Embryology

1. Embryology

2. Textbooks • The Developing Human by Kieth L.more • Human Embryology by William Larsen • Clinical Embryology by Richard Snell • Medical Embryology Langman’s 10th edition by T.W Sadler

3. Terminology • Embryology is the study of development from fertilized egg through the eighth week in utero • Developmental Anatomy is the study of development from fertilized egg to adult form • Development is a process begins with an oocyte (ovum) is fertilized by a spermatozoa (sperm).and ends at death. • Teratology is the study abnormal development (congenital malformations)

4. Pre-embryonic period : first 3 weeks after fertilization • Embryonic period ; begins 4 weeks after fertilization to the 8th week. • Fetal period: from the 9th week until birth

5. Prenatal period • in which the important changes occur before birth : • Oocyte : female germ cell (ovum) • Zygote : the resultant cell of fertilization and the beginning of the human being. • Cleavage : Mitotic divisions of the zygote. • Morula : solid ball of cells (16 or more blastomers) • Morula turns into Blastocyst in the uterus. • Embryo : composed of embryoblasts that forms the Bilaminar disc and extends to the 8th week. • Fetus : from the 9th week to birth (fetal period)

6. Postnatal period • Changes occur after birth : • Infancy : • from after birth till the first year • Newborn (neonatal) is in the first 2 weeks • The body grows rapidly during infancy • Length increase by about 50% • Weight is tripled

7. Childhood : • The period from 15 months to 12-13 years • Teeth are replaced by permanent ones • Active ossification of the bones • Puberty: • In girls between 12-15 • In boys between 13-16 • Secondary sexual characteristics develop

8. Adolescence: • The period of 3-4 years after puberty • After sexual maturity until the attainment of physical, mental, and emotional maturity. • Adulthood ; • Early adulthood 18-25 years in which ossifications and growth is virtually completed • After that developmental changes occur very slowly leading to Senility (old age).

9. Human Development

10. Development begins with fertilization • the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. • Gametes are derived from primordial germ cells • are formed in the epiblast during the second week and move to the wall of the yolk sac

11. During the fourth week these cells begin to migrate from the yolk sac toward the developing gonads and arrive at the fifth week • Mitotic divisions increase their number during their migration and also when they arrive in the gonad. • In preparation for fertilization, germ cells undergo gametogenesis • which includes meiosis, to reduce the number of chromosomes and cytodifferentiation to complete their maturation

12. Overview of the Reproductive System

13. Reproductive system • In men • The reproductive system in men has components in the abdomen, pelvis, and perineum • The major components are a testis, epididymis, ductus deferens, and ejaculatory duct on each side • and the urethra and penis in the midline • In addition, three types of accessory glands are associated with the system: • a single prostate; • a pair of seminal vesicles; and • a pair of bulbourethral glands. • The design of the reproductive system in men is basically a series of ducts and tubules

14. Testes • The testes originally develop high on the posterior abdominal wall and then descend, normally before birth, through the inguinal canal • The spermatic cord is the tube-shaped connection between the pouch in the scrotum and the abdominal wall. • Each testis is composed of seminiferous tubules (400-600) and interstitial tissue surrounded by a thick connective tissue capsule (the tunica albuginea). • Spermatozoa are produced by the seminiferous tubules

15. Epididymis • The epididymis is a single, long coiled duct that courses along the posterolateral side of the testis • During passage through the epididymis, spermatozoa acquire the ability to move and fertilize an egg • The epididymis also stores spermatozoa until ejaculation. The end of the epididymis is continuous with the ductus deferens.

16. The ductus deferens is a long muscular duct that transports spermatozoa from the tail of the epididymis in the scrotum to the ejaculatory duct in the pelvic cavity • Each seminal vesicle is an accessory gland of the male reproductive system • Secretions from the seminal vesicle contribute significantly to the volume of the ejaculate (semen).

17. The prostate is an unpaired accessory structure of the male reproductive system that surrounds the urethra in the pelvic cavity • Secretions from the prostate, together with secretions from the seminal vesicles, contribute to the formation of semen during ejaculation. • The bulbourethral glands one on each side, are small, pea-shaped mucous glands • the bulbourethral glands contribute to lubrication of the urethra and the pre-ejaculatory emission from the penis.

18. Urethra • The urethra begins at the base of the bladder and ends with an external opening in the perineum • In men, the urethra is long, about 20 cm, and bends twice along its course • The urethra in men is divided into preprostatic, prostatic, membranous, and spongy parts. • Preprostatic part is about 1 cm long, extends from the base of the bladder to the prostate

19. The prostatic part of the urethra is 3-4 cm long and is surrounded by the prostate • On each side of the prostatic urethra is the opening of the ejaculatory duct of the male reproductive system. • Therefore, the connection between the urinary and reproductive tracts in men occurs in the prostatic part of the urethra. .

20. The membranous part of the urethra is narrow and passes through the deep perineal pouch • During its transit through this pouch, the urethra, in both men and women, is surrounded by skeletal muscle of the external urethral sphincter • The spongy urethra is surrounded by erectile tissue (the corpus spongiosum) of the penis • The two bulbourethral glands in the deep perineal pouch are part of the male reproductive system and open into the bulb of the spongy urethra

21. Reproductive system • In women • The reproductive tract in women is contained mainly in the pelvic cavity and perineum, although, during pregnancy, the uterus expands into the abdomen • Major components of the system consist of: • an ovary on each side; and • a uterus, vagina, and clitoris in the midline • In addition, a pair of accessory glands (the greater vestibular glands) are associated with the tract.

22. Ovaries • Like the testes in men, the ovaries develop high on the posterior abdominal wall and then descend before birth • Unlike the testes, the ovaries do not migrate through the inguinal canal into the perineum, but stop short and assume a position on the lateral wall of the pelvic cavity • The ovaries are the sites of egg production (oogenesis). • Mature eggs are ovulated into the peritoneal cavity and normally directed into the adjacent openings of the uterine tubes by cilia on the ends of the uterine tubes. • Each of the two almond-shaped ovaries is about 3 cm long and is suspended by a mesentery (the mesovarium) from the posterior aspect of the broad ligament. • The broad ligament is a sheet-like fold of peritoneum

23. Uterus • The uterus is a thick-walled muscular organ in the midline between the bladder and rectum • It consists of a body and a cervix, and inferiorly it joins the vagina • Superiorly, uterine tubes project laterally from the uterus and open into the peritoneal cavity immediately adjacent to the ovaries. • has a rounded superior end (fundus of uterus). • Implantation of the blastocyst normally occurs in the body of the uterus. • During pregnancy, the uterus dramatically expands superiorly into the abdomen.

24. Endometrium • It is the inner layer of the uterus ; • Composed of : • Compact layer • Thick spongy layer • Basal layer ( has its own blood supply) • The compact and spongy layers are the functional ones and are shed during menses. • Menarche : it is the first time menses occurs (age of puberty) • Menopause : is a variable period in which the cyclic changes become irregular and disappear , the age of menopause is around (45-55 years ) • Between the menarche and menopause the genital system undergoes cyclic changes in structre and functional activity controlled by neurohormonal mechanisms.

25. Uterine tubes (Fallopian) • The uterine tubes extend from each side of the superior end of the body of the uterus • Because the ovaries are suspended from the posterior aspect of the broad ligaments, the uterine tubes pass superiorly over, and terminate laterally to, the ovaries. • Each uterine tube has an expanded trumpet-shaped end (the infundibulum), .

26. The margin of the infundibulum is rimmed with small finger-like projections termed fimbriae • Medial to the infundibulum, the tube expands to form the ampulla and then narrows to form the isthmus, before joining with the body of the uterus. • The fimbriatedinfundibulum facilitates the collection of ovulated eggs from the ovary. Fertilization normally occurs in the ampulla

27. urethra • In women, the urethra is short, being about 4 cm long • The inferior aspect of the urethra is bound to the anterior surface of the vagina • Two small paraurethral mucous glands (Skene's glands) are associated with the lower end of the urethra

28. The Chromosomes • Traits of a new individual are determined by specific genes on chromosomes inherited from the father and the mother. • Humans have approximately 35,000 genes on 46 chromosomes • Genes on the same chromosome tend to be inherited together and so are known as linked genes • In somatic cells, chromosomes appear as 23 homologous pairs to form the diploid number of 46 (2n)

29. There are 22 pairs of matching chromosomes, the autosomes, and one pair of sex chromosomes. • If the sex pair is XX, the individual is genetically female • if the pair is XY, the individual is genetically male • One chromosome of each pair is derived from the maternal gamete, the oocyteand one from the paternal gamete, the sperm • Thus each gamete contains a haploid number of 23 chromosomes (1n) • the union of the gametes at fertilization restores the diploid number of 46.

30. MITOSIS • Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell • Each daughter cell receives the complete complement of 46 chromosomes • Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). • During this replication phase the chromosomes are extremely long • they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope

31. Prophase • With the onset of mitosis the chromosomes begin to coil, contract, and condense • these events mark the beginning of prophase. • Each chromosome now consists of two parallel subunits, chromatids • that are joined at a narrow region common to both called the centromere. • Throughout prophase the chromosomes continue to condense, shorten, and thicken • only at prometaphase do the chromatids become distinguishable

32. Metaphase • During metaphase the chromosomes line up in the equatorial plane, and their doubled structure is clearly visible • Each is attached by microtubules extending from the centromere to the centriole, forming the mitotic spindle

33. Anaphase and Telophase • Soon the centromere of each chromosome divides, marking the beginning of anaphase • followed by migration of chromatids to opposite poles of the spindle • Finally, during telophase, chromosomes uncoil and lengthen, • the nuclear envelope reforms, and the cytoplasm divides • Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell (2n).

34. Mitosis

35. MEIOSIS • Meiosis is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells • Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23 (1n)

36. As in mitosis, male and female germ cells (spermatocytes and primary oocytes) at the beginning of meiosis I replicate their DNA • so that each of the 46 chromosomes is duplicated into sister chromatids • In contrast to mitosis, however, homologous chromosomes then align themselves in pairs, a process called synapsis • The pairing is exact and point for point except for the XY combination

37. Crossover • Crossovers, critical events in meiosis I, are the interchange of chromatid segments between paired homologous chromosomes • Segments of chromatids break and are exchanged as homologous chromosomes separate. • As separation occurs, points of interchange are temporarily united and form an X-like structure, a chiasma • The approximately 30 to 40 crossovers (one or two per chromosome) with each meiotic I division are most frequent between genes that are far apart on a chromosome.

38. Homologous pairs then separate into two daughter cells • Shortly thereafter meiosis II separates sister chromatids. • Each gamete then contains 23 chromosomes 1n.

39. Meiosis

40. Meiosis • As a result of meiotic divisions, • genetic variability is enhanced through crossover, which redistributes genetic material • genetic variability is also enhanced through random distribution of homologous chromosomes to the daughter cells • each germ cell contains a haploid number of chromosomes • so that at fertilization the diploid number of 46 is restored