Intruduction To Embryology Flashcards
What is a Oocyte (ovum,egg)?
Female germ cell
What is a sperm?
Male germ cell
What is a zygote?
Cell resulting from union of oocyte and sperm at fertilisation
What are blastomeres?
Early embryonic cells
What is a morula?
(from Latin – mulberry) Solid mass of 12 – 32 blastomeres
What is a blastocyst?
Stage where morula has entered uterine cavity and blastocystic cavity develops within
What is a gastrula?
Stage after trilaminar embryonic disc formation
What is a neurula?
Stage after gastrula when neural tube forms
What is an embryo?
Developing human during embryonic stage (to end of 8th week)
What is embryology?
The development of the child
What happens between stages 1-3?
Fertilisation
Zygote
Morula
Late Blastocyst
What happens between stages 3-4?
Blastogenesis
What happens between stages 4-6?
Formation of the bilaminar disc and implantation?
What happens between days 8-9?
Formation of the placenta and embryo
What happens between days 11-13?
Formation of the bilaminar disc and extraembryonic cavity?
What happens on day 13?
Formation of the umbilical cord
What happens during week 3?
Gastrulation
Bilaminar disc becomes trilaminar disc
How is the primitive streak created?
As the ectoderm grows a folding occurs along the caudal midline
How is the trilaminar disc formed?
Cells from the base of the primitive streak detach and migrate to lie between the layers of the ectoderm and endoderm
The invagination of these cells creates a new layer called the mesoderm
What are the 3 different types of germ layers?
Ectoderm
Endoderm
Mesoderm
What does the ectoderm differentiate into?
●Epidermis of skin, hair and nails
●Mammary, sweat and sebaceous glands
●Central nervous system
●Peripheral nervous system
●Pituitary gland
●Enamel of the teeth
●Lens of the eye and parts of the inner ear
●Sensory epithelium of nose, ear and eye
What does the endoderm differentiate into?
●Epithelial lining of the gastrointestinal tract, respiratory tract and urinary bladder
●Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas
●Epithelial lining of the tympanic cavity and auditory tube
●Plays a part in the development of the notochord
What does the mesoderm differentiate into?
●All of the musculoskeletal system
●Deep layers of the skin
●Abdominal and chest walls and lining
●The walls of the bowel (but not the lining of the bowel)
●The urogenital system
How is the notochord formed?
Tube develops from end of primitive streak extending towards cranial end
Tube fuses with endoderm to become a groove
Plate fold to become a tube once again – the notochord
Notochord has a central role in further midline development
What happens during Week 4?
Neurulation
- Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate, separated by the neural plate border
- Neural plate bends dorsally until the two plates convergence to form the neural crest
- The neural crest is disconnected from the epidermis by the closure of the neural tube. The cells of the neural crest differentiate to form most of the peripheral nervous system
- The notochord degenerates, only persisting as the nucleus pulposus of the intervertebral discs. Mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle
What happens if fusion doesn’t occur all the way to the caudal end?
Child is born with spinal bifida
What happens if fusion doesn’t extend all the way to the cranial end?
Child is born without cerebral cortex (anencephaly)
What happens during the differentiation of the paraxial mesoderm?
During development the paraxial mesoderm is divided into paired segments called somites on each side of the notochord, they develop in a cephalocaudal direction (head to tail)
When do the first pair of somites appear?
Day 20
Then at a rate of around 3 pairs per day until 42-44 pairs are formed
What do somites differentiate into at the beginning of the 4th week?
Myotomes (form skeletal muscle)
Sclerotomes (form bones and cartilages) some cells from the sclerotomes surround the notochord and spinal cord and give rise to the vertebral column
Dermatomes (form the dermis of the skin)
What is the oropharyngeal membrane?
Located at the cranial end of the embryonic disc
Consists of a small region of tightly adherent Ectoderm and Endoderm cells
What is the cloacal membrane?
Formed at the caudal end of the embryonic disc
Consists of tightly adherent ectoderm and endoderm cells - no intervening mesoderm
When this membrane appears the posterior wall of the yolk sac forms a small diverticulum, the allantois which extends into the connecting stalk
What is the cranial area?
Contains the buccopharyngeal membrane, the cardiogenic area and septum transverse
What is cranial flexion?
Brings the buccopharyngeal membrane, cardiogenic area and septum transverse ventrally, forming the ventral surface of the future face, neck and chest.
It brings the heart into its thoracic position and septum transversum to the diaphragm
What is caudal flexion?
Brings the cloacal membrane onto the ventral surface of the embryo
What is lateral folding?
Results in the incorporation of a portion of the yolk sac which is lined with endoderm into the embryo to form the primitive gut
It also leads to the formation of body cavities
The remaining part of the yolk sac and allantois remain outside the embryo
What is the primitive gut?
Blind ended tube
Divided into the foregut, midgut and hindgut
What is the midgut?
Remains connected temporarily to the yolk sac by means of vitelline duct (yolk stalk)
From liver bud to the end of the right 2/3 of the transverse colon
Where is the foregut?
Extends from the oropharyngeal membrane to the liver bud
Where is the hindgut?
From beginning left 1/3 of transverse colon to the cloacal membrane
What are the 3 parts if the mesoderm?
Paraxial plate
Intermediate plate
Lateral plate
What does the paraxial plate do?
Gives rise to somites which give rise to supporting tissue of the body:
Myotome
Sclerotome
Dermatome
Where is the intermediate plate found?
Urogenital system:
Kidneys
Gonads and their respective duct systems
Where is the lateral plate found?
At the periphery of the embryo
What is the lateral plate split into?
Somatic (parietal) layer: forms the future body wall
Sphlanic (visceral) layer: forms circulatory system, connective tissue for the glands, muscle connective tissue and peritoneal components of the way of the gut
What do the cells of the blastocyst differentiate into?
- Inner cell mass, called the
EMBRYOBLAST at one pole, it develops into the embryo proper - Outer cell mass, call the TROPHOBLAST, the cells surround the embyroblast and the blastocyst cavity
What does the embryoblast differentiate into?
- EPIBLAST
- HYPOBLAST
• These two layers form a flat disc - the bilaminar disc
What does the epiblast give rise to?
The epiblast gives rise to amnioblasts that line the amniotic cavity superior to the epiblast layer
What does the hypoblast give rise to?
The hypoblast gives rise to cells that line the blastocyst cavity and the inner surface of the trophoblast
What forms the EXOCOELOMIC MEMBRANE (PRIMITIVE YOLK SAC)?
Cells that originate from the hypoblast and line the blastocyst cavity and the inner surface of the trophoblast
What does the trophoblast differentiate into?
- CYTOTROPHOBLAST
- SYNCYTIOTROPHOBLAST (outer layer that erodes maternal tissue)
What happens to the lacunae by Day 9?
Develop in the syncytiotrophoblast resulting in the maternal sinusoids (vessels) to be eroded meaning maternal blood can enter the lacunar network and a primitive utero-placental circulation is established
• A new population of cells appear between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic membrane of the primitive yolk sac
• These cells, derived from yolk sac cells form a fine, loose connective tissue the extraembryonic mesoderm
What happens to the Extraembryonic mesoderm (chorionic plate) by day 9?
• Eventually fills all of the space between the cytotrophoblast and exocoelomic membrane
• Large cavities develop in the extraembryonic mesoderm forming a new space called the extraembryonic cavity or chorionic cavity
What are the two layers of the extraembryonic mesoderm?
- Splanchnopleuric (splanchnic or
visceral) layer - Somatopleuric (somatic or parietal) layer
What is the extraembryonic mesoderm?
looks and acts just like the embryonic mesoderm but is found outside of the embryo
What does the hypoblast do?
Forms additional new cells that migrate along the inside of the exocoelomic membrane, they proliferate gradually and form a new cavity within the primitive yolk sac - these are the
ENDODERMAL CELLS
What happens on further development of trophoblast?
• On further development of the trophoblast, the connecting stalk appears
• The connecting stalk contains capillaries which are connected to the chorionic plate (extraembryonic mesoderm) and the embryo
What is the chorion?
• A membrane that exists during pregnancy between the developing foetus and mother
• Formed by extraembryonic mesoderm and the two layers of trophoblast (cytotrophoblast & syncytiotrophoblast)
• Chorionic villi emerge from the chorion and invade the endometrium and allow for the transfer of nutrients from maternal blood to fetal blood - this initiates the formation of the placenta
