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Introduction to Human Embryology: The First 4 weeks of Development
BMS 9/28/2014
McMaster opened the lecture with a point that development is always happening. Development occurs as the zygote
develops to baby, from toddler to teenager, and continues until you die.
In males: Gametogenesis occurs in the testes, more specifically seminiferous tubules. In the histological side,
facing the lumen are spermatozoa. In the process of spermatogenesis, a germ cell called a spermatogonium goes
through meiosis to produce 4 haploid sperm cells (a normal meiosis daughter cell number)
In females: Oogenesis occurs in ovaries. The germ cell that undergoes meiosis is known as the oogonium, and
when it undergoes meiosis, only one haploid cell, the oocyte, is produced. The second meiotic division occurs
after fertilization. The first stage of meiosis is suspended in immature oocytes, and completes as follicles mature.
The polar bodies produced during meiosis 1 and 2 do not become oocytes.
A woman’s menstrual cycle (28 days) consists of three main events to prepare for a possible fertilization event.
First, during the follicular phase, the endometrium of the uterus (where the embryo will implant) thickens and
becomes more vascularized and glandular.
During the ovulatory phase, estrogen levels and the following luteinizing hormone (LH) surge causes the mature
egg to be released from a mature follicle in the ovaries and travels down the oviduct in hopes of being fertilized.
Lastly, the luteal phase involves the corpus luteum, the ruptured follicle from which the ovum emerged, begins
secreting progesterone to continue to keep the uterus in good shape for implantation. However, if implantation
does not occur, the endometrium sloughs off and the cycle begins again.
Fertilization and Pre-implantation Development
The sperm fuses with the egg in the oviduct, producing a zygote! The zygote then begins going through cleavage,
a special type of cell division where the number of cells increases, but the overall size of the embryo does not
Eventually, the cleavage forms a solid ball of cells called the morula.
To form the next stage of the embryo, the morula compacts to form a tight cell mass. The resulting structure is
called the blastocyst (blastula). It consists of an inner cell mass and a hollow space known as the blastocoel.
The blastocyst has two types of cells
1. Trophoblasts
2. The inner cell mass cells
This is the first differentiation of cells, as now they will take on different tasks. Eventually, the blastocyst
implants into the endometrium. The trophoblasts will become part of the placenta, while the inner cell mass
will form all parts of the fetus.
When the trophoblast cells invade the endometrium, nutrition for the embryo is provided by endometrial
glands. (Remember the whole endometrial thickening during the menstrual cycle?)
Development during Week 2 of pregnancy: The Embryonic Disc
The blastocyst expands and the amniotic cavity forms. The structure is now called the bilaminar embryonic disc.
The previous blastocoel is now the yolk sac. The epiblast (primitive ectoderm) and hypoblast (primitive
endoderm) are the two layers of the embryonic disc. All the parts of the embryo are derived from EPIBLAST.
At first the embryos nutritional needs are met by diffusion. This isn’t feasible as development continues, so for a
period of time, placental development proceeds faster than embryonic development.
Gastrulation (Week 2-3)
Epiblast cells invaginate inward to form the three germ layers. The invagination forms a line called the
“primitive streak”. The resulting three germ layers are ectoderm, mesoderm, and endoderm.
The notochord is a bar of differentiated mesoderm that runs through the length of the embryo. It chemically
induces the overlying ectoderm to differentiate into neuroectoderm and form the neural tube! The neural plate
folds upward and pinches to form the neural tube!
Neural crest cells emerge from the pinching of the neural tube. These cells go from epithelial to mesenchymal.
The neural crest cells form the peripheral nervous system, which is all neurons that do not have their cell bodies
in the spinal cord or brain. Some non-neural derivatives of the neural crest cells are melanocytes, head
mesenchyme, and pharyngeal arch mesenchyme.
Sadly, the neuralation process can sometimes have errors. Some neural tube defects include spinal bifida
(where the neural tube failed to close at an end) and anencephaly (where the head end of the neural tube did
not form correctly). Fortunately, folic acid supplements have cut neural tube defect cases in half!
Germ Layers
Mesoderm has 3 divisions: Paraxial (innermost), intermediate (middle), and lateral (outermost).
The lateral mesoderm splits to form the somatic mesoderm (somatopleure) and the visceral (splanchnopleure)
mesoderm. The split of these types of mesoderm yields the intraembryonic coelom.
The paraxial mesoderm makes somites. Somitic subdivisions include myotome (makes muscle) sclerotome
(makes bone) and dermatome (makes the dermis of the skin)
The somatic mesoderm makes the body wall
The visceral mesoderm makes the visceral wall.
The intermediate mesoderm makes the urogenital and cloaca
BONUS: somebody asked what the notochord’s adult “form” is. Turns out it forms the nucleus pulposus!
Body Morphogenesis (Week 4)
The embryo goes from being a flat disc to being a “tube within a tube” and crescent shaped. The processes that
allow this are cranio-caudal folding, lateral folding, and gut and body cavity formation.
Cranio-caudal folding: The growth of the neural tube causes the cranial part of the embryo to curve ventrally,
which forms the head fold. The caudal end curves ventrally as well. This is why the embryo looks curled up with
its head and tail tucked in.
Lateral folding: The edges of the embryo also fold ventrally to make a tubular embryo!
Gut and Body Cavity formation: As the cranial and caudal ends of the embryo curve ventrally, they pull the
endoderm with them, forming the foregut and hindgut. The yolk sac is in the middle of the foregut and hindgut,
and together they make the inner tube of the body. The intraembryonic coelom formed by the splitting of the
lateral mesoderm will form the peritoneal cavity!