3.4. Reproduction & embryology Flashcards
Define fertilisation
Fertilisation is a series of processes which culminate with the union of the male gamete, the sperm, with the female gamete, the oocyte, to form a DIPLOID zygote.
Where do sperm develop?
Seminiferous tubules
Describe AZOOSPERMIA and give a possible cause
No sperm present within the ejaculate
possible cause- blockage to the vas deferens, primary testicular failure
Describe OLIGOZOOSPERMIA and give a possible cause
Reduced number of sperm in ejaculate (low sperm count)
possible cause- gonadotrophin insufficiency, drugs
Describe IDIOPATHIC OLIGOZOOSPERMIA and give a possible cause
Low sperm count but physiological parameters normal
possible cause- unexplained
Describe TERATOZOOSPERMIA and give a possible cause
Abnormal morphology (giant head, double tail, bent tail)
possible cause- genetic, toxins, viral infection
Describe ASTHENOSPERMIA and give a possible cause
Reduced/lack of mobility
possible cause- toxins, viral infection
Describe SPERM AGGLUTINATION and give a possible cause
Sperm clump together
possible cause- infection, autoimmune response
What is CAPACITATION?
A series of biochemical and functional changes undergone by the sperm in the uterus and uterine tubes, in readiness for fertilisation.
Name the 2 layers of the oocyte to be penetrated
- Corona Radiata (outermost layer)
2. Zona Pelucida
What is special about the ZONA PELLUCIDA?
It allows only species-specific sperm-egg interaction
Describe the process of the sperm binding to the zona pellucida
- Capacitated sperm loosely and reversibly adhere to surface of the ZP.
- Sperm become strongly and irreversibly bound to the ZP.
Many sperm bind to ZP, but usually only a few sperm permeate into perivitelline space and only one will fuse with oocyte plasma membrane.
Describe the ACROSOME REACTION
- Outer acrosomal membrane fuses with plasma membrane of sperm.
- Small membrane vesicles, containing acrosomal enzymes, are pinched off and contents released. This exposes the inner acrosomal membrane.
- A tunnel is digested through the ZP by acrosin and the acrosomal enzymes.
- The sperm uses lurching movements to propel it forward through the ZP and perivitelline space, so that its head is in contact with the oocyte surface membrane.
What is POLYSPERMY?
The fertilisation of the occyte by more than one sperm.
Usually fatal and often detected in spontaneous miscarriage.
Polyspermy is prevented by the CORTISOL REACTION and the ZONA REACTION.
Describe the zona reaction.
The zona pellucida loses its ability to bind to sperm and therefore to induce the acrosome reaction. This changes the texture of the ZP, known as zona hardening.
Describe first mitotic division during fertilisation
The membrane of both pronuclei break down, and the haploid chromosomes from male and female gametes become organised around a mitotic spindle.
Fertilisation is complete 18-24 hrs after fusion.
During the first mitotic division in fertilisation, what is the combination of male and female chromosomes called?
Syngamy
What is an embryo?
first 8 weeks in utero
What is a fetus?
from the 9th week in utero to birth
What is cleavage?
Repeated mitotic cell division of the Zygote
Draw out the pathway of cell division
Draw out the pathway of cell division
Define the Morula
Ball of cells resulting from division of a fertilised ovum
Define the Blastocyst
Contains a cluster of cells called the inner cell mass from which the embryo arises
After how many days does implantation occur?
c. 7
Trophoblast proliferates, leaving 2 layers and invades the endometrium. What are the layers?
Outer layer= SYNCYTIOTROPHOBLAST
Inner layer= CYTOTROPHOBLAST
What hormone is produced in the synctiotrophoblast?
hCG
This promotes the survival and growth of corpus luteum.
It is detectable by the end of the 2nd week and detected in pregnancy tests.
Draw out implantation 9 days post-fertilisation
Draw out implantation 9 days post-fertilisation
At week 2, what does the inner cell mass of the Blastocyst become?
It becomes flattened into the Bilaminar embryonic disc consisting of 2 layers.
EPIBLAST- floor of the amniotic cavity
HYPOBLAST- embryonic yolk sac
Week 3- Gastrulation… what happens to the bilaminar embryonic disc?
It becomes trilaminar
What are the 3 layers of the embryonic disc?
- Ectoderm
- Mesoderm
- Endoderm
The 3 germ layers of the embryonic disc develop into specific organs and tissues.
What does the ECTODERM develop into?
Neural tube, CNS, Retina, Pineal body, Posterior pituitary (neural ectoderm)
Epidermis, hair and nails (surface ectoderm)
The 3 germ layers of the embryonic disc develop into specific organs and tissues.
What does the MESODERM develop into?
Muscle of trunk, skeleton (paraxial)
Urogenital system (intermediate)
Connective tissue, muscle of limbs (lateral)
The 3 germ layers of the embryonic disc develop into specific organs and tissues.
What does the ENDODERM develop into?
Epithelium of trachea, bronchi and lungs
The NOTOCORD is a cellular rod-like tube. What are its 2 functions?
- Strength to Bilaminar embryonic disc
2. Defines the central head-tail axis of the embryo
Weeks 4-8, Organogenesis occurs. Define this.
- Production and development of the organs of an animal
- Trilaminar disc folds into a C-shaped cylindrical embryo
- All major structures and organ systems are established, although few are functional
Human development can be classified into 3 types…
Growth- achieved by hyperplasia (cell division) and hypertrophy (increase in cell size)
Morphogenesis- development of form (movement of sheets and masses of cells)
Differentiation- maturation of cells forming tissues and organs capable of specialised functions
What organ develops from the mesodermal germ layer?
Primitive heart
What happens to the heart at 21 days post fertilisation?
cells surrounding the heart have become differentiated as myocardial cells capable of eliciting an organised response… the heart begins beating
What are SOMITES?
Masses of mesoderm that eventually become dermis, skeletal muscle, vertebrae, tendons and cartilage
NEURULATION is completed by the end of the 4th week… what is it?
The formation of the neural tube from the ectoderm of the embryonic disc.
It follows gastrulation.
List what happens during the 5th week of development
- Upper limb buds become paddle-shaped
- CRL 7-9mm
- Lower limb buds are flipper-like
- Rapid brain development and head enlargement
- Facial prominences develop
- Mesonephric ridges denote position of mesonephric kidneys
List what happens during the 6th week of development
- Head very large, projects over heart prominence
- Joints of upper limbs differentiate
- Digital rays of upper limbs evident
- Retinal pigment formed so eye is obvious
- External ear canal and auricle formed
- Reflex responses to touch
- CRL 11-14mm
List what happens during the 7th week of development
- Notches between digital rays partially separate future fingers
- CRL 16-18mm
- Liver prominent
List what happens during the 8th week of development
- Digits of hands separated (but still webbed)
- Ears characteristic shape but still low-set
- CRL 27-31mm
- Purposeful limb movements occur
- Eye lids closing
- Notches visible between digital rays of feet
- Head still disproportionately large (1/2 of total embryo length)
- External genitalia evident (not enough for sex determination)
- Ossification begins in lower limbs
