Unit 3: Before Birth

Question 1

Prenatal:

Answer 1

•”before birth”.
•divided into 3 trimesters:
-3 months each.

•3 phases:
1. Zygote: sperm + egg.
2. Embryo: weeks 1-8.
3. Fetus: weeks 9-40.

Question 2

Fertilization:

Answer 2

1. Oocyte released:
   •viable for 12-24 hours.
2. In oviduct:
   •need many sperm to penetrate egg coat.
   •egg membrane changes, only 1 sperm can enter.
   •sperm (23 chromosomes) + egg (23) = zygote (46).

Question 3

Embryonic development stages:
(1st stage):

Answer 3

1st stage:
•zygote begins cleavage:
-rapid cell division (mitosis) without enlargement.
•called a morula:
-in oviduct.
-16 cell stage (or 32).
-3-4 days to reach uterus.

Question 4

Embryonic development stages:
(2nd stage):

Answer 4

2nd stage:
•called a blastocyst: (in uterus).
-made up of 2 groups of cells:

1. Trophoblast:
   •becomes the chorion (outer membrane).
   •secrete enzymes that digest the endometrium and allow implantation (10th-14th day).
   •secrete human chorionic gonadotropin (hCG) which maintains the corpus luteum:
   -no menstruation.
   -high amounts for first 2-3 months.
   •pregnancy tests: hCG.
2. Embryoblast:
   •inner cell mass (ICM).
   •develops into embryo.

Question 5

Inner cell mass (ICM):

Answer 5

-aka embryoblast.

Question 6

Blastocyst:

Answer 6

•outer layer of blastocyst (trophoblast) gives rise to 2 structures:

1. The chorion: produces human chorionic gonadotropin (HCG).
   •maintains the CL until the placenta starts to produce progesterone then the HCG levels drop and the CL degenerates (about end of 4th month).
2. The amnion: surrounds the amniotic cavity (inside of chorion).
   •it is fluid filled and insulates the embryo/fetus.

Question 7

Implantation:

Answer 7

•once in the uterus, the blastocyst becomes attached to the wall of the endometrium.
•this is called implantation.
•the trophoblast secretes enzymes that digest tissues and blood vessels of endometrium, and the blastocyst sinks into the uterine wall.
•once implantation occurs, the trophoblast begins secreting human chorionic gonadotropin (hCG)…
•hCG maintains the corpus luteum so that it continues secreting estrogen and progesterone.
•this maintains the endometrium and prevents menstruation.
•hCG levels decline after the second month of pregnancy, when the placenta is able to secrete these hormones on its own.

Question 8

Pregnancy and hormonal balance:

Answer 8

•for pregnancy to continue, menstruation cannot occur. Any shedding of the endometrium would dislodge the embryo from the uterus.
•to prevent menstruation, progesterone and estrogen levels must be maintained.
•the CL is maintained in the early part of the pregnancy to produce the progesterone needed to maintain the endometrium.
•should the corpus luteum deteriorate, the levels of estrogen and progesterone would drop, stimulating uterine contractions.
•this is avoided by secretion of hormones from the blastocyst itself.

Question 9

Embryonic development stages:
(3rd stage):

Answer 9

3rd stage:
•gastrulation (gastrula):
•start of morphogenesis.
•differentiation: cells form 3 layers:
-ectoderm: nervous system and skin (epidermis).
-mesoderm: skeleton, muscles and reproductive structures.
-endoderm: lining of digestive and respiratory systems and endocrine glands.

Question 10

Gastrulation:

Answer 10

-invagination of tissue to form 3 germ layers.
1. Ectoderm = forms all nervous tissue, skin.
2. Mesoderm = muscle, bone, reproductive system.
3. Endoderm = linings of the digestive and respiratory systems, endocrine glands.

Question 11

Ectoderm:

Answer 11

-outer layer.

Question 12

Mesoderm:

Answer 12

-middle layer.

Question 13

Endoderm:

Answer 13

-inner layer.

Question 14

Extra-embryonic structures:

Answer 14

-structures that support the developing embryo.

Question 15

Extra-embryonic structures:
(1. Chorion):

Answer 15

•formed from trophoblast cells.
•outermost membrane.
•forms fetal part of placenta:
-chorionic villi: finger like projections of chorion that go into endometrium.

Question 16

Extra-embryonic structures:
(2. Placenta):

Answer 16

•temporary organ.
•made from chorion and endometrium (mom); fully formed by 10-12th week.
•allows exchange between mom and baby:
-O2 and nutrients from mom to baby.
-CO2 and waste from baby to mom.
-mom and baby’s blood DO NOT mix.
•provides passive immunity: mom’s antibodies pass into baby’s blood.
•produces progesterone and estrogen and hCG:
-prevents further ovulation; maintains endometrium.

Question 17

The many roles of placenta:
(What are they?):

Answer 17

-nutritional functions.
-excretory functions.
-respiratory functions.
-endocrine functions.
-immune functions.

Question 18

The many roles of placenta:
(Nutritional functions):

Answer 18

-transports nutrients from mother to fetus.
-stores nutrients.

Question 19

The many roles of placenta:
(Excretory functions):

Answer 19

-transports wastes from fetal blood to the mothers blood.

Question 20

The many roles of the placenta:
(Respiratory functions):

Answer 20

-transports oxygen from the mother to the fetus and CO2 from the fetus to the mother.

Question 21

The many roles of placenta:
(Endocrine functions):

Answer 21

-secretes hormones.
-allows hormones from the fetus to diffuse into the mother’s blood (and vice versa).

Question 22

The many roles of placenta:
(Immune functions):

Answer 22

-transports antibodies from the mother to the fetus.

Question 23

Extra-embryonic structures:
(3. Amnion):

Answer 23

•sack filled with amniotic fluid that:
-cushions embryo; temperature regulation.

Question 24

Extra-embryonic structures:
(4. Allantois):

Answer 24

•forms umbilical cord:
-connects fetus to placenta.
-1 umbilical vein + 2 umbilical arteries.
•becomes part of urinary bladder.

Question 25

Extra-embryonic functions:
(5. Yolk sac):

Answer 25

•produces 1st blood cells.
•produces future gamete cells.
•becomes part of digestive tract.