I.
Cell Division: Mitosis & Meiosis
Review
A. Usual state of cells: interphase (metabolic
phase) or G0 - "day-to-day" activity
B. Mitosis: cellular duplication - diploid (2N) parent
cells give rise to 2N daughter cells
1. occurs in all tissues of the body - main mechanism
of growth and regeneration following injury
C. Meiosis: reductive cell division - diploid cell
(2N) gives rise to haploid (1N) daughter cells
1. two distinct stages: meiosis I & II
a. interkinesis - between stages I & II
→ no DNA replication occurs - results in
specialized cells = gametes - ovum or sperm
b. gametes – haploid
2. only in specific portions of ovary or testis
Meiosis – formation of gametes
Meiosis – formation of gametes
II. Human early embryology
A. Gametogenesis
1. gametes – haploid - contain 23 chromosomes –
produced via meiosis
2. oogenesis - occurs in ovaries – oogonia enter
prophase I during fetal development → primary
oocytes
a. arrested until puberty – after puberty, primary
oocytes resume meiosis I → secondary oocyte and
polar bodies (degenerate) – 2˚ arrested at
metaphase II & will be ovulated
b. if fertilized – completes meiosis II – 2nd polar body↓
1) not fertilized – degenerates in ~24 hr.
3. spermatogenesis – occurs in testes
a. spermatogonia – diploid → mitosis – 2 primary
spermatocytes
b. spermatocytes → meiosis – 4 haploid spermatids
c. spermiogenesis – maturation → sperm
B. Fertilization
1. following copulation, deposited sperm undergo
capacitation – conditioning for fertil.
2. 3 phases of fertilization:
a. corona radiata penetration
b. zona pellucida penetration
c. fusion of sperm and oocyte plasma membranes
3. nuclear fusion → zygote – diploid – undergoes mitosis
4. cleavage – increases # of cells, size stays same
a. 16 cell stage = morula
b. enters uterine cavity – forms blastocyst (fluidfilled)
5. differentiates to form trophoblast (outer – give rise to
chorion) and embryoblast - pluripotent
C. Implantation – by day 7,
zona pellucida has degraded
1. trophoblast contacts
endometrium cells lining uterus
a. blastocyst invades
superficial functional layer
of endometrium –
embedded by day 9
D. Formation of extraembryonic membranes
1. yolk sac – evol. remnant – site for early blood cell &
vessel formation
2. amnion – surrounds developing embryo – fluid-filled
3. chorion – forms placenta (with allantois)
E. Placenta development
1. placenta - highly vascularized organ - serves as
physical & biochemical interface - embryo & mom
2. 1˚ functions
a. exchange of nutrients, waste products & gases
b. protection - maternal antibodies to embryo
c. hormone production – 1˚ estrogen, progesterone
3. consists of embryonic and maternal tissues
a. connection – stalk – develops into the umbilical
cord
4. chorionic villi – finger-like projections that appear at
leading edge of chorion (CVS?)
a. project into endometrium – contain branches of
umbilical blood vessels from embryo
b. outside villi – maternal blood – metabolic
exchange occurs from embryonic blood → across
villar wall → to maternal blood
III. Embryonic period
A. Initiation – primary germ layer formation – will give
rise to all adult human structures
1. gastrulation – starts with formation of primitive
streak – 3 germ layers form via cell movement –
invagination through primitive streak
2. forms endoderm, mesoderm and ectoderm →
embryonic disc forms
3. differential movement of cells during weeks 3 & 4 folded layers form - gives rise to neurula stage
Gastrulation → neurulation
Synthetic “embryo” - mouse
Natural
Synthetic
Stem cells developed into embryo-like form - beating heart, a gut tube
and neural folds - Amadei, G. et al. Nature https://doi.org/10.1038/s41586-022-05246-3 (2022).
B. Neurulation – formation of neural tube
1. forms from ectoderm – will develop into CNS
a. neural crest ectoderm cells form ridge – edges
approximate (L & R) – closes into neural tube
b. some drugs/pathogens interfere w/closure –
opening persists – spina bifida
2. primary germ layers – give rise to characteristic
structures of the mature body – in general:
a. ectoderm – skin epidermis and nervous system
b. mesoderm – skeleton, muscle
c. endoderm – linings of digestive organs, viscera
d. many structures – composites
The Three Primary Germ Layers
and Their Derivatives
IV. Organogenesis & Fetal Period
Organogenesis – development of organs
1. rudimentary forms – present by week 8 (end of
embryonic period)
2. fetal period – week 9 → birth
a. characterized by growth and maturation of
tissues and organs
A.
Fetal Development