone   named “human chorionic gonadotropin”, HCG,  to instruct the corpus luteum to continue the   production of progesterone. Progesterone  is the hormone that maintains pregnancy.   Among its many functions, progesterone  stimulates the growth of nutrient-rich   decidual cells to feed the early embryo. Around the middle of week 2, the embryoblast   starts to form a two-layered embryonic disc 

• hypoblast and epiblast, at the mid-plane   of blastocyst, with a cavity on either side. This  bilaminar disc sets the dorsal/ventral axis of the   embryo, with the epiblast being the dorsal side. By the end of second week, the 2 layers grow   around the cavity on their respective side and  form the yolk sac and amnion, respectively.  The yolk sac provides nutrients absorbed from  the trophoblast, as well as gas exchange for   the embryo during weeks 2 and 3, before the  placenta takes over these functions at week   4. It is also the first site of embryonic  blood cell production, before the liver.  In week 3, the two-layered disc  transforms into 3 germ layers,   through the process of gastrulation, during which  the cells lose their pluripotency and become   committed to a smaller number of cell types. Gastrulation begins with the emergence of the   primitive streak on epiblast surface.  This event sets the cranial/caudal,   or head/tail axis, of the embryo. A so-called primitive node at the cranial   end of primitive streak produces signaling  factors that control the movements of cells.  Epiblast cells migrate toward and through  the primitive streak into the embryo. The   first set of cells displaces the hypoblast and  forms the endoderm. The second set of cells   fills in the space between the endoderm  and epiblast, creating the mesoderm. The   remaining epiblast cells form the ectoderm. Each of the 3 germ layers differentiates to   become different organs and tissues. All organ  systems are present in their primitive form by the   end of the embryonic period, but most of them are  not yet functional, or only partially functional.  The yolk sac gives rise to the  allantois, a primitive urinary   bladder and part of the umbilical cord. The amnion grows to surround the embryo   by week 4. It is filled with amniotic fluid which  protects the embryo from trauma and temperature   fluctuations. The fluid also allows the fetus to  move freely and prevents adhesion of body parts.  The yolk sac, amnion, allantois and  chorion are the 4 embryonic membranes.