Gametogenesis

• Anterior/Cranial – front end

  Posterior/Caudal – rear/tail end

  Dorsal – upper surface

  Ventral – lower surface
• Transverse section – a section taken across the long axis of the animal; divides the animal into anterior and posterior parts
• Longitudinal section – a section parallel to the long axis
• Vertical longitudinal section - divides the animal into left and right parts

  • Sagittal section – a vertical longitudinal section taken at the midline
  • Parasagittal section – a vertical longitudinal section but not taken at the midline (beside the midline or any other parallel area
• Horizontal longitudinal section – also called frontal section; divides the animal intro dorsal and ventral parts
• 4 Phases of Gametogenesis

  1. the extraembryonic origin of the germ cells and their migration into the gonads
  2. an increase in number of germ cells by mitosis
  3. reduction in chromosomal number by meiosis
  4. structural and functional maturation of eggs and spermatozoa
• Oogenesis

  • Phase 1: Origin and Migration of Germ Cells
  • Primordial germ cells are the precursors of gametes that arise outside the gonads, then migrate into the gonads during early embryonic development
  • Germ cells exit from the yolk sac → hindgut → dorsal mesentery → primordia of the gonads
• Oogenesis

  • Phase 2: Increase in number of germ cells by mitosis
  • The gametes are formed from germ cells in the embryo; these germ cells eventually become oogonia (which continues mitosis for a certain period)
• Oogenesis

  • Phase 3: Reduction in chromosomal number of meiosis
  • Meiosis in females is different than in males
• Meiosis in Females

  • As the oogonia enter meiosis I, they are called primary oocytes
  • The primary oocytes proceed to the leptotene, zygotene, and pachytene stages of prophase I
  • However, meiosis is arrested at diplotene. It is during this time the primary oocyte prepares for the needs of the embryo
  • All primary oocytes remain arrested in diplotene until puberty
  • During the reproductive years, small numbers of primary oocytes complete meiosis I with each menstrual cycle
• Meiosis in Females

  • When meiosis I has been completed, there is unequal distribution of the cytoplasm resulting to the formation of secondary oocyte (more cytoplasm) and the first polar body
  • The secondary oocyte enters meiosis II, but gets arrested again during metaphase. This arrest can be lifted when a sperm enters and fertilizes the egg
  • When the secondary oocyte gets fertilized by a sperm, it will then complete meiosis II
  • Upon completion of meiosis II, unequal cytoplasmic division occurs again, thereby forming the fertilized egg (ovum) and second polar body
• Meiosis in Females

  • Ovulation refers to the resumption of the meiotic divisions and the release of the oocyte from the ovary.
  • It is provoked by hormonal stimulation and involves a breakdown of the oocyte nucleus (the germinal vesicle)
• Oogenesis

  • Phase 4: Development of the Follicle
  • The egg, along with its surrounding cells, is called a follicle
  • The development of the egg and its surrounding follicular cells is an integrated unit
  • In the embryo, the oogonia are naked. After meiosis begins, cells from the ovary partially surround the primary oocytes to form primordial follicles
• Oogenesis

  • Phase 4: Development of the Follicle
  • By birth, the primary oocytes become completely surrounded by a layer of follicular cells. The primary oocyte and the follicular cells constitute the primary follicle
  • Arrest at diplotene of prophase I occurs because of high concentration of cAMP in the cytoplasm of the oocyte
  • This cAMP is intrinsically produced by the oocyte, and the follicular cells also produce cAMP that are transported into the oocyte
• Development of the Follicle

  • A prominent, translucent membrane called zona pellucida forms between the primary oocyte and its enveloping follicular cells
  • The zona pellucida also contains sperm receptors and other components important in fertilization and early post-fertilization development
  • An oocyte with more than one layer of granulosa cells is called a secondary follicle
  • A basement membrane called membrana granulosa surrounds the epithelial granulosa cells of the secondary follicle
• Development of the Follicle

  • An additional set of outer coverings derived from the ovarian connective tissue (called stroma) begins to form around the developing follicle after it has become 2-3 cell layers thick. This is initially called theca folliculi
  • The theca folliculi eventually differentiates into:
  • theca interna – highly vascularized and glandular
  • theca externa - more connective tissue-like outer capsule
• The early thecal cells secrete an angiogenesis factor which stimulates growth of blood vessels that serves as nutritive support of the follicle
• Identify the parts of an oocyte

  A) zona pellucida

  B) theca interna

  C) theca externa

  D) nucleus

  
• Development of the Follicle

  • By puberty, the granulosa cells have developed receptors for follicle stimulating hormone (FSH). When FSH binds to the FSH receptors, the granulosa cells produce estrogen
  • An indication that follicles has al;ready further developed in the formation of antrum, a cavity filled with fluid called liquor folliculi
  • Formation of the antrum divides the follicular cells into two groups:
  • cumulus cells – cells that immediately surround the oocyte
  • mural granulosa cells – cells between the antrum and membrane granulosa
• Development of the Follicle

  • On the other hand, the cells of theca interna have developed receptors for luteinizing hormone (LH), thereby stimulating the theca interna cells to secrete androgens
  • The FSH induces granulosa cells to produce the enzyme aromatase, which then converts the androgens into estroge
• Development of the Follicle

  • Follicle enlarges rapidly and at this point the follicle is now called tertiary (graafian follicle)
• Development of the Follicle

  • Recall that the primary follicle has been arrested in diplotene of prophase I. Because of the LH surge, the primary follicle resumes meiosis, thus forming the secondary oocyte
  • Now, the secondary oocyte is located in a small mount of cells called cumulus oopherous which lies on one side of the greatly enlarged antrum
• Development of the Follicle

  • Factors secreted by the oocyte pass through the surrounding cumulus cells and stimulate them to secrete hyaluronic acid
  • The hyaluronic acid binds water molecules and enlarges the intracellular spaces, thereby expanding the cumulus oopherous
• Identify the parts of an frog ovary

  A) primordial follicles

  B) theca

  C) vitelline membrane

  D) cytoplasm

  E) nucleus

  
• Identify the parts of the ovary

  A) secondary follicle

  B) zona pellucida

  C) atretic follicle

  D) primary follicles

  E) primordial follicles

  
• Identify the parts of the primary follicle

  A) theca externa

  B) theca interna

  C) granulosa cells

  D) primary oocyte

  E) zona pellucida

  F) antral cavity

  G) atretic follicle

  
• Identify the parts of a secondary follicle

  A) zona pellucida

  B) secondary follicle

  C) antrum

  D) cumulus oopherus

  E) oocyte

  F) granulosa cells

  G) theca interna

  
• Identify the parts of a primary oocyte

  A) theca

  B) granulosa

  C) antrum

  D) corona radiata

  E) primary oocyte

  F) cumulus oopherus

  
• Identify the parts of a primary follicle in a cat ovary

  A) nucleus

  B) theca

  C) zona pellucida

  
• Identify the parts of a secondary follicle in a cat ovary

  A) secondary oocyte

  B) secondary follicle

  C) zona pellucida

  D) theca

  E) antrum

  F) granulosa cells

  
• Identify the parts of a secondary follicle in a cat ovary

  A) theca externa

  B) theca interna

  C) granulosa cells

  D) secondary follicle

  E) antrum

  F) secondary oocyte

  G) cumulus oopherus

  H) corona radiata

  
• Spermatogonia

  • Phase 1: Origin and Migration of Germ Cells
  • Primordial germ cells are the precursors of gametes that arise outside the gonads, then migrate into the gonads during early embryonic development
  • Germ cells exit from the yolk sac → hindgut → dorsal mesentery → primordia of the gonads
• Spermatogonia

  • Phase 2: Increase in number of germ cells by mitosis
  • The gametes are formed from germ cells in the embryo; these germ cells eventually become spermatogonia (which continues mitosis for a certain period)
• Spermatogonia

  • Phase 3: Reduction in chromosomal number by meiosis
  • Meiosis in males is different than in females
• Meiosis in Males

  • Begins in the seminiferous tubules of the testes during puberty
  • At the base of seminiferous epithelium are large numbers of spermatogonia
  • Type A spermatogonia – the stem cell population that mitotically maintains proper number of spermatogonia throughout life of a male
  • ) Type B spermatogonia – arise from type A spermatogonia, and are destined to enter meiosis
• Meiosis in Males

  • Entry to meiosis by spermatogonia is stimulated by retinoic acid. Spermatogonia that has entered meiosis I are now called primary spermatocytes
  • The Sertoli cells sre regularly distributed throughout the periphery of the seminiferous epithelium. They support progression of spermatogonia and nourish them
  • After meiosis I, a primary spermatocyte gives rise to 2 secondary spermatocytes, which will immediately enter meiosis II
• Meiosis in Males

  • After meiosis II, the secondary spermatocytes give rise to 4 immature haploid spermatids
  • Spermatids undergo a series of changes that transform them into highly specialized spermatozoa (sperm)—this process is called spermiogenesis
• The blood-testis barrier is an immunological barrier between the sperm cells and the rest of the body, created by the processes of Sertoli cells
• The blood-testis barrier is an immunological barrier between the sperm cells and the rest of the body, created by the processes of Sertoli cells
• Spermatogonia

  • Phase 4: Events during Spermatogenesis
  • Progressive reduction in size of nucleus and tremendous condensation of chromosome
  • This condensation of chromosome is facilitated by replacement of histone by protamines
  • These protamines have been synthesized during the long meiosis I. Protamines are Arg-rich and Cys-rich proteins that displace the Lysrich histone
  • ) The cytoplasm streams away from the nucleus (except Golgi apparatus)