Reproduction Flashcards
Gametogenesis
The production of sperm and eggs, takes places through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an early stage of cell’s life cycle. Meiosis produces haploid cells with half of each pair of chromosomes normally found in diploid cells.
Oogenesis
occurs in the most outermost layers of the ovaries. As with sperm production, it starts with a germ cell, called oogonium, but this call undergoes mitosis to increase in number, eventually resulting in up to about one to two million cells in the embryo. Results in 1 large ovum.
- Oogenesis: production of egg cells (ova)
- Oogonia (stem cells) multiply by mitosis and store nutrients.
o Primary oocytes
o Develop in primordial follicles (sack of cells that surrounds ovum)
o At birth, female presumed to have lifetime supply of primary oocytes
o Primary oocytes are arrested in the beginning stages of meiosis
Meiosis is different for oocytes. Primary oocyte undergoes meiosis
Meiosis I resume and completes during evolution.
Meiosis II resumes and completes after fertilization. Only one ovum produced.
Spermatogenesis
occurs in the wall of the seminiferous tubules, with stem cells in the periphery of the tube and the spermatozoa at the lumen of the tube. Has diploid ells and result in 4 sperm cells.
Yolk amount across species
- Alecithal eggs – no yolk (placental mammals)
- Microlecithal eggs – small amounts (marsupials)
- Mesolecithal eggs – intermediate amounts (amphibians, fish)
- Macrolecithal eggs – large amounts (insects, reptiles, birds, some fish)
In egg laying animals, more yolk = more development time in egg
Vitellogenesis
deposition of lipids and proteins (vitellogenin). Vitellogenin is a marker of environmental estrogens when seen in males.
Oviparity
Fish/amphibians: produce gelatinous eggs, water needed throughout development, fertilization occurs after egg laying (internal or external fertilization)
Reptiles/birds/monotremes: Produce eggs with calcium carbonate layer, fertilization occurs before eggshell formed (internal fertilization)
Amniotic egg: embryo produces 4 extraembryonic membranes
Chorion: the outermost membrane surrounding and embryo of a reptile, bird, or mammal. In mammals it contributes to the forming of the placenta.
Amnion: a membrane forming a fluid-filled cavity (the sac) that encloses embryo ( bag of waters)
Allantois: the fetal membrane lying below the chorion in m any vertebrates, formed as an outgrowth of the embryos gut. In birds it grows to surround the embryo, in eutherian mammals it forms part of placenta.
Yolk sac: sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk.
Placenta
provides supply of nutrients, oxygen, and hormones to fetus.
Ovaries (female gonads)
produce female gametes (ova).
Oviduct
contains ciliated fimbriae that creates current to move oocyte into uterine tube
Uterus (placental mammals)
The endometrium is the innermost layer of the uterine wall. Myometrium is the middle layer of the uterine wall. Endometrium is composed of columnar epithelium. Myometrium is composed of muscle layer which developed by the uterine myocytes.
Follicle recruitment
after puberty, multiple primordial follicles develop (cohorts). At each stage, many follicles die (atresia). Takes ~1 year to primordial follicle to reach graafian follicle stage. Birth: ~700, 000 potential eggs. Only ~ 500 eggs ovulated.
The ovarian cycle
follicle growth takes ~1 year for follicle to grow into graafian follicle that will ovulate egg. Humans- monthly (~28) series of events associated with maturation of one oocyte.
Three phases
- Follicular phase: period of vesicular follicle growth (days 1–14)
- Ovulation: Release of oocyte from the follicle occurs between phases
- Luteal phase: period of corpus luteum activity (days 14–28); time during which fertilization needs to occur
Endocrine control over oogenesis
the hypothalamus secretes gonadotropin releasing hormone (GnRH). GnRH stimulates the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary gland. FSH and LH stimulate different cells of the follicle. Thecal cells and granulosa cells.
Follicular phase
Hormonal regulation: FSH stimulates granulose cells too release estrogen and inhibin. LH stimulates thecal cells to produce androgens are converted to estrogen by the enzyme aromatase. At the beginning of the ovarian cycle, several mature follicles are stimulated to grow.
Hormonal regulation
Estrogens regulate GnRH, FSH, and LH via negative feedback. Inhibin inhibits FSH release only. One of the mature follicles becomes the dominant follicle. The dominant follicle continues to secrete estrogens even as FSH and LH levels decrease.
Estrogen levels continue to increase. When estrogen levels rise high enough, estrogen switches to stimulating the hypothalamus. LH surge occurs- stimulates ovulation.
The uterus
hollow, thick walled, muscular organ. Function is to receive, retain, and nourish fertilized ovum. The endometrium (innermost layer) thickens during the ovarian cycle and develops numerous glands and blood vessels.
Luteal phase
Shortly after ovulation:
- Estrogen levels decline
- Ruptured follicle transforms into the corpus luteum. Corpus luteum secretes progesterone (needed to maintain pregnancy if fertilization and implantation occurs), (progesterone inhibits FSH and LH secretion; inhibits another ovarian cycle if there is a pregnancy.)
If no fertilization occurs: corpus luteum breaks down FSH and LH secretion no longer inhibited; a new ovarian cycle starts.
Implantation (placental mammals)
If a fertilized egg implants into the endometrium the development of a placenta in the uterus is triggered. The placenta produces hCG, human chorionic gonadotropin. hCG stimulates the corpus luteum and keeps it from breaking down. Placenta eventually produces its own progesterone after ~2 months of pregnancy.
Uterine cycle
Days 1-5: menstrual phase – coincides with beginning of the ovarian cycle
- Ovarian hormones are at lowest levels
- Uterine lining sheds (bleeding)
- Many species reabsorb endometrium, so very little bleeding occurs
Days 6-14: preovulatory phase
- Rising estrogen levels prompt generation of new uterine layer
Days 15-28: postovulatory phase
- Endometrium thickens and prepares for impanation of fertilized egg
Testes
sperm-producing male gonads that lie within the scrotum.
seminiferous tubules have thick, stratified epithelium surrounding central fluid-containing lumen. Sperm cells develop and are pushed into lumen.
Spermatogenesis: occurs after puberty and throughout life.
Spermatogonia- cells that divide by mitosis (create an identical copy)
- One copy will develop into a primary spermatocyte
- This maintains a population of cells that can undergo meiosis.
A primary spermatocyte undergoes meiosis: product: 4 spermatids which develop into mature sperm cells.
Sperm: acrosome contains hydrolytic enzymes.
The scrotum
3 degrees calcium lower than core body temperature. Lower temperature needed for sperm development. Sperm development takes place within seminiferous tubules.
Sertoli cells
- Guide spermatocytes during development and provide nutrients
- Produce chemical mediator
- Secrete testicular fluid
- Phagocytize faulty germ cells and excess cytoplasm
- Produce proteins that form tight junctions to form the blood-testis barrier – regulates environment for sperm production.
Endocrine control over spermatogenesis
the hypothalamus secretes gonadotropin releasing hormone (GnRH).
- GnRH stimulates the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary gland.
- FSH and LH stimulate cells in the testes: Leydig and Sertoli cells
- LH stimulates interstitial cells to produce testosterone
- FSH stimulates Sertoli cells to release androgen binding protein (ABP), binds to testosterone and traps it in testes
- High testosterone concentration in seminiferous tubules promotes spermatogenesis.
- Testosterone regulates GnRH, LH, and FSH production through negative feedback testosterone.
- Inhibin: released by Sertoli cells when sperm count high; inhibits FSH release only.
Testosterone NF
Testosterone in blood ranges from 300-1000ng/dl (average ~650ng/ml)
Testosterone induces features in nonreproductive organs (pubic/axillary, facial hair, voice deepens, skeletal muscle growth)
Circulating testosterone induces negative feedback.
Anabolic steroids: exogenous steroids disrupt testosterone production in testes.
Genetic and env sex differentiation
- Simultaneous hermaphrodites: are capable of producing both eggs and sperm.
- Sequential hermaphrodites: switch from one sex to the other (regulated by aromatase activity.
- XX/XY system
SRY gene on Y chromosome initiates development of testes; blocks ovary development
Temperature sex determination: - Temp must be maintained for several days during a critical period
- Temp appears to regulate expression of aromatase
Atrazine- commonly used herbicide in US
- One example of an endocrine disruptor
- Increases aromatase production
- Demasculization occurs as testosterone/androgens converted to estrogens
Excess estrogens/xenoestrogens
- Earlier onset of puberty
- Links to female and make infertility -excess Es disrupt natural regulation of hormones
- Links to breast cancer
Sources: Bisphenol A (BPA) /phthalates
- has been found in many plastics and tin can linings
- Low levels of BPA can seep into food (especially when heated)
- ~ 92% of Americans have BPA in body, high levels in children (banned in 2012)
- DDT banned in 1972
- Many pesticides
- Contraceptives in urine: ethinylestradiol
- Animal manure
Excess androgen
- Many agricultural farms and fish farms use androgens to create larger and more profitable animals
- Runoffs into connected waterways can affect natural populations of animals
Mammary glands
Mammary glands are modified sweat glands
Each gland consists of lobules that produce and secrete milk through duct system
Myoepithelial cells around ducts contract and cause milk ejection
Strogen effects:
- Mammary gland cells differentiate during fetal development
- Breast tissue develops and grows during puberty
- Hypertrophy of cells occur during the last 3rd of pregnancy
Hormones promote mammary gland growth (mitosis and cell growth) during pregnancy: estrogen and GH
Lactation
Suckling stimulates both oxytocin and prolactin release
Prolactin increases milk production
- Remember how prolactin is regulated? Prolactin inhibiting hormone! PIH decreases during pregnancy which removes inhibition.
- PIH decreases during late pregnancy
Oxytocin increases milk ejection
- Other stimuli for oxytocin include thoughts about baby and sensory stimuli
Prolactin in parental care
Prolactin has been shown to regulate both maternal and paternal parental behaviors
