Morphology can differ slightly depending on the scientific field of study you’re discussing. It can mean the phenotype (physical appearance) of an animal or, as the general definition determines, the study of living organisms and the relationships between their physical structures.
When we put this into the context of human development, our focus shifts to anatomical development during the embryonic, fetal, and postnatal stages. This is because this is the time frame in which the major milestones of human development take place.
Before we get to the good stuff we have to lay some groundwork and start with the fundamentals. Most living organisms begin with some sort of sexual process that requires fertilization of one cell type from another cell type.
In humans, those, of course, are the sperm and the egg which come together to form the zygote: the very first stage of human development. Where all the organ systems originate.
During what is called the “preimplantation” period, mammalian embryos spend the first few days of their existence floating around in the mother’s oviduct. (Plusa & Hadjantonakis, 2018). The embryo has not developed – or shed – the proper anatomy to anchor itself to the appropriate maternal tissues via a process known as “implantation.”
In order to achieve implantation, the embryo needs two different types of “lineages.”
After the deed is done and the race is won, the fertilized contains genetic material from both mother and father in two distinct pronuclei. Up until these two combine to form the one-cell nucleus, any transcriptional and translational activities in the oocyte are completely stopped.
When they begin again, a major transcription activates the zygotic genes, transferring “genetic control” (Kanenko & Choudhuri, 2017) to the maternally-derived components.
This transfer requires that all maternally-derived proteins are discarded and any epigenetic markers on the two parental genomes are removed. This is so the zygote can attain what is called “totipotency:” the ability of a single cell to divide and serve as the source for all differentiated cells in a given organism.
In the case of the human zygote, this capability allows the zygote to form the three embryonic germ layers from which all organ systems are developed.
Once the zygote levels up to totipotency, it can then become known as the embryo, the next major milestone in human morphological development.
The Extraembryonic Lineages
I know, it sounds a bit Game-of-Throne-sy, right? These lineages are heir to a different purpose, though, and – like an inverse version of Tyrion – are dispensable at the end of the process but are extremely important in the beginning.
This is because they don’t contribute anything useful postnatally, but do give rise to the fetal portion of the placenta and extraembryonic membranes, structures which are central to embryo survival.
Separately, the extraembryonic lineages are the trophoectoderm (TE) and the primitive ectoderm (PrE). They ultimately give rise to external embryonic tissues such as the placenta and the yolk sac (Mammals have yolk, too, weird right? More on this later).
The TE is a preplacental (before the placenta is developed) cell layer that differentiates from blastomeres (totipotent cells from the zygotic stage) about 2.5 days after fertilization. The TE is instrumental to the embryo’s ability to latch onto the uterine wall.
Together the TE and PrE form a structure called the pluripotent epiblast. The pluripotent epiblast is the tissue responsible for the development of somatic cells (cell type that makes up all organs outside of the reproductive system) and germ cells (cell type exclusive to the reproductive system).
Just before implantation of the embryo, the inner cell mass (ICM, a mass of pluripotent cells in the embryo) gives rise to the three embryonic germ layers, the endoderm, mesoderm, and ectoderm, and the primitive endoderm. Together, these all go on to form the yolk sac that was mentioned earlier.
The mammalian yolk sac is a little different than that of a chicken, in that it is essential to embryo survival before the formation of the fully functional placenta. The chicken’s yolk sac, on the other hand, is meant to last the entire prenatal developmental process and tastes particularly good when boiled then sprinkled with a dash of S&P.
There’s something about neotony – which is a baby-like appearance: giant head and huge, buggy yet twinkling eyes – that makes people all warm and fuzzy. Seeing a tiny human, a chubby puppy or lion cub just makes everybody want cuddles!
That all changes as well get older though: the smaller our heads get in relation to our body, the less cute we are.
In a newborn, the head accounts for one-quarter of the total length of the body, whereas in adults, it only makes up one-seventh. In a reverse manner, though, the muscles make up a very small percentage of the total body mass and increase with age.
Tanner (2001) refers to the rate of change of the anatomical growth rate throughout a human’s lifetime as the velocity of growth (VOG). VOG decreases from birth onward, starting as early as the fourth month of fetal life and peaking at about four months post-menstrual age (days after mother’s last menstrual cycle).
The VOG of weight and length of the fetus generally follow the same pattern. Weight, though, reaches its peak VOG about two months later than length, meaning the baby gets just a little bit chubbier right before birth.
As the fetus runs out of space in the uterus, the VOG slows down (fun fact: twins’ and other multiples’ growth slows down sooner because they run out of space a lot faster than an individual baby!).
The rapid VOG in fetal development compared to children’s is due to the fact that cells are still undergoing mitosis – constituting a large portion of what is considered in the determination of the growth rate. Muscle and nerve cells of the fetus are significantly different than in a child or adult because both have cytoplasm around the nucleus.
There is also a much higher proportion of water in the muscle cells of a fetus. In fact, the later fetal and postnatal growth periods consist primarily of building up the cytoplasm of muscle cells, incorporating salts, and forming contractile proteins (which are, as the name suggests, proteins that allow muscles to expand and contract).
Key systems begin establishing themselves after the first three weeks of the embryonic stage of life. Over the next five weeks, “templates” are being laid to form the body’s major structures and organ systems.
The brain develops earlier than any other part of the body. So early that, at birth, it is already 25% of its adult weight. Because this system is highly complex, for our purposes here, we’ll only mention that the development of the nervous system starts during week three of fetal development and continues on into the postnatal period.
The nervous system arises from the ectoderm, starting with the formation of the neural plate.
Early vascular development
the heart develops from cardiogenic mesoderm that originates from above the cranial end of the developing neural tube. The formation of the vascular system continues all the way up until week 38.
Made up of the:
- Pituitary gland
- Thyroid gland
- Parathyroid gland
- Adrenal gland
This system is formed from the ectoderm and endoderm, known collectively as the epithelia. The endocrine system is a system within systems – it resides within specific endocrine organs and other major organs and tissues such as reproductive organs and the adrenal glands.
Gastrulation is the process of gut formation and comes the endoderm. In week four, the foregut, midgut, and hindgut develop in the embryo. The foregut becomes the mouth, the midgut is formed by lateral embryonic folding which “pinches off” a pocket of the yolk sac, and the hindgut will separate into the urogenital and rectal regions.
Consists of the hair, nails, teeth, glands and sensory receptors. Major tissue organizations:
- Epithelial: epidermis, from the ectoderm.
- Mesenchyme: dermis and hypodermis, from the mesoderm.
- Melanocytes: type of cells adjacent to the neural tube, from the neural crest.
- Sensory nerve endings
Derived from the neural crest, the skeleton truly begins to take shape between weeks 9 and 10, although some primary components are made before then.
The vertebral column begins to develop in week five along with the ribs, skeletal muscles, and subcutaneous tissues. These are all differentiated from the mesoderm.
The process of development is from week 4 to week 6. The portions of the lung arise from two of the germ layers, endoderm (tubular ventral growth from foregut pharynx) and mesoderm (mesenchyme of lung buds).
The respiratory system remains entirely fluid-filled until birth, although there are fetal respiratory movements (FRM) or fetal breathing movements (FBM) which occur in the 3rd trimester.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development. (2018, October 1). What are the parts of the nervous system?. Retrieved from https://www.nichd.nih.gov/health/topics/neuro/conditioninfo/parts
- Hill, M.A. (2019, October 15). Embryology. Human System Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Human_System_Development
- Kaneko, K., & Choudhuri, S. (2017). Epigenetics in reproduction and development. In Reproductive and developmental toxicology (2nd ed., pp. 1005-1021). Retrieved from https://www.sciencedirect.com/book/9780128042397
- Tanner, J. M. (2001). Human development | Description, rate, growth, & puberty. In Encyclopedia Britannica. Retrieved from https://www.britannica.com/science/human-development