The first week of Human development Flashcards
what is gametogenesis
formation of gametes
what are homologous chromosomes
a pair of similar autosomes
define ploidy
number of chromosomes
how does gametogenesis begin
meiosis
what does meiosis result in
formation of gametes (sex cells)
what occurs during meiosis I
homologous chromosomes are separated after synapsis and crossing over occurs
what occurs in Meiosis II
sister chromatids are separated in a sequence of phases that resembles mitosis
define chromatid
one of the 2 strands of a chromosome joined together by a centromere
define chromosomal crossover
important step in genetic recombination in which 2 chromosomes, normally homologous, pair up and exchange DNA. Shuffling of genes (called independent assortment), is heritable and increases the variation
what occurs during prophase I
Homologous double-stranded chromosomes pair up (synapsis), and the pair forms a tetrad. Crossing over occurs between maternal chromosomes and paternal chromosomes ensuring genetic diversity
what occurs during metaphase I
homologous double-stranded chromosomes line up above and below the equator of the cell, forming a double line of chromosomes. Spindle fibers attach to the chromosomes.
What occurs during anaphase I
maternal and paternal pairs of chromosomes are separated and pulled the the opposite ends of the cell (reduction division). The sister chromatids remain attached in each -double stranded chromosome
what occurs during Telophase I and cytokinesis
Nuclear division finishes and the nuclear envelopes re-form. The cytoplasm divides and 2 new cells are produced, each containing 23 chromosomes only. The chromosomes are still double-stranded
what occurs during Prophase II
nuclear envelope breaks down, and the chromosomes gather together
what occurs during metaphase II
double-stranded chromosomes line up along the equator of the cell, spindle fivers extend from the centrioles to the chromosomes
what occurs during anaphase II
Sister chromatids of each double-stranded chromosome are pulled apart at the centromere. Sister chromatids migrate to opposite ends of the cell
what occurs during Telophase II and cytokinesis
Nuclear division finishes, and the nuclear envelopes re-form. The four new daughter cells that are produced each contain 23 single-stranded chromosomes only
what are spermatogonia
parent or stem cells that produce sperm
define spermatids
haploid cells produced from primary spermatocytes that have undergone meiosis
what starts the process of meiosis
oogonia
what occurs after the primary occyte completes the first meiotic division
2 cells are produced
define polar body
nonfunctional cell that eventual degenerates that is the second cell (besides the 2nd oocyte) that results from Meiosis
define nondisjunction
process of separation of homologous chromosome pairs fails
Define Teratogens
external agents that produce developmental malformations
where does fertilization occur
widest part of the uterine tube
define cleavage
division of zygote and formation of blastocyst
define organogenesis
germ layers differentiate so as to give rise to all organs in the human body
define gamete
germ cell that originate in the wall of the yolk sac of the embryo and migrate into the gonad region
when does ovulation occur
When the secondary oocyte enters meiosis and the chromosomes align at metaphase
what secretes gonadotropin-releasing factor (GnRF)
hypothalamus
what hormone stimulates ovulation
LH
what hormone stimulates the endometrium of the uterus to enter the secretory phase
progesterone
Define endometriosis
Appearance of foci of endometrial tissue in abnormal locations outside of the uterus
what must degenerate in order for implantation to occur
zona pellucida
what does the syncytiotrophoblast produce?
human chorionic gonadotropin (hCG)
what does hCG do
it is a glycoprotein produced by syncytiotrophoblast that stimulates the production of progesterone by the corpus luteum of the ovary
what are the stages of embryogenesis and define them
- cleavage: series of mitotic divisions of the zygote
- gastrulation: process that establishes 3 germ layers
- organogenesis: germ layers differentiate so as to give rise to all organs in the human body
what does Sonic Hedgehog Protein (Shh) do?
It helps to determine the L/R determination (activity is only on the left because Shh activity is suppressed on the future right side by activin)
what is the result when nondisjunction occurs during meiosis I
gametes divide into 2 that have n+1 and 2 that have n-1
what is the result when nondisjunction occurs during meiosis II
2 normal offspring, one with n+1 and one with n-1
list the major events in fertilization
- Penetration of corona radiate
- attachment to zone pellucida
- acrosomal reaction and penetration of ZP
- binding and fusion of sperm and eggs
- prevention of polysperm
- metabolic activation of the egg
- decondensation of the sperm nucleus
- completion of egg meiosis
- development and fusion of male and female pronuclei (now have embryo)
What are the events of implantation
- zone pellucid must degenerate for implantation to occur
- blastocyst implants within the posterior superior wall of the uterus (and trophoblast differentiation)
- blastocyst implants within the functional layer of the endometrium during secretory phase of the menstrual cycle
- trophoblast differentiates into cytotrophoblast and syncytiotrophoblast
Explain ectopic pregnancy
Occurs when the blastocyst implants within the uterine tube due to delayed transport (ampulla of uterine tube is most common)
clinical signs: abnormal uterine bleeding, unilateral pelvic pain, increased levels of hCG and massive first trimester bleed
what is the main difference between dizygotic twins and monozygotic?
dizygotic results from fertilization of 2 different secondary oocytes by 2 different sperm. monozygotic results from fertilization of one secondary oocyte by one sperm
what are the steps of oogenesis
- primordial germ cells arrive in the ovary at week 6 & differentiate into oogonia
- oogonia enter meiosis I and undergo DNA replication to form primary oocytes
- ovarian cycle: primary oocyte completes meiosis I and secondary oocyte and first polar body form
- secondary oocyte enters meiosis II
- at fertilization= secondary oocyte completes meiosis Ii to form a mature oocyte and 2nd polar body
Explain the proliferation and differentiation of the trophoblast
trophoblast becomes the outer cell mass
- trophoblast separates into a cellular cytotrophoblast (closer to inner cell mass) and a multinucleate syncytiotrophoblast (closest to endometrium)
- syncytiotrophoblast projections penetrate the endometrial basal lamina and then begin to invade the endometrial sertoma
- trophoblastic lacunae appear
- trophoblast is less invasive at this point
what is the mother’s contribution to the baby
endometrium
