Embryology: The First Week of Human Development Lecture 2 Syed Flashcards
How many chromosomes to human somatic cells contain?
They contain 23 pairs of chromosomes for a total of 46
What is a chromatid?
One of the two strands of a chromosome joined together by a centromere
During which phase does crossing over occur?
They only cross over during meiosis I
What do you call the parent cells that produce sperm?
Spermatogonia - diploid cells
What do spermatogonium cells do at the very beginning of the cycle?
Each one divers by mitosis to an exact copy of itself called a primary spermatocyte.
Understand overall process of gametogenesis
Sir, yes sir (add pic)
What do primary spermatocytes ultimately produce?
They undergo meiosis and ultimately make haploid cells called spermatids, which have 23 chromosomes
How are initial spermatids different than sperm cells?
The spermatids must undergo further changes to form a sperm cell - they lose their cytoplasm and grow a flagellum
How many sperm are formed per spermatocyte?
Four new sperm are formed - each have 22 autosomes and either an X or Y chromosome
R-selection vs. K-selection
R-selection makes a species prone to numerous reproduction at low cost per individual offspring, while K-selected species expend high cost in reproduction for a low number of more difficult-to-produce offspring
Explain the process of oogenesis up till puberty
The parent cells (oogonia) are diploid. They start the process of meiosis and form primary oocytes prior to birth. They are arrested in Prophase I (month 5 in utero) and stay here until puberty
What happens in oogenesis after puberty?
Only one secondary oocyte usually develops. It is expelled (ovulated) from one of the two ovaries each month (they may not switch from side to side though)
What happens to the secondary oocyte after Meisosis I?
The secondary oocyte receives the bulk of the cytoplasm and is the cell that is arrested in metaphase II. The second cell becomes a polar body because it doesn’t get much cytoplasm
What happens if the secondary oocyte is not fertilized?
The secondary oocyte is degenerated, still arrested in metaphase II
What happens if the secondary oocyte is fertilized?
The secondary oocyte finishes meiosis. Two new cells are produced, with unequal cytoplasm. The lesser one becomes another polar body and degrease. The other one becomes an ovum which can support a fertilized zygote.
What is aneuploidy?
When there are numerical errors in # of chromosomes and there not exactly 46 chromosomes
What is nondisjunction?
When the process of separation of homologous chromosome pairs fails. Can happen in either meiosis I or II. It’s worse if it happens in the firs division because then non of them will have there irate number of chromosomes. (pic)
What are some common Trisomies/monosomies?
- Down syndrome, trisomy 21
- Klinefelter syndrome, 47 XXY = There’s a Y, so you’re a male, but because of the two X’s, it affects physical appearance
- Turner’s Syndrome, 45 X0, Have morphological issues. Can have more cardiovascular issues
Why do problems arise in offspring of older women?
- Prolonged dormancy of oocytes, put to 40 yrs
- Incidence of trisomy 21 increases with maternal age. Main cause is maternal meiotic nondisjunction
- Men don’t have nearly as many problems
What are teratogens and what are some examples?
They’re external agents that produce developmental malformations. Literally anything that can cause a birth defect.
Ex: radiation, infections, temp, drugs, toxins, malnutrition, mechanical forces
What are the four reasons for the variations in teratogen responses?
- Concentration and method of teratogen delivery
- Timing of exposure during development (When were you exposed? Week 8, 20?
- Variation in susceptibility due to genetic variation (Can you metabolize alcohol better than others?)
- Synergistic interactions between the teratogen and other compounds (It could be a minor problem, but can lead to other bad things when combined with other factors)
What are the two protective layers that form around the secondary oocyte?
The corona radiata and the zona pellucida form protective layers around the secondary oocyte
Where in the uterine tube does fertilization occur?
Occurs in the widest part of the uterine tube - the ampulla
How does the egg stop multiple sperm from entering?
Once one sperm enters, the egg changes the resting potential of the membrane, which zaps the rest of the sperm. Then the receptors are destroyed to stop anything else form coming in.
What is the pre-embryonic period?
It is the first 2 weeks of development when the zygote becomes a spherical, multicellular structure
What is the embryonic period?
It is the third through the eighth week of development during which all major organs systems appear.
What is the critical period of development?
The first 3 to 8 weeks are the most important. All the organs are developing, etc??
What are the three major events in embryogenesis?
Cleavage: Division of zygote and formation of blastocyst
Gastrulation: Formation of germ layers. Everything in the body comes from some form of this
Organogenesis: Germ layers differentiate so as to give rise to all organs in the body
What happens to the cells during cleavage?
Divisions increase the number of cells, but it remains the same size. Cells get smaller and smaller
What is a morula?
It is the 8-cell stage where the cells become tightly compacted into a ball
Inner cell mass vs. Outer cell mass of blastocyst
The inner cell mass is called the embryoblast. It is the part that actually becomes the embryo. The outer cell mass is called the trophoblast cells. These are the ones that help with the placenta
- When this forms, the zona pellucida also begins to dissipate
What are the three types of Twinning and what do they consist of?
A. Cleavage of early embryo - They can start as one embryonic egg, but can split into two separate ones
B. Splitting of ICM (inner cell mass) - This happens later in cycle. Can be problematic if one twin becomes dominant and gets all the resources
C. Incomplete division of ICM - When it tries to split but doesn’t happen all the way. Can lead to conjoined twins.
What is implantation?
It is the process by which the blastocyst brows into and embeds within the endometrium. Two cavities are present (trophoblast cells and embryoblast cells)
What is an ectopic pregnancy?
It is when the implantation of the blastocyst occurs outside the uterine cavity.
What two cavities form from each, the embryo and trophoblast?
From the embryo, the epiblast and hypoblast are formed
From the trophoblast, the syncytiotrophoblast and cytotrophoblast are formed.
What is hCG?
Human chorionic gonadotrophin. It signals the ovary to continue to secrete hormones to maintain the pregnancy. Ex: Telling it to continue sending estrogen
