Reproductive Embryology Flashcards
describe fertilisation: generalised acrosomal process
- sperm makes contact with egg
- acrosome reacts with zona pellucida
- acrosome reacts with perivitelline space
- plasma membranes of sperm and egg fuse
- sperm nucleus enters egg
- cortical granules fuse with egg plasma membrane, which renders the vitelline layer impenetrable to sperm
what happens during the first 5 days of embryo development?
- day 1: formation of pronuclei
- day 2: first cell division
- day 3: cell division continues
- day 4: formation of morula
- day 5: blastocyst forms
how does hatching of the blastocyst occur?
trophoectoderm cells produce protease to dissolve the zona in preparation for implantation
- 5th day after fertilisation
what does the apposition step of implantation refer to?
first connection between blastocyst and endometrium, apposes to microvilli like structure called pinopodes expressed on receptive endometrium
what does the adhesion step of implantion refer to?
trophoblast of the blastocyst adheres to the epithelial layer of maternal endometrium, embryonic tissue starts to actively secrete hCG.
what does the invasion step of implantation refer to?
trophoblast proliferation, differentiation, crossing of the epithelial basement membrane and invasion of endometrial stroma to form the placenta. Uterine spiral arteries remodelled by the invasive trophoblast.
describe decidualisation that occurs prior to invasion of the blastocyst
Pre-requisite for trophoblast invasion and placentation involves transformation of stromal cells of maternal endometrium every month (during menstrual cycle, before pregnancy) under the effect of hormone progesterone.
Changes:
- Stromal cell differentiation (elongated fibroblast like cells) converted into decidual( rounded epithelial cells).
- Angiogenesis - new vessel formation, increase vascular permeability
- Increased macrophages, lymphocytes and decidual leukocytes(uterine natural killer cells) for maternal immune tolerance
These changes transform the endometrium into a vascular receptive tissue for blastocyst invasion.
the mature placenta is formed by which components
The mature placenta is formed by several components:
- chorionic villi
- intervillous space
- placental septae
- cotyledons (subunits of placenta)
what are the functions of the placenta?
- Acts as an immunological barrier
- Gas exchange
- Nutrient exchange
- Waste excretion
- Endocrine functions
define embryogenesis
the first 8 weeks of embryo development post fertilisation
- beyond the end of week 8, the embryonic period ends and the foetal period begins.
Fertilised egg forms blastocyst on day 5 with two organised cell groups- what are these groups called?
inner cell mass (embryoblast) and trophoblast (outer cells)
what happens during weeks 2-3 of embryoblast development?
- formation of bilaminar disc and development of amniotic sac, yolk sac, extraembryonic mesoderm and chorionic cavity.
what happens during weeks 3-4 of embryoblast development?
differentiation of bilaminar disc into trilaminar structure - three germ layers (endoderm, mesoderm and ectoderm) through process of gastrulation and neural tube formation.
what happens during weeks 5-8 of embryoblast development?
organogenesis
descibe the differentiation of the inner cell mass of a blastocyst
The inner cell mass differentiates into bilaminar epiblast (columnar epithelial cells) and the hypoblast (cuboidal epithelial cells). Hypoblast is the layer facing the blastocoele, while the epiblast is on the other side
Two cavities form, yolk sac on the side of the hypoblast and amniotic cavity on the side of epiblast
describe the process of epiblast differentiation/gastrulation
- Epiblast differentiates into the three germ layers- ectoderm , mesoderm and endoderm in a process called gastrulation (D13 to 16 of embryo development)
- Gastrulation is largely controlled and orchestrated by the primitive streak. The primitive streak is a groove in the midline of the epiblast which appears during the third week. Within the primitive streak lies a primitive node at the cranial end, and within the primitive node lies the primitive pit.
- Cells of the epiblast layer break off and migrate toward the primitive pit. Here, they detach and penetrate through the epiblast layer to form three new germ cell layers:
Endoderm – formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells.
Mesoderm – formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm.
Ectoderm – formed by the epiblast cells that remain in position.
describe the devlopment of the indifferent gonad
- The indifferent gonad develops in the gonadal ridge which is closely related to the mesonephros.
- Primordial germ cells do not originate in the gonadal ridge but migrate to the gonads from endoderm lining of yolk sac via hindgut around 6 weeks of gestation
- Epithelium of gonadal ridge forms primitive sex cords
- Combination of germ cells and primitive sex cord forms the indifferent gonad which is capable of developing into a testis or ovary
detail the development of the testis beginning with migrating germ cells
The migrating germ cells enter the testis
The primitive sex cords extend into medulla and form testis cords (medullary cords) which transform into future seminiferous tubules and rete testis
Sertoli cell are derived from surface epithelium under effect of AMH
Leydig cells are produced from intermediate mesoderm and produce testosterone from 8 week onwards
