Lectures of Human Embryology - Faculty...
Transcript of Lectures of Human Embryology - Faculty...
Lectures of Human Embryology
By
DR. ABDEL-MONEM AWAD HEGAZY M.B. with honor 1983,
Dipl."Gynecology and Obstetrics "1989,
Master "Anatomy and Embryology" 1994,
M.D. "Anatomy and Embryology" 1999
Associate Professor of Anatomy and Embryology Faculty of Medicine, Zagazig University (Egypt) &
College of Medicine, Majmaah University (Saudi Arabia)
Fertilization
Definition:
It is the union of a sperm with a mature ovum to form
fertilized ovum (called zygote).
Site:
Fertilization occurs in the lateral third of the uterine tube (in
the ampulla). N.B.: The ovum (or 2ry oocyte) is viable for 24 hours after its release from the ovary
(ovulation), while the sperm retains its fertilizing power within the female genital
tract for about 48 hours. Therefore, for occurrence of fertilization, sexual intercourse
must occur not more than 48 hours before ovulation and not more than 24 hours after
ovulation. The oocyte reaches the site of fertilization by contraction and movement of
cilia of the uterine tube while the sperms that are motile cells reach this site through
propelling movements of their tails (the trip of sperms to reach the site of fertilization
is about 5-7 hours).
Site of Fertilization in Uterine Tube and Ovulation
Process of Fertilization
It occurs as follows:-
1. Several sperms reach the 2ry oocyte and surround it.
2. Only one sperm penetrates the oocyte (the other sperms
are prevented from entering it).
N.B.: Acrosome reaction: It is the reaction occurring
when the sperm becomes in contact with zona pellucida to
facilitate its penetration. This results in release of contents
of acrosome including enzymes needed for penetration,
e.g. acrosin- and trypsin-like substances.
3. Immediately after entry of the sperm, the 2ry oocyte
divides by the 2nd
meiosis to give the ovum and small 2nd
polar body (that degenerates).
4. The nucleus of the entering sperm loosens to form the
male pronucleus.
5. The nucleus of the ovum loosens to form the female
pronucleus.
6. Then the 2 pronuclei fuse to form a single nucleus,
forming zygote.
Diagrams showing the Process of Fertilization
Results:
1. Restoration of the diploid number of chromosomes (46).
2. Initiation of cleavage (or cell division), to form the fetus.
3. Specious variation, i.e. the new man is different from other
population, even from his mother and father, because he
represents a new mixture of chromosomes (one half form
the mother and the other half from the father).
4. Sex determination; as follows:
If the sperm carries Y-chromosome, the fetus will be
male.
If the sperm carries X-chromosome, the fetus will be
female.
Twins
Definition: One fertilization results in more than one foetus.
Types:
There are two types; uniovular and binovular twins.
Uniovular (monozygotic)
twins
Binovular (dizygotic) twins
-It results from fertilization of
one ovum by one sperm
(Splitting the zygote at
different stages of
development).
-They have one (or single)
placenta.
-They are of the same sex and
characters (identical twins). N.B.: If splitting occurs early in development "at
the two-cell stage", each embryo will has its own
placenta, amniotic cavity and chorionic cavity.
This case resembles that of dizygotic twins.
-It results from fertilization of
2 ova by 2 sperms.
- They have two separate
placentae.
-They are of the same or
different sex, but the
characters are not identical.
Opened Pregnant Uterus, showing Identical Twins
N.B.: Conjoined twins occur as a result of partial splitting of
the developing embryo at later stages of development (at the
stage of primitive node and streak)
Opened Pregnant Uterus, showing Full-Term Fetus
Early Stages of Embryonic Development
The human development is a complex process. It starts with a
single cell (fertilized ovum) and ends with an extremely
complex human being consisting of trillions of cells.
In this subject, the only first major events will be mentioned.
1. Cleavage
The fertilized ovum (zygote) begins to divide by mitotic
cell divisions, forming; firstly 2 cells then 4 cells, and so
on to reach a mass of cells formed of 32 cells, called
morula. The developing embryo passes to the uterine
cavity, whilst cell divisions occur. (It reaches the uterine
cavity, about 3 days after fertilization).
2. Blastocyst
The mass of cells (morula) on reaching the cavity of
uterus, it begins to absorb some fluids from this cavity.
The fluid collects in-between the cells, dividing them into
2 groups of cells; outer cells surrounding the developing
embryo called trophoblast and an inner cell mass,
displaced to one side called embryoblast. The developing
embryo is then called blastocyst. (tropho= nourishing,
blast = primitive cells).
Early stages of cleavage development
Implantation
Site: -It occurs in the endometrium of the body of uterus
(superior, posterior part),
Time -It occurs at about the 7th
day after ovulation.
-The blastocyst erodes the thick endometrium. Then, it
becomes completely hidden inside it (by about the 12-14th
day after ovulation i.e. the day of the expected next
menses to start).
Process:
a. The blastocyst increases in size, due collection of
more fluid from the uterine cavity.
b. This size increase leads to rupture and disappearance
of the zona pellucida.
c. This leads to direct contact of the trophoblast
covering the inner cell mass (which is sticky) with
the endometrium.
d. Attachment to the endometrium causes the cells of
trophoblast to divide repeatedly forming 2 layers;
inner called: cytotrophobalst and outer layer of cells
without cell boundaries called: syncytiotrophoblast.
e. Syncytiotrophoblast erodes the endometrium.
f. As the blastocyst sinks into the endomerium, it
becomes completely surrounded by 2 layers
(cytotrophobalst and syncytiotrophoblast).
g. After completion of implantation, the site of
penetration is closed by fibrin clot that is replaced by
epithelium, hence the endometrium becomes
continuous.
N.B.: The placenta is formed at the site of implantation
and formed by two components; maternal part
(endometrial part) and fetal part (trophoblastic part).
Site of Implantation in Endometrial lining of Uterine body
2ND
WEEK
I. Development of bilaminar embryonic disc:
- The cells of the inner cell mass (embryoblast)
differentiate into 2 layers:
1. Epiblast layer (ectoderm): tall columnar cells-
dorsally (adjacent to trophoblast).
2. Hypoblast layer (endoderm): cuboidal or flattened
cells- ventrally (facing the blastocele).
Stage of bilaminar embryonic disc
II. Development of 2 new cavities within the
blastocyst:
A -Formation of amniotic cavity:
a. Small cavities appear dorsally between the
ectoderm and trophobalst.
b. Then these cavities fuse together forming a
single amniotic cavity.
c. The cavity is then roofed by a layer of cells,
called amnioblastic cells, derived from the
epiblast cells adjacent to trophoblast.
B -Formation of yolk sac:
a. Then, the blastocele changes into the
primary yolk sac, through the appearance of
flattened layer of cells its floor, called
Hauser membrane, (probably derived from
the cytotrophoblast).
b. Then, the primary yolk sac changes into
secondary yolk sac, through the appearance
of another flattened layer of cells, derived
from the endoderm. Then the new small
cavity is pinched off from the 1ry yolk sac. It
is completely surrounded by endoderm. This
occurs about at the day 12.
Development of amniotic cavity and secondary yolk sac
Development of amniotic cavity, secondary yolk sac, exocoelomic cyst and chorionic
villi
III. Development of extra-embryonic mesoderm and
coelom:
A. Loose cellular tissue detach from the inner
layer of cytotrophoblat. It proliferates and fills the
space between trophoblast externally and the
amnion and yolk sac internally, forming
"extraembryonic mesoderm" (EEM).
B. Then the intercellular spaces coalesce to form a
single cavity called extraembryonic coelom (EEC). This cavity enlarges to surround the amnion and primary
yolk sac, except at the part connecting the embryonic
disc with trophoblast called connecting stalk.
The EEM linning the trophoblast and covering the
amnion is called somatic or parietal layer of EEM, while
that covering the the yolk sac is known as viscral layer of
EEM.
Development of extra-embryonic mesoderm and coelom
3RD
WEEK
(Gastrulation) or
(Development of Trilaminar Embryonic disc) The most charcteristics of the 3
rd week is the gastrulation (or formation of
trilaminar embryonic disc)
I. Development of primitive streak and primitive node:
- The ectodermal cells in the middle of cuadal half of the
embryonic disc proliferate and migrate between the cctoderm
and the endoderm, forming elnogated column of cells called
primitive streak.
- As the strak elongates caudally, its cranial end thickens to form
the primitive node (Hensen's node).
II. Development of notochord:
- The cells of the primitive node proliferate and migratecranially
between the ectoderm and endoderm till reaching the
prochordal plate, forming a mid-line cord called primitive
notochord or notochordal process.
- An invagination extends from the amion passing through the
primitive streak and then through the primitive notochord to
form the notochordal canal.
- The floor of the notochordal canal and the underlying endoderm
diaspprears resulting in a temporary connection between the
amiotic cavity and yolk sac called neurenteric canal.
- Later, the roof of the notochordal canal folds longitudinally and
proliferstes to form the definitive notochord.
- Soon afterwards, the continuity of the endoderm is restored and
the neurenteric canal is obliterated.
N.B.: Notochord is a solid cylinder of cells, extending from the
primitive streak to end at the prechordal plate.
Embryological importance of notochord:
1. It induces the differentiation of the overlying ectoderm
to form the neural plate.
2. It induces the formation of the vertebral column.
3. It forms the nucleus pulposus of the intervertbral discs.
N.B.: Over small areas; one at the cranial end and the other at the caudal
end of the germ disc, the ectoderm and endoderm are closely adherent.
The area at the cranial end is called prochordal plate and it later gives
rise to oropharyngeal membrane. On the other hand, the area at the caudal
end is called cloacal membrane.
Development of notochord
III. Development of 3rd
germ layer (intraembryonic mesoderm):
- The cells of th e primitive streak migrates laterally, forwards
and backwards to form a 3rd
layer of cells called
intraembryonic mesoderm (IEM). This layer occupies all the
space between the ectoderm and endoderm except 3 regions.
The regions do not occupied by intraembryonic mesoderm are:
1. Prochordal plate
2. Notochord
3. Cloacal membrane.
- The IEM differentiates into 3 longitudinal regions on each side
of the midline (or notochord and primitive streak):
1. Medial mesoderm called Paraxial mesoderm: It
becomes organized into segments called somites.
2. Intermediate mesoderm: It forms the dorsal longitudinal
ridge called urogenital ridge that gives rise the future
kidneys and gonads.
3. Lateral plate mesoderm into which the intraembryonic
coelom develops.
Development of intra-embryonic mesoderm
Differentiation of intra-embryonic mesoderm
IV. Development of the intraembryonic coelom (IEC)
- A single U-shaped cavity called intraembryonic coelom
develops in the lateral plate mesoderm and infront of the
prochordal plate.
- It divides the mesoderm into 2 layers:
1. Dorsal layer called somatic (parietal) layer. It gives rise to
the parietal layer of the serous membranes (pleura,
pericrdium and peritoneum).
2. Ventral layer called splanchnic (visceral) layer. It gives rise
to the visceral layer of the serous membranes (pleura,
pericrdium and peritoneum).
- Fate of IEC:
During the 2nd
month the IEC is divided as follows:
1. The cranial transverse part (infront of prochordal plate)
gives rise to the pericardium.
2. The cranial part of the lonitudinal limb gives rise to
pleura.
3. The caudal part of the lonitudinal limb gives rise to
peritonium.
Derivatives of intra-embryonic mesoderm
V. Development of Somites - By the end of the 3
rd week (about at the 20
th day), the paraxial
mesoderm starts to be divided into blocks of cells called
somites.
- The 1st pair of somites appear just caudal to the cranial end of
the notochord.
- Then new somites appear in a cranio-caudal sequence, nearly 3
pairs of somites per day.
- About 30 pairs of somites appear during the so-called somite
period (day 20 to day 30). During this period the age of the
embryo can be roughly estimated from the somites.
- Total 42-44 pairs are formed by the end othe 5th week, as
follows:-
4 pairs –occipital
8 pairs – cervical
12 pairs – thoracic
5 pairs – lumbar
5 pairs – sacral
8-10 pairs – coccygeal.
- Fate (derivatives) of somites:
1. The first occipital and last 5-6 coccygeal pairs disappear
2. Each of the remaining somites divides into 3 parts:
Dorsolateral part called dermatome. It gives rise to the
dermis and subcutaneous tissue of skin.
Intermediate part called myotome. It gives rise to
skeletal muscles of tongue, thorax, abdomen and
prabably limbs.
Ventromedial part called sclerotome. It gives rise to
bones, cartilages and ligaments of vertebral column and
ribs.
VI. Folding of the Embryo
- It begins by the end of the 3rd
week and becomes complete
during the 4th week.
- The folding is due to rapid development of the central part of
the embryo, especially the central nervous system than the
peripheral part, as well as the more expansion of the amniotic
cavity.
Types of foldings
1. Craniocaudal foldings:
a. Crainal (Head) fold: The cranial end (the
or0pharyngeal membrane, cardiogenic area and
transverse septum) of the embryonic disc folds
ventrally.
b. Caudal (Tail) fold: The caudal end (cloacal
membrane and connecting stalk) of the embryonic disc
folds ventrally.
2. Lateral folds (2): The right and left sides of of the
embryonic disc, containing the longitudinal limbs of
intraembryonic coelom folds ventrally.
Then the 4 folds meet ventral to the embryo.
Results of folding:
1. Result of 4 foldings:
- The embryo becomes cylindrical in shape.
- A ring called primitive umbilical ring develops
ventral to the embryo.
- Part of yolk sac is incorporated within the embryo
forming the gut.
N.B.: The part of gut incorporated in the head fold is
called foregut, part in the tail fold called hindgut
and the part inbetween (connected to the definitive
yolk sac through the umbilical ring is the midgut.)
2. Result of cranial folding:
- The forebrain grows dorsal to the oropharyngeal
membrane.
- The cardiogenic area and the septum transversum
becomes ventral in postion.
3. Result of caudal folding:
- The cloacal membrane becomes the most caudal part.
- The connecting stalk becomes ventral to the embryo.
4. Result of lateral foldings:
- The caudal parts of the longitudinal limbs of IEC fuse
together forming the pertioneal cavity.
- The intermediate mesoderm forming the urogenital
system becomes dorsal in postion.
Folding of embryo
N.B.: Trnsverse septum is the mesoderm connecting the
the amion and yolk sac cranially.
Opened Pregnant Uterus, showing Embryonic Development
Clinical Correlations
Implantation in the lower part of the uterus results in a condition called
placenta previa, that commonly results in bleeding before delivery. It
includes:
placenta previa lateralis, the placenta is low in position, but it
does not reach the margin of the cervix.
placenta previa marginalis; the placenta reaches the margin of
the cervix.
placenta previa centralis; the placenta reaches the center of the
cervix. This last case usually needs surgical abdominal delivery
(caesarian section).
Abnormal sites of implantation (sites of ectopic pregnancy)
include the following sites:
The uterine tube (represents about 95% of cases of ectopic
pregnancy, mostly in the ampulla of the tube),
The ovary,
The abdominal (peritoneal) cavity: In this case, it may implant
in any site covered by peritoneum. However, it mostly implant
in Douglas (recto-uterine) pouch. N.B.: Cases of ectopic pregnancy mostly die about the 2
nd month pregnancy, causing
internal hemorrhage and severe lower abdominal pain.
Hydatidiform (vesicular) mole: The uterus is distended by thin walled,
translucent, grape-like vesicles of different sizes. This case is caused due
to excessive proliferation of the trophoblast. In this condition, there is
excessive secretion of gonadotrophins. It may change into malignant
tumour called chorion epithelioma.