Topic 11.4 - Sexual reporduction Flashcards
Oogenesis
Production of egg cells in the ovaries.
Germ cells in the fetal ovaries divide by mitosis and distribute themselves to the cortex of the ovary.
By 4/5 months, the cells begin meiosis.
By 7 months they are still in the first stage of meiosis and a layer of follicle cells surround the cells.
No further progress occurs until puberty
Spermatogenesis
Production of sperm cells in the testes which are composed of narrow tubules called seminiferous tubules.
There are small gaps between tubules - called interstices which are filled with interstitial/Leydig cells.
Outer layer of seminiferous tubules is germinal epithelium. Germinal epithelium contains sperm being produced, most mature ones are nearer to liquid in seminiferous tubules.
Sperms with tails - spermatozoa
Tubule wall contains Sertoli (nurse) cells
Germ cells
Founder cells of all sexually reproducing organisms. During development, they are set aside from all somatic cells of the embryo.
In many species, germ cells form at the fringe of the embryo proper and then traverse through several developing somatic tissues on their journey to the emerging gonad.
Once in the gonad, they acquire sex-specific morphologies and the ability to undergo meiosis to generate egg and sperm.
Primary follicles
The first cell to divide by meiosis with surrounding follicle cells.
Around 400,000 produced at birth, no more produced.
During the start of each menstruation cycle, several are stimulated to develop by FSH but only one usually matures and developed a secondary oocyte.
Sertoli cells
Supply essential nutrients to sperm cells and do phagocytosis to get rid of excess spermatid cytoplasm.
Gonad
Something producing gametes (testes or ovaries)
Epidermis
Outermost layer - skin
Epithelium
Inner lining that doesn’t connect to the outside (vascular)
Endothelium
Inner lining that also connects to the outside (seminiferous tubules / pulmonary / digestive system)
Sperm and egg cells
(page 500-501 in biobook)
Differences between spermatogenesis and oogenesis
Sperm cells -> once differentiated, cytoplasm mostly removed - only haploid nucleus, mechanisms for movement and proteins/enzymes for entering egg cell remain.
Meiosis leads to 4 identical sperm cells.
Production is constant once puberty starts and millions may be in some development stage at any time.
Egg cells -> once differentiated, haploid nucleus and all requirements for embryo growth are in the egg.
Meiosis 1 produces large cell and small polar body (degenerates and dies) and then the larger cell in meiosis 2 produces another large cell ready for fertilisation and another small polar body (ded agen :() ~ this allows the egg to be so large.
Production occurs once per menstruation cycle and from puberty to menopause, only a few hundred gametes are produced
Polyspermy
Multiple sperm cells fusing with the egg cell during fertilisation.
Bad as this would cause infertility due to incorrect chromosome numbers or possibly even death.
Because of this, there are methods to prevent polyspermy occuring.
Methods preventing polyspermy
Acrosome reaction -> zona pellucida (protective egg layer containing glycoproteins) is bound to by sperm cells. The acrosome is a large membrane-bound sac of enzymes that are released to break down the zp
Penetration of egg membrane ->
The first cell to enter the zp has its membrane fuse with the membrane of the egg - fertilisation occurs.
Cortical reaction ->
After fertilisation, the egg becomes activated. Cortical granules (located near the egg membrane) have their contents released by exocytosis. Enzymes are released that digest binding proteins - sperms can no longer bind.
Internal fertilisation
To prevent drying out, terrestrial animals do int fert.
Allow for close proximity for gametes and protection.
External fertilisation
Drying out is not an issue so aquatic animals release the gametes directly into the water.
There are several risks with ext fert: pH levels changing, predation, temperature variations, and pollution etc
Why blastocyst implantation occurs
48 hours in - zygote has 4 cells, after this mitosis may lead to uneven division and cells may migrate.
Once a hollow ball of cells is created, the zygote has become a blastocyst
7 days in - blastocyst has 125 cells and has reached the uterus due to cilia in oviduct wall. At this point, the zp has broken down and the reserves from the ova (egg cell) have been used up - external sources of food needed.
Because of this, the blastocyst implants in the endometrium (lining of the uterus)
How blastocyst implantation occurs
Blastocyst develops finger-like projections (villi) that sink into the endometrium.
After this, materials can be exchanged between embryo and the mother’s blood, including oxygen and food.
8 weeks in, bone tissue developed, fetus now
Role of hCG in pregnancy
Pregnancy - depends on the maintenance of the endometrium which is maintained by oestrogen and progesterone preventing its breakdown.
hCG produced by the embryo, signalling corpus luteum in the embryo to continue to secrete oe & p
Materials exchanged by the placenta
Because body surface area to volume ratio increases as size increases, a placenta is used to supply necessary materials to the developing fetus.
Amniotic sac -> produced by the fetus which produces amniotic fluid that protects the fetus and regulates temp.
Placental villi -> More as the placenta develops (to support fetus growth), have mother’s blood and fetal blood very close together (< 5μm), the placental barrier separates the bloods and is selectively permeable. Microvilli are on placental villi to further increase SA.
The materials exchanged across the placenta are: glucose, oxygen, water, amino acids, carbon dioxide, urea, waste materials, antibodies
Types of mammal
Placental -> mammals with a placenta which is used to supply their young with necessary materials for life. Young birthed when body functions independently.
Includes - most mammals tbh
Monotremes -> mammals that produce eggs
Includes - Platypus and Echidna
Marsupials -> mammals that birth their young early and have them enter a pouch on the mother and continue development
Includes - kangaroos, koalas, wallabys
Release of hormones during pregnancy
Ninth week of pregnancy - placenta takes the job of the corpus luteum and secretes oe & p in order to supply sufficient amounts of these hormones.
If this switch over fails, a miscarriage may occur.
Role of hormones in parturition
Birth is mediated by positive feedback of oe and oxytocin
During pregnancy, progesterone inhibits the release of oxytocin from the pituitary gland and the contractions of the muscular wall of the uterus (myometrium)
Progesterone is inhibited by hormones produced by the fetus near the end of the pregnancy.
Hormones on contractions during parturition
second part
Endometrium muscle fibre contractions stimulated by oxytocin, these are then detected by stretch receptors, which signal for the increase in oxytocin secretion.
More oxytocin - more vigorous and frequent contractions.
This positive feedback loop allows for a gradual increase in myometrial contractions, allowing minimal contraction intensity for fetus birth.
Relaxation of muscle fibres in the cervix cause dilation, uterine contractions then cause the amniotic sac to burst and the fluid to be released.
The next few hours contain uterine contractions which push the baby out. Once the baby is out, the umbilical cord is broken off and the baby starts breathing itself and has physiological independence from its mother.
Open circulatory system
Blood not confined to blood vessels, found in insects.
Closed circulatory system
Blood confined to blood vessels, found in humans.
Haemoglobin (adults)
Haemoglobin (fetuses)
HbA - contains two alpha and two betas (α₂β₂)
HbF - contains two alpha and two gammas (α₂γ₂)
Fetus has gammas as it needs a higher affinity for oxygen absorbtion.
Glucose across the placental barrier
Occurs by facilitated diffusion
Water across the placental barrier
Occurs by osmosis
Oxygen across the placental barrier
Occurs by diffusion
Carbon dioxide across the placental barrier
Occurs by diffusion
Antibodies across the placental barrier
Occurs by endocytosis
Urea across the placental barrier
Occurs by diffusion
Amino acids across the placental barrier
Occurs by facilitated diffusion
