Other problems on sex differentiation and calcium homeostasis:

4.  Fill in , circle correct answer, etc.  (5 points each)
A.  The sex chromosomes in Turnerís syndrome a
re ______________
B.   After the ovum is ovulated, the follicle becomes a structure called the
______________

C.  The muscle of the uterus is called the ____________________

D.   During very early fetal development, Mullerian ducts are found in
a.  males only      b.  females only       c.  both males and females   

E.   Inhibinís role is to inhibit the secretion of   ___________
F.  Spermatogonia develop from germ cells that migrated to the  a. cortex   b. medulla  of the undifferentiated gonad.

5.   Can a woman with Androgen Insensitivity Syndrome become a mother by In Vitro Fertilization, that is, by having a fertilized egg implanted directly into her?  In other words:  Does she have a uterus?   Explain briefly the factors that are responsible for her having or not having a uterus.  (20 pts)     

5.  Consider the situation of an individual who has normal XY chromosomes, but has a mutation in the gene for the LH receptor, so that this receptor cannot function at all.    Describe the sexual differentiation of this person below.  For each part, explain what causes differentiation in this direction.  Your explanation should make it clear that you understand what causes normal differentiation of this structure, and how it is (or is not) different in this person. 

(32 points:  3 pts each structure, 5 pts each explanation)

A.  What kind of gonad will this person have? 

B.  What will happen to the Mullerian ducts?

C.  What will happen to the Wolffian ducts? 

D.  What kind of genitals will this person have?  

 

1.  One kind of male infertility occurs when there are small deletions in the Y chromosome which result in a smaller than usual number of sperm being produced -- not enough to ensure that sperm will reach and fertilize an egg.  To treat this, Intracytoplasmic Sperm Injection (ICSI) is used, in which a single sperm is injected directly into the egg in the lab, then the fertilized egg is implanted in the mother.  Some have criticized this technique, because the father's genetic defect would be passed on to the offspring, but prospective parents don't care; they just want a kid, and if their son turns out to have the same infertility, he can use the same technique to have his own kids.  This month, scientists reported that these microdeletions imply an instability in the Y chromosome, and in some cells, the Y chromosome is so unstable that it is completely lost from the cell.  When an XY baby is born to these men using ICSI, the baby may be a genetic mosaic, with most cells having the expected  XY, but something like 10% of the cells having no Y chromosome at all. 

Explain why babies born by ICSI have a greater than usual chance of having Turner's syndrome.  Assume that the offspring is a genetic mosaic, in which some cells are XY and some have no Y.  Your explanation should include a description of which are the embryonic cells that must have lacked the Y chromosome for Turner's syndrome to develop (5) AND draw a flowchart that summarizes the factors that are responsible for sexual differentiation at the level of the gonads (4), hormones (4), ducts (8), and external genitals (4) in this individual. 

 

ANSWERS

4.  Fill in , circle correct answer, etc.  (5 points each)
A.  The sex chromosomes in Turnerís syndrome are
XO
B.   After the ovum is ovulated, the follicle becomes a structure called the
corpus luteum

C.  The muscle of the uterus is called the myometrium. 

D.   During very early fetal development, Mullerian ducts are found in
a.  males only      b.  females only      
c.  both males and females   

E.   Inhibinís role is to inhibit the secretion of   FSH
F.  Spermatogonia develop from germ cells that migrated to the  a. cortex  
b. medulla  of the undifferentiated gonad.

5.   Can a woman with Androgen Insensitivity Syndrome become a mother by In Vitro Fertilization, that is, by having a fertilized egg implanted directly into her?  In other words:  Does she have a uterus?   Explain briefly the factors that are responsible for her having or not having a uterus.  (20 pts)   

No  (5).  Androgen Insensitivity syndrome, the condition that Terry had in the case study, occurs when an XY fetus lacks the receptors to respond to androgens.  Because she has a Y chromosome, the testes (5) develop and secrete Mullerian Inhibiting Hormone (5) , which causes the Mullerian ducts to degenerate.  Since the uterus develops from Mullerian ducts (5), she will lack a uterus.

5.  Consider the situation of an individual who has normal XY chromosomes, but has a mutation in the gene for the LH receptor, so that this receptor cannot function at all.    Describe the sexual differentiation of this person below.  For each part, explain what causes differentiation in this direction.  Your explanation should make it clear that you understand what causes normal differentiation of this structure, and how it is (or is not) different in this person. 

(32 points:  3 pts each structure, 5 pts each explanation)

A.  What kind of gonad will this person have? 

Testes.  Testes normally develo in males if the Y chromosome, containing the SRY gene, is present.  Here, the Y chromosome is normal, so testes should develop.

B.  What will happen to the Mullerian ducts?

Regress.  Testes normally secrete MIH, which causes Mullerian ducts to regress.  This should occur normally here, since MIH is not dependent on LH.

C.  What will happen to the Wolffian ducts? 

Regress.  In response to LH, testes secrete testosterone, which causes the Wolffian ducts to develop into the vas deferens and epididymis.  Since the person here canít respond to LH, no testosterone will be secreted, so the Wolffian ducts will regress.

D.  What kind of genitals will this person have? 

Female.  Genital differentiation in the male direction depends on DHT, which is produced from T.  Since thereís no T (as explained in part C.), thereís no DHT, so the genitals follow the default pattern of development, which is female. 

1.  This individual has some cells that have, others that lack the Y chromosome.  So an X ovum has been fertilized by a Y sperm, and due to instability of the Y chromosome, the Y has disappeared in 10% of the cells.  The father has the same problem, but appears male, while the offspring appears to have Turner's syndrome, which is a female phenotype.  For this to have happened, the cells that must have lost their Y chromosomes are the cells of the indifferent gonad (5).  That is, the cells that will become a gonad must be XO, while the other cells (which will become the ducts, the external genitals) could be either XO or XY.  Differentiation of these other structures depends only on external signals (T, DHT, MIH) and not on the presence or absence of Y chromosome in the cell.  The factors that would have been responsible for differentiation at each level are: gonad will become an ovary due to no SRY/TDF (4), and would therefore secrete estrogen/progesterone. (4). No testes, so a) no MIH secretion, and Mullerian ducts become oviducts(4); b) no T secretion, and Wolffian ducts degenerate(4); and c) no DHT, and external genitals become vagina/clitoris (4)