Transcript Chapter 7 Power Point Slides

Michael Cummings
Chapter 7
Development and
Sex Determination
David Reisman • University of South Carolina
7.1 The Human Reproductive System
 The human reproductive system
• We will not cover most of this information in this
course because it is focused on anatomy rather than
human genetics.
Timing of Meiosis and
Gamete Formation in Males and Females
 Males
•
•
•
•
Spermatogenesis begins during puberty
Millions of sperm are always in production
Spermatogenesis takes about 48 days
Each cell that undergoes meiosis produces 4 sperm
 Females
• Primary oocytes produced during embryonic
development remain in meiosis I until ovulation
• Ovulation begins during puberty
• Meiotic division produces 1 large oocyte and 2-3 polar
bodies
The Largest Cell
 The human oocyte is the largest cell produced in the
body. It is large enough to be seen with the naked
eye
Fig. 7-5a, p. 155
7.2 From Fertilization to Birth
 Cell divisions in the zygote form an early embryonic
stage called the blastocyst
 Blastocyst
• The developmental stage at which the embryo
implants into the uterine wall
• Stem cells are derived from a blastocyst
 Inner cell mass
• A cluster of cells in the blastocyst that gives rise to the
fetus
Implantation
 Implantation
 The embryo implants in the uterine wall, and
membranes develop to support the embryo
 Trophoblast
• Outer layer of cells in the blastocyst that gives rise to
the membranes surrounding the embryo
Embryonic Membranes and Placenta
 Chorion
• formed from trophoblast
• Releases human chorionic gonadotropin (hCG)
hormone which maintains uterine lining and stimulates
endometrial cells to produce hormones—hCG is what
pregnancy tests detect
• Grows to eventually form the placenta
Trophoblast
(surface layer
of cells of the
blastocyst)
Fertilization
Endometrium
Blastocoel
Implantation
Endometrium
Inner cell mass
Inner cell
mass
Uterine
cavity
1
2
3
4
5
Fig. 7-6a, p. 156
Start of Start of
amniotic embryonic
cavity
disk
Chorion
Blood-filled spaces
Chorionic villi
Chorionic
cavity
Amniotic
cavity
Start of
yolk sac
Start of
chorionic cavity
Actual
size
Yolk sac
8
7
6
Connective
tissue
Actual
size
Actual
size
Fig. 7-6b, p. 156
Development is Divided into Three
Trimesters
 First trimester
• First month: basic tissue layers form; most of the body
is divided into paired segments
• Second month: most major organ systems are formed
• Third month: embryo becomes a fetus; sexual
development is initiated
Development is Divided into Three
Trimesters
 Second trimester
•
•
•
•
Increase in size and organ-system development
Bony parts of skeleton form
Heartbeat is heard with a stethoscope
Fetal movements begin
 Third trimester
• Rapid growth
• Circulatory and respiratory systems mature
• Birth is a hormonally induced process at the end of the
3rd trimester
WEEKS 5–6
Head growth exceeds growth of other regions
Retinal pigment
Future external ear
Upper-limb differentiation (hand plates develop, then
digital rays of future fingers; wrist, elbow start forming)
Umbilical-cord formation between weeks 4 and 8
(amnion expands, forms tube that encloses the
connecting stalk and a duct for blood vessels)
Foot plate
(b)
Actual length
Fig. 7-7ab, p. 158
WEEK 8
Final week of embryonic
period; embryo looks
distinctly human
compared to other
vertebrate embryos
Upper and lower limbs well
formed; fingers and then
toes have separated
Primordial tissues of all
internal, external structures
now developed
Tail has become stubby
(c)
Actual length
Fig. 7-7cd, p. 159
Placenta
WEEK 16
Length: 16 centimeters (6.4 inches)
Weight: 200 grams (7 ounces)
WEEK 29
Length: 27.5 centimeters (11 inches)
Weight: 1,300 grams (46 ounces)
WEEK 38 (full term)
Weight: 50 centimeters (20 inches)
Length: 3,400 grams (7.5 pounds)
(d)
During fetal period,
length measurement
extends from crown to
heel (for embryos, it is
the longest measurable
dimension, as from
crown to rump).
Fig. 7-7cd, p. 159
7.3 Teratogens Are a Risk
to the Developing Fetus
 Teratogen
• Any physical or chemical agent with the potential to
cause birth defects
• Radiation, viruses, medications, alcohol
Alcohol is a Teratogen
 Fetal alcohol syndrome (FAS)
• A range of birth defects caused by maternal alcohol
consumption during pregnancy
 Alcohol is the most common teratogenic problem
and leading cause of preventable birth defects
• There is no “safe” amount of alcohol consumption
during pregnancy
Tetatogens and their impact on organ formation Defects in physiology;
Major morphological
abnormalities
Weeks: 1
2
Cleavage,
implantation
3
4
Future
heart
Future
Future
brain eye
Limb
buds
5
6
physical
abnormalities minor
7
8
9
16
20–36
38
Future Palate
ear
forming
Teeth
External genitalia
Central nervous
system
Heart
Upper limbs
Eyes
Lower limbs
Teeth
Palate
Insensitivity to
teratogens
External genitalia
Ear
Fig. 7-8, p. 160
Mechanisms of Sex Determination
Mechanisms of sex
determination vary
from species to
species
XX/XY system
XX/X0
ZW/ZZ
Temp.
Human Sex Ratios
 Sex ratio
• The proportion of males to females changes
throughout the life cycle
• The ratio at conception is slightly higher for males.
(**prenatal deaths most likely due to lethal X-linked recessive alleles)
• The ratio at birth is about 105 males/100 females
• The ratio of females to males increases as a
population ages
Sexual Development begins in the
Seventh week of Gestation
7.5 Defining Sex in Stages: Chromosomes,
Gonads, and Hormones
 Sex of an individual is defined at three levels
• Chromosomal sex (established at fertilization)
• Gonadal sex (begins around week 7 or 8 of
embryogenesis)
• Phenotypic sex
 Gonadal and phenotypic sex depend on the
interaction of genes and environmental factors,
especially hormones
Gonadal Sex Differentiation
 For the fist 7 or 8 weeks, the embryo is neither male
nor female
• Both male and female reproductive duct systems
begin to develop
 Genes cause gonads to develop as testes or
ovaries, establishing gonadal sex
• Alternative pathways produce an intermediate sex for
1 out of every 2000 births.
Y Chromosome and Testis Development
 SRY gene
• Sex-determining region of the Y chromosome
• Plays a major role in causing the undifferentiated
gonad to develop into a testis
• Testis development causes secretion of testosterone
 Müllerian inhibiting hormone (MIH)
• Hormone produced by developing testis that causes
breakdown of Müllerian (female) ducts in the embryo
Female Development
 Requires the absence of the Y chromosome and the
presence of two X chromosomes for the embryonic
gonad to develop as an ovary
 In the absence of testosterone, the Wolffian duct
system degenerates
 In the absence of MIH, the Müllerian duct system
forms female reproductive system
Egg with X sex
chromosome
Male
Sperm with Y
chromosome
Fertilized by
Embryo with XY sex
chromosomes
Sex-determining region of
the Y chromosome (SRY)
brings about development
of undifferentiated
gonads into testes.
Fertilized by
Sperm with X
chromosome
Chromosomal
sex
Embryo with XX sex
chromosomes
Gonadal
sex
No Y chromosome, so
no SRY. With no
masculinizing influence,
undifferentiated gonads
develop into ovaries.
Testes secrete
masculinizing hormones,
including testosterone, a
potent androgen.
In presence of testicular
hormones, undifferentiated
reproductive tract and
external genitalia develop
along male lines.
Female
No androgens secreted
Phenotypic
sex
With no masculinizing
hormones, undifferentiated
reproductive tract and
external genitalia develop
along female lines.
Fig. 7-14, p. 167
Androgen Insensitivity
 Androgen insensitivity (CAIS)
 A mutation in the X-linked androgen receptor gene
(AR) causes XY males to become phenotypic
females
• Testosterone is produced, but not testosterone
receptors; cells develop as females
XY Female with Androgen Insensitivity
Fig. 7-15, p. 168
Exploring Genetics:
Joan of Arc or John of Arc?
 Joan of Arc fought with the French at the Battle of
Orleans, and was burned as a heretic by her
enemies, the English, in 1431
 From an examination of trial evidence and records of
her physical examinations, R.B. Greenblatt proposed
that Joan had phenotypic characteristics of androgen
insensitivity
Mutations can cause Sex Phenotypes to
Change at Puberty
 Pseudohermaphroditism
• Mutations in several different genes cause XY
individuals to develop the phenotype of females
• Affected individuals have structures that appear
female at birth
• At puberty, testosterone burst causes a change into
the male phenotype
7.7 Equalizing the Expression of X
Chromosomes in Males and Females
 Lyon hypothesis (proposed by Mary Lyon)
• How do females avoid getting a double dose of protein
from X-linked genes?
• Random inactivation of one X chromosome in females
equalizes the activity of X-linked genes
 Barr body
• A densely staining mass in the somatic nuclei of
mammalian females
• An inactivated X chromosome, tightly coiled
X Chromosomes and Barr Bodies
Fig. 7-16, p. 169
Female Mammals are Mosaics for X
Chromosome Expression
 In females, some cells express the mother’s X
chromosome and some cells express the father’s X
chromosome
• Inactivated chromosome can come from either mother
or father
• Inactivation occurs early in development
• Inactivation is permanent; all descendants of a
particular cell have the same X inactivated
Female Mammals are Actually Mosaics for X
Chromosome Expression
Female Mammals are Mosaics
for X Chromosome Expression
Unaffected skin
(X chromosome with
recessive allele was
condensed; its allele
is inactivated. The
dominant allele on
other X chromosome
is being expressed in
this tissue.)
Affected skin with no
normal sweat glands
(yellow). In this tissue,
the X chromosome
with dominant allele
has been condensed.
The recessive allele
on the other X
chromosome is
being transcribed.
(a)
(b)
Fig. 7-18, p. 171
Mosaic Expression in Female Mammals
The gene for fur
color in cats is on
the X chromosome.
Fig. 7-17, p. 170
Inactivation of X Chromosome
by XIST RNA
XIST gene codes for
RNA that binds to
the X chromosome
and inactivates it
Fig. 7-19, p. 171
7.8 Sex-Related Phenotypic Effects
 Sex-limited trait - affects a structure or function of
the body that is present in only males or females
• Women do not get prostate cancer, women do not
grow beards but pass on the gene for beard growth on
to their sons
 Sex-influenced trait - an allele is dominant in one
sex and recessive in the other
• Baldness—the allele is dominant in males and
recessive in females
Imprinting
 Imprinting
• difference in expression of a gene depending upon
whether it was inherited from mother or father
• More discussion to follow on this topic in Chapter 11.