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GAMETOGENESIS, FERTILIZATION AND EMBRYONIC DEVELOPMENT IN HUMAN

NEELAM DEVPURA(M.Sc., NET, GATE)

Ph.D. Scholar

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CONTENTGametogenesis

a. spermatogenesis

b. oogenesisHormonal control of spermatogenesis and

oogenesisFertilizationCleavageGastrulationEmbryo development

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GAMETOGENESIS Both mitosis and meiosis play a role in gametogenesis.

Mitosis provides the precursor cells. Meiosis brings about the reduction divisions that result in gametes.

Special cells, primordial germ cells, in the gonads divide, grow, divide again and then differentiate into the gametes.

Sperm produced in testes, Ova produced in the ovaries. In human males gametogenesis continues constantly from

puberty. In females the mitotic divisions take place before birth. The meiotic divisions take place in a few oocytes each monthly cycle from puberty to menopause.

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Reproductive anatomy of human male & female

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MALE GAMETOGENESIS

SPERMATOGENESIS SPERMIOGENESIS

• 1st stage: formation of spermatocytes– Spermatogonia are stem cells

• Least differentiated (earliest in the process)

• Lie in basal lamina– Divide continuously by mitosis

(result 2n or diploid): daughter cells A (remains a stem cell) or B (goes on)

– When start to undergo meiosis are by definition called spermatocytes

• 2nd stage: meiosis I– Each primary spermatocytes

(2n) undergoes meiosis I to become 2 secondary spermatocytes:

– Each secondary spermatocyte undergoes meiosis II to become 2 spermatids

– Therefore 4 total spermatids from each spermatogonium

• 3rd stage: spermiogenesis– Spermatids differentiate into

sperm

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Sperm can swim only after they have left the testis Process of spermatogenesis is controlled by two hormones

FSH (follicle stimulating hormones) from anterior pituitary Testosterone

primary male hormone produces by testes

Sperm surrounded by Sertoli (sustenacular) cells: tight junctions • “Nurse” cells of the testes.• Activated by FSH.• Located in convoluted seminiferous tubules.• Main function is to nourish the developing sperm cells.

Interstitial or Leydig cells: secrete androgens Male sex hormones Main one is testosterone Into blood, sustain all male sex characteristic and sex organs

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OOGENESIS

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OogenesisGeneration of eggs

• Starts in fetal period– No more oocytes made after about 7th month– Developed only to early stage of meiosis I by birth and stops (called

primary oocyte)• 6-12 primordial oocytes each cycle selected to develop for ovulation (most die)

– Only then is meiosis I completed– Secondary oocyte is then arrested in meiosis II

• Meiosis II not completed (now an ovum) unless sperm penetrates its plasma membrane

• Of the 4 daughter cells, only one becomes ovum (needs a lot of cytoplasm)– The other 3 become “polar bodies”

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Hormonal Control of Spermatogenesis

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Hormonal Control of Oogenesis

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Spermatogenesis differs from oogenesis in

three ways

– All four products of meiosis develop into

sperm while only one of the four becomes an

egg.

– Spermatogenesis occurs throughout

adolescence and adulthood.

– Sperm are produced continuously without the

prolonged interruptions in oogenesis.

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Ovarian Cycle

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Fertilization After ejaculation into the vagina, sperm swim to meet an egg

Sperm live 5-7 days (need cervical mucus) Eggs live about 12-24 hours, so conception only occurs during this

short window Fertilization occurs in the fallopian tube

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Events leading to fertilization:•Sperm binds to receptors on zona pellucida•Acrosomal reaction – enzymes digest a slit•Sperm passes through zona•Fusion of a single sperm’s plasma membrane with oocyte’s plasma membrane•Cortical reaction: sperm receptors destroyed in zona so no more enter; sperm nucleus engulfed by egg’s cytoplasm

Fertilization occurs at the moment the chromosomes from the male and female gametes unite

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PREVENTION OF POLYSPERMY

Initial days• Cleavage (cell division)• Blastocyst stage by day 4: now in uterus

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Conception, fertilization of an egg by a sperm, occurs in the oviduct.

The resulting zygote begins to divide by mitosis in a process called cleavage.

Division of cells gives rise to a blastocyst, a ball of cells with a central cavity.

After blastocyst formation, the embryo implants into the endometrium

The embryo releases human chorionic gonadotropin (hCG), which prevents menstruation

Pregnancy, or gestation, is the condition of carrying one or more embryos in the uterus

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• Cleavage produces a ball of cells from the zygote– Cleavage

• Rapid succession of cell divisions that produces a ball of cells – a multicellular embryo – from the zygote

• PLANES OF CLEAVAGE:

CLEAVAGE

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TYPES OF CLEAVAGES

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• The process in which a gastrula develops from a blastula by the inward migration of cells

• Gastrulation produces a three-layered embryo• Gastrulation Second major phase of embryonic development;

adds more cells to the embryo; sorts the cells into distinct cell layers.

• "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life."  Lewis Wolpert (1986)• Gastrula

o Three-layered stage of the embryoo Three layers are labeled

1. Ectoderm2. Endoderm3. Mesoderm

GASTRULATION

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• Although the details of gastrulation differ between various groups of animals, the cellular mechanisms involved in gastrulation are common to all animals. Gastrulation involves changes in cell motility, cell shape, and cell adhesion.

• The major types of cell movements that occur during gastrulation are:

Invagination: a sheet of cells (called an epithelial sheet) bends inward.Ingression: individual cells leave an epithelial sheet and become freely migrating mesenchyme cells. Involution: an epithelial sheet rolls inward to form an underlying layer. 

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Epiboly: a sheet of cells spreads by thinning. The movement of epithelial sheets (usually of ectodermal cells) that spread as a unit, rather than individually, to enclose the deeper layers of the embryo.

Intercalation: rows of cells move between one another, creating an array of cells that is longer (in one or more dimensions) but thinner.

Convergent Extension: rows of cells intercalate, but the intercalation is highly directional. 

Delamination. The splitting of one cellular sheet into two more or less parallel sheets

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Gastrula Layers Develop Into..

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Implantation

Blastocyst floats for 2 days: “hatches” by digesting zona enough to squeeze out

6-9 days post conception - burrows into endometrium

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Embryo Development

The embryo and placenta take shape during the first month of pregnancy

GESTATIONPregnancy; the carrying of developing young within the female reproductive tract

AN OVERVIEW OF DEVELOPMENT EVENTSBlastocyst mammalian equivalent of a blastula

Trophoblast outer layer that allows uterine implantation

Placenta combination of trophoblast and maternal cells

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Overall: 1st Week of Development

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2nd Week of DevelopmentDevelopment of Trophoblast

Development of Bilaminar Embryonic disc

Development of Amnion

Development of Yolk sac

Development of Sinusoids

Development of Extra embryonic mesoderm

Development of Chorions

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Gastrulation

Primitive Stalk formation, Primitive Node formation

Trilaminar Disk formation

Notochordal process, Notochord, Archentron

Oropharangeal & Cloacal membrane

NeurulationDevelopment of Chorionic VilliDevelopment of Placenta

3rd Week of Development

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Gastrulation in birds and mammals

During gastrulation in birds and mammals, epiblast cells converge at the midline and ingress at the primitive streak. Ingression of these cells results in formation of the mesoderm and replacement of some of the hypoblast cells to produce the definitive endoderm.

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Development of Placenta

Placenta is an organ that connects the

developing fetus to the uterine wall to

allow –

• Nutrient uptake

• Waste elimination

• Gas exchange via the mother’s blood

supply

• Hormone production

• Formation of a barrier.

Function as a fetomaternal organ with 2

components-

1. Fetal placenta

2. Maternal placenta

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The “Placental Barrier”• Sugars, fats and oxygen diffuse from mother’s blood to fetus

• Urea and CO2 diffuse from fetus to mother

• Maternal antibodies actively transported across placenta

• Some resistance to disease (passive immunity)

• Most bacteria are blocked

• Many viruses can pass including rubella, chickenpox, mono, sometimes HIV

• Many drugs and toxins pass including alcohol, heroin, mercury

• Placental secretion of hormones

• Progesterone and HCG (human chorionic gonadotropin, the hormone tested

for pregnancy): maintain the uterus

• Estrogens and CRH (corticotropin releasing hormone): promote labor

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Extra embryonic membranes

• Amnion protective fluid

• Yolk sac no yolk in humans, but same name; nourishment for non- placentals.

• Chorion become embryos part of placenta Releases human chorionic gonadotropin (hcg) Causes corpus luteum to stay; prevents menstruation

• Allantois functions in waste disposal; will become the umbilical cord