Transcript DISC 23

Quiz 15
Quiz 11
7.2
The trilaminar
germ disk (3rd
Genesis
of the notochord
week)
Quiz
Around the
19th day cells that invade into the primitive node region and migrate along the median line cranially form the chordal
process
7
(also known as the axial
Legend
•Introduction
process
). One can compare this phenomenon with the sliding of a finger into a glove. Thanks to the transparency of the ectoblast this cell migration can be observed in the
Fig. 5 - Primitive streak seen dorsally
embryos of experimental animals.
•Formation
The
chordalofprocess
the primitive
growsstreak
longer through proliferation of the primitive node cells at its front end up to the prechordal plate
6
. At the same time the primitive streak recedes
•Genesis
back
into the
of the
caudal
germregion
layers 6 .
•Genesis of the notochord
•Location of the epiblast cell target and the development of the primitive
streak
•The bilaminar membranes
•Evolution of the mesoblast
•The paraxial mesoblast and the differentiation of the somites
•The intermediate mesoblast
•The lateral mesoblast
•The intraembryonic coelom
•Induction of the neural plate - neurulation
• The First Week
• Gametogenesis
• The ovum
• The formation of primary oocytes, from
which ova are developed, is complete
before birth. About 2,000,000 primary
oocytes are present at birth, but by
adolescence only about 40,000 remain.
Over the reproductive period about 400 of
these pass through maturation to
ovulation.
• The maturation of ova occurs in follicles
within the ovary, under hormonal control.
• All primary follicles are arrested in the
prophase of the first meiotic division.
• The first meiotic division is completed just
before ovulation but progress through the
second meiotic division is halted in
metaphase.
• If the ovum is fertilized the second meiotic
division is completed.
• During the menstrual cycle, Follicle
Stimulating Hormone (FSH) promotes
growth of several primary follicles, with
only one of these reaching ovulation. The
final stages of maturation require
Luteinizing Hormone (LH).
• Estrogen is produced by the growing
follicle, contributing to the growth and
development of the reproductive organs,
and to the production of the LH surge
causing ovulation.
•
• At ovulation the secondary oocyte is
expelled together with follicular fluid from
the surface of the ovary. The fimbriated
end of the fallopian tube becomes closely
applied over the swelling follicle so that on
rupture of the follicle the ovum normally
passes into the fallopian tube.
• It is possible for the ovum to miss the
fallopian tube and to be free in the
peritoneal cavity.
• Oocytes are usually fertilized
within 12 hours after ovulation,
and may not survive for more
than 24 hours before
degenerating.
• The sperm
• Sperm are formed by the germinal
epithelium of the seminiferous tubules of
the testes.
• At puberty spermatogonia in the germinal
epithelium divide, grow and transform into
primary spermatocytes.
• Each spermatocyte then undergoes a
meiotic division to form two haploid
secondary spermatocytes.
• The secondary spermatocytes
then undergo a second meiotic
division to form spermatids which
mature into sperm.
• Before sperm are functional and
able to fertilize an ovum two
events must take place.
• Capacitation is the process of sperm
conditioning in which the glycoprotein coat
and seminal proteins are removed from
the surface of the sperm's acrosome.
• Sperm are capacitated in the uterus and
fallopian tubes by uterine secretions.
• The Acrosome Reaction must be
completed before sperm may fuse with the
ovum.
• Contact with the corona radiata
surrounding the ovum triggers the
development of perforations in the
acrosome, allowing the release of
enzymes such as hyaluronidase which
facilitate fertilization.
• Fertilization
• Fertilization of the ovum involves
penetration of the sperm through the
corona radiata, fusion of the oocyte
and sperm cell membranes,
completion of the second meiotic
division of the secondary oocyte, and
formation of the zygote.
• The fertilization process lasts about
24 hours. Fertilization usually occurs
in the ampulla of the fallopian tube.
• A sperm contacts the zona
pelucida and enters the ovum.
• This signals the completion of
second meiosis and the second
polar body is formed.
• The male and female pro-nuclei
then fuse.
• The Zygote and the Blastocyst
• The zygote, formed by fusion of the two
haploid gametes, is the first form of a new
human being.
• The zygote undergoes a rapid series of
mitotic divisions called cleavage.
• As division proceeds two cells are followed
by four then eight etc.
• The zygote divides first into two cells
then four, eight etc until it forms a ball
of cells, the morula.
• The ball then develops a fluid filled
core to form an early blastocyst.
• As the blastocyst matures the inner
cell mass expands at one end.
• The blastocyst will attach itself to the
endometrium at this pole.
• About three days after fertilization the
zygote forms a ball of cells known as the
morula.
• By four days the morula enters the uterus
and develops a central cavity.
• At this stage the structure is termed a
blastocyst.
• At six days the blastocyst attaches to the
uterine wall to begin the process of
implantation.
• Cell division continues rapidly until the
blastocyst is formed into outer
syncytiotrophoblast and inner
cytotrophoblast layers.
• The cells of the syncytiotrophoblast invade
the uterine epithelium.
• By seven days after fertilization
the blastocyst is securely
attached to the endometrium and
the inner cell mass has divided to
form the hypoblast.
• The inner cell mass and
hypoblast will together form the
embryonic disc which gives rise to
the germ layers of the embryo.
• Clinical Application
• Spontaneous abortion
• Not all zygotes implant and survive. About
15% are aborted spontaneously, probably
due to the presence of chromosomal
abnormalities.
In-vitro fertilization
• Secondary oocytes removed from mature
ovarian follicles may be fertilized outside
of the body and the zygotes implanted in
the uterus.
• Formation of the embryonic disc
• The Second Week
• Implantation
• The process of development from a onecelled zygote to a late blastocyst takes
about a week.
• During this time the zygote is travelling
passively along the fallopian tube towards
the uterus.
• However, the zygote may not enter the
fallopian tube and may either attach to the
ovary or float freely in the peritoneal cavity
and attach to the mesentery or an organ
such as the liver.
•
• The progress of the zygote
through the fallopian tube may
also be impeded so that by the
time the blastocyst is ready to
implant, it is still in the fallopian
tube.
• Ectopic pregnancies arise from
implantation within the peritoneal
cavity or in the fallopian tube.
• Ectopic pregnancies represent a
significant risk for the mother,
paricularly from tubal rupture and
associated haemorrhage.
• Few ectopic pregnances progress
further than eight weeks.
• Normally, implantation occurs in the
endometrium in the upper body of the
uterus.
• The late blastocyst attaches to the
endometrium at its embryonic pole (inner
cell mass).
• At the attachment site the blastocyst forms
a syncytiotrophoblast - a syncitium formed
by the fusion of several cells.
• The syncytiotrophoblast erodes and
invades the endometrium.
• The syncytiotrophoblast expands and
contacts the glands and blood
vessels of the endometrium. From
this stage onwards the developing
embryo has access to a source of
nutrition.
• The remaining cellular blastocyst is
then termed the cytotrophoblast.
• The cells of the cytotrophoblast divide
to form new cells which migrate into
the syncytiotrophoblast and fuse with
it.
• The syncytiotrophoblast begins to
produce human chorionic
gonadotrophin (hCG), detectable by
the end of the second week.
•
• If an ectopic pregnancy has occurred,
the amount of hCG will be
significantly less.
• The inner cell mass is then
rearranged so that a cavity appears
above it.
• The mass is then thinned out into the
form of a two-layered disc.
• A layer of cells lines the cavity above the
disc. These cells will become the amnion,
and the cavity will become the amniotic
cavity.
• The two layers of the disc begin to
differentiate so that the upper layer is
composed of larger cells forming the
epiblast and the small cells of the lower
layer forming the hypoblast.
• The cells of the hypoblast
divide and migrate to form an
inner lining for the
cytotrophoblast.
• The chamber which is now
enclosed is the primary yolk
sac.
A - amniotic cavity
C -cytotrophoblast
ED - embryonic disc
G - glands
Ssyncytiotrophoblast
YS - primary yolk
sac
• Development of the trophoblast
• As the syncytiotrophoblast expands,
spaces appear within it, filled with
secretions from the endometrial
glands and blood from the eroded
vessels.
• Expansion of the syncytiotrophoblast
and continued erosion of vessels
results in the development of a
sponge-like syncytiotrophoblast.
• Since erosion of the vessels in the
endometrium has occurred in both
arterioles and vennules, a primitive
circulation develops.
• There is now a large surface area of
syncytiotrophoblast bathed in
circulating maternal blood - a primitive
placenta.
• Access to this source of nutrition
allows for further growth of the
embryo.
• As the trophoblast is growing, a
separation occurs between the
cytotrophoblast and the amnion
and yolk sac.
• The cytotrophoblast grows rapidly
and pushes out towards the blood
spaces in the syncytiotrophoblast.
• The arrangement of
syncytiotrophoblast and
cytotrophoblast forms primary
chorionic villi, present by the end of
the second week.
• These villi will eventually be invaded
by fetal blood vessels and will
become the site of gas, nutrient and
metabolite exchange between the
embryo and mother.
A - amniotic cavity
C - Developing chorion
• The embryonic disc
• The embryonic disc is at first uniform, then
later becomes polarised as a group of
hypoblast cells enlarges as the prechordal
plate.
• This indicates the head-end of the future
embryo.
• At the end of the second week - beginning
of the third week changes occur at the
opposite (caudal) end of the epiblast.
• Cells from around the periphery of the disc
migrate towards the midline forming a
ridge caudally, the primitive streak.
• The primitive streak grows in a cranial
direction, with an expanded cranial end,
the primitive node.
• Cells produced in the primitive streak
migrate to form a layer between the
epiblast and the hypoblast, the
mesenchyme, and to replace some of the
hypoblast cells.
• This is the process of gastrulation in which
the epiblast forms the ectoderm,
• the mesenchyme forms the mesoderm
and
• the cells which migrate into the hypoblast
form the endoderm.
• The ectoderm will form the skin and its
glands and appendages, the nervous
system, and parts of the mouth, nose,
anus and external genitalia.
• The endoderm will form the epithelial
lining of the digestive system and its
glands, the lining of the respiratory
tract down to the alveoli, the
epithelium of the bladder and urethra.
• The mesoderm will form the rest of
the tissue of the body including all of
the connective tissues, muscle, blood
and lymphatic vessels.
• Teratogenesis
• During development the embryo and fetus are
susceptible to the effects of a wide range of
agents such as drugs, chemicals and viruses.
• Each body system has a critical period during
which it is vulnerable. In the period covered so
far, environmental disturbances may affect
cleavage of the zygote and implantation of the
blastocyst.
• The end result is likely to be death and
spontaneous abortion rather than congenital
anomalies.