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30 Cards in this Set

  • Front
  • Back
What is the difference between an embryo and a foetus?
Embryo = from fertilisation to the 8th week
Foetus = From the beginning of week 9 to term (40 weeks)
Define morphogenesis
Morphogenesis = the development of 'form' and organisation, involving mass cell movements. The cells interact to form tissues and organs
Define differentiation
Cells, originally identical, become distinct from each other and specialise to perform specific functions
Define growth
Relates to the structural embryonic growth involving cells division and elaboration of cell products
What is the difference between age from conception and gestational age
- Age from conception is the age of the embryo from the time it was fertilized during ovulation.
- Gestational age is the age from conception plus 2 weeks - this is important as there is usually 2 weeks between ovulation and the 1st day of menstruation, and a woman measures her pregnancy from the first day of her last period
Describe the process of fertilization
- Sperm are deposited in the vagina and migrate through the cervix to the uterus and fallopian tubes
- Eggs are ovulated into fimbria and then into oviducts
- Fertilization usually occurs in the 1st 1/3 of the oviduct - although can occur outside it = ectopic pregnancy
- Egg and sperm membranes fuse allowing the sperm head and tail to enter egg cytoplasm
- Fusion of the membranes causes the permeability of the zona pellucida to change, so that further sperm cannot bind (polyspermy)
- Also stimulates the egg to finish its Meiosis II (it has be frozen in metaphase II) = forms 2nd polar body
- Male and female pronuclei approach each other, their nuclear membranes break down and they fuse and DNA replicates = zygote
Describe how a morula develops
- As a zygote passes along the uterine tube to the uterus it undergoes cleavage due to repeated mitotic divisions of the zygote
- These cells are know as blastomeres
- Blastomeres become smaller after each cleavage division
- At ~8 cells, the blastomeres undergo compaction in which the cells form a compact ball of cells, which is prerequisite for segregation of internal cells to form the inner cell mass or embryoblast
- Increased contact between cell membranes cause the formation of tight junctions between cells
- This forms the morula ~3 days post-fertilization
Describe early developmental abnormalities of the blastomeres that may cause failure to implant
- Fragmentation
- Irregular cleavage
- Slow cleavage
- Arrest
When does the embryonic genome switch on?
- Day 3
- Until then embryonic events are preprogrammed by maternal mRNA and transmitted via oocyte cytoplasm
What is human chorionic gonadotrophin?
- Hormone detected in pregnancy tests
- Originates from oocytes
- Stimulates the fallopian tubes so that menstral bleeding is ceased
Describe the formation of the blastocyst (blastulation)
- As the morula enters the uterus it draws in fluid and forms fluid-filled extracellular spaces which fuse to form a central fluid-filled cavity
- Embryo is now a blastocyst with 2 distinct types of cells
1. Trophoblasts = gives rise to the embryonic part of the placenta
2. Inner cell mass - which is the primordium of the embryo proper
How does the blastocyst increase in size?
1. Expanding the internal blastocoelic cavity, stretching and thinning the zona pellucida and eventually escaping from it ('hatching')
2. Continuing mitotic division and starting to build mass
Describe the process of implantation
- The polar trophoectoderm (made by trophoblast cells) outgrow and adhere themselves to the endometrial walls
- Some trophoblast cells differentiate in syncytiotrophoblasts which branch deeper into the endometrium
- Mural trophoectoderm (i.e. the trophoectoderm not in contact with the endometrium) forms cytotrophoblasts
- The inner cell mass becomes covered with a new differential cell layer called the hypoblast which lines the blastocoelic cavity
- The remaining inner cell mass becomes the epiblast
- The continuing burrowing of the syncytiotrophoblasts results in the blastocyst being fully implanted by day 10. A fibrin plug is formed by the damaged endometrial tissue
Give a summary of events occurring in the 1st week of pregnancy
- Fertilisation
- Cleavage of the zygote
- Morula formation
- Formation of the blastocyst
- Hatching
- Adhesion
- Implantation
Describe the embryonic disk
- Composed of 2 layers = epiblasts (relating to the amniotic cavity) and hypoblasts (primordial endoderm)
- The amniotic cavity and yolk sac develop in association with these layers
- Formation of the embryonic disk stimulates changes in the blastocoelic cavity, and it becomes known as the exocoelomic cavity (primary yolk sac)
- The amniotic cavity forms within the epiblast
Describe the formation of the primary and secondary yolk sacs
- Primary yolk sac was the blastocoelic cavity
- The yolk sac is involved in the transfer of nutrients and oxygen to the embryo from the maternal blood
- The yolk sac reduces in size and is referred to as the secondary yolk sac, which buds off a piece
What is the connecting stalk?
- A slender band which attaches the amniotic and yolk sacs to the chorion (the enveloping membrane of the embryo)
- Primordium of the umbilical cord
Summarise the significant events of the second week of development
- Completion of implantation
- Formation of the amniotic cavity and primary yolk sac
- Inner cell mass forms the embryonic disk with hypoblastic and epiblastic layers (bilaminar)
Describe the process of gastrulation
- Gastrulation is the process by which the flat bilaminar disk is converted into a trilaminar embryonic disk between days 15 & 16
- Formation of germ layers and the beginning of embryogenesis
- At day 15 a faint groove appears within the midline of the embryonic disc which has now assumed an oval shape
- The groove continues to deepen, causing a depression (primitive pit) by day 16, and a mound of cells (primitive node) at the cranial end of the groove near the centre of the germ disc = primitive streak
- This structure clearly establishes the anatomical regions of the head and tail ends of the embryo
- Cells of the epiblast migrate towards the groove and drop between the epiblast and hypoblast, differentiating into mesoderm cells
- Hypoblast cells differentiate into endoderm cells and the remaining epiblast cells differentiate into ectoderm cells
What is induction?
- Induction is when one germ layer causes the cells of another layer to become specialised or differentiate to form a specific tissue type or organ
Describe the formation of the notochord and what it becomes
- As cell migration occurs during gastrulation the primitive streak expands
- When the primitive streak has reached its definitive length the cells invaginating the primitive node form a collection of mesodermal cells called the 'notochordal process' - which is a flexible rod or a cylindrical mass of mesodermal tissue
- The notochordal process 'unzips', converting it to the Notochordal Plate
- The notochordal plate eventually detaches from the endoderm and forms a solid cylindrical structure know as the notochord
- The notochord forms in the region of the future vertebral column and forms the intervertebral discs
What tissues does the endoderm give rise to?
Forms the primitive gut which gives rise to the lungs, liver, pancreas, and digestive tube
What tissues does the ectoderm give rise to?
- Epidermis = skin, hair, mammary glands
- Epidermal placodes = lens, inner ear
- Neural tube = brain, spinal cord
- Neural = peripheral nervous system
What tissues does the mesoderm give rise to?
Axial
- Notochord

Para-axial
- Scletotome = axial skeleton
- Myotome = skeletal muscle
- Dermatome = connective tissue of skin

Intermediate
- Mullerian ducts = oviducts, uterus
- Mesonephros = kidneys, ovaries, testis

Lateral
- Somatic = connective tissue of body wall and limbs
- Visceral = mesentries, heart, blood vessels

Head
Describe the formation of the buccopharyngeal and cloacal membranes and what they become
- During week 3 of development 2 faint depressions appear in the ectoderm - one at the cranial end (buccopharyngeal membrane) and one at the caudal end (cloacal membrane)
- In week 4 the buccopharyngeal membrane forms the opening to the oral cavity
- The cloacal membrane disintegrates in week 7 to form the openings of the anus, urinary and genital tracts
Summarise the major events of the 3rd developmental week
- Gastrulation occurs
- Neurulation begins (completes day 23)
- Heart development begins day 20
- Formation of the allantois (day 16)
What is the allantois?
- Appears day 16
- Sausage-shaped tubular outgrowth of the endoderm of the yolk sac
- Involved in the formation of the urinary bladder in the early embryo and early blood formation
- Blood vessels in the allantois become the umbilical arteris and veins and the connection is the umbilical cord
Describe the origins of the nervous system
- Begins to develop in day 18
- Neural plate tissue forms from ectoderm cephalic to the primitive knot induced by the notochord = neuroectoderm
- Neural folds of the lateral neural plate fuse together, converting the plate into a neural tube
- Formation of the tube begins near the middle of the embryo and progresses both cranially and caudally (zip up)
- The neural folds are particularly prominant at the cranial end of the embryo and show the first signs of brain development
- The neural crest, formed from cells lateral to the neural folds, lies between the neural tubes and the overlying surface of ectoderm. It separates into right and left parts and migrates widely in the embryo, giving rise to spinal ganglia (dorsal root ganglia) and ganglia of the ANS
- Neural crest cells also give rise to the ganglia of some cranial nerves, the sheaths of the peripheral nerves, the meninges of the brain and spinal cord, the suprarenal medulla and several skeletal and muscular components of the head
What are somites and what do they become?
- First pair arise day 20
- Derived from mesoderm arranged in dense columns either side of the developing nervous system (neural tube)
- First appear in occipital region and extend craniocaudally
- GIve rise to most of the axial skeleton and musculature
Describe the formation of the primitive cardiovascular system
- Migration of the heart-forming mesoderm through the primitive streak to form the bilateral areas of the precardiac splanchnic mesoderm
- During week 3 two tubes begin to develop in horse-shoe shaped area of the mesoderm known as the cadiogenic area
- The two endocardial tubes are initially situated on either side of the foregut and are brought into the thoracic region due to the lateral and cephalic folding of the embryo late in week 3
- The tubes fuse to form a single heart tube
- By the end of week 3 the early vascular system and heart tube have linked and anatomic arrangements for the physiological exchanges between mother and embryo are established
- The heart begins to beat and propel blood through the embryo and placenta by day 22
- At the end of week 4 there is a complex vascular network in place which facilitates maternal-embryonic exchange of gases, nutrients and metabolic waste products