ch 29: development and inheritence Flashcards
what are the three periods (in order)
- gestation period
- prenatal period
- neonatal period
when is the gestation period
fertilization to birth (38 weeks)
what happens in the first week of development?
what is sperm capacitation?
a series of functional changes.
Sperm need about 7 hours after ejaculation before its ready for fertilization
Occurs with the aid of secretions from the female reproductive tract
◦ Flagellum beats more rapidly
◦ Removal of cholesterol, proteins and glycoproteins from plasma membrane
near acrosome that prepare it for fusion with that of the oocyte
define fertilization
= haploid sperm and secondary oocyte merge →
zygote with single diploid nucleus
where and when does fertilization occur
Occurs in the uterine tube 12 to 24 hours post ovulation
◦ Oocyte viable in uterine tube for about 24 hours post-ovulation, sperm viable
for about 48 hours
how many sperm enter the female and how do they get to where they need to go?
300 million sperm enter vagina at ejaculation → 2 million reach cervix → ~ 200
reach oocyte
* Sperm swims through vagina into cervical canal by whip-like movements of
flagella
* Muscle contractions of uterus and uterine tube keep sperm moving along
what does a sperm need to do to fertilize the oocyte? how does this happen?
To fertilize an oocyte, a sperm must penetrate two layers around the oocyte:
The corona radiata and Zona pellucida
ZP3 is a glycoprotein in the zona pellucida
which acts as a sperm receptor
Upon binding to ZP3, sperm cells release
enzymes from acrosome, digesting a path
through the zona pellucida
if multiple sperm reach the egg, why don[t more fuse with the oocyte?
Multiple sperm may bind to ZP3, but only the first sperm to make its way
through the entire barrier and reach the oocyte’s plasma membrane will fuse
with the oocyte
what does the fusion of spem and oocyte cause
- Fast Block: Depolarization of the oocyte’s cell membrane
◦ Depolarized oocyte cannot fuse with another sperm
◦ Inactivates ZP3 molecules - Slow Block: Hardening of the zona pellucida
Polyspermy is prevented by
a) depolarization of the oocyte
b) hardening of the zona pellucida
c) inactivation of ZP3
d) a and b
e) a, b and c
E
describe fraternal twins
Fraternal twins (dizygotic)
◦ Independent release of 2 oocytes fertilized by 2 separate sperm
◦ Genetically as different as any 2 siblings
describe identical twins
Identical twins (monozygotic)
◦ 2 individuals that develop from a single fertilized ovum
◦ Genetically identical (always the same sex)
◦ Separation of zygote must occur within 8 days post-fertilization
◦ If split before day 2, will develop separate placenta and separate amniotic sac
◦ If split between day 2 and 8, will share placenta but will have separate amniotic sacs
events within the egg once the sperm enters
Sperm entry triggers oocyte to complete meiosis II
◦ It divides into a larger ovum and a polar body that
degenerates
Nucleus of sperm becomes male pronucleus
Nucleus of ovum becomes female pronucleus
Fusion of male and female haploid pronuclei produce
single diploid nucleus with 46 chromosomes
Fertilized ovum (2n) is called a zygote
describe cleavage in a zygote
Rapid mitotic cell division of a zygote is called
cleavage
By day 4, has formed a solid ball of 32
blastomere cells called a morula
The morula is the same size as the original
zygote – blastomeres get smaller and smaller as
they divide
A morula is a solid sphere of cells about the
size of the original zygote, and still
surrounded by the zona pellucida.
a) True
b) False
true
development of the blastocyst
morula enters the uterine cavity by ~day 5
fluid enters morula, separates blastomeres
and forms the blastocyst cavity
a blastocyst is a ball of cells filled with fluid
◦ outer covering is the trophoblast – will
develop into the chorion
◦ embryoblast (inner cell mass) – will
develop into the embryo
how and when does the blastocyst escape from the zona pellucida
After ~ 5 days, blastocyst escapes from zona pellucida by digesting a small hole
in it using an enzyme, and then squeezing through the hole
◦ Must do so in order to implant
describe implantation
The attachment of a blastocyst to the endometrium
occurs ~6 days after fertilization and is called implantation
The embryoblast is oriented to face the endometrium
Cells of trophoblast also secrete human
chorionic gonadotropin (hCG), which
rescues corpus luteum from degeneration
Allows sustained secretion of estrogens
and progesterone, preventing
menstruation
describe an ectopic pregnancy. Why can it happen? How to treat
development of an embryo outside of the uterine cavity
normally happens in the uterine tubes
◦ due to scarring caused by previous infection
◦ destruction of cilia (smoking)
◦ defect in peristalsis
◦ abnormal tubal anatomy
can also occur in ovaries, abdominal cavity or uterine cervix
unless removed, developing embryo will cause rupture of uterine tube and
death will follow because of hemorrhage
All the following statements about a
blastocyst are true EXCEPT
a) Develops about 5 days after fertilization
b) Surrounded by the zona pellucida
c) Still about the size of the original zygote
d) Organized as embryoblast and trophoblast
B
Identical twins:
a) Share a placenta
b) Do not share a placenta
c) Sometimes share a placenta
d) Can be opposite sexes
e) Are caused by multiple ovulation events
C
describe the development of the trophoblast
Trophoblast cells in contact with
endometrium rapidly multiply and divide into
2 layers
Syncytiotrophoblast - outermost layer
secretes enzymes that allows blastocyst to
push through epithelial cells lining uterus
◦ Blastocyst burrows into endometrium
◦ Epithelial cells cover gap
what is the endometrium called when zygot implants and what are the 3 parts?
Endometrium now called decidua and has 3 parts:
1. Decidua basalis is endometrial layer between
embryo and stratum basalis
◦ Provides glycogen & lipids
◦ Later becomes maternal portion of placenta
2. Decidua capsularis is thin layer between embryo
and uterine cavity
3. Decidua parietalis is remaining endometrium
lining uterine cavity
The trophoblast
a) becomes part of the chorion
b) secretes enzymes which help penetration of the uterine lining by
the blastocyst
c) secretes estrogen and progesterone
d) a and b
e) a, b and c
D
describe the development of the embryoblast
Inner mass cells differentiate about 8 days after
fertilization into 2 layers:
Hypoblast- layer of cuboidal cells
Primitive endoderm (lining of GI and resp. tract)
Epiblast - layer of columnar cells
Primitive ectoderm (epidermis and nervous
system)
Bilaminar embryonic disc cells of hypoblast and
epiblast form flat disc
Amniotic Cavity
◦ Small cavity appears within epiblast and eventually
will enlarge
what is the amnion? What are its functions and what is it made of?
Amnion - membrane which develops from the epiblast
Creates amniotic cavity filled with amniotic fluid, which eventually
surrounds the entire embryo
Functions of the amniotic fluid
◦ Absorbs shock , regulates body temperature, and prevents
adhesions with surrounding structures
Fluid is filtrate of mother’s blood + fetal urine
May be examined for embryonic cells (amniocentesis)
describe the umbilical vesicle and its functions
Still on day 8, cells from hypoblast migrate over inner surface of blastocyst wall
Flatten and form thin exocoelomic membrane
◦ This membrane and the hypoblast forms the wall of the umbilical vesicle
Functions:
1. During the 1st and 2nd week, the umbilical vesicle supplies nutrients to the embryo
(then the endometrium takes over)
2. Contains primordial germ cells that will migrate to developing gonads to become
spermatogonia or oogonia in 3rd week
3. Also, site of blood cell formation during 3rd to 6th weeks of life (then liver takes over)
4. Forms part of gut in 4th week
describe the development of the chorion
Extraembryonic mesoderm develops from umbilical vesicle and forms a layer of
CT that surrounds umbilical vesicle and amnion
Extraembryonic mesoderm together
with trophoblast will form chorion =
embryonic contribution to the placenta
The chorion surrounds the embryo/fetus
Secretes hCG and suppresses the
mother’s immune system
Bilaminar disc connected to trophoblast by band of extraembryonic mesoderm
called connecting stalk
◦ Will become umbilical cord
The bilaminar embryonic disc is formed
from the
a) Outer cell mass
b) Amnion
c) Embryoblast
d) Syncytiotrophoblast
C) embryoblast
The _________ supplies nutrients to the embryo
and is the source of blood cells during early
embryonic development.
a) trophoblast
b) hypoblast
c) epiblast
d) bilaminary embryonic disc
e) umbilical vesicle
E
describe the third week of development
During the third week, the three primary germ layers are established and lay
the groundwork for organ development in weeks 4-8
During the process of induction, one tissue (inducing tissue) stimulates the
development of an adjacent unspecialized tissue (responding tissue) into a
specialized one
during gastrulation the bilaminar disc will form 3 primary germ layers. What are they?
Bilaminar disc will form 3 primary germ layers:
1. Endoderm will become lining of G.I. tract, respiratory tract and other
organs
2. Mesoderm will become muscle, bone, connective tissue, peritoneum
3. Ectoderm will become skin epidermis, and nervous system
Endoderm and ectoderm are tightly packed epithelial cells
Mesoderm much looser connective tissue
describe gastrulation
Gastrulation involves the
rearrangement of cells from the
bilaminar disc
◦ Epiblast becomes ectoderm
◦ Hypoblast becomes endoderm
◦ In between is mesoderm (new, red)
~ Day 16, mesoderm cells migrate towards head
◦ Form hollow tube = notochordal process
~ Days 22-24, tube forms solid cylinder = notochord
Induction drives notochord to secrete substances that
induces nearby unspecialized mesoderm cells to start to
become vertebral bodies
◦ Notochord ends up as nucleus pulposus of
intervertebral discs
Small depressions at dorsal head
and tail form oropharyngeal
membrane & cloacal membrane
◦ 4th week oropharyngeal
membrane becomes oral cavity
◦ 7th week cloacal membrane
becomes openings for anus and
urinary/reprod. tracts
describe neruolation
Thickening of the ectoderm above mesoderm is called neural plate
Plate folds inward and forms a longitudinal neural groove
The raised edges of the neural plate are called neural folds
Neural folds increase in height and meet to form the neural tube
Neural tube cells will eventually
develop into brain and spinal cord
describe the development of somites
About day 17, mesoderm near the notochord & neural tube form paired
longitudinal columns
This mesoderm will break up into chain of cube-shaped blocks of tissue =
somites
Somites differentiate into:
◦ Myotomes – skeletal muscle of neck, trunk, limbs
◦ Dermatomes – dermis & connective tissue
◦ Sclerotomes – vertebrae
describe labor
A decrease in progesterone levels and elevated levels of estrogens,
prostaglandins, oxytocin, and relaxin are all involved in the initiation and
progression of labor.
Progesterone inhibits uterine contractions
◦ levels must therefore drop
A rise in estrogens overcomes this inhibition
