1
Q
What is the preformation theory?
A
- All creatures were created at the same time
- No evolution
- Adam contained all the humans within him (homunculuses)
- This theory was disproved when cell fertilisation was introduced
2
Q
What is the Carnegie stages of development?
A
- Standarises development studies
- Uses morphological criteria to define stages of development instead of relying on time post-fertilisation
- Allows meaningful comparisons between experiments/observations and between experimenters/observers
3
Q
What is an oocyte?
A
- Fertilised oocyte
- Zona pellucida (hardened glycoproteins layer) surrounds the zygote
- Cortical reaction is when enzymes are released from the oocyte’s cortical granules to harden the zona pellucida, preventing polyspermy
- Zygote has 2 pronuclei (haploid) at the centre which fuse to from a single diploid nucleus
- Within zona pellucida, there is fluid filled perivitelline space, within which we find polar bodies
- Cumulus cells (cumulus oophorus) surround the zona pellucida
- Cumulus cells originate from granulosa cells and must be penetrated by spermatozoa for fertilisation. It aids the acrosome reaction
4
Q
What are polar bodies?
A
- These are small haploid cells formed during meiosis
- They have a small cytoplasm and contain genetic information
- They don’t have the ability to be fertilised
- Meiosis produces 1 mature ovum and 1 polar body. This polar body forms 2 polar bodies.
- They are found within the perivitelline space within the zona pellucida
- They often die by apoptosis
5
Q
What is a review of meiosis?
A
- Primordial germ cells migrate to ovaries to form oogonia which divide by mitosis to produce more oogonia
- Oogonia arrest at meiosis I at spindle fibres where they are called primary oocytes now which are stored in the cortex of the ovary
- Granulosa cells surround the primary oocyte and produce the basal lamina —> now it’s called the primordial follicle
- After puberty, the granulosa cells of the primordial follicles produce a protective layer called the zona pellucida which stays attached during ovulation —> this is the primary follicle
- The basal lamina becomes surrounded by internal and external theca cells
- Follicles increase in diameter and antrum forms
- So far, everything is FSH independent
- Then you have FSH causing further growth of a group of primary follicles every month; most die due to negative feedback involving oestrogen, FSH and LH. Only the dominant follicle survives every month
- The dominant follicle is the follicle that is ovulated
- After ovulation, dominant follicle enters meiosis II and arrests at the spindle fibres there
- Meiosis II is only complete when fertilisation occurs
6
Q
What is the difference between spermatogenesis and oogenesis?
A
- Spermatogenesis is an ongoing meiotic process where one germ cell produces 4 sperm cells
- Oogenesis occurs only in utero. One germ cell gives rise to one oocyte (mature ovum). Once a female is born, she can’t make any more eggs
7
Q
What happens during fertilisation and implantation?
A
- The dominant follicle is collected by the fimbriae upon ovulation
- Cilia and peristalsis move this oocyte along the uterine tubes to the ampulla where fertilisation occurs
- Fertilisation occurs after 24 hours. It triggers meiosis 2 to be complete, expelling the 2nd polar body
- The haploid pronuclei fuse to form diploid nucleus in the zygote and then it enters mitosis
- The diploid zygote divides rapidly but doesn’t increase in size as it occurs within zona pellucida —> this is cleavage
- At 16 cell stage, it becomes a morula
- Here, cells begin to differentiate. Trophoblasts (form placenta) form outside of an inner cell mass composed of embryoblastst (forms embryo proper)
- An epithelial barrier forms around the embryo and the cells begin to flatten and squash together —> this is compaction
- At day 5, embryoblasts clump at one end of embryo, leaving a fluid filled cavity (blastocoele) at the other end - this is a blastocyst
- Whilst all of this is happening, the zona pellucida is dissolving outside the embryo due to enzymes from embryo and decidua (mucosal lining of uterus) —> this is hatching
- At day 6-7, the blastocyst implants into decidua
- The trophoblast layer begins to differentiate into inner cytotrophoblasts (essentially stem cells) and syncytiotrophoblasys
- The cytotrophoblasts fuse to form the syncytiotrophoblast, this then invades the decidua and implants there
- The embryo becomes completely engulfed by the decidua so as the embryo grows, one side of the uterus expands across to the other
8
Q
What is bilaminar germ disc formation (week 2)?
A
- Formation of epiblast and hypoblast from embryoblasts
- As the embryo is implanting, the embryoblast begins to differentiate into 2 layers - hypoblast and epiblast
- Epiblast is columnar cells on dorsal surface. Form embryo proper
- Hypoblast is cuboidal cells on ventral surface. Form extraembryonic tissues e.g yolk sac, chorion, amnion
- The blastocoel on the ventral side becomes primitive yolk sac
- Hypoblasts migrate into primitive yolk sac to form the true yolk sac. This is in charge of nourishing embryo until placental circulation is developed enough to take over
- It is supplied by the primitive aorta and drained by Vitelline veins to the developing heart
9
Q
What is gastrulation (week 3)?
A
- Formation of trialaminar germ disc
- It begins with the appearance of primitive streak on the dorsal surface of epiblast (primitive streak is invagination of epiblast cells)
- At the tip of the primitive streak, we find Hensen’s node - this is where gastrulation is initiated
- The notochord is a cartilageneous structure that become IV discs. The notochord develops in the midline.
- The cells of epiblast undergo epithelial mesenchyme transition (EMT) and migrate towards the primitive streak and dive down to replace the hypoblast cells and form a 3rd germ layer - this is the separation step where epiblast and hypoblast separate
- The 3 layers are ectoderm (the epiblast that didn’t involute), mesoderm and endoderm
- All body tissues develop from either ectoderm, mesoderm or endoderm.
10
Q
What does the ectoderm give rise to?
A
- Epidermis and associated derivatives (e.g skin and hair)
- CNS and PNS
- Some bones, muscles and connective tissues of head and neck
11
Q
What does the mesoderm give rise to?
A
- Notochord
- Skeletal, smooth and cardiac muscle
- Connective tissues and parenchyma of organ systems
- Reproductive and urinary systems
- Skeletal structures
- Dermis
12
Q
What does endoderm derivatives?
A
- Lining of GI tract and accessory GI organs
- Lining of airways
- Pharyngeal pouch derivatives
- Embryonic blood cells
- Cloacal derivatives
13
Q
What is neurulation?
A
- This is the folding process in vertebrae embryos
- It occurs concurrently with gastrulation (week 3 and 4)
- As primitive streak recedes (moves back) caudally, the midline surface ectoderm thickens to form the neural plate
- The neural plate then invaginate to form the neural groove and fuses in the midline as the groove sinks below the surface and pinches off to form the neural tube - there are anterior and posterior neurotubes
- Neural crest forms - these are stem cells created near the neural plate
- The neural tube gives rise to neurones of CNS, supporting cells of CNS, somatomotor neurones of PNS (innervation skeletal muscle muscle), presynaptic autonomic neurones of PNS
- The neural crest gives rise to sensory neurones of PNS, postsynaptic autonomic neurones, Schwann cells of PNS, adrenal medullary cells, head mesenchyme and portions of the heart, melanocytes in the skin, arachnoid and pia mater
14
Q
What does the neural tube give rise to?
A
- Neurones of CNS
- Supporting cells of CNS
- Somatomotor neurones of CNS (innervate skeletal muscles)
- Presynaptic autonomic neurones of PNS
15
Q
What does the neural crest give rise to?
A
- Sensory neurones of PNS
- Postsynaptic autonomic neurones
- Schwann cells of PNS
- Adrenal medullary cells
- Head mesenchyme and portions of the heart
- Melanocytes in skin
- Arachnoid and pia mater
16
Q
What are somites?
A
- Somites are blocks of mesoderm on either side of neural tube
- As the neural groove invaginate along the posterior midline of the embryonic disc (primitive streak), it is flanked bilaterally by masses of mesoderm called somites
- 42 to 44 pairs of somites develop along this central axis and eventually give rise to: dermomyotomes (which divide further into dermatomes which become dermis), myotomes (which become segmental masses of skeletal masses of skeletal muscle) and sclerotomes (medial part of each somite, which along with the notochord migrate around the neural tube to form the cartilaginous precursors of the axial skeleton)
17
Q
What other processes occur in week 4?
A
- Gut formation and body folding
- Folding of embryo is down to different rates of cell proliferation in different regions and layers
- At day 16, the embryo begins to fold inwards at both cranial and caudal ends as well as bilaterally
- This folding can be described by the ‘cloth purse model’. The embryo behaves like a cloth purse, with an open cord around the margins to begin with. As the cells proliferate at different rates, it acts like it’s pilling the cords to create the neck of the purse
18
Q
What is the ‘cloth purse’ model?
A
- The embryo behaves like a cloth purse, with an open cord around the margins to begin with
- As the cells proliferate at different rates, it acts like it’s pulling the cords to create the neck of the purse
- Septum and heart move from margin to centre
- Yolk sac, Allantois and stalk make umbilical cord
- Prochordal and cloacal plates delimit gut tube
