Gametogenesis

• The zygote will increase in size and complexity by dividing to produce more cells.
• Meiosis halves the number of chromosomes to keep the same of number of chromosomes and occurs in gonads. Male gonads is testes and female gonads is ovary.
• Primordial germ cells are an early population of cells that give rise to the germ cells. Primordial germ cells are typically identifiable early in development.
• Germ cell is a generic term for any cells that are, or will become, gametes (sperm or eggs).
• In both spermatogenesis and oogenesis, it starts with a single diploid cell in the Gonial stage (a spermatogonia or an oogonia). These cells divide by mitosis to increase the number of gonial cells. The gonial cells grow and then enter meiosis 1 as primary (1') gonocytes (1' spermatocyte or 1' oocyte). The 1' gonocytes then completes meiosis I. • Once these cells have completed meiosis 1 they become haploid secondary (2') gonocytes (2' spermatocytes or 2' oocytes).
• Primary spermatocyte is the same size as the original spermatogonia while the primary oocyte is larger to obtain nutrients for the first few divisions after its fertilization.
• All of the cytoplasm that is rich in nutrients and organelles are given to the large oocyte by the three small polar bodies.
• The testes are surrounded by tough sheath of connective tissue called the Tunica albuginea.
• The testes

  A) lumen

  B) early spermatids

  C) spermatogonia

  D) primary spermatocytes

  E) leydig (interstitial) cells

  F) sertoli cells

  
• Germ cells at all stages of development are nestled on indentations in Sertoli cells. The Function of Sertoli cells is to support and nourish the germ cells and translocation of sperm toward lumen for eventual release. The Sertoli cells create a blood-testis barrier which isolate sperm from the immune system.
• Spermatogonia (2n):

  • Dark, granular, round or slightly oval nuclei near the basement membrane
  • Multiply by mitosis
• Primary (1') spermatocytes (2n):

  • Found in the layer above spermatogonia
  • In a period of growth
  • Large nuclei that stain relatively dark
• Secondary (2') spermatocytes (1n):

  • Almost immediately go onto 2nd meiotic division
  • Cells that are 1/2 the size of the 1' spermatocyte
• Spermatids:

  • 1/4 size of 1' spermatocytes
  • Closest to the lumen
• Spermatozoa:

  • Easily identified by their long tails extending into the lumen
• SPERMIOGENESIS

  • Is the process by which spermatids mature into spermatozoa.
  • Spermatids in some tubules form a layer 3-4 cells thick are undergoing the complex process of differentiating into spermatozoa.
• During spermiogenesis, spermatids undergo the following changes:
• Elongation of nucleus in what is becoming the head of the sperm and formation of the acrosomal cap
• Formation of flagellum extending into lumen
• Excessive shedding of cytoplasm which is pinched off and appears as dark residual bodies near lumen. These residual bodies will be phagocytized by Sertoli cells.
• When fully formed, the tip of the head remains attached to Sertoli cells for a short period. Eventually, spermatids are released into the lumen.
• The entire process takes about 48 days in rats and about 64 days in humans (from spermatogonia to spermatozoa)
• Parts of Mature Sperm

  A) Acrosome

  B) Plasma membrane

  C) Nucleus

  D) Mitochondria

  E) Centriole

  F) Terminal disc

  G) Axial filament

  H) Head

  I) Mid(connecting)piece

  J) Tail

  K) Endpiece

  
• Oogenesis in mammals occurs in the Ovary
• Females are born with about 800,000 eggs, most in oogonial stage (undergo mitosis) before birth. At birth, most germ cells exist as primary (1') but some are still oogonia.
• During the 30-40 years of a female's menstrual cycle, about 400 eggs will be ovulated
• The remaining 99% of eggs will degenerate
• The Inner medulla of the ovary contains large blood vessels, lymphatic tissue, nerves, and connective tissue
• The Outer cortex of the ovary contains many circular follicles
• Parts of Ovary

  A) corpus albicans

  B) corpus luteum

  C) tunica albuginea

  D) developing corpus luteum

  E) hilum

  F) early primary follicle

  G) late primary follicle

  H) primary oocyte

  I) fluid filled vesicles

  J) secondary follicle

  K) mature follicle

  L) antrum

  M) ovulation

  N) corona radiata

  O) ovulated secondary oocyte

  P) time

  
• Follicles contain a single egg surrounded by one or more layers of nurse or follicle cells.

  The function of the nurse or follicle cells is to support and nourish the developing germ cells.
• Follicle

  A) Primordial follicle

  B) early primary follicle

  C) late primary follicle

  D) early secondary follicle

  E) small secondary follicle

  F) large secondary follicle

  G) Graafian follicle

  H) Gonadtropin dependent growth

  I) FSH

  J) Gonadtropin independent growth

  
• • Oocytes develops inside a layer of cells called the follicle cells.
  • The primordial follicle contains a primary oocyte surrounded by a single layer of squamous epithelial cells arrested in first meiotic division.
  • Most follicles in the ovarian cortex will be at this stage.
• Starting at puberty until menopause, each menstrual cycle begins with the growth of 20-50 primordial follicles are stimulated by Follicle Stimulating Hormone (FSH) to begin to grow. As soon as it begins to grow it is now called a -Primary Follicle.

  The Follicle cells become cuboidal and/ or columnar and increase in number.

  The many layers of follicle cells surrounding the primary oocyte are called the Stratum granulosum.

  These follicle cells provide nutrients and secrete fluid, called liquor folliculi that accumulates in spaces between the follicular cells.
• • Once this space, called the Antrum, appears the follicle is now a Secondary Follicle.
  • The Antrum eventually becomes enormous and fills most of the follicle.
  • The primary oocyte is still surrounded by follicle cells.
  • These cells are called the Cumulus oophorous.
  • Once the antrum becomes huge the follicle is now considered a mature Tertiary Follicle or Graafian follicle.
• Follicle Parts

  A) corona radiata

  B) zona pellucida

  C) follicle cells of cumulus oophorous

  D) theca externa

  E) oocyte

  F) antrum

  G) theca interna

  
• • Graafian follicle contains large amount of fluid, some to keep oocyte in meiotic arrest.
  • Not all growing follicles become Graafian follicles. In humans, usually only one will reach this stage each month.
  • The other follicles degenerate (or atresia - atretic follicles)
  • As follicles mature cells outer follicle cells coalesce around the follicle and form the theca
• Theca interna secretes a nutritive fluid for follicle cells and some estrogen and androgen intermediates that follicle cells convert to estrogen. Theca externa is more fibrous outer layer
• Zona pellucida is the translucent area surrounding the oocyte.
• •  The zona pellucida contains glycoproteins called ZPI, ZP2, and -ZP3.
  • These glycoproteins play a major role in fertilization, inducing activating the acrosome reaction in the sperm and eventually being altered by cortical granule substances released by the fertilized egg.
  • In their altered form they prevent poLyspermy.