Embryology 1 and 2 Flashcards
Give a brief description of SPERMATOGENESIS
- spermatogonium (46 chromosomes)
- Meiosis occurs
- 4 sperm cells produced (22+X or 22+Y)
Give a brief description of OOGENESIS
- Oogonium (46 chromosomes)
- Meiosis occurs
- 1 ovum (22+X) and 3 polar bodies
How is a zygote formed?
Several sperms surround the ovum, only one penetrates.
(pro)Nucleus of sperm enters and fuses with the (pro)nucleus of ovum to give a diploid cell called the ZYGOTE.
Week 1 pre-embryonic phase
- Zygote (diploid = 46 chromosomes) is formed
- Divides to form a blastocyst.
- Moves through uterine tube, to reach uterine cavity.
How are mitochondrial diseases inherited from?
Mothers
Why is a BLASTOCYSTIC CAVITY formed?
As the number (and size) of cells increase, getting nutrition to the central core of cells starts becoming difficult. Therefore a cavity (blastocystic cavity) develops.
- Cells accumulate at one end to form an inner cell mass.
- The outer lining of cells is called the trophoblast.
What causes ectopic pregnancy?
abnormal function of cilia
What happens in week 2 of pre-embryonic phase?
Implantation occurs in the uterine endometrial layer and placenta begins to develop (this may start by day 6 of 1st week)
Cells that later form the embryo form a bilaminar disc
Sacs, membranes and cord to nourish the human conceptus (baby) start to form
What are trophoblasts?
Cells that form the outer cell mass
- divides to form 2 layers
- ultimately forms CHORION
- develops finger like processes: chorionic villi
What do CHORION do?
- Implantation process (chorionic villi)
- Forms part of the placenta in due course
- Secretes human Chorionic gonadotropin (HCG) (used to detect pregnancy)
Describe briefly IMPLANTATION
At ~ 7 days the blastocyst begins to burrow into the uterine wall (endometrium)
The chorion plays an important role in burrowing.
Describe what happens in the Endometrium After Implantation Has Occurred
Decidua basalis: part of the endometrium deep to the implanted conceptus.
hCG (Human Chorionic Gondaptropin) secreted by the chorion is going to help maintain the decidua
Maternal blood (and urine) levels of hCG increase till around 12 weeks gestation
What happens to the inner cell mass?
The cells of the inner cell mass form a 2–layered flat disc called the BILAMINAR DISC
2 cavities now begin to form:
- Amniotic cavity (A)
- Yolk sac (Y)
What is the main functions of the PLACENTA?
Main functions:
- Fetal nutrition
- Transport of waste and gases
- Immune etc
What are the two parts of the placenta?
Foetal part and surface: smooth with foetal blood vessels and end of umbilical cord
Maternal part: Decidua basalis of endometrium
Rough and has maternal blood vessels
Describe the formation of 3 germ layers in week 3
Epiblast cells migrate to lie inbetween epiblast and hypoblast layers
Cells then displace Hypoblast
Forms 3 germ layers :
- Ectoderm
- Mesoderm
- Endoderm
Trilaminar disc is formed!
Cells have now become specialised!
What is NEURULATION?
Neural tube formation
A solid tube of cells called the notochord is formed
Notochord INDUCES ectodermal cells in the midline to form a neural tube
What induces the mesoderms to thicken?
The neural tube
What are the 3 separate parts of the mesoderm?
- Paraxial mesoderm
- Intermediate plate mesoderm
- Lateral plate mesoderm
Lateral plate mesoderm splits
Each somite (paraxial mesoderm) divides into 3, what are they?
Dermatome
Myotome
Sclerotome
Define TERATOLOGY
Study of when things go wrong during development
Define TERATOGENS
Environmental factors that cause abnormal development.
Causes of abnormal development - ENVIRONMENTAL
- Drugs – prescription / other
- Alcohol/ tobacco
- Infectious agents: ToRCH (Toxoplasma, Rubella, Cytomegalovirus, Herpes ) can transfer through placenta and affect developing embryo
- Others – eg: radiation
Causes of abnormal development - GENETIC FACTORS
- Too many/too few chromosomes
- Turner’s syndrome – 45 chromosomes single X
- Down’s syndrome – 47 chromosomes trisomy 21
- Structural changes – deletions of genes, segments of chromosomes
- Causes:
- Increased maternal age
- Damage from environmental factors such as radiation (eg: X-rays)
What does the risk posed by teratogen dependent on?
- Exposure during critical periods of development
- Dosage of drug/chemical/factor
- Genetic constitution of embryo ie some more susceptible than others at equivalent doses etc
Diagnosis of malformation - PRENATAL
- Blood - AFP
- Ultrasound scan – 12 week anomaly scan
- Invasive tests: chorionic villus sampling and amniocentesis
Diagnosis of malformation - POSTNATAL
- Hip stability
- Testes (descent)
- Fingers and toes
- Hearing
What happens from week 4-8
- Folding into a tube (lateral folds) which started in 3rd week completes
- Neural tube – forebrain, midbrain, hindbrain and spinal cord development
- Heart starts to beat on Day 24!
- Gut formation from endoderm
- Urogenital system formation from intermediate mesoderm
- Body cavities from lateral plate mesoderm
- 43 pairs of somites form in the paraxial mesoderm and differentiate further
- Limb buds form
- Neck development – pharyngeal arches
