L27: Mammalian Gametogenesis, Fertilisaton and Embryogenesis

what is the vagina

an internal structure linking vulva to cervix

what organs produce male ejaculatory fluids

• seminal vesicles

• prostate gland

• bulbourethral glands

Female Oogenesis (Meiosis)

• Primordial germ cells in embryo differentiate into Oogonium (stem cell in foetus)

• Differentiation occurs and the onset of meiosis 1 leading to a primary oocyte which remains arrested in prophase until puberty

• Meiosis I complete and onset of meiosis II starts

• One primary oocyte develops into secondary oocyte and first polar body is formed

• Ovulation occurs

• Entry of sperm triggers completion of Meiosis II and secondary oocyte develops into ovum and releases a second polar body

Describe Spermatogenesis in males (meiosis)

• Primordial germ cell in embryo differentiates at birth into spermatogonium stem cell

• mitotic divisions produce large numbers of spermatogonia

• Differentiation occurs and the onset of Meiosis I

• Spermatogonium divide into 1 primary spermatocyte during prophase of Meiosis I

• Meiosis I is completed and the primary develops into 2 secondary spermatocyte

• Meiosis II occurs and secondary divide to form 4 early spermatids

• These differentiate into sperm cells (spermatozoa)

Differences in sex specific human gametogenesis

• Modified female meiosis produces a single large ovum compared to 4 tiny sperm

• Human ovum is 10million times the volume of a sperm

• Ovum packed with maternal mRNAs, proteins and nutrients to drive early development. Earliest embryonic development has NO paternal contribution to function as it takes time for the paternal genome to be transcribed and translated (ovum kick starts life of new embryo)

• Sperm is the smallest human cell which has a nucleus, mitochondrion and a tail

• There are limited mitotic divisions of oogonia and a loss of these trigger menopause

How many primary oocytes are selected to mature

One per 28 day ovarian cycle

how often do spermatogonia divisions occur

continued from puberty until old age: 160,000,000 sperm mature a day

where do mature sperm collect

centre of the seminiferous tubule

spermatogonia

germline stem cells which proliferate mitosis

role of sertoli cells in males

promote spermatogenesis

how long does it take spermatogonia to become mature spern

70 days

describe the ovarian cycle

• Day 0-1: Primary oocyte selected to development and surrounded by follicle cells

• Day 2-12: oocyte growth and follicle maturation

• Day 14: ovulation, meiosis I is completed and secondary oocyte released

• Day 15-26: Follicle remnants develop into corpus luteum which produces progesterone and estradiol

• Day 28: corpus luteum breaks down (loss of progesterone and estradiol)

• Primary oocyte within follicle → Growing follicle → mature follicle → Ruptured follicle → secondary oocyte ovulated → corpus luteum → degenerating corpus luteum

Ovulation to Implantation

1. Ovulation

2. Fertilisation

3. Cleavage

4. Continued cleavage

5. Blastocyst implants

Parts of the sperm

• Acrosome: top of head

• Nucleus: haploid in head

• Mitochondria: for energy

• Tail: Flagella function to swim

Features of the sperm acrosome

• organelle

• surrounded by plasma membrane

• filled with hydrolytic enzymes which: 1.digest carbohydrate bonds 2.digest peptide bonds 3.sperm receptor for egg protein

What is capacitation

• Occurs within the female tract as sperm swims out of seminal fluid into female tract secretions

• Acrosome membrane fuses with sperm cell membrane and exposes sperm receptor for egg protein

• sperm is now “primed” and ready to fuse with egg

Describe fertilisation within a sea urchin

• Jelly coat surrounds the fertilisation envelope of the egg cytoplasm and a molecule within the coat binds to the sperm receptors on the egg cytoplasm, triggering the sperm acrosome reaction

• The acrosome protein is released from the head of the sperm and the hydrolytic enzymes breaks down the jelly coat to fuse with the sperm binding receptors (fertilisation)

• Sperm nucleus enters the cytoplasm and the corticle granules within the egg begin to fuse to the plasma membrane and release enzymes

• The viteline layer is altered (increased) which prevents polyspermy from happening

what are ovum surrounded by in mammals

follicle cells

what is the zona pellucida in mammals

functions like the jelly coat of aquatic ova

what triggers the acrosome reaction in mammalian sperm

sperm receptor binding to zona pellucida

what triggers cortical reaction in mammalian ova

sperm binding to sperm binding receptors in plasma membr

what destroys sperm receptors in mammals

enzymes in the cortical granules which alter the zona pellucida and ablocks polyspermy

what is a blastocyst

6 day old human embryo which has an inner cell mass and a trophoblast (outer layer of cells which contribute to the placenta)

Trophoblast role

secretes enzymes to break down molecules in endometrium which allows the blastocyst to invade the endometrium and trophoblast to proliferate into endometrium

what does the inner cell mass of a blastocyst form

2 layers:

• epiblast and hypoblast

What is the epiblast

a disk of cells in which the embryo develops almost entirely from

(everything else develops from placenta, umbilical chord and membranes)

why are aquatic embryos simpler than terrestrial

they are just an embryo and nothing else

What are terrestrial embryos

both the embryo proper and the extra-embryonic structures/functions required to keep the embryo alive in a terrestrial environment (placenta, amniotic sac, umbilical chord etc…)

what is cleavage

a period of rapid cell division without growth which follows fertilisation

• it partitions the cytoplasm of one large cell into many smaller cells called blastomeres

What is the blastula

a ball of cells (blastomeres) with a fluid filled cavity called a blastocoel

Where does what we know about early embryonic development come from

amphibians (aquatic embryos)

Describe cleavage in aquatic embryos

fertilised egg → four cell stage → early blastula (cells are totipotent stem cells) → later blastula

• Early blastula stem cells can differentiate into any kind of cell

• Cells progressively lose totipotency as they commit to specific cell fates

• The blastula develops further and generate the new organism

what facilitates colonisation of land by vertebrates

• the shelled eggs of birds, reptiles and monotremes (egg laying mammals)

• the uterus of marsupial and eutherian mammals

What is the amnion

a fluid filled sac surrounding the embryo which has the same salt concentration as the worlds oceans

amniotes

• mammals reptiles and birds

• their blastocyst generates the embryo proper plus the extraembryonic tissues so terrestrial zygotes generate embryo and extra embryonic structures

what does the placenta support

foetal life from around week 4 of development

what does trophoblasts secrete

Human Chorionic Gonadotropin hormone (hCH) which acts like LH by maintaining the corpus luteum (continues to secrete estradiol and progesterone to maintain endometrium)

what is a common pregnancy test

testing hCG (human chorionuc gonadotropin hormone) levels which are secreted in urine. it may work around 1 week after implantation and around 14 days after contraception

what does the placenta do in the second and third pregnancy trimesters

stops making hCG and directly makes progesterone and estradiol so the corpus luteum degenerates and is no longer needed

what does maternal blood do

• flows out of arteries and bathes the foetal capillaries (foetal blood is restricted to capillaries so maternal and foetal blood dont mix)

• It is in direct contact with the foetal tissues

• Has a large surface area so maternal/foetal transport and exchange of nutrients can occur

why do most embryos not get rejected

• during pregnancy, sufferers of various autoimmune diseases experience remission

• the regulation of the immune system is significantly altered during pregnancy to prevent rejection

full human prenatal development

• Day 1: fertilisation in oviduct, cleavages start

• Day 4: embryo in uterus

• Day 7: blastocyst

• Day 12: Implants into endometrium of uterus

• Week 2-4: Differentiation and development start

• Week 5: Placenta develops and makes estrogen and progesterone

• Week 6-7: sex determination

• End of tri 1: most development and organogenesis complete

• 2nd and 3rd tri: growth, movement, prep for birth

• Week 38: birth

describe how the SRY gene was identified

1. Early cues: - Turner syndrome (Individuals with X0 developed as females suggesting the presence of a male-determining factor on the Y chromosome) - Klinefelter syndrome (XXY developed as male)

2. Researchers compared normal and abnormal Y chromosomes in XX males and XY females and they narrowed down the male determining factor to the Yp11 region of the Y chromosome

3. Peter Goodfellow led a team which isolated a single gene from the Yp11 region (SRY gene) and they found that XX males had a small piece of the Y chromosome containing SRY while XY females had SRY deletions or mutations

4. Experimental proof- Scientists inserted the SRY gene into genetically female mouse embryos and these XX embryos developed as males

How does the SRY gene work

• transcription factor

• activates SOX9 which is a gene that drives testis development

• in the absence of SRY, ovaries develop instead of testes

what are transcription factors

• proteins which bind to specific DNA sequences to control the transcription of genes

• they act as molecular switches which determine whether a gene should be activated or repressed

• essential for development, cell function and adaptations to environmental changes

describe how transcription factors work

1. DNA binding: they recognise specific DNA sequences

2. Recruitment; once bound to dna, they recruit RNA polymerase and some enhance transcription while others block it

what is the indifferent gonad

• early stage gonad in mammalian embryos which has not yet developed into testes or ovaries (bipotential as it can differentiate into eother)

• gonads start as indifferent around 4-6 weeks

• by week 7 sex determination begins and gonad starts differentiating into testes or ovaries

how do mammalian indifferent gonads develop

develop from the genital ridge which contains precursor cells which can either become sertoli and leydig cells (testes) or granulosa and theca cells (ovaries)

Development of the Male gonads (testes)

• In XY embryos, the SRY gene is activated

• The SRY gene triggers SOX9 which directs the gonad to develop into testes

• Testes produce testosterone(leydig cells produce) and AMH (produced by sertoli cells) which prevents female organ development and cause the Mullerian ducts to regenerate

• Testosterone promotes the development of the Wolffian duct system into the epididymis, vas deferens and seminal vesicles

Development of the female gonads (ovaries)

• in XX embryos there is no SRY gene and therefore SOX9 is not activated

• Primordial germ cells differentiate into oogonia which begin meiosis and become primary oocytes for birth

• somatic cells develop into granulose cells which support oocytes and theca cells which produce oestrogen

• WNT4 gene is activated to promote ovarian development

• The gonads differentiate into ovaries which later produce oestrogen

• Wolffian ducts degenerate and the Mullerian ducts develop into the uterus, fallopian tubes and upper part of vagina

role of AMH

• secreted by sertoli cells which are induced by the SOX9 gene in the developing gonad

• causes degeneration of the Mullerian ducts

• suppresses the development of the female reprouctive structures in the embryo

sertoli cells

nurse cells for sperm development and play a role in the formation of the seminiferous tubules

role of testosterone

• prouced by the leydig cells

• promotes development of Wolffian ducts into epididymis, vas deferens and seminal vesicles

• directs the formation of external male genitalia (e.g. penis)

what are the indifferent genitalia

the early stages of genital development in embryos where the external genital structures are indistinguishable between males and females. these structures have bipotential so can develop into male or female genitalia depending on influences of genetic factors and hormonal signals

male genital development

• SRY gene → formation of testes → testosterone secretion → masculinisation of external genitalia

• Genital Tubercle elongates to form the phallus and the penis

• Urethral folds fuse along the midline and form the urethra and the ventral surface of the penis

• genital swellings fuse in the midline to form the scrotum which will house the testes

• scrotum forms as the testes descend from initial position near the kidney

female genital development

• absence of testosterone → indifferent genitalia develop into female external genitalia

• genital tubercle remains shorter in females and becomes the clitoris

• urethral folds for the labia minora

• genital swellings do not fuse, they develop into labia majora

• lower portion of mullerian ducts fuse to form the vagina

Derivation of male structures

• Gonads → testes

• Epididymis, vas deferens, seminal vesicles from wolffian ducts

• penis from genital tubercle elongation

• urethra from urethral folds

• scrotum from genital swellings

derivation of female structures

• gonads → ovaries

• gonadal ridge → ovaries

• mullerian ducts → oviduct, uterus, upper vagina

• genital tubercle → clitoris

• urethral folds → labia minora

• genital swellings → labia majora

polyspermy

the fertilisation of an egg vy more than one sperm cell which leads to an abnormal number of chromosomes and causes genetic abnormalities or embryo development failures

main steps of fertilisation

1. sperm capacitation; within the female reproductive tract, sperm undergo changes to become capable of fertilisation

2. Sperm travel through the female reproductive tract to the oviduct where fertilisation occurs.

3. Sperm move towards the egg via chemotaxis (movement in response to chemical signals released by the egg/surrounding cells)

4. Sperm bind to the protective glycoprotein layer surrounding the egg (zona pellucida)

5. Acrosomal reaction occurs where enzymes from the sperm acrosome are released to break down the zona pellucida allowing the sperm to reach the egg membrane

6. Sperm head fuses with egg plasma membrane allowing DNA to enter the egg

7. The egg undergoes a depolarisation event which prevents polyspermy

8. after sperm enters, cortical granules inside the egg are released and alter the zona pellucida to make it impermeable to other sperm

9. sperm releases DNA into egg cytoplasm and the egg nucleus completes 2nd meiotic division to form the female pronucleus

10. Male and female pronucleus move toward eachother and fuse to form a zygote

describe sperm capacitation

• removal of glycoproteins and proteins from the sprem head

• changes in the sperm membrane to be mire receptive to the egg

• increased motility so sperm will swim faster and reach the egg

how is polyspermy avoided

1. The egg undergoes a depolarisation event which prevents polyspermy after the sperm head fuses with the plasma membrane

2. After sperm enters, cortical granules inside the egg are released and alter the zona pellucida to make it impermeable to other sperm

describe a totipotent cell

has the potential to differentiate into any cell type in the organism including both somatic cells and the extra embryotic tissues(e.g. placenta)

describe a pluripotent cell

can differentiate into any cell type which makes up the body (somatic cells) but it cannot form the extra embryonic tissues (like placenta)

The key steps of early animal development

1. fertilisation; sperm and egg fuse to form a zygote and the genetic material from both the sperm and egg combine to form the complete genome of the future organism

2. cleavage; mitotic divisions which occur after fertilisation and result in the muticellular embryo. the 1st cleavge produces 2 cells (blastomeres)

3. Blastulation; as cleavage continues, the embryo forms a hollow sphere of cells called the blastula (undifferentiated cells arranged in a single layer and interior cavity)

4. Gastrulation; blastula is reorganised into a gastrula resulting in the formation of the 3 germ layers

formation of the primary germ layers (gastrulation)

• Ectoderm; outermost layer, gives rise to skin, nervous system, sensory organs, hair and nails

• Mesoderm; middle layer, gives rise to variety of tissues and organs e.g. muscles, skeleton, circulatory system, kidneys and gonads

• Endoderm; innermost layer, forms lining of igestive tract, lungs, liver pancreas etc..

importance of the trophoblast

• crucial in the development of mammalian embryos, particularly those who develop placentas

• specialised layer of cells which forms part of the extra-embryonic tissue and is integral to the placenta formation

• invade the uterine lining to form structures like the villi which help to establish a robust connection with the mother’s blood supply

• produces hormones such as hCG which maintains the corpus luteum and supports early pregnancy

trophoblast role in placenta formation

• trophoblast is the outermost layer of the blastocyst and has 2 main roles:

• 1. Implanting the embryo into the maternal uterus by interacting with the uterine lining and facilitating the attachment and embedding of the embryo

• 2. trophoblast cells develop into the chorion and contribute to the formation of the placenta

development of animals without a placenta

• No trophoblast

• Nutrients are supplied by egg yolk sac in marsupials/ monotremes or direct maternal provisioning

• Hormonal roles limited or not needed in animals without a placenta

• Shorter gestation or external development

structure of the placenta

• fetal components; Chorion (outermost fetal membrane derived from the trophoblast). Chorionic villi (extend from chorion to maternal uterine lining, are rich in blood vessels and the primary site for nutrient and gas exchange).

• maternal components; Decidua (modified endometrial lining of uterus during pregnancy which is involved in providing initial nutrients and helps to anchor the developing placenta to the uterine wall)

• placental barrier; formes by layers of cell whoch separate fetal blood from maternal blood (syncytiotrophoblast, endothelium of fetal blood vessels, extra-embryonic mesoderm)

functions of the placenta

1. nutrient exchange- nutrients pass from mothers blood to the fetus for growth and development via diffusion, active transport and facilitated diffusion

2. gas exchange - oxygen delivered from mothers blood to the fetus while carbon dioxide is transferref from fetal blood to maternal circulation

3. waste removal- removes metabolic waste e.g. urea from the fetus and transfers them to maternal circulation for excretion

4. hormonal regulation- produces hCG, progesterone, estrogen and hPL

5. immunological protection to the fetus by blocking most maternal immune cells but allowing antibodies to pass

6. filters harmful substances (bacteria, viruses, toxins etc) from reaching fetus