Formation Of Germ Layers

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germ layer

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is a group of cells, formed during animal embryogenesis, give rise to all of an animal’s tissues and organs through the process of organogenesis.

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Fertilization

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-of the ovum takes place in the ampulla of the uterine tube.
--Fertilization - Egg activation - Zygote - Cleavage - Morula - Blastula (Blastomere) - Blastocyst - Inner cell mass.

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morula

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16 cells after division
-Becomes a blastocyst

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the TROPHOBLAST.

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It has an inner cell mass covered by an outer layer of cells,

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embryonic disc

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-when cells of inner cell mass multiply and rearrange
-3 germ layers ( ectoderm, mesoderm, endoderm)

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amniotic cavity

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cavity on ectodermal side of disc

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yolk sac

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cavity appears on the endodermal side

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extra embryonic mesoderm

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separates amniotic cavity and yolk sac

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extra embryonic coelum

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cavity that splits EEM into somatopleuric layer and splanchopleuric layer

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CHORION.

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The trophoblast & underlying somatopleuric mesoderm from a membrane

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AMNION

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The cells forming the wall of the amniotic cavity

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connecting stalk

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formed when amniotic cavity is attached to trophoblast by some mesoderm (EEC not extended)

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PROCHORDAL PLATE

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in the embryonic disc where ecto & endo are not seperated by mesoderm

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PRIMITIVE STREAK

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is also seen on the embryonic disc.

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cytotrophoblast

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trophoblast cells that maintain cellular integrity (they do not form a syncytium)

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The Hypoblast

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-is a tissue type that forms from the inner cell mass.
- It lies beneath the epiblast and consists of small cuboidal cells.

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the Epiblast

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-is a tissue type derived from the inner cell mass.
-It lies above the hypoblast.

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amniotic cavity (2)

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-formed by fusion of parts of the amniotic fold
-forms as amniotic fold rises and fuses over the dorsal aspect of embryo

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Syncytiotrophoblast

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-multinucleated cells found in the placenta of embryo
-increase S.A. for nutrient exchange between mother and fetus
-screte progesterone and hCG

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progesterone

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maintain integrity of uterine lining

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hCG

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prevents degeneration of CL

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The Cytotrophoblast

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-(or layer of Langhans)
-is the inner layer of the trophoblast, -interior to the syncytiotrophoblast in an embryo.
-It serves to anchor the embryonic chorion to the maternal endometrium.

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chorionic villi

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-outgrowths of throphoblast from the chorion project into endometrium
-give rise to placenta

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secondary villus

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Mesodermal cells penetrate the core of the primary villi and grow in the direction of the Decidua

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chorionic villi

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-outgrowths of trophoblast form the chorion project into endometrium
-give rise to placenta

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Tertiary villus

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- or Definitive placental villus.
-3rd week,
-the Mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, thus forming the villous capillary system.

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A blastocoel(e)

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or blastocele
- (also called blastocyst cavity
-,cleavage cavity
- or segmentation cavity)
- is the fluid-filled central region of a blastocyst.
-formed during embryogenesis

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The endometrium

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is the inner membrane of the mammalian uterus.

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gastrulation

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-3rd week
-most important event
-creates 3 germ layers

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Archenteron

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marks the onset of gastrulation.

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primitive streak (2)

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-first sign of gastrulation
-a furrow in the midline of the embryonic disk at the future caudal end of the embryo
-establishes a visible longitudinal axis of bilateral symmetry around which all embryonic structures will organize and align.
-error:germ cell tumors
-primitive knot starts as a regional knot of cells that forms on the blastodisc immediately anterior to where the outer layer of cells will begin to migrate inwards

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furrow

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-formed by ingression of epiblast cells
-will form endoderm and mesoderm by replacing hypoblast cells

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epiblast cells

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will develop into ectoderm

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Neurulation

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is the series of developmental events that result in the beginnings of the central nervous system CNS

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notochord

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-developes in mesoderm
-marks head/tail axis in embryo

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neural crest cells

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give rise to variety of nerve cells
-dorsal root/ spinal
-autonomic nervous system ganglia
-endocrine structures

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ectoderm

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-the start of a tissue that covers body surfaces
-emerges first and forms from the outermost of germ layers
-forms: CNS
PNS
SKIN EPIDERMIS
HAIR
NAIL
PITUITARY
MAMMARY
SWEAT GLANDS
SENSORY EPITHELIUM OF EAR NOSE AND EYE
THE LENS OF THE EYE
PIGMENT CELLS.

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MESODERM

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-layer forms in the embryos .
-cells migrating inward during gastrulation
-lead to development of coelom
-forms: SOMITES
THE SKELETON
Muscle Tissue ,Cartilage & Bone (CT)
VASCULAR SYSTEM—Heart, Arteries, Veins, Lymph Vessels, Blood Cells and Lymph Cells
UROGENITAL SYSTEM---Kidneys, Gonads(Except bladder)
SUPRARENAL GLANDS & SPLEEN
SKIN DERMIS.

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endoderm

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-formed during embryogenesis
-cells migrating inward along the archenteron from inner layer of gastrula
-consists of flattened cells that turn columner
-forms epithelial lining of digestive tube (not mouth, pharynx,rectum)
-forms lining cells of all glands that open into digestive tube
-GIT (STOMACH, COLON,INTESTINES)
EPITHELIAL PARTS OF TRACHEA
THE LUNGS, PHARYNX
Urinary Bladder
LINING OF THE URETHRA
THYROID
PARATHYROID
LIVER
PANCREAS

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twins

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-monozygotic (MZ, collequially identical)
-dizygotic (D2, colloquially fraternal)

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singleton

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fetus that develops alone in womb

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dyzgotic twins

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-fraternal twins
-biovular twins
-2 fertilized eggs are implanted in the uterine wall at same time
-when 2 eggs are independently fertilized by 2 dif sperm cells
-look dif from each other
-dif sexes

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monozygotic twins

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-identical twins
-share same womb
-single egg fertilized to form 1 zygote which then divides into 2 separate embryos
-same sex
-sim triats
-nearly identical DNA

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Complications of twin pregnancy

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Vanishing twins
Conjoined twins or "Siamese twins“
Parasitic twins
Low birth weight
Twin-to-twin transfusion syndrome