Lecture 17- Development II Flashcards
What happens to the cell cycle after cleavage?
The cell cycle resumes normal functions. Cell division slows.
Morphanogenesis
Cellular & tissue-based process by which animal body takes shape. Occurs during gastrulation and organogenesis.
Gastrulation
Hollow blastula becomes gastrula (layered embryo)
3 germ layers:
Ectoderm
Endoderm
Mesoderm
Gastrulation in Sea Urchins
Involves cell migration, invagination- infolding
Forms archenteron- early gut tube
Blastopore- open end of archenteron- becomes anus
2nd opening becomes mouth
Cells at vegetal pole (bottom) migrate into blastocoel
Vegetal plate forms-> cells flatten, fold inwards
Archenteron forms-> cells at tip extend toward opposite wall, contact opposite wall
Archenteron fuses with blastocoel wall-> digestive tube formed. present: 3 germ layers
mouth
anus
cilia
Gastrulation in Birds
Doesn’t occur like we see in echinoderms because there is too much yolk. Instead we have an active cap where gastrulation takes place. Meroblastic cleavage.
2 layers in active cap: Hypoblast and epiblast
(Epiblast is on outer part, furthest from egg, hypoblast is closest to egg)
Epiblast cells migrate inward. They either go up and push the hypoblast (becomes endoderm), move laterally and go to the sides (becomes mesoderm), or stay at the bottom of the epiblast (becomes ectoderm).
Primitive Streak
Blastopore, directs movements associated with gastrulation
Involution
Cell Migration
Hypoblast Cells
Contribute to sac around yolk
Connection between yolk and embryo
Do not contribute to embryo itself
Gastrulation in Humans
End of cleavage-> blastocyst has formed
Mammalian version of blastula
Arrive at uterus ~6 days after fertilization
Blastocyst
100 cells around central cavity
Trophoblast: Outer single layer of cells
Inner cell mass: Cluster of cells. The inner cell mass becomes the embryo, and is the source of embryonic cell lines.
Implantation
Trophoblast contacts uterine lining- secretes enzymes
-> Erode area of endometrium
-> Embryo penetrates
-> Thickens, extends projections into endometrium
Gastrulation
Takes 1 week- 2nd-3rd week of pregnancy
Inner cell mass forms flat disc- 2 layers (Epiblast and Hypoblast)
Cell Fates
Some epiblast stays in place
-> Ectoderm
Some epiblast migrates in between
-> Mesoderm
Some epiblast migrates & mixes with hypoblast
-> Endoderm
Human Gastrula
3 layers
- Ectoderm
- Endoderm
- Mesoderm
Also contains extraembryonic membranes: Chorion, amnion, yolk sac, allantois
Functions:
- Contain fluid
- Form blood vessels (Gas exchange, food/waste)
Organogenesis (basic)
Process of organ formation
Embryo begins to increase in size
Nervous system 1st to develop in most vertebrates
Neurulation
Process of forming nervous system
Cells of dorsal mesoderm (notochord) induce dorsal ectoderm to flatten and thicken.
Dorsal ectoderm becomes neural plate, then folds inward to become neural tube
Becomes central nervous system. Anterior portion is brain, and remainder is spinal cord.
Folic acid deficiency can cause problems with development.
Cells of neural crest
Form nerves, teeth, and skull bones
Somites
Blocks of mesoderm cells lateral to notochord
- Segmented/serially repeated structures
- Ribs, vertebrae, associated muscles
Early Human Development
Most radical changes for mother and embryo
Implanted embryo secretes hCG (Human chorionic gonadotropin)-> Acts like LH
Maintains progesterone and estrogen from corpus luteum during 1st trimester
Placenta
1st 2-4 weeks, nutrients directly from endometrium. Trophoblast mixes with endometrium -> placenta.
Placenta
Organ of exchange between mother and embryo
Provides nutrients, O2
Removes waste, excreted by mother
Umbilical cord connects embryo to placenta
Twins
Identical (Monozygotic)- Shared placenta
Fraternal (Dizygotic)- Separate placentas
Inner cell mass can split during 1st month-> 2 groups of cells-> identical
Birth
Begins with labor
Uterine contractions- push fetus, placenta out of body
Regulated by prostaglandins, hormones (estradiol, oxytocin)
Positive or negative feedback during labor?
Positive
What is the function of human chorionic gonadotropin (hCG)?
Promotes the maintenance of the corpus luteum which allows the corpus luteum to secrete progesterone.
Which type of cleavage pattern do sea urchins undergo?
Holoblastic
The ____ is the bag of waters in which a human fetus develops.
Amniotic sac
_____ is the process whereby an embryo develops neural folds.
Neurulation
In which region of the body does cleavage occur in humans?
Oviduct
What is the early blastocyst composed of?
Inner cell mass / Trophoblast / Blastocoel
When is the first time an embryo physically grows following gastrulation?
Neurulation
The _____ is a layer of cells forming the outer ring of the blastocyst that combines with the maternal endometrium to form the placenta.
Trophoblast
During neurulation, the mesoderm gives rise to the _____.
Notochord
The _____ secretes proteolytic enzymes to break down the endometrial extracellular matrix to allow for implantation of the blastocyst in the uterine wall.
Trophoblast
The notochord induces the formation of the _____ .
Neural plate
The neural plate undergoes a series of folds and becomes the _____.
Neural tube
_____ is the process whereby the trophoblast thickens and extends fingerlike projections into the endometrium.
Implantation
What does the neural tube develop into?
Anterior → brain / Posterior → spinal cord
_____ are blocks of cells that lie in the mesoderm, bilateral to the neural tube (notochord) and contribute to repeating, segmented structures.
Somites
Which germ layer does the neural crest develop from?
Ectoderm
What is the main function of the placenta?
Gas exchange (both oxygen and waste)
The placenta and the developing fetus are connected by the _____.
Umbilical cord
_____ is a hormone for the maternal recognition of pregnancy produced by trophoblast cells that are surrounding a growing embryo, which eventually forms the placenta after implantation.
Human chorionic gonadotropin (hCG)
