Prenatal Development and Gametogenesis

Stages of Prenatal Development

• Three stages: Pre-embryonic, Embryonic, Fetal
• Most visible development occurs during the embryonic period

Gametogenesis

• Definition: Germ cells transform into gametes
• Meiosis: Chromosome number halves (46 diploid → 23 haploid)
• Gametes: Sex cells capable of reproduction
  • Male: Sperms/Spermatozoa
  • Female: Oocytes

Spermatogenesis

• Spermatogonia: Diploid germ cells transforming into primary spermatocytes during puberty
• Meiosis I: Two haploid secondary spermatocytes
• Meiosis II: Four haploid spermatids
• Spermiogenesis: Spermatids elongate and mature into sperms

Oogenesis

• Oogonia: Diploid cells surrounded by a follicular layer
• Meiosis I: Begins before birth, arrests in prophase, and completes monthly beginning at puberty, forming one secondary oocyte and one polar body
• Meiosis II: Begins at ovulation, arrests at metaphase, and completes upon sperm penetration, forming one fertilized oocyte and one polar body

Ovarian Cycle

• Begins at puberty, terminates at menopause
• Hypothalamus: Stimulates the pituitary gland to release gonadotropins
  • FSH: Stimulates follicular maturation
  • LH: Stimulates estrogen production and ovulation
• Ovulation: Follicle becomes corpus luteum, secreting estrogen and progesterone, thickening the endometrium of the uterus

Pre-embryonic Period

• Week 1: Fertilization, cleavage, blastocyst formation, implantation
• Week 2: Bilaminar disc forms, lacunae form, extraembryonic mesoderm develops, hCG secretion begins

Week 1

• Fertilization: Union of genetic material, beginning when a capacitated sperm penetrates a secondary oocyte
• Cleavage: Repeated mitotic divisions of the zygote, increasing the number of blastomeres, forming a morula
• Blastocyst Formation: Development of a fluid-filled cavity within the morula, pushing the inner-cell mass to one side, forming the inner cell mass/embryoblast and the trophoblast
• Implantation: Blastocyst embeds in the uterine endometrium, the trophoblast differentiates into two layers: cytotrophoblast and syncytiotrophoblast

Week 2

• Bilaminar Embryonic Disc: Embryoblast differentiates into a bilaminar disc (epiblast and hypoblast)
• Amnionic Cavity: Forms within the epiblast, amnioblasts surround the amniotic cavity forming the amnion
• Primary Umbilical Vesicle: Cells continuous with the hypoblast line the cytotrophoblast forming the primary umbilical vesicle
• Lacunar Network: Syncytiotrophoblast erodes uterine blood vessels forming lacunae, allowing diffusion of nutrients and gases
• Extraembryonic Mesoderm: Cells migrate between the exocoelomic membrane and cytotrophoblast forming spaces, combining to form the future chorionic cavity
• Prechordal Plate: Localized thickening of the hypoblast, an organizing center for head structures

Human Chorionic Gonadotropin (hCG)

• Produced by the syncytiotrophoblast at the end of Week 2
• Maintains the corpus luteum of the ovary, responsible for progesterone and estrogen production

Early Embryonic Period

• Week 3: Gastrulation, neurulation, intraembryonic mesoderm differentiation
• Week 4-8: Organogenesis

Gastrulation

• Bilaminar disc → trilaminar disc (ectoderm, endoderm, mesoderm)
• Establishment of body plan: Cranial-caudal axis, left-right sides
• Begins with the appearance of the primitive streak

Primitive Streak

• Epiblast cells migrate to the median plane
• Primitive node forms, marking the cranial side of the primitive streak
• Invagination of cells creates the primitive groove

Trilaminar Disc

• Ectoderm: Epithelial tissues derived from the epiblast
• Endoderm: Epithelial tissues derived from the hypoblast
• Mesoderm: Mesenchymal or epithelial tissues derived from cells migrating between the ectoderm and endoderm

Notochord Formation

• Migrating mesenchymal cells form the notochord process
• Notochord process binds to the underlying endoderm
• Fused layers degenerate leaving the notochord plate
• Notochord plate folds inward to form the notochord

Neurulation

• Notochord signaling induces the neural plate
• Neural Plate: Thickening of the overlying ectoderm (neuroectoderm)
• Neural Groove: Neural plate invaginates, forming the neural groove and neural folds
• Neural Tube: Neural folds meet in the midline, forming the neural tube

Closure of the Neural Tube

• Begins in the cervical region and occurs bidirectionally
• Complete by the end of Week 4
• Incomplete closure can result in anencephaly or spina bifida

Neural Crest Cells

• Undergo epithelial-to-mesenchymal transition
• Migrate away from the neural folds, forming ganglia, Schwann cells, pharyngeal arches and their derivatives, leptomeninges, melanocytes, odontoblasts, suprarenal medulla

Ectoderm Summary

• Derived from the epiblast
• Neuroectoderm: Thickened region over the notochord that becomes the neural tube and neural crest
• Surface Ectoderm: Becomes the epidermis

Intraembryonic Mesoderm

• Germ layer between the ectoderm and endoderm
• Derived from the epiblast
• Differentiates into three parallel columns:
  • Paraxial Mesoderm: Forms somites
  • Intermediate Mesoderm: Forms kidneys & gonads
  • Lateral Plate Mesoderm: Forms the heart, blood vessels, and lining of body cavities

Paraxial Mesoderm

• Forms paired columns flanking the notochord
• Columns condense and segment into somites around day 22, when the embryo is about 2.5 mm
• Development of 38-39 somites occurs craniocaudally between days 26 and 32
• Somites give rise to:
  • Sclerotome: vertebrae, ribs, posterior skull
  • Myotome: skeletal muscles of limbs and trunk
  • Dermatome: skin over the back and limbs

Intermediate Mesoderm

• Continuous laterally with the paraxial mesoderm
• Precursor to components of the urinary and reproductive systems
• Forms:
  • Kidneys and ureters
  • Gonads, reproductive ducts, glands
  • Uterus

Lateral Plate Mesoderm

• Extends laterally and connects with extraembryonic mesoderm
• Covers the umbilical vesicle and amnion
• Divides into two layers:
  • Somatic (parietal) mesoderm
  • Splanchnic (visceral) mesoderm
• Transverse folding fuses the right and left sides in the midline
• Intraembryonic coelom forms (future thoracic and abdominopelvic cavities)

Mesoderm Summary

• Derived from the epiblast
• Paraxial mesoderm gives rise to somites
• Intermediate mesoderm forms urogenital structures
• Lateral plate mesoderm splits into somatic and splanchnic layers

Endoderm

• Germ layer surrounding the umbilical vesicle
• Derived from the epiblast
• Forms the lining of the gut tube
• Most internal organs develop from the gut tube
• Divided into three sections:
  • Foregut, connected to the oropharyngeal membrane
  • Midgut, connected to the secondary umbilical vesicle
  • Hindgut, connected to the cloacal membrane

Oropharyngeal & Cloacal Membranes

• Locations where ectoderm directly fuses with endoderm
• Oropharyngeal membrane is at the cranial end:
  • Future site of the mouth
  • Connects to the foregut
• Cloacal membrane is at the caudal end:
  • Future site of the anal and urogenital openings
  • Connects to the hindgut

Week 4 Embryonic Development

• Characterized by embryonic folding:
  • Neural tube closure
  • Conversion of elongated disc into curved cylinder driven by differential growth

Embryonic Folding

• Results in the formation of a curved cylinder from the elongating disc
• Driven by differential growth along the longitudinal axis and lateral parts of the disc
• Craniocaudal and transverse folding occur concurrently

Craniocaudal Folding

• Initiated by rapid growth of the neural tube
• Oropharyngeal and cloacal membranes migrate to the ventral surface
• The endoderm of the umbilical vesicle is internalized, establishing the foregut, midgut, and hindgut

Transverse Folding

• Driven by rapid growth of the somites
• Bilateral edges of the conceptus fuse at the midline
• Splanchnic mesoderm and endoderm fusion forms the gut tube
• Somatic mesoderm and surface ectoderm fusion forms the body wall, except at the vitelline duct

Weeks 3 & 4 Summary

• Gastrulation establishes body planes and generates the three germ layers
  • Ectoderm, mesoderm, endoderm
• Notochord acts as a signaling center involved in neurulation
• Neurulation establishes the nervous system and neural crest cells
• Intraembryonic mesoderm differentiates into three paired columns:
  • Paraxial, intermediate, lateral plate
• Endoderm forms the lining of the gut tube
• Oropharyngeal and cloacal membranes are formed
• The conceptus transitions from a disc to a tube via folding:
  • Craniocaudal folding internalizes thoracoabdominal structures
  • Transverse folding forms the gut tube, body wall, and surrounds the conceptus with the amnionic cavity

Teratogens

• Most infant deaths result from birth defects caused by genetic factors, environmental factors, or a combination
• Teratogens are substances, organisms, or physical agents that disrupt normal development
• Consequences of teratogen exposure:
  • Slowed growth
  • Loss of function
  • Malformations
  • Death

This quiz covers the stages of prenatal development, including the pre-embryonic, embryonic, and fetal stages. It also delves into gametogenesis, detailing the processes of spermatogenesis and oogenesis. Test your understanding of these fundamental biological concepts.