Third week of development.pdf

Third week of human development

1. 3
rd
week of development

2. Events :
•Gastrulation
•Neurulation
3rd week ofembryonic development is characterized by:
•Appearance of primitive streak
•Development of notochord
•Differentiation of three germ layers

3. •The most characteristic event occuring during third week of
gestation is Gastrulation.
•It is the process of formation of all 3 germ layers in the
embryo (trilaminar embryo):
1. Ectoderm
2.Mesoderm
3.endoderm
•Gastrulation begins with the formation of primitive streak on
the surface of the epiblast. Initially the streak is vaguely
defined but in a 15-16 day embryo it is clearly visible as a
narrow groove with slightly bulging regions on either side.
The cephalic end of streak the primitive node ,consist of
slightly elevated area surrounding the small primitive pit.
Gastrulation:

7. •Cells of the epiblast migrates toward the
primitive streak. On arival they become flask
shaped, detached from epiblast and slip beneath
it. This invard movement is called invagination.
•Cell migration and specification are controlled by
fibroblast growth factor 8 (FGF8), which is
synthesized by streak cells themselves. This
growth factor controls cell movement by
downregulating E-cadherin(a protein that bind
epiblast cells together).
•FGF8 then control cell specification into
mesoderm by regulating Brachyury (T)
expression.

8. •Once the cells have invaginated , some displace
the hypoblast creating endoderm and other
come to lie between the epiblast and newly
created endoderm to form mesoderm cells
remaining in the epiblast then form ectoderm.
•Thus the epiblast through the process of
gastrulation is the source of all of germ layers
and cells in these layers will give rise to all of
the tissues and organs in the embryo.

12. •As more and more cells move between the epiblast
and hypoblast they begin to spread laterally and
cranially. Gradually they migrate beyond the margin of
disc and establish contact with extraembryonic
mesoderm covering the yolk sac and amnion.
•In the cephalic direction they pass on each side of the
precordal plate.
•The precordal plate itself forms between the tip of
notocord and oropharyngeal membrane. And is
derived from some of the first cells that migrate
through the node in midline and move in cephalic
direction.later the precordal plate will be important for
formation of forebrain.
•The oropharyngeal membrane at the cranial end of
disc consist of small region of tightly adherent
endoderm cells that represent future opening of oral
cavity.

14. •Ectoderm develops into CNS
•Mesoderm forms the CVS
•Endoderm forms the Alimentary system and
respiratory system.

15. Significance of gastrulation
•Convert the bilaminar to trilaminar disc.
•Establishes the cranio-caudal axis and bilateral
symmetry.
•Induce embryonic cells to form organ system.

16. Notochordal process and notochord:
•Notochordal process:
•Some mesenchymal cells that have ingressed through
the streak, acquired mesodermal cell and migrate
cranially from the primitive node and pit, forming a
median cellular cord, the notochordal process.
•This process soon acquires a lumen, the notochordal
canal.
•Prenotochordal cells invaginating in the primitive node
move forward cranially in the midline until they reach
the prechordal plate.
•The notochordal process grows cranially between the
ectoderm and endoderm until it reaches the
prechordal plate.

17. •As the hypoblast is replaced by endoderm cells moving in at
the streak, cells of the notochordal plate proliferate and
detach from the endoderm. They then form a solid cord of
cells, the definitive notochord, which underlies the neural
tube and serves as the basis for the axial skeleton.
• The prechordal plate is the primordium of the oropharyngeal
membrane, located at the future site of the oral cavity.
•Caudal to the primitive streak there is a circular area-the
cloacal membrane, which indicates the future site of the anus.

19. •cranially on each side of the notochordal process is a region
of mesoderm called the cardiogenic region(area).
•This region lies rostral(anterior/cranial) to the prochordal
plate, and it is where the primordium heart begins to develop
at the end of the third week.
•The embryonic disc remains bilaminar at the cloaca and
oropharyngeal membrane
•This is because the embryonic ectoderm and endoderm are
fused at these sites, thereby preventing migration of
mesenchymal cells (which form mesoderm) between them.
•By the middle of the 3rd week, intraembryonic mesoderm
separates the ectoderm and endoderm everywhere except at
the:
1.Oropharyngeal membrane cranially
2.In the median plane cranial to the primitive node, where the
notochordal process is located
3.At the cloacal membrane caudally

27. 5

30. •The cloacal membrane is formed at the caudal end of
the embryonic disc. This membrane, which is similar in
structure to the oropharyngeal membrane, consists of
tightly adherent ectoderm and endoderm cells with no
intervening mesoderm. When the cloacal membrane
appears, the posterior wall of the yolk sac forms a
small diverticulum that extends into the connecting
stalk.
•This diverticulum the allantoenteric diverticulum or
allantois appears around 16
th
day of development .
Although in some lower vertebrates the allantois
serves as a reservoir for excretion product of the renal
system, in humans it remains rudimentary but may be
involve in abnormalities of bladder development.

33. •The embryonic period or period of
organogenesis occurs from 3
rd to 8
th weeks of
development and is the time when each of
the three germ layers, ectoderm, mesoderm,
and endoderm, gives rise to a number of
specific tissues and organs
•By the end of the embryonic period, the main
organ systems have been established,
rendering the major features of the external
body form recognizable by the end of the
second month.

34. Neurulation
•It is the process by which the neural plate forms neural
tube.
•During neurulation, the embryo may be referred to as
a neurula.
•The stages of neurulation include the formation of:
1.Neural plate
2.Neural groove
3.Neural folds & their fusion
4.Neural crest cells
5.Neural tube

35. •Under the inducing
effect of the
developing
notochord, the
overlying
ectodermal cells
thickens to form the
neural plate.

43. Congenital anomalies
•Disturbance of neurulation may result in severe
abnormalities of the brain and the spinal cord.
•Most defects are the result of non-closure or defective
closure of the neural tube:
ØIn the brain region (e.g. anencephaly: total absence of the
brain
ØMeroencephaly (partial absence of the brain) is the most
severe neural tube defect and is also the most common
anomaly affecting the CNS.
ØIn the spinal cord regions (e.g. spina bifida)

45. Spina bifida

46. Derivatives:
qEctoderm:
ØSurface ectoderm:
•Epidermis of the skin
•Hair
•Nail
•Sweat & Sebaceous glands
•Mammary glands
•Enamel of the teeth
•Lens of eye
•Internal ear
•Anterior lobe of the pituitary gland
ØNeuroectoderm:
•Neural Tube
•Neural Crest Cells

47. ØNeural Tube Derivatives:
•Central nervous system
•Peripheral nervous system
•Retina
•Sensory epithelia of nose & ear
•Pineal gland
•Posterior lobe of the pituitary gland
ØNeural Crest Cells Derivatives:
•Sensory ganglia (cranial & spinal)
•Autonomic ganglia
•Meninges (Pia mater & Arachnoid mater) of the brain & spinal cord
•Schwann cells
•Satellite cells
•Melanoblasts
•Suprarenal medulla
•Several skeletal & muscular components in the head (derived from
pharyngeal arches)

48. qMesoderm:
Differentiates into the:
1.Paraxial mesoderm
2.Intermediate cell mass
3.Lateral plate mesoderm

50. ØParaxial mesoderm:
•By the beginning of the 3rd week, paraxial mesoderm is
organized into segments called somitomeres.
•Somitomeres appear first in the cephalic region of the embryo,
and their formation proceeds cephalocaudally (extends
cranially and caudally /from head to tail)
•From the occipital region caudally, somitomeres further
organize into somites.
•Toward the end ofthe 3
rd
week (at approximately the 20
th
day
of development), the 1st pair of somites arises in the occipital
region.
•From here, new somites appear in craniocaudal sequence at a
rate of approximately 3 pairs per day.
•The somite period of human embryo development is from days
20 – 30.
•About 38 pairs of somites are present on day 30
•By the end of the 5th week, 42 to 44 pairs of somites are present.

52. •There are:
ü4 occipital,
ü8 cervical,
ü12 thoracic,
ü5 lumbar,
ü5 sacral,
üand 8 to 10 coccygeal pairs.
The 1st occipital and the last 5-7 coccygeal somites later disappear.
The remaining somites divide into:
üVentromedial part called sclerotome gives rise to the bones,
cartilages and ligaments of the vertebral column & ribs.So, the
somites give rise to most of the axial skeleton.
üMiddle part called myotome gives rise to skeletal muscles of the
chest and abdomen.
üDorsolateral part called dermatome It gives rise to dermis and
subcutaneous tissue of the skin.

54. ØIntermediate mesoderm:
•connects paraxial mesoderm with the lateral plate
differentiates into urogenital structures.
•Excretory units of the urinary system (kidney) and the
gonads (testis and ovary) develop from the intermediate
mesoderm
ØLateral mesoderm
•is a thin plate of mesoderm located along the lateral
sides of the embryo. Large spaces develop in the lateral
mesoderm and from the intraembryonic coelom
•The intraembryonic coelom divides the lateral mesoderm
into 2 layers:
1.somatic/ parietal layer of lateral mesoderm
2.splanchnic or visceral layer of lateral mesoderm

57. •Somatic/parietal layer : is located beneath the
ectodermal epithelium and continuous with the
extraembryonic mesoderm covering the amnion.
•Splanchnic or visceral layer: is located adjacent to the
endoderm and continuous with the extraembryonic
mesoderm covering the umbilical vesicle (yolk sac).
üThe somatic mesoderm and overlying embryonic
ectoderm form the embryonic body wall or
somatopleure.(serous membrane inner lining of
coelom)
üwhereas the splanchnic mesoderm and underlying
embryonic endoderm form the embryonic gut or
splanchnopleure (wall of gut )

58. üDuring the second month, the intraembryonic
coelom is divided into three body cavities:
•Pericardial cavity
•Pleural cavities
•Peritoneal cavity

59. Blood and Blood Vessels:
•Blood cells and blood vessels also arise from mesoderm.
•Blood vessels form in two ways:
1.Vasculogenesis: whereby vessels arise from blood islands and
2.Angiogenesis: which entails sprouting from existing vessels
ØA) Vasculogenesis ( blood vessels)Mesenchymal cells differentiate
into endothelial precursors called angioblasts (vessel-forming cells).
•Angioblasts aggregate to form blood islands.
•Small cavities appear within the blood islands.
•Angioblasts flatten and arrange themselves around the cavities to
form endothelial cells of blood vessels.
These endothelium – lined cavities soon fuse to form networks of
endothelial channels
ØB : Angiogenesis: entails sprouting from existing vessels.

61. Blood cells formation:
•Blood cells(hamatoblasts) develop from the
endothelial cells of vessels develop on the
umbilical vesicle(yolk sac) and allantois at the
end of the 3rd week.
•Blood formation (hematogenesis) does not
begin in the embryo until the fifth week.
•It occurs first along the aorta and then in
various parts of the embryonic mesenchyme,
mainly, the liver, and later in the spleen, bone
marrow, and lymph nodes.

62. Primordial cardiovascular system:
•Heart & great vessels develop from mesenchymal
cells in the cardiogenic area.
•Paired longitudinal endothelial lined channels or
endocardial heart tubes develop during the 3rd
week.These tubes fuse to form the heart tube.
•The tubular heart joins with blood vessels in the
embryo, connecting stalk, chorion and yolk sac to
form a primordial cardiovascular system.
ØHeart begins to beat on 21-22 days and blood
circulates
ØCVS is the first organ system to reach a functional
state.

63. qEndoderm:gives rise to the epithelial lining
of the gastrointestinal and respiratory tracts,
parenchyma of the tonsils, thyroid and
parathyroid glands, thymus, liver, and
pancreas, epithelial lining of the urinary
bladder and most of the urethra, and the
epithelial lining of the tympanic cavity,
tympanic antrum, and pharyngotympanic
(auditory) tube.

64. Major derivatives of the embryonic germ layers: