357( lectures-1) 2013 dr mathew

1. INTRODUCTION TO
EMBRYOLOGY
357
GROWTH AND DEVELOPMENT

2. Embryology is the study of
development of an organism from
fertilization of the ovum-the single
cell stage-through the period of
organogenesis.
In the human, this time frame
encompasses the first 8 weeks of
pregnancy.

3. Earliest Development
Week 1: Fertilization , Morula, Blastocyst, Implantation
Week 2: Bilaminar Embryo, Placenta and Membranes,
Gastrulation
Embryonic Period
Week 3: Gastrulation, Neurulation, Early Circulatory System
Week 4: Protoheart beats, Gut tube, Branchial Arches, Limb
Buds
Week 5-8: Organogenesis, Face, Limb differentiation, Ear,
External Genitalia
Foetal Period
Weeks 9-38: Brain growth, neural connections, lots of growth

5. The development of an animal embryo can be divided into
five major processes:
1) GAMETOGENESIS:
the process of gamete production.
2) FERTILIZATION:
the fusion of male and female gametes to form
a single-celled zygote capable of undergoing
development.
3) CLEAVAGE:
the mitotic divisions that divide the
cytoplasm into increasingly smaller
cells, without an increase in the total size
of the cell mass.

6. 4) GASTRULATION:
a stage of cell movement and rearrangement resulting
in three different germ layers of cells.
The three germ layers have different potentials for
tissue specialization and development. Yolk greatly
influences gastrulation and the development of these
three germ layers: ectoderm, endoderm, and
mesoderm.
5) ORGANOGENESIS:
the process whereby organs develop from the three
germ layers.

7. Several cellular events are essential to the process:
(1) Cell proliferation increases cell number in
preparation for cell differentiation. Cell division
(cycle) times in the embryo are as little as 4 hours, so
there can be a 32-fold increase in cell number in a
24-hour period.
(2) Cell migration occurs as cells move into position
to create differentiated cell types.
(3) Cell differentiation is the completion of cell
development, when cells assume their ultimate
phenotype.

8. Oogonia are not shown in this
figure because they
differentiate into primary
oocytes before birth.
Note that
(1) following the two meiotic
divisions, the diploid number
of chromosomes, 46, is
reduced to the haploid number,
23;
(2) four sperms form from one
primary spermatocyte, whereas
only one mature oocyte results
from maturation of a primary
oocyte; and
(3) the cytoplasm is conserved
during oogenesis to form one
large cell, the mature oocyte.
The polar bodies are small
nonfunctional cells that
eventually degenerate.

9. OogenesisOogenesis

10. OVARIAN CYCLE AND OVULATION
The ovarian cycle is regulated by the
gonadotrophin releasing hormone produced
by the hypothalamus.
This acts on the pituitary and stimulate the
production of gonadotrophins, the follicle
stimulating hormone (FSH) and the
leutinizing hormone (LH).

11. Ovulation
During each cycle, 5-15 primordial follicles begin to grow
by the action of FSH.
Usually only one of these, mature and only one oocyte is
released.
The others degenerate and the oocyte and follicular cells
are replaced by connective tissue, forming corpus
atreticum.
The follicular and thecal cells produce estrogens which
induces follicular or proliferative phase of the
endometrium and stimulates the production of LH
(required for follicular maturation and ovulation).

12. TheThe
OvarianOvarian
CycleCycle

14. Ovulation and subsequent changes
 The primordial follicle matures into graffian follicle under
the influence of FSH.
 The primary oocyte completes the first meiotic division.
 The surface of the ovary bulges locally and an avascular sopt
(stigma) appears.
 The oocyte together with the cells of the region of cumulus
oopherus is released from the ovary.
 Cumulus oophorus cells rearrange around zona pellucida to
form zona radiate.

15. Ovulation and subsequent changes
 The oocyte begins the second meotic
division.
 The fimbriae collect the oocyte and
guide into the uterine tube.
 The granulos cells remaining in the
wall of the follicle gives rise to corpus
luteum that secretes progesterone.
 Progesterone together with other
hormones causes the uterine mucosa to
enter progestational or secretory stage.
 If fertilization does not occur, corpus
luteum form a scar tissue called corpus
albicans.
 If fertilization occurs it remains as
corpus luteum of pregnancy
(graviditatis) and continues to produce
progesterone.

16. Fertilization
Fertilization (fusion of the
sperm and egg) normally
occurs in the ampullary
region of the uterine
(fallopian) tube within 24
hours of ovulation.

17. Fertilization
Once the sperm enters
the egg, the male and
female pronuclei come
into close contact and
replicate their DNA, and
cell division then occurs,
creating a two-cell
embryo.

20. Cell division continues as the embryo proceeds along the
uterine tube toward the uterus.
Three days after fertilization, the embryo consists of a
ball of cells called the morula (mulberry).

21. The cells of the morula undergo compaction, a
process whereby cell-to-cell contacts are
maximized through tight junctions, and inner cells
are segregated from outer cells.
As subsequent cell divisions occur, a small group of
inner cells (the inner cell mass, or embryoblast)
becomes segregated from the outer cells (the outer
cell mass or trophoblast).
Over the next 2 days, fluid is pumped from the
outside to the inside, and the morula is
transformed into a hollow blastocyst.

23. Blastocyst
Trophoblast
(outer cells)
Inner Cell Mass
(foetal cells)
Chorion
(outer
embryonic
membrane)
Placenta
(embryonic
contribution
to nutrient
exchange)
Epiblast
(Amnion
and
ectoderm)
Hypoblast
(Yolk sac
and
endoderm)

24. FETAL MEMBRANES
There are four fetal membranes—the
amnion, chorion, yolk sac, and
allantois.

25. FETAL
MEMBRANES
These are thin layers of
tissue which surround
the embryo or fetus and
provide for its nutrition,
respiration, excretion
and protection; they are
the yolk sac, allantois,
amnion, and chorion.

26. FETAL MEMBRANES
In the course of development, the
chorion becomes the outermost, and the
amnion the innermost, membrane
surrounding the developing embryo.
As the allantois increases in size, it
expands and becomes closely
associated, if not fused, with the
chorion. The two membranes together
are known as the chorioallantoic
membrane.
The yolk sac gradually decreases in size
and is eventually incorporated into the
midgut of the embryo.

27. PLACENTA
Placenta is the structure in most mammals that
develops in the uterus along with a fetus to
mediate metabolic exchange.
Nutrients and oxygen in the mother's blood pass
across the placenta to the fetus, and metabolic
wastes and carbon dioxide from the fetus cross in
the other direction; the two blood supplies do not
mix.

28. UTERUS, UTERINE TUBES, AND OVARIES
The body of the uterus narrows from the fundus, the rounded, superior part of the body, to
the isthmus, constricted region between the body and cervix.
The cervix of the uterus is its tapered vaginal end that is nearly cylindrical in shape.
The lumen of the cervix, the cervical canal, has a constricted opening at each end.
The internal os communicates with the cavity of the uterine body and the external os
communicates with the vagina.
Uterus The uterus
a thick-walled, pear-shaped muscular organ
The uterus consists of two major parts:
Body, the expanded superior two thirds
Cervix, the cylindrical inferior one third

29. UTERUS, UTERINE TUBES, AND
OVARIES
The walls of the body of the uterus consist of
three layers
Perimetrium, the thin external layer
Myometrium, the thick smooth muscle layer
Endometrium, the thin internal layer
The perimetrium is a peritoneal layer that is
firmly attached to the myometrium.
During the luteal (secretory) phase of the
menstrual cycle, three layers of the
endometrium can be distinguished
microscopically (see Fig. C)
A thin, compact layer consisting of densely
packed, connective tissue around the necks of
the uterine glands
A thick, spongy layer composed of
edematous connective tissue containing the
dilated, tortuous bodies of the uterine glands
A thin, basal layer containing the blind ends
of the uterine glands

30. During the luteal (secretory) phase of the menstrual
cycle, three layers of the endometrium can be
distinguished microscopically (see Fig. C)
A thin, compact layer consisting of densely
packed, connective tissue around the necks of the
uterine glands
A thick, spongy layer composed of edematous
connective tissue containing the dilated, tortuous
bodies of the uterine glands
A thin, basal layer containing the blind ends of the
uterine glands
The walls of the body of the uterus
consist of three layers
Perimetrium, the thin external layer
Myometrium, the thick smooth
muscle layer
Endometrium, the thin internal layer
The perimetrium is a peritoneal
layer that is firmly attached to the
myometrium.

31. About the sixth day, the blastocyst implants by
attaching itself to the uterine epithelium
The uterus consists of three layers, endometrium, the
mucosal lining, myometrium, thick layer of
smooth muscles and perimetrium, the peritoneal
covering of the outside wall.
During the ovarian cycle the endometrium passes
through the follicular or proliferative phase,
secretory or progestational phase and the
menstrual phase.

32. UTERUS, UTERINE TUBES, AND
OVARIES
At the peak of its development,
the endometrium is 4 to 5 mm
thick.
The basal layer of the
endometrium has its own blood
supply and is not sloughed off
during menstruation.
The compact and spongy layers,
known collectively as the
functional layer, disintegrate
and are shed during
menstruation and after
parturition (delivery of a baby).

33. Proliferation phase:
It begins at the end of the menstrual phase under the
influence of estrogen.
Secretory phase:
This phase begins 2-3 days after ovulation in response to
progesterone produced by the corpus luteum. Implantation
occurs during this phase. During this period three layers of
the uterine mucosa can be distinguished (superficial
compact layer, intermediate compact layer and a thin
basal layer).
Menstrual phase:
If fertilization does not occur the shedding of compact and
spongy layer begins, making the initiation of the menstrual
phase.

34. The Uterine CycleThe Uterine Cycle

36. 2nd week of development
After implantation, over the next several
days, the blastocyst invades this tissue.
By this time, the trophoblast has
differentiated into two layers: an invasive
outer multinucleated cytoplasmic mass
called the syncytiotrophoblast, and an inner
proliferative, the cytotrophoblast.
SYNCYTIOTROPHOBLAST
CYTOTROPHOBLAST

39. The embryoblast reorganizes into two layers, the
epiblast dorsally and the hypoblast ventrally
EPIBLAST DORSALLY
HYPOBLAST VENTRALLY

40. Two cavities are formed, the amniotic cavity dorsal to the epiblast and the
yolk sac cavity ventral to the hypoblast
AMNIOTIC
CAVITY
AND THE
YOLK SAC
CAVITY

41. The epiblast and hypoblast appear as a slightly elongated disc (the
bilaminar germ disc) and it is the epiblast that will give rise to all of the
tissues of the embryo.

42. Proliferation of epiblast
cells at the margins of
the disc forms
amnioblasts that line
the amniotic cavity.
In a similar fashion, a
primitive yolk sac is
created by proliferation
of hypoblast cells at the
disc margins.
Thus, the embryonic disc is
suspended between
these two cavities.

43. Two layers of
extraembryonic
mesoderm are
formed between
the embryo and
its cavities and
cytotrophoblast.
Extraembryonic mesoderm is formed by the cells derived from the yolk sac and forms a connective
tissue network.
Initially, this tissue forms as a single layer, but it soon separates into two layers: a layer around the
yolk sac, which is the extraembryonic splanchnic mesoderm, and a layer over the amnion and on
the inner surface of the cytotrophoblast, which is the extraembryonic somatic mesoderm.

44. By 12 to 14 days, cells of
the syncytiotrophoblast
erode uterine blood
vessels, and maternal
blood fills spaces
(lacunae) that form in the
syncytium, bringing
nutrients closer to the
developing embryo.

46. 3rd week of development
Gastrulation which results in the
development of three germ layers,
ectoderm, endoderm and mesoderm,
occurs during this week.

47. 3rd week of
development
Gastrulation begins
with the formation of a
primitive streak, which
later appears as a
narrow groove, on the
surface of the epiblast.

48. 3rd week of
development
The cephalic end of the
primitive streak is called
primitive node.
The primitive node
consists of a slightly
elevated area surrounding
the small primitive pit.

49. 3rd week of development
Cells of the epiblast migrate toward the primitive streak.
Upon arrival at the primitive streak they become flask
shaped and detach from the epiblast and lie beneath it.
This type of cell movement is called invagination.

50. 3rd week of development
Some of the invaginating cells displaces the hypoblast,
giving rise to endoderm.
Some of the other cells occupy the region between the
newly formed endoderm and the epiblast forming the
mesoderm.
The cells remaining in the epiblst forms the ectoderm.

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