gastrulation and germ layers Flashcards
what is gastrulation
process of epiblastic cells movement (ingression or invagination) that leads to the transformation of the Bilaminar Germ Disc into a Trilaminar Germ Disc… with the establishment of all the three primary germ layers – Ectoderm, Mesoderm and Endoderm.
Gastrulation commences with the formation of a
morphogenic inductive centre” called the Primitive Streak on about the 15th Day
morphogenic inductive centre- primitive streak
-through which the epiblastic cells ingress or invaginate to form the mesoderm and endoderm, while the remaining epiblastic cells become the ectoderm.
The formation of the Primitive Streak and gastrulation establish
all the major body axes: cranio-caudal axis, medio-lateral axis, dorso-ventral axis and left-right axis.
One of the congenital malformations resulting from errors in Gastrulation is
Caudal Dysplasia, also called Caudal Regression Syndrome, Caudal or Sacral Agenesis (e.g. Sirenomelia).
derivatives of the germ layers
very complex cross overs
the Ectodermal Germ Layer gives rise to
organs and structures that maintain contact with the outside world.
what is induced to for the neuroectoderm?
ectodermal layer
neuroectoderm leads to the formation of
the neural tube (via Neurulation) from where the nervous system (Central and Peripheral) is derived.
Also gives rise to the Neural Crest cells and pituitary gland.
other ectoderm formed structures
- sensory epithelium of the ear, nose and eye.
- epidermis of the skin and other integumentary structures – hair, nails, sweat glands, mammary glands, etc.
- enamel of the teeth.
- oral and proctodeal (anal canal) epithelium
what 3 layers make up the mesoderm?
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Paraxial mesoderm form
segmental blocks called somitomeres which in the head region is called neuromeres in assoaciation with the neural plate.
Paraxial mesoderm form
segmental blocks called somitomeres which in the head region is called neuromeres in assoaciation with the neural plate.
The neuromeres contribute to
the mesenchyme (connective tissue) in the head
Somites
From the neck (occipital) region downwards, the somitomeres form segmemented pair of blocks of mesoderm, on either side of the developing neural tube
the first pair of somite arise
in the occipital region of the embryo at about the 20th day of development.
New somites appear craniocaudally at the rate of about
3 pairs per day
at the end of the 5th week how many pairs of somites have formed
about 42 to 44 pairs of somites would have been formed.
number and distribution of somites
There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8-10 coccygeal pairs of somites.
how many commits would a 23 day old embryo have
12 somites
how can the age of an embryo be accurately determined
by counting somites numbers during this period of development because somites are formed with a specific periodicity.
The Intermediate mesoderm:
Contribute in the formation of the structures of the urogenital system including the primordial germ cells, gametes and gonads.
The Lateral Plate mesoderm:
splits into parietal (somatic) and visceral (splanchnic) layers.
The parietal layer lines the body (intraembryonic) cavity walls.
Mesoderm of the visceral layer together with the endoderm form the wall of the gut tube
Mesoderm from the parietal layer together with the overlying ectoderm, forms
the lateral body wall
what does the parietal layer of lateral plate mesoderm also form?
the dermis of the skin in the body wall and limbs, bones and connective tissue of the limbs and sternum.
the costal cartilage, limb muscles and most of the body wall muscles.
the mesothelial or serous membranes (parietal layer) that line the peritoneal, pleural and pericardial cavities and secret serous fluid.
what does the visceral layer of lateral plate mesoderm also form
the serous membrane (visceral layer) that surrounds organs.
the blood cells and blood vessels.
The main derivative of the endodermal germ layer is the
Gastrointestinal Tract including the pharynx.
The main derivative of the endodermal germ layer is the
Gastrointestinal Tract including the pharynx.
what does the endodermal germ layer also form
the epithelial lining of the respiratory tract, the urinary bladder and urethra, and the tympanic cavity and auditory tube.
the parenchyma of the thyroid, parathyroids, liver, and pancreas.
that walls of the GIT – smooth muscles and other connective tissues are derived from
the mesoderm - because muscles
