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Stages of Human Development
Ilse Pienaar
Lecture 4
Embryology
Developmental Periods Prenatal Period
Developmental changes before birth
SpermGr. sperma = seed
SpermatozoonL. spermatos = seed; zōon = animal
Zygote 1×sperm * 1×ovum = 1 cell
An embryo
EggOocyteOvum (s), Ova (p)
Stage 1
Developmental Periods Gametogenesis
MeiosisLLLL
Stage 1
2*Diploid46 X
2*Haploid23 X
Developmental Periods- Meiosis I Meiosis I
Interphase
Stage 1
Developmental Periods- Meiosis I Meiosis I
Prophase
Stage 1
Developmental Periods- Meiosis I Meiosis I
Metaphase
Stage 1
Developmental Periods- Meiosis I Meiosis I
Anaphase
Stage 1
Developmental Periods- Meiosis I Meiosis I
Telophase
Stage 1
Developmental Periods- Meiosis II Meiosis II
Prophase II
Stage 1
Developmental Periods- Meiosis II Meiosis II
Metaphase II
Stage 1
Developmental Periods- Meiosis II Meiosis II
Anaphase II
Stage 1
Developmental Periods- Meiosis II Meiosis II
Telophase II
Stage 1
Developmental Periods End stage of Meiosis I & II
23 X
Diploid46 X
23 X 23 X 23 X
Stage 1
Developmental Periods Zygote
Mitotic cell divisions / cleavage
Stage 2
Zygote Morula Early blastocyst
Stage 3
Late blastocyst
Developmental Periods
Stage 4
Implantation Gastrula
The Organogenic Period Week 4 – 8
The major external and internal structures are established
Minimal function
The Organogenic Period Week 4 – 8 Phases of Embryonic Development
Growth
Stage 2 Stage 3
Morphogenesis Differentiation
Stage 1
Germ Layer Derivatives Formed during gastrulation Pimordia of all tissues and organs
Germ Layer Derivatives Ectoderm
Central Nervous System (CNS) Sensory epithelial
Eye, Ear, Nose, Epidermis, Hair, & Nails
Mammary glands Pituitary gland Subcutaneous glands Teeth enamel
Ectoderm
Mesoderm
Endoderm
Germ Layer Derivatives NeuroEctoderm
Neural Crest Cells Spinal Cranial
V, VII, IX, X Autonomic ganglia Ensheathing cells of PNS Muscle Connective tissues Meninges (brain & spinal cord)
Ectoderm
Mesoderm
Endoderm
Germ Layer Derivatives Mesoderm
Connective tissue Cartilage Bone Muscle
Striated & smooth Heart Vessels
Blood & lymph
Ectoderm
Mesoderm
Endoderm
Germ Layer Derivatives Mesoderm
Kidneys Ovaries & testes Genital ducts Serous membranes
Line body cavities Spleen Cortex of suprarenal glands
Ectoderm
Mesoderm
Endoderm
Germ Layer Derivatives Endoderm
Epithelial lining Gastrointestinal, respiratory tracts, tympanic cavity & bladder
Parenchyma of the tonsils Thyroid and parathyroud glands Thymus Liver Pancreas
Ectoderm
Mesoderm
Endoderm
The Organogenic Period Embryonic Folding
From flat trilaminar embryonic disc Cylindrical embryo Median and Horizontal planes
Head Fold Tail Fold
Cranial
CaudalSi
de
Side
The Organogenic Period Median Plane Folding
The Head Fold Brain: Primordium Developing forebrain grows
beyond oropharyngeal membrane Heart: Ventral move – septum transversum,
primordial heart, pericardial coelom, oropharyngeal membrane
Foregut: Primordium of pharynx, esophagus… - lie between the brain and heart – develops into the central tendon of the diaphragm
Embryonic coelom – Primordium of body cavities
The Organogenic Period Medial Plane Folding
Tail fold Growth of the distal part of the neural tube –
primordium of the spinal cord Caudal eminence projects over the claucal membrane Part of the endodermal germ layer is incorporated into
the embryo to become the hindgut – primordium of descending colon
Terminal part of cloaca dilates – primordium of urinary bladder and rectum
The Organogenic Period Horisontal Plane Folding
The Tail Fold Right and left lateral folds Produced by the rapidly growing spinal cord and
somites Midgut: Primordium of small intestine – part of
the endoderm germ layer is incorporated into the embryo
Initial wide connection between the midgut and yolk sac
The Organogenic Period Horisontal Plane Folding
The Tail Fold After folding – connection reduced Reduce communication between intra- and
extraembryonic coelomic cavities
