^ Chapter 6 - 3rd to 8th weeks: The embryonic period Flashcards
What happens in the embryonic period, or the period of organogenesis?
The three germ layers (ectoderm, mesoderm, and endoderm) gives rise to a number of specific tissues and organs.
When does the embryonic period, or period of organogenesis, occure?
It occurs from 3rd to 8th weeks
When does the main organ systems become established, rendering the major features of the external body form recognizable by the 2nd month?
By the end of embryonic period
When does the germ layer have the shape of a disk that is broader in the cephalic than in the caudal region?
At the beginning of the third week of development
When does the neural plate form?
At the beginning of the third week
Neuroectoderm
- Cells of the neural plate make up the neuroectoderm
- Their induction represents the initial event in the process of neurulation
What cause the induction of the neural plate?
The upregulation of fibroblast growth factor (FGF) signaling together with inhibition of the activity of bone morphogenic protein 4 (BMP4), a transfoming growth factor-B (TGF-B) family member responisble for ventralizing ectoderm and mesoderm, causes induction of the neuronal plate.
BMP4
BMP4 permeates the mesoderm and ectoderm of the gastrulating embryo, ectoderm is induced to form epiderims, and mesoderm forms intermediate and lateral plate mesoderm
Which proteins inactivate BMP (bone morphogenic protein)
Noggin, chordin, and follistatin
Where can we find noggin, chordin, and follistatin?
They are present in the organizer (primitive node), notochord, and prechordal mesoderm
What cause the mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm)?
Noggin, chordin, and follistatin neuralize ectoderm by inhibiting BMP and cause the mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm).
Induction of caudal neural plate structures (hindbrain and spinal cord) depends on which two secreted proteins?
- WNT3a
- FGF
Retinoic acid (RA)
Appears to play a role in organizing the cranial-to-caudal axis because it can cause respecificaton of cranial segments into more caudal ones by regulating expression of homeobox genes.
What is neurolation?
Neurolation is the process whereby the neuronal plate forms the neural tube.
When does the neural plate become elevated to form neural folds and the depressed midregion forms the neural groove?
By the end of the 3rd week.
What does the neural crest undergo as it leaves neuroectoderm by active migration and displacement to enter the underlying mesoderm?
Neural crest undergoes an epithelial-to-mesenchymal transition as it leaves the neuroectoderm
Mesoderm def.
Refers to the cells derived from the epiblast and extraembryonic tissue
Mesenchyme def.
Refers to the loosely prganized embryonic connective tisuue
Dorsal pathway through the dermis
Is where the crest cells will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles
Ventral pathway through the anterior half of each somite to become (the crest cells):
- Sensory ganglia
- Sympathetic and enteric neurons
- Schwann’s cells
- Cells of the adrenal medulla
Neural crest cells that leaves the neural tube before closure contribute to the:
- Craniofacial skeleton
- Neurons for cranial ganglia
- Glial cells
- Melanocytes
- Other cell types
Neural crest also refered to as
the fourth germ layers, because it is important and it contributes to so many organs.
SNAIL and FOXD3
specify cells as neural crest
SLUG
promotes crest cell migration from the neuroectoderm
The fate of the entire ectodermal germ layer depends on….
BMP concentrations
High level of BMP
induce epidermis formation
Intermediate levels of BMP at the border of the neural plate and surface of ectoderm
Induce neural crest
Very low concentration of BMP
Cause formation of neural ectoderm
What regulates the neural crest cell migration, proliferation, and differentiation
- BMPs
- other members of TGF-B family
- FGFs
Ectodermal thickenings
- The otic placodes
- The lens placodes
Where does the structures needed for hearing and maintenace of equlibrium come from?
Otic placodes -> Otic vesicles -> structures needed for hearing and maintenace of equlibrium come from
Placodes –> lenses for eyes
During the 5th week
The ectoderm gives rise to:
gives rise to organs and structures that maintain contact with the outside world:
● The central nervous system
● The peripheral nervous system
● The sensory epithelium of the ear, nose, and eye
● The epidermis, including the hair and nails.
In addition, it gives rise to:
● Subcutaneous glands
● The mammary glands
● The pituitary gland
● And enamel of the teeth
Neural crest derivates
- Connective tissue and bones of the face and skull
- Cranial nerve ganglia
- C cells of the thyroid gland
- Conotruncal septum in the heart
- Odontoblasts
- Dermis in face and neck
- Spinal (dorsal root) ganliga
- Sympathetic chain and preaortic ganglia
- Parasympathetic ganlia of the gastrointestinal tract
- Adrenal medulla
- Schwann’s cells
- Glial cells
- Meninges (forebrain)
- Melanocytes
- Smooth muscle cells to blood vessles of the face and forebrain
Paraxial mesoderm is formed by the
17th day
The lateral plate (thin mesoderm layer) is divided into 2 layers:
- The somatic or parietal mesoderm layer
2. The splanchnic or visceral mesoderm layer
Which lateral plate layer covers the yolk sac?
The splanchnic or visceral mesoderm layer
intraembryonic cavity is formed by
the layers of the lateral plate
What connects the paraxial and lateral mesoderm?
The intermediate mesoderm
The intraembryonic cavity is continous with the…
extraembryonic cavity
When does the paraxial mesoderm to be organized into segments?
By the beginning of the 3rd week
Somitomeres
segments that the mesoderm
From the occipital region caudally, somitomeres further organize into..
somites
When does the first pair of somites arise?
the 20th day
How many pairs of somites to we have by the end of 5th week?
42-44 pairs:
- 4 occipital
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 Sacral
- 8-10 coccygeal pairs
The first occipital and the last 5-7 coccygeal somites later dissapears
What regulates the boundaries of each somite?
- Retinoic acid (RA)
- a combination of FGF8 and WNT3a
Each somite forms its own
- Sclerotome
- Myotome
- Dermatome
Sclerotome
- The tendon cartilage and bone component
- Differentiate into vertebrae and ribs
Myotome
- Providing the segmental muscle component
Dermatome
Forms the dermis of the back
Does the myotome and dermatome have their own segmental nerve component?
Yes
What is the noggin genes and sonic hedgehog (SHH) by?
They are produced by the notochord and floor plate of the neural tube.
Which transcription factor does the sclerotome cells express?
Sclerotome cells express the transcription factor PAX1
PAX1
Initiates the cascade of cartilage- and bone-forming genes for vertebral formation
What regulates the expression of PAX3?
WNT proteins from the dorsal neural tube
PAX3
marks the dermomyotome region of the somite
Muscle-specific genes
MYOD
MYF5
Neurotophin 3 (NT-3)
- Secreted by the dorsal region of the neural tube, to form dermis
- Direct the midportion of the dorsal epithelium of the somite
Intermediate mesoderm
- Differentiate into urogenital structures
- Temporarely connects paraxial mesoderm with the lateral plate
Where does it form segmental cell clust regions?
In cervical and upper thoracic region (future nephrotome)
Where does it form unsegmented mass of tissue, the nephorgenic cord?
More caudally
Excretory units of the urinary system and the gonads develop from…..
the party segmented, partly unsegmented intermediate mesoderm
Lateral mesoderm splits into..
- Parietal (somatic) layer
- Visceral (Splanchnic) layer
The parietal (somatic) layer
Lines the intraembryonic cavity
The visceral (splanchnic) layer
Surrounds the organs
Lateral body fold =
Mesoderm from parietal layer + ectoderm
What forms the dermis of the skin in the body wlal and the limbs, the bones and the connective tissue of the limbs, and the sternum?
The parietal layer of lateral plate mesodem
What forms the costal cartilages, limb muscles, and most of the body wall muscle?
Sclerotome and muscle precusor cells that migrate into the parietal layer of lateral plate mesoderm
Mesothelial membranes, or serous membranes….
- Line the perioneal, pleural, and pericardial cavities and secret serous fluid
- Thin membranes formed by mesoderm cells of the parietal layer surrounding the intraembryonic cavity
What forms a thin serous membrane around each organ?
Mesoderm cells of the visceral layer
Where does blood cells and blood vessles arise from?
mesoderm
Which two ways does the blood vessles form in?
- Vasculogenesis
2. Angiogenesis
Vasculogenesis
Vessels arise from blood islands
Angiogenesis
Blood vessles forming from already existing vessles
When and where does the first blood islands appear?
- They appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions.
Where does the blood islands arise from?
The islands arise from mesoderm cells that are induced to form hemangioblasts, a common precusor for vessles and blood cell formation.
Hemangioblasts
- Directed to form blood cells and vessles by vascular endothelial growth factor (VEGF)
- A common precusor for vessles and blood cell formation
Where does the first blood cells arise in?
The first blood cells arise in islands in the wall of the yolk sac, this population is transitory (lasts only for a short period of time)
Where does the hematopoietic cells derive from?
They derive from mesoderm surrounding aorta in a site near the developing mesonephric kindey called aortagonad-mesonephros region (AGM)
A major hemopoietic organ of the embryo and fetus?
- The liver -> from the 2nd to 7th months of development
When does the stem cells from the liver colonize the bone marrow?
In the 7th month of gestation. After that the liver loses its blood-forming function
When does the liver lose its blood-forming function?
After the 7th month of gestation
FGF2
Induces blood island development from competent mesoderm cells that form hemangioblasts
Vascular endothelial growth factor (VEGF)
- Is secreted by surrounding mesoderm cells
- Stimulates proliferation of endothelial cells at points where new vessles are to be formed
Peripheral hemangioblasts differentiate into…
Angioblast
Hemangioblasts in the center of blood islands form…
Hematopoietic stem cells
Where is SHH secreted?
It is secreted by notochord
PROX1
Appears to be master gene for lymphatic vessle differentiantion.
What is the main organ system derived from the endodermal germ layer?
The gastrointestinal tract is the main organ system derived from the endodermal germ layer.
What forms the roof of the yolk sac?
The gastrointestinal tract
Ventral body wall defects
Failure of the lateral body folds to close the body wall results in ventral body wall defects.
The gut tube
The gut tube is divided into three regions:
- Foregut
- Midgut
- Hindgut
How does the midgut communicate with the yolk sac?
Through vitelline (yolk sac) duct
What is the ectodermal-endodermal membrane called?
At its cephalic end, the foregut is temporarily bounded by an ectodermal–endodermal membrane called the oropharyngeal membrane.
Oropharyngeal membrane
This membrane separates the stomadeum, the primitive oral cavity derived from ectoderm, from the pharynx, a part of the foregut derived from endoderm.
What happens with the oropharyngeal membran in the 4th week?
In the fourth week, the oropharngeal membrane ruptures, establishing an open connection between the oral cavity and the primitive gut. The hindgut also terminates temporarily at an ectodermal–endodermal membrane, the cloacal membranen.
The cloacal membrane
This membrane separates the upper part of the anal canal, derived from endoderm, from the lower part, called the proctodeum, which is formed by an invaginating pit lined by ectoderm.
Proctodeum(?)
The proctodeum breaks down in the seventh week to create the opening for the anus.
When is the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region
By the 5th week, the yolk sac duct, allantois, and umbilical vessels are restricted to the umbilical region
Endoderm gives rise to:
● The epithelial lining of the respiratory trac
● The parenchyma of the thyroid, parathyroids,
liver, and pancreas
● The reticular stroma of the tonsils and the
thymus
● The epithelial lining of the urinary bladder
and the urethra
● The epithelial lining of the tympanic cavity and
auditory tube
What is the homeobox genes known for?
Homeobox genes are known for their homeodomain, a DNA-binding motif, the homeobox.
Homeotic cluster
Many homeobox genes are collected into homeotic clusters
Hom-C in drosophila
Is the name of the hemopoietic gene complex that is an important cluster of genes specifying the craniocaudal axis
How is the genes, which contans Antennapedia and Bithorac classes of homeotic genes, organized?
They are organized on a single chromosome as a functional unit
