Stages of H um an D evelopm ent - Austin …

1 human Anatomy and Physiology: human Development; Lecture Notes, Ziser, DevelopmentDuration & Stages of PregnancyHuman gestation last an average of 266 days(38 weeks, ~9 months)is divided in 3 month intervals called trimesters1st trimester (1st 3 months: wk 1 - 12)preembryonic and embryonic developmentstress, drugs and nutritional deficiencies aremost threatening during this timesome suggest that morning sickness is correlated withthis critical period has the evolutionary advantage of making mom less likely to ingest potentially dangerous materialseg.

2 Cabbage, brussel sprouts, potatoes,overcooked meat all contain poisons that could potentially do damage to embryo;eg. coffee contains over 1,000 different toxinswomen who do not experience pregnancy sickness are significantly more likely to miscarriage2nd trimester (2nd 3 months: wk 13 - 24)fetal development beginsorgans complete most of their development3rd trimester (3rd 3 months: wk 25 - birth) human Anatomy and Physiology: human Development; Lecture Notes, Ziser, development is completedtremendous increase in size of fetusHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, of human Development0.

3 Fertilizationa. zygote1. Preembryonica. cleavage divisionsb. morulac. blastocystd. implantatione. primitive streak2. Embryonica. neurulab. tailbudc. metamorphosis3. Fetal0. Fertilizationduring intercourse 1/4 of sperm die immediatelythe rest can remain viable for 28-48 hoursthere is typically a high percentage of defective sperm cellsfemale defense system attacks spermsperm make their way to the cervixhalf the sperm go up the uterine tube containing the eggHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, uterine tube sperm must swim against the currentfew sperm cells actually make it to the egg!

4 Of 200-300 M sperm in a typical ejaculationonly 2000-3000 ( ) actually make it to the eggthe egg is fertilized in the uterine tubethe egg has 2 layers of cells around itwhen sperm contacts eggs membrane, acrosome secretes enzymes that digests a hole to break through these layerssperm penetrates the cell membrane of the eggsegg prevents more than 1 sperm from penetration:!egg membrane depolarizes and prevents other sperm from binding!egg generates a fertilization membrane that pushes other sperm awayupon fertilization, egg completes meiosisthen 23 chromosomes of the egg and 23 of the sperm mix to produce a zygote!

5 46 chromosomesHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, Preembryonic Stagea. Cleavage1st cleavage occurs in about 30 hrs afterfertilizationmitotic divisions continue for ~ 3 daysas egg divides each cell gets smaller! overall size stays the sameeach cell ! blastomere= all cells are identicalb. Morulaby ~72 hrs, reaches uterus, is a morula= solid ball of about 16 or more cellsno larger than original zygotec. Blastocystmorula continues to divide for ~ 4-5 days anddevelops into ~100 cell blastocyststill ~ same size as original eggblastocyst is hollow & filled with liquidHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, = trophoblast !

6 Will help form placentathickened clump of cells = inner cell masswill become embryo and membranesd. Implantationwithin ~10 days after fertilization,blastocyst begins to implant in endometriumimplantation takes ~ 1 weekcells of trophoblast secrete enzyme allowingembryo to eat a hole into the uterine liningEctopic Pregnancies~1 in 300 pregnancies blastocyst implants somewhere otherthan in the uterusmost cases are tubal pregnancies! if not detected can rupture and kill the motherconceptus can also implant somewhere in the abdominalcavity ! anywhere there is an adequate blood outside of uterus, colon or bladderusually are life threatening and require abortions tosave motherbut: ~9% of abdominal pregnancies result in live births bycaesariane.

7 Primitive StreakHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, the human embryo is 2-2 1/2 weeks old it is~1/10th of an inch longa groove forms along surface of epiblast= primitive streakcells migrate into this streak and forms 3 celllayers=primary tissue layers:ectodermmesodermendodermEach of these tissue layers will give rise to a veryspecific set of ectoderm will differentiate into the skinand nervous mesoderm develops into the skeletal,muscular and circulatory systems andparts of the urinary and reproductivesystemsAnd the endoderm gives rise to the digestiveand respiratory systems and portionsof the urinary and reproductivesystemsHuman Anatomy and Physiology: human Development.

8 Lecture Notes, Ziser, the 3 tissue layers are formed = embryoMiscarriageonly about 1/3rd of all zygotes develop to termmost miscarriages are early spontaneous abortions! easily mistaken for a late or heavy menstrual periodestimates:25-30% of blastocysts fail to implant42% of implanted blastocysts die by the end of thesecond week16% of those that make it through 2 weeks areseriously abnormal and abort within the nextweek61% of early spontaneous abortions were due to chromosomal abnormalities2. Embryonic Stagesbegins about day 16organogenesis one of main processesa.

9 NeurulaThe nervous system is one of the first systemsto develop in the embryo!By the fourth week its formation is well under wayFolds form along each side of the primitive streakHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, curve upward to join forming a closed tubealong the length of the tube is expanded in the front and will formthe brainThe smaller region further back will form thespinal cord and neural tube doesn t close properly along its length! spina bifidacirculatory system also is establishedA simple tubular heart begins pumping bloodfrom the placenta through the umbilicalcord to the developing embryo bringingoxygen and nutrients and returning wastesto the !

10 Will form vertebrae, ribs, spinal nervesand trunk musclesb. Embryonic Membranesduring embryonic development embryonicmembranes form around the embryoeg. amnion (bag of waters)surrounds the embryo; becomes filled with fluid!acts as shock absorberbreaks just before birthHuman Anatomy and Physiology: human Development; Lecture Notes, Ziser, chorion eventually it becomes principal part of placentaplacenta provides an exchange of nutrients and wastes between mom and babythe maternal and fetal blood vessels are next to each other but blood does not mix!