Introduction To General Embryology
The Human Development is a continuous process that begins when an ovum from a female is fertilized by a sperm from a male. Cell division, growth and differentiation transform the fertilized ovum (zygote) into a multicellular human being.
Developmental Periods can be divided into: prenatal & postnatal periods.
Normal somatic cells and primordial germ cells contain 46 single chromosomes (23 homologous pairs) and this is called diploid number. 22 pairs of the chromosomes are autosomes and one pair is sex chromosome; XX for female and XY for male. Gametes contain 23 single chromosomes (haploid number). The male gamete (sperm) produced by meiosis in the testis, contains 22 autosomes and either an X or a Y chromosome therefore, the male gamete determines the genetic sex of the individual. The female gamete (ovum) produced by meiosis in the ovary, contains 22 autosomes and an X chromosome.
Chromosomes are the carriers of genes. Each chromosome consists of two arms separated by a centromere. During meiosis I, single chromosomes undergo DNA replication, which essentially duplicates the arms. This forms duplicated chromosomes, which consists of two sister chromatids attached at the centromere Autosomes contain genetic information for most human characteristics. Homologous chromosomes: pair of similar autosomes
Gametogenesis Definition: It is a process by which mature gametes (sperms or ova) are formed from primitive sex cells. Site: It takes place in the testis in male and ovary in female. Gametogenesis is divided into : Spermatogenesis : the formation of male gametes (sperms). Oogenesis: the production of female gametes (ova).
Gametogenesis
Cell Division:1- Mitosis It is a process by which one somatic or primitive germ cell divides giving rise to 2 daughter cells that are genetically identical to parent cell. Each cell contains 46 chromosomes. Before somatic cell enters mitosis, each chromosome replicates its DNA. Mitosis is divided into phases: A-Prophase: Chromosomes begins to coil, contract and condense. Each chromosome now consists of two parallel subunits, chromatids, that are joined at the centromere.
DNA Replication
B-Prometaphase: The chromatids become distinguishable. C-Metaphase: The chromosomes line up in the equatorial plane of the cell. D-Anaphase: The centromere of each chromosome divides longitudinally and each chromatid migrates to the opposite pole of the cell. E-Telophase: The chromosomes uncoil and lengthen, the nuclear envelope reforms and the cytoplasm divides.
Various stages of mitosis. In prophase, chromosomes are visible as slender threads. Doubled chromatids become clearly visible as individual units during metaphase. Blue, paternal chromosomes; red, maternal chromosomes.
2-Meiosis It is a specialized process of cell division that occurs only in reproduction. It is the cell division that takes place in the germ (sex) cells to generate male and female gametes, sperm and egg cells, respectively. Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23. The first of which is a reduction division. The second meiotic division looks like mitosis. The result is the formation of four gametes; each contains half the number of chromosomes (23 single chromosomes).
Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I
Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II
Meiosis I (1) First meiotic division: - The 46 chromosomes in each cells arrange themselves in 23 pairs. - The members of each pair move apart and migrate to the opposite poles of the germ cell. - The cytoplasm divides to form two cells each of which contains 23 chromosomes. a. DNA replication: forming a primary gametocyte (46 duplicated chromosomes, 4N). b. Pairing of each two homologous chromosomes of the 46 duplicated chromosomes. c. Chiasmata formation: where crossing over occurs (large chromatid segments are exchanged, contributing to genetic variability). d. Arrangement: the 46 homologous chromosomes arrange at the metaphase plate.
e. Disjunction: the 46 chromosomes separate from each other, centromeres do not split. f. Cell division: two secondary gametocytes (23 duplicated chromosomes, 2N) are formed. (2) Second meiotic division: - Follows the first meiotic division. - The two chromatids of each chromosomes move apart to the opposite poles of the cells where they form a chromosome. - The cytoplasm then divides to form two cells each contains 23 chromosomes. Results of meiotic division: A primitive germ cells with 46 chromosomes gives four daughter cells each containing 23 chromosomes. 1.Each germ cell contains haploid no. of chromosomes. 2.Genetic variability is enhanced through cross over which redistributes genetic material.
First and second meiotic divisions. A. Homologous chromosomes approach each other. B. Homologous chromosomes pair, and each member of the pair consists of two chromatids. C. Paired homologous chromosomes interchange chromatid fragments (crossover). Note the chiasma. D. Doublestructured chromosomes pull apart. E. Anaphase of the first meiotic division. F and G. During the second meiotic division, the double-structured chromosomes split at the centromere.
Male Genital System
2 testes 1. Primary sex organs: Enclosed in the scrotum. Each testis consists of about 250 lobules. Each lobule contains 1-3 seminiferous tubules. The seminiferous tubules join together forming a network called rete testis in the mediastinum testis (the posterior thick part of the testicular capsule). Functions of testis: formation of sperms & male sex hormone (testosterone).
2. Genital ducts: a) Vasa efferentia: 15-20 tubules connecting the rete testis to the epididymis. b) Epididymis: Comma-shaped organ posterior to the testis. It is formed of head, body & tail which is continuous with the vas deferens. c) Vas deferens: Enters the abdominal cavity where it forms a dilatation called ampulla of the vas that lies behind the urinary bladder. d) Ejaculatory ducts: 2, each is formed by the union of the ampulla & seminal vesicle of one side. They pierce the prostate to open in the urethra.
3.Glands: 1) Seminal vesicles: 2, that lie lateral to the ampulla of the vas. 2) Prostate: Large gland that surrounds the beginning of the urethra. 3) Bulbourethral glands: 2, that open into the penile urethra. 4.External genitalia: the penis.
Spermatogenesis Definition: It is the process by which the spermatogonia are transformed into spermatozoa (sperms). Site: Seminiferous tubules of the testis. Onset: It begins at puberty and continues till very old age. Duration: The time required for a spermatogonium to develop into a mature spermatozoon is about 64 days.
Steps - The spermatogonium which is the primitive male sex cell (46 chromosomes) divides by mitosis into 2 daughter cells (type A and type B). - The type A cell remains in contact with the basement membrane, while the type B continues maturation and is transformed into a larger cell called primary spermatocyte (46 chromosomes). - The primary spermatocyte undergoes the first meiotic division thus giving rise to two secondary spermatocytes each of which has 23 chromosomes. - Each secondary spermatocyte undergoes the second meiotic division thus giving rise to two spermatids each of which has 23 chromosomes. - The spermatids are transformed into complete sperms by a process called spermiogenesis.
Type A spermatogonia, derived from the spermatogonial stem cell population, represent the first cells in the process of spermatogenesis. Clones of cells are established and cytoplasmic bridges join cells in each succeeding division until individual sperm are separated from residual bodies.
Spermiogenesis It is the morphological transformation of spermatids into spermatozoa (sperm). These changes are: 1. The nucleus: enlarges and condenses to form the head of the sperm. 2. The golgi apparatus: forms the acrosomic cap. 3. Formation of neck, middle piece, and tail. 4. Shedding of most of the cytoplasm. 5. The cell membrane: encloses the whole sperm..
The sperms are ejaculated in the seminal fluid which consists of secretions of the sex glands (prostate, seminal vesicles and bulbo-urethral glands). The fluid of an ejaculate measures about 3.5 ml. and contains about 100 million sperms in each 1ml. The sperms are genetically of two types: one carries X chromosome while the other carries Y chromosome. All ova are of one type and carry X chromosome
Hormonal control of spermatogenesis: 1-Luteinizing hormone (LH): secreted by the pituitary gland: it binds to receptors on Leydig cells and stimulates testosterone production, which binds to Sertoli cells to promote spematogenesis. 2- Follicle stimulating hormone (FSH): It binds to Sertoli cells stimulating testicular fluid production. Abnormal sperm: They are frequent. Up to 10%have defects. The head or tail may be abnormal. Spermatozoa may be giant or dwarf and sometimes they are joined. Sperms with morphologic abnormality lack normal motility and don't fertilize oocytes.