Dev Bio Exam 2

Chromosomal sex determination in mammals

• Male: XY

• Female: XX

• Mechanism: Father’s Y determines sex

Chromosomal sex determination in birds

• Male: ZZ

• Female: ZW

• Mechanism: Female’s W determines

Chromosomal sex determination in honeybees and ants

• Male: unfertilized, haploid egg

• Female: fertilized, diploid egg

• Mechanism: whether sperm is used

Primordial germ cells

germline stem cells that give rise to gametes in vertebrates

What do the primordial germ cells migrate into?

genital ridges

Genital ridges

early undifferentiated precursors to the gonads (testes or ovaries)

—> Biopotential gonads

What happens when the primordial germ cells migrate into genital ridges?

the genital ridges are able to become functional gonads

—> can become testes or ovaries

Bipotential gonad

can develop into either an ovary or a testis

Mullerian vs. Wolffian duct

• Mullerian: develops into female internal genitalia —> uterus, oviduct, cervix, upper vagina

• Wolffian: develops into male internal genitalia —> epididymis, vas deferens, seminal vesicles

Primary sex determination

the determination of the gonad as female or male

• largely dependent on genetics

• factors on Y chromosome

Outline the process of male primary sex determination (fetus is XY)

1. gonadal mesoderm cells initiate differentiation into sertoli cells

2. developing sertoli cells secrete anti-Mullerian hormone (AMH), blocking the development of female ducts

3. epithelial sertoli cells surround incoming germ layers and organize into testis cords that form loops in central region of developing tetsis

4. testis cords become surrounded by a thick ECM, the tunica albuginea to help protect it

5. wolffian duct becomes the epididymis and vas deferens to allow sperm to be flushed out

Outline the process of female primary sex determination (fetus is XX)

1. pre-granulosa cells are surrounded by ovarian cells

2. germ cells become developing ovum (eggs)

3. epithelial cells surrounding oocyte differentiate into granulosa cells

4. surrounding mesenchymal cells of the ovary differentiate into thecal cells

5. thecal and granulosa cells form the female sex cords (ovarian follicles)

What is the Mullerian duct and how does it form?

• present in all early embryos but degenerates in males due to AMH

• the precursor to oviducts, uterus, cervix, upper vagina in females

• develop from invagination of epithelial and mesenchymal cells

Where does fertilization occur in the female?

the oviduct

Invagination

sheet of cells bend inward to form a cavity

• epithelial to mesenchymal transition

• primarily forms mesoderm and endoderm

Involution

sheet of cells rolls inward, spreading over the internal surface of the outer cell layer to form an underlayer

Ingression

cells apart of a sheet break away and stay individual

Delamination

cells apart of a sheet break away individually and form new sheets

Epiboly

surface sheet of cells spread out and expand to cover the entire embryo

• primarily forms ectoderm

Convergent extension

group of cells converge (narrow) towards the midline and line up on midline to extend perpendicularly

Genetic mechanism of gonadal sex determination

bipotential gonad has not yet been committed to male or female and transcription factors are all expressed —> loss of any transcription factor will prevent normal development of male or female gonads

Outline the ovary pathway

1. transcription factors (Wt1, Lhx9, Gata4, Sf1) in bipotential gonad activate Wnt4 and Rspo1

2. Rspo1 acts with Wnt4 to produce beta-catenin transcription factor

3. this activates other ovary-forming genes and blocks synthesis of of testis-promoting transcription factor Sox9

4. beta-catenin induces Fox12 gene which stimulates ovary development

Outline the Testis pathway

1. transcription factors (Wt1, Lhx9, Gata4, Sf1) in bipotential gonad activate SRY

2. SRY transcription factor activates the autosomal gene (not on chromosome) Sox9

3. Sox9 stimulates synthesis of Fgf9 protein, binds to genes encoding AMH to degenerate Mullerian duct, activates other testis-forming genes, induces Dmrt1 for sperm production

4. Sox9 also blocks the ability of beta-catenin to induce ovary formation

5. Fgf9 stimulates testis development

Can Sox9 be activated without SRY?

Yes —> Sox9 can activate its own promoter to be transcribed for long periods of time independent of SRY

Why is the timing of SRY activation important?

If SRY is turned on too late, Sox9 cannot drive testis formation, allowing granulosa cell differentiation causing ovary formation

• SRY triggers a cascade of events and if not activated at the right time, default ovarian pathway occurs

XX and XY pathway visual

What is considered the testis determining factor and how was it determined?

SRY transcription factor —> tested with mice

• XX mouse with transplanted active SRY gene developed male genitalia

What is secondary sex determination?

the development of the female and male phenotypes (features) in response to hormones secreted by ovaries and testes

• reproductive duct system starts out bipotentially —> can become Mullerian duct (female) or Wolffian duct (male)

What hormones promote male development?

• Anti-Mullerian Hormone (AMH)

• Testosterone

• 5 alpha-dihydrotestosterone (DHT)

What hormones promote female development?

• Estrogen

Outline the female phenotype pathway

• Wolffian duct degenerates due to absence of testosterone

• fetal ovaries secrete estrogen to induce differentiation of Mullerian ducts into uterus, oviduct, cervix, upper vagina

Outline the male phenotype pathway

SRY gene is activated —> bipotential gonad becomes testis:

• sertoli cells form —> secrete paracrine factor AMH —> Mullerian duct degenerates

• leydig cells form —> secrete testosterone —> Wolffian duct differentiates into epididymis, vas deferens, seminal vesicles

What is the function of the steroid hormone testosterone?

• inhibits breast development

• promotes formation of Wolffian duct structures

What is the function of the derivative of testosterone, 5 alpha-dihydrotestosterone (DHT)?

• more potent hormone than testosterone

• promotes formation of external genitalia: penis, urethra, prostate, scrotum

What is Androgen Insensitivity Syndrome?

• individual has XY genotype and therefore an SRY gene

• they form testes that secrete testosterone

• mutation causes target cells to lack receptors for testosterone —> not competent

• they can respond to estrogen made by adrenal glands

• they develop external female sex characteristics with internal male gonads because testes are still producing AMH that degenerates Mullerian duct

Outline the Androgen Insensitivity Syndrome pathway

SRY gene is activated —> bipotential gonad becomes testis:

• sertoli cells form —> secrete paracrine factor AMH —> Mullerian duct degenerates

• leydig cells form —> secrete testosterone but cannot stimulate Wolffian duct leading to failure of internal male development

Fertilization

the process by which sperm and egg meet and fuse to begin the creation of a new organism

What are the two main purposes of fertilization?

1. sex: the combining of 2 parental organisms

2. reproduction: the creation of a new organism

What are the 6 main events fertilization usually consists of?

1. Attraction: movement of the gametes towards each other

2. Interaction between sperm and ECM of egg: egg has ECM that binds to and activates sperm in a species-specific manner

3. Exocytosis of sperm acrosome contents: sperm digests egg’s ECM and egg extends membrane towards sperm to become one zygote

4. Passage of sperm across egg membrane: only one sperm can unite with egg; other sperm is inhibited

5. Fusion of pronuclei: haploid nuclei of sperm and egg fuse and diploid number of chromosomes is reestablished

6. Egg activation: zygote’s mitotic apparatus forms and cell division is initiated

Meiosis

cell division that reduces the chromosome number by half, producing four genetically diverse haploid gametes from one diploid parent cell

Identify what stage of maturation the oocyte is in at the time of sperm entry for dogs/foxes

Primary oocyte

• paused at diploid before Meiosis I

Identify what stage of maturation the oocyte is in at the time of sperm entry for insects/starfish

Metaphase I

• fertilization triggers completion of Meiosis I

Identify what stage of maturation the oocyte is in at the time of sperm entry for mammals/amphibians/fish

Metaphase II

• fertilization triggers completion of Meiosis II

Identify what stage of maturation the oocyte is in at the time of sperm entry for sea urchins

Completed Meiosis

• fully developed haploid egg before fertilization

• no meiotic arrest at time of sperm entry

Compare the structure of a mammalian and urchin oocyte

• sea urchin: outer jelly layer that surrounds inner vitelline envelope

• mammalian: lacks jelly layer and instead has outer cumulus layer that surrounds inner zona pellucida

Fertilization envelope

a hardened, protective barrier formed around a sea urchin egg shortly after sperm entry to prevent polyspermy; not present in mammals

Describe how sea urchin sperm locate and fertilize an egg

1. Chemotaxis is used to attract sperm

2. Acrosome reaction when sperm reaches jelly layer

3. Bind to vitelline envelope with proteins from acrosomal process

4. Fusion of egg and sperm cell membranes

What happens during chemotaxis?

sperm follow gradient of chemical released by oocyte

• resact is released by egg jelly and sperm moves to higher concentration of chemical

Outline the acrosome reaction

• initiated by contact of sperm with glycoproteins in egg jelly

• enzymes are released from the acrosome to break down the jelly coat on the egg cell surface

• changes in pH allow actin to extend forward and create the acrosomal process containing species-specific molecules for binding to vitelline envelope

Explain recognition of egg’s extracellular coat in sea urchins

proteins on the sperm (Bindin) bind to specific receptors on the egg surface to ensure only sperm of the same species can fertilize the egg

What happens when sperm and egg fuse in sea urchins?

sperm and egg membranes fuse —> egg forms fertilization cone that engulfs sperm —> sperm nucleus enters egg cytoplasm

Describe how sperm can move through the mammalian reproductive tract and fertilize an egg