embryonic
TRANSCRIPT
-
7/27/2019 embryonic
1/40
Embryonic Period
By
Dr Daw Khin Win
-
7/27/2019 embryonic
2/40
Objectives
1. Describe the formation and derivativesof secondary mesoderm.
2. Define notochord and name its fate and
functions.3. Name the germ layers and list the
derivatives of each layer.
4. Define somite and describe thederivatives of somites.
-
7/27/2019 embryonic
3/40
Gastrulation
The process of the formation of three germ layersis called Gastrulation.
This trilaminar germ disc is the basis of later
embrynic development that gives rise to derivativesof germ layers- tissues, organs and organ-systems.
It takes place in the third week after
fertilization.
Folding of embryo and organogenesis follow in thefourth week.
-
7/27/2019 embryonic
4/40
2nd Week of Development(Bilaminar Germ Disc)
At the time of implantation, the embryoblast consists of only one typeof cells.During the 2nd week of development this embryoblast differentiatesinto 2 types of cells:Epiblast columnar cells; faces the amniotic cavityHypoblast cuboidal cells; faces the yolk sac
-
7/27/2019 embryonic
5/40
2nd Week of Development(Bilaminar Germ Disc)
Cells of epiblastgive rise toanother layersuperficially thatbecomes the
amnion or amnioticmembrane and theamniotic cavity isformed between itand the epiblast.
Epiblastproliferates toform two layer ofcells- epiblast andhypoblast.
Hypoblastproliferates andformsextraembryonicmembrane enclosingthe cavity, primary
yolk sac.
-
7/27/2019 embryonic
6/40
Primary or Extraembryonic Mesoderm
-
7/27/2019 embryonic
7/40
Extraembryonic Coelom
-
7/27/2019 embryonic
8/40Primitive streak
BilaminarGerm disc
Syncytiotrophoblast
Cytotrophoblast
ExtraembryonicMesodermDefinitive Yolk Sac
Buccopharyngeal MembraneCut Edge of Amnion
Hypoblast Epiblast
Wall of Yolk Sac
-
7/27/2019 embryonic
9/40
Primitive nodeand pit
Primitive streak
Prochordalplate
Cloacal plate
Primitive StreakAt the beginning of the 3rd week, the development of the embryoblast is markedby the appearance of the primitive streak.The primitive streak appears as a midline linear streak bilateral symmetryOn the epiblast surface of the bilaminar germ disc dorsal/ventralAt the caudal half cranial/caudal
-
7/27/2019 embryonic
10/40
Epiblast
Invaginating cells Hypoblast
Formation of Secondary MesodermPrimitive Pit Primitive Streak
Amnion
YolkSac
At the beginning of the 3rd week, a midline linear groove called the
primitive streak appears on the epiblast surface in the caudal half of the embryo.Cells of the epiblast migrate in the direction of the primitive streak.
On arrival they become flask-shaped, detach from the epiblast and invaginatebetween the epiblast and hypoblast layers.
This migration of cells into the primitive streak occurs until the end ofthe 4th week after which the primitive streak disappears.
-
7/27/2019 embryonic
11/40
The primitive streak consists of 2 parts;
Primitive groovePrimitive node (at cephalic end)
Epiblast cells migrate laterally and forward between the epiblast & hypoblast layers.In doing so, they do not cross:
Prochordal plate epiblast & hypoblast are firmly fused at this pointMidline where the notochord is developingCloacal plate epiblast & hypoblast are also fused here.
-
7/27/2019 embryonic
12/40
Prochordal plate
Cloacal plate
(Ruptures at endof 3rd week)
(Ruptures at end of 8th
week)
NotochordalProcessPrimitive
Node &
Pit
PrimitiveGroove
Formation of NotochordThe primitive node is seen at the cephalic end of the primitive streak.In the center is a pit called primitive pit.The cells of the primitive node also proliferates and the pit also extends into thisas a lumen. The structure so formed is called the notochordal process.The notochordal process extends cranially until it reaches the prochordal plate.
As the primitive streak regresses, the notochordal process lengthens caudally untilit reaches cloacal plate.
-
7/27/2019 embryonic
13/40
Origin & Derivatives of Three Germ Layers
-
7/27/2019 embryonic
14/40
NotochordThe lower layer of the notochordal process fuses with the
underlying hypoblast and perforations appear, gradually leading to its
disappearance.Thus a temporary connection between the amniotic and yolksac cavities is formed by way of the neurenteric canal.
The hypoblast on each side of the perforation closes up thedefect and the connection is then lost.
-
7/27/2019 embryonic
15/40
-
7/27/2019 embryonic
16/40
Function and Fate of Notochord
The remaining upper layer of the notochord proliferates to
form a solid cord called the definitive notochord.This extends as a solid cord between the prochordaland cloacal plates.
Function1. To serve as a temporary axis2. To induce the development of the neuro-ectoderm &
neural plate3. To initiate the formation of the vertebra or axial
skeleton
Fate of notochordThe notochord in the region of the vertebra bodies
disappearIn the region of the inter-vertebral disc it remains as
nucleus pulposus.
-
7/27/2019 embryonic
17/40
Formation of Neural tube
The midline ectodermal cells of epiblast proliferate and form a plate calledneural plate.
It folds and forms the neural tube.Some cells of neural plate remains laterally to the neural tube and form the
neural crest.The neural tube sinks into the mesoderm and surface ectoderm grows over it.The neural tube passes throug the stage of neuropores, anterior andosterior that close later.
-
7/27/2019 embryonic
18/40
Formation of Neural tube
-
7/27/2019 embryonic
19/40
Formation of Secondary MesodermOnce the cells of primitive streak have invaginated, some displace the
hypoblast creating the embryonic endoderm, and others come to liebetween the epiblast and newly formed endoderm to formmesoderm.
Cells remaining in the epiblast then form ectoderm.Thus, the epiblast, through the process of gastrulation, is the source
of all of the germ layers.
-
7/27/2019 embryonic
20/40
Trilaminar Germ DiscThe newly replaced
hypoblast layer is now called
endoderm.The cells filling thespace between the epiblastand endoderm cranio-caudallyare now called secondarymesoderm.
The primitive streakregresses in size as thisfulfillment is completed andat the end of the 4th weekit completely disappears.
After its
disappearance, the epiblastis now called ectoderm. Theembryo is now a trilaminargerm disc.
-
7/27/2019 embryonic
21/40
Secondary mesoderm differentiates into3 parts on each side of the embryo.These 3 parts are-1. Paraxial mesoderm2. Intermediate mesoderm3. Lateral plate mesoderm
These 3 divisions of mesoderm give rise to specific structures in the adult.
-
7/27/2019 embryonic
22/40
Initially, cells of the mesodermal germlayer form a thin sheet of loosely
woven tissue on each side of themidline.By 17th day, cells close to the
midline proliferate and form a
thickened plate of tissue known asparaxial mesoderm.More laterally, the mesoderm layer
remains thin and is known as the
lateral plate.Intermediate mesoderm connectsthe paraxial & lateral plate mesoderm
-
7/27/2019 embryonic
23/40
Paraxial mesoderm
- Lies on each side of the notochord.This will form:-
The supporting tissues such as:
Connective tissue,Bone and cartilages of the axialskeleton,
Skeletal muscle andDermis of the skin.
-
7/27/2019 embryonic
24/40
Intermediate mesoderm- Lies lateral to the paraxial mesoderm.- This connects the paraxial mesoderm and lateral plate mesoderm.- Also known as Urogenital ridge
This forms:1. Urinary system (kidneys & ureters) and
2. Genital system (gonads and genital ducts)
-
7/27/2019 embryonic
25/40
Lateral plate mesoderm- Lies lateral to the intermediate
mesoderm.
On Day 17, this splitsinto 2 layers: a ventral layerassociated with endoderm and adorsal layer associated withectoderm.
The ventral layer iscontinuous with the extra-embryonicmesoderm covering the yolk sac andis called the splanchnic mesoderm(visceral).
The dorsal layer iscontinuous with the extra-embryonicmesoderm covering the amnioticcavity and is called the somaticmesoderm (parietal).
Together these layers linea newly formed cavity, theintraembryonic cavity (afterembryonic foldings) which iscontinuous with the extra-embryoniccavity on each side of the embryo.
-
7/27/2019 embryonic
26/40
The lateral plate mesodermwill form:-
1. The blood and lymphaticcells: heart and vessels;
2. Skeletal muscles of thebody walls,3. Smooth muscles,4. Cortex of the adrenal gland5. Spleen6. Lining of the body cavities.
-
7/27/2019 embryonic
27/40
-
7/27/2019 embryonic
28/40
Somites
SOMITESBy the beginning of the 3rd week, the paraxial mesoderm is
organized into segments called somitomeres (Somites).The somitomeres appear first in the cephalic region and then
proceed cranio-caudally.Somitomeres are mesodermal cells arranged in concentric whorls
around the center of the unit
-
7/27/2019 embryonic
29/40
From the occipital region to caudally,somitomeres are organized into
somites.The 1st pair of somites appear in
the cervical region at approximately
on the 20th day of development.From here new somites appear in
cranio-caudal sequence at a rate of
approximately 3 pairs/day until theend of the 5th week.
-
7/27/2019 embryonic
30/40
Altogether 42 44 pairs of somites areformed.
There are:-- 4 occipital,- 8 cervical ,- 12 thoracic,- 5 lumbar,- 5 sacral and- 8 10 coccygeal somites.
The 1st occipital and the last 5 8 coccygealsomites disappear. Remaining occipitalsomites form the tongue muscles.
-
7/27/2019 embryonic
31/40
Derivatives of Somites
At the beginning of the 4th week, the rest of thesomites lose their compact organization and becomepolymorphous and differentiate into:-a. ventromedian (sclerotome) andb. dorso-lateral (dermo-myotome).
The cells forming the sclerotomewill surround thespinal cord and notochord to form the vertebral
column.The dermo-myotomegives rise to a new layer of cellscalled myotomewhich will contribute to theformation of the skeletal muscles; while thedermatomeform the dermis and subcutaneoustissue of the skin.
Hence each somite forms its own sclerotome, its ownmyotome and its own dermatome.Each myotome and dermatome has its own segmental
nerve component.
-
7/27/2019 embryonic
32/40
Intermediate mesoderm- Lies lateral to the
paraxial mesoderm.- This connects the paraxial
mesoderm and lateralplate mesoderm.
- Also known as urogenital
ridge
This forms:1. Urinary system (kidney &
ureter) and2. Genital system (gonads
and genital ducts).
-
7/27/2019 embryonic
33/40
HEAD FOLD and TAIL FOLD
Head fold
Tail fold
Allantois
-
7/27/2019 embryonic
34/40
Head fold
Tail fold
Note: the allantois(a) isfolded with the tail into theembryo
Folding of the EmbryoEmbryonic folding
1. Head and tail folds2. Lateral foldin
As a result of cephalo-caudal folding , a continuously large portion of the endoderm-lined cavity isincorporated into the body of the embryo proper as the primitive gut.
The middle part of the gut temporarily communicates with the yolksac by way of a broad stalk, the vitelline duct.
This duct is wide initially, but with further growth of the embryo,
it becomes narrow and much longer.
V aduct
Head Fold
-
7/27/2019 embryonic
35/40
HEAD and TAIL FOLDING
-
7/27/2019 embryonic
36/40
-
7/27/2019 embryonic
37/40
-
7/27/2019 embryonic
38/40
-
7/27/2019 embryonic
39/40
Applied Anatomy
Cells of primitive streak are pluripotent, so remnants ofthe streak give rise to sacrococcygeal teratomacontaining many types of tissue that change to malignanttumor.
Neural tube defects are very common as it takes placein the 3rd week.
Spina bifida and anencephaly are two common anomaliesof CNS.
Abnormal development of trophoblats lead to
choriocarcinoma, a malignant tumor.
-
7/27/2019 embryonic
40/40
Sacrococcygeal Teratoma Anencephaly & Hydrocephalus