Embryology Flashcards
What are the different stages of embryonic development
Weeks 0-3 : conceptus / embryo
Weeks 3-8 : Embryonic period
Weeks 9-40 : fetal period
What does gametogenesis consist of?
Mitosis and meiosis
Describe meiosis
- chromosomes double and pair up then chiasmata form
- the nucleus divides and chromosome pairs divide
- the daughter nuclei divide again and each chromosome is singular
What is spermatogenesis?
- Formation of male gametes
- occurs at puberty and continues throughout life
- each cycle takes 64 days
- production rate decreases with age
What is oogenesis?
- formation of female gametes
- starts in development and ends at menopause
- by week 28-30 of embryonic development primary oocytes begin meiosis but arrest in prophase until puberty
- reproductive life span determines in fetal life.
What is trisomy 21?
=down syndrome
- usually caused by nondisjunction leading to an additional copy of chromosome 21
- 75% of nondisjunction cases occur during oocyte formation
What is cleavage?
A period of rapid cell division with no increase in size
It results in the formation of morula then a blastocyst
What is gastrulation?
The formation of germ layers (week 3 embryonic development)
What are the three germ layers?
Endoderm (inner layer)
Mesoderm (middle layer)
Ectoderm (outer layer)
How is the body initially formed in embryos?
Embryonic folding leads to a tube within a tube ( one tube being the gut tube and the other being the skin/body wall)
What is organogenesis?
The formation of organs and organ systems
The basis of all organs is in place by the end of the embryonic period (week 8) and will continue to develop through the foetal period (40 weeks)
What happens in the foetal period (weeks 9-40)
- Mainly grows in length during the 2nd trimester and adds weight in the 3rd trimester.
- tissues become mature and functional
- overt sexual differentiation
- bone laid down and connections made in the CNS
What are the anatomical terms used in embryology ?
Nearer the head = cranial
Nearer the feet = caudal
Nearer the front = ventral
Nearer the back = dorsal
What are the primary process which take place from zygote formation to embryo?
Cell division, differentiation, cell attachment, apoptosis, induction (ability to cause another cell to differentiate) and cell migration
What are secondary processes in embryonic development?
Axis formation/ polarity (cells knowing which way is up)
Folding/ rotation (gives embryo 3D form and formation of organs)
What is epigenetic control ?
Preferential expression of either the maternal or paternal copy of a gene
Describe the sperms role in fertilisation
1- the sperm binds with the zone pellucida glycoprotein (ZP3)
2- acrosomal enzymes released from the sperm head, the sperm digests its way into egg
3- egg and sperm plasma membrane fuse and sperm contents enter the egg
4- sperm entry triggers completion of meiosis 2 and the realease of cortical granules by oocyte
When are blastomeres thought to be totipotent ?
Up until the 8 cell stage
What is mosaicism?
Individuals with two or more cell lines with different chromosome complements due to mitotic disjunction (trisomy 21)
In a zygote with trisomy 21 some cells will have a normal complement of cells and some with trisomy making the features less prominent
Describe the compaction of the morula
Cells are confined within the zona pellucida (ZP) and maximise available space by coming into closer contact with each other and begin to form cell junctions.
Outer cells form extensive gap junctions and fluid enter through the ZP
What are the parts of the blastocyst?
The inner cell mass = embryoblast
Outer cell mass =trophoblast
Blastocyst cavity
When does blastocyst hatching occur?
When the embryo interacts directly with the endometrial lining of the uterus for implantation
When and where does implantation occur?
Typically by day 6 on the posterior or anterior uterine wall (usually posterior)
What happens around day 7 of embryonic development?
(Bilaminar embryo)
The active process is driven by the trophoblast which divides into the cytotrophoblast (many individual cells) and the syncytiotrophoblast (single multinucleated cell)
What happens around day 9 of embryonic development?
- Formation of two cavities; the amniotic cavity (epiblast) and the primitive yolk sac (hypoblast)
- implantation complete and closure by the fibrin Coagulum
- vacuoles appears in the syncytiotrophoblast and unite to form lacunae (small space)
What happens around day 12 of embryonic development?
Establishment of the uteroplacental circulation where maternal blood flows into lacunae to diffuse to the embryoblast
The extraembryonic mesoderm develops and eventually degenerates forming the chorionic cavity
What happens on day 13 of embryonic development?
Further development of the chorionic cavity and presence of the connecting stalk (eventually umbilical cord)
Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac
What happens at week 3 of embryonic development?
Formation of the primitive streak =appears in the midline at caudal end of epiblast
At the cranial end of the primitive streak is the primitive pit and node
What is gastrulation?
Movement of epiblast cells through the primitive streak to form three primary germ layers and body axes are established.
Three layers = endoderm, mesoderm and ectoderm
What does the ectoderm do?
It is responsible for the epidermis of skin and the nervous system
What are the layers of the mesoderm?
The mesoderm becomes divided into 3 regions either side of the notochord - the paraxial mesoderm, intermediate mesoderm and the lateral plate mesoderm (somatic layer and visceral layer)
What does the paraxial mesoderm do?
Responsible for the axial skeleton, voluntary skeletal muscle and parts of the dermis (back)
What does the intermediate mesoderm do?
Forms urogenital systems (kidney, ureter, gonads etc)
What does the somatic part of the lateral plate mesoderm become?
Most of the dermis, lining of body wall and parts of limbs
What does the visceral part of the lateral plate mesoderm become?
Cardiovascular system, mesothelial covering of organs and smooth muscle
What does the endoderm become?
Lining of the gut tube, lining of the respiratory tract and lining of the bladder and urethra
what happens around day 8 of embryonic development?
The embryo organised into 2 cell layer; the epiblast (dorsal surface) and the hypoblast (ventral surface) and the amniotic cavity begins to form
when is the risk of birth defects being induced the most likely?
day 5
describe the process of heart development to a single tube.
- cardiac progenitor cells migrate through the primitive streak into the visceral layer of lateral plate mesoderm
- two endocardial tubes come together to form one tube. proliferation of mesoderm leads heart into thorax.
- the bulbus cordis moves ventral, caudual and to the right. the atrium moves dorsal and cranial. looping and folding occurs.
describe how the heart goes from a single tube to a four chambered heart.
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