Topic 7

True/False all human embryos undergo a hermaphroditic period

True

In what week of gestation do gonadal primordia arise

week 5

Primordial germ cells become what which become what

Primordail germ cells become cortex/inner medulla

– Cortex – develop into ovary

– Medulla – develop into testis

When is chromosomal sex determined

Conception

At what stage of gestation do undifferentiated gonads develop into phenotypic male or female gonads

7 weeks

(sexually bipotential until then)

Precursor of female internal sex organs

Mullerian system

Precursor of male internal sex organs

Wolffian system

Tjio and Levan (1956) contribution to genetics

Discovered 46 diploid for humans; 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes

X and Y Chromosomes are Easily Distinguishable Using a Technique Called _________

Karyotyping

Karyotyping

analysis of chromosomes using a light microscope

Karyotype

A pictorial display of metaphase chromosomes from a mitotic cell

Cytogenetics

The study of chromosome

Karyotyping steps

Treat with colchicine which arrests the cells in metaphase

Stain to observe the chromosomes – Giemsa Stains

Photograph or visualize using a computer

Analysis of Chromosomes (Size, position of the centromere, banding and staining regions)

Fluorescent dyes can also be used in karyotyping. For what?

Bind to Specific Regions of Chromosomes

Variations in Color Detected by Computer Program - Result in Digital Image

Pairing of the Chromosomes is easier - Homologous Pairs Identical Colors

Aberrations and Crossovers Easily Recognized

Detect Translocations not Previously Recognizable

Karyotypes are often prepared using what cells

Lymphocytes

The Y chromosome structure in includes 2 main regions with 1 sub-region. Name all 3

PAR - Pseudoautososmal region

MSY - Male specific region

• SRY - sex-determining region

What regions on the Y chromosome that share homology with regions on the X chromosome

Pseudoautosomal Regions (PARs)

What is the function of PARs during meiosis

They synapse and recombine with the X chromosome

Why are PARs important for male gametogenesis

They ensure proper segregation of X and Y chromosomes

How can an XY female be possible

Deletion of the SRY region on Y Chromosome

How can an XX male be possible

Translocation of the SRY Region of Y to the X chromosome

If the SRY gene was given to a female mouse, what would happen

Converts Chromsomal Female Mouse to Phenotypic Male

At what stage of development does expression of SRY begin (by what cells along with development of what)

Sertoli Cells at Time of Testes Development

Where is SRY transcribed that it leads to Male specific Neural Properties

Hypothalamus & midbrain

What chromosome is SF1 gene on

Chromosome 9

Where is SF1 initially expressed & where does it remain expressed

Genital Ridges of both Sexes

Remains Solely in Developing Testes

What happens to SOX9 expression after SRY is expressed

SOX9 is upregulated in Sertoli cells just after SRY expression

What phenotype results from altered SOX9 expression in an XY individual

The individual is phenotypically female despite having an XY genotype

What leads to the regression of female ducts (what hormone by what cells)

Activation of Genes Endcoding Anti Mullerian Hormone secreted by Sertoli Cells

What is DAX1

Gonad Determining Genes

A lack of DAX1 (NR0B1) can lead to absent or underdeveloped gonads

What role does the DAX1 gene play in XY females

DAX1 region on the Y chromosome can interfere with testis development, even in individuals who have the SRY gene

What is DAX1 generally repressed by

SRY

What genes are located on the sex chromosomes & autosomes in females vs males

Effect of SRY on a Y chromosome on Sox9

SRY binds to & elevates the enhancer Sox9 gene in the testis determining pathway

How does Sox9 block development of female phenotype?

Block the ovary-forming pathway by blocking β catenin

Binds to the promotor site on the gene for anti-Mullerian Hormone (AMH) - can’t be produced without SRY

What does Sox9 activate that stimulates testis development

Sox9 activates Fgf9 synthesis

Effect of Sox9 on Sertoli cells

Sox9 activates other genes that help generate Sertoli cells

How is the Wnt pathway stimulated in females

Absence of the Y chromosome allows Rspo1 to bind to its cell membrane receptor & stimulate the Wnt pathway (Wnt4)

What is β catenin and how is it produced

It is a transcriptional regulator produced by the Wnt pathway

How does β catenin contribute to positive feedback

Activates genes for Rspo1 & Wnt4

How does β catenin contribute to female phenotype development

Initiates the ovarian pathway of development

Prevents the production of Sox9

Activates other genes in the ovary → production of granulosa cells

How, if there are 2 X chromosomes, is there not an overloading of the genes on those chromosomes

Dosage compensation by inactivation of 1 X chromosome

What is a Barr Body

An inactivated X chromosome

(XX females & XXY males have a Barr body)

Where & when does x inactivation occur

X inactivation occurs outside the ovary around day 12 in utero

What is XIST

X-inactive specific transcript

Plays role in X chromosome activation

True/False XIST is present on both X chromosomes

Kinda True - Initially, XIST is transcribed from both X chromosomes

The X-inactivation center (Xic) is only active on the inactive X..

Only one X will continue to produce XIST – Inactive.

Inactive X is condensed/decondensed 

highly condensed

In what phase can you stain a cell and see the inactive X (Barr bodies)

Stained interphase cells

Give an example of anhidrotic ectodermal dysplasia

Depiction of the absence of sweat glands in the shaded regions in a female due to them being Heterozygous for the X-linked condition

The locations vary from female to female, based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes.

Why do males with Androgen insensitivity syndrome develop phenotypically female

Males who inherit this condition produce testosterone and DHT but are unable to respond to either hormone due to a mutation in the gene encoding the androgen receptor.

These individuals have normal male chromosomes

The low level of estrogen that is produced by the adrenal glands is enough to stimulate female secondary sex characteristics

Summary slide

Does development of gonads in males & females begin at the same time

Male development of gonads occurs about 2 weeks before female gonadal development

When preparing a karyotype, why can’t we use RBCs

They don’t have a nucleus

In what stages of the cell cycle can FISH be done

Interphase & Metaphase