General Bio 101 - Embryonic development part 2
|Human Development|| Pre-embryonic period (first 2 weeks) |
-- includes cleavage, implantation
Embryonic period (3rd - 8th week)
-- includes induction.
Fetal period (3rd to 9th month)
-- includes growth and development.m
| Pre-embryonic period|
| Begins 1.5 days after fertilization.|
Completed in 5-6 days, before implantation.
Zygote becomes a hollow blastula.
Late blastula composed of:
-- inner cell mass
-- trophoblast layer
| Late blastula|
| Trophoblast cells "digest" their way into the uterine lining.|
Trophoblast cells will become the chorion.
Urine tests for pregnancy detect hCG (human chorionic gonadotrophin).
|Gastrulation|| Formation of the three primary germ layers |
Epiblast migrates inward through the primitive streak to form both mesodern and endoderm.
|Gastrulation|| Begins with 2-layered embryo.|
Hypoblast (lower layer) grows down
around "yolk" to form yolk sac.
Epiblast (upper layer)
-- grows up to form amnionic sac.
--invaginates through primitive streak
to form mesoderm and endoderm.
|Embryonic period|| Basic structures of all organ systems are completed during this phase (3-8th week)|
Induction of organ systems from interaction of three primary germ layers.
Tubular "heart" beats at 3 weeks.
Embryo the size of a pea at 5 weeks.
|Extraembryonic membranes|| Three grow out of the embryo.|
The fourth (the chorion) surrounds but has no contact with the embryo. It is formed from the trophoblast layer.
|Extraembryonic membranes: Chicken|| Yolk sac = encloses yolk; has blood vessels for food transport to embryo.|
Amnion = holds fluid around embryo.
Allantois = stores wastes;
blood vessels for gas exchange.
Chorion = gas exchange.
|Extraembryonic membranes: Human|| Yolk sac = not functional |
Amnion = "bag of waters" around embryo
Allantois = its blood vessels used in umbilical cord.
Chorion = from trophoblast; forms placenta with allantois; secretes hCG; boundary in placenta between mother and fetus.
|Primary induction|| The interaction of any two primary germ layers.|
For example, formation of the spinal cord.
|Formation of spinal cord|| Mesoderm (specifically, the notochord) induces the overlying ectoderm to form the neural tube, which becomes the spinal cord and brain.|
This is primary induction because the interaction is between primary germ layers.
|Spina bifida|| If closure of neural tube is incomplete,|
child is born with exposed neural tissue.
|Secondary induction|| The interaction of two cell layers derived from the same primary germ layer.|
For example, formation of eye.
|Formation of eye|| Optic cup (outgrowth of brain) induces overlying ectoderm to become lens of eye.|
Optic cup becomes retina, its lip the iris.
This is secondary induction because both the optic cup and the skin are from ectoderm.
|Tertiary (etc.) induction|| For example, the structural similarities of early embryos of vertebrates.|
|Vertebrate embryos are similar|| Does "ontogeny recapitulate phylogeny"?|
Does embryonic development retrace the path of evolutionary history?
If not, then why do human embryos have gill pouches?
|Gill pouches in humans|| The pouches never function as gills.|
Gill pouches serve as intermediates in the development of other structures.
|Derivatives of gill pouches|| 1st pair become Eustachian tubes from throat to middle ear behind eardrum.|
2nd pair of pouches becomes the tonsils.
3rd pair becomes thymus gland.
4th pair the parathyraoids (Ca++ levels)
|Fetal period|| Third to Ninth month.|
Growth (from 1 ounce to 7.5 pounds).
Head to body ratio decreases.
| Abdomen at 0, 20 and 30 weeks|
| 12 wk = swallowing & sucking reflexes.|
16 wk = fills womb, abdomen expands.
17 wk = "quickening" (abortion deadline
according to English Common Law).
23 wk = can survive with intensive care.
40 wk (+ 2) is considered full term.
|Weight gain (28 lb.)|| Baby (7.5)|
Amnionic fluid (2) & placenta (1.5)
Uterus (3) & breasts (1)
Blood (3) and body fluid (2.5)
"Unnecessary" (not really) (7.5)
|U.S. Supreme Court (1973)|| 1st trimester = States cannot outlaw abortion in first 3 months of pregnancy.|
2nd trimester = States may prohibit abortions except those for health reasons.
3rd trimester = States may prohibit abortions all together.
|Childbirth: Stages of labor|| 1st = dilation of cervix (3 mm to 10 cm)|
= 8-20 hours for 1st baby (3-8 later)
= breathing to avoid premature push
2nd = delivery of baby (0.3-1.5 hr)
including crowning, episiotomy, and
cutting umbilical cord.
3rd = delivery of placenta (5-10 min later)
|Prepared Childbirth|| Breathing & relaxation to break |
cycle of pain-tension-pain
Father is involved.