Human Embryology Lecture Notes PDF

HUMAN EMBRYOLOGY MOHAMED ELADL Visit the Facebook page for questions and discussions  Objectives Understand and explain the process of gametogenesis, including the formation of male and female game...

HUMAN EMBRYOLOGY MOHAMED ELADL Visit the Facebook page for questions and discussions  Objectives Understand and explain the process of gametogenesis, including the formation of male and female gametes. Outline the stages of spermatogenesis, including spermatogonia multiplication, meiotic divisions, and spermiogenesis. Explain the stages of oogenesis, including prenatal and postnatal maturation of oocytes. Identify the differences in structure, cytoplasm content, motility, and chromosomal composition between spermatozoa and ova. Describe the sequence of events during fertilization, including capacitation, acrosome reaction, and the formation of the zygote. Identify common fertilization errors, such as dispermy, superfecundation, and superfetation, and explain their genetic implications. Utilize knowledge of spermatogenesis and oogenesis to analyze & solve clinical problems related to fertility, genetic disorders, & embryonic development.  GAMETOGENESIS Definition: It is the process of formation of gametes It occurs only in gonads Characters: 1. Preparation of gametes for fertilization 2. Reduction of number of chromosomes into haploid number (23 chromosome).  GAMETOGENESIS Gametogenesis includes 2 processes: Spermatogenesis: Formation of sperms in the testis. Oogenesis: Formation of the ovum in the ovary.  Spermatogenesis Definition: It is the process of formation of mature sperms. Occurs in seminiferous tubules in the testis. Duration: 2 months (roughly 64 days). Onset: From puberty (13 –16 Y) onwards.  Spermatogenesis Stages: 2 stages: 1) Cell growth & multiplication (Spermatogenesis proper). 2) Cell metamorphosis (Spermiogenesis).  Spermatogenesis Spermatogenesis proper: 1) The spermatogonia (44 + X + Y) divide mitotically, to give rise to more spermatogonia (DAUGHTER CELLS). 2) The spermatogonia grow & enlarge to form PRIMARY SPERMATOCYTEs. 3) The primary spermatocyte (44 + X + Y) divides by the first meitoic division into two SECONDARY SPERMATOCYTES, each cell contains half the number of chromosomes but one with 22 + X and the other with 22 + Y. 4) Each secondary spermatocytes has 22 + X or 22 + Y chromosomes. It divides by second (Yan et al., 2022) meiotic division to form two SPERMATIDS.  Spermatogenesis Spermiogenesis Morphological changes transform the spermatids to mature sperms. The spermatids become attached to the Sertoli cells and undergo a series of morphological (metamorphic) changes. 1) The nucleus: enlarges and forms the head of the sperm. 2) The golgi apparatus comes to lie over the anterior 2/3 of the nucleus to form the head cap or acrosomal cap of the sperm. 3) The mitochondria & centrioles: come to lie on the other pole of the nucleus and an axial filament originates from the anterior centriole to form the core of the body and tail. 4) The mitochondria collect around the axial filaments to form the spiral sheath. 5) Most of the cytoplasm disappears. (Yan et al., 2022) 6) The cell membrane: encloses the whole sperm.  Spermatogenesis The approximate 64 day cycle of the spermatogenesis can be subdivided into four phases that last differing lengths of time: Mitosis of the 16 days Up to the spermatogonia primary spermatocytes First meiosis 24 days For the division of the primary spermatocytes to form secondary spermatocytes Second meiosis A few hours For engendering the spermatids Spermiogenesis 24 days Up to the completed sperm cells Total 64 days  Oogenesis Definition: It is the process of formation of mature ovum. Occurs in the outer cortex of the ovary. Stages: 1) Prenatal maturation of oocytes (formation of primary oocyte). 2) Postnatal maturation of oocytes (formation of mature ovum).  Prenatal maturation of oocytes (Primary Oocyre Formation) 1) OOGONIA (Lie in the cortex of the ovary) Each contains 22 pairs of chromosomes and one pair of sex chromosomes (two X- chromosomes). 2) The daughter oogonia enlarge to form THE PRIMARY OOCYTE. Each primary oocyte is surrounded by a single layer of flat cells to form the primordial follicles. 3) The primary oocyte starts the first meiotic division before birth but remains in suspended prophase till puberty.  Postnatal maturation of oocytes (Mature Ovum Formation) ❖ AT PUPERTY: 5 : 12 primary oocytes resumes the first meiotic division few hours before ovulation to form two haploid cells: - A large cell called SECONDARY OOCYTE. - A small cell called first polar body. ❖ AT OVULATION: The 2ry oocyte undergoes the 2nd meiotic division but arrested in the metaphase till fertilization  Postnatal maturation of oocytes (Mature Ovum Formation) ❖ AT FERTILIZATION: The sperm penetrates the 2ry oocyte and it completes the 2nd meiotic division from two cells: - A large cell called mature ovum. - A small cell called the second polar body.  Postnatal maturation of oocytes (Mature Ovum Formation) 2 million 1ry oocytes in the ovaries of newborn females. Only 40000 primary oocytes during adolescence, Only about 480 become 2ry oocytes and are expelled at ovulation during reproductive life.  Comparison between male & female gametes Female gametes Male gametes No primary oocytes after Production of 1ry birth sparmatocyts after puberty The mature oocyte is The sperm is highly motile. immotile cell. The mature oocyte is a The sperm have a little massive cell. cytoplasm. The cytoplasm of the mature The cytoplasm of the sperm oocyte contains yolk granules contains no yolk granules. which provide nutrition to the dividing zygote during the 1st week of development. There is one type of ovum: There are two types of normal 22 + X. sperm: 22 + X and 22 + Y.  Prenatal periods Intrauterine life Definition: It is the period between the time of fertilization and the delivery. Duration: 280 days from the first day of the last normal menstrual cycle (10 lunar months); or 266 days from fertilization.  Phases of Prenatal periods Intrauterine life The prenatal periods divided into 3 phases: 1. The germinal phase: The first 3weeks after fertilization. 2. The embryonic phase: From the 4th to 8th weeks of development. 3. The fetal phase: From the 9th to birth.  First Week Two important events occur in the 1st week: 1. Fertilization. 2. Cleavage.  Fertilization Definition: It is the process of meeting and union between the sperm and the ovum to form the zygote. Site: Usually in the AMPULLA OF THE UTERINE TUBE, which is the widest part of the tube and is located close to the ovary.  Process of Fertilization 1)200 to 300 million sperms deposited in the female genital tract 2)Only 200 reach the site of fertilization 3)Only one sperm is capable of fertilization of the ovum (THE FERTILIZING SPERM) 4)The other sperms help the fertilizing sperm by digesting the membrane, which surround the oocyte and so, FACILITATE THE ENZYMATIC REACTIONS OF THE FERTILIZING SPERM. 5)The sperm can stay alive in the female genital tract for about 2 days. 6)The ovum dies within 12 to 24 hours after ovulation. 7)The sperms pass rapidly from the vagina to the uterus & then to the uterine tube by the contractions of muscles of the uterus & uterine tube is the process of meeting and union between the sperm and the ovum to form the zygote.  Process of Fertilization For fertilization of the ovum, the sperms must undergo 2 processes: A. Capacitation. B. Acrosome reaction Capacitation: SITE: In the cervix of the uterus DURATION: 7 hours in human. PROCESS: The seminal plasma proteins are removed from the plasma membrane that overlies the head region of the spermatozoa  Process of Fertilization Acrosome reaction: SITE: In the immediate surrounding area of the oocyte PROCESS: The following substances are released: 1. Hyaluronidase: For penetration of the corona radiata. 2. Trypsin like substances: For digestion of the zona pellucida. 3. Zona lysin: help the spermatozoa to cross the zona pellucida.  Fertilization Phases 1. Passage of sperm through corona radiata (By enzyme hyaluronidase). 2. Penetration of zona pellucida 3. Zona reaction: Once the sperm penetrates the zona pellucida, a zonal reaction occurs that makes the zona pellucida impermeable to other sperms results from the action of lysosomal enzymes released by the oocyte  Fertilization Phases 4.Fusion of plasma membranes of the oocyte and sperm. The head and tail of the sperm enter the cytoplasm of the oocyte but the plasma membrane of the sperm remains behind.  Fertilization Phases 5. Formation of male pronucleus. The nucleus of the sperm enlarges to form the male pronucleus and the tail of the sperm degenerates. 6. Union of the 2 pronuclei: The membranes of pronuclei break down and the chromosomes condense and become arranged for a mitotic division. The combination of 23 chromosomes in each pronucleus results in a ZYGOTE WITH 46 CHROMOSOMES.  Results of Fertilization 1. Stimulates the secondary oocyte to complete the 2nd meiotic division. 2. Restores the diploid number of chromosomes (46) in the zygote. 3. Determination of the sex of the embryo. 4. Determination of characters and the state of health and disease of the embryo. 5. Initiation of cleavage.  Fertilization Errors 1. Superimposed pregnancy: Ovulation and fertilization occurs during an early established pregnancy ✓ Superfetation:. Involves two embryos that form during two separate menstrual cycles ✓ Superfecundation: Two or more eggs during the same menstrual cycle. Each egg is fertilized by sperm, 2. Dispermy and triploidy Two sperms may participate in fertilization, resulting in a zygote with an extra set of chromosomes i.e. 69 chromosomes. 3. Parthenogenesis: Unfertilized oocyte is activated to cleavage WITHOUT SPERM PENETRATION and development may start. Parthenogenesis Please Study this lecture today