reproduction & embryotic development
what came first, the chicken or the egg?
Figure 17. Life Cycle Of A Chicken. Chris Lesley, July 2, 2020. Retrieved on October 16, 2021, from https://www.chickensandmore.com/life-cycle-of-a-chicken/
Chickens participate in sexual reproduction, and embryotic development within a fertilized egg takes approximately 21 days.
The anatomy of chickens' reproductive tracks is interesting, it is quite different from mammalian reproductive tracks.
In males (roosters) there are the testes, which produce sperm, and the cloaca, which allows the sperm to exit the body.
In females (hens) there is a single ovary and an oviduct. The oviduct is comprised of multiple parts: infundibulum, magnum, isthmus, uterus, vagina, and the cloaca. Both reproductive tracts are shown below (Minard, 2018).
Figure 18. Reproductive Tract Of A Male Chicken. Dr. Jacquie Jacob, University of Kentucky. Retrieved on October 16, 2021, from https://poultry.extension.org/articles/poultry-anatomy/avian-reproductive-system-male/
Figure 19. Reproductive Tract Of A Female Chicken, Dr. Jacquie Jacob, University of Kentucky. Retrieved on October 16, 2021, from https://poultry.extension.org/articles/poultry-anatomy/avian-reproductive-female/
Figure 20. Chick Embryo Development. Poultry Hub Australia, 2021. Retrieved on October 23, 2021 from https://www.poultryhub.org/anatomy-and-physiology/body-systems/embryology-of-the-chicken
Copulation allows for the transfer of gametes. Gametes are germ cells that contain half of the chromosomes required to form an individual offspring. Copulation in bird species is commonly known as the "cloacal kiss".
Within a hen's oviduct are storage tubules that can hold viable sperm for up to 70 days (Noll, 2017). This allows for hens to continuously lay a fertilized egg for several days. Fertilization occurs when a single gamete from both parent chickens fuse together to form a zygote. Within the next few hours after fertilization, the zygote undergoes cell division and becomes an embryo. The nutritious yolk sac and the protective egg shell forms surrounding the embryo. The image to the left provides great visual representation of how the chicken embryo utilizes the nutrients within the egg as it grows over time (Poultry Hub Australia, 2021).
chickens' reproduction is controlled by the endocrine system
According to the article "Reproduction in Hens: Is Testosterone Necessary For the Ovulatory Process?", hormones produced by the endocrine system play an important role in reproduction. The endocrine system maintains biological processes and is responsible for producing hormones. Although testosterone is typically thought of as being a male hormone, it is very significant in females too. Testosterone stimulates the production of progesterone, which is another hormone that prepares the female body for pregnancy.
In the study, the researchers observed the effects of administering intramuscular injections of testosterone. These injections were done in vivo (within the body) and in vitro (outside the body). The researchers found that the concentration of testosterone needed to increase prior to ovulation, in order for ovulation to proceed (Rangel et al, 2014).
many other aspects of chicken reproduction have been studied
Another article titled "Chicken Embryo Development: Metabolic and Morphological Basis For in ovo Feeding Technology" compiled and reviewed a variety of studies involving the addition of supplemental nutrients into developing chicken embryos. Naturally, a chicken embryo already has sufficient access to many nutrients within the yolk sac; such as carbohydrates, proteins, lipids, and minerals (Givisiez et al, 2020).
The phrase "in ovo" refers to the technique of injecting supplements into the egg before the chicken hatches. This is commonly done with vaccines at hatcheries, as it reduces stress from animal handling and is less labor-intensive (Zoetis, 2011). A video example of in ovo vaccine injections is linked below.
One of the many studies discussed in the aforementioned article was about the effect of in ovo administration of Asorbic Acid, more commonly known as Vitamin C. The results concluded that the Vitamin C supplement caused improvement to the chickens' immune system and had positive effects on muscle development (Givisiez et al, 2020).
A different study by researchers in Shanghai focused on the changes in the gut microbiome of chicken embryos as they grew. The image to the right shows the variety of bacteria that the researchers found and observed.
The researchers noted that there was an increase in population of gut microbiota during days three through twelve of embryotic development, and that the diversity of the gut microbiome changed throughout embryotic development. Their results concluded that, "Proteobacteria (65.6%) was the predominant phylum in all the samples, and Actinobacteria (16.6%) emerged as the second phylum, followed by Firmicutes (6.1%), Bacteroidetes (5.4%), and Cyanobacteria (2.2%)" which indicates that there is a wide variety of things within baby chicken guts (Akinyemi et al, 2020).
Figure 21. Composition of gut microbiota in the chicken embryo. (A) Phylogenetic tree constructed from the taxa. Each classification unit is distinguished with different colors. Colored blocks in the outermost circle indicate phyla and in the inner circle indicate genera. (B) The abundant microbes at each level (genus, family, order, class, and phylum). Akinyemi et al, July 3, 2020. Retrieved on October 16, 2021, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7598139/
interesting videos
