Development & Ageing - Early fetal development Flashcards
What is fertilization age?
The age of a fetus measured from the time of fertilization
What is gestational age?
- The age if calculated from the time of the beginning of the last menstrual period
- Fertilization period + 14 days
What is the Carnegie stage?
- 23 stages of embryo development based on embryo FEATURES not TIME
- Covers the window of 0-60 days fertilization in humans
What is the embryogenic stage?
- 14-16 days post fertilization
- Establishing the early embryo from the fertilized oocyte
- Determining to cell types:
- Pluripotent embryonic cells
- Extraembryonic cells
What is the embryonic stage?
- 16-50 days post fertlization
- Establishment of the germ layers and differentiation of tissue types
- Establishment of the body plan
What is the fetal stage?
- 50-270 days
- Major organs now present
- Migration of some organ systems to final location
- Extensive growth and acquisition of fetal viability (survival outside the womb)
What stages occur in the first, second and third trimesters?
First:
- Embryogenic stage
- Embryonic stage
Second & third:
- Fetal stage
What occurs in the maternal-to-zygotic transition?
- Embryo is dependent on maternal proteins and mRNAs to get through the first divisions
- These mRNA and proteins are synthesized and stored during oocyte development
- Transcription of of embryonic genes occurs at the 4-8 cell stage
- This increases protein synthesis and organelles are matured
What occurs in compaction?
- Around the 8 cell stage
- Outer cells press against zona pellucida
- Outer cells change to wedge shaped from spherical
- Outer cells connect to each other through tight gap junctions and desmosomes
- This forms a barrier to diffusion between inner and outer embryo
- Outer cells become polarised (apical and basal polarity)
- Inner cells remain spherical
Describe the structure of the blastocyst
- Zona pellucida outer shell
- Trophoectoderm under zona pellucida made of extra-embryonic cells
- Inner cell mass of pluripotent embryonic cells
- Blastocoel next to inner cell mass
What is hatching?
- When the blastocyst escapes the zona pellucida
- Occurs by ezymatic digestion and cellular contactions
Explain what occurs in the peri-implantation events
- The trophoectoderm separates into the syncitiotrophoblasts and the cytotrophiblasts
- Syncitiotrophoblast invade the endometrium and destroy blood vessels to allow them to be bathed in maternal blood
- The cytotrophoblasts continue to divide to provide more cells to the syncitiotrophioblasts
- The inner cell mass also differentiates into the epiblast and hypoblast
- The epiblast is what fetal tissue will be derived from
- The hypoblast will form the yolk sac
Explain what occurs in the bi-laminar embryonic disc formation
- The syncitiotrophoblasts continue to expand into the endometrium
- Some of the epiblast cells become separated from the main block by the formation of the amniotic cavity
What is gastrulation?
When the bi-laminar disc differentiates into 3 primary germ layers
Explain the process of gastrulation
- After 25 days a thickened structure forms along the midline of the epiblast, called the primitive streak
- At the cranial end of the embryonic disc, the primitive streak expands and is called the primitive node which contains the primitive pit
- Once formed, cells of the epiblast migrate inwards to the caudal end, detatch from the epiblast and slip beneath it, known as invagination
- The hypoblast cells are eventually completely replaced by a proximal cell layer called the definitive endoderm
- The remaining cells of the hypoblast are called the ectoderm
- Some of the invaginated cells remain in the layer between the definitive endoderm and the ectoderm and form the mesoderm
Where is the notochord formed?
It forms along the embryo midline, under the ectoderm, from the primitive streak to the head end
What is the function of the notochord?
Acts as a key organiser centre for neuralation and mesoderm development
What occurs in neurulation?
The notochord send signals to the neural plate to invaginate in the middle to form the neural groove and this creates two ridges on either side called the neural folds. Within the folds are neural crest cells.
Later on the neural folds rise and move in to meet eachother and eventually fuse over the neural groove which creates a hollow tube where the groove was, which is called the neural tube. The neural crest cells then migrate away
On what days do the tail end and head end of the neural tube close?
Tail end - 27 days
Head end - 23 days
What is somitogenisis?
The formation of somites
Somites arise from a paired block of paraxial mesoderm flanking the neural tube and notochord
What tissue types do somites form?
Sclerotome:
- Forms vertebrae and rib cartilage
Dermomyotome:
- Sub divides to form:
- Dermatome:
- Forms the dermis of the skin, some fat and connective tissue of neck and trunk
- Myotome:
- Forms the muscles of the embryo
- Dermatome:
How is the gut formed?
Ventral and lateral folding of the embryo cuases part of the yolk sac to be pinched off which forms the primitive gut
The primitive gut is the patterned into the foregut, midgut and hindgut
What structures does each part of the gut form?
Foregut:
Esophagus, stomach, upper duodenum, liver, gallblader, pancreas
Midgut:
Lower duodenum and remainder of small intestine, ascending colon and first two thirds of the transverse colon
Hindgut:
Last thrid of transverse colon, descending colon, rectum and upper anal canal
