developmental conditions- embryology Flashcards
fertilisation
=occurs in the distal portion of fallopian tube
The spermatozoa utilize acrosome (sac of enzymes) to digest through the corona (outer cells of ovum)
prenatal development
=process from formation of embryo to the birth
3 stages:
1.Germinal period
(first 2 weeks of development that result in formation of primitive germinal layers)
2.Embryonic period
(2nd to the end of the 8th week- where the organ systems develop. Where the greatest potential for congenital defects arise)
3.Foetal period
(last 30 weeks, organ systems grow and mature sufficiently to allow the new born baby to survive outside of the womb)
the germinal period
-The period that occurs in the first 2 weeks post fertilization.
-In the first 18-39 hours, the zygote divides into 2 cells (first cleavage)
-The cells then keep dividing to form more cells
-The first 6-7 days of development occur in the fallopian tube since the ovum is being transported to the uterus via ciliated cell lining
-If there has been previous damage to the tissues in the fallopian tubes, then the conceptus may become implanted there (ectopic pregnancy)
morula
=a ball of 12 or more cells that formed due to the repetitive cell division. Occurs on day 3-5
The cells in the centre of the blastocyst migrate towards the periphery that leave the blastocoel (hollow cavity)
day 6 of germinal period (blastocyst formation)
The blastocyst consists of 2 major regions:
- Trophoblast- develops in placenta
- Inner cell mass- develops in embryo
implantation (day 5-7)
=where the blastocyst arrives in the uterus
-Progesterone stimulates production of a sticky mucus (uterine milk) which encourages adherence to the uterine lining (endometrium)
-The trophoblast begins to invade the endometrial tissue
-This may lead to a small blood loss as the blood vessels beneath endometrium can become ruptured
late germinal period
-Cells of the inner cell mass divide repeatedly and the mass thickens
-The inner cell mass differentiates into 2 distinct layers that form a convex embryonic disc:
1. The ectoderm (outer layer)
2. The endoderm (inner layer)
embryonic period
-In the 3rd week, the 2 layered embryonic disc begins to thicken
-Ectoderm cells rapidly divide to form the primitive streak (thickening)
-Lower ectoderm cells differentiate to make a third layer of tissue (middle) that’s called the mesoderm
-The 2 layered embryonic disc is now a 3 layered embryo
-These are the germinal layers (ectoderm, mesoderm and endoderm)
Organogenesis
=formation of body organs (all organ systems are recognizable by the 8th week)
Organ systems development (from germinal layers):
Ectoderm the outer region of tissue that gives rise to the nervous system and epidermis of the skin
Mesoderm–> middle region of tissue that gives rise to: skeletal system, muscles, dermis od skin, reproductive system, urinary system and cardiovascular system
Endoderm–> the inner region of tissue that gives rise to: gastrointestinal tract and epithelial linings of hollow organs and respiratory system
formation of CNS (neurulation):
=first event of organogenesis is development of brain and spinal cord
-The ectoderm thickens and form the neural plate
-The neural plate then folds inwards forming a neural groove that has 2 prominent neural folds and neural crests
-By the 22nd day, the neural crests come together and fuse which results in the neural tube which pinches off into the body
-The anterior part becomes the brain and the rest becomes the spinal cord
-The neural tube closing relies on adequate levels of folic acid being available
-If the neural tube fails to close, this can result in spina bifida
types of Birth defects
Genetic–> inherited and usually follow predictable mendelian ratios
Congenital–> developmental defects that are more difficult to predict and may arise spontaneously as a result of dietary deficiency or exposure to teratogens (environmental factors)
-Often are idiopathic
-Common congenital defects are cleft palates or cardiac septal defects and these can be treated through surgery
-A large number are fatal and may lead to spontaneous miscarriage in the 1st trimester
-The mothers immune system is consistently screening for genetic and congenital defects throughout pregnancy
spina bifida (split spine)
= where the neural tube fails to close (3-4 weeks) which results in exposed neural tissue
-Usually is closed around day 22 and the overlying tissues and skin form over this closed tube
-If there is a lack of folic acid or exposure to teratogens, then the closure may not occur and these overlying tissues and skin can’t form- resulting in an open neural tube
-Spina bifida may result in permanent physical disability- often will require a wheelchair
types of spina bifida
- Spina bifida occulta (hidden):
-Neural tube closes and meninges form around the brain and spinal cord but one or more of the posterior portions of vertebra fail to fully close
-Many individuals are asymptomatic and are unaware of the condition which may only be discovered on x-ray or mri
-Most frequent in lumbar or thoracic vertebrae
-May predispose to back pain - Meningocoele:
-Here neural tube closes but meninges are frequently herniated through spaces between the vertebrae or portions of the skull
-Can lead to cerebrospinal fluid accumulating in herniated meninges and forming a cyst
-This cyst can enlarge dramatically
-Can occur at any point along the neural tube and can affect the spine and skull
-When skull is affected can lead to facial disfiguration and will often require surgery to correct - Myelomeningocele:
-Most serious and most common type
-Neural tube has failed to closewhich has led to exposure of neural tissue
-Can result in an incomplete spinal cord and paralysis
-Cannot repair fully - Hydrocephalus (water on the brain):
-Cerebrospinal fluid is produced by ependymal cells which line hollow ventricles of the brain
-CSF then leaves ventricles and drains into subarachnoid space surrounding spinal cord through cerebral aqueduct
-Commonly occurs when fluid does not drain fully through occluded cerebral aqueduct
-Leads to CSF collecting in ventricles and expanding brain
-Head can expand
-Mild- severe
anencephaly
=absence of cranial vault (acrania)
-The brain will form and the meninges and major bones of the skull will form around the brain
-If the anterior portion of the neural tube doesn’t fuse then the brain cannot develop and thus the skull and cranial vault are not there
-This is often detected early through ultrasounds
-The babies will usually not survive
-However some are born with vital centres of the brainstem and can survive for varying periods of time outside the uterus
common teratogens
= Any environmental factor which can interfere with prenatal development and lead to birth defects
- Ionizing radiations
- noxious chemicals
- certain drugs e.g. thalidomide
- certain infectious diseases e.g. Rubella
Exposure to teratogens during the embryonic stage of prenatal development is particularly harmful since it is at this time that the major organs and organ systems are forming
In most cases where a birth defect has occurred it is difficult to trace this back to exposure to a particular teratogen
cytomegalovirus (CMV)
- CMV is transmitted by sexual contact and secretions, adult infections may be asymptomatic
- About 1% of pregnant women are infected, and only a small percentage of these will have babies with clinical symptoms
- Symptoms include hearing loss (around 25% of deaf children), cerebral palsy, microcephaly (see diagrams below), intellectual disability (used to be termed mental retardation), visual defects and dental anomalies
