Developmental Biology Exam 2 Flashcards
Primary sex determination
gonads: testees or ovaries
genetics
Secondary sex determiantion
Phenotype- male vs female internal and external organs
Hormones and paracrine factors from the gonads
Bipotential gonads
genital ridge: gonad rudiment will become gentile duct
bipotent: can become ovaries and testees
Have indifferent development takes place followed by differentiation into either testees or ovaries (based on XX and XY genotype
Have both Mullerian and Wolffian ducts
Hormones made by genotype will fully generate one duct and disintegrate the other
How genitals are affected with XY genotype
Gonads become the testees
Wolffian duct differentiates into sperm transport duct
Mullerian duct is degenerated
How genitals are affected with XX genotype
Gonads become ovaries
Mullerian duct becomes oviduct which become the fallopian tubes
Wolffian duct is degenerated
Primary Sex determination for XY
Gene SRY promotes testis formation
SRY is a transcription factor which binds enhancer region of Sox9 gene. Sox9 is then expressed by activating itself via SRY
Sox9 also activates anti-Mullerian hormone (AMH) and promotes degradation of beta-catenin.
Primary Sex determination for XX
no SRY gene present
wnt4 paracrine factor leads to wnt/beta-catenin signaling and target gene expression
wnt4 leads to beta-catenin stabilization
Results in expression of ovary differentiation genes
Beta-catenin promotes maintenance of ovarian structures and blocks expression of Sox9
Secondary sex determination female
Bipotential gonad leads to wnt4 factor leads to ovary leads to granula cells and thecal cells leads to follicles leads to estrogen leads to differentiation of Mullerian duct leads to female sex phenotype
In the absence of testosterone the Wolffian duct regresses
Secondary Sex determination male
Bipotential gonad leads to SRY, Sox9 leads to testees leads to:
1. Sertoli cells which make AMH which leads to regression of Mullerian duct
AND
2. Leydig cells which make testosterone which causes differentiation of Wolffian duct which causes male sex phenotype
Germ Plasma theory (1892)
Weismann proposed
Some germ cells contain heritable information and somatic cells carry out ordinary body functions
germ plasm, which is independent from all other cells of the body (somatoplasm), is the essential element of germ cells (eggs and sperm) and is the hereditary material that is passed from generation to generation.
Background of Test of Germ Plasma Theory (1910)
Done by Theodore Boveri
Used parascaris aequorum which is a round worm parasite with chunky chromosomes you can see under a microscope
Have two chromosomes per haploid cell
Cleavage of first embryonic development separates animal half from vegetal half of the zygote.
Animal chromosomes during first two blastomeres: chromosome diminution
vegetal blastomere: chromosomes remain normal
During second cleavage animal cells split meridionally and the vegetal cell divides equatorially. Both vegetal cells created have normal chromosomes
Therefore at the fourth cleavage only one cell (vegetal) will have a full set of genes
At the 16th cell stage there will only be two cells with undiminished chromosomes. One of them becomes a germ cell and the other does chromosomes diminution to form stomatic cell
Chromosome diminution
When chromosome blastomere ends fragment before cell division
Only portion of original chromosome survives
Genes are lost and are therefore not present in new nuclei
Meridional division
a type of cell division that occurs during the early stages of embryonic development when a furrow cuts through the center of an egg, bisecting both poles
Equatorial division
is a term used to describe the process of cell division where chromosomes are divided equally into two daughter cells during metaphase of mitosis or meiosis:
Test of Germ Plasma Theory (1910)
Boveri set out to study the part of the cell plasma that caused the chromosomes to not diminish and see if this so called section of the cytoplasm existed
He centrifuged the eggs before their first cleavage to shift orientation of mitotic spindle. Therefore each cell formed should have portion of the vegetal portion
After first division no nucleus underwent chromosomal diminution. Only the animal ones for the second division underwent chromosomal diminution.
He concluded the vegetal cytoplasm contains a factor that protects nuclei from chromosomal diminution and determines germ cells
Primordial germ cells (PGCs)
Germ cells (aka. germ line) = cells that make gametes. Totipotent
Germ cells derived from PGCs. Can become either sperm or eggs. Therefore PGCs are bipotent
Where do PGCs come from?
In mammals they come from the ICM
ICM made primitive endoderm. epiblast are all cells of the embryo which also make PGCs
On P side becomes PGCs and are more likely to be mesoderm but signals tell them they are going to be germ cells
Therefore induction occurs
Migrate to gentile ridge and during this migration the PGCs also proliferate
Mammalian spermatogenesis
- Proliferation
- Meiosis
- Differentiation
Spermatogenesis: Proliferative Phase
- primordial germ cells (PGCs) migrate to the gentile ridge. While migrating they become gonocytes. Once at destination they become seminiferous tubules
- Contact with the seminiferous tubules leads to gonocyte differentiation into spermatogonial stem cells (SSCs). SSCs are type A spermatogonia.
The type A spermatogonia (stem cells) will either not divide, will divide to make asymmetric cell or through mitosis will make type B spermatogonia which will differentiate and mitosis to have two primary spermatocytes
sertoli cells
men mesodermal cells differentiate into these
Their role is to secrete AMH
Will also form seminiferous tubules
during week eight they surround germ cells to make testis chords
Testis chords
Form loops in central region of developing testis and are connected by thin canals called rete testis near developing kidney duct
What happens when male germ cells enter the gonads?
They develop within the testis chord, proliferate and then arrest in mitosis
When puberty hits, the testis cords develop into seminiferous tubules. The germ cells migrate to the periphery of the tubules to make SSC’s
Two directions men mesodermal cells can go?
Sertoli cells (epithelial) or Leydig cells (Mesenchymal)
Leydig cells
Secrete testosterone
What does fully developed testis have?
epithelial tubules of Sertoli cells surrounding germ cells and a mesenchymal cell population that secrete testosterone
To protect the testis each is surrounded by thick ECM which is called the tunica albuginea
XX fetus
Germ cells in gonad are organized in clusters surrounded by pre-granulosa cells
Germ cells enter meiosis
When XX is birthed, the pre-granulosa cells all degenerate only leaving the ones at the cortex of the gonad left
Each germ cell is surrounded by pre-granulosa cells
Germ cells will become oocytes
pre-granulosa cells will become granulosa cells. The rest of the mesenchymal cells will become thecal cells which forms follicles with granulosa cells.
The follicles envelop the oocyte and secrete steroid hormones such as estrogens and during pregnancy progesterone.
Germ cells and somatic cells of gonad
Germ cells are biopotential but are told what to do when they are in either male or female sex chords
They are told to begin mitosis and become eggs or arrest in mitosis and become spermatogonia (sperm stem cells)
Importance of germ cells in XX and XY
XX: the follicle cells would degenerate without germ cells
XY: The germ cells help support the differentiation of Sertoli cells but are not required for the maintenance of testis structure
Steps of spermogenesis
- Proliferative phase where sperm stem cells spermatogonia increase by mitosis
- Meiotic phase involves two divisions to create a haploid state
- postmeiotic phase called spermiogenesis during which round cells (spermatids) eject most of their cytoplasm and become the streamlines sperm
Proliferative phase of spermogenesis
PGCs arrive at genital ridge
Gonocytes are in sex chords that will be seminiferous tubules
Gonocytes become undifferentiated spermatogonia near the basal end of the tubular cells. Are then true stem cells
Spermatogonia reside in stem cell niches at the junction of Sertoli cells, the Leydig cells and the testicular blood vessels
Adhesion molecules join the spermatogonia directly to the Sertoli cells which will nourish the developing sperm
Mitotic proliferation of stem cells in XY
Produces type A spermatogonia which is held together by fragile cytoplasmic bridges
Glial derived neurotrophic factor (GDNF) which is secreted from Sertoli cells keeps the stem cells in mitosis
BMPs and Wnts can induce type A spermatogonia to differentiate into further sperm
Developmental process of sperm
Undifferentiated spermatogonia goes through differentiation to make differentiating spermatogonia. These cells initiate meiosis which makes spermocytes which undergo second meiosis. Then the cells undergo spermiogenesis to become spermatids
Type B spermatogonia
precursors of the spermocytes and contain high levels of Stra8
Spermocytes
last cells to undergo mitosis and divide out to generate primary spermatocytes which enter meiosis
primary spermatocytes
Undergoes meiotic division to male a pair of haploid secondary spermatocytes which complete the second division of meiosis
Spermatids
Formed after secondary spermatocytes complete a second division of meiosis
Are connected together by cytoplasmic bridges
Are haploid cells but since they are connected together they can diffuse into the cytoplasm of their neighbors and function as a diploid
As the divisions happen the cells go from the seminiferous tubule to its lumen
Oogenic meiosis
Primary oocyte goes through unequal cytokinesis after telophase I to generate second degree oocyte and a polar body
The secondary oocyte goes through another unequal cytokinesis after telophase II which gives mature ovum (egg) and another (2nd) polar body
Oogenesis
- PGCs (gonocytes) proliferate to make oogonia before birth
- In gentile ridge, oogonia associate with somatic cells to form follicles. One forms one follicle
- Surviving oogonia in a follicle enter meiosis I before birth. Become primary oocytes
- Primary oocytes pause in diplotene of prophase I
- Resumed at sexual maturity. ~12-40 years will be resumed meiosis in response to luteinizing hormone (LH) who is co-secreted along with follicle-stimulating hormone by the gonadotrophin cells in the adenohypophysis (anterior pituitary).
- Meiosis pauses again at meiosis II until fertilization
Dictyate resting phase
Meiosis arrested at first meiotic prophase and reinitiated in a smaller population of cells
Resumes during puberty
Retinoic acid (RA)
Determines timing of meiosis and sexual differentiation of mammalian germ cells
How Retinoic acid (RA) affects XX
RA activates Stra8 which is a txn factor what causes initiation of meiosis
After meiosis female germ cell fate/female differentiation occurs in the gentile ridge
How Retinoic acid (RA) affects XY
Cyp26b1 causes degradation of an acid which causes RA to not work
Nanos (late) degrades Stra8
These two factors causes meiosis to not occur which causes male fate and delayed meiosis
Fertilization goals
- Sexual reproduction. Genetic information from parents to offspring
- Initiate the development of the egg metabolism (maturation)
What happens during fertilization?
Sperm and egg contact and recognition
Regulation of sperm entry
Fusion of genetic material
Activation of the egg metabolism
Anatomy of sperm
From flagella to acrosomal vesicle:
Axoneme (made of tubulin and mitochondria), Mitochondria, Centriole, Nucleus (haploid), Cell membrane, Acrosomal vesicle (derived from Golgi; contains digestive enzymes)
Egg (ovum) Anatomy
Female pronucleus, Plasms membrane, vitelline envelope (outer jelly layer which is ECM around the egg; is important for sperm and egg recognition. Called Zona Pellucida in mammals), Jelly coat
