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Lecture 6: Animal Reproduction Lecture 7: Animal Development
the creation of offspring without fusion of egg and sperm
the creation of offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote
4 types of asexual reproduction
: separation of a parent into 2 or more individuals of about the same size
: breaking the body into pieces, which develop into adults
: new individuals arise from outgrowths of existing ones
: the development of a new individual from an unfertilized egg
Cost of sexual reproduction
Advantages of sexual reproduction (4)
diversity is advantageous in catastrophic events
increases in rate of adaptation
higher reproductive success, offspring live longer
shuffling of genes and elimination of harmful genes
each individual has male & female reproductive systems
What does meiosis do?
reduces the chromosome number in half
External vs. Internal Fertilization
: eggs shed by the female are fertilized by sperm in the external environment
: sperm are deposited in or near the female reproduction tract, fertilization occurs within the tract
the release of mature eggs at the midpoint of female cycle
seconds after sperm binds to egg, vesicles beneath the plasma membrane release their contents and form a fertilization envelope
How is the cortical reaction initiated?
by fusion of egg and sperm
What is the role of the fertilization envelope?
To prevent polyspermy (entry of multiple sperm nuclei into the egg)
What is a calcium wave? When does it occur?
Ca2+ spreads across the egg
correlates with the appearance of the fertilization envelope
when secretions in the mammalian female reproductive tract alter sperm motility and structure
Spermatogenesis vs. Oogenesis (3 differences)
all 4 products of meiosis develop into sperm while only one of the 4 becomes an egg
spermatogenesis occurs throughout adolescence and adulthood
sperm are produce continuously unlike the prolonged interruption in oogenesis
Ovarian Cycle vs. Menstrual Cycle
changes in the ovaries vs. changes in the uterus
Events in ovarian cycle
sequential release of FSH and LH stimulates follicle growth
follicular phase ends at ovulation, and secondary oocyte is released
after ovulation, the corpus luteum secretes progesterone and estradiol
Events in the menstrual cycle
Estradiol and progesterone promote endometrium to thick with blood vessels in preparation for embryo implantation
If an embryo is not implants, the endometrium is shed (menstruation)
Steps in Female Reproductive Cycle
What conditions are necessary for formation of sperm?
temperature must be lower than body temp.
a partially developed egg
consists of an oocyte surrounded by support cells
process by which an oocyte develops into an ovum
What phase is the oocyte stopped at before birth of the female?
primary oocyte paused at prophase of meiosis 1
What happens to the oocyte when a female starts puberty?
In vitro fertilization
eggs are mixed with sperm in culture dishes and the embryo is returned to the uterus at the eight-cell stage
What are the two sex hormones? functions?
testosterone & progesterone
development of secondary sex characteristics
Why does cortical reaction occur?
to prevent polyspermy
Why are testes located outside of the body?
Because spermatogenesis cannot occur at body temperature
Where does spermatogenesis take place?
in the seminiferous tubule
When does spermatocyte go from 2n to n?
when it becomes a secondary spermatocyte
What happens during meiosis II of spermatogenesis?
4 early spermatids are formed
If thickening of endometrium is blocked, what else is blocked?
implantation of the fertilized embryo
What do progesterone and estradiol do?
promote thickening of the endometrium
When does ovulation occur in the reproductive cycle?
when LH is at its peak
After ovulation occurs, what happens to the follicle tissue?
the leftover tissue forms the corpus luteum
When progesterone and estradiol levels increase, what happens to the levels of FSH & LH? why?
FSH & LH decreases
this prevents another egg from maturing when a pregnancy may be under way
growing follicle starts to secrete more and more estradiol
left over corpus luteum starts to secrete progesterone and estradiol
estradiol signals follicle to grow, uterine tissue starts to thicken and rebuild itself
maximal thickness of endometrium is achieved here
uterine lining disintegrates, release blood and endometrial tissue
Maternal side of placenta comes from ________ while the fetal side of placenta comes from _______
Where does fertilization occur?
In the fallopian tubes
In the blastocyst, the inner cell mass forms the _____ while the trophoblast forms the ______
Low levels of estradiol inhibit what?
they inhibit secretion of LH and FSH, keeping them at low levels too
What do LH and FSH stand for?
Follicle Stimulating Hormone
What does the hypothalamus secrete in order to signal the anterior pituitary to secrete FSH/LH ?
GnRH (Gonadotropin-Releasing Hormone)
What inhibits the anterior pituitary from releasing FSH/LH?
low levels of estradiol
What gland secretes FSH and LH?
What stimulates/inhibits the hypothalamus from releasing GnRH?
stimulated by high levels of estradiol
inhibited by combination of estradiol/progesterone
In the sperm there lives a little human and after fertilization the little human grows and grows
Morphogenesis involves (2 processes)
What is morphogenesis?
the process by which cells occupy their appropriate locations
What is gastrulation?
cells of a blastula are rearranged into a three-layered embryo
What is organogenesis?
the formation of organs
What are the 3 embryonic germ layers & what do they form?
Ectoderm- outer layer
Endoderm- lines the digestive tract
mesoderm- partly fills space in between endoderm and ectoderm
human equivalent of the blastula
Inner Cell Mass
a cluster of cells at one end of the blastocyst
outer epithelial layer of the blastocyst
Function of the trophoblast? What does it generate?
generates the placenta
the formation of primitive CNS structures
process by which a cell or group of cells becomes committed to a particular fate
refers to the resulting specialization in structure and function
What are fate maps?
diagrams showing organs and other structures that arise from each region of an embryo
How can we create a fate map?
can take early stage embryo and label the different cell, map what they will become
by imaging - using a green marker to label cells
What happens if you ablate a cell before gastrulation?
will not affect it
What are germ cells?
Specialized cells that give rise to sperm or eggs
Conclusion about P. Granules?
they act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development
What happens when blastocyst reaches the uterus?
blastocyst implants itself into endometrium
inner cell mass goes through gastrulation, produces three-layered embryo
neural plate curves inward and forms neural tube
can develop into all the possible cell types (fertilized egg)
can develop into some cells (committed germ cell)
The two ways a cell decides to become something else?
: mother cell gives each daughter different materials, intracellular method (extreme case in c. elegant)
: a group of cells tells other cells what to do
How can cells influence other cells?
signaling molecules from the organizer induce the host cell to adopt different fates
What kind of signaling is involved when cell sends signals to other cells to do something?
What does ZPA do? How many in a regular cell?
only 1 in a regular cell
produces an inductive signal that conveys positional info indicating “posterior”
Is the neural tube formed only by grafted cells?
No, the graft also induces transformations in the host tissue and changes it as well
What do AER and ZPA stand for?
AER = apical ectodermal ridge
ZPA = sone of polarizing activity
What is the first stage in development after the embryo has been fertilized?
What happens during cell Cleavage?
rapid cell division
During Cleavage, __________ increases but ______ stays the same
Where does Cleavage take place?
in the fallopian tube
How are genetically identical twins made?
clones from the same egg
when embryo is split during early development
early blastocyst splits into two
How are fraternal twins made?
release and fertilization of two different eggs
How do fraternal and identical twins each deal with the placenta?
identical - share the same one, fight for nutrients
fraternal- two placenta
How are P. Granules distributed in a newly fertilized egg?
they all move to the posterior end before the first cleavage division
Why can C. Elegans not have monozygotic twins?
because there is no grace period, the cells are committed early in development and P. Granules are not distributed equally
Asymmetric Cell Division
the two cells do not inherit the same things
happens at the expense of becoming another tissue