Early Embryo Flashcards
endocrine
diffusible molecules that act globally
paracrine
diffusible molecules that act over short distances
- action is often determined by diffusion gradient
- target cells may be under the influence of multiple inducers
- ex: hormones, neurotransmitters, chemokine, cytokines
Juxtacrine
non-diffusible molecules
- direct interaction of cell surface proteins or extracellular matrix components with receptors
- gap junctions- direct connection between the cytoplasm of adjacent cells
autocrine
cells act on themselves. This is a type of paracrine signal
gametogenesis
production of sperm and ova
diploid 2 N stem cells -> (meiosis) -> Haploid 1 N cells -> (maturation) -> viable gametes
Primordial germ cells (PGCs)
- reside in the wall of the yolk sac
- form the gametes
- migrate around the developing GI tract and onto the posterior body wall to the genital ridge (form gonads)
- migration is guided by chemokine released by the genital ridge
Spermatogenesis
- Primordial germ cells (PGCs) lie dormant in the seminiferous tubules of the testis until puberty
- elevated testosterone levels have an effect on Sertoli cells that influence them to become spermatogonia, the stem cell population for sperm
Spermatogenic sequence
Primordial germ cells (PGCs) -> spermatogonia (mitotic; diploid 2N) -> primary spermatocytes (diploid 4N; crossover time) -> meiosis 1 -> secondary spermatocytes (haploid 2N) -> Meiosis II -> spermatids (haploid 1N = 22 + X or Y)
Relation of Sertoli cells to developing sperm
- creates isolated microenvironment formed by tight and gap junctions (blood-testis barrier)
- produce growth factors and other control substances
- absorb excess cytoplasm
- released sperm are still immotile and unable to fertilize an ovum
sperm cell
Head: contains nucleus; capped by the acrosome (hydrolytic enzymes: across, hyaluronidase)
mid piece: mitochondria; anchors tail
tail: big flagellum
Spermiogenesis
refers to sperm maturation
- sperm is stored in the epididymis
- sperm moves through the vas deferens to the urethra where seminal secretions are added- produced by the seminal vesicles, prostate, and bulbourethral glands
- sperm must undergo capacitation in the female tract before it is able to fertilize the ovum.
- capacitation: changes in the acrosome that prime it for release of its enzymes
Oogenesis
- Begins in the 5th month of gestation
- Oogonia remain arrested at prophase in primordial/primary follicle until the menstrual cycle starts
- dependent on menstrual cycle so the process stops at menopause
Oogenic sequence
Primordial germ cells (PGCs) -> mitosis -> oogonia (diploid 2N) -> initiation of meiosis -> primary oocyte (diploid 4N) -> progression of meiosis 1 -> secondary oocyte (haploid 2N) -> Meiosis II -> definitive oocyte (haploid 1N = 22 + X)
process produces 1 definitive oocyte and 3 polar bodies
folliculogenesis
- Puberty increases FSH
- primordial follicle -> primary follicle (cuboidal and surrounded by zone pellucida, a glycoprotein coat that acts as a sperm barrier
- follicle becomes multi-layered then forms a cavity (astral follicle) -> mature follicle surrounded by thecae cells (steroidogenic)
- primary oocyte -> secondary oocyte and first polar body
- second meiotic arrest occurs before ovulation (metaphase II)
ovulation
involves the proteolytic breakdown of the follicular wall and thecae smooth muscle contraction
-meiosis II does not start until ovum is fertilized
Pituitary hormones
FSH: peaks at ovulation and promotes follicle and oocyte maturation; steroids produced by the follicle control the Fallopian tube and uterus to produce an hospitable environment
LH: stimulates ovulation and converts follicle into corpus luteum (CL) which produces progesterone for a short period
ovarian hormones
Estrogen peaks before ovulation and increases again during the corpus luteum phase
Progesterone peaks at the corpus luteum
both estrogen and progesterone tapers off if fertilization does not occur
coverings of the ovum
- Cumulus remains in contact with the oocyte via processes and it regulated meiosis. There is also a hyaluronic acid-rich ECM associated with it that is a barrier to sperm
- Zona Pellucida is a more significant barrier to the sperm; may facilitate transportation of the embryo down the Fallopian tube
post-ovulatory events
oocyte is picked up by the Fallopian tube; transported to the ampulla where it meets capacitated sperm. sperm are attracted to the ampulla through chemotropic factors
- follicle is converted to a corpus luteum- source of progesterone needed to maintain pregnancy
fertilization (time zero)
capacitated sperm must penetrate the cumulus oophorus and bind to receptors on the zone pellucida (ZP3 protein), thereby releasing acrosomal enzymes
- cortical reaction: release of cortical granules into the perivotelline space; enzymes make the ZP3 protein non receptive to other sperm
- completion of meiosis in the ovum -> female pronucleus (1N haploid genome)
pronuclei
- the sperm nucleus enters the ovum, forming the male pronucleus
- male and female pronuclei fuse to form a diploid 2N nucleus = zygote
preimplantation development
zygote -> morula -> blastocyst
embryo is still encased in ZP until implantation
initial differentiation of the embryo
- blastomeres are identical and totipotent up until the 8 cell stage blastomeres
- compaction occurs at 8-cell stage -> gap junctions dorm between cells and the blastomeres become polarized so that they have an outer surface and a n inner surface
outer cells -> trophoblast -> placenta
inner cells -> inner cell mass (ICM) -> embryo
formation of the blastocyst
blastocyst cavity forms from the result of tight junctions forming between outer cells
fluid (H2O) and ion (Na+) transport leads to cavity formation
implantation
hatching: combination of embryonic and uterine protease activity breaks down the ZP
attachment: occurs at the ICM end of the blastocyst
invasion: occurs by proteolytic breakdown of the endometrium
in vitro fertilization (IVF)
Indications: Fallopian tube blockage; some male factor infertility
ova and sperm collected then combined in vitro where fertilization takes place
embryo grown in vitro to 4-16 cell stage, then transferred to uterus
Zygote intrafallopian transfer (ZIFT)
similar to IVF except time of transfer is just after fertilization and transfer is to the ampulla of the Fallopian tube
Gamete intrafallopian transfer (GIFT)
indications: low sperm count or poor sperm quality; unexplained female infertility
sperm and eggs mixed and placed in ampulla
sperm collection
if the vas deferent is obstructed or absent sperm can be obtained from the testis or epididymis
- microepididymal sperm extraction (MESA): microsurgical procedure performed under general anesthesia; results in enough sperm for IVF
- Percutaneous epididymal sperm aspiration (PESA): a needle is guided into the epididymis to collect just a few sperm. few and relatively immature sperm are obtained
- Testicular sperm extraction (TESE): immobile sperm are obtained; they are not as viable as epididymal sperm
- intracytoplasmic sperm injection (ICSI): when PESA or TESE are used to collect few immature sperm, IVF will not work. Sperm must be directly inserted into the ovum. A single sperm is injected directly into the egg cytoplasm. ICSI results in higher a incidence of chromosomal anomalies than natural fertilization
sex selection
- based on X sperm having more DNA than Y sperm
- stain DNA and sort by flow cytometry- individual sperm are directed to a collection container based on the amount of DNA detected
Analysis during IVF
- remove one or more cells from an 8-cell embryo
- chromosomal analysis: looking for abnormal numbers of chromosomes or gross abnormalities
- polymerase chain reaction: amplification of small amounts of DNA for specific gene analysis
Bilaminar disc
ICM becomes two layers:
epiblast + hypoblast = bilaminar disc
basement membrane develops between these two layers
development of the trophoblast
Original trophoblast has two pathways:
1) cytotrophoblast: dividing stem cell population
2) syncytiotrophoblast: many nuclei in an undivided cytoplasm; formed by the fusion of cytotrophoblast cells. it can not undergo further mitosis; very invasive tissue going into endometrium; source of hCG
Invasion of the conceptus -Day 8
trophoblast: syncytium grows into the endometrium and cytotrophoblast continues to divide
ICM: formation of the amnion and amniotic cavity begins within the epiblast. this cavity will eventually become a fluid-filled space surrounding the embryo
first step in formation of the yolk sac (day 9)
hypoblast: migrates out into former blastocoel as the exocoelomic membrane
epiblast (day 9)
expansion of the amnionic cavity; the roof of this cavity is called the amnion and is composed of amnioblasts
