Fertilization, implantation and embryo development

Dr Isameldin Elamin MD DOWH MBBS

Assistant Professor

Obstetrics & Gynaecology

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Oogenesis (the ovum, the egg)

• Primordial follicles undergo sequential development, differentiation, and maturation until a mature graafian follicle is produced. The follicle then ruptures, releasing the ovum.

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Embryonic and fetal development of oogonia

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ovulation

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Schematic representation of the sequence of events occurring in the ovary during a complete follicular cycle.

Uterus becomes receptive

• PROLIFERATIVE PHASE

• The proliferative phase is characterized by endometrial proliferation or growth secondary to estrogenic stimulation

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• SECRETORY PHASE

• Following ovulation, progesterone secretion by the corpus luteum stimulates the glandular cells to secrete glycogen, mucus, and other substances.

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Spermatogenesis (Production of sperm)

• The testis provides the dual function of spermatogenesis and androgen secretion.

• FSH is predominantly responsible for stimulation of spermatogenesis and LH for the stimulation of Leydig cells and the production of testosterone.

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• Spermatogenesis requires about 74 days.

• The average ejaculate contains 2 to 5 mL of semen; 40 to 300 million sperm may be deposited in the vagina.

• Fewer than 200 sperm achieve proximity to the egg. Only one sperm fertilizes a single egg released at ovulation.

The maturation cycle of spermatozoa

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Structure of the mature spermatozoon

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Fertilization• Fertilization, or conception, is the union of

male and female pronuclear elements.

• Conception normally takes place in the fallopian tube, after which the fertilized ovum continues to the uterus, where implantation occurs and development of the conceptus continues.

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Egg surrounded by spermatozoa

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male gamete is sperm contains 23 chromosomes (haploid no)Female gamete is the ovum contains 23 chromosomes. (haploid)Human cell contains 46 chromosomes.(diploid)

Fertilization restores the diploid number of chromosomes and determines the sex of the zygote.

Fertilization and early development

        

 

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I. Fertilization• Path of sperm from the male to the female:   Penis    Vagina   Cervix    Uterus    Fallopian Tube• If an egg is passing down the Fallopian Tube,

fertilization can occur if sperms are present.• The egg secretes a chemical that attracts the

sperm.• After fertilization, membranes around the egg

change to prevent penetration by other sperm.• Sperm nuclei and egg nuclei unite, forming a

zygote ( with 46 chromosomes).

Egg

Sperms

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Cleavage, Morula, Blastocyst

• Following fertilization, cleavage occurs That produce a mulberry-like mass known as a morula.

• Fluid is secreted by the outer cells of the morula, and a single fluid-filled cavity develops, known as the blastocyst cavity.

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Implantation

• Implantation

• The fertilized ovum reaches the endometrial cavity about 3 days after ovulation.

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Early stage of implantation

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Ectoderm Mesoderm Endoderm

•Nervous system.

•Lining of mouth, nostrils, anus

•Epidermis, hair, nails.

•Bones and muscles.

•Blood and blood

vessels.

•Reproductive and

excretory systems.

•Dermis.

•Digestive tract.

•Respiratory system.

•Liver, pancreas.

•Glands.

•Gall bladder.

Germ layers of the embryo

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IV. Control of Development

• The nucleus contains DNA and all cells have the same DNA.

• DNA controls all cell processes. 

 •Control of development involves an interaction between Cytoplasm and DNA.•The path of development is determined by the gastrula stage.

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Placenta

•The nutrients, oxygen, and waste diffuse between the mother’s and the Fetus’ circulatory system by the placenta.

•The placenta is attached to the wall of the uterus.

•The embryo is attached to the placenta by the umbilical cord.

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V. Nourishment

• Before implantation, the embryo gets food from the cytoplasm of the egg and the secretions in the uterine cavity.

• After implantation, the embryo gets food from the mother’s blood through the placenta.

• Nutrients are transferred from Mother to the Fetus by diffusion through the placenta.

• NOTE: The blood of Mother and Fetus do not mix.

• The amniotic sac contains amniotic fluid to protect the Fetus , to allow movement and help in proper growth.22

Development of the fetus

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Quickening• Definition: The first movement of the fetus felt by the

mother, usually occurring during the fourth or fifth month of pregnancy is called “quickening”.

• By seventh month the fetus is quite active.

• During the last month the fetus becomes less active (usually due to less space inside the uterus to move around).

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• Q: If DNA controls cell activities and all cells of an organism contain the same DNA, how are so many different cells made?

• A: Different sections of DNA are turned on and off to make different cells.

THANK YOU

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