Study Notes on Embryonic Development
STAGES OF EMBRYONIC DEVELOPMENT IN MAMMALS
1. Lesson Plan
2.1. Fertilization, formation of the zygote
2.2. Segmentation (Cleavage)
2.3. Gastrulation
2.4. Differentiation of embryonic germ layers
2.1. FERTILIZATION, FORMATION OF THE ZYGOTE
Definition of Fertilization:
Fertilization is the biological process that occurs when the male gamete (sperm cell) meets and fuses with the female gamete (ovum).
The purpose of this process is to restore the genetic complement specific to the species.
As a result of fertilization, a new cell called the zygote (fertilized egg cell) is formed, which is the first cell of the new organism that will later develop into all tissues and organs.
Sexual Maturity and Hormonal Regulation:
Fertilization can occur only after the female has reached sexual maturity, characterized by cyclic morphological and functional changes in the female reproductive system.
These changes are regulated by hormones produced by the pituitary gland (hypophyseal hormones) and the ovaries (ovarian hormones).
Cyclic Changes in the Female Reproductive Tract:
The cyclic changes involve several organs including:
Ovaries
Oviducts (fallopian tubes)
Uterus
Cervix
Vagina
Collectively, these changes form the sexual cycle or estrous cycle.
Types of Fertilization:
Internal Fertilization: Occurs inside the female reproductive tract, characteristic of mammals and birds.
External Fertilization: Occurs outside the female body in species such as fish, typically in water.
Insemination Process:
Internal fertilization occurs after insemination, the deposition of spermatozoa inside the female reproductive tract.
After ovulation, the secondary oocyte is captured by the infundibulum of the oviduct and directed through its lumen, taking approximately 8 hours to reach the distal third of the oviduct.
Transport Mechanisms:
Directional flow of fluid toward the uterus
Coordinated beating of cilia lining the oviduct
Peristaltic contractions by smooth muscles of the oviduct wall
If the oocyte does not meet spermatozoa in this region, it degenerates before reaching the uterus.
Spermatozoa Migration and Survival:
Spermatozoa migrate toward the site of fertilization, which takes approximately 1 to 5 hours.
Survival times vary by species:
Up to 6 days in mares
Up to 11 days in bitches
Up to 21 days in domestic birds (hens)
Factors affecting survival include unfavorable pH conditions, entrapment in uterine folds/glands, and destruction by leukocytes.
Decapacitation and Capacitation:
Upon contact with seminal plasma, sperm undergo decapacitation, where glycoproteins attach to the sperm membrane, inhibiting acrosomal enzyme activity, preventing fertilization.
To become capable of fertilization, sperm must undergo capacitation, which includes:
Removal of decapacitation factors by enzymes like fertilizin, β-glucuronidase, and β-amylase.
Occurs in the uterus and oviducts, increasing spermatozoa's penetrating ability.
Changes in the acrosome during capacitation enhance the release of acrosomal enzymes.
Sperm-Oocyte Interaction:
Attachment between oocyte and sperm is controlled by specific cellular receptors ensuring species-specific fertilization.
Spermatozoa penetrate the corona radiata and zona pellucida using flagellar movement and acrosomal enzymes.
Upon contact with the oocyte membrane, zona pellucida reaction occurs, preventing multiple sperm from fertilizing (monospermic fertilization).
Entry of Spermatozoon into Oocyte:
The sperm enters as a whole through fusion of membranes; the sperm flagellum separates while the head and neck remain.
Pronuclei Formation:
The proximal centriole detaches and duplicates within the sperm neck.
The oocyte completes its second meiotic division, forming the mature ovum and second polar body.
The female pronucleus enlarges, while the male pronucleus forms from the sperm head.
Both pronuclei fuse during amphimixis, forming the diploid zygote.
Consequences of Fertilization:
Activation of the oocyte, completion of its meiotic division, restoration of species-specific diploid chromosomes, and determination of genetic sex.
While fertilization is species-specific, hybrids can occur between closely related species (e.g., donkey and horse), often resulting in infertility, contrasting hybrids like dogs and wolves that remain fertile.
2.2. SEGMENTATION (CLEAVAGE)
Definition:
After fertilization, the zygote is surrounded by the zona pellucida, which protects it during transit through the oviduct where cleavage occurs.
Cleavage Characteristics:
Mammalian ova contain some yolk, resulting in complete cleavage that may be equal or unequal and asynchronous.
During total cleavage, all cellular components of the zygote are distributed to daughter cells called blastomeres; early blastomeres continue to develop.
Equal and Unequal Cleavage in Various Species:
In cattle, horses, sheep, and pigs, equal cleavage occurs during the first two divisions transitioning to unequal:
Larger blastomere termed macromere
Smaller termed micromere
In primates and rodents, cleavage is unequal and asynchronous from the beginning.
Cell Size and Character:
With increased blastomere numbers, their size decreases due to the constraint of the zona pellucida.
Blastomeres show totipotency, each capable of forming an individual embryo if separated.
Morula Formation:
Continued multiplication of blastomeres forms a morula, resembling a mulberry.
Composed of approximately 32 or 64 blastomeres in most mammals, while about 16 in humans, the formation time varies among species.
Development Timing:
The morula travels through the oviduct within about 3 days in primates and rodents.
In some species, the morula stage is incomplete during oviduct transit but reaches blastula stage subsequently.
Birds develop to gastrula stage before exiting the oviduct, with protective membranes for development outside the maternal body.
Blastocyst Formation:
Micromere numbers increase as they compress and lose spherical shape, forming polyhedral tissue structures.
Major processes in blastocyst formation include the formation of the trophoblast and the embryoblast (inner cell mass) leading to the embryonic disc formation.
Trophoblast Formation:
Pressure from micromere fluid causes peripheral macromeres of the morula to flatten and completely surround the structure, forming the primary trophoblast.
At this point, the zona pellucida disintegrates.
Embryonic Disc and Fluid Accumulation:
Micromeres compact, acquiring secretory ability, and fluid accumulates, creating the blastocoelic cavity.
Micromeres relocate to the animal pole, becoming the embryonic disc.
Nutritional Shift to Uterine Support:
As yolk reserves are depleted, trophoblasts contact uterine mucosa to absorb nutrients (carbohydrates, proteins, etc.) from uterine secretory cells.
Trophoblast differentiation occurs for absorption and processing of nutrients.
Implantation and Nidation:
Following yolk depletion, close contact between blastocyst and uterine mucosa occurs, facilitated by progesterone secreted by the corpus luteum.
In primates and rodents, blastocyst nidation occurs with deep penetration into the endometrium, whereas in other mammals, adherence occurs without deep penetration.
Cellular Changes During Implantation:
For attachment, the blastocyst orients the embryonic disc toward the uterine mucosa; trophoblasts differentiate into cytotrophoblast and syncytiotrophoblast.
Syncytiotrophoblast projections invade the uterine epithelium, causing maternal connective cells to enlarge and differentiate into decidual cells, completing the nidation process.
Completion of implantation initiates gestation, where embryo nutrition shifts to placental support.
2.3. GASTRULATION
Importance of Gastrulation:
Gastrulation represents a critical stage of early embryonic development when the embryonic disc reorganizes and acquires a structural plan.
Three primary germ layers are formed: ectoderm, mesoderm, and endoderm, along with establishment of the body's primary axes.
Embryonic Disc Formation:
The bilaminar embryonic disc consists of the epiblast (upper) and the hypoblast (lower).
Epiblast: Columnar cells form this layer and give rise to all three germ layers.
Hypoblast: Cuboidal cells contribute primarily to extra-embryonic structures, not directly forming embryonic tissues.
Basic orientation of the disc is identifiable with cranial (anterior) and caudal (posterior) regions.
Primitive Streak Development:
Gastrulation begins with the appearance of the primitive streak on the dorsal epiblast, marking early embryonic organization.
The primitive streak appears as a longitudinal thickening with a central primitive groove for epiblast cell migration, determining the embryo's cranial-caudal axis.
Primitive Node:
At the cranial end of the primitive streak lies the primitive node (node of Hensen), which regulates migrating epiblast cell patterns and positions.
It is essential for proper future notochord positioning, influencing embryonic axial development.
Epiblast Cell Migration:
Epiblast cells migrate through the primitive streak, interposing themselves to form the intra-embryonic mesoderm, resulting in a trilaminar embryo.
Notably, two areas remain without mesoderm: prochordal plate (cranially) and cloacal membrane (caudally).
Notochord Formation:
The notochord arises from cells migrating cranially from the node of Hensen, establishing the embryo's longitudinal axis and serving as structural support.
The notochord stimulates the neural plate formation and influences early development patterning, though it is eventually replaced by the vertebral column.
2.4. DIFFERENTIATION OF EMBRYONIC GERM LAYERS
Embryonic Gut Formation:
As embryonic folding occurs, the amniotic cavity expands around the embryo and part of the yolk sac wall is integrated into the embryo, forming the primitive gut.
Divided into:
Foregut: Anterior intestine, closed by the pharyngeal membrane.
Midgut: Middle intestine, temporarily connected to yolk sac via omphalomesenteric duct.
Hindgut: Posterior intestine, closed by the cloacal membrane.
Endodermal Cells Differentiation:
Structures formed from differentiating endodermal cells include:
Epithelium of the digestive tract (excluding oral cavity and anal canal)
Epithelium of the respiratory tract and lungs
Epithelium of the tympanic cavity and Eustachian tube
Epithelium of urinary bladder and urethra
Epithelium of vagina
Germ cells
Parenchyma of the thyroid, parathyroid, liver, pancreas, and salivary glands
Thymic reticulum
Epithelium of tonsils.
Mesoderm Evolution:
Around the fourth week, the mesoderm differentiates into:
• Paraxial Mesoderm: Located alongside the notochord.
• Intermediate Mesoderm: Found laterally, forming nephrotomes and urogenital system structures.
• Lateral Mesoderm: Divided into somatopleure (associated with the amniotic cavity) and splanchnopleure (associated with the yolk sac).
Somite Formation from Paraxial Mesoderm:
Paraxial mesoderm begins regular division into somites, starting cranially and progressing caudally.
Somites differentiate into:
Sclerotome: Forms cartilage, bone of the axial skeleton, vertebrae, and ribs.
Dermomyotome: Cells adjacent to sclerotome become myoblasts for skeletal muscle, while superficial cells form dermis and hypodermis.
Intermediate Mesoderm Contributions:
Converts into nephrotomes and nephrogenic cords, forming nephrons and extra-renal urinary ducts, contributing to the urogenital system.
Lateral Mesoderm Contributions:
Splits into two layers with distinct forms:
Somatopleure: Forms parietal serous membranes (e.g., pleura, pericardium).
Splanchnopleure: Forms visceral serous membranes and connective layers of internal organs and contributes to body cavity formation.
Main Derivatives of the Mesoderm:
Develops multipurpose structures including connective tissues, bones, cartilage, muscles, blood cells, heart walls, serous membranes, kidneys, reproductive systems, adrenal cortex, and lymphatic components.
Ectoderm Development (Neurula Stage):
Neurulation marks ectoderm thickening resulting in the formation of the neural plate.
Neural folds elevate and fuse to form the neural tube, giving rise to the brain (enlarged cranial region) and spinal cord (narrower tail region).
Neural crest cells detach during neural tube closure, giving rise to cranial/spinal ganglia, autonomic ganglia, and adrenal medulla structures.
Derivatives of the Ectoderm:
From the neural plate and tube:
Entire nervous system
Sensory receptors of sensory organs
From surface ectoderm:
Skin epidermis; hair, nails, hooves; sweat and sebaceous gland epithelium; mammary gland components; oral and nasal epithelium; pituitary and pineal glands; dental enamel; and other specialized structures.