Topic 11.4: Sexual Reproduction Flashcards
Gametogenesis
Process by which diploid precursor cells undergo meiotic division to become haploid gametes (sex cells)
Process of gametogenesis
1) Multiple mitotic divisions and cell growth of precursor germ cells
2) Two meiotic divisions (meiosis I and II) to produce haploid daughter cells
3) Differentiation of the haploid daughter cells to produce functional gametes
Spermatogenesis
Production of spermatozoa (sperm) in the seminiferous tubules of the testes (induced by testosterone in Leydig) cells
Spermatogenesis process
It involves mitosis, cell growth, two meiotic divisions and differentiation
1) Germline epithelium divides (Mitosis)
2) Spermatogonia become spermatocytes
3) Spermatocytes undergo two meiotic divisions (spermatids)
4) Spermatids differentiate to become spermatozoa
Oogenesis
Production of female gametes (ova) within the ovaries
Oogenesis process
It occurs in the ovaries and involves mitosis, cell
growth, two (unequal) meiotic divisions and differentiation
1) Foetal development , a large number of primordial cells are formed by mitosis
2) Oogonia undergo cell growth until they are large enough to undergo meiosis (1 oocytes)
3) Primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles
4) Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes
5) These cells will complete the first meiotic division to form two cells of unequal size. One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body
6) The secondary oocyte begins the second meiotic division but is arrested in metaphase II The secondary oocyte is released from the ovary and enters into the oviduct
7) The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte. If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body
8) Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote
Spermatogenesis
1) Location
2) Meiotic divisions
3) Germ line epithelium
4) Number of gametes produced
5) Size of gametes
6) Duration
7) Onset
8) Release
9) End
1) Occurs entirely in testes
2) Equal division of cells
3) Involved in gamete production
4) Four
5) Sperm smaller than spermatocytes
6) Uninterrupted process
7) Begins at puberty
8) Continuous
9) Lifelong
Oogenesis
1) Location
2) Meiotic divisions
3) Germline epithelium
4) Number of gametes produced
5) Size of gametes
6) Duration
7) Onset
8) Release
9) End
1) Occurs mostly in ovaries
2) Unequal division of cytoplasm
3) Not involved in gamete production
4) One (3 polar bodies)
5) Ova larger than oocytes
6) In arrested stages
7) Begins in foetus
8) Monthly from puberty
9) Until menopause
Seminiferous Tubule
1) Basement membrane (Germline epithelium)
2) Sertoli cell
3) Spermatogonia
4) 1° Spermatocyte
5) 2° Spermatocyte
6) Spermatid
Ovary
1) Primordial follicles
2) Primary follicle
3) Secondary follicle
4) Mature follicle
5) Secondary oocyte
6) Corpus luteum
7) Corpus albicans
Male and female gametes
1) Sperm is small and motile and contributes the male’s haploid nucleus to the zygote
2) Egg is large and non-motile and contributes all the organelles and cytoplasm to the zygote
Sperm diagram
1) Head
- Haploid nucleus
- Acrosome (hydrolytic enzymes for jelly coat)
- Centriole (Cellular division)
2) Midpiece
- Mitochondria (ATP source)
3) Tail
- Flagellum (facilitate movement)
Egg diagram
1) Corona radiata
(Follicular cells that nourish the egg)
2) Zona pellucida (jelly coat that mediates sperm entry)
3) Haploid nucleus
4) Cortical granules (prevent polyspermy)
5) Cytoplasm
External fertilization
The fusion of gametes outside of the body of a parent
1) Common in aquatic animals
2) Susceptible to environmental influences
3) Species usually release large quantities of gametes
(spawning)
Internal Fertilization
Fusion of gametes inside of the body of a parent
1) Gamete of one parent can be introduced inside the body of another
2) Prevent exposure and desiccation of gametes or embryos
3) More protection to the gametes and embryos,
Fertilization
Fusion of male and female gametes
Human fertilization
1) Capacitation
2) Acrosome Reaction
3) Cortical Reaction
Capacitation
Uterine chemicals dissolve the sperm’s cholesterol coat, improving its mobility
Acrosome Reaction
The acrosome releases hydrolytic enzymes which soften the glycoprotein matrix of the jelly coat (enables penetration)
Cortical Reaction
Cortical granules release enzymes to destroy the sperm binding sites on the zona pellucida (prevents polyspermy)
Blastocyst formation
1) The egg and sperm nuclei combine to form a diploid nuclei and the fertilized cell (zygote)
2) The zygote will undergo several mitotic divisions to form a solid ball of cells (Morula)
3) As the morula continues to divide, it undergoes differentiation and cavitation to form a blastocyst
Blastocyst Sections
1) An inner cell mass (embryo)
2) A surrounding outer layer (trophoblast => placenta)
3) A fluid filled cavity (blastocoele)
Implantation of Blastocyst
1) The blastocyst breaches the jelly coat that was surrounding it and preventing its attachment to the endometrium
2) Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall
hCG
1) When a blastocyst implants within the endometrium, it begins to secrete hCG (human chorionic gonadotropin)
2) hCG prevents the degeneration of the corpus luteum in the ovary (which continues to produce estrogen + progesterone)
3) Progesterone maintains the endometrium until the placenta develops (at which point, levels of hCG will begin to drop)
Placenta
Life support system for the foetus
1) It facilitates the exchange of materials between the mother and foetus
2) It secretes hormones to maintain the pregnancy after the corpus luteum has degenerated
Structure of the Placenta
Disc-shaped and connected via an umbilical cord
1) Fetal vein
2) Fetal artery
3) Umbilical cord
4) Chorionic villus
5) Maternal vein
6) Maternal artery
7) Intervillous space (lacunae)
Material exchange in placenta
1) Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
2) Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels
Hormonal role in placenta
1) Begins producing estrogen and progesterone at 12 week
2) Estrogen stimulates the growth of uterine muscles and the development of the mammary glands
3) Progesterone maintains the endometrium, as well as reducing uterine contractions and potential maternal immune responses
4) Both estrogen and progesterone levels drop near the time of birth
Birth
1) Fetal growth causes stretching of the uterine walls, which is detected by stretch receptors
2) This triggers the release of oxytocin that induce uterine muscles to contract
3) This causes more stretching and hence more contraction until the origin stimulus (the foetus) is removed
Hormonañ control in birth
1) After 9 months, the baby is fully grown and stretches the walls of the uterus
2) This stress induces the release of chemicals which trigger a rise in the levels of estrogen (estriol in particular)
3) Estriol increases the sensitivity of the uterus to oxytocin
4) Estriol inhibits progesterone, which was preventing uterine contractions from occurring
5) The brain triggers the release of oxytocin from the posterior pituitary gland
6) Oxytocin stimulates the uterine muscles to contract, initiating the birthing process
7) The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contractions
8) The uterine contractions trigger the release of chemicals that cause further contractions
9) Contractions will stop when labour is complete and the baby is birthed
Gestation period
Time taken for a fetus to develop
• Altricial animals are born helpless (need extensive rearing)
• Precocial animals are born developed (no rearing needed)
While other factors contribute, there is a positive correlation between animal size and development of young at birth
