(dev&age) early fetal development Flashcards
1
Q
how do we measure time in embryo-fetal development?
A
fertilisation age
gestational age
Carnegie stage
2
Q
what is fertilisation age?
A
age of embryo measured from the time of fertilisation (assumed to be + 1 day from last ovulation)
3
Q
what is fertilisation age also known as?
A
conceptual age
4
Q
what is gestational age?
A
calculated from the time of the beginning of the last menstrual period (LMP)
determined by fertilization date (+ 14 days) if known, or early obstetric ultrasound and comparison to embryo size charts
5
Q
what is a Carnegie stage?
A
the 23 stages of embryo development based on embryo features and structure rather than time
6
Q
how is fertilisation age calculated?
A
measured from the time of fertilisation (assumed to be + 1 day from last ovulation)
7
Q
how is gestational age calculated?
A
calculated form the beginning of the last menstrual period (LMP)
determined by either fertilisation age + 14 days OR using an early obstetric ultrasound and comparing results w embryo size charts
8
Q
what is the main limitation of fertilisation age?
A
difficult to know exact time of fertilisation (unless method of conception was IVF, not natural) = variable time between intercourse and fertilisation
9
Q
what is Carnegie staging based on?
A
based on embryo features, structure and development rather than time
10
Q
why is it difficult to measure fertilisation age?
A
difficult to know exact time of fertilisation as variable time between intercourse and fertilisation
11
Q
how does fertilisation age compare to gestational age and why?
A
gestational age is always 14 days longer than fertilisation age as it begins at the start of the last menstrual period (usually approx 14 days prior to fertilisation day)
12
Q
what is an advantage of Carnegie staging?
A
allows comparison of developmental rates and events between species
13
Q
in which scenario can fertilisation be definitively calculated?
A
if the method of conception was IVF
14
Q
when should fertilisation occur in comparison to ovulation?
A
fertilisation should occur within 24 hours of ovulation
15
Q
what are the three embryo-fetal stages in pregnancy?
A
embryogenic stage (14-16 days post fertilisation)
embryonic stage (16-50 days post fertilisation)
fetal stage (50-270 days post fertilisation)
16
Q
how do the embryo-fetal stages compare to the trimesters in pregnancy?
A
embryogenic + embryonic stages = first trimester (first 50 days)
fetal stage = second and third trimester (50-270 days)
17
Q
how long does the embryogenic stage last?
A
approx 14-16 days post fertilisation
18
Q
how long does the embryonic stage last?
A
approx 16-50 days post fertilisation
19
Q
how long does the fetal stage last?
A
50-270 days post fertilisation
20
Q
which embryo-fetal stages does the first trimester encompass?
A
embryogenic + embryonic stages
21
Q
which embryo-fetal stage do the second and third trimester encompass?
A
fetal stage
22
Q
what does the embryogenic stage involve?
A
establishing the early embryo from the fertilised oocyte
determining two populations of cells: pluripotent embryonic cells and extraembryonic cells
23
Q
at which point does an embryo become a foetus?
A
end of the first trimester
24
Q
what is the function of the pluripotent embryonic cells?
A
contribute to the development of foetus
25
what is the function of the extraembryonic cells?
contribute to the development of support structures (e.g. placenta)
26
what does the embryonic stage involve?
establishment of the germ layers and differentiation of tissue types
establishment of body plan
27
what does the fetal stage involve?
major organ systems established
migration of some organ systems to final location
extensive growth and acquisition of fetal viability (survival outside the womb)
28
how do pluripotent embryonic cells compare to extraembryonic cells?
pluripotent embryonic cells contribute to the foetus while extraembryonic cells contribute to the supporting structures (e.g. placenta)
29
in which stage is the body plan establised?
embryonic stage
30
in which stage does the foetus grow and develop foetal viability?
fetal stage
31
what is foetal viability?
the ability of the foetus to survive independently outside the womb
32
in which stage does the germ layer develop?
embryonic stage
33
in which stage do major organ systems develop and migrate to their final positions?
fetal stage
34
name the terms used to describe the fertilised oocyte in the first few days of life
zygote (1 cell)
cleavage stage embryos (2-8 cells)
morula (16+ cells)
blastocyst (200-300 cells)
35
what is a zygote?
the first diploid cell that forms as a result of the fusion of an egg and sperm in fertilisation
36
what are the cleavage stage embryos?
the division of the zygote into two, then four then eight cells
37
what is a morula?
the stage of the embryo where it consists of more than 16 cells
38
what is a blastocyst?
the stage of the embryo where it is made up of 200-300 cells
39
where does the development of a zygote into a blastocyst take place?
happens as the embryo migrates from the ampulla, down the fallopian tubes towards the uterus
40
what specific structure present from the zygote to the blastocyst stage and for what reason?
zona pellucida encases the cells undergoing cell division from the zygote to the blastocyst stage
41
how does an ovulated oocyte become a zygote?
fertilisation with a sperm cell
42
how many days post fertilisation is the embryonic 2 cell stage?
1 day post fertilisation
43
how many days post fertilisation is the embryonic 4 cell stage?
2 days post fertilisation
44
how many days post fertilisation is the embryonic 8 cell stage?
3 days post fertilisation
45
when do embryonic genes begin to be transcribed?
in the 4-8 cell stage of embryonic development
46
if not by transcribing embryonic genes, how does the embryo get through the first few mitotic divisions?
relies on maternal mRNAs and proteins for the first few mitotic divisions
47
when are maternal mRNAs and proteins synthesised?
synthesised and stored during oocyte development (i.e. pre-ovulation)
48
what happens if the synthesis/storage of maternal mRNA and proteins is impaired?
if synthesis or storage of these maternal mRNAs/proteins is impaired during oogenesis = impaired embryonic development
49
what occurs during maternal-to-zygotic transition?
change of transcription from maternal mRNAs and proteins to embryonic genes (zygote genome activation)
increased protein synthesis
organelle maturation (Golgi, mitochondria)
50
when does maternal-to-zygotic transition take place?
between the 4 cell and 8 cell stage (i.e. between day 2 and 3 post-fertilisation)
51
what is zygote genome activation?
transition to transcription of embryonic genes (from transcription of maternal mRNAs and proteins)
52
when is there much less reliance on maternal mRNAs and proteins for transcription in the embryo?
in the 4-8 cell stage when zygote genome activation occurs and transcription of embryonic genes begins
53
what is compaction?
the change in shape from spherical to wedge-shaped that occurs when outer cells become firmly attached to each other via tight gap junctions or desmosomes AND give rise to the first two cell types
54
what does compaction primarily cause?
the development of the first two cell lineages: inner and outer cells
55
when does compaction occur?
around or after the 8-cell. stage
56
what happens during compaction?
outer cells connect to each other through tight gap junctions and desmosomes = become pressed against the zona pellucida + become polarised
inner cells attach firmly to each other
57
what are the two cell types that are given rise to as a result of compaction?
outer cells and inner cells
58
why is compaction important in embryology?
first morphological change that takes place in embryogenesis to change embryo from spherical to wedge-shaped
+ gives rise to the first two cell types (trophectoderm and inner cell mass lineages)
59
what kind of morphological change takes place as a result of compaction and how?
embryo changes from being spherical to being wedge-shaped
as the outer cells connect to each other via tight gap junctions and desmosomes and pull in to compact the inner cells + deform the spherical shape
60
how does compaction affect diffusion?
forms a barrier to diffusion between the inner and outer embryo
61
describe the structure of a compacted morula
the 16-cell stage wherein there are two distinct cell lineages: inner cells and outer cells
62
what happens to the two lineages when a compacted morula forms a blastocyst?
the two cell population reorganise themselves alongside the formation of the blastocoel cavity
63
what is the blastocoel?
fluid-filled cavity that forms osmotically in the blastocyst stage
(helps with the growth of cells and structural support)
64
how does the blastocoel form?
the trophectoderm pumps Na+ ions into the cavity (centre of the blastocyst) and water follows to maintain osmotic balance forming a large fluid-filled space in the centre
65
what is the zona pellucida?
hard protein outer covering of the developing embryo
66
what is the function of the zona pellucida?
inhibits polyspermy and protects the early embryo
67
what does the zona pellucida prevent?
polyspermy
68
describe the structure of a blastocyst
the zona pellucida encases the outer trophectoderm
within the trophectoderm, the inner cell mass and the blastocoel are found
69
what are the two cell lineages that are found in a blastocyst?
trophectoderm contains extra-embryonic cells AND
| inner cell mass contains the pluripotent embryonic cells
70
what is the function of the trophectoderm in blastocoel formation?
pumps Na+ ions into the blastocoel cavity and to maintain the osmotic balance, water follows, creating a fluid-filled cavity
71
what is the trophectoderm?
collection of extra-embryonic cells that contribute to developing support structures
72
what is the inner cell mass?
collection of pluripotent embryonic cells that contribute to the developing organism
73
how are the trophectoderm and inner cell mass arranged?
trophectoderm forms. a sphere which surrounds the inner cell mass, pushed to one side of the sphere
74
what cell type is found in the trophectoderm?
extra-embryonic cells
75
what cell type is found in the inner cell mass?
pluripotent embryonic cells
76
differentiate between the function of pluripotent embryonic cells and extra-embryonic cells
pluripotent embryonic cells contribute to the developing foetus whereas the extra-embryonic cells contribute to the developing support structures
77
what do pluripotent embryonic cells do?
contribute to the developing foetus
78
what do extraembryonic cells do?
contribute to the developing support structures
79
what is hatching?
the process by which the blastocyst must be extruded from the zona pellucida to implant itself into the endometrium
80
when does hatching occur?
day 5-6 post fertilisation
81
why is hatching important?
to allow the blastocyst to implant into the endometrium and ensure a pregnancy can happen
82
how does hatching occur?
the blastocyst secreted enzymes that digest the zona pellucida
the embryo undergoes cellular contractions that will weaken a point on the zona pellucida through which the blastocyst will extrude itself
83
what are the implications of no hatching occurring?
the blastocyst cannot implant itself into the endometrium and so a pregnancy cannot take place
84
what two things occur to enable hatching?
enzymatic digestion (of the zona pellucida)
cellular contractions (of the embryo)
85
what secretes the enzymes that digest the zona pellucida to enable hatching?
blastocyst
86
what two cell lineages are present in the morula?
inner cell mass (pluripotent embryonic cells)
trophectoderm (extra-embryonic cells)
87
when do the peri-implantation events take place?
days 7-9 post fertilisation
88
what are the peri-implantation events that take place between days 7-9 post fertilisation?
division of the trophectoderm lineage into the syncytiotrophoblast and the cytotrophoblast cells
division of the inner cell mass into the epiblast and the hypoblast
89
what does the trophectoderm divide into?
syncitiotrophoblast and cytotrophoblast
90
what does the inner cell mass divide into?
epiblast and hypoblast
91
what is the function of the syncitiotrophoblast?
invades and destroys maternal cells and capillaries of the endometrium
= creates an interface between the embryo and maternal blood supply
92
what impact does the syncitiotrophoblast have on the endometrium and maternal cells?
invades and degrades the maternal endometrial cells and capillaries
(to create an interface between the embryonic cells and the maternal blood supply)
93
what is the function of the cytotrophoblast?
remain individual to provide a source of syncitiotrophoblast cells
94
from where do the syncitiotrophoblast and cytotrophoblast originate?
trophectoderm
95
what is the function of the epiblast?
structure from which the fetal tissue is derived
96
what is the function of the hypoblast?
structure from which the yolk sac (extraembryonic structure) develops
97
what is the function of the yolk sac?
gut development and early haemotopoiesis
98
from where do the epiblast and the hypoblast originate?
inner cell mass
99
where is the syncitiotrophoblast found?
invading the endometrial tissue
100
where is the cytotrophoblast found?
outer surface of embryo
101
where is the epiblast found?
between the cytotrophoblast and the hypoblast (in the region of the inner cell mass)
102
where is the hypoblast found?
underside of the epiblast, adjacent to the blastocoel
103
when does the bi-laminar embryonic disc form?
after day 12
104
what important stage does bi-laminar disc formation precede?
gastrulation
105
what structure forms resulting in the subsequent formation of the bi-laminar disc?
amniotic cavity
106
how does the amniotic cavity form?
develops by separating two layers of the epiblast to form the amniotic cavity
107
where does the amniotic cavity form?
between two layers of the epiblast cells
108
what is the amnion?
structure that forms from the epiblast cells of the blastocyst
grows and surrounds the developing embryo, creating a fluid-filled cavity (amniotic sac)
109
where is the amniotic cavity found?
between the amnion cells and the bi-laminar disc
110
what is the function of the amnion?
grows and surrounds the developing embryo to create a fluid-filled cavity (amniotic sac)
111
what is the structure of the bi-laminar disc?
single inner layer of epiblast cells adjacent to a single outer layer of hypoblast cells = two layer disc
112
between which two cavities is the bi-laminar disc found?
amniotic cavity and the blastocoel
113
what does the syncitiotrophoblast secrete?
hCG (human chorionic gonadtrophin)
114
why is the secretion of hCG by the syncitiotrophoblast important?
hCG is the hormone detected to give a positive result on a pregnancy test
115
why is the syncitiotrophoblast important for pregnancy testing?
secretes hCG that is tested for in a pregnancy test
116
what is the embryo ready to undergo following the formation of the bi-laminar disc?
gastrulation
117
what two structures surround the amniotic cavity?
amnion and bi-laminar embryonic disc
118
what does the epiblast divide into?
some cells remain as epiblast while some form amnion cells
119
what is gastrulation?
the process by which the b-laminar embryonic disc undergoes reorganisation to from a trilaminar disc due to the inwards migration of cells
leading to the formation of the three germ layers (endoderm, mesoderm and ectoderm)
120
what does gastrulation form?
the three germ layers: ectoderm, mesoderm and endoderm
121
when do the three germ layers form?
week 3 of development
122
what is the primitive streak?
a structure that forms at the midline of the epiblast, extending from the caudal end
123
when does the primitive streak form?
approximately 15 days post fertilisation (after bi-laminar disc formation)
124
where does the primitive streak from?
in the midline of the epiblast layer (elongating from the caudal end)
125
how does the primitive streak develop?
at cranial end, primitive streak expands to create a primitive node
primitive node contains a circular depression called the primitive pit
depression continues along the midline of the epiblast back towards the caudal end of the primitive streak, forming a primitive groove
126
why is the formation of the primitive streak important?
establishes the cranial and caudal ends of the embryo as well as the right and left axis
127
how does the primitive streak become the primitive groove?
the depression of the primitive pit (in the primitive node) continues caudally forming the primitive groove
128
how do the epiblast cells respond to the formation the primitive groove?
detach from the epiblast layer and migrate inwards towards the primitive streak, slipping beneath it into the interior of the embryo
129
what is invagination?
the process by which the epiblast cells detach from the epiblast layer and migrate inwards to the primitive streak, slipping beneath it into the inner embryo
130
how does the endoderm form?
the first group of epiblast cells to invaginate the primitive streak will invade the hypoblast, displacing the cells to form a new cell layer = endoderm
131
by when is the hypoblast completely displaced and how?
by day 16, the hypoblast cells are completely replaced by the invagination of the invading epiblast cells
132
what does the displacement of hypoblast cells form?
the endoderm
133
post endoderm formation, what do the remaining epiblast cells form?
the outermost, distal layer = the ectoderm
134
how does the ectoderm form?
once the endoderm forms, the remaining cells in the epiblast form the ectoderm
135
how does the mesoderm form?
some of the epiblast cells remain sandwiched in the space between the ectoderm and the endoderm, forming the mesoderm
136
what stops the migration of epiblast cells to the primitive streak?
the completion of the formation of the endoderm and the mesoderm
137
what does the epiblast give rise to?
ectoderm, mesoderm and endoderm
138
what organs does the endoderm give rise to?
liver, pancreas, lung, thyroid and GI tract
139
what organs does the ectoderm give rise to?
skin epithelia, tooth enamel, CNS, neural crest
140
what organs does the mesoderm give rise to?
blood (endothelial cells, WBCs, RBCs), muscle (all types), gonads, kidneys, adrenal cortex, bone, cartilage
141
what is the first major event after gastrulation?
notochord formation
142
when does notochord formation take place?
after day 13 post fertilisation
143
what is the notochord?
rod-like tube structure formed from cartilaginous cells
144
where does the notochord form?
under the ectoderm, in the midline of the embryo
145
from where AND in which direction does the notochord grow?
from the edge of the primitive streak towards the cranial end of the embryo
146
what is the function of the notochord?
acts as a centre of organisation for neurulation (CNS formation) and mesoderm development
147
which two processes rely on the notochord?
neurulation (CNS formation)
mesoderm development
148
what is neurulation?
process in which the neural plate bends up and later fuses to form the hollow tube that will eventually differentiate into the brain and the spinal cord of the central nervous system (i.e. formation of the CNS)
149
what is the neural plate?
a section of thickened ectoderm
150
why is the neural plate important for notochord function?
notochord signals through the neural plate to develop the neural system
151
how does the notochord lead to neural tube formation?
signals from the notochord (from under the ectoderm) will move up through the embryo and will direct the neural plate to form the neural tube
152
when does neurulation take place?
approximately day 14 post fertilisation
153
what do the signals from the notochord stimulate?
stimulate the inner part of the neural plate to move down towards the notochord, forming a neural groove
stimulate the outer parts of the neural plate to move up to form ridges that run along the cranial-caudal axis of the embryo, forming neural folds
154
what two structures are formed as a result of notochord signaling in neurulation?
neural groove
neural folds (x2)
155
how is the neural groove formed?
signals from the notochord stimulate part of the neural plate to move downwards (towards the notochord) creating the neural groove
156
how are the neural folds formed?
signals from the notochord stimulate the other parts of the neural plate to move upwards, forming two ridges (along the cranial-caudal axis of the embryo) called neural folds
157
which cells are found in the neural folds?
neural crest cells
158
what is the function of the neural crest cells?
multipotent cells that migrate away and populate many different tissues
159
where are the neural crest cells found?
in the neural folds of the neural plate
160
how does the neural tube form from the neural fold and groove?
neural folds move together over neural groove and fuse together
forming a underlying hollow tube (i.e. neural tube)
161
what is the neural tube overlaid with?
epidermis (i.e. ectoderm layer)
162
what happens to the neural crest cells when the neural tube forms?
migrate away from the neural folds to other tissues
163
what is essential for neurulation?
coordinating signals from the notochord
164
what happens to the neural tube over time in a physiologically normal embryo?
closes off at the cranial and caudal ends
165
what covers the top of the neural tube?
ectoderm-derived epidermis
166
when does closure of the head end of the neural tube take place?
approx day 23
167
when does closure of the tail end of the neural tube take place?
approx day 27
168
which closes first: the head end or the tail end of the neural tube?
the head end closes 3-4 days prior to the tail end closing
169
why does the closure of the head end occur before the closure of the tail end in the neural tube?
to allow for the development of head structures (i.e. spinal cord, brain)
170
what are two common neural tube developmental defects?
anencephaly
spina bifida
171
what is anencephaly?
the absence of most of the skull and brain structures due to the failure of the cranial end of the neural tube to close
172
how does anencephaly present?
absence of most of the skull and brain structures
173
how common is anencephaly?
approx 1/10,000 births
174
what is spina bifida?
protruding and opened spinal cord at birth due to failure of the caudal end of the neural tube to close
175
how does spina bifida present?
opened end of spinal cord that protrudes out at lower end (varying severity)
176
how common is spina bidifa?
approx 0.4-0.5/10,000 births
177
what do neural crest cells do once the neural tube forms?
migrate out of the neural folds extensively and populate other tissues
178
where are the neural crest cells derived from?
ectoderm-derived
179
what are the categories of neural crest cells?
cranial
cardiac
trunk
sacral & vagal
180
what do cranial neural crest cells give rise to?
cranial neurones, glia, lower jaw, middle ossicles (ear bones), facial cartilage
181
what do cardiac neural crest cells give rise to?
aortic arch, pulmonary artery septum, large artery walls, musculo-connective tissue
182
what do trunk neural crest cells give rise to?
dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
183
what do vagal and sacral neural crest cells give rise to?
parasympathetic ganglia, enteric nervous system ganglia
184
what can defects of neural crest migration and specification lead to?
various birth defects such as pigmentation disorders, deafness, cardiac and facial defects, and failure to innervate the gut
185
give examples of birth defects due to failure of neural crest migration and specification
pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut
186
what are the neural crest cells that migrate to the skull called?
cranial neural crest cells
187
what are the neural crest cells that migrate to the developing heart called?
cardiac neural crest cells
188
what are the neural crest cells that remain in the trunk called?
trunk neural crest cells
189
where are neural crest cells specified?
in the neural folds
190
what is somitogenesis?
formation of somites
191
what are somites?
bilaterally paired blocks of paraxial mesoderm that form along the cranial-caudal axis of the developing embryo
192
how does somitogenesis occur?
blocks of paraxial mesoderm condense AND bud off from either side of the neural tube and notochord in somite pairs, in a synchronised manner
line up from the cranial to the caudal end, progressing down the long axis of the embryo
193
what is the next key stage after neurulation?
somitogenesis
194
in which direction does somitogenesis progress?
from the cranial end to the caudal end, down the long axis of the embryo
195
what parts of somitogenesis make it species-specific?
number of pairs formed
rate of 'budding off'
appearance of somite pairs
196
what is the rate of budding in human somitogenesis?
one pair buds off per 90 mins
197
how many pairs form in human somitogenesis?
exactly 44 pairs
198
when does human somitogenesis conclude?
when the full 44 pairs have formed and lined up
199
why is somitogenesis important?
lays out the body plan for the vertebra
essential for segmentation, bone and musculature development
200
where are the somite pairs more defined: head end or tail end?
head end
201
what do somites give rise to?
two different types of embryonic tissue: sclerotome and dermomyotome
202
what does the sclerotome give rise to?
rib cartilage and vertebrae
203
what is the dermomyotome?
one of the types of embryonic tissue formed from somites
| other one is sclerotome
204
what does the dermomyotome subdivide to form?
dermatome and myotome
205
what does the dermatome give rise to?
dermis of the skin, fat and connective tissue of the neck and trunk
206
what does the myotome give rise to?
muscles of the embryo
207
what must occur for the primitive gut to form?
both ventral and lateral folding
208
what two types of folding result in the formation of the primitive gut?
ventral and lateral folding
209
what is ventral folding?
folding where the head and tail ends curl together
210
what is lateral folding?
folding where the two sides of the embryo curl together
211
what do ventral and lateral folding result in?
result in the yolk sac being pinched off the front of the embryo
212
what are the three parts of the primitive gut?
foregut, midgut and hindgut
213
what structure is responsible for the development of the primitive gut?
the yolk sac
214
what is the yolk sac?
membranous sac attached to the front of the embryo, formed from the cells of the hypoblast
involved in early haemopoiesis
215
where is the yolk sac found?
attached to the front of the embryo
216
how does the yolk sac give rise to the primitive gut?
ventral and lateral folding will cause a membranous sac to be pinched off the front of the embryo = yolk sac
217
when does the formation of the gut tube occur?
approx after day 16 after fertilisation
218
which cells give rise to the yolk sac?
hypoblast cells
219
what does the foregut include?
oesophagus, stomach, upper duodenum, liver, gallbladder, pancreas
220
what does the midgut include?
lowed duodenum, ileum, ascending colon and first two-thirds of the transverse colon
221
what does the hindgut include?
last third of the transverse colon, descending colon, sigmoid colon, rectum and upper anal canal
222
how does the heart develop?
begins as a tube of mesoderm around day 19 and begins to pump blood and beat at around day 22
223
which day does the heart begin to develop?
approx day 19 post fertilisation
224
which day does the heart begin to pump blood?
approx day 22 post fertilisation
225
when can a fetal heartbeat be detected?
approx at 6 weeks gestational age
226
when do the fetal lungs develop?
approx week 4 of development
227
where do the fetal lungs arise from?
arise from the lung bud
228
how do the lungs develop?
arise from the lung bud at around week 4 of development
lung bud splits into two and progressively branches through development
229
where do the gonads originate?
from the mesoderm
230
how does gonad development occur in XY embryos?
in XY embryos = SRY gene present
causes gonadal cells to become Sertoli cells
the Sertoli cells stimulate testes development, Leydig cell formation and testosterone production
231
how does gonad development occur in XX embryos?
in XX embryos = SRY gene is absent
so gonadal cells will adopt granulosa cell fate and stimulate ovary development (requires reinforcement by FOXL2)
232
what causes the masculinisation of the embryo?
the present of the SRY gene (only present of the Y chromosome) that causes gonadal cells to become Sertoli cells (then consequent masculinisation)
233
what aspect of the Y chromosome causes masculinisation and why?
the presence of the SRY gene
234
what is the function of the SRY gene?
causes the development of gonadal cells into Sertoli cells
which then cause testes development, Leydig cell formation and testosterone production
235
once Sertoli cells form due to SRY gene activation, what are the next steps?
they then cause testes development, Leydig cell formation and testosterone production
236
what causes the feminisation of the embryo?
the absence of the SRY gene (as no Y chromosome is present)
237
what is the result of the SRY gene being absent?
feminisation of the embryo
instead of forming Sertoli cells and causing testes development, the gonadal cells adopt the granulosa cell fate and stimulate ovary development
238
what does the feminisation of the embryo require reinforcement from?
the transcription factor FOXL2
239
what is FOXL2?
a transcription factor that is essential for ovarian development and function
240
what are the gonadal ridges?
the the initial structures that form from the mesoderm, that go on to form the gonads
241
what do the gonads develop from and how?
develop from the mesoderm as bipotential (i.e. neither ovaries nor testes) structures known as gonadal/genital ridges
242
what are the bipotential structures, from the mesoderm, that give rise to the gonads?
gonadal/genital ridges
243
what structure is the main organiser of post-gastrulation embryo development?
notochord
