Ch. 46 and 47 Reproduction and Development
1) Animal reproduction can be either sexual or asexual. Sexual reproduction in animals
involves the fusion of a sperm and an egg to form a zygote. Asexual reproduction
involves reproduction without the fusion of sperm and egg.
2) One form of asexual reproduction is fission, where an adult splits into two or more
individuals of similar size.
3) Another form of asexual reproduction is budding, where an adult splits unequally,
and the fragments develop into offspring.
4) Fragmentation occurs when an adult animal is broken into pieces, which regenerate
into new adults.
5) Parthenogenesis is the development of offspring from an unfertilized egg. Some
species (e.g. many whiptail lizards) are exclusively parthenogenic. Some are both
sexual and parthenogenic. Aphids are usually all female and parthenogenic, but in an
unstable environment, they will produce sexual female and male offspring.
6) Sexual reproduction is something of a mystery in evolution. Most eukaryotes
reproduce sexually at least part of the time, despite the fact that sexual reproduction is
much less fecund than asexual reproduction (the rate of offspring production is
halved). It is presumed that the genetic recombination produced by sexual
reproduction must provide a great advantage in survival and reproductive success.
7) Ovulation is the release of mature eggs at the midpoint of a female cycle. Most
animals exhibit reproductive cycles related to changing seasons. Reproductive cycles
are controlled by hormones and environmental cues. Animals may reproduce
asexually or sexually, or they may alternate these methods.
8) Mechanisms of fertilization vary across different animals species. External
fertilization occurs outside the body in the environment (always in water, or some
kind of protected enclosure (e.g. earthworm cocoon)). Internal fertilization occurs
in the female reproductive tract, and requires compatible copulatory organs.
9) Different species employ different strategies for reproductive success. Some species
favor producing massive quanitites of gametes and or offspring to increase the chance
that some will be successful. This is kind of “broadcast” reproduction is common
among species with external fertilization (e.g. fish). Others produce fewer gametes
and or offspring, but protect their offspring to increase their chances of success (e.g.
mammals).
10) In humans, spermatogensis is a continuous process, with production of each sperm
cell taking about seven weeks. One spermatogonial stem cell undergoes meiosis to
produce four sperm cells. Oogenesis (egg production) is a much longer process. The
cells that undergo meiosis to produce eggs cells are all present before birth, and halt
the meiotic process at the beginning of meiosis I. At puberty, FSH hormone
stimulates a small group of follicles to proceed to meiosis II metaphase, then stop
again. One of the daughter cells produced by meiosis I is called a polar body, which
degenerates. Meiosis is only completed after sperm enters the egg cell. A second
polar body is produced following the completion of meiosis II.
11) Female mammals have a uterine cycle. Oogenesis is coordinated with the
development of the endometrium. The endometrium is a lining of the uterus that
provides a hospitable environment for an embryo.
12) Human woman and some other primates have menstrual cycles. Most other
mammals exhibit estrous cycles (commonly called “heat”). In menstrual cycles, the
endometrium is shed and expelled out of the vagina. In estrous cycles, the
endometrium is reabsorbed. Unlike menstrual cycles, females with estrous cycles
typically only copulate during the fertile phase of the cycle.
13) When sperm penetrates an egg cell, a fertilization envelope forms within a minute.
This protects the egg from being penetrated by multiple sperm.
14) In humans, fertilization occurs in the Fallopian tube. Usually, the embryo migrates
to the body of the uterus, where it implants in the endometrium. If the embryo
implants anywhere else, it is referred to as an ectopic pregnancy. Most ectopic
pregnancies occur in a Fallopian tube (this usually requires surgery to remove the
Fallopian tube). Some ectopic pregnancies occur in other parts of the body. In very
rare cases, ectoptic pregnancies where the embryo implants in the liver can result in
live births by Caesarian section!
15) Vertebrate embryos have an animal pole and a vegetal pole. The animal pole
develops into the adult organism, while the vegetal pole develops into the extraembryonic membranes that support the embryo (e.g. the yolk sac)