Embryology Flashcards
What are the three prenatal development stages?
- Pre-embryonic period (0 to 2 weeks)
- Embryonic period (3 to 8 weeks)
- Fetal period (9 weeks to birth)
Which is the most sensitive and critical prenatal development stage?
Embryonic period: all major organs are formed, higher chance of miscarriage
What is gametogenesis ?
The formations of gametes (sperms and ova)
Two haploid cells form a diploid one containing both maternal and paternal genetic material.
What happens during gametogenesis?
Replication of DNA in chromosomes (mitosis)
The duplicated chromosomes are called sister chromatids, they are connected at the centromere.
Crossing over can occur to exchange genetic material causing variation.
Then two different divisions of meiosis occur in order to produce the haploid cells.
What is spermatogenesis?
The formation of sperm cells
What happens during spermatogenesis?
There is a primary spermatocyte that comes from the spermatogonia in the seminal vesicles. This undergoes the first meiotic division to form secondary spermatocytes (23X and 23Y ). These then undergo the second meiotic division forming spermatids. Spermatids are immature sperm cells so they have to undergo spermiogenesis to develop into normal sperms.
How many sperm cells are produced at the end of spermatogenesis?
4 sperms
Where does spermiogenesis take place?
In the seminal vesicles
How many spermatozoa are released and how many actually reach the uterus?
About 300 million spermatozoa are released but only 3 million reach the uterus.
How many spermatozoa actually reach the uterine tube?
Only 1.4 million reach the uterine tube, which is where fertilization takes place
How long do spermatozoa survive?
Most survive 24-48 hours, however, some can last up to 4 days.
What is capacitation?
The removal of glycoprotein and seminal proteins from the surface of the acrosome.
What is the function of capacitation?
To increase mobility and metabolism
Where does capacitation occur?
Female reproductive tract
What are the stages of development of follicles?
Primordial follicle: 2 million at birth, about 400 thousand are left by puberty.
Primary follicle: 15 to 20 primordial follicles develop into primary follicles monthly.
Secondary follicle: primary follicles develop into secondary.
Graafian follicle: one secondary follicle develops into a Graafian follicle.
What is oogenesis?
The formation of egg cells
What happens during oogenesis?
Primary oocyte: 46 XX in growing follicle (surrounded by follicular cells)
Primary oocyte: 46 XX in larger follicle (surrounded by zone pellucida and follicular cells)
After the first meiotic division: a secondary oocyte is formed: 23 X in mature follicle (first polar body is visible and antrum)
Second meiotic division occurs if fertilisation occurs: Second polar body is visible, fertilised oocyte and corona radiata
What is ovulation?
The release of the ovum from the Graafian follicle, for it to enter the fallopian tube
When does the Graafian follicle rupture?
When the follicle is the same size as the ovary
What is the corona radiata?
Radially arranged follicular cells around the zone pellucida
Where does fertilization happen?
In the ampulla of the uterine tube
What are the three phases of fertilization?
- Penetration of corona radiata: about 1.4 million sperms reach the ampulla of the uterine tube, only capacitated sperms pass through the corona radiata freely, and only one sperm fertilizes the oocyte
- Penetration of the zone pellucida
- Fusion of the oocyte and sperm cell membranes to form zygote
What is the acrosomal reaction and why does it happen?
- Membrane of acrosome breaks.
- Release of enzymes within the acrosome facilitate penetration of zona pellucida.
- Establishment of a pathway for sperm through zona pellucida
What are the enzymes released from the acrosome during acrosomal reaction?
Across, hyaluronidase, esterase
What is the cortical reaction and why does it happen?
A change in properties of the oocyte membrane and zona pellucida the make it impermeable to other sperms.
- Release of lysosomal enzymes from cortical granules.
It is necessary to prevent polyspermy
What happens to the zygote in the case of polyspermy?
The zygote will not survive
What is the process of formation of a zygote?
- The pronuclei approach each other, and the membranes break.
- The pairing of tech paternally and maternally derived chromosomes restore the diploid number of 46.
- The zygote us unique as it contains a new combination of chromosomes
- The zygote starts dividing by mitosis.
What is cleavage?
The mitotic divisions of the zygote, which result in a rapid increase in the number of cells.
What are the embryonic cells known as after the cleavage?
Blastomeres
What happens to the size of the cells as their number increases (zygote)?
They become smaller in size
What is a morula?
The spherical cluster of blastomeres
What is compaction?
It begins at the 8-cell stage (3 mitotic divisions later)
Blastomeres form a tightly packed ball of cells
Outer cells maximize their contact with each other and seal the interior of the mould
What cells are formed after compaction?
Trophoblasts and embryoblasts
What are trophoblasts?
They are the cells responsible for implantation and placental formation
They make no contribution to the embryo