Human Development

" Cleavage, it's not what you think !! "

Fertilization - Only about 100 - 2,000 make it to the egg in the fallopian tube and some as fast as 5 minutes. Once there it requires at least 7 more hours to be able to gain the ability to fertilize an egg. The window of fertilization is 3 days before to 1 day after ovulation.

Zygote - A fertilized egg

          Cleavage - The phase during which the zygote divides without the embryo growing.

Morula - The 32 cell stage we have a solid ball of cells. The morula enters the top of the uterus.

Blastula - A hollow ball of cells. As cleavage continues all the cells stay to the outside and the center becomes empty.

          Blastocyst - A hollow ball of cells in mammals. The stage that implants into the endometrium of the uterus on the 7 th day after fertilization.

Gastrulation - One end of the blastula starts to indent. A new cavity is formed which becomes the digestive tract. Cell differentiation first begins during gastrulation.

Germ layers - A gastrula forms 3 primitive germ (tissue) layers.

          Ectoderm - (outer most layer) Gives rise to central nervous system and epidermis (skin).

          Mesoderm - (middle layer) Becomes skeletal muscle, skeleton, and circulatory, and reproductive systems.

          Endoderm - (inner most layer) becomes the lining of the digestive tract.

Organogenesis - organ formation - the 3 tissue layers turn into their respective/specific organs. This is about week 3. The first organ system to develop in the embryo is the nervous system.

          Neurulation - The infolding of the ectoderm to form a groove which becomes a neural tube

                    Neural tube - Becomes the brain and spinal chord.

Human chorionic gonadotropin - hCG - After implanting the embryo secretes a hormone hCG. This keeps the corpus luteum active and keeps estrogen and progesterone levels high. This is what a pregnancy test is testing for.

Placenta - A temporary organ, fully formed about the 10th week. The placenta is a mix of embryo and mothers tissues. It provides nutrients and oxygen from the mother. It also removes wastes and produces estrogen and progesterone for pregnancy. The embryo now no longer needs the corpus luteum.

First Trimester - Organogenesis takes place and all organ systems are developing starting with the nervous system. Movement and heartbeat begin (not felt by mother).

          Teratogens - Drugs or other compounds which interfere with normal development of the fetus. The embryo is most susceptible to damage from drugs, alcohol, and cigarette smoke.

Second Trimester - Face takes on human features, eyes open, and movement is felt by the mother. At the end of the 6th month the fetus is now 10 - 12 inches long and 1.5 pounds in weight.

Third Trimester - The fetus adds some 6 - 8 inches in length and about 4 or 5 pounds of weight. It is a period of rapid growth.

Gestation period - Duration of pregnancy - in humans is 280 days (266 days from conception).

Dilation - Dilate the cervix

Expulsion - Push out the baby

          Oxytocin - Increases the contractions of the uterus. A doctor may inject more if there is difficulty with contractions not being big enough.

Placental - Cut and tie off the umbilical cord and then deliver the placenta (a temporary organ).

Fetus in Fetu - The twin survives as a parasite inside brother or sister. It has an umbilical cord like structure and leaches on to the host blood supply. 92 cases total. Mostly 1 twin inside but have seen 2 and even 5.

Zygote -->(Cleavage) -->Morula -->Blastocyst -->(Gastrulation) Gastrula -->(Organogenesis) Embryo

Return to Greg Sievert's GB 100 Syllabus
Last updated on 5 January 2018