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The Physiology of conception. Yasser Orief M.D. Lecturer of Obstetrics & Gynecology, Alexandria University Fellow, L ϋ beck University, Germany DGOL, Auvergné University, France. Hypothalamo – Pituitary – Gonadal axis. The Reproductive System. Gonads – primary sex organs
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The Physiology of conception Yasser Orief M.D. Lecturer of Obstetrics & Gynecology, Alexandria University Fellow, Lϋbeck University, Germany DGOL, Auvergné University, France
The Reproductive System • Gonads – primary sex organs • Testes in males • Ovaries in females • Gonads produce gametes (sex cells) and secrete hormones • Sperm – male gametes • Ova (eggs) – female gametes
Folliculogenesis • It begins by the recruitment of a cohort of primordial follicles into a pool of growing follicles and ends by either ovulation or atresia. It requires almost one year for a primordial follicle to grow and develop to the ovulatory stage.
Gougeon A: Dynamics of follicular growth in the human: A model from preliminary results. Hum Reprod 1:81, 1986. Reproduced with permission from Oxford University Press.)
Folliculogenesis, starting from pre-antral (late secondary) stage.
Menstrual (Uterine) Cycle • Cyclic changes of the endometrium • Regulated by cyclic production of estrogens and progesterone • Stages of the menstrual cycle • Menses – functional layer of the endometrium is sloughed • Proliferative stage – regeneration of functional layer • Secretory stage – endometrium increases in size and readies for implantation
Stages of Pregnancy and Development • Fertilization • Embryonic development • Fetal development • Childbirth
Fertilization • The oocyte is viable for 12 to 24 hours after ovulation • Sperm are viable for 12 to 48 hours after ejaculation • Sperm cells must make their way to the uterine tube for fertilization to be possible
Fertilization • Divided into 4 steps: • 1. Contact and recognition (“Casual Introductions”) • Sperm undergo capacitation (further maturation) within the female reproductive tract • Recall that sperm were produced in the testes and matured in the epididymus until ejaculation • Sperm reach the egg in the oviduct where fertilization will occur
2. Sperm Entry • Only ONE sperm is allowed to enter • Fast block - electrical charge in egg plasma membrane prevents polyspermy • Slow block - depolarization of egg plasma membrane due to Ca++ release
3. Egg Activation • The release of calcium ions in egg plasma membrane also triggers protein synthesis • 4. Fusion • The sperm nucleus is propelled to the egg nucleus by microtubules
Cleavage follows fertilization • Cleavage is a series of rapid mitotic divisions (without cell growth) • The two-celled zygote divides repeatedly until a ball of 32 cells is formed • This is the morula - 32 cells
Continued divisions make the hollow blastula • These few cells are pluripotent (have the potential to become ANY of the 220 types of cells in the human body). • These are embryonic stem cells
Gastrulation • At the end of the cleavage stage, cells making up the blastula move about and surface proteins help cells recognize each other • The gastrula is formed, which consists of 3 “germ layers” • Endoderm • Mesoderm • Ectoderm
Extraembryonic Membranes • These membranes develop from the germ layers, but are NOT part of the embryo (they are lost at birth) • They lie outside of the embryo & provide protection and nourishment • Four exist in terrestrial vertebrates: • Chorion, amnion, allantois (stores nitrogenous waste in reptiles), & yolk sac (not found in humans, but yolk aids in formation of RBC
Human Development • The gestation period lasts 266 days from fertilization to birth • Organogenesis (development of the organs and organ systems) begins with the nervous system
