L3: Third week of development (Gastrulation) Flashcards
What are the changes during the 3rd week?
β Changes in the structure of bilaminar embryonic plate:
ο² Development of prochordal plate
ο² Development of primitive streak (Groove)
ο² Development of notochord
β Formation of 3 germ layers:
ο² Gastrulation
prochordal plate
Site:
β near the cranial end in the midline
Cause:
β elongation (in height) of the primary endoderm (hypoblast): β’ It is the site of the oropharyngeal membrane
What is the site of the primitive streak?
β Opacity formed caudally in the midline (from caudal end to middle of the embryo)
Formation of the primitive streak
β proliferation and migration of the epiblastic cells:
β’ Primitive node: the proliferation of cranial end of the primitive streak
β’ Primitive pit: depression in the middle of the primitive node
What is the fate of the primitive streak?
β Diminishes in size and becomes an insignificant structure in the sacrococcygeal region of the embryo
β Disappears by the end of the fourth week (if it persists, it will lead to the formation of sacrococcygeal teratoma)
What is the definition of notochord?
β The core around which the vertebral column formed
What is the source of notochord?
β epiblast of primitive streak & nodes
What is the site of notochord?
β in the midline between ectoderm & endoderm
What are the stages of formation of notochord?
β Notochordal process: 17th day
β Notochordal canal
β Notochordal plate
β Notochord: 20th day
What is the function of notochord?
β Axis for the formation of the vertebral column
β Produce intraembryonic mesoderm
β Initiate (stimulate) neutralization (neural tube formation)
What is the fate of notochord?
β The cranial part: some parts of bones forming skull base
β Parts in the center of the bodies of vertebrae: Degenerate and disappear
β Parts in the intervertebral discs: Undergo mucoid degeneration to form nucleus bulbosus.
What is the definition of gastrulation?
β the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc.
What is the time of gastrulation?
During the 3rd week
What are the steps of the process of gastrulation?
1) Cells from primitive streak migrate and are arranged between epiblast & hypoblast forming intraembryonic mesoderm.
2) The epiblast of primitive streak displaces the hypoblast, forming the embryonic endoderm
3) The cells remaining in the epiblast from the embryonic ectoderm
What is the first morphological sign of gastrulation?
β The first morphologic sign of gastrulation is the formation of the primitive streak
What happens by the middle of 3rd week?
β By the middle of the 3rd week, the intraembryonic mesoderm separates the ectoderm and endoderm everywhere except:
ο² At oropharyngeal membrane cranially.
ο² In median plane cranial to the
primitive node (site of notochord)
ο² At the cloacal membrane caudally.
What is the origin of the intraembryonic mesoderm?
Primitive streak, Primitive node & Notochord
What are the steps of the development of the intraembryonic mesoderm?
β A longitudinal groove appears on each side of the notochord divides the intraembryonic mesoderm into:
β’ paraxial mesoderm: medial to the longitudinal groove
β’ intermediate cell mass: the floor of the longitudinal groove
β’ lateral plate mesoderm: lateral to the longitudinal groove
What is the definition of paraxial mesoderm?
β the medial part of the intraembryonic mesoderm on each side of the notochord
What are the steps of the development of paraxial mesoderm?
β The intra-embryonic mesoderm proliferates to form a thick column of paraxial mesoderm
β At end of 3week: the paraxial mesoderm divides into segments (somites) by transverse grooves
What is the number of Somites?
β 42 to 44 pairs of somites:
β’ 4 occipital
β’ 8 cervical
β’ 12 thoracic 5 lumbar
β’ 5 sacral
β’ 8-10 coccygeal
What is the sequence of appearance of somites?
β Appear in the craniocaudal direction
What is the fate of somites?
β Ventromedial part (sclerotome):
β’ Give axial skeleton (vertebral column& ribs
β Dorsolateral part (dermo-myotome):
β’ Dermatome: gives dermis & fascia
β’ Myotome: gives skeletal muscles
What is the definition of intermediate cell mass?
β mesoderm corresponds to the floor of the longitudinal groove
What is the fate of intermediate cell mass?
β Kidney & Cortex of suprarenal gland
β Testis or ovary
β The male and female genital duct system
What is the definition of the lateral plate mesoderm?
β mesoderm lateral to the longitudinal groove
What are the steps of the development of lateral plate mesoderm?
β Small cavities appear in it
β These cavities unite to form a single horse-shoe cavity (intraembryonic coelom) that divide the lateral plate mesoderm into somatic (partial), splanchnic (visceral), and intraembryonic coelom
What is the function of the somatic layer?
β Give rise to:
β’ Muscles of the chest and abdominal wall
β’ Parietal layer of pericardium, pleura and peritoneum
What is the function of the splanchnic layer?
β Give rise to:
β’ Muscles of heart & Smooth muscles of the gut and bronchial tree
β’ visceral layer of pericardium, pleura, and peritoneum
What is the function of the intraembryonic coelom?
give rise to:
β’ pericardium, pleura and peritoneum
What are the derivatives of endoderm?
β3 Epi + pptt + salahβ
ο The epithelial lining of:
ο² Gastrointestinal tract
ο² Respiratory tract
ο² Urinary tract and reproductive system
ο Thymus& thyroid glands
ο Parathyroid glands & pancreas
ο Liver
What are the derivatives of mesoderm?
ο The dermis of skin and fascia
ο Skeletal system
ο Muscular system (all muscles)
ο Circulatory and lymphatic system
ο Excretory system
ο Reproductive system except for germ cells
ο Suprarenal cortex
ο The lining of the body cavity
What are the derivatives of ectoderm?
ο Nervous system
ο Cornea and lens of the eye
ο Pituitary and pineal gland
ο Epidermis & skin appendage (sweet glands& hair follicles)
ο Epithelium of anterior part of the mouth and lower part of the anal canal
ο Derivatives of neural crest
