Lecture title: GENERAL EMBRYOLOGY 2018/2019

Dr Odetola Amoo Amos   (Published 2019)

Dr Amoo Amos
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Lecture Note

EDO UNIVERSITY IYAMHO
Department of Anatomy
ANA 213: General Embryology
Instructor: Dr. Odetola Amos A., email:odetola.amos@edouniversity.edu.ng
Lectures: Friday, 8am –10 am, LT1, phone: (+234) 8032072958
Office hours: Thursday, 1.30 to 2.30 PM, Office:
General overview of lecture: This course aims at giving the students a thorough knowledge
of general embryology. It covers the following topics:Gametogenesis:Spermatogenesis and
Oogenesis; Fertilization; Implantation, Cleavage, Morula, Blastocysts, Primitive Streak,
Bilaminal and Trilaminal Germ Disc, Development of tissues, organs and systems of the
embryo, the chorionic and amniotic cavities, Foetal Membranes, Placenta formation and
functios, molecular regulation in differentiation, Patterning of body axis, and Birth defects,
Twins and Twin defects, embryonic environment and foetal periods.
Topic: Gametogenesis: Mitosis & Meiosis
Learning outcomes:At the completion of this lecture, students should be able to:
• describe the cell division
• mention the source of the primordial germ cells
• differentiate between mitotic and meiotic cell division
• understand the meaning of haloid number of chromosome
Assignments: students are going to havethree (3) individualand one (1) group assignments
throughout the course in addition to a Mid-Term Test and a Final Exam. Assignments are
organized and structured as preparation for the midterm and final exam, and are meant to be a
studying material for both exams.
Grading: We will assign 10% of this class grade to homeworks, 10% for the programming
projects, 10% for the mid-term test and 70% for the final exam. The Final exam is
comprehensive.
Textbook: The recommended textbook for this class are as stated:
Title: Langman’s Medical Embryology
Author: T.W. Sadler
Publisher: Wolters Kluwer Health
ISBN: 978-1-4511-9164-6
Title: Larsen’s Human Embryology
Authors: Gary C.S., Steven B.B., Philip R.B.,andPhilippa H.F.
Publisher: Churchill Livingstone
ISBN: 978-1-4557-0684-6
Title: The Developing Human, Clinically Oriented Embryology 8th Ed.
Author: K.L. Moore and T.V.N.Persaud
Publisher: Saunders
Main Lecture: Below is a description of the contents. We may change the order to
accommodate the materials you need for the projects.
Introduction
v Gametogenesis is the process of formation and development of specialized generative
cells, gametes.
v This process, involving the chromosomes and cytoplasm of the gametes, prepares
these sex cells for fertilization.
v During gametogenesis, the chromosome number is reduced by half and the shape of
the cells is altered.
v Primordial germ cells: give rise to the gametes
Chromosome Theory of Inheritance
v Traits of a new individual are determined by specific genes on chromosomes inherited
from the parents
v Humans have ? 23,000 genes on 46 chromosomes
v Genes on the same chromosome tend to be inherited together and called linked genes
v In somatic cells – 23 homologous pairs = 46 Diploid number
v Autosomes – 22pairs matching chromosomes (XX)
v Sex chromosomes – one pair. XX or XY
v One chromosome of each pair is from the maternal, Oocyte and the other from the
paternal, Sperm.
v Each gamete contains haploid number of chromosomes
Mitosis Cell Division
v Process by which one cell divides and gives rise to two daughter cells that are
genetically identical to the daughter cells.
v Before mitosis, chromosome replicates its DNA
v Chromosomes become extremely long and spread diffusely within the nucleus
v At this stage, chromosome cannot be visualised with light microscope
v Consists of six phases
§ Interphase
§ Prophase
§ Prometaphase
§ Metaphase
§ Anaphase
§ Telophase
§ Prophase
• Chromosome continues to condense, shorten and thicken
§ Prometaphase
• Chromosome becomes visible under light mricroscope
§ Metaphase
• Xsome line up in the equator
• Each xsome is attached by microtubules extending from the centromere to the
centriole
§ Anaphase
• Centromere of each xsome divides
• Migration of chromatids to the opposite pole of the spindle
§ Telophase
§ Cytokinesis
Meiotic Cell Division
v Cell division that takes place in the germ cells in which diploid number of
chromosome is reduced to haploid
v It requires two cell divisions – Meiosis I & II
First Meiotic Cell Division
v Germ cells replicate their DNA so that each of the 46 chromosomes is duplicated into
sister chromatids
v Synapsis – homonogous chromosomes align themselves in pairs
v Points of interchange are temporarily united and form an X-like structure, Chiasma
v Homonogous pairs then separate into two daughter cells
v Thus reducing chromosome numbers from diploids to haploid
v Points of interchange are temporarily united and form an X-like structure, Chiasma
v Homonogous pairs then separate into two daughter cells
v Thus reducing chromosome numbers from diploids to haploid
2nd Meiotic Cell Division
v The cell divisions of meiosis are equal and yield four identical spermatozoa in male
• In females, it yields a single, massive, haploid definitive oocyte and three-minute,
non-functional, haploid polar bodies.
v Metiotic cell divisions brings about:
Ø Genetic variability through
§ Crossover which re-distributes genetic materials
§ Random distribution of homologous chromosomes to the daughter cells
Ø Germ cells with haploid number of chromosome
Morphological Changes During Maturation of the Gametes
v Oogenesis
v Spermatogenesis
Oogenesis
v The process whereby oogonia differentiate into mature oocyte
v Maturation of oocytes begins before birth
v Complete after puberty
Prenatal Maturation of Oocytes
v Once primordial germ cells arrived in the gonad of a genetic female they differentiate
into oogonia
v Oogonia then undergo a number of mitotic divisions
v By the end of the third month, they are arranged in clusters surrounded by a layer of
flat epithelial cells
v The flatten epithelial cells are called follicular cells
v They are derived from the surface (Germinal) epithelium of the ovary
v Many oogonia continue mitotic division
v some are arrested in the prophase of meiosis I and become primary oocytes
v Oogonia proliferate and increase rapidly in number
v By 5th month of development, the total number of germ cells in the ovary reaches its
maximum – 7millions
v Many oogonia and primary oocytes degenerate and become atretic.
v By 7th month majority of oogonia have degenerated
§ At birth, only seven hundred thousand to two million remain
§ During childhood, most oocytes become atretics.
§ Only about 40,000 oocytes are present at the beginning of puberty
§ All surviving primary oocytes have entered prophase of meiosis I
§ Each primary oocyte becomes surrounded by a layer of flat follicular epithelial cells
§ Primary oocyte with its surrounding flat epithelial cells is called primordial follicle
§ the primary oocyte enlarges, the follicular epithelial cells become cuboidal in shape
and then columnar, forming a Primary follicle.
§ The primary oocyte soon becomes surrounded by a covering of amorphous acellular
glycoprotein material, the zonapellucida
§ Near the time of birth, all the primary oocytes have started prophase of meiosis I and
become arrested at diplotene stage.
§ Completion of prophase does not occur until puberty.
§ The follicular cells surrounding the primary oocyte secrete a substance, oocyte
maturation inhibitor (OMI), which keeps the meiotic process of the oocyte arrested.
Postnatal Maturation of Oocytes
v As primordial follicles begin to grow, the surrounding follicular cells change from flat
to cuboidal and proliferate to produce a stratified epithelium of granulosa cells and
the unit is called a primary follicle
v Granulosa cells rest on a basement membrane which separate it from the ovarian
connective tissue called theca folliculi
v Granulosa cells and the oocytes secrete a layer of glycoprotein called Zonalpellucida
on the surface of the oocytes
v Theca folliculi organise into two layers:
§ Theca interna – an inner secretory cells
§ Theca externa – an outer fibrous capsule
v As the development continues, fluid-filled spaces appear between granulosa cells
v These spaces coalesce to form antrum and the follicle is now called Antral or
vesicular follicle
v Granulosa cells surrounding the oocyte remain intact and form the cumulus oophorus
v Mature vesicular follicle is called Graafia follicle
v At maturity, Meiosis I is completed resulting in the formation of two daughter cells of
unequal size
v Each with 23-double stranded chromosomes
v One cell, secondary oocyte, receives most of the cytoplasm while the first polar body
receive little or none
v The 1st polar body lies in the perivitelline space (space between zonalpellucidal and
the cell membrane of the secondary oocyte)
v Secondary oocyte then enters meiosis II but arrests in metaphase 3hrs before
ovulation
v Meiosis II is completed only if fertilization occurs.
v If no fertilization, the egg degenerates about 24hrs after ovulation
• First polar body may undergo second division
Nuclear Maturation of Germ Cell in Meiosis
Spermatogenesis
v Def:is the sequence of events by which spermatogonia are transformed into mature
sperms.
v This maturation process begins at puberty.
v Primordial germ cells give rise to spermatogonia stem cell
v Cells emerge from the stem cell to form type A spermatogonia
v Type A cells undergo limited number of mitotic divisions to form clones of cells
v The last cell division produces type B spermatogonia which then divide to form
primary spermatocytes
v Primary spermatocytes enter a prolonged prophase (22days)
v Followed by rapid completion of meiosis I and formation of secondary spermatocytes
v Second meiotic division immediately begin to form haploid spermatids
v Throughout this series of events cytokinesis is incomplete so that successive cell
generations are joined by cytoplasmic bidges
Spermiogenesis
v Def:The series of changes resulting in the transformation of spermatids into
spermatozoa
v These changes include
§ Formation of acrosome which covers half of the nuclear surface and contains enzyme
§ Condensation of the nucleus
§ Formation of neck, middle piece and tail
§ Shedding of most of cytoplasm as residual bodies that are phagocytized by sertoli
cells
v In human, it takes about 74days for spermatogonium to develop into mature
spermatozoa
v Spermatozoa attain full motility in the epididymis
Conclusion
v The first meiotic division produces haploid double stranded chromosome while the
second produces haploid single stranded chromosome
v Male and female gametes are haploid cells which are produced after second meiotic
division
References
• Sadler TW. Langman’s Medical Embryology. 13th Ed. Philadelphia: Wolters Kluwer,
2015.
• Gary CS, Steven BB, Philip RB, Philipa HF, Larsen’s Human Embryology. 5th Ed.
Philadelphia:Churchilllivingstone. 2015.
• Keith LM, Persaud TVN. The developing human clinically oriented embryology. 8th
Ed.


Item Type: Lecture note(non-copyrighted)
Format: PDF document,   334.38 KB
Copyright: Creative Commons LicenseCreative Commons license
Keywords: anatomy
Department: Medicine and Surgery
Field of Study: Human Anatomy
Uploaded By: Uwaifo Ferdinand
Date Added: 04 Feb 2019 7:40am
Last Modified: 04 Feb 2019
Lecture URL: https://www.edouniversity.edu.ng/oer/lecturenotes/general_embryology_20182019


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