general embryology dentistry- 1 (1).pdf

General Embryology Embryology refers to the study of embryos; the term generally means prenatal development of embryos, fetuses, and neonates. SIGNIFICANCE OF EMBRYOLOGY Bridges the gap between prenatal development and obstetrics, perinatal medicine, pediatrics, and clinical anatomy Develops...

General Embryology Embryology refers to the study of embryos; the term generally means prenatal development of embryos, fetuses, and neonates. SIGNIFICANCE OF EMBRYOLOGY Bridges the gap between prenatal development and obstetrics, perinatal medicine, pediatrics, and clinical anatomy Develops knowledge concerning the beginnings of life and the changes occurring during prenatal development Builds an understanding of the causes of variations in human structure Supports the research and application of stem cells for treatment of certain chronic diseases DEVELOPMENTAL PERIODS 1-Prenatal period: a)Embryonic period: from fertilization to the end of the 8th week. b)Fetal period: from the end of the 8th week to birth. 2-Postnatal period: a-Newborn or neonate: period of the first four weeks (first month) of postnatal life. b-Infancy: first year of postnatal development. c-Childhood: period between infancy and puberty. d-Puberty: 12-15 years in girls and 13-16 years in boys. Development of primary and secondary sex characters is apparent. In females it presents with menstrual cycle, and in males by production of mature spermatozoa. e-Adulthood: Attainment of full growth and maturity is generally reached between the ages of 18 and 21 years General embryology Female genital system The uterus A thick-walled, pear-shaped muscular organ 3x2x1 Three parts: fundus, body and cervix The walls of the body of the uterus consist of three layers ❑Perimetrium, the thin external layer ❑Myometrium, the thick smooth muscle layer ❑Endometrium, the thin internal layer Uterine Tubes The uterine tubes, approximately 10 cm The uterine tube is divided into four parts: infundibulum, ampulla, isthmus, and uterine part.  Ovaries The ovaries are almond-shaped reproductive glands located close to the lateral pelvic walls on each side of the uterus. The primary sex organ in female The ovaries produce oocytes and estrogen and progesterone, the hormones responsible for the development of secondary sex characteristics and regulation of pregnancy  OOGENESIS Definition: Oogenesis is the sequence of events by which oogonia (primordial germ cells) are transformed into mature oocytes. Before Birth Once primordial germ cells have arrived in the gonad of a genetic female, they differentiate into oogonia. These cells undergo a number of mitotic divisions and, by the end of the third intrauterine month, are arranged in clusters surrounded by a layer of flat epithelial cells. The flat epithelial cells are known as follicular cells, originate from surface epithelium covering the ovary. The majority of oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis I and form primary oocytes.  During the next few months, oogonia increase rapidly in number, and by the fifth month of prenatal development, the total number of germ cells in the ovary reaches its maximum, estimated at 7 million. At this time, cell death begins, and many oogonia as well as primary oocytes become atretic. By the seventh month, The majority of oogonia have degenerated. All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat epithelial cells. A primary oocyte, together with its surrounding flat epithelial cells, is known as a primordial follicle. Maturation of Oocytes Continues at Puberty - Near the time of birth, all primary oocytes have started prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase The fact that the risk of having children with chromosomal abnormalities increases with maternal age indicates that primary oocytes are vulnerable to damage as they age..  At puberty, a pool of growing follicles is established and continuously maintained from the supply of primordial follicles. Each month, 15 to 20 follicles selected from this pool begin to mature.