How Does Mammal Fetal Development Relate to Developmental Disability?Guest post by Margot
How Does Mammal Fetal Development Relate to Developmental Disability?
By Margot Cole
Neourdiversity portions of this essay were consulted on by Dylan Rothbein.
In the kingdom Animalia we can gain a lot of information about how an animal functions and it’s evolution by studying embryology. In biology animals are grouped according to their development as embryos. Development is defined biologically as irreversible from the time an animal is a zygote to the time it passes away. As a zygote develops the cells organize into different parts of the body. Human embryos have 3 tissue layers, the ectoderm, mesoderm and endoderm. Humans are deuterostomes which means the anus develops first. In humans the nervous system forms from the ectoderm and neural tube (Elshazzly 2022). The nervous system is complex in mammals with sensory organs and a brain all centered in the head of the animal in cephalization.
The embryonic and fetal development of the brain interests me the most because I have Cerebral Palsy and my friends who I met in the process of my disability rights work all have brains that formed differently during fetal development for different reasons.
In Down syndrome a person receives a third copy of chromosome 21. In humans and most other living creatures there are 2 copies of each chromosome, this is called a diploid organism. Sometimes a creature will have 3 copies of all of their chromosomes, this is called triploid. Some common fruits like bananas and other plants are triploid. However being triploid in humans is always fatal. Down syndrome by contrast is where a person inherits a third copy of only the 21st chromosome while the other chromosomes remain diploid called trisomy, so people with Down Syndrome survive into adulthood. Having a third chromosome 21 affects the development of every part of the body (Delany et al 2021). In about 50% percent of people with Down Syndrome the person’s the heart is smaller affecting how much blood flow each body part receives in the womb. This affects how much blood flow the brain receives during embryonic and fetal development contributing to intellectual impairment, lung problems, balance and gait problems, and eye problems. People with Down syndrome tend to pass out from low blood pressure related to the autonomic nervous system, and have blood sugar problems like diabetes and hypoglycemia which also affects how the brain functions throughout the person’s life (Delany et al 2021). People with Down Syndrome tend to develop neurologically to be very social. Unfortunately soon-to-be mothers are very frightened by the health problems associated with Down syndrome and will opt for prenatal testing to diagnose Down syndrome. If the tests for Down syndrome come back positive the mother almost always decides to abort her baby at a rate of about 90% in the USA. In Iceland the abortion rate for Down Syndrome is so high almost no babies with Down syndrome are born there anymore.
In Autism during embryonic development, the brain develops differently, with some parts of the brain having more connections than in a non autistic baby, and other parts of the brain having less connections than a non autistic baby (Rothbein 2021). This unique, highly individualized wiring affects sensory integration and developmental milestones, outside the womb, during childhood (Rothbein 2021). This developmental aspect is why some people with Autism also retain some of the primitive involuntary reflexes. This means an autistic person will have issues with motor control, proprioception and depth perception (Rothbein 2021). Autism is a physical disability in many ways (Rothbein 2021). A condition that often co occurs with autism is dyspraxia that affects balance, motor control and leads to low muscle tone and issues with executive function and motor planning. This unique brain development also means that epilepsy is 40% more common in the autistic population. In autism emotions are heightened (Rothbein 2021) and so is the feeling of pain which relates to an area of the brain called the thalamus (Hedrith 2023) .
I have Cerebral Palsy which is a brain injury that happens before, during or shortly after birth, and causes motor disability with spasticity, lack of proprioception, issues with balancing, coordination, speech, heightened reflexes, or reduced reflexes (Futagi, at al 2012), and can affect a person differently depending on where the brain damage occurred. I have spasticity on both sides of my lower body so I use crutches but some people with Cerebral Palsy use wheelchairs or walk independently with a limp. Most people with Cerebral Palsy have an exaggerated startle reflex. Some research suggests this is because babies with Cerebral Palsy retain something called the Moro Reflex which in non disabled babies disappears by 6 months of age because more voluntary reflexes take over the more primitive reflexes, but people with Cerebral Palsy never develop the more voluntary reflexes. Other studies suggest people with Cerebral Palsy develop a more adult startle reflex but it remains exaggerated because of spasticity. The brain injury in Cerebral Palsy is sometimes caused by environmental factors during pregnancy and delivery (Martha et al 2007).
The endocrine system can affect how the brain develops and functions throughout life. Some scientists have hypothesized that being gay and/or being gender dysphoric can occur in the womb from a baby receiving different levels of hormones in the womb during fatal development of the brain. For example some male babies may receive too much estrogen in the womb resulting in an attraction to the same sex, or male to female gender dysphoria and vice versa (In the Womb 2008). This scientific hypothesis is very controversial and some worry that some mothers would abort a gay child or gender dysphoric child if it could be detected prenatally (In the Womb 2008). Intersex is an entirely different condition with a baby processing a mix of male and female sex chromosomes resulting in having both male and female sex organs (In the Womb 2008).
How do other internal organs develop? In the womb one of the most important organs to develop is the lungs (Doubilet & Benson 2008). The developing fetus must practice breathing by taking amniotic fluid in the developing lungs to strengthen the diaphragm and chest (Doubilet & Benson 2008) . As the kidney’s develop they release amniotic fluid back into the amnion (Doubilet & Benson 2008). When this process does not occur properly it can effect the development of other organs, including the brain, and a baby can die.
In conclusion, embryonic development leads to neurodiversity. Neurodiversity is a natural variation is brain structure and brain function driven by environmental factors as well as natural selection, and neurodiversity should be celebrated. Neurodiversity is beautiful.
References
Delany, D.R., Gaydos, S.S., Romeo, D.A. et al. Down syndrome and congenital heart disease: perioperative planning and management. J Congenit Heart Dis 5, 7 (2021). https://doi.org/10.1186/s40949–021–00061-3 https://link.springer.com/article/10.1186/s40949-021-00061-3
Doubilet, Peter. Benson, Carol. “Your Developing Baby: Conception to Birth” Mcgraw Hill. 2008.
Elshazzly, Mohamed, Lopez, Michael. “Embryology, Central Nervous System” 2022. https://www.ncbi.nlm.nih.gov/books/NBK526024/
Edwards, Christopher. Khalili, Yasi. “Moro Reflex” 2022. https://www.ncbi.nlm.nih.gov/books/NBK542173/
Futagi, Yasuyuki. Toribe, Yasuhisa. Suzuki, Yasuhiro. “The Grasp Reflex and Moro Reflex in Infants: Hierarchy of Primitive Reflex Responses” International Journal of Pediatrics. 2012. https://www.hindawi.com/journals/ijpedi/2012/191562/
Martha Wilson Jones, Elaine Morgan, Jean E. Shelton, Christine Thorogood,
“Cerebral Palsy: Introduction and Diagnosis (Part I)”, Journal of Pediatric Health Care, Volume 21, Issue 3, 2007, 146–152,
Rothbein, Dylan. “Autism” Medium. 2021. https://medium.com/@dylanrothbein/autism-cbd75769943d
Hedrith, Vladimir. “Preschoolers with autism who have sleep problems tend to show excessive connectivity in the thalamus” Psypost. 2023. https://www.psypost.org/2023/06/preschoolers-with-autism-who-have-sleep-problems-tend-to-show-excessive-connectivity-in-the-thalamus-164369
“In the Womb: Identical Twins” National Geographic. 2008.