Prenatal human development - periods, stages, and processes of development after fertilization

Prenatal development lasts 10 lunar months and runs through three periods. The first week is the proliferative period, weeks 2 to 8 are the embryonic period, and from week 9 to birth is the fetal period.

The points to hold on to:

• Fertilisation = union of egg and sperm, in the upper third of the uterine (fallopian) tube; the result is one large cell, the zygote (about 140 μm).
• The zygote cleaves: 2 cells, 4 cells, then the morula (10 to 32 cells), which reaches the uterus at the end of the first week and becomes the blastocyst (inner cell mass plus trophoblast).
• In the second week the inner cell mass becomes the bilaminar disc (epiblast above, hypoblast below) with an amniotic cavity above and the yolk sac below.
• Gastrulation (days 14 to 20) turns the bilaminar disc into a trilaminar disc: ectoderm, mesoderm, endoderm. The primitive streak and primitive node drive it.
• Late in the third week three events follow: the notochord forms (day 17), the pharyngeal (oropharyngeal) membrane marks the beginning of the primary mouth, and the neural plate forms. Neurulation (about day 20) closes the neural plate into the neural tube.
• The neural crest cells separate from the neuroectoderm and migrate. In the head they form the ectomesenchyme, the "fourth germ layer"; every tissue of the tooth except enamel, and its supporting apparatus, comes from them.
• The face and oral structures form between the 4th and 7th weeks. This is the time of greatest susceptibility to teratogens (weeks 2 to 8).

Prenatal development is the process of growth and development within the womb, during which the zygote becomes an embryo, then a fetus, and finally a baby. The prenatal period lasts 10 lunar months (40 weeks). Development passes through three phases (periods):

The proliferative period is also called the preimplantation period. Its events are fertilisation, implantation and the embryonic disc, together with the formation of the blastocyst.

The three phases in one line: 1st week = proliferative stage (cellular proliferation, migration and differentiation); 4th week = differentiation stage (morphogenesis of internal and external organs); 8th week onward = fetal stage (growth and maturation of the fetus), then birth.

Exam tip: the proliferative period is the first week (fertilisation to implantation). Learn it as: proliferative = week 1, embryonic = weeks 2 to 8, fetal = week 9 to birth.

Day-by-day schedule

A day-by-day outline (from the student notes, for the days the lecture does not date) fits the lecture's periods:

Fertilisation is the union of egg and sperm (fusion of a sperm cell and an egg cell, the oocyte). The origin of the tissues begins with it, when the sperm contacts the egg in the distal part of the uterine tube. It happens in the upper third of the oviduct (fallopian tube), in its widest part, the ampulla, which lies close to the ovary.

• The ovum is a big round cell. The sperm is small and highly mobile.
• After ovulation the ovum (0.15 mm in diameter) is picked up by the fimbriae of the tube and moved along by the cilia and the peristaltic movement of the tube.
• Under the influence of oestrogen the cervix secretes a flow of alkaline mucus. Of the sperm deposited in the vagina only thousands, capacitated sperm, enter the uterine tube; only 300 to 500 reach the ovum. The rest are destroyed by the acid medium of the vagina. It takes about 1 hour for sperm to reach the site.

The three phases of fertilisation

1. Phase 1: the sperm passes through the corona radiata cells by the effect of the enzyme hyaluronidase secreted from the sperm, and by the movement of its tail. In the acrosome reaction the acrosome releases its contents.
2. Phase 2: the inner acrosomal membrane dissolves and the sperm penetrates the zona pellucida by acrosin.
3. Phase 3: the plasma membranes of oocyte and sperm fuse. The secondary oocyte (arrested at metaphase II) immediately restarts and completes its second meiotic division, and a male and a female pronucleus form.

Polar bodies, seen in division in the figure, are cast off by the oocyte during the maturation divisions.

After fertilisation, the single large cell with a complete (diploid) set of 46 chromosomes is called the zygote. The fertilised egg grows and is termed the zygote; it is a large single cell, 140 μm in diameter. In the centre of the zygote the male and the female genetic material (DNA) can be seen as two nucleoli (the male and female pronuclei).

The continuous process of cell division follows fertilisation. The zygote divides in the primary (first) division: rapid cell division forming two new cells. The next division gives the four-cell stage.

The cell mass produces a ball of cells, the morula, in the uterine tube. It has 10 to 32 cells. The morula grows and begins its migration medially towards the uterus at the end of the first week. Once in the uterine cavity the morula floats freely for the next 2 days, covered by endometrial fluid and mucus.

Sequence: zygote -> 2 cells -> 4 cells -> morula (10 to 32 cells) -> blastocyst.

On the slide: the morula is a solid ball of 10 to 32 round cells inside the zona pellucida, with no cavity yet. Slide 3 (below) has a cavity and an eccentric clump of cells.

Shortly after the morula enters the uterus from the fallopian tube it becomes the blastocyst through cellular differentiation and cavitation. As the blastocyst swells it becomes hollow and develops a small inner cell mass. The morula's cells differentiate into two types:

• an inner cell mass (embryoblast), growing on the interior of the blastocoel;
• trophoblast cells, growing on the exterior.

In the student notes the blastocyst is formed by the trophoblasts pumping sodium ions so that water enters by osmosis. The fluid is absorbed through the canaliculi of the zona pellucida; the morula starts to accumulate fluid and forms a cavity between its cells. Once the cavity appears it is called a blastocyst. The zona pellucida becomes stretched, thinned and gradually disappears shortly before implantation.

The blastocyst has an outer layer of cells (trophoblast, which becomes the placenta), an inner cell mass (embryoblast, which forms the embryo) and a fluid-filled cavity, the blastocoel. The formation of the blastula (blastocyst, a hollow ball of cells) marks the beginning of morphogenesis, the shaping of the embryo, and of the migration of dividing cells to specific locations.

Implantation is the process in which the blastocyst digests the uterine endometrium, permitting deeper penetration. It begins about 6 to 7 days after fertilisation (about the 20th day of a regular menstrual cycle). The blastocyst attaches to and burrows into the thickened uterine lining (the endometrium) to establish a connection with the maternal blood supply. That connection is essential for the exchange of nutrients, waste and gases between mother and embryo.

• As the zona pellucida disappears the trophoblast cells become more adhesive to the endometrium.
• The trophoblast cells invade the stromal cells lying between the secretory endometrial glands, by histolytic action of the blastocyst, bringing about deeper penetration into the decidua.
• The trophoblast cells facilitate this adhesion by penetrating the endometrium with protrusions of the cytoskeleton of their cells.
• The trophoblast cells differentiate into syncytiotrophoblast (where hCG, human chorionic gonadotropin, is produced) and cytotrophoblast. The syncytiotrophoblast ruptures maternal capillaries and establishes a route of passive exchange between mother and embryo (lacunae filled with maternal blood).
• The trophoblast also begins to differentiate to form a primitive placenta; the embryo connects to the developing placenta through a stalk.

hCG

hCG (human chorionic gonadotropin) is produced by the trophoblast starting on day 6. It causes the endometrium of the uterus to grow and proliferate and prevents the menstrual cycle from occurring. This is why a woman misses her periods when pregnant.

In the second week the blastocyst increases in size by proliferation. A blastocyst cavity forms between the inner cell mass and the trophoblast, and differentiation of the inner cell mass begins.

Formation of the embryonic disc

• Two small cavities develop on either side of the inner cell mass. They reach each other in the centre, where a small disk is formed: the embryonic disk.
• The embryonic disk becomes the embryo, composed of the common walls of the two adjacent sacs.
• One sac is lined with ectodermal cells, which will form the outer body covering (the epithelium). The other sac is lined with endodermal cells.
• The embryonic disc is also called the embryoblast (lecture term).

The bilaminar disc

The result is a bilaminar embryonic disc of two cell layers:

Above the disc is an upper amniotic cavity, below it is the lower yolk sac (a primitive haematopoietic organ for the embryo and fetus). The trophoblast also begins to differentiate to form a primitive placenta, and the embryo connects to the developing placenta through a stalk.

Exam tip: bilaminar = two layers (epiblast + hypoblast), trilaminar = three layers (ectoderm, mesoderm, endoderm). Do not mix up the second and third week.

Gastrulation is the process by which the bilaminar embryonic disc is converted into a trilaminar one. It lasts from the 14th to the 20th day and establishes the three germ layers by stratification. A single-layered blastula is reorganised into a multilayered gastrula.

• Towards day 14 the primitive ectoderm thickens, forming the primitive plaque.
• A primitive groove forms in it (by invagination), and at its cranial edge the primitive node (Hensen's node) forms. Together the groove, node and pit are the primitive streak.
• The cells of the primitive groove and node acquire mitotic activity. They divide and penetrate between the ectoderm and the endoderm.
• Formation of the primitive streak within the embryonic disc is critical. The epiblast cells migrate through the primitive streak towards the hypoblast, which eventually creates three tissue layers: ectoderm, mesoderm, endoderm.

The beginning of mesoderm formation is the invagination of the ectoderm under the primitive streak: the inserted cells divide ectoderm and endoderm.

Reading the figure of cell movements

The numbered scheme of the embryonic disc seen dorsally has these labels: 1 primitive groove, 2 primitive pit, 3 primitive node, 4 oropharyngeal membrane, 5 cardiac plate, 6 sectional edge of the amniotic membrane, 7 mesoderm, 8 endoderm, 9 future cloacal membrane; 1+2+3 together form the primitive streak. The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

The mesoderm has a twofold origin:

• the majority is derived from the proliferation (inward movement) of the ectoderm through the primitive streak;
• a smaller part is formed later, from the migrating cells of the neural crests (the ectomesenchyme).

The germ layers are the ectoderm, the mesoderm and the endoderm. The initial process starts with the primitive streak and the primitive node.

The third week of development: the bilaminar embryonic disk converts into a trilaminar disk of ectoderm, mesoderm and endoderm (gastrulation). The primitive streak is critical.

Fate of the germ layers

The mesoderm in detail

Through complex movements the embryo develops from a simple bilaminar disk into different forms. Three fateful events occur:

1. Notochord formation: the organiser of the whole embryo.
2. Formation of the pharyngeal (oropharyngeal, buccopharyngeal) membrane: the beginning of the primary mouth.
3. Formation of the neural plate: an incentive for the formation of tissues, organs and organ systems.

The notochord

Towards day 17 of the primitive plaque a new proliferation of cells forms in depth: the notochord. In front of the primitive streak lies the primitive node, mesodermal cells that will form the notochord. The notochord activates (induces) the overlying ectoderm to form the neural plate. Portions of the mesoderm that do not form the notochord divide into sections called somites.

By the end of the third week the trilaminar embryonic disc has a definite orientation. During the fourth week the embryo begins to form a tubular structure.

Cross sections

In cross section the notochordal plate lies in the midline between the intraembryonic mesoderm on both sides, beneath the ectoderm and above the endoderm; the notochord then separates and the neural groove, neural tube, somites and notochord appear in order.

During the fourth week portions of the mesoderm that do not form the notochord divide into sections called somites. They are specific body regions and structures. There are 38 somite pairs; they give rise to most of the skeletal structures of the head, neck and trunk.

From the student notes: somites are segmented blocks of mesodermal tissue that form on both sides of the neural tube. They are first observed during the third week and are one of the earliest visible signs of body segmentation.

• Formation: the notochord induces the overlying mesoderm to differentiate into paraxial mesoderm, which becomes segmented into somites (somitogenesis).
• Order: the somites form in a cranio-caudal (head-to-tail) sequence along the length of the embryo.
• Derivatives: the sclerotome (medial part of each somite) forms the axial skeleton, vertebrae and ribs; the dermatome (lateral part) forms the dermis of the skin; the myotome (intermediate part) forms skeletal muscles of the trunk, limbs and diaphragm.
• Each pair of somites is a segment that contributes to a specific region of the body.
• Abnormalities in somitogenesis can lead to vertebral defects, scoliosis and other musculoskeletal abnormalities.

The embryonic period runs from the second to the eighth week. The mass of cells becomes an embryo, the different tissues develop, and the organs and organ systems are organised. The heart is formed in the fourth week. The face and the oral structures are formed between the 4th and 7th week.

Folding of the embryo

The embryo is folded by the head fold, the lateral folds and the tail fold. The development of a head fold is critical to the formation of the primitive oral cavity. Details are given with the 4th week in topic I.02.

Clinical comment: teratogens

Environmental teratogens may affect the development of normal cells, tissues, organs or organ systems. The smaller and less complex the development, the less extensive the problem created. Development is also related to timing. Tissues are most susceptible to defective development when they begin to differentiate in the embryonic period (2 to 8 weeks).

Epithelial-mesenchymal interactions

Epithelial-mesenchymal interactions are the necessary interactions of an epithelium and the underlying mesenchyme that determine the terminally differentiated tissue. They matter for the face and the teeth.

Formation of the neural tube and the fate of the germ layers

The key events of the first weeks: the series of events leading to the formation of the three-layered (triploblastic) embryo during the first 3 weeks of development involves cell proliferation and migration. During the next 3 to 4 weeks the major tissues and organs differentiate from the triploblastic embryo. The key events are the differentiation of the nervous system and neural crest tissues from the ectoderm, the differentiation of the mesoderm, and the folding of the embryo.

The cranial part of the embryo grows first. In the ventral direction the pharyngeal membrane is formed. Distally over the notochord starts an ectodermal thickening, the neural plate. The neural folds appear during the third prenatal week: the lateral edges of the neural plate begin to elevate as folds, arising dorsally. These folds represent the first change in shape of the embryo's body from the flat sheet of cells.

At approximately 20 days the gastrula enters the next phase, neurulation. The neural folds reach the midline; the neural tube closes first in the cervical region and then closes both anteriorly and posteriorly.

Elements of neurulation

The elements are the neural plate, the neural tube and the neural crests. The steps:

1. A neural plate forms in the cranial end (neuroectoderm: ectodermal cells above the notochord thicken and elongate).
2. At the end of the third week the lateral edges of the plate become elevated and move together to form the neural folds. The space created is the neural groove.
3. The neural folds fuse and the plate becomes the neural tube. Fusion begins at the future cranial end and proceeds towards the caudal end.
4. During closure the cells at the crest of the neural folds detach, forming the neural crest cells.
5. When the neural tube has fused completely, neurulation is complete and the tube is separated from the surface ectoderm.

The neural groove deepens and is surrounded by the two neural folds, and the neural crest cells develop from these folds. The neural tube gives rise to the central nervous system (brain and spinal cord). It has a hollow lumen.

Primitive brain

In the cranial part the neural tube builds the primitive brain. At this stage it consists of the prosencephalon, the mesencephalon and the rhombencephalon. The bilateral neural crests stimulate the maxillofacial area. In the head region the anterior part of the tube expands as forebrain, midbrain and hindbrain; the hindbrain forms 8 bulges, the rhombomeres.

In the student notes: the cranial (anterior) portion expands to form the prosencephalon (forebrain), the middle portion the mesencephalon (midbrain) and the caudal portion the rhombencephalon (hindbrain). These primary vesicles differentiate into secondary vesicles: the prosencephalon gives the telencephalon and diencephalon, the mesencephalon remains relatively unchanged, and the rhombencephalon gives the metencephalon and myelencephalon.

As the neural tube forms, a group of cells separates from the neuroectoderm. These neural crest cells have the capacity to migrate and differentiate extensively within the developing embryo. They are the basis for nerve structures. The neural crests are bilaterally paired strips of cells arising in the ectoderm at the margins of the neural tube.

The neural crest in the head

Neural crest cells in the head region have an important role. They differentiate to form most of the connective tissue of the head. Embryonic connective tissue elsewhere is derived from mesoderm and is known as mesenchyme; in the head it is known as ectomesenchyme, reflecting its origin from neuroectoderm. In the head region neural crest cells migrate into the pharyngeal arches and form ectomesenchyme, contributing tissues that in the body region are typically derived from mesoderm.

Neural crest cells are a migratory cell population that gives rise to the majority of the cartilage, bone, connective tissue and sensory ganglia in the head. Many different systems (neural, skin, teeth, head, face, heart, adrenal glands, gastrointestinal tract) have a contribution from neural crest cells.

Exam tip: All tissues of the tooth (except enamel) and its supporting apparatus are derived directly from neural crest cells. The proper migration of the neural crest cells is essential for the development of the face and the teeth.

Three flows

The neural crest cells divide rapidly. Three cell flows with different biological functions form:

• the first group develops at the site of its formation;
• the second group forms migratory flows;
• the third group is intended to be absorbed and stimulates differentiation of the surrounding tissue.

Ectomesenchyme

The migration flows of the neural crest are called ectomesenchyme. They represent the fourth germ layer. They are another source (besides the primitive mesoderm) for mesenchymal tissue.

Migration pathway

Ectomesenchymal cells transmit active products into the intercellular spaces: fibronectin and glycosaminoglycans. The migration pathway of the neural crest cells from the neural folds is achieved by these products. Their broadcasting stops when the cells reach their destination.

From the student notes: when the crest cells migrate they differentiate into sensory neurons, autonomic neurons, glial cells, pigment cells (melanocytes), parts of the skull and face and other tissues. Dysfunction of the neural crest can lead to a variety of congenital conditions known as neurocristopathies.

The embryo in the fourth week is a tube with three parts:

The cranial part is the largest. At about day 22 the embryo carries the stomodeum, pharynx, thyroid, lung, heart, liver, yolk sac, body stalk, tailbud, cloacal plate and the anterior neuropore.

The embryo at day 26 to 27 shows: 27 somites, forebrain, site of the lens placode, site of the otic placode, stomodeum, 1st, 2nd and 3rd pharyngeal arch, heart prominence and somites. At week 4, day 26, the embryo is 5 mm long and has 25 somites.

The fetal period continues until birth. Both values are found in the lectures for its start: the 8th week (p173) and the 9th week (p3). During this period growth continues, the already formed tissues and organs differentiate, and the fetus is prepared for function. All major structures are already formed but they continue to grow and develop.

As all major organs are formed, the fetus is not as sensitive to damage from environmental exposure as the embryo was. Instead, toxic exposure often causes physiological abnormalities or minor congenital malformation. The major organs continue to develop and mature, and the fetus becomes increasingly capable of independent survival outside the womb as it nears the end of the pregnancy.

Morphogenetic mechanisms for the formation of the three-dimensional embryo

A summary of what has happened until now, in five steps:

1. Developing mesoderm converts the bilaminar embryonic disk to a trilaminar disk; this involves cell proliferation and migration.
2. Formation of the neural plate and primary brain ventricles.
3. Formation of the neural tube and neural crests; formation of migratory flows, stimulating differentiation.
4. Multiple stimulation between the neural crests and the ectoderm.
5. Expression of different biophysical impacts associated with morphogenetic changes.

Sequence: fertilisation -> zygote -> morula -> blastocyst -> implantation -> bilaminar disc -> gastrulation (trilaminar disc) -> notochord -> neural plate -> neural tube and neural crest -> pharyngeal arches and face.