Asexual and Sexual Reproduction

Animals can reproduce by means of asexual or sexual reproduction that best suit their population, the environment, or other prerequisites. In this article, we will discuss the different mechanisms regarding the asexual and sexual reproduction of invertebrates and vertebrates.

Mechanisms of asexual reproduction

Invertebrates carry out the several simplest forms of asexual reproduction.

Budding: new individuals arise from the outgrowth of existing ones; in any case of budding, you can observe the trait “connected individuals,” because the offspring and parents are connected together.

Fission: the splitting and separation of a parent organism into two individuals of about the same size; e.g. binary fission in bacteria.

Fragmentation and regeneration: as the name confirms, this type of reproduction is a two-step process; appears in annelids, sponges, corals, cnidarians, and tunicates.

Parthenogenesis: happens when an egg develops without being fertilized; it can be observed in both invertebrates and vertebrate (much rare). Among invertebrates, this can occur in species of bees, wasps, and ants. Male bees, for instance, are fertile haploid adults that are not fertilized, while female bees are fertile diploid adults. Among vertebrates, such as the Komodo dragon and hammerhead shark, females can produce genetically-identical offspring as a rare response to low population density.

Variations of sexual reproduction

Normally, among vertebrates, as in humans, sexual reproduction involves simply the mating between a male and a female individual. However, for many sexual animals, finding a mate can be difficult, and this is the reason for the emergence of deviations of sexual reproduction.

Hermaphroditism: characterized by an individual having both the male and female reproductive system; this type of adaption makes the finding of a mate easier in some vertebrates (any two individuals can mate).Sometimes, self-fertilization is possible.

Sex reversal: individuals can transform between male and female; this is especially useful for sedentary animals like oysters and corals, because by transforming into males during the time of ovulation, more gametes (sperm) is released into the environment so chances of fertilization can greatly increase.

Parthenogenesis of bees.

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Axonal Transport

Recall that in neurons, peptide transmitters such as glycine and GABA are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals. But how is this process carried out? In this article, you will learn the specifics of axonal transport, which is responsible for the transport of substances across a neuron.

Preface

Axonal transport relies on microtubules that run along the neural axon and serve as ‘tracks’ for motor proteins like kinesin and dynein. They are capable of binding to and transporting mitochondria, cytoskeletal polymers, and synaptic vesicles containing neurotransmitters. Axonal transport is categorized by fast or slow, anterograde (away from the cell body) or retrograde (to the cell body).

Fast and slow transport

Motor proteins carrying vesicular cargoes move relatively fast compare with those transporting cytoskeletal proteins or organelles such as mitochondria.

Through fluorescence microscopy, it is discovered that slow transport is not indeed slower than fast transport. Instead, slow transport processes with a mechanism called "Stop and Go," in which motor proteins constantly pauses during the transport, making the overall transit rate much slower.

Anterograde transport

Anterograde transport moves molecules/organelles outward from the cell body to the axonal terminal or cell membrane. Kinesin is responsible for both the fast and slow transport of cargo during an anterograde transport. It is good to remember that kinesin is an ATPase and is also involved in the separation of chromosomes during mitosis and meiosis. Lipids, proteins, and substances packed in vesicles that cannot be synthesized in the axon or the axonal terminal, as well as organelles like mitochondria, uses anterograde transport to travel across the neuron.

Retrograde transport

Retrograde transport, on the other hand, carries molecules/organelles away from the axonal terminal towards the cell body. Utilizing dynein as the motor protein, retrograde transport returns used synaptic vesicles, deteriorated organelles, and carries signals from the synapse back to the cell body.

A view of axonal transport.

Reference:

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.

Summary of Neurotransmitters

Neurotransmitters are molecules responsible for the carrying of messages (signals) between neurons. It involves various areas, including biology, psychology, and chemistry, making it an especially crucial topic. Today, we will discuss the major neurotransmitters used by the human body and also their significances.

Acetylcholine

Acetylcholine is a transmitter used by all motor axons, autonomic preganglionic neurons, postganglionic parasympathetic fibers, and some cells of the motor cortex and basal ganglia. It is the chief neurotransmitter of the parasympathetic nervous system, which contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows the heart rate (vasodilator).

*preganglionic neurons connect the CNS to the ganglia (groups of neuron cell bodies in the peripheral nervous system), and postganglionic neurons connect the ganglia to the effector organ. The terms autonomic, parasympathetic, motor cortex (the region of the cerebral cortex involved with voluntary movements), and basal ganglia (a group of subcortical nuclei in the brains of vertebrates) will be discussed in later articles.

In the central nervous system, acetylcholine appears to have multiple roles, such as thought, memory, and learning; it is in abnormally short supply in the brains of persons with Alzheimer's disease.

Biogenic Amines

Biogenic amines include epinephrine, norepinephrine, dopamine, histamine, and serotonin. The first three belong to the category catecholamine (neurotransmitters containing the catechol, benzene with two side-by-side hydroxyl groups). Histamine and serotonin have otherwise composition.

The catecholamines are all derived from the amino acid tyrosine that is catalyzed by the enzyme tyrosine hydroxylase into DOPA. Information about this detailed process will be explained in another article regarding adrenergic transmission [click to understand more].

Dopamine is produced by the action of the enzyme DOPA carboxylase on DOPA. It is present in several brain regions. Dopamine coming from the substantia nigra into the corpus striatum represents the major dopamine activity, and it plays an essential role in the coordination of body movements. Dopamine is also believed to be involved in motivation, reward, and reinforcement.

In Parkinson's disease, the dopaminergic neurons of the substantia nigra degenerate, leading to characteristic motor dysfunction.

Norepinephrine is synthesized from dopamine by the enzyme dopamine β-hydroxylase. It is the primary transmitter for postganglionic sympathetic neurons, which is released into the blood from the adrenal medulla. In the brain, norepinephrine is synthesized by the locus coeruleus, a nucleus (cluster of neurons in the CNS) in the pon, where it influences sleep and wakefulness, attention, and feeding behavior.

Epinephrine is produced from norepinephrine by the enzyme phenylethanolamine-N-methyltransferase (PNMT). It is released by chromaffin cells (neuroendocrine cells) of the adrenal medulla and the rostral ventromedial medulla (RVM), a group of neurons in the medulla oblongata. Epinephrine plays an important role in the fight-or-flight response, as well as regulating visceral functions (e.g. respiration)

Histamine is present in neurons of the hypothalamus. In the brain, histamine mediates arousal and attention, similar to that of ACh and norepinephrine. It is also released from mast cells in response to allergic reactions or tissue damage.

Serotonin is found in high concentrations in the raphe region of the pons and upper brainstem and has a wide projection to the forebrain. It involves a wide range of behaviors, including sleep and wakefulness, cognition, the feeling of happiness, etc. (unnecessary to really know its specific effect)

Reuptake by the presynaptic membrane is a major factor in terminating transmitter action of the biogenic amines.

Amino Acids

Amino acids include glycine,y-aminobutyric acid (GABA), glutamate, and aspartate.

Glycine is an inhibitory transmitter in spinal interneurons. GABA is an inhibitory transmitter of the central nervous system. Both of them generate IPSPs via ligand-gated Cl- channels. Glycine is produced by the mitochondria, whereas GABA is synthesized from glutamate.

Glutamate and aspartate are excitatory transmitters of the CNS that generate EPSPs, and are products of the Krebs cycle. Both glutamate and aspartate are produced in the mitochondria

While non-peptide transmitters are synthesized in nerve terminals, peptide transmitters are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals [click to understand more]. Both glutamate and aspartate are produced in the mitochondria

Reuptake by presynaptic membranes is a major factor terminating the transmitter action of the amino acids.

Nitric Oxide (NO)

Unlike other transmitters, NO is neither packaged in vesicles nor released by exocytosis. As a gas, it readily diffuses across cell membranes to adjacent target tissue once being synthesized. NO is an inhibitory transmitter in the central and enteric nervous systems. In smooth muscles, NO is responsible for vasodilation and increasing blood flow (endothelial-derived relaxing factor).

List of neurotransmitters and their locations.

Reference:

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.

The Editors of Encyclopaedia Britannica. “Acetylcholine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 26 Dec. 2019, www.britannica.com/science/acetylcholine.