Lecture 5 & 6: Early Embryology Flashcards
What is the definition of pre-embryonic period?
Weeks 1 & 2 of pre-natal development
What is the definition of embryonic period?
Weeks 3 to 8 of pre-natal development
What is the definition of fetal period?
Weeks 9 to term of pre-natal development
What are the changes taking place as the fertilized human ovum (zygote) travels down the Fallopian tube towards the uterus?
- Undergoes cleavage: successive cell divisions within the zona pellucida
- Takes 4 days
What is a zygote?
A single cell, the product of fertilization of an ovum by a sperm, produced by fusion of the male & female pronuclei
What is cleavage?
Successive cell divisions
What is a zona pellucida?
Glycoproteins shell that surrounds the oocyte
What is a morula?
- The ‘mulberry’
- Solid ball of cells resulting from cleavage of cells of the zygote
What is an ovary?
Organ that produces an ovum
What is a Fallopian tube?
Tube that connects the uterus to the ovaries
What is a uterus?
Hollow organ where a fetus develops and grows
What is the process of formation of the blastocyst?
- After third cleavage, compaction occurs
- Blastomeres maximize contact with each other, forming a compact ball of cells (morula)
- Spaces between the cells of the morula merges
- Forms one large central space (blastocoele)
- Morula is converted into a hollow sphere of cells (blastocyst)
What are the initial stages of implantation?
- Blastocyst hatches from zona pellucida
- Can now interact with uterine surface to implant
- Inner cell mass becomes the embryo
- Outer cell mass becomes the trophoblast
- Trophoblastic placenta develops the syncytiotrophoblast
- Syncytiotrophoblast first adheres to, and then invades the modified endometrium
What is the blastocyst?
Hollow sphere of cells formed from the morula through the process of compaction whereby fluid is pumped in to create the blastocoele, leaving an inner cell mass and an outer cell mass
What is the trophoblast?
- Outer cell mass that was pushed to the outside of the blastocyst during compaction
- Destined to produce the tissues supporting the embryo/fetus during development
What is the inner cell mass?
- Embryoblast
- Group of cells inside the blastocyst that goes on to form the embryo proper
What is implantation?
When the sycyntiotrophoblast adheres to and invades the modified endometrium
What is the cytotrophoblast?
Inner layer of trophoblast
What is the syncytiotrophoblast?
- Outer layer of trophoblast
- Determines which substances cross the placenta and which do not
How does the embryonic disc form?
- Cell differentiations occurs in the inner cell mass
- Blastocyst formed 2 layers (ectoderm and endoderm)
What is the definition of a bilaminar disc?
Two layered embryonic disc consisting of epiblast and hypoblast derived from the inner cell mass
What is the definition of a germ layer?
The three primary tissue layers of the trilaminar disc (ectoderm, mesoderm & endoderm) that arise through gastrulation and subsequently give rise to all the tissues of the body
What is the definition of an ectoderm?
The outermost layer of the three embryonic germ layers of the trilaminar disc formed during gastrulation
What is the definition of a mesoderm?
The middle layer of the three embryonic germ layers of the trilaminar disc formed during gastrulation
What is the definition of a primitive streak?
Begins as a thickened region of epiblast at the causal end of the bilaminar disc and is the site from which gastrulation proceeds
What is the definition of gastrulation?
The process occurring during the third week of development resulting in the formation of the three germ layers of the trilaminar disc from epiblast migrating through the primitive streak
What is the definition of a trilaminar embryonic disc?
The three-layered germ doc produced by gastrulation consisting of the three germ layers (ectoderm, mesoderm and endoderm)
What are the important derivatives of the ectoderm?
Organs and structures that maintain contact with outside world (e.g. nervous system, epidermis)
What are the important derivatives of the mesoderm?
Supporting tissues (muscle, cartilage, bone, vascular system including heart and vessels)
What are the important derivatives of the endoderm?
Internal structures (e.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands)
How does the mesoderm differentiate into the notochord?
- Cells that ingress through the cranial region of the node become prechordal plate and notochord
- Prenotochordal cells invaginating in the primitive node move until they reach the prechordal plate
- They form a solid cord of cells (notochord)
How does the mesoderm differentiate into somites?
- Regions of the epiblast that migrate and ingress through the primitive streak have been mapped during gastrulation
- Cells that migrated at the lateral edges of the node and from the cranial end of the streak become paraxial mesoderm
- By beginning of third week, paraxial mesoderm becomes organized into segments (somitomeres)
- Somitomeres further organize into somites
How does the mesoderm differentiate into the intermediate mesoderm?
- Regions of epiblast that migrate and ingress through the primitive streak have been mapped
- Cells migrating through the midstreak region become intermediate mesoderm
How does the mesoderm differentiate into the lateral plate?
- Regions of the epiblast that migrate and ingress through the primitive streak have been mapped during gastrulation
- Cells migrating through the more caudal part of the streak form lateral plate mesoderm
How does the intra-embryonic coelom form?
- Lateral plate divided into 2 layers (somatic mesoderm layer and splanchnic mesoderm layer)
- Somatic and splanchnic mesoderm layer line the intraembryonic coelom
What are the derivatives of the paraxial mesoderm?
Forms somitomeres which organize into somites, which give rise to:
- myotome (muscle tissue)
- sclerotome (cartilage and bone)
- dermatome (dermis of the skin)
AKA all the supporting tissues of the body
What are the derivatives of the intermediate mesoderm?
Urogenital structures
What are the derivatives of the lateral plate mesoderm?
Splits into 2 layers that line the intraembryonic cavity and surrounds the organs
- Somatic layer: dermis of the skin in the body wall and limbs, bones and connective tissue of the limbs and the sternum, parts that are surrounding the intraembryonic cavity form mesothelial membranes that secrete serous fluid
- Splanchnic layer: forms the wall of the gut tube with the embryonic endoderm
How does the neural plate form?
Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate
How does the neural plate form the neural tube?
- Process is neurulation
- Lateral edges of the neural plate become elevated to form neural folds
- Depressed mid region forms the neural groove
- Neural folds approach each other in the midline, where they fuse
- Fusion begins in the cervial region and proceeds cranially and caudally
What is head fold?
Bending of cranial aspect of the embryo ventrally, bringing the mouth and heart into their ventral positions
What is lateral fold?
The lateral edges of the embryonic disc flex sharply, edges of each germ layer make contact at head and tail regions and zipper toward the umbilicus
- Ectoderm now covers the entire body surface except at the umbilical region where the connecting stalk and yolk sac merge
What is tail fold?
Bending of the caudal aspect of embryo centrally to reposition the cloaca and allantois
How do somites differentiate into dermatome, myotome and sclerotome?
- Somites undergo further changes which underline the pattern of outgrowth of spinal nerve roots from the spinal cord, the formation of vertebral column and the innervation of muscles and skin
What does the dermatome become in layer development?
An area of skin
What does a myotome become in later development?
A block of skeletal muscle
What does a sclerotome become in later development?
A vertebra and rib
How does innervation of skin and muscle work?
- Soinal nerve roots growing out from the neural tube enter and innervate the dermatomes and myotomes
- Sclerotomes are rearranged to lie between the nerve roots and become the vertebrae
How are the terms ‘dermatome’ and ‘myotome’ used to describe the innervation of skin and muscles in the adult?
Used clinically to define an area of skin or muscles supplied from a single spinal nerve root
What is situs inversa?
- Complete mirror-image viscera
- Commonly results from immobile cilia
- No associated morbidity
